Wei-Kang Song

Prof Yu-Jun Cui
email: yu-jun.cui@enpc.fr

Anh Minh Tang

Wen-Qi Ding

Qiong Wang

Experimental study on water evaporation from compacted clay using environmental chamber

Keywords: environmental chamber, compacted clay, air/soil parameters, evaporation, surface crack ratio

Water evaporation induces large volume change for clayey soils, often causing problems to geotechnical and geoenvironmental constructions. To better understand this process, an evaporation test on a compacted clay was conducted in a large-scale environmental chamber under controlled atmospheric conditions. Atmospheric parameters (wind speed, air temperature and relative humidity) and the response of soil parameters (volumetric water content, temperature, soil suction as well as desiccation cracks) were monitored. The results show that the soil temperature is strongly related to the air conditions, evaporation process and desiccation cracks.

Unlike for sand, the evolution of volumetric water content is governed by both the high water retention capacity of clay and the effect of cracks. A three-stage evolution can be observed for not only the actual evaporation rate but also the surface crack ratio. Thus, the surface crack ratio can be considered as one important parameter in the evaporation analysis taking into account the effect of desiccation cracks.

Introduction

Water evaporation from clayey soils can result in significant changes in soil suction, water content and temperature, giving rise to significant soil volume change and hence strongly influencing the performance of the involved geotechnical and geoenvironmental constructions: the long-term behavior of pavement and embankments/dams can significantly change due to the desiccation cracks and settlement induced by the decrease of water content [START_REF] Cui | Simulating the water content and temperature changes in an experimental embankment using meteorological data[END_REF][START_REF] Puppala | Threshold moisture content and matric suction potentials in expansive clays prior to initiation of cracking in pavements[END_REF], Tang et al. 2011a); buildings with shallow foundations and other geotechnical constructions can be seriously damaged due to the differential settlement induced by water evaporation [START_REF] Silvestri | Foundation problems in Champlain clays during droughts. Ⅰ: Rainfall deficits in Montréal (1930-1988)[END_REF][START_REF] Cui | Water balance and evapotranspiration monitoring in geotechnical and geoenvironmental engineering[END_REF][START_REF] Corti | Simulating past droughts and associated building damages in France[END_REF][START_REF] Corti | Drought-induced building damages from simulations at regional scale[END_REF][START_REF] Qadad | Influence of the soil-atmosphere exchange on the hydric profile induced in soil-structure system[END_REF]; the behavior of soil covers used in mine tailings or other hazardous waste landfills (Wilson 1990;Wilson et al. 1994Wilson et al. , 1997;;Yanful and Choo 1997;Yang and Yanful 2002;Yanful et al. 2003;[START_REF] Cui | Water balance and evapotranspiration monitoring in geotechnical and geoenvironmental engineering[END_REF] and also the landfills for municipal solid waste disposal [START_REF] Blight | Graded landfill requirements in South Africa-the climatic water balance classification[END_REF][START_REF] Blight | Solar heating of the soil and evaporation from a soil surface[END_REF] can be also strongly affected by water loss due to evaporation; the safety of high-level radioactive waste repository as a result of changes in soil hydro-mechanical properties induced by ventilation during the construction and operation periods is also an important issue [START_REF] Bond | Approaches for representing hydro-mechanical coupling between sub-surface excavations and argillaceous porous media at the ventilation experiment, Mont Terri[END_REF][START_REF] Millard | Accounting for anisotropic effects in the prediction of the hydro-mechanical response of a ventilated tunnel in an argillaceous rock[END_REF]). These problems clearly show the importance of well understanding the mechanisms of water evaporation from clayey soils. Indeed, when these mechanisms are well identified, it is possible to develop relevant water evaporation models for predicting soil deformation/settlement (e.g., [START_REF] Hemmati | Soil-vegetation-atmosphere interaction by a multiphysics approach[END_REF][START_REF] Hemmati | Thermo-hydro-mechanical modelling of soil settlements induced by soil-vegetation-atmosphere interactions[END_REF][START_REF] Cui | Experimental and numerical investigation of soil-atmosphere interaction[END_REF], and for designing geotechnical and geoenvironmental constructions such as soil covers (e.g., Yanful and Choo 1997;Yanful et al. 2003).

Up to now, many experiments were conducted to investigate water evaporation from clayey soils. For instance, an evaporation experiment was performed by [START_REF] Garnier | Determining the hydraulic properties of a swelling soil from a transient evaporation experiment[END_REF] on a swelling soil sample (48 mm in diameter and 50 mm in height) to determine the evaporation rate and soil hydraulic properties of soil. Yanful and Choo (1997) performed an evaporation test using an environmental chamber on a clay considered as a candidate cover soil. A clear evolution of drying front was observed through the water content profile; moreover, it was found that the soil temperature firstly decreases and then increases during evaporation and the evolution of evaporation rate presents typical three stages. Later, Yang and Yanful (2002) and Yanful et al. (2003) investigated the evaporation and drainage phenomena on a compacted sample of Halton clayey till (115 mm in diameter and 255 mm in height) with both constant (Yanful et al. 2003) and variable water table (Yang and Yanful 2002). The results showed that water table change has no significant effect on the evaporation and drainage processes, verifying the effectiveness of this soil for an oxygen barrier in sulfide-bearing mine waste covers. Wilson et al. (1997) studied the Regina clay with a small sample of 0.2-0.3 mm thick and found that the ratio of actual evaporation rate to potential evaporation rate and soil suction show a unique relationship, independent of soil properties and drying time. [START_REF] Lee | Evaporation theory for deformable soils[END_REF] performed evaporation tests on Yulchon clay using a column of 240 mm diameter and 800 mm height and the results were used to verify a model of water evaporation rate from the surface of a deformable material.

On the other hand, desiccation cracking occurs easily for clayey soils upon drying.

Many studies were conducted to the evolution of soil cracking with changes in water content. [START_REF] Nahlawi | Laboratory experiments on desiccation cracking of thin soil layers[END_REF] conducted a series of desiccation cracking tests on a thin clayey soil layer in some narrow perspex and metal molds under controlled relative humidity and temperature conditions. They observed that the water content at crack initiation generally increases with the increase of clay layer thickness. The water content values at the end of the tests under different initial conditions are similar when the air condition is the same. [START_REF] Tang | Experiment evidence on the[END_REF] performed desiccation tests on soils from slurry state in a glass cup. They found that the initial critical water content which corresponds to the initiation of desiccation cracking increases with temperature. By contrast, the final critical water content which corresponds to the transition point where the surface crack ratio trends to reach a stable value is not significantly affected by temperature. [START_REF] Péron | Experimental study of desiccation of soil[END_REF] carried out both free desiccation tests and constrained desiccation tests on a slurry clayey soil for investigating the mechanisms of desiccation cracking. They reported that the local suction and water content values at cracking initiation are close to the air entry values. Tang et al. (2011b) conducted desiccation tests on different soils for investigating the wetting-drying effect. They found that the water content at cracking initiation increases rapidly during the first three drying paths, and change slightly during the subsequent drying paths. [START_REF] Li | Study of desiccation crack initiation and development at ground surface[END_REF] studied the initiation and development of crack geometric parameters at the compacted soil surface and excavated soil surface in the field. They reported that the evolution of desiccation cracks can be divided into three stages: initial stage, primary stage, and steady state stage. Moreover, when the water content reaches a critical value cracks develop quickly, and when the water content approaches the shrinkage limit cracking reaches a steady state. Yesiller et al. (2000) reported a significant effect of fine content. In general, high suctions, rapid increases in suctions, and high amount of cracking were observed in soils with high fine contents. Summarizing, the above-mentioned studies mainly focused on the effect of temperature, thickness, soil type and wetting-drying cycles on the development of cracks, the effect of cracks on evaporation being rarely mentioned. [START_REF] Ta | Etude de l'interaction sol-atmosphere en chambre environmentale[END_REF] and [START_REF] Cui | Experimental and numerical investigation of soil-atmosphere interaction[END_REF] conducted evaporation tests on a large Romainville clay sample (1000 mm length, 800 mm width, and 1000 mm depth) using an environmental chamber, showing significant influence of desiccation cracks on the evolution of evaporation rate.

Further examination of the experiments reported in literature shows that few tests were conducted with full control of atmospheric conditions and complete measurement of soil response to evaporation. Indeed, in most experiments the soil was poorly instrumented (e.g., Wilson et al. 1997;[START_REF] Garnier | Determining the hydraulic properties of a swelling soil from a transient evaporation experiment[END_REF]Yanful and Choo 1997;Yang and Yanful 2002;Yanful et al. 2003;[START_REF] Lee | Evaporation theory for deformable soils[END_REF], although the water evaporation process has been recognized to be governed by both atmospheric and soil conditions for bare soils.

In this study, the process of water evaporation was investigated on a compacted clay sample (1000 mm×800 mm×250 mm) in a large-scale environmental chamber which was previously used for investigating water evaporation from sand [START_REF] Song | Development of a large-scale environmental chamber for investigating soil water evaporation[END_REF][START_REF] Song | Experimental study on water evaporation from sand using environmental chamber[END_REF] under controlled atmospheric conditions (i.e., controlled air relative humidity, temperature and air flow rate) and with a steady water table. The atmospheric parameters (air temperature, relative humidity) and the response of soil parameters (soil temperature, volumetric water content, matric suction and surface crack ratio) were monitored by various sensors for 83 days, including the soil parameters on the surface (soil surface temperature and matric suction). The obtained results help better understand the mechanisms of water evaporation from clayey soils, and they can also be used for further theoretical and numerical analyses.

Materials

The clayey soil selected for this investigation was used for the construction of an experimental embankment in Héricourt, France. Its geotechnical properties are presented in Table 1. The soil is a highly plastic clay according to the Casagrande's classification criterion and belongs to the CH group following the unified soil classification system (USCS). The grain size distribution curve determined by wet sieving and sedimentation is shown in Fig. 1.

Experimental set-up

The evaporation test was performed in a specially designed environmental chamber system (Fig. 2). The environmental chamber system is a large acrylic transparent chamber combined with wind supply unit (Fig. 3), air collection unit (Fig. 4a), photograph collection unit (Fig. 4b), water supply unit (Fig. 4c) and data logging system (Fig. 4d), controlling the changes of atmospheric conditions and monitoring the evolution of soil responses. In the evaporation test, the compressed air was firstly adjusted by the air flow regulator and the real flow was measured by a flowmeter (Fig. 3a). Then, it was heated using heating hoses (Fig. 3b) with the temperature controlled by a temperature regulator (Fig. 3c). The air temperature and relative humidity were measured before air was diffused into the chamber by an air distributor. After passing through the chamber, the air stream was gathered by the air collection unit for measuring its temperature and relative humidity (Fig. 4a). During the test, the evolution of desiccation cracks was captured by the photograph collection unit (Fig. 4b). The water table was kept constant by the water supply unit, and the change of water table was monitored using a graduated tube (Fig. 4c). All data were recorded using the data logging system (Fig. 4d).

For monitoring the atmospheric conditions and the response of soil, various sensors were buried inside the soil or installed at different positions of chamber (see Figs. 2 and5). In particular, four high-capacity tensiometers were installed at different depths along the chamber wall for measuring the soil matric suction; six soil Page 8 of 57 Can. Geotech. J. Downloaded from www.nrcresearchpress.com by HARRIS LEARNING LIBRARY on 05/01/16

For personal use only. This Just-IN manuscript is the accepted manuscript prior to copy editing and page composition. It may differ from the final official version of record. moisture sensor namely ThetaProbe (ML2x) were buried at different depths for measuring the volumetric water content; five soil temperature sensors (PT1000) were installed in the soil sample at different depths; five T3111 transmitters were installed at different locations for monitoring air temperature and relative humidity; five thermistors were fixed at different heights above the soil surface for monitoring air temperature; an infrared thermometer (Pyropen-D) was installed at the chamber cover for measuring the soil surface temperature. A digital camera (Canon EOS400D) was used for taking photos of soil surface. A flowmeter (MAS-3120) and an anemometer (Testo 435-2) were used to measure the air flow rate and the corresponding wind speed inside the chamber, respectively. More details of this environmental chamber and the sensors used can be found in [START_REF] Song | Development of a large-scale environmental chamber for investigating soil water evaporation[END_REF].

Note that this chamber paid more attention to the parameters in the near soil surface zone than the one reported by [START_REF] Ta | Etude de l'interaction sol-atmosphere en chambre environmentale[END_REF].

Test procedure

The clayey soil transported from the Hércourt site was air-dried, sieved at 2 mm and then stocked in a large sealed plastic box. Note that the gravimetric water content of the clay powder was 6.4 %. Prior to soil compaction, a 6.5 mm thick gravel layer was compacted on a geotextile layer above the bottom of chamber (Fig. 6a). The gravel layer surface was controlled by a level bar (Fig. 6a). This layer is termed as drainage layer. Another geotextile layer overlaid this layer and the edges of geotextile were taped to the chamber wall, avoiding migration of clay particles.

A mass of 59.58 kg of soil powder was poured into the chamber, smoothed using a wood plate with level bar (Fig. 6b) and compacted manually using a steel plate to have an uniform layer of 50 mm thick (Fig. 6c) with a dry density of 1.4 Mg/m 3 . Note that this is also the in-situ dry density of the embankment soil at the site of Héricourt [START_REF] Dong | Investigation of aggregates size effect on the stiffness of lime and/or cement treated soil: from laboratory to field conditions[END_REF]. This procedure was repeated for other layers until reaching the total height of 250 mm.

During the compaction, various sensors were installed in the soil between the layers.

Five PT1000 sensors for soil temperature were installed every 50 mm (i.e., 50, 100, 150, 200 and 250-mm depths) (Fig. 6d). All these sensors were buried in the zone 300 mm far from the chamber wall (Fig. 6d) in order to minimize the effect of laboratory temperature changes. Six ThetaProbe sensors were buried at different depths. Three of them were at 80 mm, 130 mm and 230 mm below the soil surface, and the other three were in the near surface zone at 25, 40 and 55-mm depths, respectively. For the sake of minimizing the effect of sensor installation on the soil density, a hole with similar size as ThetaProbe was created at the defined depth for inserting the sensor (Fig. 6e). Then, the hole was backfilled by the same soil powder with the calculated quantity and then compacted manually to reach the same dry density of 1.4 Mg/m 3 . Notably, the ThetaProbe was calibrated before installation according to the method proposed by [START_REF] Tang | Development of a large-scale infiltration tank for determination of the hydraulic properties of expansive clays[END_REF].

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For personal use only. This Just-IN manuscript is the accepted manuscript prior to copy editing and page composition. It may differ from the final official version of record.

Three high-capacity tensiometers were installed at various depths (i.e., 25, 77 and 173 mm below the soil surface) along two sides of chamber wall for the matrix suction measurements during the saturation process. Five thermistors measuring the air temperature were fixed at different elevations (i.e., 50, 170, 235, 330 and 425 mm above the soil surface) along one inside wall of the chamber. Two relative humidity sensors (T3111 transmitters) were installed at 50 and 275-mm heights for monitoring the air relative humidity, while three other relative humidity sensors were installed for recording the relative humidity at inlet, outlet and in the laboratory. One tensiometer was installed at a depth of 15 mm below the soil surface for measuring the near surface matric suction. An anemometer with a telescopic handle was fixed at one side of the chamber cover, allowing measuring the wind speed at 50 mm above the soil surface. After these sensors were installed, the chamber cover was sealed by silicon to ensure the air-tightness. Moreover, an infrared thermometer was fixed on the cover to monitor the soil or water surface temperature at the center. Four Light Emitting Diodes (LEDs) were installed around the four edges of the transparent chamber cover, allowing the soil surface to be lighted for better photographing.

Finally, the bottom of chamber was connected to a water tank with its water level kept at the level of the soil surface. The saturation process was then performed with water flow from soil bottom firstly and then from soil surface for accelerating this process.

Finally, a water layer was formed on the soil surface and the volumetric water content sensors showed stable values. After the saturation, the evaporation experiment was started by imposing an air flow of 155 L/min and with a thin water layer (6-15 mm in thickness) on soil surface. The inlet air was heated at a temperature as high as 200 °C, so that a quite low relative humidity (1.5 % to 3 %) and a high temperature (56 ± 4 °C) at the inlet of the chamber were obtained. Furthermore, photographs of soil cracks were taken every 90 min. The surface crack ratio defined as the ratio of the surface area of cracks to the total initial surface area can be then determined using the digital image processing technique (see [START_REF] Tang | Influencing factors of geometrical structure of surface shrinkage cracks in clayey soils[END_REF]. Note that the water level was controlled at the initial location by regularly adding water to the tank and the quantity of water added was also recorded.

The evaporation rate was determined based on the change of absolute humidity at the inlet and outlet of chamber and the air flow rate, as follows:

[1] 86400 ( ) /( )

outlet inlet w E Q h h A ρ = -
where E is the actual evaporation rate (mm/day), h outlet is the absolute humidity of air flow at the outlet of chamber (Mg/m 3 ), h inlet is the absolute humidity of air flow at the inlet of chamber (Mg/m 3 ), Q is the air flow rate through the chamber (L/s), ρ w is the water density (Mg/m 3 ), and A is the evaporative surface area of soil in the chamber (m 2 ).

More details about this method can be found in the works of [START_REF] Mohamed | Solute transport through unsaturated soil due to evaporation[END_REF], [START_REF] Aluwihare | Measurement of evaporation on bare soil and estimating surface resistance[END_REF] and [START_REF] Song | Development of a large-scale environmental chamber for investigating soil water evaporation[END_REF][START_REF] Song | Experimental study on water evaporation from sand using environmental chamber[END_REF].

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For personal use only. This Just-IN manuscript is the accepted manuscript prior to copy editing and page composition. It may differ from the final official version of record. The changes of air temperature at the inlet and outlet and in the laboratory are shown in Fig. 9. On the whole, the air temperature was relatively constant. The highest air temperature was for the inlet with a value as high as 56 ± 4 °C, whereas the value at the outlet was lower and was increasing during the test from 26.5 °C to 33 °C. The laboratory room temperature varied from 17 °C to 24.5 °C and was lower than those at the inlet and outlet. All temperature data recorded are used to plot the air-soil temperature profiles (Fig. 12). Generally, the air temperature was significantly higher than the soil temperature and large temperature gradient was observed at the air-soil interface. For the air temperature, the elevations above 170-mm height presented similar values and a large temperature gradient was observed from 50-mm height to the soil surface. Regarding the soil temperature changes, at the beginning of evaporation, the temperature at the soil surface was the lowest value and the temperatures below the 50-mm depth were quite similar, around 20.8 °C. Furthermore, the soil temperatures in the zone below 50-mm depth showed a linear distribution over depth: the soil temperatures in this zone increased over depth before t = 16 days while the trend was inversed after this time. On the other hand, a large temperature gradient occurred between the soil surface and the 50-mm depth and it became larger and larger after t = 16 days.

Results and discussion

However, the temperature gradient below this depth was small. Beyond the phenomena listed above, the soil temperature also increased with the increase of air temperature, suggesting that the energy for evaporation was from the hot air and that part of the energy was used for heating soil and air. Moreover, the decrease of temperature gradient between 50-mm height and soil surface can be attributed to the increasing energy used for heating soil and air when the evaporation rate decreased.

The enlarged temperature gradient between the soil surface and the 50-mm depth showed a deepening of the drying front, as observed in the sand evaporation test [START_REF] Song | Development of a large-scale environmental chamber for investigating soil water evaporation[END_REF][START_REF] Song | Experimental study on water evaporation from sand using environmental chamber[END_REF]. For the soil temperature evolution, the changes of soil temperature with air temperature changes were found larger in clay than in sand (see [START_REF] Song | Experimental study on water evaporation from sand using environmental chamber[END_REF]. This phenomenon may be attributed to the effect of desiccation cracks: cracks change the evaporation process from one-dimensional to three-dimensional situation, allowing the air condition to affect the soil temperature in deeper zones through them.

The changes of air relative humidity are shown in Fig. 13. The imposed relative humidity at the inlet was extremely low, ranging from 1.5 % to 3 %. The values at other locations were much higher and showed a decreasing trend: the values at the outlet and at 235-mm height decreased from 40 % to 11 %; the value at 50-mm height was the highest and varied from 13 % to 55 %. On the other hand, the relative humidity in the laboratory varied with a large fluctuation between 10 % and 50 %, and presented a quite different evolution pattern with respect to other positions. On the whole, the variations of relative humidity in the chamber (50, 235-mm depth and outlet) can be divided into three parts: (1) a decrease with quite low rate in the first 15 days; (2) a sharply decline until t = 28 days; and (3) a slow decrease followed by a stabilization at the end of test. More precisely, the relative humidity at 50-mm height declined slowly from 55 % to 41 % during the first 15 days, then significantly from 41 % to 19 % until t = 28 days, and finally reached a value as low as 13 % at the end of test. Furthermore, the value at 235-mm height was a little higher than that at the outlet in the first 16 days and then they became similar during the rest of time. Note that the similar three-stage evolution of air relative humidity inside the chamber was found in the sand evaporation test [START_REF] Song | Experimental study on water evaporation from sand using environmental chamber[END_REF]) and clay evaporation test [START_REF] Ta | Etude de l'interaction sol-atmosphere en chambre environmentale[END_REF]. This phenomenon can be attributed to the loss of water vapor from soil with the decreasing evaporation rate. a linear relationship between water content and depth can be identified and this gradient developed progressively toward the deeper zones. For instance, the linear profile appeared in the first 55 mm depth zone from the beginning to t = 60 days; it was observed from the 40-mm depth to the 80-mm depth after this time. This gradient was also the maximum for the whole depth.

The evolutions of volumetric water content at various depths can be clearly evidenced through the contour map (Fig. 16). On the whole, the lines with low water content values appeared later than with higher ones: the line with 60 % water content appeared at the initiation of evaporation; the line with 50 % water content at t = 13.5 days; the line with 30 % water content at t = 26 days and the line with 15 % water content at t = 39 days. Furthermore, the lines advanced toward deeper zones, indicating that water loss gradually deepened. On the other hand, the densely-distributed contour lines in the zone of 80-mm depth indicated a large water loss. In other words, water evaporation occurred mainly in this zone. The evolutions of volumetric water content at each depth can also be observed in this figure. For the water content at 25-mm depth, the value decreased to 60 % at t = 6.4 days; it declined to 50 % at t = 13 days, to 40 % at t = 14.2 days, to 30 % at t = 26.3 days and to 20 % at t = 32.8 days. Finally, it became lower than 15 % after t = 39 days.

Fig. 14 confirms that the dense installation of sensors in the near surface zone allows a good monitoring of volumetric water content, avoiding determining the water content by the destructive oven-drying method (e.g., [START_REF] Ta | Etude de l'interaction sol-atmosphere en chambre environmentale[END_REF]. In general, the surface soil layer lost water firstly and the deeper zone started to lose water only by the end of test. Similar results were obtained in the sand evaporation test by [START_REF] Song | Experimental study on water evaporation from sand using environmental chamber[END_REF].

Comparison between the case of Fontainebleau sand and the case of clayey soil in this study shows that the scenarios are quite different: for the clay, only the water content at 25-mm depth decreased quickly at the initiation of evaporation, and then at deeper locations. However, for sand, the entire zone from the surface to the 55-mm depth lost water quickly. This can be attributed to the higher water retention capacity and lower permeability of the clay.

Regarding the water content profiles (Fig. 15), the change of profile in the zone from the surface to the 55-mm depth at t = 12 days indicated a rapid decrease of water content and suggested a possible transition of evaporation mode from one-dimensional to three-dimensional: the water inside soil evaporates only from soil surface firstly, and then from both the crack walls and soil surface. Similar phenomenon was observed by [START_REF] Konrad | Desiccation of a sensitive clay: field experimental observations[END_REF] in a field experiment. Similar evolution of water content profile was also observed in their study. A linear relationship between water content and depth was observed in the near surface zone, and this linear relationship extended to deeper zone as also observed in the sand evaporation test by [START_REF] Song | Experimental study on water evaporation from sand using environmental chamber[END_REF]. This linear relationship was also observed by [START_REF] Ta | Etude de l'interaction sol-atmosphere en chambre environmentale[END_REF] through measuring the water content by oven-drying.

The evolutions of soil matric suction at different depths are presented in Fig. 17. All values at various locations were increasing with water loss: the deeper the location the lower the suction and the later the initiation of increase of suction. The soil matric suction at 15-mm depth increased quickly from 10 kPa at t = 11 days to 1000 kPa at t = 15.3 days. Similarly, the suction at 25-mm depth also sharply increased up to the maximum value of 1305 kPa at t = 19 days. However, the suction at 77-mm depth increased slowly from approximately 10 kPa at t = 13.1 days to 30 kPa at t = 26 days, and then it sharply increased up to the maximum value of 1074 kPa at t = 29.5 days.

As far as the 173-mm depth is concerned, the value reached 10 kPa until t = 22.3 days, and then it increased at a low rate up to 37 kPa at t = 55 days, and finally reached its top value of 369 kPa at t = 63 days. It is noted that the suction at 25-mm depth increased more quickly than that at 15-mm depth. This can be attributed to the shrinkage of soil body when undergoing evaporation. Actually, the tensiometer at 25-mm depth was installed at the edge of chamber and in contact with soil, but the one at 15-mm depth was inserted into the soil from the surface. Thus, during evaporation the soil lost water and shrank. As a result, the water evaporation from the edge of soil was quicker than inside the soil.

The profiles of soil suction are shown in Fig. 18 (without considering the data at 25-mm depth). It can be observed that the matric suction decreased over depth with the largest suction gradient when approaching the soil surface. The suction gradient from 15-mm to 77-mm depth increased sharply after t = 16 days, from 0.06 kPa/mm at t = 16 days to 20.8 kPa/mm at t = 19 days. This phenomenon corresponds to a large variation of volumetric water content during these days (see Fig. 15). Furthermore, the suction gradient between 77-mm depth and 173-mm depth became larger after t = 19 days, indicating a deepening of evaporation process and thus an acceleration of water loss in this zone. Note that the fact that the largest suction gradient being at the surface zone was also observed in the sand evaporation experiment [START_REF] Song | Development of a large-scale environmental chamber for investigating soil water evaporation[END_REF][START_REF] Song | Experimental study on water evaporation from sand using environmental chamber[END_REF].

Page 21 of 57

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The simultaneous measurements of suction and volumetric water content at various depths allow the determination of the soil water retention curve, as shown in Fig. 19.

It appears that the relationships between the water content and suction at 15-mm and 25-mm depths are quite different with those at other depths. This is probably due to the difference in soil density over depth which was strongly affected by the swelling during saturation process on one hand and the development of surface desiccation cracks on the other hand. Indeed, the surface desiccation cracks increased quickly when the suction was larger than 10 kPa which corresponds to a volumetric water content of 53%.

The actual evaporation rate determined following eq. ( 1) is plotted in Fig. 20. Three stages can be identified: a stage of constant value around 2.3 mm/day during the first 15 days (constant-rate stage); a stage of sharp decrease down to 0.5 mm/day at t = 45 days (falling-rate stage); and a stage of decrease at a quite low rate of 0.3 mm/day followed by stabilization (slow-rate stage). Note that the similar three-stage evolution of actual evaporation rate was observed in the evaporation test on clayey soil conducted by Yanful and Choo (1997).

Figure 21 presents the evolution of the surface crack ratio during evaporation. It is observed that the evolution of the surface crack ratio can also be divided into three stages as for the actual evaporation rate (Fig. 20) and the relative humidity inside the chamber (Fig. 13): (1) an increase stage until t = 10 days with low values close to zero;

(2) an rapid increase stage from t = 10 days to t = 25.5 days and (3) a steady stage with a value around 25.3% until the end of test. Comparing the evolutions of actual evaporation rate and surface crack ratio, a clear phenomenon can be identified: with the development of cracks, more water can be evaporated, leading to or keeping in a high evaporation rate (the constant-rate stage). With further development of cracks, there is less water being evaporated, leading to a decrease of evaporation rate (the falling-rate stage). Finally, the evaporation rate reaches the slow-rate stage and the evolution of cracks approaches a steady state when the soil water content is extremely low. This is consistent with the observation of [START_REF] Li | Study of desiccation crack initiation and development at ground surface[END_REF]. Furthermore, it is worth noting that the characteristic days for surface crack ratio do not correspond to those for the actual evaporation rate, the evolution of actual evaporation curve lags behind that of the surface crack ratio curve, suggesting that cracks are not the only factors affecting the evaporation process. It is important to mention that the surface crack ratio is an important parameter in the evaporation analysis taking into account the effect of cracks [START_REF] Cui | Experimental and numerical investigation of soil-atmosphere interaction[END_REF]). In Fig. 21, the corresponding photographs of surface cracks at different times during evaporation are also shown in this figure for better appreciating the evolution of surface crack ratio.

Direct measurements of the depths and widths of cracks were carried out using a ruler, and the results are presented in Fig. 22. It is observed that the largest crack of 32 mm was in the central part of the soil surface, and the deepest crack of 230 mm was at the edges. Moreover, at the edges, cracks presented limited width variations (15-20 mm) but significant depth variations (170-230 mm). This represents the influence of the chamber wall. There is no clear relationship between depth and width, their ratio varying from 1.2 to 14.

Conclusions

A comprehensive laboratory experiment on water evaporation from a clayey soil was investigated using a large scale environmental chamber, with controlled atmospheric conditions and constant water table. The atmospheric parameters (wind speed, air temperature, and relative humidity) and the response of soil parameters (soil temperature, volumetric water content, matric suction and surface crack ratio) were monitored. The following conclusions can be drawn:

The soil temperature is strongly affected by the air conditions, evaporation process and desiccation cracks. The soil temperature varied with the variation of air temperature. With the decrease of evaporation rate, more energy was available for heating soil. The development of cracks increased the evaporating surface and therefore enhanced the influence of air condition on the soil temperature.

An obvious temperature decrease was observed at three locations: 50-mm height, soil surface and 50-mm depth. The soil temperature increased with the increase of air temperature, suggesting that the energy for evaporation was from the hot air, and that part of the energy was used for heating soil and air. The decrease of temperature gradient between the 50-mm height and the soil surface is attributed to the increasing energy used for heating soil and air along with the decreasing evaporation rate. The enlarged temperature gradient between the soil surface and the 50-mm depth suggests a deepening of the drying front.

The air relative humidity decreased with the decreasing evaporation rate. Three stages were identified: (1) a decrease at a quite low rate;

(2) a sharp decline stage;

(3) a slow decrease followed by stabilization.

The dense installation of sensors in the near surface zone allowed a rich measurements of volumetric water content, avoiding determining the water content by the destructive oven-drying method. The surface soil layer lost water firstly and the deeper zone started to lose water by the end of test. The evolution of volumetric water content in the near surface zone was quite different from that for sand, to be attributed to the higher water retention capacity and lower permeability of this clayey soil.

The rapid change of volumetric content profile during the early stage of evaporation in the zone from the soil surface to 55-mm depth suggested a rapid decrease of water content and a transition of evaporation mode from one-dimensional to three-dimensional. The linear relationship between water content and depth was identified for the near surface zone, as opposed to the case of sand where the linear relationship was observed for much deeper zone. Furthermore, the large change of Page 25 of 57

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For personal use only. This Just-IN manuscript is the accepted manuscript prior to copy editing and page composition. It may differ from the final official version of record.

water content in the near surface zone suggests that evaporation mainly affected this zone. Obviously, the development of cracks helped to the evaporation to be extended to deeper zone.

The soil matric suction was found to decrease over depth, and the suction gradient increased over time especially in the surface zone. The evolution of suction gradient is consistent with the volumetric water content changes.

The evolution of the surface crack ratio can be divided into three stages as for the

List of Tables

Table 1. Geotechnical properties of the soil studied

List of Figures

Figure 7

 7 Figure 7 depicts the evolution of air flow rate versus elapsed time. The air supply unit

Figure 10

 10 Figure 10 shows changes in air temperature over time at various positions. The values

Figure 14

 14 Figure 14 presents the evolutions of volumetric water content at different depths. It

Fig. 1 .

 1 Fig. 1. Grain size distribution of Héricourt clay

Fig. 1 .

 1 Fig. 1. Grain size distribution of Héricourt clay

Fig. 2 .

 2 Fig. 2. Sketch of the environmental chamber system

Fig. 3 .Fig. 4 .

 34 Fig. 3. Wind supply unit: (a) high-pressure compressed air source and air flow rate

Fig. 5 .Fig. 6 .Fig. 7 .Fig. 8 .Fig. 9 .Fig. 10 .Fig. 13 .Fig. 14 .Fig. 15 .

 5678910131415 Fig. 5. Distribution of sensors

Fig. 16 .Fig. 18 .Fig. 19 .Fig. 20 .Fig. 22 .

 1618192022 Fig. 16. Contour map of volumetric water content at different times

  Can. Geotech. J. Downloaded from www.nrcresearchpress.com by HARRIS LEARNING LIBRARY on 05/01/16For personal use only. This Just-IN manuscript is the accepted manuscript prior to copy editing and page composition. It may differ from the final official version of record.

	Page 18 of 57

  Can. Geotech. J. Downloaded from www.nrcresearchpress.com by HARRIS LEARNING LIBRARY on 05/01/16For personal use only. This Just-IN manuscript is the accepted manuscript prior to copy editing and page composition. It may differ from the final official version of record.

	Page 32 of 57
	temperature dependence of desiccation cracking behavior of clayey soils. Research, 7(4): 783-801. doi: 10.1016/S1093-0191(02)00053-9.
	Engineering Geology, 114(3-4): 261-266. doi: 10.1016/j.enggeo.2010.05.003. Yang, M., and Yanful, E.K. 2002. Water balance during evaporation and drainage in
	Tang, C., Shi, B., Liu, C., Gao, L. and Inyang, H.I. 2011a. Experimental investigation cover soils under different water table conditions. Advances in Environmental
	of the desiccation cracking behavior of soil layers during drying. Journal of Research, 6(4): 505-521. doi: 10.1016/S1093-0191(01)00077-6.
	Materials Yesiller, N., Miller, C.J., Inci, G., and Yaldo, K. 2000. Desiccation and cracking in Civil Engineering, 23(6): 873-878. doi:
	10.1061/(ASCE)MT.1943-5533.0000242. behavior of three compacted landfill liner soils. Engineering Geology, 57(1-2):
	Tang, C.S., Cui, Y.J., Shi, B., Tang, A.M., and Liu, C. 2011b. Desiccation and 105-121. doi: 10.1016/S0013-7952(00)00022-3.
	cracking behaviour of clay layer from slurry state under wetting-drying cycles.
	Geoderma, 166(1): 111-118. doi: 10.1016/j.geoderma.2011.07.018.
	actual evaporation rate: (1) a slow increase stage with values close to zero; (2) a rapid Wilson, G.W. 1990. Soil evaporative fluxes for geotechnical engineering problems.
	increase stage and (3) a steady stage. The evolution of actual evaporation curve lags PhD. thesis, Department of Civil Engineering, University of Saskatchewan,
	behind that of the surface crack ratio curve, indicating that cracks are not the only Saskatoon, Sask.
	factors affecting the evaporation process. Wilson, G.W., Fredlund, D.G., and Barbour, S.L. 1994. Coupled soil-atmosphere
	modelling for soil evaporation. Canadian Geotechnical Journal, 31(2): 151-161.
	doi: 10.1139/t94-021.
	Wilson, G.W., Fredlund, D.G., and Barbour, S.L. 1997. The effect of soil suction on
	evaporative fluxes from soil surfaces. Canadian Geotechnical Journal, 34(1):
	145-155. doi: 10.1139/t96-078.
	Yanful, E.K., and Choo, L.P. 1997. Measurement of evaporative fluxes from candidate
	cover soils. Canadian Geotechnical Journal, 34(3): 447-459. doi: 10.1139/t97-002.
	Yanful, E.K., Mousavi, S.M., and Yang, M. 2003. Modeling and measurement of
	evaporation in moisture-retaining soil covers. Advances in Environmental

Table 1 .

 1 Geotechnical properties of the soil studiedCan. Geotech. J. Downloaded from www.nrcresearchpress.com by HARRIS LEARNING LIBRARY on 05/01/16For personal use only. This Just-IN manuscript is the accepted manuscript prior to copy editing and page composition. It may differ from the final official version of record.

	Page 35 of 57
	Physical property	Value
	Specific gravity	2.70
	Plastic limit (%)
	Liquid limit (%)
	Plasticity index
	Clay (<2µm) content (%)
	Blue methylene value	7.5

Acknowledgments

The authors wish to acknowledge the support of the National Natural Science of China (Grant No. 51509017).

(2)

(3) (4)


Thomas Østerlie
email: thomas.osterlie@samfunn.ntnu.no

Co-materialization: Digital Innovation Dynamics in the Offshore Petroleum Industry

Keywords: digital innovation, industrial transformation, materiality, performativity

This paper empirically explores the concept of co-materialization to explain the digital innovation dynamics in offshore petroleum production. The central insight developed is that the very nature of subsurface processes and phenomena that may be monitored and controlled is transformed as offshore petroleum production is digitalized. The paper shows how digital technologies are intrinsic to this transformation as material reality and abstract concepts take on meaning together through digital technologies. The central dynamic driving this transformation is the process wherein digital technologies, physical phenomena, and work processes for monitoring and controlling these phenomena evolve together in continuous interplay.

Introduction

Digitalization 1 remained low within offshore petroleum production in the North Sea region well into the 1990s. Once available only by boat or helicopter, offshore installations in these inhospitable waters are today electronically available from the mainland. Massive developments of subsurface data communication cables have increased data transfer speed and capacity between offshore and onshore facilities [START_REF] Henderson | What is a capability platform approach to Integrated Operations? An introduction to key concepts[END_REF]. Advances in data communication technology during the late 1990s enabled real-time communication between down-hole sensors and topside facilities, sparking a proliferation of remote sensor technologies connected in large sensor networks [START_REF] Watson | Information Systems and Environmentally Sustainable Development: Energy Informatics and New Directions for the IS Community[END_REF]. These sensor networks stretch across the seabed and deep into individual wells for entire oil fields. By connecting on-and offshore facilities through subsurface cables, these remote sensor networks have become constituent parts of a larger digital infrastructure [START_REF] Tilson | Digital Infrastructure: The Missing IS Research Agenda[END_REF] that the offshore petroleum industry has built up over the past decade or so. Significant re-organizations of petroleum production in the North Sea have taken place during the same period, and personnel previously located at offshore installations have been moved to onshore operations centers [START_REF] Gulbrandsøy | Virtual collaboration in oil and gas organizations[END_REF]. Onshore personnel that used to focus on developing plans and recommendations that were handed over to offshore personnel to implement are today an integral part of daily offshore activities. With the availability of real-time sensor data and new engineering applications for visualizing and manipulating this data, onshore engineers can actively participate in monitoring, diagnosing, and controlling offshore processes. Since this new organization of offshore petroleum production is geographically distributed, communication technologies such as video conferencing, e-mail and instant messaging have become integral to the collaboration between offshore and onshore personnel [START_REF] Bayerl | Technology Effects in Distributed Team Coordination: High-Interdependency Tasks in Offshore Oil Production[END_REF].

The digitalization of offshore petroleum production is creating wakes of digital innovations [START_REF] Boland | Wakes of Innovation in Project Networks: The Case of Digital 3-D Representations in Architecture, Engineering, and Construction[END_REF] within the distributed network of petroleum companies, technology vendors, service companies and research institutions [START_REF] Acha | Governance and Co-ordination of Distributed Innovation Processes: Patterns of R&D Co-operation in the Upstream Petroleum Industry[END_REF], and these wakes have spawned both actual and anticipated changes in the organizing logic [START_REF] Sambamurthy | Research Commentary: The Organizing Logic for an Enterprise's IT Activities in the Digital Era-A Prognosis of Practice and a Call for Research[END_REF] of the offshore petroleum industry. With a basis in the ongoing digitalization of the offshore petroleum industry, both the actual and anticipated changes have collectively functioned as an organizing vision [START_REF] Swanson | The Organizing Vision in Information Systems Innovation[END_REF] for the future organization of petroleum production. Heralded under different labels, such as e-Fields, Smart Fields, i-Fields, Digital Energy, and Integrated Operations, the core vision is fully digital oil fields in which mass volumes of sensor data are used for computer-assisted, or even completely automated decision-making.

This dominant view of the ongoing transformation of offshore petroleum production focuses on the innovation outcome, namely that of increased digitalization, though it says little about the dynamics driving the innovation processes related to digitalization. Yet, understanding these processes is important in order to achieve a better understanding of the relationship between digitalization and the ongoing transformations in the offshore petroleum industry. In a first step towards unpacking this relationship, this paper asks: What are the dynamics driving digital innovation in the offshore petroleum industry?

Through an analysis of empirical data from studies within the offshore petroleum industry, this paper explores a possible answer to the question. Building and elaborating upon the insight that digital technologies play an integral part in creating the materiality of the physical world [START_REF] Østerlie | Dual Materiality and Knowing in Petroleum Production[END_REF], this paper contends that the impact of digital innovation in offshore petroleum production lies in a transformation of the fundamental relationship between work and the physical phenomena to be monitored and regulated. The central dynamic driving this transformation is the process wherein digital technologies, physical phenomena, and work processes for monitoring and controlling these phenomena evolve together in continuous interplay. Comaterialization is forwarded as a concept describing this process.

Co-materialization is offered as the main contribution of this paper. The paper contributes towards current developments of a performative agenda within IS research [START_REF] Orlikowski | Sociomateriality: Challenging the separation of technology, work, and organization[END_REF]. This agenda is a response to continued calls to better account for the constitutive role played by computing technologies in studies of work and organizing [START_REF] Leonardi | Materiality and change: Challenges to building better theory about technology and organizing[END_REF][START_REF] Monteiro | Social Shaping of Information Infrastructure: On Being Specific about the Technology[END_REF][START_REF] Orlikowski | Research Commentary: Desperately Seeking the 'IT' in IT Research-A Call to Theorizing the IT Artifact[END_REF]. Much of the existing work towards a performative agenda has, therefore, been to conceptualize the relationship between technology, work, and organizing through concepts such as sociomateriality [START_REF] Orlikowski | Sociomateriality: Challenging the separation of technology, work, and organization[END_REF] and imbrication [START_REF] Leonardi | When Flexible Routines Meet Flexible Technlogies: Affordance, Constraint, and the Imbrication of Human and Material Agencies[END_REF]. This paper contributes towards the performative agenda by showing the necessity of expanding upon the notion of materiality to encompass the materiality of the physical world when studying the role of digital sensor technologies in transformations of work and organizing. This insight should be of general interest to researchers for the continued development of a performative agenda within both IS and IS-related fields.

The remainder of the paper is organized as follows: First, a section outlining a performative approach to studying digital innovation dynamics in offshore petroleum production is presented. The attention is then turned towards the methods and materials that the paper is based on. A case story is presented, before the concept of co-materialization is empirically elaborated based on the case story. The paper concludes by drawing some implications of the concept.

Digital Innovation and Materialization

While innovation is a much studied topic within the IS literature over the past decades, Lyytinen and Rose [START_REF] Lyytinen | Disruptive information systems innovation: the case of internet computing[END_REF] observe that this literature is predominantly concerned with IS adoption and diffusion patterns. With basis in the observed lag between first availability of novel computing technologies and their adoption within organizations, the IS innovation literature has focused on how information systems come to be applied in novel ways to transform organizations' administrative, IS, or core business functions [START_REF] Swanson | Information Systems Innovation among Organizations[END_REF]. Focusing predominantly on the social processes related to adoption and diffusion dynamics, the IS innovation literature do not account for the constitutive role played by computing technologies as a driver in IS innovation. The broader innovation literature has been concerned with what Dosi [START_REF] Dosi | Technological paradigms and technological trajectories: A suggested interpretation of the determinants and directions of technical change[END_REF] refers to as the "'prime mover' of inventive activity" (p.148), i.e. innovation dynamic. These theories follow two broad categories: demand-pull and technology-push. Demand-pull theories see the marketplace and the identification of needs as the central dynamic driving technological innovation. These theories have been criticized for treating technology as a black box, failing to account for just how technology contributes to innovation. Technology-push theories, on the other hand, see innovation driven by technological development. Somewhat crudely put, this group of theories typically conceptualizes innovation as a linear process of science-technology-product. While technology development is the prime mover in technology-push theories, these theories tend to ignore the environmental feedback that shapes technologies as they are taken into use, which has been a central concern within the IS literature.

As such, neither the IS innovation literature, with its focus on adoption and diffusion patterns rather than innovation dynamics, nor the broader demandpull/technology-push theories on innovation dynamics, offer much traction for studying the dynamics driving digital innovation in offshore petroleum production.

More recently, Tilson et al. [START_REF] Tilson | Digital Infrastructure: The Missing IS Research Agenda[END_REF] forward content digitizing coupled with functional convergence of computing and media platforms as the driving dynamic in transforming communication and media industries. Digitizing is also central to the transformation of offshore petroleum production. Drawing upon the works of Barad [START_REF] Barad | Getting real: Technoscientific practices and the materiality of reality[END_REF][START_REF] Barad | Posthumanist Performativity: Toward and Understanding of How Matter Comes to Matter[END_REF][START_REF] Barad | Meeting the universe halfway: Quantum physics and the entanglement of matter and meaning[END_REF], Østerlie et al. [START_REF] Østerlie | Dual Materiality and Knowing in Petroleum Production[END_REF] show how digital sensor technologies used in petroleum production play an integral part in creating the materiality of the physical world when digitizing physical phenomena. As such, digitizing the physical world is significantly different from content digitizing, which Tilson et al. [START_REF] Tilson | Digital Infrastructure: The Missing IS Research Agenda[END_REF] defines as "converting analog signals into a digital form" (p. 750).

Building upon the insight developed by Østerlie et al. [START_REF] Østerlie | Dual Materiality and Knowing in Petroleum Production[END_REF], this paper further mines Barad's work to further develop a performative approach for analyzing the dynamics driving digital innovation in offshore petroleum production. Specifically, this section will, with some help from Rouse [START_REF] Rouse | Barad's Feminist Naturalism[END_REF], elaborate on Barad's use of the concept of materialization. Barad uses materialization to describe the processes through which the world takes on meaning. Central here is that meaning is not inherent in the world, but rather, Barad assumes the position that meaning, which she refers to as "local intelligibility," is performed within phenomena in the world. Three elements in this position are of particular relevance here: the primacy of phenomena, material (re)configurations and discursive-materiality.

First and foremost, phenomena are the basic ontological unit in Barad's work: "phenomena are constitutive of reality" ( [START_REF] Barad | Meeting the universe halfway: Quantum physics and the entanglement of matter and meaning[END_REF], p. 205). The world exists independent of humans and our understanding of it; however, this does not mean that it is made up of pre-existing entities with more or less clearly delineated boundaries and properties that are to be grasped and represented. On the contrary: Rouse [START_REF] Rouse | Barad's Feminist Naturalism[END_REF] contents that Barad's position is that "the world is articulated by overlapping, intra-acting phenomena, but most of these fail to disclose any pattern of local intelligibility" (p. 149).

If the world is not made up of delineated phenomena with fixed properties, what is it that we can have knowledge of? This is the point where Barad pushes performativity to encompass material reality. Instead of representations that mirror preexisting phenomena, phenomena take on meaning through processes of diffraction. While the world appears as phenomena with more or less clearly delineated boundaries and properties, these are not basic properties of the world. Instead, they are enacted through material (re)configurations in the world. When the world does disclose patterns of local intelligibility, i.e. when the world takes on meaning, it does so through particular material (re)configurations. Throughout her works, Barad uses various experimental scientific setups as pedagogic examples to illustrate such material (re)configurations:

[C]onsider an experiment in which light is scattered from a particle. The scattered light may be directed towards a photographic plate rigidly fixed in the laboratory and therefore used to record the position, or the light may be directed towards a piece of equipment with movable parts used to record the momentum of the scattered light. The first case essentially describes the process of taking a picture of the particle with a flash camera. In that case, the light is part of the measuring apparatus. In the latter case, the light's momentum is being measured and hence it is part of the object in question. ( [START_REF] Barad | Getting real: Technoscientific practices and the materiality of reality[END_REF], p. 171)

In this example, position is not a property of light as a preexisting entity with properties, but rather is enacted through the scientific instrument setup. Yet, the boundary between light as a phenomenon with properties such as position or momentum to be measured, and the instrumental setup, is not fixed either. When measuring momentum, light is part of the instrumental setup, and to this end, Barad argues that scientific instruments are not external to the reality being grasped, but are always part of the material (re)configuration that makes reality intelligible. But, if "the belief in the power of words to mirror preexisting phenomena" ( [START_REF] Barad | Posthumanist Performativity: Toward and Understanding of How Matter Comes to Matter[END_REF], p. 802) is misguided because phenomena are enacted through material (re)configurations, what then is the function of concepts such as position and momentum?

This leads to the third element of Barad's concept of materialization: discursivemateriality. Instead of refuting the dualism between discursivity (in this example understood as abstract concepts such as position and momentum) and materiality, Barad posits that phenomena are discursive-material. In so doing, Rouse [START_REF] Rouse | Barad's Feminist Naturalism[END_REF] observes that Barad includes concepts as part of the material (re)configuration that makes reality intelligible: "the natural world only acquires definite boundaries and concepts only acquire definite content, together" (p. 146). Barad offers an interpretation of Bohr's quantum physics philosophy to illustrate this point. For Bohr, position is not a property of a particle, nor is it an abstract concept. Instead, the concept of position only has meaning through an experimental setup or a material (re)configuration in Barad's vocabulary, with fixed rulers and a particle.

To summarize, with a basis in an ontology of phenomena, reality takes on meaning through discursive-material (re)configurations in the world, a process Barad refers to as materialization. Nyberg [START_REF] Nyberg | Computers, Customer Service Operatives and Cyborgs: Intra-actions in Call Centres[END_REF] juxtaposes Barad's work with actor network theory [START_REF] Law | Object Lessons[END_REF]. He observes that whereas ANT seeks to explain how phenomena become stable, Barad retains the poststructuralist sensitivity towards reality as becoming rather than being. As such, her work emphasizes the fluidity and constant unfolding character of phenomena.

A central premise for studying digital innovation in offshore petroleum production is that the physical phenomena that petroleum professionals monitor and control are physically inaccessible to human inspection. It is therefore meaningless to distinguish between material reality, understood as the stuff the world is made up of [START_REF] Iedema | On the Multi-modality, Materiality and Contingency of Organizational Discourse[END_REF], and the digital technologies for knowing about it. Thus, the attraction of materialization for studying digital innovation in offshore petroleum production lies in Barad's formulation of a performative ontology that creates no such separation between phenomena and the physical arrangements for knowing about them.

Methods and Materials

This paper draws upon data collected through two consecutive studies within the Norwegian petroleum industry. The first of these studies did not focus on digital innovation, but rather on ICT use for safe and reliable petroleum production. This study was conducted as an independent part of a larger joint industry research and development project (JIP) within the Norwegian petroleum industry [START_REF] Verhelst | Digital Platform for the Next Generation IO: A Prerequisite for the High North[END_REF]. The JIP is a form of generic project organization used within the petroleum industry to facilitate cooperation between petroleum companies, the vendor industry and research institutions to build competency and develop technologies regarded as particularly critical to the industry as a whole. The particular JIP reported from here gathered participants with long and extensive experience from the ongoing digitalization of the North Sea in order to study the feasibility of expanding petroleum production into the polar region. I participated in workshops and meetings organized regularly within the JIP. Seeing the JIP as a rich site for learning more about the ongoing digitalization of offshore petroleum production, I approached project workshops and meetings as fieldwork using breaks and after hour activities to learn more about other project participants' experiences from the ongoing digitalization of offshore petroleum production.

In parallel with attending project activities, I conducted 10 months of ethnographic fieldwork [START_REF] Fetterman | Ethnography: Step by Step[END_REF] in the onshore operations center of an international petroleum company operating in the Norwegian sector of the North Sea. This operations center houses the onshore personnel of several offshore petroleum fields. Onshore and offshore personnel work closely with each other, communicating through different media such as instant messaging, e-mail, phone, and video conferencing. At the time of conducting the fieldwork, the petroleum company was in the final stages of implementing a new engineering application for the real-time monitoring of wells for use in the onshore operations center. I had, therefore, collected quite a lot of data on the relationship between digitalization and work transformation when asked to do the second study this paper builds upon. I had even developed the outlines of a model on digital innovation dynamics as part of the analysis for the first study.

Digital innovation in offshore petroleum production was the topic of the second study. Financed as an independent study, I was asked by the same petroleum company where I had previously conducted ethnographic fieldwork to come up with policy advice to improve the organization's ability to implement research-based software in their operational units. Deciding to base this study on the model of digital innovation dynamics I had previously outlined, I conducted a total of 24 interviews with people who had been involved in developing new digital technologies used within the petroleum company: 13 researchers and 4 software engineers working in the corporation's R&D division, 3 engineers from different operational units who had been the customer for new digital technologies, 1 engineer working in the corporation's central IT division, as well as 3 software engineers working in vendor companies who develop new digital technologies put to use within the petroleum company under study. The interviews focused on the respondents' experiences from research-based software development projects. Instead of focusing on single projects, I inquired about general experience. Nevertheless, throughout the period of the interviews, I acquired insight into a number of specific projects and digital technologies.

This paper, therefore, draws upon a diversity of materials. I made daily field notes [START_REF] Emerson | Writing Ethnographic Fieldnotes[END_REF] throughout the ethnographic fieldwork in the onshore operations center. Jotting down notes in a pad throughout the day, I would transcribe the field notes during periods of calm or latest the same day upon returning from the fieldwork. Similarly, I made field notes during JIP project workshops and meetings, which I transcribed later the same day. Conducting the first study as a grounded theory study [START_REF] Charmaz | Constructing Grounded Theory: A Practical Guide Through Qualitative Analysis[END_REF], I coded the field notes shortly after writing them up, writing theoretical memos to record conceptual insights emerging during coding. In addition to the field notes taken during the first study, the theoretical memos related to innovation were used as materials for the analysis presented in this paper. From the second study I have transcribed each of the interviews conducted. I also conducted a document search of OnePetro (http://www.onepetro.org) to collect supplemental data on the projects and technologies mentioned during the interviews. OnePetro archives all papers published through the Society of Petroleum Engineers' conferences and journals, making it a key source for reports on the experience and development of new technologies from the global petroleum industry.

The concept of co-materialization emerged from the analysis a large number of projects and technologies developed over the past 20 years. For the ease of presentation, I choose to empirically ground the elaboration of co-materialization in a single case story. This particular case has been chosen because it intensely manifests central properties and dimensions of co-materialization.

Case Story: Predictive Maintenance of Topside Chokes

Petroleum is produced from hydrocarbon molecules contained within fluids trapped in subsurface reservoirs. On the Norwegian Continental Shelf (NCS), a subsea plateau that forms the Norwegian sector of the North Sea, these reservoirs are located thousands of meters beneath the seabed. Wells have been drilled to drain the fluids containing hydrocarbon molecules out of the reservoirs. Reservoir pressure pushes the fluids, referred to by petroleum professionals as the well flow, out through the wells and through kilometers of pipelines leading topside towards an offshore production platform. Onboard the platform, a petrochemical processing plant separates crude oil and gas from the hydrocarbon molecules contained within the well flow. The gas is exported by pipeline to onshore refineries, while supertankers feed refineries across the globe with crude oil from the NCS. This section outlines the major steps towards one of the more recent innovations in offshore petroleum production: predictive maintenance of offshore chokes. It starts with an outline of sand influx, the basic problem addressed by predictive maintenance. It then outlines the major developments towards predictive maintenance of offshore chokes. The purpose is to outline the case story that forms the backdrop for the discussion of co-materialization as a concept describing and explaining key dynamics driving digital innovation in offshore petroleum production.

Sand Influx and Sand Control

NCS reservoirs are typically found within geological sand stone layers. The porous sand stone structures function as sponges, trapping fluids containing hydrocarbon molecules. Over time, as increasing amounts of fluids are drained out of a reservoir, changes in the fluid balance may cause these sand stone structures to loose integrity. As the sand stone looses integrity, large areas of the reservoir start crumbling and may even collapse. Sand particles from the crumbling reservoir is then swept along with the fluids that are drained out of the reservoir, into wells and through the pipelines towards the topside platform.

Sand swept along with the well flow end up as sand deposits in the petrochemical processing equipment on the topside platform. Sand deposits stand the danger of contaminating the crude oil and thereby degrading its quality. In addition, sand clogs up the processing equipment. Parts of the processing plant have to be taken offline to clean the processing equipment of sand deposits. This reduces the plant's processing capacity for the duration of the cleanup procedure. More critically, though, sand particles create a sand blasting effect on chokes (the valves used to control fluid flow rates within the piping) and in pipeline bends as they are swept along with the fluids streaming at high speeds through the piping. Sand can, in extreme cases, erode through piping and choke casing and thereby puncturing the equipment. A punctured pipeline subsea may cause immense environmental damage as oil gushes into the ocean. On the topside platform, a punctured pipe or choke casing will send high-speed fluids jetting onto the deck. This is a significant safety risk as the high-speed fluids may cause human injury or even death. The well flow also contains gas. Punctured equipment topside will, therefore, also cause a gas leak. Leaking gas is considered among the most dangerous situations on a petroleum installation as it may ignite and explode.

Sand problems are not particular to the NCS. Referred to as sand influx, sandrelated problems are reported in the American petroleum literature as early as the late 1940s. Sand influx as a concept describes the physical processes in the subsurface reservoir that cause sand to be swept into the well. To this end, well screening technologies for preventing sand from entering wells have been developed. It comes with a set of technologies and work practices aimed at preventing sand from entering wells. These technologies and work practices are collectively referred to as sand control. Sand control routines builds on the premise that wells can only produce at rates where there is no sand influx. As such, these routines focus on establishing the maximum fluid flow rate where sand is not swept from the reservoir into the well. While this may significantly reduce the production rates of a single well, this is usually not a significant problem as long as only a few wells produce with such restrictions.

Sand Content and Sand Monitoring

Sand deposits started appearing in the topside petrochemical processing equipment of offshore platforms on the NCS in the late 1980s early 1990s. By then, the fluid balance within reservoirs on fields developed in the early 1970s and 80s was causing sand stone formations to collapse. Production restrictions related to producing at maximum sand free rate on a single well has limited impact on a field's overall production volumes. However, when a large number of wells experience sand influx it significantly reduces the field's overall production capacity. This was the situation, or at least the prospective situation, for an increasing number of fields on the NCS at the beginning of the 1990s. Sand monitoring emerges in the mid 1990s as a response to this problem. Sand monitoring builds on the premise that limited amounts of sand in the well flow constitutes no significant risk to safety or operations. Instead of establishing the maximum sand free rate on a well, sand monitoring seeks to ensure that wells produce within what is called maximum safe sand rate. However, producing with maximum safe sand rate requires real-time monitoring of the amount of sand in the well flow.

Sand monitoring as an alternative strategy for dealing with the effects of sand is proposed in a period where early digital sensor technologies are being put in use on offshore platforms. In parallel with developing safe operational constraints for producing with sand in the well flow, different digital sensor technologies for quantifying sand content were being experimentally developed. Several vendors sought to offer this technology, but in the end acoustic sand sensors are chosen. Acoustic sand sensors detect the ultrasonic sounds of sand particles hitting the inside piping in bends. Based on an algorithm that combines well flow velocity with the frequency of ultrasonic sound signals, the sand sensor quantifies the number of sand grains passing across it per second.

Sand monitoring requires significant changes in the operating conditions for the platform. Producing with sand in the well flow increases wear and tear on pipes and chokes. Sand monitoring is therefore coupled with more frequent and extensive inspections and maintenance of pipes and chokes. Sand monitoring required pipe and choke vendors to develop new inspection and maintenance frequencies, and changes in the maintenance regimes of the subcontractors in charge of plant maintenance.

Erosion Potential and Predictive Choke Maintenance

Chokes are used to control fluid rates within the pipelines. The valve opening and thereby accuracy degrades as the choke wears out. This gives offshore operators less control over fluid rates, making it increasingly difficult to regulate the production process in such a way that it optimizes the plant's processing capacity. Chokes on fields with the sand monitoring therefore tended to be replaced well before they were worn out. This is expensive and causes unnecessary downtime. By the turn of the millennium petroleum companies operating on the NCS had started building up onshore operations centers. Data communication capacity between on-and offshore installations was steadily increasing, and operators were exploring the possibilities of involving onshore personnel more closely with daily offshore activities. A key task for the onshore personnel was to optimize daily production. Production optimization seeks to prioritize between wells in such a way that the offshore petrochemical plant's processing capacity is used most efficiently within the restrictions incurred through the gas and crude oil export capacity.

As part of these efforts, one operator along with a software vendor and an engineering company joined forces to develop predictive choke maintenance to give onshore engineers better control with the accuracy of the offshore chokes used to optimize production. This was developed for a field already running with sand monitoring strategy. To this end, the engineering company developed a set of models describing and prescribing the degree of wear and tear on individual wells over time depending on the amount of sand having passed through the choke. This, they called erosion potential. The software vendor developed a tool that used these models together with sand sensor and other data sources to predict the current state of offshore chokes.

While the basic models for erosion potential remained unchanged, the software tool underwent a number of revisions. The greatest challenge was related to poor data quality of the input data. A number of visualizations and work processes were developed to allow onshore engineers to determine accuracy of the choke erosion models.

Discussion: Digital Innovation and Co-materialization

With basis in the case story above, I will now explore a possible answer to the question 'what are the dynamics driving digital innovation in offshore petroleum production?'. The difference between innovation, on the one hand, and technology adoption and organizational change in general, on the other, is that some form of novelty accompanies innovation [START_REF] Lyytinen | Disruptive information systems innovation: the case of internet computing[END_REF]. To answer the research question, we, therefore, first have to establish what constitutes novelty in the case story. Having established this, we can then turn our attention to the dynamics driving digital innovation in offshore petroleum production.

With the organizing vision of fully digital oil fields in which mass volumes of sensor data are used for computer-assisted, or even completely automated decisionmaking, most of the petroleum industry literature forwards novel digital technologies as 'prime-mover' [START_REF] Dosi | Technological paradigms and technological trajectories: A suggested interpretation of the determinants and directions of technical change[END_REF] of the ongoing transformation of offshore petroleum production. In the case story above, however, neither the acoustic sand sensor that measures sand in the well flow nor the software developed to monitor erosion on individual offshore chokes is based on novel digital technologies. The acoustic sand sensor is based on a technology for detecting ultrasonic sounds. While never previously used to measure sand content in fluids, the technology itself was, at the time, by no means a novel technology. Rather, it had been developed for other uses and used for a number of applications prior to developing digital sensors for detecting and quantifying sand. Similarly, the software developed to predict choke erosion was crafted around a series of familiar visualization techniques such as plotting data in graphs for comparison between data sources and breaking down the physical infrastructure in a tree hierarchy to ease navigation. Instead, novelty in the case story lies in a transformation of the very nature of subsurface processes and phenomena that may be monitored and controlled. The case story above traces two such transformations: the transformation of sand and of erosion from approximated to quantified phenomena. We can use the transformation of sand from an approximated, delayed action phenomenon to sand as a real-time, quantifiable characteristic of the well flow to exemplify this. Prior to the introduction of digital sand sensors, sand in the well flow materialized as accumulated deposits in the processing equipment. It would take hours from measures to limit sand influx had been taken before sufficient deposits had accumulated in the processing equipment so that offshore personnel could determine the effect of their actions; sand was a delayed action phenomenon. Offshore personnel would, furthermore, refer to sand deposits in approximate and relative terms such as significant or small amounts of sand observable when cleaning up the processing equipment.

Digital technologies are intrinsic to transforming sand from an approximated, delayed-action phenomenon to a real-time, quantified characteristic of the well flow. Physical phenomena materialize through complex discursive-material performances that involve undifferentiated matter, physical artifacts and digital technologies. Sand content emerges through the material setup of an acoustic sand sensor, along with a pipeline bend, and the undifferentiated matter of the well flow rushing from the well towards the topside platform. The acoustic sand sensor is mounted on the outside of this bend. The well flow creates a sound as it hits the outer bend of the piping, where the sand sensor is mounted, at high speeds. The acoustic sand sensor is designed to isolate the sound of solid particles hitting the piping, and transforms this sound into a digital signal. Through laboratory experiments, the sensor vendor had developed an algorithm that combines well flow velocity with frequency of ultrasonic sound signals to calculate sand content, measured as grains of sand passing across the sand sensor per second. The acoustic sand sensor's controller software implements this algorithm to transform the digital signals generated by the acoustic sand sensor into a measurement of sand content. Sampling the digital signals each second, the material setup of acoustic sand sensor, controller software, piping, and well flow performs sand as a real-time, quantified characteristic of the well flow.

In this analysis, sand content as a quantifiable characteristic of the well flow is not a property of the physical world simply waiting to be represented. Instead, it is performed through the material setup where digital technologies -both acoustic sand sensor hardware and controller software -play a constitutive role in creating the materiality of the physical world [START_REF] Østerlie | Dual Materiality and Knowing in Petroleum Production[END_REF]. As such, sand content is not merely an abstract concept (i.e. discursive). Rather, material reality and abstract concepts take on meaning together through digital technologies; they co-materialize. This is not to be interpreted as a form of radical constructivism claiming that sand in the well flow as a figment of social construction. Such a position would reduce physical phenomena to being purely discursive. The argument pursued is, on the contrary, a refutation of reducing physical reality to constructs and representations, and instead to offer a realist analysis of how the world takes on meaning through the use of digital technologies. There were undeniably solid particles in the well flow before introducing the acoustic sand sensor, with accumulated sand in the processing equipment being a testament to this. Yet, it is through the discursive-material reconfigurations outlined above that sand content, as a quantitative characteristic of the well flow, comes to be performed as a pattern of local intelligibility, to use Barad's terminology.

Having established what constitutes novelty in the case story, we can turn our attention towards the dynamic driving digital innovation in offshore petroleum production. Sand had been a well-known operational problem within the global petroleum industry for almost half a century when platforms on the NCS started experiencing sand influx in the late 1980s. While measures for handling sand in the well flow along with research on reservoir geology to better understand the causes of sand influx had been in continuous development since the 1970s, it is with the advent of digital technologies in the 1990s that sand content emerges as a real-time, quantified characteristic of the well flow. This does not mean that some sort of technological imperative drives the emergence of sand content. An engineer working with a vendor company offering a sand sensor technology competing with the acoustic sand sensor in the case story, explained that their sand sensors built on a technology that the vendor had developed for significantly different purposes. They had never thought of using the technology to detect and quantify sand. Yet, when petroleum companies operating on the NCS started developing sand content as a concept, the vendor saw the possibility of adopting their technology to implement the sand content concept.

As such, digital technologies emerge as part of an evolving understanding of how to regulate subsurface processes and phenomena. Subsurface processes play an important role in shaping this understanding. Digital sensor technologies for quantifying sand in the well flow emerges at a point in time when a growing number of oil fields on the NCS were beginning to experience significant reduction in production capacity due to sand influx. Petroleum companies came to identify sand control as a significant production limitation to be addressed. Sand monitoring, as a concept emerges from the convergence of aging oil fields on the NCS and the technological possibilities of technological developments at the time. The senior research engineer commonly attributed as the key originator of the sand monitoring concept participated in the industry R&D project that this paper draws from. He described the sand monitoring strategy as an egg of Columbus: once formulated, everybody was wondering why nobody had come up with the idea before. Clearly proud to be attributed as the originator of sand monitoring, he carefully pointed out that formulating the concept was "a matter of timing". The timing was right in that increasing computerization of offshore installations opened up for the possibility of instrumenting the production process with digital sensors, which made the production restrictions caused by sand control to be a solvable problem.

The evolving understanding of how to regulate subsurface processes and phenomena emerges from recognized needs and relevant technologies for responding to these needs. Sand content co-materializes with technologies for measuring sand content and sand monitoring in a space of possibilities where changes in fluid distribution in maturing reservoirs lead to increased sand influx in wells, where existing sand control strategies significantly reduce production volumes on fields with much sand influx, and where measuring sand content is technologically feasible. It is, therefore, possible to say that recognized needs and relevant technologies for responding to these needs co-materializes out of the same space of possibilities.

Conclusion

This paper has empirically explored the concept of co-materialization to explain the digital innovation dynamics in offshore petroleum production. The central insight developed is that the very nature of subsurface processes and phenomena that may be monitored and controlled has been transformed as part of digitalizing offshore petroleum production. The paper has shown how digital technologies are intrinsic to this transformation through a process where material reality and abstract concepts take on meaning through digital technologies. The central dynamic driving this transformation is the process wherein digital technologies, physical phenomena, and work processes for monitoring and controlling these phenomena evolve together in continuous interplay. As such, this paper offers an interpretation of the ongoing digitalization focusing on change processes. This supplements the focus on innovation outcome predominant in the organizing vision of fully digital oil fields (see Section 1). While this organizing vision has been central to securing top-level commitment to the massive investments required for developing the digital infrastructure and making the organizational changes required to transform offshore petroleum production. Now that the infrastructure is more or less in place, the industry is entering a new phase: that of capitalizing on the digitalization. Co-materialization offers a perspective that may contribute towards this second phase. By emphasizing the creation of new phenomena forms the thrust of digital innovation, co-materialization suggests that focus for this second phased of digitalization should lie on finding new ways of using existing data sources to create phenomena that may be regulated in order to improve and optimize production.

Through the concept of co-materialization, this paper suggests that digital innovation not only changes the nature of work but the very phenomena to be regulated through technology and work. Common to both sand monitoring (Section 4.2) and predictive maintenance (Section 4.3) is that they constitute a fundamental shift in the industry's relationship to the materiality of offshore petroleum production. Both sand monitoring and condition monitoring constitute a shift away from static towards a more dynamic relationship with the materiality to be monitored and controlled. Such a shift is a common denominator to digital innovation in offshore petroleum production. This may suggest that the ongoing digitalization of offshore petroleum production constitutes a shift towards an increasingly dynamic relationship that transforms the offshore petroleum industry as it redistributes competency within petroleum companies, as well as between petroleum companies and other actors within the offshore petroleum industry on the one hand, while transforming the nature of work, technology and organizing within the offshore petroleum industry on the other. This, however, will have to be explored further in later publications.

This paper draws upon Tilson et al.'s[START_REF] Tilson | Digital Infrastructure: The Missing IS Research Agenda[END_REF] distinction between digitizing, understood as "the process of converting analog signals into a digital form", and digitalization, described as "a socio-technical process of applying digital techniques to broader social and institutional contexts that render digital technologies infrastructural" (p.750).