Geetanjali Das

Andry Razakamanantsoa

Gontran Herrier

Dimitri Deneele

Influence of pore fluid-soil structure interactions on compacted lime-treated silty soil

Keywords: lime-treated soil, kneading action, pore fluid, tortuosity, leaching

Introduction

Soil stabilization is a current practice for the efficient and effective management of natural resources in any land and infrastructure development project. The process of soil stabilization involves two main categories: mechanical improvement and chemical stabilization [START_REF] Houben | de l'article/du chapitre Earth construction[END_REF].

Mechanical improvement involves the use of proper implementation techniques such as proper mixing of soil with chemical binder and water, compaction of soil through a suitable mechanism, and at an appropriate water content and compaction energy [START_REF] Le Runigo | Performance of lime-treated silty soil under long-term hydraulic conditions[END_REF][START_REF] Le Runigo | Impact of initial state on the fabric and permeability of a lime-treated silt under long-term leaching[END_REF][START_REF] Little | Fundamentals of the Stabilization of Soil with Lime[END_REF]. Chemical stabilization consists of the use of inorganic or organic binders, such as slags [START_REF] Nguyen | Effect of lime treatment on the hydraulic conductivity of a silty soil[END_REF][START_REF] Wild | Effects of partial substitution of lime with ground granulated blast furnace slag (GGBS) on the strength properties of lime-stabilised sulphatebearing clay soils[END_REF], fly ashes produced from coal-burning [START_REF] Kolias | Stabilisation of clayey soils with high calcium fly ash and cement[END_REF][START_REF] Show | Reuse of incinerator fly ash in soft soil stabilization[END_REF], cement kiln dust [START_REF] Miller | Influence of soil type on stabilization with cement kiln dust[END_REF], and lime [START_REF] Akula | Evaluating the Long-Term Durability of Lime Treatment in Hydraulic Structures: Case Study on the Friant-Kern Canal[END_REF][START_REF] Ali | Assessment of lime treatment of expansive clays with different mineralogy at low and high temperatures[END_REF][START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF][START_REF] Das | Compressive strength and microstructure evolution of lime-treated silty soil subjected to kneading action[END_REF][START_REF] Makki-Szymkiewicz | Evolution of the properties of lime-treated silty soil in a small experimental embankment[END_REF].

Lime is one of the most versatile [START_REF] Dowling | Lime in the limelight[END_REF], low-cost [START_REF] Inkham | Low-cost alkaline substances for the chemical stabilization of cadmium-contaminated soils[END_REF], and readily available chemicals. It was shown to be paramount in several applications using environmentally friendly techniques [START_REF] Dowling | Lime in the limelight[END_REF]. Soil treated by lime can be used repeatedly, thus promoting its reuse and improving cost-effectiveness [START_REF] Hopkins | Stockpiling Hydrated Lime-Soil Mixtures[END_REF]. Upon mixing soil with lime, two effects are observed:

(i) instant reduction in moisture content and consequent flocculation/agglomeration of the clay particles, further lowering soil plasticity and increasing workability of the lime-treated soil [START_REF] Bell | Lime stabilization of clay minerals and soils[END_REF][START_REF] Diamond | Mechanisms of soil-lime stabilization[END_REF][START_REF] Little | Stabilization of pavement subgrades and base courses with lime[END_REF][START_REF] Little | Fundamentals of the Stabilization of Soil with Lime[END_REF]; (ii) long-term increase in strength of the lime-treated soil due to the formation of cementitious compounds coming mainly from pozzolanic reactions [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF][START_REF] Lemaire | Effects of lime and cement treatment on the physicochemical, microstructural and mechanical characteristics of a plastic silt[END_REF][START_REF] Pu | Mechanical characteristics and water stability of silt solidified by incorporating lime, lime and cement mixture, and SEU-2 binder[END_REF][START_REF] Verbrugge | Strength and micro observations on a lime treated silty soil[END_REF].

Several laboratory studies and few field studies are available that explain the importance of the hydromechanical performances of the lime-treated soil towards the maintenance of the long-term durability of lime-treated earth structures [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF][START_REF] Das | Compressive strength and microstructure evolution of lime-treated silty soil subjected to kneading action[END_REF][START_REF] Deneele | Experimental assessment regarding leaching of lime-treated silt[END_REF][START_REF] Le Runigo | Performance of lime-treated silty soil under long-term hydraulic conditions[END_REF][START_REF] Le Runigo | Impact of initial state on the fabric and permeability of a lime-treated silt under long-term leaching[END_REF][START_REF] Lemaire | Effects of lime and cement treatment on the physicochemical, microstructural and mechanical characteristics of a plastic silt[END_REF][START_REF] Verbrugge | Strength and micro observations on a lime treated silty soil[END_REF]. The hydromechanical behavior is heavily influenced by compaction conditions such as compaction procedure, energy, and water content [START_REF] Cuisinier | Microstructure and hydraulic conductivity of a compacted lime-treated soil[END_REF][START_REF] Le Runigo | Impact of initial state on the fabric and permeability of a lime-treated silt under long-term leaching[END_REF]Mitchell et al., 1965;[START_REF] Watabe | Influence of compaction conditions on pore-size distribution and saturated hydraulic conductivity of a glacial till[END_REF], which plays a central role in the mechanical improvement process. According to le [START_REF] Le Runigo | Performance of lime-treated silty soil under long-term hydraulic conditions[END_REF][START_REF] Le Runigo | Impact of initial state on the fabric and permeability of a lime-treated silt under long-term leaching[END_REF] and Mitchell et al. (1965), lime-treated soil compacted at different initial moisture contents and compaction energies shows different magnitudes of hydraulic conductivity, k. This is because the hydraulic conductivity of soil depends on the macropore structure of the soil, which was shown to be a direct function of compaction conditions by [START_REF] Ranaivomanana | Effects of cement treatment on microstructural, hydraulic, and mechanical properties of compacted soils: Characterization and modeling[END_REF][START_REF] Ranaivomanana | Permeability prediction of soils including degree of compaction and microstructure[END_REF]. A higher magnitude of k indicates a greater quantity of water percolation throughout the compacted soil in a short time. This can enhance the leaching of cementitious compounds as well as the available lime, which consequently decreases the compressive strength of the lime-treated soil, as reported by Le [START_REF] Le Runigo | Performance of lime-treated silty soil under long-term hydraulic conditions[END_REF] and [START_REF] Deneele | Experimental assessment regarding leaching of lime-treated silt[END_REF].

Although the consideration of moisture content and the implemented energy during compaction were shown to have a significant impact on the hydromechanical evolution, how well the laboratoryimplemented compaction mechanism represents the in-situ compaction mechanism is an important question. In-situ fine-grained soil is often compacted by a pad foot roller, especially hydraulic earthen structures (dikes, levees, dams, reservoirs). The pad-foot roller often consists of pads attached to the drum surface of the wheel. During compaction, these pads generate a kneading action in the soil. The impact of kneading action in natural soil was evaluated by [START_REF] Kouassi | A new technique of kneading compaction in the laboratory[END_REF], who demonstrated an equivalent generation of soil characteristics such as dry density and elastic stiffness between laboratory and field compacted soil. A similar observation was also reported by [START_REF] Clegg | Kneading compaction[END_REF]. [START_REF] Cuisinier | Microstructure and hydraulic conductivity of a compacted lime-treated soil[END_REF] evaluated the evolution of hydraulic properties of lime-treated soil compacted by two procedures: static and kneading.

A lower k was observed in the lime-treated kneaded soil. Beyond these few studies, further evaluation of the kneading mechanism, particularly with respect to lime-treated fine-grained soil, remains less investigated. Thus, it is reasonable to investigate the influence of the kneading mechanism in the hydromechanical evolution of lime-treated soil.

Another essential aspect that has remained less investigated with respect to the hydromechanical evaluation is the effect of the chemistry of pore water solution during its percolation through lime-treated compacted soil. Since lime-treated soil is susceptible to increased leaching, it may cancel out the improvement brought by lime treatment [START_REF] Deneele | Experimental assessment regarding leaching of lime-treated silt[END_REF]. Hence, the leaching mechanism of limetreated soil should be evaluated by considering the chemistry of porewater to which the selected soil is subjected to. The effect of the chemistry of pore fluid such as leachates on the hydraulic behavior of natural clay in the context of liner material used in underground nuclear disposal repository was widely investigated. Several studies have shown how the aggressive effect of leachates induces cracks in clay-liner and causes an increased hydraulic conductivity during the soil-leachate interactions [START_REF] He | Effects of leachate infiltration and desiccation cracks on hydraulic conductivity of compacted clay[END_REF][START_REF] Sunil | Soil-leachate interaction and their effects on hydraulic conductivity and compaction characteristics[END_REF][START_REF] Vaverková | Chemical Composition and Hazardous Effects of Leachate from the Active Municipal Solid Waste Landfill Surrounded by Farmlands[END_REF]. Currently, DW is used as a conventional permeant solution in most of the existing studies to investigate the hydraulic and leaching behaviors of lime-treated soil, whereas in the field, water from natural sources influences such behaviors. Thus, it is essential to consider the interactions of pore solution chemistry with the lime-treated soil while investigating the soil's hydromechanical evolution.

In this context, the present study investigates the influence of compaction modes and pore solution chemistry on the hydromechanical performances of a lime-treated silty soil. The compaction modes involved static and kneading compactions, and DW and a low ionic strength solution were used as pore solutions. The investigation includes evaluating the hydraulic conductivity and mechanical performances based on the leaching mechanism and microstructural modifications.

Materials and Methodologies

Soil, Lime, and permeant solutions properties

The soil selected in this study was silty soil imported from Marche-Les-Dames (Belgium). The main geotechnical properties of the soil were obtained from the studies reported by [START_REF] Charles | An experimental full-scale hydraulic earthen structure in lime treated soil[END_REF] and [START_REF] Makki-Szymkiewicz | Evolution of the properties of lime-treated silty soil in a small experimental embankment[END_REF]. The soil is composed of 12% clay and 82% silt fraction. Its liquid limit is 31%, and the plasticity index is between 8 to 12. The Methylene blue value is 2.5 g/100 g. The mineralogy of the soil consists of Illite, Kaolinite, and Chlorite as clay minerals along with Quartz and Feldspars.

The quicklime (CaO) used for the treatment was supplied by Lhoist. The lime consists of 90.9% of available CaO and a reactivity (t60) of 3.3 min. The Lime Modification Optimum (LMO) of the silt, which defines the minimum lime content required to initiate the pozzolanic reactions, was determined by [START_REF] Eades | A quick test to determine lime requirements for lime stabilization[END_REF]. The LMO was found to be 1% by weight of lime. Three different lime contents were used, lime content equal to LMO, 2.5%, and 4%. Two permeant solutions were chosen DW, and DW+10 -3 M NaCl. The latter has been used previously by [START_REF] Razakamanantsoa | Long term chemo-hydro-mechanical behavior of compacted soil bentonite polymer complex submitted to synthetic leachate[END_REF] and [START_REF] Sato | Comparison of prehydration and polymer adding effects on Na activated Ca-bentonite by free swell index test[END_REF] as a reference fluid to highlight the negative effect of leachates on Bentonites in the context of landfill. The pH and Electric Conductivity (EC) of the DW and DW+10 -3 M NaCl, latter regarded as Low-mineralized Water (LW) is presented in Table 1.

Sample preparations

The maximum dry density, ρd(max), and Optimum Moisture Content (OMC) of the untreated and the 3 different lime-treated silt were obtained by standard Proctor test as per ASTM, D698-12e2 and are presented in Table . 2. The silt was air-dried, sieved, and was then mixed with distilled water at OMC. The soil mixture was stored in sealed plastic bags for about 24 hours to allow moisture content homogenization. The wet soil and the respective lime were then mixed and were rested for 1 hour before compaction.

Cylindrical specimens of dimensions 0.05 m in height and 0.05 m diameter were prepared by Static (S) and Kneading (K) compaction methods at the constant compaction parameters mentioned in Table 2.

The static compaction involves compression of the specimens from top and bottom, as demonstrated by [START_REF] Holtz | An introduction to geotechnical engineering[END_REF]. The kneading compaction was performed by a laboratory-developed kneading tool. The process of kneading compaction was conducted, as demonstrated by [START_REF] Kouassi | A new technique of kneading compaction in the laboratory[END_REF] for natural soil. [START_REF] Kouassi | A new technique of kneading compaction in the laboratory[END_REF] does not consider compaction energy during kneading compaction, whereas, in the present study, compaction energy corresponding to the one mentioned in ASTM D698-91 was applied for specimens preparation. The application of the compaction load was made successively with the rotation of the 3-kneading feet by an angle of 45° between 2 successive loadings.

A total of 18 specimens was prepared, 9 specimens corresponding to each compaction method.

After compaction, specimens were wrapped in plastic film and cured for 28-days at a laboratory temperature of 20 ± 1 ˚C.

Laboratory tests

Hydraulic conductivity test

Percolation setup

Hydraulic conductivity test was performed with flexible wall permeameter by a constant hydraulic head. The percolation system established was similar to the one used by [START_REF] Ranaivomanana | Permeability prediction of soils including degree of compaction and microstructure[END_REF]. A schematic design of the permeability setup is presented in Fig. 1. At the end of the curing time, specimens were sealed in a cellulose membrane and placed inside cylindrical transparent Polyvinyl chloride (PVC) cell with porous stones on its top and bottom (Fig. 1). A water inlet valve was equipped at the center of the PVC cell for applying the confining stress. The PVC cell, along with the membrane and specimen, was then placed within an upper and a lower base plate to avoid vertical deformation. Both the base plate are accompanied by a hole at its center for allowing outflow and inflow of solution in and from the specimen, respectively. The valve connected to the hole at the lower base plate was linked with the Mariotte bottle (Fig. 1). During the hydraulic test, the solution from the Marriott bottle passes through the base of the specimen. This is to ensure uniform flow throughout the specimen by reducing any presence of entrapped air within the compacted specimen. The percolated fluid was collected in an effluent collection bottle linked with the valve connected to the hole in the upper base plate (Fig. 1).

A total of 12 setups for hydraulic conductivity tests were designed, of which 6 setups consist of specimens submitted to DW, and the remaining were submitted to LW.

Hydraulic conductivity test protocol

During the hydraulic conductivity test, a confining pressure of 88±2 kPa was applied for at least 24-48 hours before the application of hydraulic head pressure to ensure the homogeneity of the stress distribution. The hydraulic head applied (H) was 170±5 cm. Thus, the applied confining stress was about 5 times higher than the hydraulic head pressure. This was done (i) to maintain the structure of the compacted specimen against any microstructure change due to the hydraulic head, and (ii) to avoid any preferential flow between the soil and the membrane [START_REF] Ranaivomanana | Permeability prediction of soils including degree of compaction and microstructure[END_REF].

The hydraulic conductivity test was conducted in accordance with the flow conditions provided by Darcy's law. According to Darcy, the hydraulic conductivity, k (m/s) in terms of the total discharge, Q = dV/dt, through a specimen; the cross-sectional area of the flow, A (m 2 ); and the hydraulic gradient, i = H/e can be expressed as follows (Equation 1).

𝑘 * * * (1)
where H H 2 -H 1 is the hydraulic head (elevation datum) in m, and e is the specimen thickness in m; dV is the incremental volume of percolated water between two measurements in m 3 ; dt is the time elapsed between two measurements in seconds.

The hydraulic conductivity test was carried out in two phases: saturation and percolation. The saturation phase involves the wetting of specimens until full saturation, and the percolation phase involves the renewal of the entire porewater in the specimens. During the saturation phase, the outlet of the permeability setup was kept closed until a volume of influents corresponding to 1 Pore Volume Flow (PVF)

of each specimen enter the specimens. One PVF is defined as the volume of pore water required to fill and renew the total volume of pore water and void initially present in the soil once [START_REF] Katsumi | Long-term barrier performance of modified bentonite materials against sodium and calcium permeant solutions[END_REF]. This level of PVF was selected to ensure about 90-95% saturation of the specimens before initiating the percolation phase. Mathematically, 1 PVF is the product of the volume of soil solids, V s, and the void ratio, e of a given specimen. Then the accumulated volume of flow passing through the soil is denoted in terms of PVF by dividing the total flow by the volume corresponding to 1 PVF.

The concept of PVF was proposed to consider the duration of contact between the permeant solution and the lime-treated soil. Longer contact between soil and the permeant solution might be favorable to the development of dissolution or precipitation mechanism inside the soil structure. Thus, the PVF can be called an important index to assess the durability of the lime treatment. PVF was less often used for hydraulic performance studies related to lime-treated soil. However, it was widely implemented in the literature related to the hydraulic performance of Bentonite used in waste storage management [START_REF] Katsumi | Long-term barrier performance of modified bentonite materials against sodium and calcium permeant solutions[END_REF][START_REF] Shackelford | Evaluating the hydraulic conductivity of GCLs permeated with non-standard liquids[END_REF].

The hydraulic conductivity measurements were stopped in accordance with the following termination criteria: (i) after 40 PVF was reached; and (ii) the last 5 values of EC measured during percolation became almost stable. Such termination criteria were also referred to in a study reported by [START_REF] Shackelford | Evaluating the hydraulic conductivity of GCLs permeated with non-standard liquids[END_REF].

Calcium analysis

During the percolation phase, effluents were collected at different PVF. The effluents were then filtered using 0.45 µm syringe prior to Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES) analysis for determining the elementary concentrations of Calcium (Ca). The analysis of Ca concentration was made to investigate the difference in the concentration of Ca leached from the limetreated soil under the influence of DW and LW.

Unconfined compressive strength (UCS) test

Specimens after 28 days of curing and at the end of the percolation phase were subjected to compressive strength analysis using a mechanical press with a load sensor of 10 kN. The load was applied to the specimens at a constant axial displacement rate of 1 mm/min.

Pore structure determination

Soil specimens sampled from 28 days cured soil and from specimens at the end of hydraulic conductivity test were freeze-dried and then subjected to Mercury Intrusion Porosimetry (MIP) test using Micromeritics Auto Pore IV.

In the MIP test, specimens were first evacuated via heating inside a sealed penetrometer. Through incremental hydraulic pressure, mercury was then progressively introduced into the specimens. The applied pressure, p (MPa), and the volume of mercury intruded were registered progressively [START_REF] Romero | Microstructure investigation in unsaturated soils: a review with special attention to contribution of mercury intrusion porosimetry and environmental scanning electron microscopy[END_REF]. Pore diameter, D was obtained according to the Washburn equation (Equation 2):

𝐷 . .
(2)

D is the diameter of the entrance pore where mercury intrudes, 𝛾 is the surface tension of mercury, and 𝜃 represents contact angle.

In this study, pore classifications were made as per the International Union of Pure and Applied Chemistry (IUPAC) [START_REF] Rouquerol | Recommendations for the characterization of porous solids (Technical Report)[END_REF], which classifies pores based on their pore-width as macropores (> 500 Å), mesopores (20-500 Å), and micropores (< 20 Å).

The following nomenclatures are referred to for specimens' identifications: type of specimen (untreated, 1%/2.5%/4% lime-treated)-compaction mode (S/K)-type of permeant solutions (DW/LW). For example, 1%-S-DW represents 1% lime-treated statically compacted soil submitted to DW percolation.

Results

Comparative evaluation of UCS in the lime-treated unleached and leached soil

The UCS obtained from the lime-treated 28 days cured specimens (unleached) were compared with the UCS obtained from the lime-treated leached soil at the end of the hydraulic conductivity test (Fig. 2).

The testing conditions during UCS test for each specimen were kept similar. The UCS of 1% lime-treated kneaded specimens leached by DW decreased by about 60% (Fig. 2b). The UCS measurement of the corresponding LW submitted soil was not possible as the specimen broke at the end of the hydraulic conductivity test. About 40% of the initial UCS was lost in the 2.5% lime-treated DW leached kneaded specimen, whereas this decrease in UCS was about 20% in the corresponding LW leached specimen. The UCS for 4% lime-treated leached specimens increased by about 36%.

Comparative evolution of Pore size distribution 3.2.1. In 28-days cured lime-treated specimens

The Pore Size Distribution (PSD) of the statically and kneading compacted lime-treated 28-days cured specimens is summarized in Fig. 3. Three different peaks were observed at pore diameter 10 3 , 10 4 , and 10 5 Å. The total pores of diameter 10 3 Å were minimum in 1% lime-treated soil (Fig. 3a), slightly higher in 2.5% lime-treated soil (Fig. 3b), and maximum in 4% lime-treated soil (Fig. 3c). At the same time, pores of diameter 10 4 Å were minimum in 4% lime-treated soil and maximum in 1% lime-treated soil. This difference in the evolution of pores is attributed to the lime content used. At higher lime content, the evolution of cementitious compounds was enhanced [START_REF] Lemaire | Effects of lime and cement treatment on the physicochemical, microstructural and mechanical characteristics of a plastic silt[END_REF][START_REF] Little | Fundamentals of the Stabilization of Soil with Lime[END_REF][START_REF] Verbrugge | Strength and micro observations on a lime treated silty soil[END_REF], and hence more pores of diameter 10 3 Å were developed. The trend of pore evolution of diameters 10 3 and 10 4 Å remained at a similar level in both types of compacted soil.

However, the presence of the macropores of diameter 10 5 Å was comparatively significant in all the statically compacted specimens than the corresponding kneaded soil.

In unleached and leached lime-treated specimens

At the end of the hydraulic conductivity test, the evolution of PSD in the two types of compacted soil after being leached by DW and LW was compared with the unleached specimens in Fig. 4. A decrease in the pores of diameter 10 4 Å and generation of pores of diameter lower than 10 3 Å in the 2.5% and 4% lime-treated leached specimens was observed compared to the unleached specimens (Fig. 4b, c, e & f). This generation of pores lower than 10 3 Å was relatively significant in the 4% lime-treated soil.

However, the macropores of diameter 10 5 Å increased in the 1% lime-treated statically compacted specimen after being leached by DW (Fig. 4a). At the same time, no significant change occurred in the corresponding LW leached specimen compared to the unleached specimen. In the 2.5% and 4% lime-treated soil, the evolution of macropores of diameter 10 5 Å remained constant after being leached by both DW and LW (Fig. 4b &c).

In the kneaded specimens, a constant evolution of the macropores of diameter 10 5 Å was observed in the leached and unleached 1% lime-treated specimens (Fig. 4d). On the other hand, the 2.5% lime-treated specimens showed an increase in the total 10 5 Å diameter macropores after being leached by DW, whereas in the corresponding LW submitted specimen, the total macropores with a similar diameter decreased and were lower than that of the unleached soil (Fig. 4e). In the 4% lime-treated soil, the total macropores with a diameter of 10 5 Å increased slightly in the DW leached specimen, while the total of this macropore feature remained almost constant in the corresponding LW leached specimen (Fig. 4f).

Hydraulic conductivity measurements in untreated and lime-treated soil

The analysis of the hydraulic conductivity was conducted after verifying the level of saturation in the lime-treated 28 days cured soils and at the end of the hydraulic conductivity test, i.e., on completion of the renewal of 40 PVF. The measured degree of saturation, Sr is summarized in Table 3. The S r of all the 28 days cured lime-treated soils were less than 80%. This saturation level reached a value greater than 90%, when measured at the end of the hydraulic conductivity test. This difference in saturation values indicates that the hydraulic conductivity measurement was conducted in saturated specimens. It is worth noting that in-situ lime-treated soil may rarely reach a similar saturation level as the one obtained herein (Table 3) before the percolation happens.

The evolution in the k of the lime-treated and the untreated soil is presented in Fig. 5 with respect to the PVF measured during the hydraulic conductivity test. The duration taken by each specimen of the present dimensions to renew the volume of solution that corresponds to 40 PVF is provided in Table 4. Fig. 5a shows that the magnitude of k in the untreated statically compacted specimens, submitted to both types of solutions remained within the range of 10 -8 to 10 -9 m/s. This k then increased by about one and two orders of magnitude in the 1% lime-treated LW and DW submitted soil, respectively. The duration taken by the LW submitted soil to renew 40 PVF was about 5 days higher than the corresponding DW submitted soil (Table 4). However, it took about 49 to 76 more days to circulate a similar volume of solutions through the untreated soil.

In the 2.5% and 4% lime-treated statically compacted soil, the k was in between 10 -7 to 10 -8 m/s for both DW and LW submitted soil. Thus, k was about one order of magnitude higher than the untreated soil (Fig. 5b &c). The total duration corresponding to the percolation of 40 PVF was only 1.5 and 5 days for the 2.5% and 4% lime-treated soil, respectively, which was much lower than the duration taken by the corresponding untreated soil (Table 4).

For the lime-treated kneading compacted soil, the level of k measured for the untreated soil was in the range of 10 -8 to 10 -9 m/s (Fig. 5d). In the untreated LW submitted soil, the level of k further decreased after about 20 PVF. This decrease is attributed to the grain rearrangement within the soil matrix during the percolation phase [START_REF] Young | Soil properties and behaviour[END_REF]. However, the obtained level of k in the 1% lime-treated kneaded specimens submitted to DW and LW remained almost at a similar level as the untreated soil (Fig. 5d). At the same time, the k increased by an order of magnitude in the 2.5% and 4% lime-treated kneaded specimen submitted to DW, whereas it remained constant for the corresponding LW submitted specimen (Fig. 5e & f). The 2.5% and 4% lime-treated kneaded soil took 75 and 40 days, respectively, to renew 40 PVF, whereas the corresponding DW submitted soil took only 4-6 days to circulate the similar volume of solutions (Table 4). However, the duration of the percolation phase was relatively higher in the untreated kneaded soil than in the lime-treated kneaded soil.

Based on the evolution of k obtained for each specimen in Fig. 5, Table 5 presents the estimated hydraulic life expectancy of an in-situ lime-treated earth structure of a unit reference height if built with the present soil configuration. The duration of life expectancy was expressed both in years and days (Column D) and was calculated by dividing the reference unit height of the structure by the average of the last 10 values of k measured in each specimen (Fig. 5).

As per Table 5, the estimated hydraulic life expectancy was 14.5 years for 2.5% lime-treated kneaded soil percolated by LW (Column D), while it was just 1.1 years for the corresponding DW percolated soil. For the 4% lime-treated soil, the life expectancy of the LW percolated sample was 6 years higher than that of the DW percolated sample. The difference in life expectancy based on the types of solution percolated was less significant for the 1% lime-treated kneaded soil. In the statically compacted soil, the difference in life expectancy for soil treated with 2.5% and 4% lime and percolated by both types of the solution was less significant. However, the hydraulic life expectancy of 1% lime-treated soil percolated by LW was 0.9 years higher than the corresponding DW percolated soil. It is worth noting that the above-estimated life expectancy did not consider the soil structure-pore fluid interactions during percolation. The obtained duration represents the duration taken by a given pore fluid to reach the bottom of a unit height structure if percolated from the top, considering k as the flow velocity. Each hydraulic conductivity test subjected specimens of given configuration took different durations to renew 40 PVF (Table 4). This duration was used to calculate the required duration to renew 40 PVF in a unit height hydraulic structure (Table 5, Column E). These obtained durations were further expressed in percentage with respect to the percentage of the total life expectancy estimated for the structure of unit height (i.e., Column E/Column D) in Fig. 6. on being submitted to 40 PVF of LW reached only 28% of its total life expectancy (Fig. 6b). Besides, the renewal of 40 PVF has reached about 30 to 50% of the total estimated life of the structure, built with the remaining configuration. The above evaluation of hydraulic performance was in terms of PVF, which presents the advantage of considering the pore fluid-soil structure interactions. Based on this evaluation, it was shown that the total life expectancy was yet to be reached even after the renewal of 40 PVF (Fig. 6).

Thus, such an assessment helps to investigate the life scale of in-situ earth structures at laboratory scale.

However, it is worth noting that the above estimation of life expectancy was based on the hydraulic performances and did not consider the mechanical performances of the compacted soil. Permeant solution flows mostly through the widest connecting pore available in soil [START_REF] Hunt | Flow, transport, and reaction in porous media: Percolation scaling, criticalpath analysis, and effective medium approximation[END_REF]. Hence, the leaching mechanism in a saturated lime-treated soil (Table 3) is more likely to be pronounced on the available minerals around the inter-aggregate pores constituting the flow-path compared to the intra-aggregate pores.

Evolution of calcium concentrations in the effluents of untreated and lime-treated soils

In the 1% lime-treated statically compacted soil, most of the Ca from the available and added lime, particularly around the flow-path, was lost on renewal of 10 PVF of DW and LW (Fig. 7a). Further renewal of solution triggered a release of soluble Ca from the soil. In the 2.5% and 4% lime-treated specimens, a sharp concentration of Ca was leached up to about 10 PVF, and the rate of leaching then slowed down.

However, the overall trend of Ca leaching for the treated soil remained much above the trend obtained from the untreated soil, thus, indicating that the loss of Ca was either from the cementitious compounds or residual lime available, particularly the available Ca around the flow path (Fig. 7b &c).

For the kneading compacted soil, Ca available around the flow-path in the form of cementitious compounds or residual lime was lost completely at about 5-7 PVF from the 1% lime-treated soil (Fig. 7d).

Then a concentration of soluble Ca was leached from the soil. The 2.5% lime-treated DW submitted specimen leached a significant Ca concentration, particularly coming from the lime added during the renewal of 40 PVF of DW. Besides, a part of soluble Ca was also lost from the soil (Fig. 7e). At the same time, the trend observed in the loss of Ca concentration in the corresponding LW submitted soil remained above the DW submitted soil, thus, indicating a part of Ca was lost only from the added lime (Fig. 7e).

For the 4% lime-treated kneaded soil, the evolution in the loss of Ca for the DW and LW submitted specimens remained at about a similar level (Fig. 7f). The leached Ca comes from the added lime.

Besides, the 2.5% and 4% lime-treated DW submitted kneaded soil leached a significant Ca concentration in just 4 and 6 days of percolation phase, respectively, whereas it took 75 and 40 days for the corresponding LW submitted soil to leach the equivalent Ca concentration (Table 4).

Discussions

Influence of pore fluid-soil structure interaction on the UCS evolution of lime-treated soil

Soil treated at LMO was shown to have a limited contribution towards the development of cementitious compounds [START_REF] Le Runigo | Performance of lime-treated silty soil under long-term hydraulic conditions[END_REF]. Hence, the addition of 1% lime to soil can be considered insufficient to maintain its strength and lost almost 60% of its initial UCS, irrespective of the compaction modes and the pore solutions the specimens were submitted to (Fig. 2).

The evolution of compressive strength in lime-treated soil enhances with the development of cementitious bonding, which depends on the lime content used (higher than the LMO), water content, and curing time [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF][START_REF] Lemaire | Effects of lime and cement treatment on the physicochemical, microstructural and mechanical characteristics of a plastic silt[END_REF][START_REF] Little | Stabilization of pavement subgrades and base courses with lime[END_REF][START_REF] Verbrugge | Strength and micro observations on a lime treated silty soil[END_REF]. Thus, in the 2.5% and 4% lime-treated soil, due to the constant contact of the compacted soil structure with the pore solution, particularly during the saturation phase, a significant evolution of cementitious compounds occurred. This was confirmed by the decrease in the total pores with a diameter of 10 4 Å, which in turn increased the formation of pores with a diameter lower than 10 3 Å in both the 2.5% and 4% lime-treated compacted samples (Fig. 4b-c, e-f). As expected, the evolution of pore with a diameter lower than 10 3 Å was more pronounced in the 4% lime-treated soil due to the greater availability of lime than that of the 1% and 2.5% lime-treated samples. As a result, the UCS of the 4% lime-treated soil increased by 57% and 36% in the statically and kneaded specimens, respectively, at the end of the hydraulic conductivity test compared to the 28 days cured specimens (Fig. 2a &b). The UCS remained unaffected in the 2.5% lime-treated statically compacted soil (Fig. 2a); however, it decreased in the corresponding kneaded soil.

The decrease in UCS of the kneaded soil was about 20% higher in the DW submitted soil than the LW submitted soil (Fig. 2b). This relatively significant decrease in UCS indicates the pronounced influence of pore solution chemistry on the kneaded soil structure. These phenomena are detailed in the later sections, which involve the mechanism of hydraulic conductivity and leaching as a result of the coupled influence of soil structures and pore solutions.

Influence of pore fluid-soil structure interaction on the hydraulic behavior of lime-treated soil

The observed hydraulic performances in the lime-treated soil appear to be in accordance with the microstructural modifications brought about by a coupled effect produced between the compacted soil structure and the chemistry of the solutions.

Influence of soil structure on the hydraulic conductivity evolution

Transport of pore fluid through a compacted soil matrix was shown to follow the pore geometry constituted by the widest connecting pores [START_REF] Hunt | Flow, transport, and reaction in porous media: Percolation scaling, criticalpath analysis, and effective medium approximation[END_REF]. Besides, previous studies have demonstrated that lime treatment increases the size of inter-aggregates pores due to flocculation, and as a result, increases the magnitude of hydraulic conductivity compared to the untreated soil [START_REF] Nguyen | Effect of lime treatment on the hydraulic conductivity of a silty soil[END_REF][START_REF] Tran | Effects of lime treatment on the microstructure and hydraulic conductivity of Héricourt clay[END_REF]. This explains the increased k from about 10 -9 m/s to somewhere in between 10 -6 and 10 -8 m/s in all the lime-treated statically compacted specimens (Fig. 5a-c).

However, according to [START_REF] Mitchell | Some Characteristics of the elastic and plastic deformation of clay on initial loading[END_REF], under kneading action, flocculated particles in natural soil break and transform into more likely a dispersed structure. This statement was confirmed in the lime-treated soil studied herein. The large inter-aggregates formed due to lime treatment were relatively deformed during kneading compaction, and consequently, the macropores were reduced.

Fig. 3 shows the presence of a relatively lower number of 10 5 Å diameter macropores in all the lime-treated kneaded soil compared to the corresponding statically compacted soil. Thus, the evolution of k remained constant, i.e., between 10 -8 to 10 -9 m/s in the 1% lime-treated kneaded soil submitted to both types of solutions, as well as the 2.5% and 4% lime-treated kneaded specimens submitted to LW (Fig. 5d-f).

However, the magnitude of k for the 2.5% and 4% lime-treated DW submitted soil was between 10 -7 and 10 -8 m/s, which was one order of magnitude higher than the corresponding untreated and LW submitted soil (Fig. 5e &f). This difference in k was because of the effect of pore solution chemistry, which is explained in the later section.

Influence of soil structure on flow characteristics

From the preceding discussion, different evolutions of 10 5 Å diameter macropores were observed under different methods of densification (i.e., static and kneading actions). Hence it will be interesting to evaluate the flow path of the permeant solutions through both types of compacted structures, which is measured by hydraulic tortuosity.

The hydraulic tortuosity, T is defined as the parameter that characterizes the heterogeneity in the flow path of the solution in a porous media [START_REF] Srisutthiyakorn | Hydraulic tortuosity: From artificial packs to natural rocks[END_REF]. In this study, T was calculated using the Kozeny-Carman equation (Equation 3), which considers T as a function of the pore geometry [START_REF] Allen | Computing and comparing effective properties for flow and transport in computer-generated porous media[END_REF]. Results of T calculated for the static and kneading compacted specimens are summarized in Table 6.

𝑇 ɸ (3)
In Equation 3, k is the coefficient of permeability in m/s, ɸ is the porosity calculated from each specimen, c is the Kozeny constant assumed to be equal to 200 for calculation convenience, and S is the specific surface area in m 2 /g measured with BET. The value of c was considered based on the plot provided by [START_REF] Allen | Computing and comparing effective properties for flow and transport in computer-generated porous media[END_REF], where c was shown to be a function of the porosity of the soil. According to Table 6, almost all the statically compacted specimens showed a comparatively lower value of T than the corresponding kneaded specimens. Thus, the consequence of kneading action on reducing total macropores of diameter 10 5 Å by deforming the flocculated particles was reflected in the greater value of T obtained in the kneaded soil. Hence, it can be said that the kneaded soil structure experiences an intimate and longer contact with the permeant solution, and the pore fluid within the kneaded soil structure has greater accessibility towards the available lime. Such intimacy is favorable for the development of pozzolanic reaction, and consequently, a significant formation of cementitious compounds can occur in the pores. This benefit of kneading action was confirmed by the observed decrease in the amount of 10 5 Å diameter macropores in the 2.5% lime-treated LW submitted kneaded soil compared to the unleached soil (Fig. 4e). As a result, the estimated hydraulic life expectancy and the duration to renew 40 PVF of LW through a unit height in-situ kneaded structure was relatively longer than the one obtained using the reference standard method (Table 5).

Based on the above discussion, Fig. 8 presents a synthetic schematic diagram that elaborates the flow paths within the lime-treated statically and kneading compacted soil. The mechanism of leaching was defined by the chemistry of the pore solutions the specimens were submitted to. As evident from Fig. 7e & f and Table 4, the leaching of Ca was comparatively higher and accelerated in the DW submitted kneaded specimens compared to the corresponding LW submitted specimens. As described earlier, due to the constant contact of soil-lime with the pore solution during the saturation phase of the hydraulic conductivity test, a significant formation of cementitious bonding occurred in the inter-and intra-aggregate pores. Once the percolation phase was initiated, the renewal of the pore solution occurred. Since DW is devoid of ions and has a low EC (Table 1), the DW dissolves a relatively significant quantity of Ca from the cementitious compounds, particularly those present around the flow path, and the soluble Ca from the soil. The dissolution of cementitious compounds from the macropores, thus, in turn, increased the magnitude of k (Fig. 5e &f). As a result, 40 PVF of DW was percolated through the 2.5% and 4% lime-treated kneaded soil only in 4 and 6 days, respectively. At the same time, the corresponding 2.5% and 4% lime-treated LW submitted specimens took 75 and 40 days, respectively, to percolate a similar volume of LW (Table 4). The above phenomena also explain the relatively significant UCS reduction observed in the DW submitted kneaded specimen (Fig. 2b).

However, the influence of pore solution chemistry in the leaching mechanism inducing a modification in the flow-path remained less pronounced in the 1% lime-treated kneaded specimen than in the 2.5% and 4% lime-treated samples. As soil treated at LMO has limited contribution towards the development of cementitious compounds, almost all Ca from the added lime was leached, and consequently, a significant amount of soluble Ca was leached from the soil (Fig. 7d). Besides, under the application of a constant load during kneading compaction, the flocculated particles in the 1% lime-treated specimens tend to deform more than the deformation of the 2.5% and 4% lime-treated soil as the addition of the higher lime dosages creates stronger flocculated particles. Thus, the evolution of k in the 1% limetreated remained at a similar level as the untreated kneaded soil (Fig. 5d).

Although the impact of pore solution chemistry on the mechanical and hydraulic evolution was pronounced in the kneaded soil, it remained less significant in the statically compacted soil. Due to the presence of additional macropores (Fig. 3) in all the lime-treated statically compacted specimens, statically compacted soil can be said to have undergone preferential flow, which was reflected in the relatively lower value of T than that in the kneading compacted samples (Table 6). In the process of preferential flow, the pore solution-soil structure interaction remained less significant, and thus the leaching mechanism remained at a similar level irrespective of the types of pore solutions the specimens were submitted to (Fig. 7a-c).

This explains the evolution of a similar level of k in the 2.5% and 4% lime-treated statically compacted soil regardless of the types of pore solution they were submitted to (Fig. 5b &c). It also explains the difference in UCS observed between the statically and kneading compacted soil (Fig. 2).

However, the k increased by 10 times in the 1% lime-treated DW submitted soil compared to the LW submitted soil (Fig. 5a). Considering that the leaching mechanism in 1% lime-treated statically compacted soil remained at a similar level (Fig. 7a), and 1% lime-treated soil has limited contribution towards the deposition of cementitious compounds around the flow path; hence, the observed increase in the k value in the DW submitted soil was due to the partial disintegration of inter-aggregates around the flow path, which in turn increased the quantity of 10 5 Å diameter macropores (Fig. 4a).

Comparison of hydraulic evolution in laboratory and in-situ cured soil

The hydraulic evolution and the estimated hydraulic life expectancy of the 2.5% lime-treated LW percolated kneaded soil (Table 5) were compared with the atmospherically cured specimens. In-situ specimens were sampled from a 7-years atmospherically cured embankment built with the same soil configuration, as reported by [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF]. The embankment was previously studied by [START_REF] Makki-Szymkiewicz | Evolution of the properties of lime-treated silty soil in a small experimental embankment[END_REF], who reported a k of 2.00E-09 m/s after 6 months of atmospheric curing. The hydraulic conductivity was evaluated again after 7 years of atmospheric curing by subjecting the sample to LW at the laboratory, and the k value was found to be 9.18E-10 m/s. The estimated hydraulic life expectancy and the evolution of k in the laboratory and the in-situ cured specimens are compared and are presented in Table 7. Table 7 shows that the obtained values of k and hydraulic life expectancy evolved in a positive manner. The k value decreased while the hydraulic life expectancy increased with curing time. The decrease in k was due to the evolution of cementitious compounds, which increased the mesopores by decreasing the available macropores, as reported by [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF]. The increased mesopores positively contribute to an increase in UCS, which has led to an average UCS of 3.29 MPa in the 7 years atmospherically cured soil, as reported by [START_REF] Das | Evaluation of the long-term effect of lime treatment on a silty soil embankment after seven years of atmospheric exposure: Mechanical, physicochemical, and microstructural studies[END_REF].

Thus, in addition to the positive evolution of k with increased curing time, an enhanced mechanical behavior can also be expected with time. Such an evolution can contribute to the increased hydromechanical life expectancy of the structure. This underlines the relevance of kneading compaction and the use of LW to evaluate the long-term hydromechanical performance of lime-treated soil.

Conclusions

This study investigates the hydromechanical evolution in lime-treated soil based on the leaching mechanism and microstructural modifications brought about by a coupled pore solutions-soil structure interaction. Based on the analysis, the following conclusions are derived:

1) The evolution of UCS in the lime-treated leached specimens was impacted by combined influence created by (i) the availability of lime, (ii) the quality of the interaction of the pore fluid with the soil-lime component and, (iii) the impact of compaction mechanism on the extent of deformation of larger-sized flocculated particles. Thus, the UCS increased by 57% and 36% in the leached 4% lime-treated statically compacted and kneaded specimens, respectively, compared to the unleached specimens. The UCS remained unchanged in the 2.5% lime-treated statically compacted specimen, while it decreased by 21% and 40% in the 2.5% lime-treated kneaded soil, percolated by LW and DW, respectively.

2) Evaluation of hydraulic conductivity in terms of PVF helps to determine the percentage of life expectancy that can be reached in an in-situ hydraulic structure by considering the pore fluid-soil structure interactions.

The renewal of 40 PVF corresponds to the deterioration of about 30 to 50% of the total estimated life of the lime-treated structure, built with the present soil configurations.

3) Kneading action reduced the number of macropores of diameter 10 5 Å in the compacted soil structure, which consequently reduces the magnitude of k. Thus, the magnitude of k was 10 -8 to 10 -9 m/s in the LW submitted kneaded soil, while it was 10 -6 to 10 -8 m/s in the corresponding statically compacted soil.

4) Higher hydraulic tortuosity obtained in the kneaded soil demonstrated the longer contact duration between the pore solution and the soil and lime component. This feature favored the development of cementitious compounds and lowered the macropores of diameter 10 5 Å in the 2.5% lime-treated LW submitted kneaded soil, which is favorable for the long-term performance of lime-treated earth structure.

5) The chemistry of pore fluid caused a significant modification in the hydromechanical evolution of limetreated soil based on its accessibility to the soil and lime component. DW, being relatively more aggressive than LW, dissolved a significant amount of Ca from the cementitious compounds, thus resulting in an increase in the k of 2.5% and 4% lime-treated kneaded soil. This influence of DW on the leaching mechanism and the hydraulic conductivity evolution remained less pronounced in the statically compacted soil than the kneaded soil due to the limited accessibility of pore fluid to the soil components.

6) The hydromechanical behavior of lime-treated kneaded soil evolved with curing time, which in turn increased the life expectancy of an in-situ earth structure. The hydraulic evolution and the life expectancy of 28 days laboratory cured 2.5% lime-treated kneaded soil, percolated by LW was 2.18E-09 m/s and 14.5 years, respectively. After 7 years of atmospheric curing, the hydraulic evolution and the life expectancy evolved to 9.18E-10 m/s and 34.5 years, respectively. Such observations were due to evolution of cementitious compounds, which also increased the UCS to 3.29 MPa. This underlines the relevance of the use of kneaded soil and LW to evaluate the long-term hydromechanical performance of the lime-treated insitu structure.

7) Soil treated at LMO (1%) has limited contribution towards the development of cementitious compounds.

Thus, the influence of pore solution-soil structure interaction remained less significant in the hydraulic, leaching, and compressive strength evolution of the 1% lime-treated soil, whereas significant modifications in these properties can be observed in soil treated with lime content of 2.5% and 4%.

Thus, the above findings showed that the hydromechanical evolution in lime-treated soil is governed by the mechanism created during the pore fluid-soil structure interactions. The duration of the pore fluid-soil structure interaction is defined by the compaction mechanism implemented, and the extent of influence is determined by the pore solution chemistry and the lime content added. Implementation of kneading compaction accompanied with 10 -3 M of NaCl concentration, a low ionic strength solution brings a significant modification in the hydromechanical performance. Thus, the selected compaction procedure and the permeant solution in the laboratory scale must be representative of the compaction mechanism and pore water that the structure is likely to be subjected to in the field. This will help to have a close prediction of the long-term hydromechanical performance of in-situ lime-treated structures. Such a prediction can contribute effectively to the efficient management of construction resources. Besides, to have a considerable modification in the hydromechanical performance, the addition of lime content higher than LMO is essential.

Fig. 1

 1 Fig. 1 Schematic diagram of flexible wall permeameter setup (Dimensions are not as per scale)

Fig. 2

 2 Fig. 2 Comparative evolution in UCS obtained in the 28 days cured and leached statically (a) and Kneading (b) compacted specimens Fig. 2a shows that 1% lime-treated statically compacted specimens lost about 60% of the initial UCS on being leached by DW and LW. On the other hand, no significant change in the UCS was observed in the 2.5% lime-treated leached specimens. However, the UCS increased by about 57% for the 4% limetreated leached specimens.

Fig. 3

 3 Fig. 3 Comparative evolution of PSD in the 1% (a), 2.5% (b), and 4% (c) lime-treated statically and Kneading compacted 28days cured specimens

Fig. 4

 4 Fig. 4 Comparative evolution of PSD between 1%, 2.5%, and 4% lime-treated unleached and leached statically compacted (a-c) and kneading compacted specimens (d-f).

Fig. 5

 5 Fig. 5 Evolution in hydraulic conductivity of untreated and 1%, 2,5%, and 4% lime-treated DW and LW submitted statically (ac), and kneading (d-f) compacted specimens

Fig. 6

 6 Fig. 6 Percentage of life expectancy that can be reached in a unit height earth structure if subjected to 40 PVF of DW (a) and LW (b) in kneading-and statically-compacted soil

Fig. 7

 7 Fig.7presents the calcium concentration, Ca (mg/l) measured in the effluents collected at different PVF from the untreated and the lime-treated statically and kneaded specimens during the percolation phase.

Fig. 7

 7 Fig. 7 Concentration of Ca leached from the untreated and 1%, 2.5%, and 4% lime-treated DW & LW submitted statically (a-c) and kneading (d-f) compacted specimens

Fig. 8

 8 Fig. 8 Schematic diagram showing differences in lime-treated compacted soil matrices after being subjected to static and kneading compactions and the expected flow paths of permeant solution during hydraulic test

Table

Table 2

 2 Maximum dry density and OMC of untreated and lime-treated silty soil

	Soil	ρd(max) (kN/m 3 )	OMC (%)
	Untreated silty soil	18.4	14.3
	Silty soil treated with 1% lime	17.4	17.6
	Silty soil treated with 2.5% lime	17.1	18.5
	Silty soil treated with 4% lime	17.0	18.7

Table 3 :

 3 Sr measured in the lime-treated soil at the end of curing time and at the end of hydraulic test

		Sr (%) of statically compacted soil		Sr (%) of kneaded soil
		1%	2.5%	4%	1%	2.5%	4%
		lime-treated	lime-treated	lime-treated	lime-treated	lime-treated	lime-treated
	After 28 days of curing	74.6	78.5	74.4	74.4	69.3	71.7
	After 40 PVF was renewed in DW submitted soil	98.2	100	99.8	100	90.2	98.8
	After 40 PVF was renewed in LW submitted soil	100	99.4	100	_	100	98.8

Table 4 :

 4 Duration of percolation phase in the lime-treated soil to renew 40 PVF of solutions

		Duration taken by statically compacted soil (days)		Duration taken by kneaded soil (days)
		untreated	1% lime-treated	2.5% lime-treated	4% lime-treated	untreated	1% lime-treated	2.5% lime-treated	4% lime-treated
	DW submitted soil	77	0.7	1.5	5	87	70	4	6
	LW submitted soil	55	6	1.5	5	93	65	75	40

Table 5 :

 5 Estimated life expectancy of a unit height in-situ earth structure, if built with the present soil configurations

	A	B	C	D	E
				Estimated life	Required duration to
	Compaction	Specimen's	Average k	expectancy for a unit	reach 40 PVF in a unit
	modes	configuration	(m/s)	height structure	height structure
				(years (days))	(days)
		1-S-OMC-28-DW	2.97E-07	0.1 (39.0)	16
		1-S-OMC-28-LW	3.30E-08	1.0 (350.6)	120
	Statically	2.5-S-OMC-28-DW	1.71E-07	0.2 (67.7)	32
	compacted	2.5-S-OMC-28-LW	2.01E-07	0.2 (57.7)	32
		4-S-OMC-28-DW	4.56E-08	1.0 (254)	100
		4-S-OMC-28-LW	4.73E-08	0.7 (244.7)	100
		1-K-OMC-28-DW	2.91E-09	10.9 (3983.5)	1400
		1-K-OMC-28-LW	3.56E-09	8.9 (3249.4)	1300
	Kneading	2.5-K-OMC-28-DW	1.49E-07	0.2 (77.7)	80
	compacted	2.5-K-OMC-28-LW	2.18E-09	14.5 (5297.0)	1500
		4-K-OMC-28-DW	3.00E-08	1.1 (385.8)	120
		4-K-OMC-28-LW	4.54E-09	7.0 (2548.3)	800

Table 6

 6 Tortuosity calculated from statically and kneading compacted specimens

	Specimen ID	k (m/s)	ɸ	S (m²/g)	T	Specimen ID	k (m/s)	ɸ	S (m²/g)	T
	1%-S-DW	2.97E-07 0.38 13.16 2.32 1%-K-DW	2.91E-09 0.37 11.75 24.84
	1%-S-LW	3.30E-08 0.35 12.02 6.81 1%-K-LW	3.56E-09 0.37 11.62 23.34
	2.5%-S-DW 1.71E-07 0.38 19.78 2.04 2.5%-K-DW 1.49E-07 0.38 18.56	2.27
	2.5%-S-LW	2.01E-07 0.39 18.42 2.08 2.5%-K-LW 2.18E-09 0.38 18.56 19.46
	4%-S-DW	4.56E-08 0.38 22.93 3.36 4%-K-DW	3.00E-08 0.37 22.10	4.12
	4%-S-LW	4.73E-08 0.37 23.73 3.12 4%-K-LW	4.54E-09 0.39 20.60 12.34

Table 7

 7 Comparison of k and life expectancy obtained for in-situ and laboratory cured 2.5% lime-treated kneaded soil, percolated by LW

	Type of specimens	Duration of curing	k (m/s)	Life expectancy (years)
	Laboratory-cured	28 days	2.18E-09	14.5
	In-situ cured	6 months 7 years	2.00E-09 9.18E-10	15.9 34.5

Acknowledgements

This work was financially supported by Association Nationale de la Recherche et de la Technologie with grant N˚2018/0219 and Lhoist Southern Europe with grant N˚RP2-E18114. The authors are very thankful to the research team of Université Gustave Eiffel, and Lhoist Nivelle for their great support in performing laboratory experiments and technical supports.


Mickael Le Vern
email: mickael.le-vern@univ-eiffel.fr

Ouardia Sediki
email: ouardia.sediki@univ-eiffel.fr

Andry Razakamanantsoa
email: andry.razakamanantsoa@univ-eiffel.fr

Frédéric Murzyn
email: frederic.murzyn@estaca.fr

Frédérique Larrarte
email: frederique.larrarte@univ-eiffel.fr

Pascal Insenga
email: pascal.insenga@vinci-construction.com

Philippe Gotteland
email: philippe.gotteland@fntp.fr

Development of a model to quantify dust emissions from truck traffic on earthmoving sites: approach and preliminary results

Keywords: Dust emission, Truck traffic, Earthworks, Soil mechanics, Wake flows, Water savings

Dust emission is a major issue in general but even more for civil engineering professionals. This is particularly important on earthmoving sites with truck traffic on compacted soils. Suspended dust causes many troubles, including visibility mitigation, which leads to safety risks. To prevent the lift of particles, soils are watered abundantly. When carried out in an arbitrary way, this solution can lead to a significant waste of this natural resource. Savings could be achieved by anticipating the necessary and sufficient quantities for dust suppression. For this purpose, a model is being developed to highlight and quantify the main factors involved in the lifting of soil particles by truck traffic. The approach adopted for the development of the model together with the preliminary results are presented. Soils degradation by traffic are studied in laboratory in order to assess the accumulation of particles on the surface. These particles are lift due to mechanical and aerodynamic forces, which are studied in test track and wind tunnel, respectively. This study allows to characterize the mechanism of dust emission on construction sites by relying on physical parameters that are not considered in the empirical models currently available.

Introduction

In an increasing stringent context of improving air quality, limiting dust emissions generated by human activities is an overriding objective for environmental and public health issues. Soil degradation is a major factor of particulate matter suspension in the atmosphere. It is caused in particular by vehicle traffic on unpaved roads. Due to the contact between the wheel and the ground, the aggregates of soil particles can be torn off and then crumbled. This leads to the formation of fine particles which are prone to fly away when vehicles pass by or because of the wind and related aerodynamic processes [START_REF] Etyemezian | Vehicle-based road dust emission measurements: I-methods and calibration[END_REF][START_REF] Gillies | Effect of vehicle characteristics on unpaved road dust emissions[END_REF][START_REF] Kuhns | Effect of soil type and momentum on unpaved road particulate matter emissions from wheeled and tracked vehicles[END_REF]. Airborne dust suspension leads to health issues [START_REF] Pope | Health effects of particulate air pollution: Lines that connect[END_REF] as well as visibility mitigation [START_REF] Moosmüller | Scattering cross-section emission factors for visibility and radiative transfer applications: Military vehicles traveling on unpaved roads[END_REF]BADDOCK et al., 2014). This involves significant safety risks, particularly in areas with vehicle traffic [START_REF] Strader | Driving blind: Weather-related vision hazard and fatal motor vehicle crashes[END_REF]. On earthmoving sites, the truck traffic on runways is a major source of dust [START_REF] Muleski | Particulate emissions from construction activities[END_REF]. The most common method to minimize particle lift is to spray the ground with a huge amount of water. This watering was estimated to be about 12 liters of water per cubic meter of earthworks in France in 2010 [START_REF] Sptf | Intensifier l'engagement pour la route durable en 2012[END_REF]. This can lead to a significant waste of water. Resource savings could be made by anticipating the necessary and sufficient quantities to prevent dust emission. In order to achieve this goal, it is require to study the mechanisms of soil degradation and particle lift by traffic. To this end, research works have been undertaken in recent years at the University Gustave Eiffel [START_REF] Sediki | Degradability of unpaved roads submitted to traffic and environmental solicitations: laboratory scale[END_REF]LE VERN et al., 2020a;[START_REF] Vern | Experimental assessment of dust emissions on compacted soils degraded by traffic[END_REF]. This paper aims at presenting the corresponding approach as well as some preliminary results.

Methodology

The dust emission mechanisms are assessed according to three main lines of study: -The soil surface degradation under traffic stress is analyzed at the laboratory scale.

-The physical, mechanical and environmental parameters that govern dust emission when a vehicle is in motion are highlighted with a test track study. -The transport of particles in the wake of trucks circulating on the earthworks runways is characterize with scale models in a wind tunnel. Figure 1 illustrates the steps that lead to the dust plume formation. Above a certain threshold of particulate concentration that needs to be identified, suspended dust will significantly reduce visibility. This nuisance requires the watering of the runways on the construction site. The aim of the study is to develop a model based on physical laws and experimental measurements for the quantification of dust concentration generated by trucks traffic on runways. A better knowledge of dust emission mechanisms would allow to limit them and to forecast the quantities of water needed during the construction project.

Preliminary results

The dust emission model under development is based on three parameters: -The number of particles likely to fly away as they are no longer bound to the granular matrix. They appear due to soil surface degradation by traffic; -The Particle Size Distribution (PSD) of these particles; -The mechanical and aerodynamic stresses on particles when vehicles are in motion. These three points are discussed below.

3.1 Origin of dust: mechanical degradation of the soil surface Earthworks runways are made by soil moistening and compaction. This process forms a granular matrix consisting of aggregates. Dust is generated by trucks traffic as these aggregates are torn off by the wheels. This phenomenon was studied with a traffic simulator reproducing the motion of a wheel on laboratory-prepared soils. They were made with two materials: Speswhite kaolin clay (denoted "K") and Hostun HN38 sand (denoted "S"). Three samples were degraded in the traffic simulator: S0K100 (100% clay), S50K50 (50% sand and 50% clay) and S75K25 (75% sand and 25% clay). Table 1 summarises their geotechnical properties.

where d is the diameter of particle (μm), %clay is the mass percentage of particles smaller than 2 μm, %sand is the mass percentage of particles between 20 and 2000 μm, dmin and dmax represent the minimum and maximum soil particle diameter (μm) respectively [START_REF] Vern | Experimental assessment of dust emissions on compacted soils degraded by traffic[END_REF]. This empirical function highlights the capacity of a soil to form aggregates (characterized by the product %clay x %sand) and their gradual crumbling when the number of wheel passes N increases. This makes it possible to know the PSD evolution of the dust to be put into suspension during the soil degradation by traffic.

Particle lift by vehicle motion

When a vehicle is in motion, the lift of particles depends jointly on physical (type and quantity of particles, vehicle geometry), mechanical (vehicle speed, tyre/surface contact dynamics) and environmental (wind, relative humidity and air temperature) parameters.

An experimental study on a test track was carried out to assess the impact of two different moving vehicles (Renault Clio III and Ford Ranger) on particle lift. Our attention was particularly focused on the effect of the above-mentioned parameters on airborne particulate matter concentration when vehicle traffic occurs over a bed made of four different types of particles:

-Montmorillonite clay -Clayed silt from Marche des Dames (France) -Silt from Val d'Europe (France) -Loess from Strasbourg (France) Each test consisted of running one of the two vehicles at a given speed (30, 45 or 60 km.h - 1 ) over one of the four types of particles previously placed on the ground at a given surface concentration (200, 400 or 600 g.m -2 ). This latter is directly related to the surface degradation (Figure 2). Figure 4-a shows the experimental setup. The concentration of resuspended particles was measured by suction of the dust plume behind the rear tire using a particle counter.

Conclusions

To date, works carried out have made it possible to characterize the appearance of dust during traffic on compacted soils. Models of surface degradation and PSD evolutions have been developed. They will need to be validated by testing a wider range of soils. This work represents a novelty in the field of dust emission studies and is a first step in the development of a robust model for the quantification of dust emissions. A particular attention will be drawn to the lift of particles due to mechanical and aerodynamic forces. Analysis of the test-track experiments results and wind-tunnel studies will be undertaken to this end.

Figure 1 .

 1 Figure 1. Processes related to dust emissions, from soil compaction to its watering.

Figure 2

 2 Figure2shows the experimental results of their surface degradation as a function of the number of wheel passes. They represent the cumulative mass of aggregates torn off per unit of ground surface covered by the tire during its movement. The results show deviations of several orders of magnitude from one soil to another. For the same number of wheel passes, high-plasticity clay generates ten times more dust than clayed sand and hundred times more than low-plasticity clay. This latter has the least propensity to generate dust under traffic because it is a well graded soil with a high shear strength(LE VERN et al., 2020a).

Figure 2 .

 2 Figure 2. Soil surface degradation as a function of the number of wheel passes, with 50% uncertainty (shading).

3. 2

 2 Particle size distribution of dust during traffic degradation By measuring the Particle Size Distribution (PSD) of the aggregates torn off during soil degradation by traffic, a typical trend can be observed. An example is given in Figure 3 for S75K25 sample.

Figure 3 .

 3 Figure 3. Particle size distributions of initial soil (S75K25) and dust particles after N passes of the wheel.

  Figure 4-b shows an example of measurements for one of the tests (PMX being the concentration of particulate matter having a diameter less than X μm). Temperature, relative humidity and solar radiation were monitored with a weather station. The Renault Clio III was equipped with a dynamometric hub enabling real-time measurements of the longitudinal, vertical and transverse forces and torques applied to the right front wheel. Overall, 74 tests were conducted corresponding to a wide range of experimental conditions. The results are currently being analyzed.

Figure 4

 4 Figure 4. a) Configuration of test track experiments; b) Particulate concentration measurements during the vehicle traffic over the bed of particles.

Figure 5 .

 5 Figure 5. Wind tunnel scale model of a) Caterpillar articulated dump truck and b) Mercedes 8x4 cab-over truck.

Table 1 :

 1 Soil geotechnical properties

	Soil	Liquid Limit (%)	Plastic Index (%)	Methylene blue value (g/100g)	Specific surface area (cm 2 /g)	Unified Soil Classification System
	S0K100	55.00	25.00	1.56	10.65	CH: High plasticity clay
	S50K50	29.90	12.60	0.74	5.28	CL: Low plasticity clay
	S75K25	23.10	7.60	0.43	2.47	SC: Clayed sand

Acknowledgments

The authors would like to express their gratitude to the Fédération Nationale des Travaux Publics (grant number RP2_E18101) and the Region Pays de la Loire (grant number GHPAI3) for their financial support. Special thanks to Erwann Rayssac, Sébastien Buisson and Olivier Burban for their help during the experiments.