Eliane Balaa
email: elianebalaa@yahoo.com

Mathieu Raynal

Youssef Bou Issa

Emmanuel Dubois

Classification of Interaction Techniques in the 3D Virtual Environment on Mobile Devices

Keywords: Interaction techniques, 3D Virtual environment, mobile devices, environment density, depth of targets, occlusion, Augmented Reality

3D Virtual Environments (3DVE) are more and more used in different applications such as CAD, games, or teleoperation. Due to the improvement of smartphones hardware performance, 3D applications were also introduced to mobile devices. In addition, smartphones provide new computing capabilities far beyond the traditional voice communication. They are permitted by the variety of built-in sensors and the internet connectivity. In consequence, interesting 3D applications can be designed by enabling the device capabilities to interact in a 3DVE. Due to the fact that smartphones have small and flat screens and that a 3DVE is wide and dense, mobile devices present some constraints: the environment density, the depth of targets and the occlusion. The pointing task faces these three problems to select a target. We propose a new classification of the existing interaction techniques, according to three axis of classification: a) the three discussed problems (density, depth and occlusion); b) the first two subtasks of the pointing task (navigation, selection); and c) the number of targets selected by the pointing technique (1 or N). In this paper we will begin by presenting a state of the art of the different pointing techniques in existing 3DVE, structured around three selection techniques: a) Ray casting, b) Curve and c) Point cursor. Then we will present our classification, and we will illustrate the classification of the main pointing techniques for 3DVE. From this classification, we will discuss the type of interaction that seems the most appropriate to perform this subtask optimally.

Introduction

Nowadays, with built-in mobile operating system, smartphones provide new computing capability and connectivity far beyond traditional voice communication [START_REF] Robbins | ZoneZoom: Map Navigation for Smartphones with Recursive View Segmentation[END_REF]. Internet connectivity and applications, games, gyroscopic sensor [START_REF] Schäfers | Designing Low-Dimensional Interaction for Mobile Navigation in 3D Audio Spaces[END_REF], built-in camera, accelerometer, touchpad and touch screen are very common on such devices. In parallel, three-dimensional technologies have been recently introduced in different applications and areas such as desktop computing and CAD [START_REF] Liang | Geometric Modeling Using Six Degrees of Freedom Input Devices[END_REF] (computer aided design systems), 3D games, VE (virtual environments), AR (augmented reality), ubiquitous computing and tele-operation [START_REF] Zhai | Anisotropic human performance in six degreeof-freedom tracking: An evaluation of three-dimensional display and control interfaces[END_REF]. Extending the use of 3D applications onto smartphone provides a greater value to both. Few applications to the concept are launched: 3D games and Head-tracking are some examples. Interesting 3D applications on mobile devices can be designed through the mapping of the device abilities to the interaction requirements in the 3D environment. Consequently, the smartphone will be promoted as the new generation of multi-use device. Researchers in 3D virtual environment have developed many new techniques and metaphors for 3D interaction to improve 3D application usability on mobile devices. But they are all facing the same kinds of problems or factors that are specific to the use of 3D on mobile devices. Indeed due to the fact that smartphones have small and flat screens and that a 3DVE is wide and dense with a large number of targets of various sizes, mobile devices present specific constraints:

1. The environment density [START_REF] Vanacken | Exploring the effects of environment density and target visibility on object selection in 3D virtual environments[END_REF]: to display a large number of targets with various sizes on the relatively small screen of mobile devices, the scene must be shrunk, and the virtual space size and its components must be minimized. Various objects are very close to each other. In this case, how to accurately select one of these objects? 2. The depth of targets [START_REF] Hwang | Design of the 3D Input Method Based on Touch Device for Mobile[END_REF]: the elements of the environment are displayed in 3D and are identified by three coordinates (X, Y, Z). They are arranged in several levels of depth while the display is a flat touch screen (x,y). But how to point at the appropriate depth to reach an object that is in depth and behind other, even transparent, layers? Depth causes two problems: On the one hand, to know where the pointer is in depth in relation to the target; on the other hand, to point target in depth quickly and with accuracy adds additional complexity to the pointing task. 3. The occlusion [START_REF] Vanacken | Multimodal selection techniques for dense and occluded 3D virtual environments[END_REF]: One or more elements of the scene can hide the target either partially or totally and thus reduce its visibility and ability to be selected. Furthermore, the user uses his finger to point a target using the touch screen which creates an additional occlusion while targeting a small object. How to reach an object if it is hidden by another or by the interaction process?

Designing an appropriate interaction technique for 3D on mobile devices must therefore consider these constraints in order to identify the most appropriate solution to the selected context. In light of these three major concerns, and to contribute to the comparison and design of interaction techniques in 3D on mobile devices, we propose a new classification space. The originality of our approach is to particularly emphasize usage constraints (density, depth, and occlusion) in the context of the pointing task. It is no longer a technologically centered approach but a real user centered approach.

Classification

As a first step we chose to focus on the pointing task only, among the three well known tasks of Bowman [START_REF] Bowman | Principles for the Design of Performance-oriented Interaction Techniques[END_REF]. Indeed, the pointing task, which is the most used, faces the three mentioned problems. Despite the large amount of different techniques proposed in the literature, these three problems are never solved at the same time. Typically, suggested interaction techniques are used to solve one or two of the three problems. To refine this analysis and better compare or design the pointing techniques in 3D, we propose to consider the pointing task as a set of three sequential subtasks:

1. Navigation refers to exploring the content of the 3DVE in different ways to search for the target and visualize it [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF]. In this phase, the user may change his viewpoint through the environment or move the scene to visualize the target. 2. Selection refers to moving a pointer to reach the target position once the target is visible.

[5] 3. Validation consists of validating or confirming the pointing task, when the pointer reaches accurately the target. When the technique selects multiple targets during the selection task, the validation task can also select the desired target from the N selected targets. [START_REF] Bowman | Principles for the Design of Performance-oriented Interaction Techniques[END_REF] In addition, the target environment can be either dense or sparse. In order to select a target in a dense environment, multiple objects close to the target may be hard to avoid while trying to select the target. As a result, the target will have to be selected from a first subset of objects. In a sparse environment, such kind of problem doesn't occur, and thus only one object is selected. Consequently we will consider two cases: a) the interaction techniques selecting multiple objects and: b) the interaction techniques selecting 1 object. Finally problems that have been raised above can therefore be involved at different steps of the pointing task. This is why we propose a new classification of the existing interaction techniques, according to three axis of classification: a) the three subtasks of the pointing tasks (navigation, selection, validation); b) the three discussed problems (density, depth and occlusion) on the first two subtasks of the pointing task, and c) the number of targets selected by the pointing technique during the validation subtask (1 or N). • The first column presents the name of the pointing technique and its reference. • Columns 2 to 4 relate to the navigation subtask o In column 2, the target is in a high density environment but it is visible. Usually, navigation will not solve this problem of density because its goal is only to display the target. However, some interaction techniques can indirectly use navigation to bring target closer and therefore make easier the selection task.

o In column 3, the target is in depth, but visible. Like density, navigation task is not necessarily required, but can be used to facilitate the selection task. o In column 4, the target is occluded. To visualize it, the user has to change the viewpoint searching for the target. This case covers interaction techniques that benefit from the Navigation to solve the occlusion problem.

• Columns 5 to 7 relate to the selection subtask o In column 5, the user must point a target that is close to several others (density problem). The technique must allow the selection with accuracy of the target among the N surrounding. o In column 6, the user must be able to point with the pointing technique the target that would be distant in depth. The technique must enable to quickly and accurately select this target. o In column 7, the position target is known but the target is occluded or partially occluded. The technique must allow the selection of the desired target despite obstacles. • Column 8 treats the validation problem. We distinguish situations where one target is selected during the selection phase and the case where multiple targets are selected. In this last case, it requires a new selection from N preselected targets.

For each cell of the table, we note "-" if the sub-task is not covered by the pointing technique. If the subtask is covered, we note "P" if the problem is present but is not resolved by the pointing technique; "PS" if the problem is partially solved and "S" if the problem is solved by the pointing technique.

Thus this table allows to quickly see the advantages and disadvantages of a pointing technique, and also allows comparing two techniques together. This representation also allows us to study more precisely the resolution of a problem for a subtask by analyzing the various solutions proposed for this particular column.

In section 3 we use this classification space and its sub-categories to compare existing interaction techniques. We highlight the differences of the various techniques to demonstrate that existing classifications do not sufficiently highlight the weaknesses and forces of each technique.

Existing Techniques

A well-established classification [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF] proposes to classify the selection technique used in 3D Virtual Environment into 3 categories: point cursor, ray casting and curve. We will present the main techniques of these three categories and show the main problems they face. The simplest pointing technique consists of moving a pointer in the 3DVE. The pointer movement is controlled by a pointing device. For example, 3D Point cursor [START_REF] Grossman | The Design and Evaluation of Selection Techniques for 3D Volumetric Displays[END_REF] is an interaction technique where a cursor is moved in the virtual environment to select a visible target. When the user press a button, the 3D position and orientation of an input device are linearly transformed into coordinates in the virtual environment where a cursor is displayed. To reach the target, the user moves his input device. Consequently, the coordinates of the cursor change. If the cursor points at the target, the target is highlighted. Then the selection is confirmed by clicking on the input device button. However, this task is complicated since it requires more accurate movements and multiple adjustments in depth.

In case of dense and crowded environment, the target can be occluded. Two problems may occur: the first is the occlusion of the target and the second is the density problem: due to the fact that the target is surrounded by multiple objects that makes the selection of this target a complicated task.

To point the target precisely, the user has to make fine displacement of the crosshair. To point the target accurately without making a big number of displacements, researchers introduce the volume selection with multiple occlusion levels. In the Silk cursor technique [START_REF] Zhai | The "Silk Cursor": Investigating transparency for 3D target acquisition[END_REF], the cursor is replaced by a semi-transparent rectangular volume cursor. The user hand is transformed into a silk cursor. When the target is inside the rectangular volume, the user close his hand and the selection is ended. The occlusion levels are used to solve the depth problem. Silk cursor provides information on where the object is placed relatively to another object with no measure of how much they are spatially separated. In the transparent sphere technique [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF], the crosshair is replaced by a transparent sphere and the target is one of the objects inside the sphere. To select it, this technique uses list menu or circulation techniques. The menu technique solves the density problem but lose the information about the depth position.

In summary, pointing techniques, based on the point cursor, do not support the navigation phase. They require an additional accuracy effort from the user to point a target in depth. In the selection phase, Silk cursor and transparent sphere partially solve the depth problems. In transparent sphere, the density problem is partially resolved because it requires the search of the target from a set of items.

With the ray casting techniques, the depth problem is solved. In ray cursor [2,5] (see Fig. 1-A), the user controls the origin position and the orientation of a virtual ray by adjusting the input device position. Then, a virtual ray is emitted from the user through the target direction. This ray intersects the targeted object. The first object intersected by the virtual ray is highlighted and the selection is confirmed by clicking the input device button. Therefore, this technique does not solve the occlusion problem because if a target is positioned in front of the desired target, it is the first target which is selected and not the desired target. In Go-Go technique [START_REF] Poupyrev | The Go-Go Interaction Technique: Non-linear Mapping for Direct Manipulation in VR[END_REF] (see Fig. 1-B), the virtual ray is replaced by a virtual hand. Go-Go is an arm-extension technique using a nonlinear transformation. The user real hand is transformed in a virtual hand in the virtual environment. The virtual hand is kept on the ray extending from the torso and going through the physical hand. Go-Go is limited by the real hand length and the difficulty of grabbing; these limitations are solved by the HOMER [START_REF] Bowman | An evaluation of techniques for grabbing and manipulating remote objects in immersive virtual environments[END_REF] interaction technique which combines a ray casting and an arm-extension techniques. HOMER can select objects at any position with no restrictions and perform easy selection and manipulation. [START_REF] Poupyrev | The Go-Go Interaction Technique: Non-linear Mapping for Direct Manipulation in VR[END_REF] In the selection phase, Ray cursor, Go-Go and HOMER solve the depth problem thanks to the virtual ray projected to infinity. These techniques are used in a sparse environment to point a visible target. However, they do not solve the occlusion problem because the pointer selects the first target encountered in the case of the ray cursor technique and a visible target in the case of the Go-Go and HOMER techniques. The density problem is not solved either: the Go-Go and HOMER techniques need multiple adjustments to point the target and the ray cursor requires a higher selection precision. Consequently, in case of a dense environment, these techniques are not accurate to select an object.

Fig. 1. A) Ray casting technique [3] B) Go-go technique

To solve the occlusion problem, when the ray passes through multiple targets, different techniques are proposed to identify and point the correct target from the group of the aligned objects. The "Ray With Variable Length" technique [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF] aims to change the ray length by changing the position of the ray endpoint. The closest objet to the endpoint is highlighted and selected when the user pushes a tracked wand button. Another technique is the Depth ray [START_REF] Grossman | The Design and Evaluation of Selection Techniques for 3D Volumetric Displays[END_REF], in addition to the ray, a depth marker travels between the objects following the input device displacements. Each time it comes close to an intersected object, the object becomes red. However, the Depth ray uses a continuous displacement of the input device that can cause modifications of the ray position. This limitation is solved by the Lock ray technique [START_REF] Grossman | The Design and Evaluation of Selection Techniques for 3D Volumetric Displays[END_REF] which is a Depth ray version aiming of locking the ray then the depth marker appears to avoid confusion between selection and disambiguation phases. Another technique consists of using a menu to select a target from the N pointed objects of the ray casting. The menu technique consists of displaying the visible and the occluded objects intersected by the ray. Then the target is selected from the menu. Different techniques exist: a 2D flower menu in the Flower Ray technique [START_REF] Grossman | The Design and Evaluation of Selection Techniques for 3D Volumetric Displays[END_REF] (which is a 3D extension of the splatter technique [START_REF] Ramos | Tumble! Splat! Helping Users Access and Manipulate Occluded Content in 2D Drawings[END_REF]) ; a virtual sphere menu in the Daisy menu technique [START_REF] Liang | Geometric Modeling Using Six Degrees of Freedom Input Devices[END_REF] ; a circular menu in the Ring menu technique [START_REF] Liang | Geometric Modeling Using Six Degrees of Freedom Input Devices[END_REF] or a list menu in the Floating menu technique [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF][START_REF] Ramos | Tumble! Splat! Helping Users Access and Manipulate Occluded Content in 2D Drawings[END_REF]. Using a ray cursor and adding a menu technique solves the occlusion and the depth problems in the selection phase. The limitation of these combined techniques is in the huge number of objects displayed in the menu. These techniques are efficient if the ray crosses exactly the target. Otherwise a density problem is detected.

To overcome the density problem, some techniques use a volume rather than a ray. A selection volume is defined by a ray and the volume around it. This volume can have different forms: a) a cone in the case of the spotlight technique [START_REF] Liang | Geometric Modeling Using Six Degrees of Freedom Input Devices[END_REF] or a cylinder

A B

in the case of the transparent cylinder technique [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF]. The transparent cylinder technique consists of displaying a cylinder around the ray (see Fig. 2 A.). All objects contained in the cylinder are selected. Then the target is selected using a circulation or Floating menu techniques. The huge number of objects inside the cylinder is solved using a dynamic selection volume (case of the spotlight). The spotlight technique consists of displaying a cone around the ray. The spread angle of the cone is adjusted to minimize the number of objects inside it. Then the target is selected using a special numeric algorithm. Once found, the area around the object is gradually shrunk to minimize the total number of the selected objects. Consequently, the transparent dynamic selection volume solves the density problem, in the selection phase.

Another way to solve the density problem consists of designing hybrid techniques. They combine techniques based on the point cursor metaphor to others based on the ray casting metaphor: for example, the hybrid technique formed by the combination of a ray cursor and a bubble cursor technique [START_REF] Grossman | The Design and Evaluation of Selection Techniques for 3D Volumetric Displays[END_REF], or the one formed by the combination of a ray cursor and a transparent sphere technique [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF] (see Fig. 2 B.). To do that, these techniques add a dynamic sphere volume at the ray endpoint to augment the selection area and minimize the pointing problem in a dense volume. The hybrid technique formed by the combination of the ray cursor and the bubble cursor provides a shape transformation and warp to select one target. And the hybrid technique formed by the combination of the ray cursor and the transparent sphere selects a group of objects. Based on the number of objects inside the dynamic selection area, researchers propose: a) circulation or menu techniques or b) a special numeric algorithm to select one target. In the selection phase, these techniques solve the depth and the density problems.

Fig. 2. A) Transparent cylinder and B) transparent sphere technique [4]

To resolve the occlusion problem during the selection phase, Curve technique presents a possible solution. Curve with fixed length [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF] is an interaction technique where a fixed curve is moved in the virtual environment to select a visible target. Unlike the curve with fixed length where the user has to move many time in order to select a target, a Curve with variable length [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF] technique move the destination endpoint along the three dimensions in order to select its target. In the selection phase, Curve with fixed length and Curve with variable length solve the depth and the occlusion problem because the curve help avoid targets which is in front of the desired B A target. In case of dense and crowded environment, the target can be occluded and the curve will select multiple objects. The curve techniques are based on the transparency of the environment. Thus, the user can detect the position of the target and make the adjustment to reach it. The Flexible pointer [START_REF] Olwal | The Flexible Pointer: An Interaction Technique for Augmented and Virtual Reality[END_REF] technique is visualized by a flexible curve. The flexible pointer direction is determined by vector formed by the two hands. The amount of curvature is defined by the orientation of each the hand. It is used to point visible, partially or fully occluded objects and to point around objects. Using a curved ray has the advantage of selecting a deep target without passing through the environment objects. The problem of density is solved by the flexibility of the curve in the case of the flexible pointer technique or by using the Virtual pointer metaphor technique [START_REF] Steinicke | Object selection in virtual environments using an improved virtual pointer metaphor[END_REF]. The Virtual pointer metaphor is a ray casting metaphor, using a dynamic area selection and a menu or selection algorithm, to point the target and then to draw a Bezier curve graph pointing the target. Curve also solves the depth problem. The transparency added to the curve metaphor solves the occlusion problem. Solving the density problem is achieved by two methods a) the form and the length of a flexible curve in a transparent environment or b) a ray based technique in a nontransparent environment. In the selection phase, the Flexible pointer length and the Virtual pointer metaphor solve the depth, the occlusion and the density problems.

Most of the presented techniques are only concerned with the selection phase. The navigation subtask is not supported by the proposed techniques and requires different approaches. This complicates the interaction task in the 3DVE. The following techniques cover the selection and the navigation phases. Using the two ray selection technique [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF], a first virtual ray is emitted. Multiple targets could be intersected. To select the target from those that have been intersected by the first ray, the user changes his point of view and emits a new ray. The target is located at the intersection of the two virtual rays. The shadow cone [START_REF] Steed | 3D Selection Strategies for Head Tracked and Non-Head Tracked Operation of Spatially Immersive Displays[END_REF] used the same principle, using virtual cone instead of rays. The Smart ray [START_REF] Grossman | The Design and Evaluation of Selection Techniques for 3D Volumetric Displays[END_REF] proposes to emit several rays in different viewpoints (see Fig. 3). For each ray emitted, the technique adds a weight to the selected objects and the weight are adjusted in function of two conditions: a) if the target is hit several times by the ray and b) if the ray is closer to the center of the selected object. The target with the highest weight can be selected. In conclusion, in the selection and the navigation subtasks, these techniques solve the occlusion and the depth problems. At last, the world-in-miniature (WIM) technique [START_REF] Stoakley | Virtual Reality on a WIM: Interactive Worlds in Miniature[END_REF] uses a small representation of the environment to manipulate and interact with the environment objects. The user holds the small version. He uses a simple virtual hand for indirect manipulations of the full-scale object by interacting with their icon in the small version. The user manipulates the miniature objects then the full-scale objects are affected. The WIM is a point cursor technique and used for selection and navigation.

Discussion

Error! Reference source not found. summarizes the classification of the techniques we have presented in Section 3. We grouped the techniques in relation to the classification of [START_REF] Dang | Visualization and interaction on flight trajectory in a 3D stereoscopic environment[END_REF]. This table allows us to draw some information about the different techniques. At first, we can see that few of them can make both selection and navigation. The techniques we present are more specific to the selection.

We also note that point cursor techniques do not solve the problem of depth: it is difficult to locate the pointer position in depth relative to the target. In addition, the pointing task in depth is difficult with a pointer.

The depth problem is solved using ray casting techniques. However, this technique causes the occlusion problem. Indeed, the virtual ray emitted can cross another target before the desired. In this case, the validation should be done in several stages. On the one hand, point to the right target with the virtual ray, and then select the desired target among those crossed by the virtual ray. This generates two selection and/or validation steps. Therefore, it may increase the pointing time.

The occlusion problem can be solved by curves techniques. Curves techniques allow user to avoid targets located in front of the desired one. However, it poses afresh the depth problem because curves must be moved, and it can cause several movements to point precisely target.

Conclusion

In light of these considerations, this work proposes a categorization of interaction techniques. Our classification offers a different way of analyzing the existing interaction techniques which may help in the process of evaluating such techniques. The use of such categorization help users to choose the ones best suited for their needs, to improve existing techniques or to design new techniques. This new classification is based on two criteria: problems and factors affecting pointing performance and the interaction phases.

Fig. 3 .

 3 Fig. 3. Smart ray[START_REF] Grossman | The Design and Evaluation of Selection Techniques for 3D Volumetric Displays[END_REF]

Table 1 .

 1 Classification structure

	1	2	3	4	5	6	7	8
	PT	Dens.	Navigation Depth	Occl.	Dens.	Selection Depth	Occl.	Validation 1 or N
	Our classification space consists in a double-entry table. Each pointing technique
	(PT) is represented in a line. Columns depict the three sub-task of the pointing task.
	Subtasks are depicted by the columns and divided in sub columns with each encoun-
	tered problems:

Table 2 .

 2

		PT	Navigation		Selection		Validation
			Dens Depth Occl	Dens Depth Occl
	Point cursor	3D Point cursor [5] Silk cursor [20] Transparent sphere [4]	---	P P S	P PS PS	S S S	1 1 2
		WIM [15]	S	P	PS	S	1
		Ray cursor [2,5]	-	P	S	P	1
		Go-Go [10]	-	P	PS	PS	1
		Homer [3]	-	P	S	S	1
		Variable Length [4]	-	P	S	S	N
	Ray casting	Depth ray [5] Lock ray [5] spotlight [8]	---	P P S	S S S	PS S S	1 2 2
		Transparent cylinder [5]	-	S	S	S	2
		Two ray selection [4]	S	S	S	S	2
		Shadow cone [14]	S	S	S	S	2
		Smart ray [5]	S	S	S	S	N
	Hybrid	Ray cursor + Bubble cursor [5] Ray cursor + Transparent sphere[4]	--	S PS	S S	P S	1 1
	Curve	fixed or variable length [4] Flexible pointer [9]	--	P P	PS S	S S	1 1
		Virtual pointer metaphor [16]	-	P	S	S	1


Isabelle Blasquez
email: isabelle.blasquez@unilim.fr

Hervé Leblanc
email: leblanc@irit.fr

A Guideline to Teach Agile Requirements

Keywords: Agile Requirements, Collaborative Workshops, User Stories

This paper presents a speci c sequence of collaborative workshops dedicated to build a rst version of a product backlog. This backlog is composed by user stories re ned from a rst idea of the product to develop.

INTRODUCTION

In agile, like traditional software development, work begins by capturing customer requirements. The specification can also be seen as a process to bridge the gap from the business problem (why?) towards clear explanations about the solution to develop (what?). Providing relevant and unambiguous requirements specification is one of the biggest challenges in software engineering [START_REF] Adzic | Speci cation by Example: How Successful Teams Deliver the Right Software[END_REF].

Agile methodologies (e.g. Scrum or Extreme Programming) provide a process framework to develop products. However, they postulate that an initial product backlog exists. Even if agile requirements engineering is a recent topic, a systematic literature review about agile requirements shows that skateholder and user involvement are critical success factor and collaboration and building a shared understanding of the user perspective are essential [START_REF] Schon | Agile Requirements Engineering: A systematic literature review[END_REF].

In the figure 1, we present a sequence of collaborative workshops to teach agile requirements from a product vision to relevant user stories describing the right product. First, we capture requirements in order to build a vision by discovering the problem and identifying first features of a solution which maximize customers benefits and value proposition. Second, we refine these features to epics and we begin to split into user stories. Third, we promote discussion around user stories to detail them, share common understandings and ensure domain language. This sequence of collaborative workshops was presented and practiced during a 3-days course about agile requirements for a professional Bachelor dedicated to Development and Software Quality. The objective was not to write code, but to produce a prototype with Marvel1 . Then, in the context of a supervised project in full agile development, team of 7 students proposed their own product ideas. Each team re-used these workshops to initialize their product backlog around one speci c product idea during two days.

This sequence of workshops is based on our experience and inspired by Speci cation By Example [START_REF] Adzic | Speci cation by Example: How Successful Teams Deliver the Right Software[END_REF] and User eXperience techniques. Each workshop will be brie y described according to its position in this sequence at the next section. At the end of the sequence, a new iteration of the elevator pitch can be requested. This new version generally shows that the vision of the project has been re ned thanks to the next workshops that helps to better target the product goal and to better de ne the main features to develop.

Product Box.

The Product Box is the next workshop used to re ne and share the vision. The goal of the product box is to identi ty the most exciting product features [START_REF] Hohmann | Innovation Games: Creating Breakthrough Products Through Collaborative Play[END_REF]. Each member of the team is asked to imagine that the product will be sold on store shelves or at a trade show, such as a product box. A blank white cardboard box is provided each team. The team must then imagine a name for the product, maketing slogans, pictures and some key arguments. They build boxes through materials (newspapers, stickers, color papers) or just by writing target phrases. Once the box is nished, each team presents their box to sell their product. A discussion is then initiated (who want to buy this box ? why ?) and the other teams give feedback on the product to improve its functionality and use.

2.1.3

Personas. Di culties raised by students are in targeting the users and in indentifying the main purposes of the product. A re ect o n p ersonas c an h elp t hem t o r e ne th eir vi sion and then make e ective design decisions. The notion of personas was intially proposed by [START_REF] Cooper | The Inmates Are Running the Asylum[END_REF]: they are de ned a s p retend u sers, not real people rather hypothetical archetypes of actual users whose behaviors, motivations and goals are close. Personas should also be discovered through an investigation phase. However, this phase is time-consuming and at rst students are simply invited to imagine personas.

Such a workshop consists of creating more or less detailed cards that include name, picture, storytelling on the life of the character (behavior, skills, attitudes, environment) to make the personas a realistic character and identify the needs, objectives, expectations, and obstructions that could incite them or not to use this product.

Impact Mapping.

Impact Mapping is a strategic planning technique used to help teams align their activities with overall business objectives and make better roadmap decisions [START_REF] Adzic | Impact Mapping: Making a Big Impact with Software Products and Projects[END_REF]. It is a graphic tool which allows clear marking of project's measures and focusing only on necessary assumptions. The impact map is a four-level mindmap created collaboratively by answering the following questions: why, who, how and what. The why focuses on a business goal that should be SMART (Speci c, Measurable, Attainable, Relevant, Time-Bases). The who depicts the actors (or personas) who contribute to achievie this goal. The how identi es the behaviors changes (impacts) on the actors to achieve the goal.

The what is about deliverables to support the required impacts. It helps to outline the scope of the product through main features to develop.

Drawing an impact map is not easy, that is why this workshop is used (or not) as rst iteration to make e ective design decisions. As the scope is generally limited to features in the map, the story mapping will then help re ne the features in user stories.

From features to user stories

Story Mapping is presented by [START_REF] Patton | User Story Mapping: Discover the Whole Story, Build the Right Product[END_REF] as an idea to use a simple map you can work with others to tell a product's story and see the big picture form as you do. It is an approach to specifying, organizing and prioritizing user stories in a two-dimensional map. This artefact is product backlog which o ers a product vision in releases. The high-level of horizontal axis represents a narrative ow where the activities performed by a user to illustrate the behaviour of the system are rst ordered. It is focus on users and their experience, as if you are telling the story about your user to someone else. Determining these major activities becomes quite easy if an impact mapping has been done previously. The activities are then decline into larger (epic) or smaller (user story) stories. An epic is a too large story that can't be delivered within a single iteration. An epic can be split into smaller user stories, either during this workshop, either thereafter during backlog re nement sessions regularly conducted in the further development. The vertical axis group the stories in release to plan the development. The backlog is also prioritized such that the most valuable stories are highest.

The rst horizontal row also represents a walking skeleton, a minimum set of user stories to implement to deliver a usable version of the product. Working through successive rows eshes out the product with additional functionality.

User Stories

User stories are functional increments that add business value to the product. User stories are well-known as the 3 C's model. As mentionned in [START_REF] Cohn | User Stories Applied: For Agile Software Development[END_REF], while the Card may contain the text of the story, the detail are worked out in the Conversation and recorded in the Con rmation. The Cards are written during the story mapping where they are lled with just enough text (like a title) to identify the requirement.

First, we focus on stories of the walking skeleton. To promote conversation, we rst recommend to collaboratively drawn wireframes on the blackboard. This work consists in illustrating requirements using concrete examples to clarify meaning. It allows a shared understanding, a needs alignment, a rst identi cation of acceptance criteria of a story and sometimes the discovery of new user stories.

To be unambiguous, a good speci cation should be precise and testable. To be useful, it should be self-explanatory, focused, and in domain language. Afterwards, to re ne details about the conrmation of a story, it is possible from previous wireframes to tell, in domain language, precise examples of scenarios of a story. Each example can then be written using a formalism based on the Gherkin language which helps to break down it in 3 steps and allows express an example as an acceptance test with a Given step (to install some initial context), a When step (to describe an occured event) and a Then step (to ensure some outcomes). This formalism leaves the opportunity to automate (or not) these acceptances tests during the development.

Figure 1 :

 1 Figure 1: A guideline from vision to user stories 2 WORKSHOPS SEQUENCE 2.1 From goal to features 2.1.1 Elevator Pitch. An elevator pitch is a short summary used to quickly and simply de ne a product, service, or organization and its bene ts to customers. It should be possible to deliver the summary in the time span of an elevator ride, or approximately 30 seconds to two minutes [5]. The workshop consists of exploring and sharing the product vision by collaboratively writing a summary based on the following template : For . . . (the product's target market) Who/That . . . (describe what need your product solves) Our Product . . . (product name, or product category) That Leads To . . . (main advantage, appealing reason for purchase) Unlike The . . . (principal competing alternatives) Our Product O ers . . . (describe main di erentiation)

https://marvelapp.com


Véronique Bouteiller
email: veronique.bouteiller@ifsttar.fr

Lucas Bourreau

Laurent Gaillet

Franck Schoefs

Benoit Thauvin

Julien Schneider

Pre

Presentation of the French Project DéCoF-Ré

Keywords: reinforced concrete, bridge, marine environment, corrosion diagnosis, concrete durability, chloride indicator

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Corrosion is considered as the main degradation of reinforced concrete structures ( [START_REF] Woodward | Bridge management in Europe (BRIME)-Deliverable D14-Final Report[END_REF][START_REF] Tilly | CONREPNET -Concrete repairs -Performance in service and current practice[END_REF]. In France, being designed for a lifespan of 50 years, the reinforced concrete bridge average is 35 years old. At the level of bridge's owners a plan is needed to prioritize the maintenance or rehabilitation or retirement. At the level of researchers, reliable tools to assess the state of the reinforced concrete structure based on the corrosion diagnosis of the steel reinforcement together with the concrete durability are to be provided in order to qualify the state of the structure, to predict its durability and to suggest further repair solutions (patch repair or electrochemical treatments).

The French Project DéCoF-Ré meaning Decision Corrosion and Reliability of the ile de Ré Bridge has two main objectives. The first objective is to perform a reliable corrosion diagnosis of the piers of the bridge which is located in a marine environment by means of non-destructive tests such as half-cell potential and resistivity. The second objective is to determine the durability of the concrete cover of the piers in terms of durability indicators [START_REF]en oeuvre d'une approche performantielle et prédictive sur la base d'indicateurs de durabilité[END_REF].

The aim of this paper is to present the French Porject DéCoF-Ré which is a collaborative research that started in 2015 involving six partners: IFSTTAR (coordinator), Cerema, Nantes University and Capacité SAS, SIXENSE-Concrete Company and the Département de la Charente-Maritime (owner of the ile de Ré Bridge).

Presentation of the ile de Ré Bridge

The ile de Ré Bridge (Figure 1), located in the French Atlantic coast, was built in 1987-1988 to provide an access to the Ré Island from La Rochelle mainland. The bridge has a total length of 2928.5 meters, a width of 15 meters and a maximum height above the sea level of 30 meters (shipping channel). The bridge deck was constructed on 28 piers, from which 24 piers were founded in the sea (Figure 2). Piers concrete composition was based on a CPJ 45 RPM cement that today would correspond to a CEM II/B cement (Table 1) and the water/cement ratio was equal to 0.5. The investigated area on a pier is described in Figure 3 and Figure 4.

Two faces (upon 8) of the piers were considered: face FC is the most exposed side with dominant winds (WSW for La Rochelle with 30 km/h mean speed), daily sunshine and strongest wetting/drying cycles and face FG is the opposite side which is more sheltered (Figure 3).

Because the investigations needed to be performed using a vessel and depended on the tide duration, three zones defined in a coordinate system X0Y (face's wide 1.85m and height 1m) were studied corresponding to a total surface area of 5.55m² (Figure 4). These three zones includes atmospheric, splash and tidal exposures that can be related to the XS1 and XS3 exposure classes in marine environment according to the EN206 standard [START_REF]18-325-1. Béton : spécification, performances[END_REF].

Corrosion diagnosis

Corrosion diagnosis was performed on 15 piers of the ile de Ré Bridge (14 in seawater using a vessel and 1 on the beach).

Corrosion diagnosis methodology was divided in six steps. The first step consisted in finding the general bridge's information from the archives. The following steps, illustrated in Figure 5, were performed using a vessel. The second step consisted in a visual inspection of the piers (after cleaning) to assess the general degradation state (rust spots, leachings, cracks, etc.). The third step aimed at locating the steel reinforcements (horizontal and vertical rebars) using a ground penetrating radar (EasyScan ® 1.6 GHz, GSSI). In the fourth step, concrete electrical resistivity was measured using a Wenner probe (Resipod ® 5 cm, PROCEQ). The fifth and sixth steps were dedicated to the electrochemical measurements, respectively half-cell potentials (Canin +® (copper/saturated copper sulphate electrode (CSE, +318 mV vs NHE)), PROCEQ) and corrosion rates (GalvaPulse ® , Germann Instrument).

North

Durability of concrete

After 29 years, the concrete durability of the piers of the ile de Ré Bridge was investigated on 11 piers (10 in seawater using a vessel and 1 on the beach).

The durability of concrete is determined largely by its deterioration over time which is affected by the environment [START_REF]en oeuvre d'une approche performantielle et prédictive sur la base d'indicateurs de durabilité[END_REF]. The study consisted, after drilling the cores from the structure, in determining the durability indicators (chloride migration tests, water porosity, gas permeability, capillary absorption and resistivity) and the penetration front of chlorides (Xd) as well as of carbonation (Xc).

Preliminary results

Corrosion reliability

The interpretation of measurement values according to different standards or literature recommendations [START_REF]Standard test method for halfcell potentials of uncoated reinforcing steel in concrete[END_REF][START_REF]Standard test method for halfcell potentials of uncoated reinforcing steel in concrete[END_REF][START_REF]COST 509 -Corrosion and protection of metals in contact with concrete -Final Report: European Commission[END_REF][8]showed that the diagnosis can be erroneous of the influence of the marine environment. In order to provide more reliable results, models of the influences of the tide, the water height and the seasons were therefore studied [9, 10].

As an example for the half-cell measurement interpretation, Figure 6 (left) presents the potential mapping as measured on the pier PN face FG during summer, considering the ASTM analysis [START_REF]Standard test method for halfcell potentials of uncoated reinforcing steel in concrete[END_REF]. This mapping shows a gradual increase of potential from the bottom zone to the top zone. From the literature it appears that the gradient is not significant of a decrease of probability of corrosion but is rather due to a lack of oxygen (mainly in the bottom zone). A model was then developed considering the average values of half-cell potentials for the horizontal reinforcements. Figure 6 (middle) shows the heights and their corresponding half-cell potentials "reference profiles" for faces FC and FG. Finally, a reliable potential mapping was obtained considering the raw measurements and subtracting the reference profiles. The resulting mapping is illustrated on Figure 6 (right). The investigated area finally appears like a no-active-corrosion region.

40th IABSE Symposium, 19-21 September Nantes, France. Tomorrow's Megastructures

Concrete durability

In a marine environment, chloride ingress is one of the major causes of the premature deterioration of the reinforced concrete structures. From the durability point of view, when exceeding a certain amount of chloride at the rebar level, this latter can be depassivated and the corrosion initiating.

In the Project DéCoF-Ré, 51 cores were drilled from the structure to determine the chloride profiles depending on the pier, the face and the height. The penetration of chlorides was then compared to the concrete cover of the reinforcements.

As an example, Table 2 gathers the amount of free and total chlorides as determined by potentiometric titration for pier PN face FC for the three zones Z1, Z2 and Z3. The critical chloride content was considered equal to 0.4% weight of concrete as described in literature. The chloride content was higher than the critical chloride content for a concrete cover of 15-25mm (the values appear in red) or 45-55mm in the case of total chlorides for zone 1. The chloride penetration index, Xd, has been determined as the intersection of the chloride profiles and the critical chloride content and is also reported in Table 2. For the free chlorides Xd values are equal to 36, 33 and 42 mm respectively for zones Z1, Z2 and Z3. For the total chloride, Xd values become 59, 48 and 47mm.

From Figure 7 which illustrates the concrete cover of horizontal and vertical reinforcements of pier PN face FC as determined by radar and an image analysis, the concrete cover values were in the range of 55 to 125mm.

For this pier, after 29 years, chlorides did not reach the reinforcement level and thus the corrosion is not alike to occur. This is in good agreement with the result from the reliable potential mapping (Figure 6 left).

Conclusions

The results of the DéCoF-Ré Project have been gathered in two databases, one dealing with the corrosion diagnosis and a second one dealing with the durability of the concrete cover.

Corrosion results have been modelled in order to provide reliable mappings of half-cell potentials considering the influences of the tide, the height and the seasons.

The durability results have been analysed in terms of chloride profiles and compared to the concrete cover of the reinforcements.

More work is under progress and the state of the piers of the ile de Ré Bridge will be ascertained based on the complementary analysis of corrosion results and durability results. Furthermore, the autopsies performed on several locations will allow to validate the conclusions.

Figure 1 :Figure 2 .

 12 Figure 1: Ile de Ré Bridge (V. Bouteiller, Ifsttar)

Figure 3 .Figure 4 .

 34 Figure 3. Top view of the pier with the investigated faces FC and FG

Figure 5 .

 5 Figure 5. Methodology to assess the corrosion state of the steel reinforcement of the piers of the ile de Ré Bridge

Figure 6 .

 6 Figure 6. Potential mapping with ASTM C876 analysis (left), model of half-cell potential reference profiles (middle) and reliable potential mapping (right) for pile PN face FC

Figure 7 .

 7 Figure 7. Concrete cover of horizontal and vertical reinforcements as determined by radar and image analysis for pier PN face FC

Table 1 .

 1 Concrete mix composition (in kg/m 3 )

	shipping
	channel

Cement Water Sand 0/3 mm Gravel 6/20 mm Additive Pozzolith 200N

	370	185	830	1 200	0.74

Table 2 .

 2 Free and total chloride content versus depth and chloride front (Xc) ; Critical chloride content is equal to 0.67% (according to concrete formulation and density).

		Free chlorides (g/100g of concrete)
	Depth (mm)	05-15	15-25	45-55	75-85	Xd free
	PN-FC-Z3-X80cm-Y67cm	0,26	0,18	0,02	0,01	42
	PN-FC-Z2-X110cm-Y66cm	0,17	0,09	0,02	0,00	33
	PN-FC-Z1-X110cm-Y68cm	0,16	0,09	0,04	0,00	36
		Total chlorides (g/100g of concrete)
	Depth (mm)	05-15	15-25	45-55	75-85	Xd total
	PN-FC-Z3-X80cm-Y67cm	0,44	0,36	0,03	0,01	47
	PN-FC-Z2-X110cm-Y66cm	0,35	0,20	0,05	0,01	48
	PN-FC-Z1-X110cm-Y68cm	0,28	0,18	0,08	0,02	59

Acknowledgements

The authors would like to thank the team of the DéCoF-Ré Project: B. Godart, W. Traverst, P. Boujard, N. Coulaty-Chin, A. Orcesi, Y. El Rabbih, X. Derobert, M. Sissoko (IFSTTAR); R. Queguiner, S. Pasquiet, P. Boulaire, M. Rebours, C. Naudat (Cerema Dter Ouest); F. Landrin and V. Queyrat (Cerema Dter Ile de France), J-F. Barthélémy (Cerema Dtec ITM); M. Roche, O. Amiri (Nantes University); M. Brouxel, S. Naar (SIXENSE-Concrete), A. Audouin-Dubreuil, F. Lavoute, M. Barbier (Département de la Charente-Maritime); F. Gazet (Dekra). They also thank the team of the vessel for their help.


Anne Gebhardt
email: anne.gebhardt-even@inrap.fr

Keywords: Belgium, Charcoal mound, Experimental archaeology, France, Soil micromorphology Micromorphologie des sols

Studying the landscape evolution under Human impact requires reference to numerous anthropic activities which influence, among others, vegetation and soils. Since people became sedentary, intentional pressure on the wood-land has become more and more strong as the need of land for cultivation or wood for domestic and artisan purposes increased.

One of these activities is charcoal burning, which was very intensive until the beginning of 20 th century. Apart from strong pressure on the vegetation, the question is here to Impact of charcoal production activities on soil profiles: the micromorphological point of view

ArcheoSciences, revue d'archéométrie, 31, 2007, p. ??-?? improve to check the impact of this charcoal production activity on the soil.

To enlarge this knowledge and to make interpretations more reliable, it is also common practice to turn to experimental case studies. Fortunately, in the frame of the traditional charcoal production, which locally was still in practice in the mid-twentieth century, such operations are still occasionally performed After a quick description of the main steps of charcoal burning, this paper will focus on the soil micromorphological study of several profiles located under ancient and recent charcoal mounds. The experimental burning of a loam profile at Gent University Laboratory of Micropedology completed this task.

MaterIal and Methods

We had the opportunity to study the three following plots.

The Paimpont Forest (Ille-et-Vilaine, France)

Archaeological investigation of an ancient charcoal mound was done in 1993 in the Paimpont forest, on the Armorican Sandstone substrate, near the village of Hucheloup. Intensive charcoal activity existed here in the past. The studied mound was chosen for its location on a gentle slope, and for its thickness of about 30 cm which made a specific preparation of the plot most likely (Hunot, 1993, unpublished field notes).

The charcoal mound was about 8m in diameter and the section shows 3 layers rich in charcoal, separated by a thin clay layer of 1 to 3cm thick, probably excavated from some local marschy areas (Fig. 1. The dark layers are shifted one to each other toward the west, indicating reorganisation of the plot for each of the 3 successive uses. The charcoal remnants were studied by J.-Y. Hunot and D. Marguerie [START_REF] Gebhardt | Meules de charbonnier et archéométrie : problématique, méthodologie et premiers résultats[END_REF]).

The soil micromorphological study concerned the evolution of the anthropic topsoil horizon since abandonment (start of the pedogenesis, integration and alteration of charcoal in the soil, P1) compared to the natural organic surface horizon nearby (A0/1 horizon, P2 -see Fig. 1).

The Zonien Forest (Belgium) [START_REF] Mees | Base maps and soil survey of undistrubed iron industry sites in the Zonien Forest (Loess Belt, Belgium)[END_REF] made an archaeo-pedological mapping of a site with traces of 7 th /9 th century metallurgic activity in the Zonien Forest (Belgium). This provided the opportunity to study an ancient charcoal production plot, most probably associated with this activity (Langohr, personal communication, 2003). The charcoal mount profile description (CM) compared to the original local reference loess forest soil (R) is given in Fig. 1, together with some analytical data (Table 2; [START_REF] Mees | Base maps and soil survey of undistrubed iron industry sites in the Zonien Forest (Loess Belt, Belgium)[END_REF].

The micromorphological investigations done by Mees were completed and standardised here in order to allow a better comparison between the different case studies presented in this paper.

The Écomusée de Haute-Alsace (Ungersheim, France)

In October 1995, the Ecomusée de Haute Alsace gave us the opportunity to study a demonstration charcoal exploitation plot, organized for the public. Traditional charcoal was made here in the ancient way. The demonstration was carried out in the Ill river alluvial forest, which has never been cultivated.

So we had the opportunity to compare some undisturbed soil blocks sampled under 2 charcoal burning plots, one used once (profile 1T) and another used 4 times (profile 4T) to the natural soil profile (profile Ref; Fig. 1).

The laboratory heating experiment of soil samples (Belgium)

In 1993, while holding a 'Human and Capital Mobility' postdoctoral contract from the EEC at the International Training Centre of the University of Gent, Belgium, some laboratory experiments where done in order to study by micromorphological observations the impact of soil heating. Thus different depth horizons of a loess profile (from the Zonien Forest, Belgium) were heated in a laboratory oven at 200°C, 400°C and 600°C (Table 3).

Soil micromorphological descriptions and data presentation

The micromorphological description follows the international terminology [START_REF] Bullock | Handbook for soil thin section description[END_REF]. For the semiquantitative counting, the thin sections are divided in 2cm deep horizontal zones. Codes are chosen for each microfeature described (Fig. 7,8,9) and the results are tabulated to provide good comparison between the studied profiles. Observation is done under low magnification (obj. x4), except for some biogenic inclusions that can only be observed under high magnification (obj. x20).

Figure 1: Location of the sites and soil profile descriptions from the Paimpont Forest (France) charcoal mound transect, the Zonien Forest (Belgium) described after Florias [START_REF] Mees | Base maps and soil survey of undistrubed iron industry sites in the Zonien Forest (Loess Belt, Belgium)[END_REF], and the Ecomusée de Haute Alsace (France). At the Paimpont Forest charcoal mound (after [START_REF] Hunot | Sondages dans la charbonnière[END_REF], the deepest charred layer (5) is 5,4m long, the middle one (3) 6,6m, the upper one (1) is 6,9m long. Bioturbations are observed mainly in the first 5cm; after 10cm depth, biogalleries (roots, worms) become rare. Figure 1 : Localisation des sites et description du profil de la meule de la forêt de Paimpont (France), de la forêt de Soigne (Belgique, d'après Florias Mees, 1989) et de l'Écomusée de Haute Alsace (France). En forêt de Paimpont, le niveau de combustion le plus profond (5) de la charbonnière fait 5,4 m de long, l'intermédiaire (3) fait 6,6 m et le supérieur (1) 6,9 m. Des bioturbations sont observées dans les premiers 5 cm ; au delà de 10 cm de profondeur, les biogaleries (racines, vers) deviennent rares.

ArcheoSciences, revue d'archéométrie, 31, 2007, p. ??-??

results

The traditional art of the charcoal-burner

The preparation of the site (Fig. 2) is very important. It should be very flat, not too dry, not too wet [START_REF] Larcher | Les charbonniers à Paimpont. Contribution à l'histoire d'une commune[END_REF]. Thus on slopes, terraces are build. It should be cleared of organic remains, and humus rich surface horizons are removed, sometimes down to 20cm depth. Vegetation and roots should be deeply extracted with hoe, spade or a motocultor, depending on traditional or modern facilities (Grosz, pers. comm.).

It is thus understandable that, when possible, the mound was rebuilt on an ancient, already prepared charcoal plot. Also, according to [START_REF] Larcher | Les charbonniers à Paimpont. Contribution à l'histoire d'une commune[END_REF], the production of charcoal increases by 10% on well prepared, already used soils. The colonisation by vegetation on ancient charcoal plots is very poor, which facilitates the reuse of the plot (Hunot, 1993, unpublished field notes).

While preparation the plot, the upper organic horizons are well mixed with lower mineral ones and kept apart for later use (Grosz, pers. com.). Turves are cut around the working area when available [START_REF] Hunot | Sondages dans la charbonnière[END_REF], or imported from nearby grassland or natural pool banks [START_REF] Bouflet | Les chaffras du Maine[END_REF].

A central chimney is build first. Then, to construct of the wood stack, the wood coppice logs are arranged around in a very precise way that mixes the size and the different tree species used in order to optimise the combustion (Fig. 2). When all the wood is set up, the stack is covered with the mixture of organo-mineral horizons and the turves [START_REF] Larcher | Les charbonniers à Paimpont. Contribution à l'histoire d'une commune[END_REF]. The very finely crumby organo-mineral mixture covering the wood should penetrate into the smallest holes when the stack settles during the burning. This is to avoid oxidation and flame formation during combustion and the consequent compaction of the mound. This phase depends on the oxygenation rate and the quality of the combustion. The stack is then topped with fresh clay.

The combustion lasts about 5 [START_REF] Larcher | Les charbonniers à Paimpont. Contribution à l'histoire d'une commune[END_REF] to 10 days [START_REF] Bouflet | Les chaffras du Maine[END_REF]Grosz, pers. com.). The temperature should reach around 530°C in the middle of the stack (Grosz, pers. com.), in order to obtain the best quality of charcoal (a lot of carbon, less gas; Table 1). 1885-1902[START_REF] Larcher | Les charbonniers à Paimpont. Contribution à l'histoire d'une commune[END_REF].

Tableau 1 : Différences de qualités des charbons obtenus à différentes températures d'après La Grande Encyclopédie de Marcelin Berthelot, 1885-1902[START_REF] Larcher | Les charbonniers à Paimpont. Contribution à l'histoire d'une commune[END_REF].

The fire is lighted and the combustion is regularly maintained, in the beginning every 4 and later every 6 hours, by adding through the chimney, wood and/or bad quality charcoal from the previous combustion. Fresh pure clay is continuously added to cover the mound. This delicate and dangerous work is done from the top (Fig. 3).

At the base of the mound, ventilation holes allow air entrance for the combustion and facilitate the escape of smoke. In the last two days, the combustion is slowed down in reduced condition by closing these holes.

When the stack has considerably compacted and the turf burnt, the combustion reaches the external bottom edge of the mount [START_REF] Larcher | Les charbonniers à Paimpont. Contribution à l'histoire d'une commune[END_REF], but under the mound at the contact with the soil, the wood should not be completely burned (Grosz, personal communication, 1995). After cooling, the stack is opened, charcoal is raked, slightly broken, sorted and bagged for selling. The place is cleaned; the fine earth is sieved and reused for a next charcoal mound. ArcheoSciences, revue d'archéométrie, 31, 2007, p. ??-??

Micromorphological modifications of soil profiles beneath charcoal burning sites

At Paimpont forest (Fig. 4), the abandoned charcoal mound profile (P1) shows some differences compared with the undisturbed profile (P2). Although the porphyric fabric seems more or less similar, the structure is definitely more massive on the mound plot (P1). The latter is also characterized by the presence of amorphous organic matter, phytoliths, some diatoms and many burned/charred fragments such as charcoal (Plate e), melted glass-like droplets (Plate g, h, i) and burned soil fragments (Plate j). The pedofeatures observed are dusty coatings and intercalations. In the undisturbed part of the plot, the horizon is very organic, showing a more intensive biological activity giving an increasingly pellety structure towards the top of the profile.

In the Zonien Forest, the sampling addressed the comparison of the natural B horizon and the position just under the black charcoal Ap horizon of the charcoal mound. Fig. 4 shows less difference between the reference profile and the charcoal mound than in the Paimpont forest case. We can note a little more charcoal and burned organic soilfragments, and an increase in dusty clay coatings and intercalations under the charcoal burning plot (Plate k). However, charcoal activities seem not to have affected the chemistry of the soils (Table 2, [START_REF] Mees | Base maps and soil survey of undistrubed iron industry sites in the Zonien Forest (Loess Belt, Belgium)[END_REF].

At the Ecomusée de Haute Alsace case, we had the opportunity to compare a one time and a four times used charcoal burning plot to the reference forest soil (Fig. 4). The latter is very well structured, with an important pellety organic top A0/A1 horizon (Plate a). The soil profile on the only once used site seems quite compacted (Plate b) compared to the plot used four times (Plate c) that has a reworked and homogenised structure. In all these profiles appear iron concretions, which however derive from the alluvial sedi- ArcheoSciences, revue d'archéométrie, 31, 2007, p. ??-?? To understand better the variation of texture, structure and colour due to the burning, 4 horizons (E1, E3, B2t, BC2) of a silt-loamy soil from the Zonien Forest (Belgium) were experimentally heated at 200°C, 400°C and 600°C (Table 3). Contrarily to the experiment from [START_REF] Mathieu | Observations pétrographiques sur la paroi d'un four à chaux carolingien creusé en sol limoneux[END_REF], our results showed no differences in the macro/micro texture and structure of any horizon at any temperature. The only difference observed is a slight increase in intensity of reddish colour, visible only above 400°C. This reddish colour change is more intense in the deeper horizons, as these have a slightly higher iron content. This characteristic is related to the higher clay content and the presence of clay coatings in the B horizon. The clay in these soils has indeed a brown colour as some iron ions, with a positive charge, are adsorbed on the negatively charged clay surfaces. This conclusion about the change in colour is the same as in the Mathieu and Stoops experiment (1972). Those authors pointed out that the iron hydrates are transformed to hematite at 800/1100°C. No transformations of the quartz (melting) where noticed in either experiment.

dIscussIon

This study has pointed out some aspects of the impact of charcoal burning on the natural soil. Regarding the charcoal burning process, this influence is not so much due to the heating of the soil but more to the preparation of the working area and the construction of the mound. The colour of the soil under a charcoal mound is not a good indicator of charcoal making, unless the wooden stack burnt out because of inattention of the charcoal burner. The fragments of burned soil observed should than be interpreted as clods of the very finely crumby organic-mineral mixture covering the mound in order to slow down the combustion temperature.

The main difference is the increase, in the upper part of the profiles, of more or less charred plants and burned soil Basic components: Mineral inclusions: bm1: gravel-size flint, ironstone, quartz, chalk…, bm2: sand <1mm, bm3: sand >1mm, bm4: loam. Biogenic inclusions: bb1: plant residues, bb2: amorphous organic matter, bb4: phytoliths, bb5: diatoms, bb7: spores/pollens. Anthropogenic inclusions: soil fragments: bas4: speckled calcitic soil fragments, bas5: reticulated clayey soil fragments; burned /charred fragments: bab1: coarse charcoal, bab2: fine charred plant remains, bab4: fused ash, bab7: burned soil fragments. Composants majeurs: Inclusions minérales: bm1: silex, roche ferrugineuse, quartz, craie graveleuse..., bm2: sable <1mm, bm3: sable>1mm, bm4: limon. Inclusions biogénique: bb1: résidu de plantes, bb2: matière organique amorphe, bb4: phytolithes, bb5: diatomées, bb7: spores/pollens. Inclusions anthropiques: soil fragments de sol: bas4: fragments de sol calcitique tacheté, bas5: fragments de sol argileux réticulé ; fragments de sol brûlé/carbonisé: bab1: gros charbons, bab2: résidus végétaux carbonisés fins, bab4: cendres fondues, bab7: fragments de sol brûlé.

Pedofeatures: Crystalline/Depletion: pc2: rounded iron concretion (PPL: black, OIL: red). Textural: pt2: dusty clay coating & infillings, pt3: intercalation. Excremental: pex2: mite pellets type, pex3: enchitraeïde pellet type, pex4: organo-mineral (<100µm), pex5: organicmineral (100-500µm), pex6: organic mineral (>500µm). Traits pédologiques: Cristallisation/dissolution : pc2 : concrétion de fer arrondies (PPL : noir, OIL : rouge). Traits texturaux : pt2 : revêtements argileux poussiéreux & remplissages, pt3 : intercalation. Formes excrémentales : pex2: déjections de type mite, pex3 : déjections de type enchitraeïde, pex4: déjection organo-minérale <100 µm, pex5 : déjection organo-minérale 100-500µm, pex6 : déjection organo-minérale >500 µm. [START_REF] Oyama | Revised Standard Soil Color Chart[END_REF]. Tableau 3 : Variation de couleur des horizons de sol limon-silteux en fonction de la profondeur et de l'intesité de combustion ;couleur d'après OYAM and Takehara (1967).

ArcheoSciences, revue d'archéométrie, 31, 2007, p. ??-?? Phytolithe fondu provenant probablement de la combustion de graminées dans des foyers domestiques des charbonniers (Forêt de Paimpont). j./k./l.: Different kinds of dusty clay coatings associated with charcoal burning activity due to the combination of deforestation and strong preparation of the soil surface (levelling, digging) from the Paimpont Forest (j), the Zonien Forest (k.) and from the four times used profile from the Ecomusée de Haute Alsace (l.). In the last site some intercalations are also developed. Différents revêtements argileux poussiéreux générés par la déforestation et la préparation des aires de charbonnage (labour, nivellement) en forêt de Paimpont (j), forêt de Soigne (k) et de la charbonnière utilisée 4 fois à l'Écomusée de Haute-Alsace (1). Dans de dernier site, on observe également des intercalations.

ArcheoSciences, revue d'archéométrie, 31, 2007, p. ??-?? fragments. In the case of plots used several times (Paimpont Forest, Ecomusée d'Alsace) thin sections showed finely mixed earth and charcoal which result from the mixing/ homogenisation and sifting of the sediment after each use.

The melted glass-like droplets observed in some cases (Paimpont, Ecomusée), are very similar to the ones often described in open air archaeological site sediments and interpreted as melted opal phytoliths [START_REF] Gebhardt | Micromorphological study of construction material and living floors in the Medieval motte of Werken (West Flanders, Belgium)[END_REF]. As the melting temperature of the quartz (800°C, [START_REF] Mathieu | Observations pétrographiques sur la paroi d'un four à chaux carolingien creusé en sol limoneux[END_REF] is not supposed to be reached in the centre of the stack, those features probably come from nearby open fires used for the lighting process of the wood stack or any other temporary domestic activity of the charcoal burner such as heating and cooking. The phytoliths observed in many cases are imported with the pieces of turves coming from nearby pool banks (Paimpont forest), used to cover the charcoal mound. Diatoms probably have the same origin.

Dusty clay coatings seem to be commonly associated with charcoal burning activities. Usually those features are explained by the splash effect of the rain on a bare surface, most often cultivated [START_REF] Jongerius | The role of micromorphology in agricultural research[END_REF]. In a forest context, charcoal-burning activity is a good explanation of those features. The combination of the local deforestation and the rigorous preparation of the soil surface (levelling, digging, trampling) necessary to this activity can generate a periodic long-term bare surface propitious to dusty clay coating and intercalation formation.

conclusIons

This micromorphological study of several soil profiles located under ancient archaeological and recent analogous charcoal production sites, completed by some laboratory soil heating experiments, helps to understand better the impact of charcoal production activities on soils.

Contrary to some common ideas, the impact of charcoal production on the soil morphology is not directly due to the heating effects of wood carbonisation but more to the preparation and the management of the plots before, during and after the production phase. Traces are detected of the clearing of the soil and of the type of earth used to cover the mound. It appears that latter material can come from somewhat distant plots as preference is given to turves with well-developed grass vegetation. The observation of phytoliths and diatoms in the soil thin sections is a useful tool in this aspect of the research. Other traces, such as melted phytoliths and some baked earth fragments appear to belong either to the domestic activities of the charcoal burners and to the charcoal production itself.

The perception of the influence of charcoal activities on the soil chemistry and vegetation remains unexplored, but this paper could be the start for more investigations.

Figure 2 :

 2 Figure 2: The different stages of construction of charcoal mounds the Ecomusée de Haute Alsace (France).Figure 2 : Les différentes étapes de la construction d'une charbonnière à l'Écomusée de Haute-Alsace (France).

Figure 2 :

 2 Figure 2: The different stages of construction of charcoal mounds the Ecomusée de Haute Alsace (France).Figure 2 : Les différentes étapes de la construction d'une charbonnière à l'Écomusée de Haute-Alsace (France).

Table 2 :

 2 Comparative analytical data between the reference (P4) and the charcoal mound (P3) from the Zonien Forest (after[START_REF] Mees | Base maps and soil survey of undistrubed iron industry sites in the Zonien Forest (Loess Belt, Belgium)[END_REF]. HZ: Horizon, C%: percentage of total carbone, pH: soil acidity, FE% and Al%: percentage of Iron and Aluminium (dithionite extraction), exch.bases: percentage of exchangeable bases (NH4OAc extraction), CEC: cation exchange capacity, expressed in meq/100g of soil, V%: base saturation). Tableau 2 : Données pédo-chimiques comparatives entre le sol de référence et la charbonnière de la forêt de Soigne (d'après[START_REF] Mees | Base maps and soil survey of undistrubed iron industry sites in the Zonien Forest (Loess Belt, Belgium)[END_REF]. HZ : Horizon, C% : pourcentage de carbone total, pH : acidité du sol, FE% et AL% : pourcentage de fer et d'aluminium (extraction à la dithionite), exch.bases : pourcentage de bases échangeables (extraction à NH 4 Oac), CEC : capacité d'échange cationique en meq/100g, V% : saturation en bases.

Figure 3 :

 3 Figure 3: Through the chimney, from the top, wood and/or bad quality charcoal from the previous combustion is regularly added, and fresh pure clay is frequently added to cover the mound.Figure 3: Du bois, ou du charbon de mauvaise qualité provenant des combustions précédentes, est apporté par la cheminée sommitale ; de l'argile humide et pure est utilisée pour recouvrir la meule.

Figure 3 :

 3 Figure 3: Through the chimney, from the top, wood and/or bad quality charcoal from the previous combustion is regularly added, and fresh pure clay is frequently added to cover the mound.Figure 3: Du bois, ou du charbon de mauvaise qualité provenant des combustions précédentes, est apporté par la cheminée sommitale ; de l'argile humide et pure est utilisée pour recouvrir la meule.

Figure 4 :

 4 Figure 4: Micromorphological results of semi-quantitative counting of the Paimpont forest, the Zonien forest and the Ecomusée de Haute Alsace sites.Figure 4 : Résultats du comptage semi-quantitatif des traits micromorphologiques associés aux charbonnières de la forêt de Paimpont, de Soigne et de l'Ecomusée de Haute Alsace.

Figure 4 :

 4 Figure 4: Micromorphological results of semi-quantitative counting of the Paimpont forest, the Zonien forest and the Ecomusée de Haute Alsace sites.Figure 4 : Résultats du comptage semi-quantitatif des traits micromorphologiques associés aux charbonnières de la forêt de Paimpont, de Soigne et de l'Ecomusée de Haute Alsace.Key of percentages of the main micromorphological features. 0: 0 none, 1: 1 piece very rare, 2: 1-2% rare, 5: 2-5% occasional, 10: 5-10% many, 20:10-20% abundant, 30: 20-30% frequent, 50: 30-50% common, 70: 50-70% dominant, 100: >70% very dominant. Pourcentage d'apparition des principaux traits micromorphologiques. 0: 0 aucun, 1: 1 très rare, 2: 1-2%rare, 5: 2-5% occasionnel, 10: 5-10% nombreux, 20:10-20% abondant, 30: 20-30% fréquent, 50: 30-50% commun, 70: 50-70% dominant, 100: >70% t dominant.

  ments of the plots. Biogenic inclusions such as phytoliths, plant residues and spores/pollen are observable in all three profiles, with a little higher abundance in the profile used only once. Charred organic and soil fragments are more abundant under the four times used plot (Plate d). Dusty coatings and intercalations are less numerous than in the former Paimpont and Zonien Forest cases. Melted glass-like droplets appear in the profile of the 4 times used site.

	Microstructure: Fabric: f1: distribution (monic/0; enaulic/20; chitonic/40; gefuric/60; porphyric/80), f2: orientation (reticulated/20;
	speckled/50; striated/80), f3: birefringence (no/0; low/20; medium/60; good/80), f4: pedality (no/0; low/20; medium/60; good/80),
	f5: connection (no/0; low/20; medium/60; good/80). Structure: sv1: total voids space, sv2: burrows, sv3: passage infills, sv4: packing
	voids, sv5: planar voids, sv6: chambers, sv7: vesicles, s10: spongy structure, s11: massive structure, s14: pellety structure, s15: crumby
	structure.

abondant, 30: 20-30% fréquent, 50: 30-50% commun, 70: 50-70% dominant, 100

: >70% t dominant. Microstructure: Fabrique: f1: distribution (monique/0 ; énaulique/20; chitonique/40; géfurique/60 ; porphyrique/80), f2: orientation (réticulée/20; tachetée/50; striée/80), f3: biréfringence (aucune/0; faible/20; moyenne/60; bonne/80), f4: pédalité (aucune/0; faible/20; moyenne/60; bonne/80), f5: connection (aucune/0; faible/20; moyenne/60; bonne/80). Structure: sv1:porosité totale, sv2: terrier pile d

'assiettes, sv3: biogaleries, sv4: vides d'entassement, sv5: fissures, sv6: chambres, sv7: vésicules, s10

: structure spongieuse, s11: structure massive, s14: structure pellety, s15: structure grumeleuse.

ArcheoSciences, revue d'archéométrie, 31, 2007, p. ??-??

Table 3 :

 3 Colour variation of the silt-loamy profile with depth and heating intensity; colour named after

ArcheoSciences, revue d'archéométrie, 31, 2007, p. ??-??

Acknowledgements

This work is a small personal unfunded project whose aim was to understand better the effects of charcoal exploitation on the soil micromorphology. I would like to thank all the collegues who kindly allowed me to sample on archaeological/experimental sites and use their field data. They are:

M. Grodwohl director of the Ecomusée de Haute Alsace (France) and M. Grosz the charcoal-burner who conducted

the experimental part of this work, J.-Y. Hunot who studied the Paimpont forest (France) ancient charcoal mound and allowed me to use his field observations, and R. Langohr from the Laboratory of Pedology of Gent (Belgium) who allowed me to use data from the Zonien Forest collected by his doctorant F. Mees.

I also thank the Laboratory of micropedology of Gent (Belgium) directed by Prof. Stoops, for the production of the thin sections and the help in the heating experiment of soil samples. I won't forget the useful discussions with R. I. Macphail (Institute of Archaeology, London) and S. Limbrey (Birmingham University), with a special thanks to the latter for the help to improve the English text.