# Electrical, pedological and geotechnical characterisation of micro-reliefs (“mottureaux”) of the Poiré-sur-Velluire marsh (France)

Laurent Caner, Clémence Pavageau, Delphine Decoene, Philippe Cosenza

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Laurent Caner, Clémence Pavageau, Delphine Decoene, Philippe Cosenza. Electrical, pedological and geotechnical characterisation of micro-reliefs (“mottureaux”) of the Poiré-sur-Velluire marsh (France). Journées Scientifiques AGAP Qualité 2024, Mar 2024, Poitiers, France. pp.04004, 10.1051/e3sconf/202450404004. hal-04687255

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# Electrical, pedological and geotechnical characterisation of micro-reliefs (“mottureaux”) of the Poiré-sur-Velluire marsh (France)

## Caractérisation électrique, pédologique et géotechnique de micro-reliefs (“mottureaux”) du marais du Poiré-sur-Velluire (France)

Laurent Caner<fnref n="1" />, Clémence Pavageau<fnref n="1" />, Delphine Decoene<fnref n="2" /> and Philippe Cosenza<fnref n="1" />

**Abstract.** A survey of electrical, pedological and geotechnical measurements was carried out on the Marais du Poiré-sur-Velluire site (Marais Poitevin, France) in 2021 and 2022 to characterise micro-reliefs (often named “mottureaux” or “gilgais”), which are home to a rich biodiversity. The electrical measurements consisted in Electrical Resistivity Tomography at various investigation depths. The summary of the results presented in this paper highlights the following two elements, which may play a definite role in the genesis of these very special micro-reliefs: (a) desiccation cracks and (b) the thickness of a very soft, plastic-to-liquid clay layer in the saturated part of the soil.

**Résumé.** Une campagne de mesures électriques, pédologiques et géotechniques a été menée dans le Marais du Poiré-sur-Velluire (Marais Poitevin, France) afin de caractériser des micro-reliefs (souvent appelés “mottureaux” ou “gilgais”) lieux d’une riche biodiversité. Les mesures électriques ont consisté à acquérir des Tomographies de Résistivité Électrique à différentes profondeurs d’investigation. La synthèse des résultats obtenus présentée dans ce papier met en lumière deux éléments pouvant jouer un rôle significatif dans la formation de ces micro-reliefs si particuliers : (a) les fissures de dessiccation et (b) l’épaisseur d’une couche d’argile très molle, plastique à liquide dans la partie saturée du sol.

<footnote n="1">¹ Université de Poitiers CNRS, Institut de Chimie des Milieux et Matériaux de Poitiers-IC2MP, 86073 Poitiers, France.</footnote>

<footnote n="2">² Parc Naturel Régional du Marais Poitevin, 2 Rue de l'Église, 79510 Coulon, France</footnote>

<footnote>*Corresponding author: laurent.caner@univ-poitiers.fr</footnote>
# 1 Introduction

The “Marais communal du Poiré-sur-Velluire” site is in the city of Velluire-sur-Vendée in the “Marais Poitevin Regional Park”. The marsh is a 241-hectare natural wet meadow that has been grazed since the 12th century and has never been ploughed. This wet meadow is only grazed for part of the year, when the marsh is “dry”, and the soil has sufficient bearing capacity. In winter, the marsh is covered with fresh water from rain and ditches during flooding episodes. The communal marshland features a micro-relief system, with dips and fairways formed by former sea channels and ditches. The topography is also marked by the presence of micro-reliefs, known locally as “mottureaux”. They are decimetric in height, with the largest having a surface area of around ten cm² or even a few m² (Figure 1). These micro-relief are mainly found in the lower areas of the marsh. The highest areas of the marsh, with a flat topography, are marked by the absence of the micro-relief. These forms are uncommon in the “Marais Poitevin” and have also been described in the “Saint-Denis-du-Payré” National Reserve and in other brackish marshes. The “mottureaux” were described by [1-3] and are like the “gilgais” [4, 5], the term used in physical geography and soil science (Figure 2) to define the microtopography of the surface of clayey soils with alternating elevations and depressions.

Ecologically speaking, these micro-reliefs create different habitats for plant and animal communities. They multiply habitats over a few square meters and host of important plants and animals biodiversity in the “Marais Poitevin Regional Park”.

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Photography of the micro-reliefs locally termed as “mottureaux”.</figcaption></figure>

In the literature, the formation of “gilgais” is linked to intrinsic factors linked to the soil and extrinsic factors such as climate and hydric regime [2, 4, 5]. Intrinsic factors correspond to soil properties and its susceptibility to shrink and swell. These properties are texture (clay fraction content), clay mineralogy and the presence, although not systematically [2] expandable 2:1 clay minerals (i.e., smectite), exchangeable cations of 2:1 clay minerals, soil structure (i.e., the spatial organization of the pore space and solid phase), soil plasticity,
hydraulic conductivity, density, and particle size. Extrinsic factors include: (a) soil moisture and, more specifically, the spatio-temporal variability of soil water content (i.e., alternating wetting and drying cycles); and, to a lesser extent, (b) vegetation through the influence roots, which can act as preferred drainage paths. The clay fraction content, the presence of expandable minerals and alternating desiccation-humidification cycles appear thus to be the major factors in the formation of these “gilgais” micro-reliefs. In the context of marsh soils, generally located close to the coast, the literature does not directly mention soil salinity as a main parameter involved in the genesis of “gilgais”. Based on articles by [4, 5], the physical mechanisms of “gilgais” formation can be schematically classified into 5 categories (Figure 2):

- heave processes between desiccation cracks (or fissures).

- heave processes over desiccation cracks.

- contraction processes on desiccation cracks due to moisture gradients.

- heave processes usually due to mechanical loading.

- ripple processes resulting from slope flow.

The poorly known formation mechanisms appear to be due to variations in soil volume associated with periodic alternation of physical states: swelling due to hydration and shrinkage due to desiccation. Category 1 in Figure 4 corresponds to the hypothesis of [2], with upward thrust caused by lateral pressures generated by water and/or soil fragments falling in the cracks. The desiccation of the soils during summer drought leads to important shrinkage of the clayey material and induces the formation of deep, wide shrinkage cracks (multi-centimetre sized). Desiccation and moisture gradients also lead to fragmentation of the large polyhedric aggregates into small aggregates in loose packing, and with some of these crumb fragments fall into the bottom of the cracks. In winter, when the soil rehydrates, these clayey materials swell and exert pressure and shearing, leading to the upward thrust of certain blocks. This is also considered process of Vertisols formation [6, 7].

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Classification of formation mechanisms of the micro-reliefs like “gilgais” (from: [4] modified).</figcaption></figure>
<figure><img src="image_4.png" /></figure>

**Fig. 2 continued.** Classification of formation mechanisms of the micro-reliefs like “gilgais” (from: [4] modified).

# 2 Materials et method

## 2.1 Location

Figure 3a show the location of the location of the Communal du Poiré sur Velluire in the Marais Poitevin, and 3b the location of the studied zones with the height of the micro-reliefs.

<figure><img src="image_5.png" /></figure>

**Fig. 3.** Location and height of the micro-relief in the “communal du Poiré-sur-Velluire”. (From: Parc Naturel Régional du Marais Poitevin ; Image capture and processing: Drone aquitaine).
## 2.2 Soil description

Test pits were sampled with hand auger at the top of the micro-reliefs and at the bottom. Soil colour according to depth, texture, and traces of hydromorphy (rust stains) were described. The presence of limestone fragments was noticed. For each profile, samples were taken every 20cm. The reference level was taken at the bottom of the micro-reliefs.

## 2.3 Measurement of soil resistance

Penetration test or penetrometer test is often used to quantify the resistance to penetration of the soil layers and soil compaction. Penetrometer test was carried out using a PANDA® (Pénétromètre Automatique Numérique Dynamique Assisté par ordinateur) portable dynamic penetrometer [8]. The test involves driving into the ground a train of 50 cm-long rods using a standardized hammer. For each blow, the speed of the hammer when it impacts the device is measured and converted into energy E supplied. At the same time, the value of the cone's penetration is measured. After processing by the datalogger, a profile of cone resistance (Qd in MPa) as a function of depth is obtained. The device allows measurements from 0.1 to 40 MPa. Measurements were carried out using steel lost cones with a conical (angle 30°) cross-section of 4 cm² to limit friction forces between the 1 cm² section rod train and the clay soil.

## 2.4 Electrical resistivity tomography

Electrical resistivity tomography (ERT) was carried out using a Syscal R1+ resistivity meter (Iris-Instrument, France). Two arrays with 48 electrodes spaced 1 m and 0.2 m apart were used in the field to obtain two ERTs of different lengths, depths, and resolutions. The 1 m inter-electrode spacing profile is 48m long and enables the investigation to a depth of around 8.75m. The 0.2m inter-electrode spacing profile is 9.6m long and can investigate a depth of around 1.75m. The electrical profiles were aligned as closely as possible, avoiding planting the electrodes at the tops of the “mottureaux”. The sequences comprised 463 four-electrodes measurements following in a Wenner-Schlumberger array. Electrical data inversion was performed using Res2DInv® software with the following options: (a) “conventional” smoothness-constrained least-squares method or $l_2$ norm inversion method, (b) a model refinement that allows to get model cells with width of half the unit electrode spacing and (c) a maximum number of iterations equal to 5 for the inversion process. For the whole set of profiles, the RMS errors we obtained were less than 5%.

## 2.5 Measurement of the gravimetric water content

Gravimetric water content was measured after oven-drying the samples at 105°C for 24 hours. On the graphs representing water content as a function of depth, the water content at the shrinkage limit ($W_s$ = 30%), the water content at the plastic limit ($W_p$ = 40%) and the water content at the liquid limit ($W_l$ = 70%) obtained on the “Marais Poitevin” and “Marais de Rochefort” [9 - 11] are given.

## 2.6 Measurement electrical conductivity of the 1:5 water extract

Electrical conductivity measurements of the 1:5 water extract (EC₁.₅) were carried out on a suspension prepared with 10g air-dried disaggregated soil and 50 mL osmosed water and stirred for 30 min on a rotary shaker. Conductivity is measured using a 3630 IDS - WTW® multi-parameter probe equipped with an electrical conductivity probe. The electrical
conductivity of the 1:5 extract can be converted to the electrical conductivity of the fluid using the following formula [13]:

$$EC_{fluid} = EC_{1:5} \times \frac{5}{W} \quad (1)$$

with:

EC<sub>fluid</sub>: electrical conductivity of the fluid [µS cm<sup>-1</sup>]
EC<sub>1:5</sub>: electrical conductivity of 1:5 water extract [µS cm<sup>-1</sup>]
W: water content of the sample [%].

The electrical conductivity of the fluid can then be converted into equivalent NaCl concentration, i.e., the NaCl concentration that would have the same electrical conductivity, knowing the temperature at which EC<sub>1:5</sub> is measured [12].

# 3 Results

The measurement campaign covered 5 areas of the marsh associated with micro-reliefs of different morphologies (Figure 2; [14, 15]). For the sake of brevity and synthesis, we present here only the results acquired in two highly contrasted zones: zone 2 without micro-reliefs and zone 4 with large micro-reliefs.

## 3.1 Area without micro-reliefs

The area to the north of the marsh in an 'elevated' position (area 2) has few micro-reliefs (Figure 3).

### 3.1.1 Soil profiles description

The soil pits were drilled in the 'highest', flat part of the marsh (Figure 3) and in an area without micro-reliefs (Figure 4). The soil profiles (Table 1) show a brown to very dark grey layer rich in organic matter around 20 cm thick, followed by dark grey layers up to 70 cm and grey between 70 cm and 120 cm with a clay to heavy clay texture. The horizons show redox features (rust stains) whose intensity increases with depth. A few black ferro-manganic concretions can be observed. At depth, the soil becomes very plastic.

<figure><img src="image_6.png" /><figcaption>Fig. 4. Area of sampling 2 without micro-reliefs.</figcaption></figure>
<table><caption>**Table. 1.** Description of the soils from area 2 without micro-reliefs. The colours are given in moist state.</caption><thead><tr><th>Depth</th><th></th><th></th></tr></thead><tbody><tr><td>0-20cm</td><td>very dark grey (2.5Y 3/1)<br>clayey texture</td><td rowspan="4"></td></tr><tr><td>20-40cm</td><td>dark grey (2.5Y 4/1)<br>clayey texture, rust stains</td></tr><tr><td>40-70cm</td><td>dark greyish brown (2.5Y 4/2)<br>clayey texture, numerous rust stains</td></tr><tr><td>70-90cm</td><td>grey (2.5Y 5/1)<br>clayey texture, rust stains<br>plastic limit at 80cm</td></tr><tr><td>100-120cm</td><td>grey (2.5Y 5/1)<br>clayey texture, numerous rust stains<br>positive reaction with HCl<br>presence of the water table</td><td></td></tr></tbody></table>

<figure><img src="image_7.png" /></figure>

### 3.1.2 Electrical resistivity tomography

The ERT acquired with the large profile (inter-electrode spacing 1m) over this zone shows a very simple structure with two superimposed layers (Figure 5a). The surface layer is relatively homogeneous and highly conductive, with inverted resistivity values below 15 ohm.m. This layer is about two meters thick and corresponds to a clay soil. The second, underlying formation is more electrically resistant, and is probably the Upper Jurassic limestone, given the geological context of the site.

Figure 5b, corresponding to the soil surface at a higher resolution, also shows the superposition of two layers. The surface layer, approximately 0.4 m thick, has resistivity values larger than 25 ohm.m. It corresponds to an unsaturated soil. The sub-surface layer has much lower resistivity values, below 15 ohm.m, and is very homogeneous. This layer corresponds to the clay soil shown in Figure 5a.

The synthesis of the two ERT presented in Figure 6 suggests a vertical organization of the soil in Zone 2 into three layers:

- from around 0 to 0.4 m, a layer of unsaturated soil.

- from 0.4 to around 2m, a layer of probably saturated clay soil.

- from 2 to over 8m, the Upper Jurassic limestone formation.

### 3.1.3 Penetration resistance profiles

Qd resistance values from four penetrometer tests in Zone 2 (Figure 7, see their locations in Figures 5 and 6) show the following characteristics:

- from 0 to 20 cm, Qd resistance varies widely from one borehole to the next. The layer corresponds to the dry “crust” at the soil surface.

- from 20 to 70-80 cm, Qd resistance varies little with depth. It ranges from 2 to 4 MPa, corresponding to a firm to stiff clay according to the classification proposed by [16].
<figure><img src="image_8.png" /><figcaption>**Fig. 5a.** Electrical resistivity tomography (ERT_1m_Z2) acquired in the area 2 (without micro-reliefs) with 48 electrodes profile in a Wenner-Schlumberger array with an electrode spacing of 1m.</figcaption></figure>

<caption>**Fig. 5b.** Electrical resistivity tomography (ERT_1m_Z2) acquired in the area 2 (without micro-reliefs) with 48 electrodes profile in a Wenner-Schlumberger array with an electrode spacing of 0.2m.</caption>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Synthetic representation of the two electrical resistivity tomographie ERT_1m_Z2 (1m inter-electrode spacing) and ERT_0.2m_Z2 (0.2m inter-electrode spacing) with the same resistivity scale (area 2, without micro-reliefs). Location of penetrometer tests are also indicated.</figcaption></figure>

- from 70-80 cm to 1.50-1.70 cm, Qd strength decreases with depth, reaching a minimum of between 0.2 and 0.3 MPa at around 150-170 cm. This latter strength range corresponds to a very soft clay according to [16].

- from 1.50-1.70 cm to around 2m, Qd strength increases to reach a peak corresponding to the soft clay/limestone interface highlighted by ERT_1m_Z2 (Figures 5 and 6).

### 3.1.4 Gravimetric water content profiles

Samples were taken at the centre (x=23m) and the end (x=1m) of the ERT_1m_Z2 profile. The surface layer has a water content of 16.6% at 23m and 20% at 1m (Figure 8). It increases progressively with depth, reaching a maximum value of 71.8% for the 23m borehole, above the 70% liquidity limit estimated for the site. The trend is the same for the 1m profile, but with higher water contents. This trend shows that the soil becomes plastic from 120cm upwards, becoming liquid at around 170cm. Beyond 170cm, the water content decreases, reaching a value of 27.8% at 230cm depth (Figure 8). From 180cm, the material contains a lot of limestone gravel and flint mixed with the clay soil. It is much more compact and contains less water. From 220-230 cm, it is no longer possible to dig with an auger.

It is interesting to note that the 120 cm depth characterizing the solid/plastic transition also corresponds roughly to the 1 MPa Qd resistance characteristic of the soft clay/very soft clay transition (Figures 7a and 7b).
<figure><img src="image_10.png" /><figcaption>**Fig. 7a.** Qd resistance profile performed at the centre of ERT_0.2m_Z2 (Fig. 5).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 7b.** Qd resistance profile performed at the end of ERT_0.2m_Z2 (Fig. 5).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Gravimetric water content of the soils from area 2. Ws: shrinkage limit; Wp: plasticity limit; Wi: liquid limit.</figcaption></figure>
### 3.1.5 Electrical conductivity profiles of the 1:5 water extract

Samples were taken at the centre (x=23m) of the ERT_1m_Z2 profile. The surface layer has a EC₁.₅ of 210-230 µS cm⁻¹ (Figure 9a). It increases to around 400-450 µS cm⁻¹ up to 180-200 cm and then rises to around 1000 µS cm⁻¹.

<figure><img src="image_13.png" /><figcaption>**Fig. 9.** (a) EC₁.₅ as a function of soil depth and (b) equivalent NaCl concentration in area 2.</figcaption></figure>

Equivalent NaCl concentrations in zone 2 range from 1.91 to 5.63 g L⁻¹ down to a depth of 2 m, increasing to 9.64 to 12.20 g L⁻¹ for the last two samples in contact with gravel and weathered limestone.

### 3.2 Area with large micro-reliefs

The area under study (Area 4) features numerous large polygonal micro-reliefs 35-55 cm high and 1 to 1.5 m diagonal (Fig. 10).

<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Photography of large micro-reliefs in area 1.</figcaption></figure>
### 3.2.1 Soil description

The surface consists of a brown to very dark grey layer rich in organic matter of 20 to 25 cm thick. The layer from 20 to 70-80 cm is dark grey to grey, with abundant rust stains from 70 to 80 cm. (Table 2) The soil is very plastic from 80 cm upwards.

<table><caption>**Table. 2.** Description of the soils from area 1 with large micro-reliefs. The colours are given in moist state.</caption><thead><tr><th>Depth</th><th></th></tr></thead><tbody><tr><td>0-25 cm</td><td>very dark grey (2.5Y 3/1)</td></tr><tr><td>25-35 cm</td><td>clayey texture</td></tr><tr><td>35-50 cm</td><td>dark grey (2.5Y 4/1)</td></tr><tr><td>50-70 cm</td><td>clayey texture, rust stains</td></tr><tr><td>70-80 cm</td><td>grey (2.5Y 5/1)</td></tr><tr><td>80-100 cm</td><td>clayey texture, numerous rust stains</td></tr><tr><td>100-120cm</td><td>positive reaction to HCl plastic limit</td></tr><tr><td></td><td>light brownish grey (2.5Y 6/2)</td></tr><tr><td></td><td>clayey texture</td></tr></tbody></table>

<figure><img src="image_15.png" /></figure>

### 3.2.2 Electrical resistivity tomography

Figure 11a shows the superposition of two layers. The surface layer (warm colors), the most heterogeneous, with a thickness of less than 1m, is more electrically resistant than the underlying formation (cold colors). Zones of large micro-reliefs observed on the surface correspond to zones of higher resistivity in the soil surface. This difference could be explained by two situations: (a) soils in large micro-reliefs would be more unsaturated than soils without micro-reliefs and/or (b) soils in large micro-reliefs would be less salty than soils without micro-reliefs. It should be noted that numerous desiccation cracks in the vicinity of the micro-reliefs (area framed in pink in Figure 12) were also observed.

Figure 11b shows the superposition of three layers. The surface layer, around 0.3-0.4 m deep, has high resistivity values of up to several hundred ohm.m (warm colors). It corresponds to an unsaturated soil. As in Figure 14a, areas of coarse mottled soil generally show higher resistivities. Beneath this first desaturated layer, a second layer 0.6-0.7 m deep (see red arrows in Figure 11b) shows lower resistivity values between 2.6 and 7.1 ohm.m. This layer and the previous one corresponds to the unsaturated soil. This layer and the previous one corresponds to the most superficial layer in Figure 11a. Finally, the third and deepest layer shows much lower resistivity values (below 2.6 ohm.m).

Comparison of the two ERT (Figure 12) suggests a vertical organization of the Zone 4 subsoil into three layers:

- from around 0 to 0.4 m, a layer of unsaturated soil.

- from 0.4 to around 0.6-0.7 m, a transition layer.

- from 0.6-0.7 to over 8m, a layer of saturated soil more saline (or richer in clay minerals) than the overlying soil layer.
<figure><img src="image_16.png" /><figcaption>**Fig. 11a.** Electrical resistivity tomography (ERT_1m_Z2) acquired in the area 4 (with large micro-reliefs) with 48 electrodes profile in a Wenner-Schlumberger array with an electrode spacing of 1m. Areas framed in pink indicate the presence of large micro-reliefs on the surface.</figcaption></figure>

<caption>**Fig. 11b.** Electrical resistivity tomography (ERT_1m_Z2) acquired in the area 2 (with large micro-reliefs) with 48 electrodes profile in a Wenner-Schlumberger array with an electrode spacing of 0.2m. Areas framed in pink indicate the presence of large micro-reliefs on the surface. The two vertical red arrows show the presence of a horizontal interface at a depth of around 60/70 cm.</caption>
<figure><img src="image_17.png" /><figcaption>**Fig. 12.** Synthetic representation of the two electrical resistivity tomographies ERT_1m_Z4 (1m inter-electrode spacing) and ERT_0.2m_Z4 (0.2m inter-electrode spacing) with the same resistivity scale (area 2, with large micro-reliefs). Areas framed in pink indicate the presence of large micro-reliefs on the surface. Locations of penetrometer tests are also indicated.</figcaption></figure>

### 3.2.3 Penetration resistance profiles

Qd resistance values obtained at the centre and at the end of the ERT profile (Figure 13) show the following features:

- from 0 to 30-40 cm, Qd resistance varies widely from one borehole to the next. The layer penetrated corresponds to the dry "crust" at the soil surface.

- from 30-40 cm to 60-90 cm, Qd resistance varies little with depth. It has values between 2 and 4 MPa, corresponding to a firm to stiff clay according to the classification proposed by [16]. The lower limit of this layer coincides with the saturated soil interface highlighted in the previous TRE_0.2m_Z4 (Figure 13 a and b).

- from 60-90 cm to 1.50/1.70 cm, Qd resistance decreases with depth, reaching a minimum between 0.2 and 0.9 MPa. This range of values corresponds to a very soft clay according to [16]. At a depth of around 1.20 m, the strength decreases below the characteristic value of 1 MPa defining the soft clay/very soft clay limit [16].

- From 1.50/1.70 cm to 3m, Qd resistance stabilizes at values slightly below 1 MPa.
<figure><img src="image_18.png" /><figcaption>**Fig. 13a.** Qd resistance profile performed at the centre of ERT_1m_Z4 (positions shown in Figures 11 and 12).</figcaption></figure>

<caption>**Fig. 13b.** Qd resistance profile performed at the end of ERT_1m_Z4 (positions shown in Figures 11 and 12).</caption>

### 3.2.4 Gravimetric water content profiles

Three water content profiles were measured at the centre (x=7m, bottom of a micro-relief), at the end (x=23m, bottom of a micro-relief) and (x=40m, top of a micro-relief) as referenced to ERT_1m_Z4 (Figures 11 and 12).

<figure><img src="image_19.png" /><figcaption>**Fig. 14.** Gravimetric water content of the soils from area 4. Ws: shrinkage limit; Wp: plasticity limit; Wi: liquid limit.</figcaption></figure>
The surface layer has a water content of between 19.6% (top micro-relief at x=0m) and 25.8% (bottom micro-relief at x=23m). The water content increases progressively with depth, reaching high values: 81.6% (bottom micro-relief at x=7m), 87.1% (bottom micro-relief at x=23m) and 86.6% (top micro-relief at x=40m). These values are above the 70% liquidity limit estimated for the site. This evolution shows that the soil becomes plastic from 100-120 cm and evolves towards a liquid state from around 180cm. From 180cm upwards, the soil appears in a liquid state over a significant thickness of 170cm (Figure 14). This soil corresponds to the very soft clay identified by penetrometer tests. In contrast to zone 2, the water content remains at high levels without decreasing with depth down to 350cm.

### 3.2.5 Electrical conductivity profiles of the 1:5 water extract

The surface layer has an EC₁.₅ between 247 µS cm⁻¹ (top microrelief at x=40m) and 637 µS cm⁻¹ (bottom micro-relief at x=7m). It rises progressively between 600 and 700 µS cm⁻¹ from a depth of 0.3 to 0.5 m, and then to around 900-1100 µS cm⁻¹ down to 1.10 m (Figure 15). It then rises sharply to around 2800 µS cm⁻¹ down to 1.70 m. Between 1.70 m and 3.50 m, values fluctuate between 3500 and 5130 µS cm⁻¹.

In the zone with large micro-reliefs, equivalent NaCl concentrations range from 3.98 to 23.06 g L⁻¹ for most points (Figure 15b). For the point sampled at the top of the micro-relief at position x = 40 m, NaCl concentrations are around 32 g L⁻¹ at depths of 80 and 200 cm. The surface crust has the lowest NaCl concentrations. The tops of the micro-reliefs have lower concentrations than their bottoms, but from a depth of 50 cm this trend reverses and the micro-relief “roots” appear saltier than the areas between the micro-reliefs down to a depth of -140 cm. At depths greater than 140 cm, the values are scattered, but with an overall trend of increasing concentrations up to values of between 21 and 23 g L⁻¹ between 310 and 350 cm depth.

<figure><img src="image_20.png" /><figcaption>**Fig. 15.** (a) EC₁.₅ as a function of soil depth and (d) equivalent NaCl concentration in area 4.</figcaption></figure>
## 4 Discussion

On the basis of these initial results from the “Marais du Poiré-sur-Velluire”, it would appear that the micro-reliefs (“gilgais”, “mottureaux”) formation mechanism described by [4], i.e. upward thrust between shrinkage cracks caused by “clay material fallen into the bottom of the cracks”, is unlikely for at least one reason. The results of penetrometer tests show that the lower zones between and below the micro-reliefs are clearly softer than the micro-reliefs themselves [14, 15]. These lower zones are insufficiently stiff to transmit the pressures that could cause the upward thrust observed.

A comparison of the various results acquired on the five zones, and more particularly on zones 2 and 4 of the site, enables us to propose a vertical organization of the soils composed of the superposition of three layers of varying thickness (Figure 16 a and b).

i- *an unsaturated surface layer*. This layer is itself vertically composed of two sub-units: (a) a layer of soil (30-40 cm thick) corresponding to a dry "crust" that is highly resistant both mechanically (Qd resistance larger than 10 MPa) and electrically (electrical resistance of several tens of ohm.m), and (b) a layer of loose soil corresponding to the zone of the surface water table fluctuation. This second sub-unit extends to a depth of 60 to 90 cm. From a mechanical point of view, this second surface unit corresponds to a firm to stiff clay according to the classification proposed by [16] (i.e., Qd resistance greater than 2 MPa). The second layer is a transition level between the overlying unsaturated and stiff formation and the underlying layer rich in very soft clay. EC₁.₅ measurements show that this layer is brackish.

ii- *a layer of saturated clay soil*, also vertically composed of two sub-units: (a) a transition layer extending to a depth of around 100-120 cm, depending on the study area, followed by (b) a layer of very soft, plastic clay (in the sense of Atterberg limits), rich in liquid clay of highly variable thickness. Although it has the mechanical properties of a soft to very soft clay, the transition layer is in a solid state in the sense of Atterberg limits. At a depth of around 100-120 cm, corresponding to the solid/plastic interface, the clay soil becomes a very soft, plastic clay with more or less thick beds of liquid clay (still liquid in the sense of Atterberg limits). This unit is saltier than the overlying transition layer. According to penetrometer tests and ERTs, this very soft, plastic clay layer is multi-metric in thickness in areas with large micro-reliefs and only multi-decimetric in thickness in the reference area (Area 2) without micro-reliefs.

iii- the third layer revealed on the marsh is a limestone formation. This unit was identified in zone 2 at a depth of around 2m. This unit most probably corresponds to the Jurassic limestone formations of the Bajocian (J2) and Bathonian (J3) present to the north of the marsh, according to the 1:50,000 Marans geological map. This superposition of three layers had been highlighted by [17] with the limestone bedrock appearing between 8 and 10 m in a zone with large micro-reliefs.

From pedological point of view, the soil, over a thickness of 1 m to 1.20 m, is like the soils described in the Marais Poitevin [18 – 20]. The brown to black surface layer is due to the decomposition and incorporation of organic matter. The colour becomes grey from 20 to 40 cm, and rust stains are present between 20 cm and 80 cm, indicating a seasonal variation of the level of the soil water table. From a depth of 80-100 cm, the soil is grey without rust stains, with a few bluish zones indicating the permanent presence of the surface water table (gleyic properties). The texture is clay to heavy clay, with no significant variations along the profile as described in the literature.
<figure><img src="image_21.png" /><figcaption>**Fig. 16a.** Schematic representation of soil profile in zone 2 without micro-reliefs; **Fig. 16b.** Schematic representation of soil profile in zone 4 with large micro-reliefs. The grey levels reflect the gradual transition from a solid fissured state (dark grey) to a plastic, soft clay state (light grey), to a liquid clay state (very light grey).</figcaption></figure>

The vertical organization of the soils highlights two elements that seem to be important in the genesis of these very special geomorphological objects:

- Desiccation cracks.

- The very soft, plastic-to-liquid clay layer in the saturated clay soil.

Regarding the first element, we consider that the significant differences in electrical resistivity between areas with large micro-reliefs (i.e., Area 4) and areas without these features (i.e., Area 2) are attributable to the effects of the numerous desiccation cracks in the vicinity of the micro-reliefs. These wide cracks, which can reach several decimetres in length, amplify the desiccation of the soil and therefore reduce the water content, the main vector of the electric current.

Concerning the second element, the very soft, plastic-to-liquid clay layer in the saturated soil (defined here as material with Qd resistance of less than 1 MPa) present in all the study areas shows two features that differ between areas with and without micro-reliefs. On the one hand, it has beds or banks of so-called liquid clay of greater thickness in areas with large micro-reliefs, as shown by the water content profiles. On the other hand, the total thickness of this very soft clay is significantly greater in the zones with micro-reliefs.

This second element then suggests that a hydrodynamic instability mechanism yet to be determined (micro-diapirism or upward vertical density gradient? see for example Figure 17. Differential vertical mechanical loading?) would also play a significant role in the formation of micro-reliefs. It does not exclude the impact of osmotic swelling [3, 14, 15] associated with desiccation cracks, but could even be coupled to the latter.

<figure><img src="image_22.png" /><figcaption>**Fig. 17.** Schematic diagram of micro-diapirism. By analogy with the diapirism observed on a larger scale, this phenomenon would cause very soft soils to upward thrust through a denser deformable soil.</figcaption></figure>
**5 Conclusion**

Like many of soils in the Marais Poitevin, the soils of the Marais du Poiré-sur-Velluire contain numerous micro-reliefs (often named “mottureaux” or “gilgais”), which are home to a rich biodiversity but whose formation mechanisms remain largely unknown. A survey of electrical, pedological and geotechnical measurements was carried out in 2021 and 2022 on the Marais du Poiré-sur-Velluire site to characterise these very special micro-reliefs. The electrical measurements consisted in carrying out ERTs at various investigation depths. Qd cone resistance as geotechnical parameter was measured as a function of depth. Auger drilling was carried out, along with measurements of water content, salinity and Atterberg’s limits.

The summary of the results presented in this paper highlights the following two elements, which may play a definite role in the genesis of the micro-reliefs: (a) desiccation cracks in soil surface and (b) the thickness of a very soft, plastic-to-liquid clay layer in the saturated part of the soil.

Investigations are currently continuing with a field “crushing” test of these micro-reliefs carried out in January 2023 on three plots on the site under different grazing management. The restoration of the micro-reliefs is also being monitored using electrical, pedological, geotechnical and topographical methods. This new experimental campaign should enable us to gain a better understanding of the formation of these micro-reliefs.

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