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This paper presents an experimental study aimed at evaluating the measuring capabilities of an innovative monocellular pressuremeter probe, the Monocell Francis Cour probe, based on calibration testing in the laboratory. The originality of the tested equipment is to allow evaluation of soil properties in both small- and large-strain domains, without the need of sophisticated point displacement measuring arms. This paper focuses on the evaluation of small-strain shear modulus G of Fontainebleau sand, NE34, based on tests carried out with this specific pressuremeter probe. Shear modulus values obtained with this probe are then compared with values resulting from more classical elementary tests, showing a fairly good consistency. It is shown, with satisfactory agreement, that it is possible to quantify the influence of the density index of the sand as well as the influence of the mean effective stress around the probe on the shear modulus, based on the analysis of specific unload−reload loops performed during the test. The experimental programme carried out under well-controlled laboratory conditions allows a validation to be proposed for a method of derivation of the shear modulus of sand at small strain levels using this prototype pressuremeter.

In order to study the behaviour of reinforced sand, a series of monotonic consolidated drained and consolidated undrained triaxial tests was conducted on Chlef sand samples and, respectively, sand ...

The results of a series of displacement-controlled cyclic axial loading tests of model pile in saturated kaolinite clay, carried out in a calibration chamber, are presented. An instrumented pile-pr...

This paper presents a study of the influence of plastic and non-plastic fines on the resistance of sands to liquefaction. The study is carried out with the aid of a triaxial apparatus on reconstitu...

The present experimental study investigates the influence of cyclic displacement amplitude and effective consolidation stress on the evolution of mobilized local shaft friction along piles submitted to large number of cycles (up to 105 cycles). Two-way cyclic displacement-controlled tests were performed on an instrumented pile-probe installed and loaded in a calibration chamber. Tests were performed on reconstituted specimens of saturated clay to examine the shaft friction evolution in the soil-pile interface during cyclic loading. Displacement-controlled static tests were also performed before and after the cyclic loading in order to quantify the influence of cyclic parameters on post-cyclic static response. It was found that the amplitude of cyclic displacement and the initial state of stress have an influence on the evolution of local friction during cyclic loading. The degradation rate of local friction increased for larger cyclic displacement amplitudes whereas with increasing the effective consolidation stress, the degradation rate decreased. The application of displacement- controlled cycles resulted in a modification in the behaviour of the interface. A significant peak of static friction followed by strain softening was observed during post cyclic static tests, which was not the case for pre-cyclic static tests. The peak value of friction obtained upon post-cyclic static loadings found to be more important for higher values of applied displacement amplitude during cyclic loading. Finally, a brief synthesis of the results is given.

This paper highlights the effect of non-plastic fines on sand matrix by performing a series of undrained triaxial tests. The silty sand specimens were reclaimed by moist tamping method and two dist...

Pile foundations are usually subjected to cyclic loading which could possibly causes a significant degradation of the friction mobilised along the pile shaft. Most of the experimental studies devot...

This paper presents an experimental study that investigates the influence of the fines on the initiation of instability of sands under cyclic loading. The materials used for this study and the expe...

To investigate local friction mobilization along piles subjected to cyclic axial loadings, a calibration chamber experiment is presented based on the implementation of instrumented probes in specimens of saturated clay. Typical results obtained with a piezo-probe are presented, allowing not only tip resistance and local friction to be measured, but also the local pore-water pressure developed on the probe shaft. In addition, specific piezometers installed in the clay specimen allow a good picture to be obtained of the pore-water pressure field around the probe during installation and loading. After installation of the probe, a succession of monotonic and cyclic displacement-controlled loading phases is applied. Upon displacement-controlled cyclic loading of the piezo-probe up to a very large number of cycles (105 cycles), an initial degradation of local friction is observed followed by a subsequent reinforcement, which continues until the end of the cyclic sequence. The friction evolution is related to the evolution of the pore-water pressure measured during cyclic loading. In particular, the influence of the cyclic loading sequence on the post-cyclic static friction resistance is evaluated. A comparison is finally made with the results obtained with another type of probe, showing a good consistency between both types of results.

International audience; In the French conventional railway track, an interlayer was created naturally through the interpenetration of ballast and subgrade under the effect of long-term train loading. Field investigation showed that the proportion of ballast grains decreased over depth in the interlayer. Moreover, the water content of interlayer soils varied depending on the weather conditions, which can strongly affect the mechanical behavior of interlayer soil. In this study, the effect of water content on the mechanical behavior of interlayer soils under various coarse grain contents was investigated by monotonic triaxial tests. Three water contents of fine soil (wf = 17.6%, 10.6%, and 7.0%), five volumetric coarse grain contents (fv = 0%, 10%, 20%, 35%, and 45%) and three confining pressures (σ3 = 30, 60 and 120 kPa) were considered. Results showed that a decrease of wf led to an increase of shear strength and soil stiffness due to the effect of suction, and to an increase of dilatancy due to the aggregation of fine soils. Moreover, the variations of maximum deviator stress qmax, Young’s modulus E0, dilatancy angle ψ and friction angle φ with fv followed a bi-linear pattern for the three σ3 values, defining a characteristic volumetric coarse grain content fv-cha value for a given wf value: fv-cha ≈ 25%, 29% and 33% for wf = 17.6%, 10.6% and 7.0%, respectively. The fv-cha corresponded to the transition from a structure dominated by fine soils to a structure dominated by coarse grains. The increase of fv-cha with the decrease of wf could be attributed to the swelling and shrinkage of fines. While drying from optimum water content of fines wopt-f = 13.7% to a lower wf value, more coarse grains were needed to constitute the global skeleton due to the increase of the global volume of macro-pores resulted from the shrinkage of fine soils. By contrast, while wetting from wopt-f = 13.7% to a higher wf value, since the global volume of macro-pores decreased due to the swelling of fine soils, less coarse grains were required to constitute the global skeleton.

International audience; This study aims to investigate the undrained shear strength of sand with added materials at different mean diameter ratios D50/d50 (D50 and d50 respectively represent the mean diameter of host sand and added materials) in loose state by performing a series of undrained triaxial tests. For this purpose, a very low density index of sand matrix IDmat equaling to 0.00 was employed and the fines of C500 silt, Illite and Speswhite with distinct mean grain sizes were respectively added into the host HN31 sand at different contents Fc (5%, 10% and 15%). For comparison and further analysis, undrained triaxial tests were also carried out on Sand-Sand mixtures, created by adding the HN31 sand into the host HN31 sand at Fc of 5%, 10%, 14% and 18.7%. Results showed that, for low value D50/d50 (e.g., 1 for Sand-Sand), the undrained shear strength Su increased as Fc increased, which might be attributed to the increase of occlusal friction, and in that case the sliding of host sand became difficult. By contrast, in the case of higher value D50/d50 (e.g., 58.33 for Sand-C500, 159.09 for Sand-Illite and 500 for Sand-Speswhite), Su decreased as Fc increased. This was explained by the fact that, with increase of Fc, the quantity of added materials with small size increased, and in that case the sliding of host sand became easy. A critical value of D50/d50 (14.5) was thus deduced, which theoretically verified the negligible effect of Fc on Su. Furthermore, for sand mixed with fines (e.g., C500 silt, Illite and Speswhite clays), Fc has an insignificant effect on the peak value of Δu, while the increase of D50/d50 facilitated the generation of excess pore water pressure after reaching about Δumax. These findings provided useful information for analyzing the mechanical behaviour of sand with added materials in geotechnical practice.

Ballast/fines mixture is often found in railway substructure and the overall behaviour of tracks is strongly dependent on the mechanical behaviour of this mixture. Since directly testing such mixture with large grains is limited with common laboratory equipment, the consideration of a model material at smaller size is advisable. Parallel gradation method is widely used for this purpose. This study assesses the validity of this method in the case of ballast/fines mixture. Large-scale cyclic triaxial tests were carried out on the ballast/fines mixture at six volumetric coarse grain contents. The results obtained were analysed, together with those obtained previously from small-scale triaxial tests on a microballast/fines mixture whose microballast grain size distribution was determined by applying the parallel gradation method. The cyclic parameters (permanent strain and resilient modulus) were obtained for the two types of mixture. Results show that for all mixtures two distinct soil fabrics can be identified according to the variations of permanent strain and resilient modulus with the volumetric content of coarse grains fv: a fine-fine contact structure for fv ≤ 20% and a grain-grain contact structure for fv ≥ 35%. In the case of fine-fine contact structure, the permanent strains and resilient modulus values of the ballast specimens are consistent with those of the microballast specimens, evidencing the validity of the parallel gradation method. In the case of grain-grain contact structure, the permanent strains and resilient modulus values are found to coincide globally at the two scales, also justifying the validity of the parallel gradation method, the slight differences between the two scales being attributed to the irregular grain sliding and the distribution of the fines soils.

In the conventional French railway substructures, a layer namely interlayer was created naturally by the interpenetration of ballast and subgrade. The lower part of this interlayer is characterized by a matrix of fines with inclusion of coarse grains in it. Moreover, the coarse-grain content is decreasing over depth. In this study, the effect of volumetric content of coarse grains f v (volumetric ratio of coarse grains to the whole sample; grain size > 2 mm) on the mechanical behavior of the lower part interlayer soil was investigated by carrying out monotonic triaxial tests. The results showed that increasing the volumetric content of coarse grains f v led to more significant dilatancy. The shear strength also increased with the increase of f v and the increasing trend appeared to be much more significant when f v >20%. Examination of the variations of peak deviator stress with f v revealed the existence of a characteristic value of fv-cha ≈ 27% that separates two zones with different coarse-grain effects: a fine-fine contact dominating zone when fv ≤ 27% and a grain-grain contact dominating zone when fv > 27%. To clarify the observed phenomena, X-ray microcomputed tomography (μCT) scans were performed on the as-compacted samples, allowing the coarse-grain distribution inside the as-compacted sample to be visualized. It was found that when f v ranged from 0% to 10%, the skeleton of the sample was mainly constituted by fines with the coarse grains floating in the fines matrix. By contrast, when f v reached 20%, the coarse grains formed some aggregates but the fine-fine contacts were still dominating. When f v increased up to 35%–45%, the skeleton of the sample turned to be dominated by the coarse grains. This suggests that the characteristic volumetric content of coarse grains identified through monotonic triaxial test results corresponds to the transition of soil fabric from fine-fine contacts to grain-grain contacts.

For the French conventional railway lines, a layer, namely interlayer, was created in the substructure mainly by the interpenetration of ballast grains and subgrade soils. The in situ investigation indicated that the content of coarse grains decreases over depth. In this study, the resilient modulus and damping ratio of the unsaturated lower part of the interlayer soil were investigated at six different volumetric inclusion contents fv (volumetric ratio of dry coarse grains to the whole sample) by carrying out cyclic triaxial tests following a multi-step loading procedure. The results show that the two dynamic parameters (resilient modulus and damping ratio) change significantly at the beginning of loading and the variation rate decreases as the number of cycles increases. Besides, a mean characteristic volumetric inclusion content fv–cha ranging from 27.8% to 28.9% was identified, separating two zones with different inclusion effects. To verify this observation, X-ray microcomputed tomography (μCT) scans were conducted on as-compacted samples. The results obtained strongly support the existence of fv–cha: when fv ≤ fv–cha, the matrix of fines constitutes the skeleton of the sample with inclusions floating in it, leading to slight changes of the two dynamic parameters with fv. By contrast, when fv > fv–cha, the inclusions dominate the skeleton of the sample, giving rise to significant changes of the two dynamic parameters with fv.

This paper was aimed at highlighting the impact of loading waveforms on the cyclic liquefaction resistance of sand. The test specimens were reclaimed by the dry tamping method with Hostun 31 sand, and the density of sand matrix was controlled to be 0.70. Three sets of undrained cyclic triaxial tests were performed by altering the loading waveforms as follows: triangular, sine and rectangular waves. The impact of the loading waveform was experimentally observed as axial cyclic stress was applied. Under the same amplitude of loading, it was found that the rectangular wave led to the most severe liquefaction as a result of greater imposed force acting for a longer duration. The triangular wave led to the lowest liquefaction softening, and the sine wave was the case in between. Consequently, in terms of the cyclic strength curves, diverse curves related to the imposed waveform were observed, and the curve for rectangular wave was located in the lowest part of the graph, thereby reconfirming its higher liquefaction potential. An in-depth analysis of the dynamic properties suggested that the development of the damping ratio with the progression of shearing is intrinsic for a given sand fabric under a certain initial confining stress. A unique relationship between the damping ratio and the amplitude of shear strain could be roughly established. However, the number of cycles required to trigger a certain level of shear strain remained a function of the amplitude and waveform. A novel parameter was proposed in this paper on the basis of the Arias intensity in order to unify the sand liquefaction response. Compared with the conventional method, one of its advantages is its ability to express the combined influence of the amplitude and waveform through a time-integral for a given imposed loading.

This study was aimed at highlighting the effect of loading frequency on the sand liquefaction response by performing cyclic triaxial tests in both stress-controlled and strain-controlled modes. For the first case, dense (IDmat = 0.70) and medium-dense (IDmat = 0.55) sand specimens were subjected to alternative and non-alternative axial stresses; whilst dense (IDmat = 0.70) sand specimens were subjected to alternative axial strain over a wide range of loading frequencies in the second case. It was experimentally found that in the case of alternative loading for test specimens with IDmat = 0.70 in stress-controlled mode, the effect of the loading frequency was not notable for dry sand specimens. In contrast, the effect of the loading frequency was significant for saturated sand specimens in both the loading modes. This was because, upon shearing, transient deformation occurred with the presence of excess pore water pressure, during which the inertia force needed to be taken into account. In the axial direction, the inertia force opposing the external loading certainly increased the liquefaction resistance by suppressing the complete development of the sand liquefaction. As for non-alternative loading in stress-controlled mode for test specimens with IDmat = 0.55, the loading frequency played a vital role in pure extensional loading, which could also be logically attributed to the presence of the axial inertia force appearing in the large strain and transient deformation stages. However, the effect of the loading frequency on the compressive pattern was unclear due to the following two reasons: (i) the inertia force in the lateral direction appeared in the form of enhancing lateral confinement, thereby impeding the relative sliding; and (ii) during shearing, the rearrangement of the sand grains occurred in a more stable direction. These two aspects both provided beneficial contribution to sand liquefaction resistance, thereby preventing the occurrence of transient deformation. Under the circumstance, the sand specimens deformed only moderately, thereby eliminating the need to consider the role of the loading frequency.

This paper presents results of a series of undrained monotonic compression tests on loose sand reinforced with geotextile mainly to study the effect of confining stress on the mechanical behaviour of geotextile reinforced sand. The triaxial tests were performed on reconstituted specimens of dry natural sand prepared at loose relative density (Dr = 30%) with and without geotextile layers and consolidated to three levels of confining pressures 50, 100 and 200 kPa, where different numbers and different arrangements of reinforcement layers were placed at different heights of the specimens (0, 1 and 2 layers). The behaviour of test specimens was presented and discussed. Test results showed that geotextile inclusion improves the mechanical behaviour of sand, a significant increase in the shear strength and cohesion value is obtained by adding up layers of reinforcement. Also, the results indicate that the strength ratio is more pronounced for samples which were subjected to low value of confining pressure. The obtained results reveal that high value of confining pressure can restrict the sand shear dilatancy and the more effect of reinforcement efficiently.

International audience; Soil improvement using fibres is widely used in soil stabilisation to prevent sand liquefaction. In order to study the undrained behaviour and liquefaction resistance of sand reinforced with polypropylene fibres, a series of triaxial compressive tests were conducted on unreinforced and reinforced Chlef sand with different contents of polypropylene fibres (0, 0.3, 0.5 and 0.8%). Samples were prepared at 30% and 80% relative densities representing loose and dense states respectively, and triaxial tests were performed at confining pressures of 50, 100 and 200 kPa. Tests results show that fibre inclusion has a significant effect on the shear strength and dilation of sandy soil. The increase in strength is function of fibre content, relative density and confining pressure. The maximum strength improvement for both loose and dense fibre-reinforced sand is more pronounced at higher confining pressure and at higher fibre content.

The construction of embankments and other earth hydraulic structures using coarse soils requires assessing their potential for internal erosion by suffusion, defined as detachment and transport of fine particles through the matrix constrictions under internal flow. For potentially erodible coarse soils containing a certain amount of clays, a possible remedial solution is the lime treatment which is studied in this work in an experimental program consisting in: erosion test, crumb test, unconfined compression test and microstructure characterization tests (SEM, mercury intrusion porosity). The experiments were carried out on a reconstituted soil owing similar characteristics to natural coarse soils. The treatment reported in this study is carried out using a minimum lime content of only 1%, which can be achieved in situ in a cost-effective manner. Comparisons of results on treated and untreated soils showed that the lime treatment is effective after only 24 h of treatment. The suffusion is stopped, the agglomeration of the particles generated by the treatment seems to be maintained after samples immersion and the unconfined compressive strength (UCS) is improved. The microstructure observations of the fine part of the soil (particles smaller than 1 mm) showed the appearance of agglomerates generating an increase of the pore volume.

When performing cyclic triaxial tests, the connection process between loading piston and specimen could have significant effect on the test results. Tensile pre-shearing effect occurs during saturation and consolidation stages in screw-based configurations and compressive pre-shearing stress is induced while making the connection between loading piston and specimen in vacuum-based configurations. In order to overcome these technical problems, a special auto compensation system was incorporated into an existing cyclic triaxial apparatus, allowing: (1) no physical contact between loading piston and specimen to achieve an isotropic consolidation state, (2) the later connection between these two components to be easily accomplished with almost insignificant disturbance on the specimen (maximum 100 N in axial force). To assess the performance of this auto compensation system, two experiments of same dense reference sand specimens were respectively conducted, and the results were found to be similar, showing the good repeatability. Moreover, as compared with screw-based configuration, the test specimen did not undergo an adverse tensile pre-shearing. As a result, the test specimen became more resistant to cyclic mobility. On the contrary, as compared with another vacuum-based configuration (without auto compensation) well-documented in the literature, the obtained result was less stable since the employment of auto compensation system ensured that there was no beneficial compressive pre-shearing during the connection stage. The results obtained in this study showed the good performance of the developed auto compensation system in the triaxial test apparatus for liquefaction analysis.

Dry and moist tamping methods are widely used for the investigation of sandy soil liquefaction. However, the effect of these two tamping-based reconstitution methods on composite soil liquefaction behavior have been rarely assessed. In this study, the dry and moist tamping methods were assessed using loose silica sand matrix (HN31) containing various amounts of non-plastic silt particles (C500), and by performing a series of undrained triaxial compression tests. Microstructural observations using a high-resolution optical microscope were also performed on loose silty sand specimens that had the similar initial states to those for the undrained triaxial tests. Triaxial results show that for silty sand specimens prepared by dry tamping method, a global dilatant behavior was observed and the undrained shear strength decreased as the fraction of silt particles increased. By contrast, a total reversed phenomenon was observed for those prepared by moist tamping method: all specimens showed more or less limited liquefaction behavior and silt particles mitigated the contractive tendency, which provided beneficial contribution to the undrained shear strength. Microstructural observations show that in the dry tamping method, overall structure formed by the host sand and silt particles was regularly deposited, promoting a more stable strain-hardening response. Moreover, some silt particles were found to be located near sand-sand contact points, significantly loosening the sand matrix. On the contrary, by using moist tamping method, the sand was uniformly covered by bulked silt particles, and the aggregated sand formed the metastable structure, facilitating the occurrence of static liquefaction. Moreover, with the increase of fines fraction, this metastable structure was weakened, characterized by the filling of large inter-sand voids, leading to the increase of undrained shear strength. Thus, a critical value of total fines fraction FcTot equalling to 21% was determined, which theoretically verified the insignificant effect of specimen preparation methods on the undrained shear strength. In that case, the metastable structure in specimen prepared by moist tamping method was expected to disappear and the overall response of specimen changed from the dominance of host sand to that of fines.

For the French ancient rail track, owing to the effect of traffic loadings, an interlayer was naturally created due to the interpenetration of ballast grains and subgrade fine soil. Under the effect of rainfall/evaporation, the water content of interlayer soil can vary, affecting its dynamic properties such as resilient modulus and damping ratio. In order to investigate the effect of water content on the resilient modulus and the damping ratio of interlayer soil, a series of cyclic triaxial tests were carried out, following a multi-step loading procedure with various deviator stress amplitudes of 10, 30, 50, 100 and 200 kPa. Two target fines water contents (17.6% and 10.6%) and five coarse grain contents (0%, 10%, 20%, 35% and 45%) were considered. Results showed that under a given coarse grain content, an increase of water content led to a decrease of resilient modulus owing to the decrease of suction while an increase of damping ratio owing to the increase of soil viscosity. Through the variations of resilient modulus and damping ratio with coarse grain content, a characteristic coarse grain content was identified, defining two distinct soil fabrics: a fine soil dominated fabric when the coarse grain content was smaller than the characteristic value, and a coarse grain dominated fabric when the coarse grain content was larger than the characteristic value. Moreover, a constant characteristic coarse grain content was obtained at a given water content: 25.5% at 17.6% water content and 32.0% at 10.6% water content. The increase of characteristic coarse grain content with the decrease of water content could be explained by the swelling-shrinkage of fine soil: with a decrease of water content, the shrinkage of fine soil led to an increase of the volume of macro-pores. More coarse grains were thus needed to constitute the global skeleton, giving rise to an increase of the characteristic coarse grain content. In addition, comparison of the values of characteristic coarse grain content obtained from cyclic tests with those from monotonic tests showed that at a given water content, cyclic loading led to a slightly larger value than monotonic loading, evidencing the more significant effect of cyclic loading on soil fabric.

We performed a geophysical survey in Mirabello, a village of Emilia Romagna Region of Northern Italy, to study the soil affected by the ML 5.9 earthquake of 2012, specifically the formation of surface ruptures by sand ejection due to the liquefaction of shallow subsurface layers. The investigation was carried out using ground penetrating radar and seismic reflection/refraction techniques. This work confirms the importance of electromagnetic waves to map the shallow subsurface extent of fractures and liquefied sand bodies, while the high-resolution seismic reflection profile allowed us to map the fractures at depth. The result obtained by tomographic inversion of first arrivals of seismic data has been compared with a ground penetrating radar section acquired in the same place, furnishing complementary information for the interpretation of the section.

International audience; The pile (and micropile) group effect in sand remains a key issue in civil engineering. This paper describes an experimental study carried out using a calibration chamber and an instrumented model pile installed by jacking. A specific methodology is proposed to study group effect through a generic pile within the group. A pile group can be modeled by a group of five piles, one central pile and four adjacent piles. The central pile represents a generic pile within a group with a large number of piles, and the calibration chamber enables the performance of a parametric study to evaluate the global and local (tip resistance and shaft friction) group effects and quantify them using the efficiency coefficient factors relative to tip resistance and shaft friction. The effect of parameters, such as the pile-to-pile spacing ratio and the installation order on pile group response has been investigated. The instrumented generic pile is designed to study independently tip resistance and shaft friction.The results show that the installation order has a significant influence on the pile group response. Some recommendations from laboratory tests are proposed to optimize pile installation in the case of a group and benefit from group effect.

An interlayer soil was identified in the conventional French railway substructure, corresponding to a mixture of fine soils and coarse grains. As the overall mechanical behavior of the interlayer soil can be conditioned by the microstructure of the fine soils, for the laboratory testing, it is important to develop an appropriate protocol for the sample preparation by compaction, which allows this microstructure effect to be minimized. In this study, two sample preparation approaches were considered for a fine/coarse mixture, with two distinct volumetric contents of coarse grains fv (volumetric ratio of coarse grain to total sample). The microstructure of fine soils was investigated using mercury intrusion porosimetry, and its effect on the overall mechanical behavior was examined through monotonic triaxial tests. Results showed that while compacted dry of optimum, the fine soils exhibited a bimodal porosity microstructure. By contrast, while compacted wet of optimum, the fine soils exhibited a unimodal porosity microstructure. When fv = 10 %, the sample was characterized by a fine matrix macrostructure with coarse grains floating in it. In that case, a strong effect of the microstructure of fine soils on the overall mechanical behavior was identified. On the contrary, when fv = 35 %, the sample was characterized by a coarse grain skeleton. In that case, a very limited microstructure effect of fine soils on the overall mechanical behavior was observed. Thus, while preparing samples of unsaturated fine/coarse soil mixture by compaction to study the overall mechanical behavior, it is important to account for the fv value. At low fv, the compaction at different water contents is to be avoided because of the significant effect of the microstructure of fines. On the contrary, at high fv, the samples at different water contents can be prepared by compacting the mixture at the desired water contents.

International audience; This work aims at investigating the evolution of local friction mobilized at the pile-soil interface, focusing on small amplitude and large number of displacement controlled cycles, corresponding to fatigue behaviour (10^5 cycles). An experimental program has been conducted in a calibration chamber with an instrumented pile-probe installed and loaded in reconstituted specimens of saturated clay.After describing the testing setup and the experimental procedure, a typical test is presented, carried out on a reference French kaolinite to evaluate the local friction mobilized upon different monotonic loadings and its evolution during the application of displacement-controlled cyclic loading. Quasi-static displacement-controlled compression tests are carried out before and after the cyclic sequence in order to evaluate the influence of the cyclic sequence on the mobilized static local friction after cycles, as compared to the initial friction mobilized. The displacement-controlled cyclic test allows to observe an initial phase of friction degradation which starts progressively from the very beginning of the cyclic sequence (cyclic softening), followed by a stabilization phase until the end of the test. The evolution of mobilized friction during the application of the cycles is quantified by introducing a coefficient of evolution which allows to clearly visualize the degradation and reinforcement phases. The local shaft friction evolution, upon the cyclic loading, is interpreted in terms of the combination of excess pore water pressure generation and dissipation which develops during the cyclic sequence and controls the evolution of effective normal stress acting on the soil-pile interface. A comparison is made between maximum static shear mobilized before the cycles and after the cycles, showing a clear influence of the cyclic sequence on this quantity. Finally, the repeatability of the test is checked, showing a fairly good level of repeatability.

International audience; An offshore wind farm will be developed in France at the Fecamp site covering in excess of 67 km². In this location, medium to high density chalk containing flint nodules is found. In this paper, some of the hydro-mechanical properties of this specific marine chalk are presented, aiming to characterize the first meters of the chalk more precisely for the design of gravity base foundations. A series of un-drained static and cyclic triaxial tests carried out on 100 mm and 300 mm diameter specimens of weathered chalk are presented. Specimens have been recovered from 100 mm, 300 and 500 mm diameter vibrocore sections. Results show, upon undrained static tests, that the material presents essentially dilative behaviour and usually generates negative excess pore water pressures during shear. Upon undrained cyclic tests, the material shows a progressive development of fairly significant positive excess pore water pressures and relatively high values of axial strains. Results also indicate that the excess pore water pressure dissipation, after a sequence of cycles, induces a densification of the material which results, upon post-cyclic undrained static shear, in a more dilative response resulting in higher values of undrained shear strength.

International audience; In the French conventional railway substructure, the interpenetration of ballast grains and subgrade fine soils that occurred over years of operation has created a new layer, namely, an interlayer. The in situ investigation on this interlayer showed that the content of ballast grains decreases over depth, with the lower part characterized by a matrix of fines with inclusions of coarse grains. In this study, the permanent deformation of a material simulating the lower part of interlayer soil was investigated at six different volumetric inclusion contents fv (volumetric ratio of the inclusion grains to the total sample) by performing cyclic triaxial tests. The results indicate that at a given cycle, the permanent deformation decreases with the increase of fv, and the evolution of permanent deformation is strongly influenced by the loading history. To further investigate the effect of inclusion content, a method that allows eliminating the influence of loading history and estimating the end-stage permanent deformations at different stress levels was applied. Quadratic polynomial and bilinear fitting methods were used to fit the variations of the estimated end-stage permanent deformations with fv, showing a proper similarity between the two methods at high loading amplitudes (higher than 15 kPa). Furthermore, the testing results revealed the existence of a characteristic volumetric inclusion content fv-cha by bilinear fitting method, separating two zones with different inclusion effects. This observation is strongly supported by the X-ray microcomputed tomography (μCT) scans performed on the as-compacted samples. When fv ranges from 0 to 20% (smaller than fv-cha), the fines constitute the skeleton of the sample, and the permanent deformation decreases rapidly with the increase of fv. By contrast, when fv increases up to 35–45% (larger than fv-cha), the inclusions dominate the skeleton of the sample, leading to a slight decrease of permanent deformation with fv.

International audience; This paper presents laboratory measurements of mobilized local friction along piles subjected to very large numbers of axial loading cycles. The experimental approach used here is of the physical modelling type and consists of testing an instrumented prototype pile-probe installed and loaded in specimens of saturated clay reconstituted in a calibration chamber. The procedure developed for evaluating the local friction mobilized upon monotonic loading and the further evolution of local friction during the application of displacement-controlled cycles is described. After the installation of the probe, a succession of monotonic and cyclic displacement-controlled loading phases, carried out on a reference kaolinite, is presented and analysed. During the cyclic sequence, carried out up to 105 cycles, an initial phase of friction degradation is observed, followed by a reinforcement phase, which continues until the end of the test. A coefficient of evolution is defined which allows for the quantification, during the application of the cycles, of the evolution of mobilized friction in terms of the degradation or reinforcement of friction. The evolution of the friction mobilized during the application of the cycles is interpreted in terms of the combination of excess pore water pressure generation and dissipation. A comparison is made between the maximum static shear mobilized before and after the cycles, which shows the influence of the cyclic sequence on this quantity. Elements are finally given on the repeatability of the test, showing a fairly good level of repeatability.

International audience; This paper deals with the resilient behavior of the interlayer soil which is created mainly by the interpenetration of ballast and subgrade soils. The interlayer soil studied was taken from a site in the southeast of France. Large-scale cyclic triaxial tests were carried out at three water contents (w = 4, 6 and 12 %) and three fines contents corresponding to 5, 10 % subgrade added to the natural interlayer soil and 10 % fine particles (

International audience; A radial injection experimental setup is presented in this paper, which allows one to study the transport and deposition of suspended particles and permeability impairment in porous media. This setup allows one to simulate the conditions of the injection of produced water in oil reservoirs from an injection well. This is achieved by a radial injection of water containing suspended particles of a few microns in size from a central well including a sand screen, through a confined cylindrical sand pack (diameter 52 cm, height 40 cm). A peripheral drainage system permits the recovery of the fluid at the outlet of the sand pack by a collector equipped with a turbidimeter for measuring the concentration of suspended particles. The injection pressure is measured at the inlet, and six pore pressure probes have been installed at different locations inside the sand pack. First experimental results presented show a significant heterogeneity of deposited particle profiles in both radial and axial directions.

In support of the InSight mission in which two instruments (the SEIS seismometer and the $\mbox{HP}^{3}$ heat flow probe) will interact directly with the regolith on the surface of Mars, a series of mechanical tests were conducted on three different regolith simulants to better understand the observations of the physical and mechanical parameters that will be derived from InSight. The mechanical data obtained were also compared to data on terrestrial sands. The density of the regolith strongly influences its mechanical properties, as determined from the data on terrestrial sands. The elastoplastic compression volume changes were investigated through oedometer tests that also provided estimates of possible changes in density with depth. The results of direct shear tests provided values of friction angles that were compared with that of a terrestrial sand, and an extrapolation to lower density provided a friction angle compatible with that estimated from previous observations on the surface of Mars. The importance of the contracting/dilating shear volume changes of sands on the dynamic penetration of the mole was determined, with penetration facilitated by the $\sim1.3~\mbox{Mg/m}^{3}$ density estimated at the landing site. Seismic velocities, measured by means of piezoelectric bender elements in triaxial specimens submitted to various isotropic confining stresses, show the importance of the confining stress, with lesser influence of density changes under compression. A power law relation of velocity as a function of confining stress with an exponent of 0.3 was identified from the tests, allowing an estimate of the surface seismic velocity of 150 m/s. The effect on the seismic velocity of a 10% proportion of rock in the regolith was also studied. These data will be compared with in situ data measured by InSight after landing.

Offshore wind turbines are slender structures with sensitive dynamics, strongly influenced by soil-structure interaction. The structure is subjected to cyclic and dynamic loads with frequencies close to the first natural frequency of the offshore wind turbine. To avoid resonance phenomenon due to external excitations, it is essential to precisely evaluate the initial first natural frequency of the wind turbine and its long term evolution. The present work deals with the design and analysis of a scaled model of an offshore wind turbine with monopile foundation. This study is aimed at experimental evaluation of the initial first natural frequency of this scaled model, followed by the comparison of experimental results with those obtained from the existing analytical models.

International audience; The conventional railway substructure in France was built by emplacing ballast directly on subgrade. Over years of operation, the interpenetration of ballast and subgrade created a soil layer between them. Under different conditions, this naturally formed layer, namely interlayer, can contain different quantities of fine particles, becoming more or less sensitive to changes in water content. As the water content changes are governed by the hydraulic behavior of the interlayer soil, assessing the influence of fine particle content on the hydraulic behavior of interlayer soil is of importance. To this end, the hydraulic behavior of an interlayer soil taken from Senissiat (near Lyon, France) was investigated using two infiltration columns, a large-scale column equipped with tensiometers and a time domain reflectometer (TDR) for suction and volumetric water content measurements, respectively, and a smaller column equipped with high-capacity tensiometers only. Different fines contents were considered and wetting-drying cycles were applied to the soil specimens. The hydraulic conductivity was determined by applying the instantaneous profile method. The results obtained showed that (i) hysteresis exists for both the soil water retention curve and the hydraulic conductivity changes with suction; (ii) the effect of wetting-drying cycles is insignificant; (iii) adding 10% fine particles to the natural interlayer soil gives rise to changes in the soil water retention curve but does not induce significant changes in hydraulic conductivity; (iv) the unsaturated hydraulic conductivity of interlayer soil with 10% fine particles added is close to that of soil sieved at 2 mm, suggesting that the unsaturated hydraulic conductivity of interlayer soil is mainly governed by fine particles through the suction effect. By contrast, in a saturated state, the value for the interlayer soil with 10% fine particles added was found to be higher, suggesting that in this case the hydraulic conductivity is mainly governed by the water transfer through macropores.

International audience; Two strong earthquakes occurred in the region of Chlef (north western part of Algeria) during the last century. From the geological context, there were several great masses of sandy soil ejections on the ground surface level and severe damages to civil and hydraulic structures. These damages were due to the soil liquefaction phenomenon. The objective of this paper is to study the resistance to static and cyclic liquefaction of Chlef sand through triaxial tests with focus on the effect of the initial state. For this purpose, a series of undrained monotonic and cyclic triaxial tests were carried out on saturated silty sand at two initial relative densities (Dr = 29 and 80 %). The initial confining pressure was kept at 200 kPa. Moist tamping and dry funnel Pluviation were used to prepare sand specimens. Test results indicated that the monotonic and cyclic resistance of soil mainly depends on initial conditions. Samples prepared by dry funnel Pluviation method have a greater resistance to liquefaction while those by moist tamping have been found to be potentially liquefiable. Further, liquefaction resistance increases with increase in initial relative density and confining pressure.

This paper presents an experimental study that investigates the influence of the non-plastic fines on the initiation of instability of sands under cyclic loading. The materials used for this study and the experimental device are first presented, then the results of typical liquefaction tests for both loose and medium dense specimens are presented with emphasis on the exhibited behaviour. The results of a series of cyclic undrained tests that were done with the aid of a triaxial apparatus for different percentages of fines are also presented and analysed. The results reveal that the increase in the non-plastic fines content lead to an increase in the liquefaction resistance of sands.

La plupart des éoliennes offshore sont fondées sur monopieux. L’augmentation constante de la capacité des turbines et l’insuffisance des procédures évoquées dans les documents normatifs, notamment en ce qui concerne la prise en compte des chargements cycliques, posent des défis pour leur dimensionnement. Dans le cas des sites français, la présence de sols et roches carbonatés, dont le comportement est encore mal connu aujourd’hui, rend la tâche encore plus complexe. Le projet SOLCYP+, lancé en 2017 pour une durée de 3 ans, doit permettre de disposer de nouvelles méthodologies fiables et robustes pour le dimensionnement des monopieux de grand diamètre y compris dans le contexte géotechnique du plateau continental français. Cet article présente une description détaillée du projet ANR SOLCYP+. On aborde, tout d’abord, le contexte, la problématique et les objectifs du projet puis, dans un second temps, on décrit plus en détail les quatre principales tâches scientifiques.

International audience; An experimental study has been carried out at CERMES on 1 g physical model made up of reinforced soil by a rigid inclusion. Loading tests were conducted at a prescribed rate of displacement by using a rigid disk. The experimental work focused on the effect of thickness and grain size distribution of the mattress material on the load transfer between the rigid inclusion and compressible soil. The variation of cover ratio has been also analyzed and the composition of compressible soil as well. Obtained results indicate the increase of cover ratio from 2.22 % to 8.88 % leads to 100% increase of efficiency and the settlement reduction is 50%. The variation of mean diameter of mattress material from 2.5 mm to 12 mm clearly led to an increase of efficiency equal to 90% and settlement reduction of 25%.

This paper presents a macroscopic model of cement grout flow in a porous medium, with special focus on the situation of radial flow filtration. The modeling is based on the mass balance equations of the different components present in the grouted medium, as well as on a general phenomenological filtration law. Closed-form solutions of these equations are derived in the particular situation of a linearized filtration law. This allows to evaluate the cement concentration of the grout and the porosity as functions of the radial coordinate and of time, as well as the extent of the zone actually reinforced by such a process. Besides, a numerical resolution of the model equations for more general filtration laws, is also presented.

International audience; The present Note describes an experimental study devoted to the strength of a sand reinforced by cement grouting. Through grouting, the granular medium gains a cohesion without significant change of the friction angle. The most significant experimental feature is that the cohesion is proportional to the volume fraction of cement in the grouted material. This result is interpreted within the framework of a periodic homogenization applied to yield design.; On présente une étude expérimentale consacrée aux capacités de résistance d'un sable injecté par un coulis de ciment. L'effet principal de l'injection est un gain de cohésion sans modification significative de l'angle de frottement. La principale observation expérimentale réside dans le fait que la cohésion est proportionnelle à la fraction volumique de ciment réalisée dans le matériau injecté. On interprète ce résultat dans le cadre de l'homogénéisation périodique en Calcul à la Rupture.

International audience; Damage and clogging of the granular structure of reservoirs are commonly observed during the re-injection of produced water containing solid particles and oil droplets. These phenomena can lead to a significant decrease of the reservoir permeability around the injectors and affect the injectivity of these wells. In this paper an experimental study on the transport and deposition of silica particles in a porous medium subjected to flow is presented. The study is performed on two different devices; the first one is an injection column 1 m long in which a sand pack with controlled density using the pluviation technique has been placed and the second one is an injection cell in which a cylindrical sand specimen (height 80 mm, diameter 80 mm) is put under isotropic stress. Water containing a low concentration of fine particles (diameter of few microns) is injected at a constant flow rate. The evolutions of the sample mass and of the injection pressure are recorded. The experimental results show an increase of particles mass retained inside the sample during the injection and an increase of the injection pressure attributed to a decrease of the permeability of the sample. They also show that despite the low concentration of particles of the injected fluid, a clogged area of a few centimetres forms at the column inlet. The analysis of this area by mercury porosimetry showed a significant decrease of the porosity and of the pore size and the formation of a new family of pores by the agglomerated particles. This study focuses on the impact of various parameters (diameter of the injected particles, pore size distribution of the porous medium, concentration of particles in the injected water, injection flow rate and surface roughness of the grains) on the reduction of the permeability.

International audience; The objective of this paper is to present a study on the mechanical behavior of three Algerian sands through cyclic triaxial tests with focus on the effect of the fines content. The particularity of this study is to keep the amount of the sand material matrix as a constant parameter for the entire range of fines content. This choice was considered due to the difficulty to determine experimentally the maximum and minimum void ratios of soil mixtures for fines content exceeding 15 %. The materials used in this investigation are originated from two different regions (Chlef and Boumerdes) in northern Algeria and are known for their higher seismicity levels. By varying the cyclic stress ratios and the proportion of fines contained naturally in the sand, a series of cyclic triaxial tests were carried out on reconstituted samples with a density index of 0.5 and an initial confining pressure of 100 kPa, on a servo-controlled dynamic machine with a sinusoidal frequency signal of 0.05 Hz and alternated symmetrical cycles. Comparing the clean and natural sands, the test results indicate that the presence of fines influences significantly the liquefaction resistance. Indeed, the fines content increases the liquefaction resistance for Zemmouri sand, decreases or stabilizes it for the Rass and Chlef sands, respectively. The effect of fines on the Chlef and Rass sands is in good agreement with the published literature, where the liquefaction resistance decreases to a threshold value and then increases with the increase of the fines content. This study can be used in soil classification and determination of liquefaction potential of seismic areas with smaller amounts of fines.

International audience; This paper presents the results of an experimental study aimed at evaluating the elementary filtration properties of a fine cement grout injected in a sand. In a first step, the experimental setup or filtration cell, specially developed for injecting thin samples of sand put under stress, is presented. Next, the results of an experimental programme carried out with this cell are presented, allowing for the study of the influence of basic parameters (density index, consolidation stress, cement concentration in the grout, and injection flow rate) on the filtration properties of a typical grout composed of fine cement. A filtration coefficient is then defined, allowing for characterization of the elementary filtration properties of the tested grout by the sand matrix. Finally, the respective influence of tested parameters on the value of this coefficient is presented and discussed.; On présente, dans cet article, les résultats d'une étude expérimentale destinée à étudier les propriétés de filtration élémentaires d'un coulis de ciment fin injecté dans une matrice sableuse. On présente, tout d'abord, un dispositif expérimental développé de manière spécifique pour ces travaux, appelé cellule de filtration, permettant d'injecter de fines éprouvettes de sable placées sous contrainte. On étudie ensuite, à partir de ce dispositif, l'influence relative de paramètres de base tels que l'état initial de la matrice sableuse (indice de densité et niveau de consolidation appliqué), la concentration en ciment du coulis et le débit d'injection sur les résultats obtenus en mettant en évidence l'influence significative de certains de ces paramètres. A partir des essais réalisés, on introduit un coefficient de filtration élémentaire permettant de caractériser les propriétés de filtration pour des caractéristiques d'injection données (matrice, coulis, débit d'injection). Finalement, on présente et l'on discute l'influence des paramètres étudiés dans le cadre de ce travail sur l'évolution du coefficient de filtration.

We present in this paper the results of a research concerning the mechanical behaviour of coarse grained soils. These materials are characterized by a spread grain size distribution curve. The presence of over sized particles makes difficult the mechanical characterization of these soils. We were more particularly interested in coarse-grained soils composed by rigid inclusions embedded in a fine-grained matrix (heterogenous soil). The studied material is a reference heterogenous soil composed of Fontainebleau sand and gravel. First, we propose a repeatable method of sample fabrication. Then we present some results on the influence of the gravel fraction on the behaviour and on the failure critrion of the soil under a triaxial stress path.

Resume A partir de deux modes de reconstitution bien differencies d'eprouvettes triaxiales de sable, le damage humide et la pluviation a sec, on met en evidence, sur un sable de reference, des differences de comportement mecanique associees significatives vis-a-vis du phenomene de liquefaction « statique ». Le damage humide favorise l'initiation de l'instabilite de liquefaction alors que la pluviation favorise un comportement plus stable, moins liquefiable. L'observation microscopique des materiaux sableux correspondants permet de mettre en evidence deux structures bien differenciees, a savoir, pour le damage humide, une structure irreguliere dans laquelle predominent des agregats (grains agreges) et des macropores, tres contractante et instable et, pour la pluviation, une structure plus reguliere, sans macropores, plus dilatante et plus stable. Ces observations montrent l'importance qu'il y aurait a caracteriser, a l'aide de parametres appropries, la structure initiale des materiaux sableux, directement liee a leur mode de formation (naturel ou artificiel). Pour citer cet article : N. Benahmed et al., C. R. Mecanique 332 (2004).

In this paper, an advanced experimental set-up is presented for studying the monodirectional grout injection process in sand matrices. The prototype presented is based on the use of sand pluviation for the reconstitution of vertical sand columns (80 mm in diameter and 104 cm in height), which makes it possible to obtain, within the whole column, a good homogeneity of the sand at a controlled preselected state of density. Also, the pore fluid pressure is measured at five equidistant points on the columns, which provides useful information on the injection process. The procedure used to perform a test is described in detail, and the result of a typical test carried out on a medium-size silica sand with a cement-bentonite grout is presented, showing the importance of the filtration mechanism with progressive plugging of the sand matrix for the injection process and for the mechanical properties of the grouted material. Finally, some comments are made on the influence of grain size distribution curve on the injection process. Dans cette étude, nous présentons un montage expérimental sophistiqué pour é tudier le processus d'injection monodirectionnel d'un coulis dans des matrices de sable. Le prototype présenté est basé sur l'utilisation de la pluviation sableuse pour la reconstitution de colonnes de sable verticales (de 80 mm de diamètre et 104 cm de hauteur) ce qui permet d'obtenir, dans toute la colonne, une bonne homogénéité du sable à un é tat présélectionné et contrôlé de densité. De plus, nous mesurons la pression de fluide interstitiel en cinq points équidistants sur les colonnes, ce qui donne une information utile sur le processus d'injection. La procédure utilisée pour effectuer un test est décrite en détail et nous présentons le résultat d'un test type effectué sur un sable de silice de taille moyenne avec un coulis au ciment bentonite, montrant l'importance du mécanisme de filtration, avec obturation progressive de la matrice sableuse, pour le processus d'injection et pour les propriétés mécaniques du produit cimenté. Enfin, nous faisons certaines observations sur l'influence de la courbe de distribution des dimensions de grains sur le processus d'injection.