Identification of a Cam Clay Model through Shear-Box and Oedometer Tests. Application to Lateritical Soils from Senegal (West Africa)

The main objective of this research is to study the mechanical behaviour of tropical soils using elasto-plastic constitutive equations in the so-called limit and critical states. Indeed, researchers of the Cambridge University had noticed that during their various experiments, the rate of volumetric deformation ( ) of the sample tending to zero every time the rupture of the specimen is reached during a test performed on a clay specimen Roscoe et al., 1958. To better understand and clarify this mechanical behaviour, a description has been proposed in the (e, p, q) representation that means void ratio, volumetric stress (spherical pressure) and deviatoric stress. This frame of theoretical study and apprehension is called: the theory of the Critical State. One of the major problems met at the time of our present research is the non-availability of triaxial apparatus allowing us to achieve some tests on tropical soils (samples from Senegal in West Africa) and to describe the behaviour of these materials easily like the researchers of the university of Cambridge in the theory of the critical state. To by-pass this difficulty, we decided to consider two very classical and simple mechanical tests: shear-box and the oedometer test as well as the interrelationship of the results given by the tests and some theoretical calculations. This is a way to identify an elasto-plastic model (the modified Cam Clay model) without any triaxial experiment. Indeed it supposes the model to be suitable to describe the mechanical behaviour of the considered clays.

Studying the stress-suction coupling in soils using an oedometer equipped with a high capacity tensiometer

In the context of research into deep nuclear waste disposal,various works have concerned the hydromechanical behavior of Boom clay,a stiff plastic clay extracted in the SCK-CEN Underground Research Laboratory near the Mol City(Belgium),at a depth of 223 m.Due to some amount of smectite minerals in the clay fraction,Boom clay exhibits swelling properties when hydrated under low stresses.To investigate some aspects of the hydromechanical behavior of Boom clay,oedometer compression tests were carried out on samples of Boom clay close to saturation and submitted to an initial suction.During oedometer compression,the changes in suction with increased vertical stress are monitored by means of a high capacity tensiometer installed at the bottom of the sample.Some aspects related to hydromechanical couplings are examined through the investigation of the changes in suction during oedometer compression,a somewhat delicate and poorly documented experimental approach.A comparison is also made with a completely different soil sample under suction,i.e.a statically compacted unsaturated low plasticity silt.Some technical difficulties typical of this new experimental approach are first described in detail so as to optimize the interpretation of the data obtained.The experiment allows the determination of the point at which suction is changed to positive pressure during compression.Below this point,the ratio between the vertical stress and the change in suction are determined.Above this point,the data show that positive pore pressures are dissipated in a common way.The suction/stress behavior during unloading is also described and discussed.Finally,an interpretation in terms of microstructure effects is provided for both samples.The experimental approach initiated here seems to provide interesting further application to better understand hydromechanical couplings in natural soils in relation with suction increase during stress release.

Effect of Sample Disturbance on Unconfined Compression Strength of Natural Marine Clays

Quantitatively correcting the unconfined compressive strength for sample disturbance is an important research project in the practice of ocean engineering and geotechnical engineering. In this study, the specimens of undisturbed natural marine clay obtained from the same depth at the same site were deliberately disturbed to different levels. Then, the specimens with different extents of sample disturbance were trimmed for both oedometer tests and unconfined compression tests. The degree of sample disturbance SD is obtained from the oedometer test data. The relationship between the unconfined compressive strength q u and SD is studied for investigating the effect of sample disturbance on q u. It is found that the value of q u decreases linearly with the increase in SD. Then, a simple method of correcting q u for sample disturbance is proposed. Its validity is also verified through analysis of the existing published data.

Feasibility of using electrokinetics and nanomaterials to stabilize and improve collapsible soils

Loess as a subcategory of collapsible soils is a well-known aeolian deposit generally characterized as a highly-porous medium with relatively low natural density and water content and a high percentage of fine-grained particles.Such collapsible soil sustains large stresses under a dry condition with natural water content.However,it can experience high and relatively sudden decreases in its volume once it reaches a certain water content under a certain load and therefore,the natural condition of the soil might not be suitable for construction if the possibility of the exposure of the soil to excessive water exists during the lifetime of the project.This research presents the utilization of an innovative method for stabilization and improvement of Gorgan loessial soil.This method uses electrokinetics and nanomaterials to instigate additives to move through soil pores,as an in situ remedial measure.To assess the acceptability of this measure,the deformability and strength characteristics of the improved collapsible soil are measured and compared with those of the unimproved soil,implementing several unsaturated oedometer tests under constant vertical stress and varying matric suction.The result emphasizes the importance of the matric suction on the behavior of both improved and unimproved soils.The test results indicate that the resistance of the soil was highly dependent on the water content and matric suction of the soil.The oedometer tests on samples improved by 3%lime and 5%nanomaterials show considerable improvement of the collapse potential.Results also reveal that stabilized samples experience notably lower volume decrease under the same applied stresses.

Initial Stable State of Ocean Floor Deposits

Extensive oedometer tests and physical tests have been conducted on remolded and reconstituted marine soils with different initial water contents and liquid limits. The oedometer test data can be well fitted with a straight line in the bilogarithmic ln(1+ e ) ～ lg p plot. The initial effective stress corresponding to the initial void ratio is determined by extrapolation of the bilogarithmic compression line. This new way of extrapolating oedometer test data to the initial void ratio overcomes the difficulty of measuring the residual effective stress of soils in the remolded state. The initial stable compression line of ocean floor deposits under different deposition environments corresponds to the compression line at the sensitivity equal to one. This initial stable compression line obtained in this study is consistent with the available compression line at the sensitivity equal to one which is proposed based on the experimental data of remolded undrained strength and the theoretical concept of Cam clay critical state line.

Experimental Study on Volume Change Indices of Bentonite Soils

This paper reports experimental results regarding statically compacted clay specimens to study the volume change behavior of bentonites. The volume change indices such as the coefficients of compressibility, volume compressibility, and consolidation （ i. e. av, mv. and cv respectively） and the saturated coefficient of permeability k at different surcharge pressures were determined with the commonly adopted procedures. The swell potentials, swelling pressures, different phases of the swollen specimens were analyzed for the volume change behavior during compression. Experimental results revealed that the swell potential is dependent on the initial dry density, the initial water content and the vertical pressure at which the clay specimens were allowed to swell. The swelling pressure was found to be similar for the specimens with varying water content, showing strong dependency on the initial void ratio. The compression indices （ viz. mv and av） of saturated specimens decreased with an increase in the vertical pressure. About 80% to 90% of the volume change occurred in the primary compression phase under any given vertical pressure. The coefficient of consolidation cr. and the saturated coefficient of permeability k decreased with an increase in the vertical pressure.

Intrinsic Compression Behavior of Remolded and Reconstituted Clays-Reappraisal

Evaluating the impacts of soil structure on mechanical behavior for natural sedimentary clays is an important issue in geotechnical engineering. Burland introduced void index for normalizing the compression curves of various remolded and reconstituted clays to obtain the intrinsic compression line, which provides a reference framework to assess the in-situ compression behavior. However, it does not quantitatively account for the effects of initial water content on compressive behavior of remolded and reconstituted clays and the initial water contents of clays are not always limited to 1.0 - 1.5 times the liquid limits defined by Burland. A modification based on collected tests data was presented on the expressions of ?and ?defined by Burland. Extensive oedometer test data were also collected on various remolded and reconstituted soils with distinct liquid limits and initial water contents to verify the validity of modified expressions. A normalized compression line deduced by intrinsic compression line is proposed in the e-log p plot, which can be used to evaluate the effects of soil structure quantitatively on the intact compressive behavior for natural sedimentary clays.

Hydromechanical Behavior of Low-Swelling Soils Compacted at Low Water Content: Laboratory Study

Fine unsaturated soils are used in many applications, particularly in road infrastructure and in construction. These materials undergo deformations according to the stresses to which they are subjected. The purpose of this paper is to study the influence of hydromechanical stresses on the behavior of low swelling soils compacted at low water content in accordance with the French standard GTR 92 (Guide des Terrassements Routiers). Then, various experimental tests on an oedometer were carried out in the laboratory. Two types of low swelling soil sampled in Nasso on the outskirts of the town of Bobo Dioulasso (Burkina Faso) were used. After shuffling, each sample was moistened to its optimum water content and then compacted to 90% and 95% of its optimum density. Behavior tests show that these soils deform very little when subjected to hydromechanical stresses. However, these deformations are swelling in nature for low mechanical stresses and when the stresses are high, they tend to collapse. When these soils are subjected to a vertical stress of 420 kPa, the primary consolidation time is of the order of one minute for NH2 (a silty soil) and about ten minutes for NH3 (a silty-clayed soil).

Investigation of the hydro-mechanical behaviour of compacted expansive clay

The hydro-mechanical behaviour of compacted expansive Romainville clay was investigated.The soil was air-dried,crushed,and passed through a 2 mm sieve before being statically compacted to a dry density of 1.35 Mg/m3.The mechanical behaviour was investigated by tests in oedometer with controlled suction using the vapor equilibrium technique(suction s=0,9,39,and 110 MPa).The vertical stress was applied in the range of 0-800 kPa.The experimental results are shown as follows:1)wetting-induced swelling was higher at lower vertical stresses;2)the vertical stress under which no swelling occurred during water flooding was estimated at 60 kPa,which can be considered as the swelling pressure of the soil tested;3)the soil compressibility(changes of volume upon stress increases)was strongly influenced by the soil suction:the lower the suction,the higher the compressibility.The hydraulic behaviour was investigated using a large-scale infiltration chamber(800mm×1000 mm in section and 1000mm high).The large size of the soil column allowed bttrying the volumetric water content sensors(ThetaProbe)without significantly affecting the water transfer and the soil swelling during infiltration.The soil suction was monitored along the soil height(every 100 mm)using various relative humidity sensors and psychrometers.In the infiltration test,water was kept on the soil surface and changes in suction and volumetric water content were monitored for 338 d.The wetting front has reached the bottom of the soil column at the end of the test.The data from the simultaneous monitoring of suction and water content were used to determine the water retention curve and the unsaturated hydraulic conductivity using the instantaneous profile method.It has been observed that the soil water retention curve depends on the soil depth;that is to be related to the soil depthdependent swelling.The unsaturated hydraulic conductiv-ity was found to be quite low,comprised between 3×10-11m/s(at saturated state)and 10-14m/s(at about 100 MPa suction).