Jubert Pineda

Shear Wave Measurements Using Bender Elements in Argillaceous Rocks

The bender element technique for the measurement of the small strain shear stiffness of soils is here extended to measurements in very stiff argillaceous rocks that will be subject to artificially induced weathering. Moduli between 3 and 17 GPa are thus determined in natural samples of two different intact materials: Lilla claystone and Opalinus clay. It is explained how practical difficulties related to bender insertion and high frequency testing were overcome. Theoretical difficulties, particularly those related with possible near field noise, are examined in detail. Travel time is established by the first break technique by jointly examining the output from several input signals of different characteristics. The error associated with the technique is bounded by making similar measurements in several dummy samples of materials (aluminum and Lucite) whose well-known elastic properties lie in a similar range to that of the tested geomaterials. The effect of suction on the small strain stiffness of the homogenous Lilla clay samples is shown to be similar to that previously observed—at a lower suction range—in unsaturated compacted soils.

Residual Strength of Clays at High Suctions

This paper presents a study on the residual shear strength of a plastic clay (Boom clay) under high suction. The device, an adaptation of Bromhead ring shear apparatus to allow for suction control during shearing, is first described and the experimental procedure detailed. Then, results of shear strength measured under a suction equal to 70 MPa, several vertical stresses and different shearing history (first-shearing and shearing on pre-sheared samples) are presented and discussed. They evidence a huge increase in the residual friction angle whose value is

Degradation of marls; two case studies from the Iberian Peninsula

15^ˆ

Effects of pore fluid salinity on the shear strength of a soft clay

greater than that measured in saturated conditions and a highly dilatant response during first-shearing. ESEM micrographs performed on samples sheared in saturated and unsaturated conditions suggest that such a response can be explained by the character more granular of the dry material as a result of particle aggregation. Comparison with results obtained by \cite{JV:17} on a low-plastic clay indicate that this effect is enhanced by the plasticity of the clay.

A New High-Pressure Triaxial Apparatus for Inducing and Tracking Hydro-Mechanical Degradation of Clayey Rocks

Abstract The strength degradation of two marl formations (Pamplona marls in Spain and Abadia marls in Portugal) is analysed in the paper. In the case of Pamplona marls, a number of large-scale direct shear tests on natural and saturated specimens of the undisturbed material were performed. Tests on marl–concrete interfaces were performed and are also presented. Both cases analysed show that the saturation of the marl leads to a strong reduction of friction and effective cohesion. Negative effects of suction changes on the mechanical behaviour of the marls were investigated in more detail in suction controlled tests on Abadia marls, a more plastic material. A numerical simulation of wetting effects on a marl fragment of finite size helps to identify the nature of degradation mechanisms, which are associated with the development of tensile surfaces inside the marl matrix. They, in turn, are due to the expansive behaviour of the clay fraction of the marl relative to suction changes.

Ageing Effects on the Stiffness Behaviour of a Microbiologically Treated and Compacted Soil

The paper presents the results from a preliminary study focused on the influence of pore fluid salinity on the shear strength of a soft clay from New South Wales (Australia). Direct shear tests (DST) were carried out on natural specimens using pore fluids with different salt concentrations. Electrical conductivity measurements were performed on the squeezed pore water after shearing to quantify changes in salinity of the soil pore water. Strength parameters showed a clear dependency of the pore fluid. Changes in c’ and ’ appear to be controlled by two factors: (i) initial salinity, and (ii) extent of the concentration gradient. Results from mercury intrusion porosimetry tests demonstrated the influence of changes in pore water salinity on the evolution of the dominant pore size. It was in agreement with macroscopic observations. Implications for further laboratory testing are highlighted in the paper.

Numerical Implementation of a Critical State Model for Soft Rocks

This paper presents the development of a new high-pressure triaxial apparatus specifically prepared for inducing and tracking the degradation of clayey rocks. Total suction—used to induce the hydraulic degradation—is applied with vapor transfer technique by controlling the relative humidity of air in contact with the material. The evolution of the degradation process along different hydro-mechanical stress paths is continuously tracked with bender elements, as well as with air or water permeability measurements on partially saturated or saturated states, respectively. Relevant test results on a low porosity clayey rock (Lilla claystone, Spain) are presented to evaluate the main capabilities of the new equipment. The results bring up the high sensitivity to water of the material, which is evidenced by the important reduction of shear wave velocity induced on first wetting and drying at low confining stress, as well as by the significant increase in the air permeability of the degraded material (around four orders of magnitude larger than the intact material). Test results also showed clear differences in the volume change and shear strength behavior of undisturbed, saturated, and degraded samples, highlighting the relevance of degradation on macroscopic behavioral features.

Getting high-quality samples in ‘sensitive’ soils for advanced laboratory tests

The paper describes an experimental study aimed at evaluating the evolution of some mechanical properties, specifically small-strain stiffness and unconfined compressive strength, induced by ageing after a microbiological treatment on a compacted soil. Bacillaceae microorganisms were used to induce microbial calcium carbonate precipitation and improve the mechanical properties of a compacted clayey silt. After compaction, samples were left to age and soil stiffness tracked every 24 hours using shear wave velocity measurements (bender elements). Parallel tests were run to determine the unconfined compressive strength at different elapsed times (just after compaction, 2, 4 and 7 days).

Characterisation of Ballina clay

This paper details the basic tasks for the numerical implementation of a simple elasto-plastic critical state model for bonded materials (i.e. soft rocks-hard soils) into the finite element program SNAC developed at the University of Newcastle in Australia. The first task described focusses on the derivation of the incremental constitutive relationships used to represent the mechanical response of a bonded/cemented material under saturated conditions. The second task presents how these stress-strain relations can be numerically integrated using an explicit substepping scheme with automatic error control. The third task concentrates on the verification of the substepping algorithm proposed. The model used to represent the saturated mechanical response of a bonded material combines the modified Cam clay with the constitutive relationships for cemented materials proposed in Gens & Nova (1993), but incorporates some flexibility on the degradation law adopted. The role of suction and other relevant aspects of unsaturated behaviour are also discussed at the end of the paper.

Environmental degradation of claystones

Laboratory tests are well recognized as highly appropriate for defining the engineering properties of geomaterials, in terms of constitutive law parameters for modeling geotechnical engineering problems. The strong development of advanced techniques, both in equipment and in data interpretation, has increased the confidence in laboratory testing, while on the other hand the limitations due to the quality of soil sampling with depth and the spatial representativeness of the samples are less consensual. Still, the development of new methods for assuring high-quality samples is increasing, together with sampling quality assessment by non-destructive methods using vibration wave velocities. Interpretation methods of in situ tests for ground characterization have also evolved significantly, increasing the reliability of these methods. Their versatility to cover large areas on site and the fact that these tests are, in principle, performed at the actual state (physical and stress) conditions, as well as the improvements in the correlations between field tests and hydraulic and geomechanical parameters, allow joining the quality of data and theoretical approaches, namely through critical state soil mechanics. This keynote paper discusses some of the aspects that can and should enable the association of ground characterization from laboratory testing over undisturbed samples used in more or less advanced tests, enhancing the determinant conditioning factor, that is, the sampling technique to get representative specimens and the way this is assessed. The confidence that we expect to have on the geomechanical parameters that we need for our geotechnical activities will mostly depend on this in view of the high uncertainties of the parametrical correlations with in situ test data, therefore, important in ground characterization. This is especially relevant in sensitive soils, such as soft fine soils, loose sandy soils, or young residuals soils. These have or can have “weak” equilibria of the interparticle micro- and macrostructures (or their arrangement, fabric) that will change substantially their properties if samples are collected and conditioned with processes that do not preserve that intrinsic “ADN”. The change in these natural conditions can be evaluated by techniques of quality assessment, which will be discussed in what follows.