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There are two widely accepted approaches to determine the shear strength of unsaturated soils: the effective stress approach (Bishop, 1959) and the independent stress state variables approach (Fredlund et al., 1978). The main difference between these two approaches lies in how to reduce the effect of increasing matric suction. Bishop’s coefficient χ can be calculated using (χ = tan / tan ) and vice versa. However, Bishop’s approach with the coefficient χ “of the matric suction efficiency” seems to be closer to the reality since there is a difference between decreasing ϕb and the experimental results, which have proved that the effective friction angle slightly increases with increasing matric suction. The coefficient χ decreases with increasing suction as a result of decreasing both the total area of water-solid contacts and number of particles connected by water menisci. This explains why the parameter χ decreases with increasing porosity. Thus, the maximum effect of matric suction can be reached with higher water content as a result of two opposite influences: decreasing matric suction and increasing number of particles connected by water menisci (matric suction efficiency χ).

The analysis of this problem is based on the experimental programme of the bentonite buffer material under unsaturated conditions. Pure bentonite as well as bentonite mixtures with siliceous sand and graphite were tested. Samples with extremely different porosities were prepared under compaction pressures ranging from 300 kPa to 100 MPa and then tested in the triaxial apparatus.

Research in the field of unsaturated soil mechanics for high plastic clays is very active. One area of considerable current interest is development of general constitutive models for unsaturated clay based sealing materials in a frame work that can be implemented in numerical modeling tools (Alonso . 1990, Delage and Graham 1995, Toll 1990). In particular, more quantitative information is required to define the features of yielding, failure and strain hardening for predictive modeling applications. Soil suction must be controlled and independently measured in laboratory tests. This will allow examination of behaviour along any stress path that can be expected to occur in engineering applications which will provide the necessary material information to calibrate and validate proposed constitutive frameworks.

This paper presents details of laboratory tests in a custom triaxial system with stress path automation and independently controlled and measured suction (Blatz and Graham 2000, 2003). Details describing the equipment will be given along with selected results for the yield, strength, and strain hardening behavior of a high-plastic sand-clay material at suctions from 5 MPa to 160 MPa and isotropic pressures from 1 MPa to 6 MPa. The results demonstrate the importance of independent measurement and control of suction along well controlled loading paths for interpreting the behaviour of unsaturated high plastic clays.

Investigation into the mechanical and hydro-mechanical behaviour of unsaturated sand requires special laboratory equipment for testing and preparing the specimens. In this paper, the requirements for a triaxial testing device for unsaturated sand are presented. With respect to the requirements for testing unsaturated sand (axis-translation technique), measuring small overall and pore-water volume change, and the control or measurement of small values of suction, a new triaxial device has been developed. It consists of a double-walled triaxial cell, a modular loading frame including the axial power unit, and a pc for controlling and data logging. The device enables the determination of the shear strength, the SWCC, and the hydraulic conductivity of unsaturated sand.

This paper deals with a theoretical description of the undrained and drained behaviour of unsaturated soils under isotropic load. By taking into account the compressibility of the solid mass and the fluid/gas mixture inside the pores a theoretical relation between the pore water coefficient (B) and the saturation (S) is derived, which is shown to be similar to measured data given in the literature. The approach described involves natural strains, schematizations of the solid mass and the fluid/gas mixture and the use of different boundary conditions for respectively B=1 and B<1.

The main goal of this paper is to present the first results of a study performed to provide insights into the relationship between the hydromechanical stress path experienced by a compacted soil and the modification of its pore space geometry. A new oedometer employing the axis translation technique was used to characterize the hydromechanical behaviour of the tested material. The fabric of the tested samples was determined using the mercury intrusion porosimetry technique under 4 stress levels and two different suctions. From this information, the macro and micropore volume variations were then determined. The results showed that mechanical loading produced a progressive reduction of the macropore volume and a significant increase of the micropore volume beyond a stress of 250 kPa. The obtained results tend to demonstrate that suction strengthens soil fabric, as the initial “double structure” of the tested material was not destroyed in the case of the unsaturated sample by the loading up to 1 000 kPa, whereas, the saturated sample exhibited a more homogenous fabric.

One-dimensional consolidation tests are often performed without accurate control of the initial degree of saturation, since most commercially used testing apparatuses do not permit back-pressuring. As a result, the initial degree of saturation is often less than unity and, thus, the measured ‘Apparent’ Compression Index can be very different than the value corresponding to a fully saturated sample. Similar differences are caused by most oedometer tests being performed up to a maximum stress lower than the maximum pre-consolidation pressure. Oedometer tests on undisturbed samples, on initially unsaturated samples of reconstituted soil, and a data-base of commercial oedometer tests are presented in order to exhibit the effect of high maximum preconsolidation pressure, cementation and initial unsaturation. Finally, expressions for the volumetric deformation of unsaturated soils proposed by Alonso et al. (1990) are used to explain different trends exhibited by the ‘Apparent’ Compression Index for low- and high-plasticity soils.

Since 1974, Belgium investigates the design for disposal of its High Level Radioactive Waste (HLW) in a deep clay formation, the “Boom Clay”. Although the clay formation is the main (natural) barrier against the transport of the radionuclides towards the biosphere, the design also involves several engineered barriers (multi-barrier principle). In the design developed in the late 1980’s, a non-saturated bentonite based material was chosen as part of this barrier system. Prior to demonstrating this design in conditions, a surface mock-up test has been operated between 1997 and 2002. This test served as a preliminary test on the performance of several components of the system, such as bentonite based backfill blocks and instrumentation. With clearly defined heating and hydration conditions, it gave us the opportunity to perform a large scale simulation of the hydration/saturation of the backfill at controlled conditions. After describing the general disposal design and the experimental set-up, this paper will detail the measurements and observations obtained during operating and dismantling the mock-up. To support the interpretation of these measurements and observations, a modelling of the experimental set-up is being performed. We further detail the characterisation programme carried out to obtain the input data for the modelling. Finally, lessons learned for the development of the design for the HLW disposal will be drawn.

This paper presents the experimental results from several conventional oedometer tests conducted on compacted and unsaturated soils. Soil samples of medium plasticity, but with different grain size distribution and degree of saturation were tested. Some of these tests were interrupted before consolidation was achieved, and the water content of the soil was measured both before and after each test. The results point out that the recorded time-settlement curves consist of two distinct stages that are characterized by a different settlement rate. The early stage should essentially be ascribed to the faster dissipation of air pressure, while the final stage should be caused by the gradual expulsion of water from the soil. The results are also analysed using Terzaghi’s theory [1] in order to ascertain whether this theory is suited to simulating the time-histories of settlement recorded during the tests.

Volume change as a result of drying is often neglected in soil mechanics and soil hydrology, despite the important influence it has in the change of mechanical stability and water flow. Therefore, processes which lead to volume change have to be understood. Tensile stresses as the main parameter for shrinkage are a result of hydraulic and mechanical mechanisms in unsaturated soils or soil substrates. Both mechanisms have to be recognised as dependent processes. Unsaturated soils are defined as 3-phase systems. Capillary forces in soil pores act as contractive forces of the liquid phase on the solid phase. The resulting tensile stress caused by water increases with decreasing degree of water saturation. This causes shrinkage in a given soil volume, including soils with small plasticity. Mechanical stress parameters will simultaneously be changed with shrinkage, which as a result also change the hydrological parameters altering the pore system. The separation of the mechanical from the hydraulic stressis difficult. Therefore, a method was developed, which allows the determination of tensile stress under defined boundary conditions and is based on the general stress equation. Also a method is described by which this information is used for general modeling of volume change by hydraulic stress and general empirical functions used in hydraulic modeling.

The paper presents experimental results linking matric suction and tensile strength of compacted clays. Test results from a cohesive soil are presented and discussed with respect to the soil structure and the interaction of soil and water. It is assumed that two main groups of pores can be clearly identified in compacted clays; the pores between aggregates (interaggregate pores) and pores between particles (intraaggregate pores ). Based on a description of soil-water-interaction an expected behaviour, describing tensile strength as a function of matric suction, is derived and compared with the experimental results. The laboratory test results indicate that there is a strong correlation between the pore size distribution (assessed by interpretation of the soil water characteristic curve SWCC) and the tensile strength of compacted soils. Furthermore, the test results are compared by using micro-mechanical considerations of the interaction between the skeleton of unsaturated soils (interparticle contact force) and by using numerical calculations with an elastic relationship.