William Van Impe

Advances in geosynthetic clay liners: polymer enhanced clays

Bentonite clay is widely used in Geosynthetic Clay Liners (GCLs), because of its elevated sealing capacity in presence of water. However, exposure to high concentrated inorganic solutions can change the clay fabric increasing its hydraulic conductivity. To improve the hydraulic performance of GCLs, a chemical-resistant clay (HYPER clay) was developed through treatment of a natural bentonite with an anionic polymer, Sodium Carboxymethyl Cellulose (Na-CMC). The swelling ability, water adsorption capacity and hydraulic conductivity of the HYPER clay were evaluated. Test results showed the beneficial effect of the anionic-polymer treatment on the swelling, water adsorption and hydraulic performance of the clay.

Hydraulic conductivity of a dense prehydrated GCL subjected to partial desiccation

A dense prehydrated GCL (DPH) was subjected to partial but increasingly severe drying cycles alternated with permeation with a calcium solution. The permeability remained low for desiccation water contents higher than the manufacturing (prehydration) water content. Increasingly long permeation was however necessary to recover low permeability. The permeability increased irreversibly for desiccation water content lower than that of prehydration. Test results indicate the permeability of DPH GCLs subjected to dry/wet cycles coupled with cation exchange strongly depends on the extent of desiccation.

Considerations on the Stiffness of Sensitive Soft Soils

The present paper comments on the evaluation of the initial stiffness of sensitive soft soils as an outcome of several testing methods: Resonant Column (RC), Bender Element (BE) tests, Seismic cone tests and Empirical relationships (correlations). The RC, BE and common geotechnical laboratory tests were carried out on soft clays adjacent to the Tunis Lake in Tunisia. The database was completed making use of existing studies carried out on other types of Swedish soft clays from the literature related to seismic cone tests. A number of empirical correlations for determining the initial value of the shear modulus of soils in the literature was adopted in order to re-analyze the available data set. The authors suggested an adapted empirical lower bound correlation aiming at evaluating the analyzed soft soils’ small strain shear modulus (Gmax), probably even useful for less undisturbed sampling. The proposed equation aims at giving an indication of the effect of, for example, the mean effective stress (p′) and the corresponding void ratio (e) on the initial soil stiffness more clearly.

Crushability on granular materials by strain controlled oedometer testing at high stress levels

The crushability of artificial and natural granular materials has been analyzed using one-dimensional strain-controlled compression tests and standard oedometer tests at high stress levels to study the effect on the breakage of a number of parameters such as: grain mineralogy, grain size, uniformity coefficient, strain rate and loading time. Measuring the breakage factor at different stages of compression, it was demonstrated that the crushing started before reaching the point of maximum curvature (yield point) in the compressibility curve. Beyond yielding the crushing increased abruptly. Among the natural materials analyzed, the lowest yield stress value occurs in the carbonate sand, and the highest in the silica sand. We found that the breakage increases with increasing size of the grains. Coarse uniform samples showed higher breakage than uniform fine samples and well-graded samples. The breakage increased with decreasing strain rate or increasing load application time. For natural soils it is difficult to study separately the influence of isolated parameters on the breakage. The study of homogeneous artificial materials facilitated this analysis.

Stress-strain behavior of artificially cemented Kaolin clay

In this paper, the strength and compressibility of a reference material such as Kaolin clay after treatment with binders was studied with the aim of identifying key behavior features and differences with respect to noncemented Kaolin clay. The water content of the soil was fixed at a high value to represent a very soft consistency. Portland cement was used as binder at dosages varying from 5% to 20%. A number of samples were prepared in the laboratory and were allowed to cure under controlled conditions. The shear and compression behavior of natural and cement-treated kaolin clay samples was assessed by triaxial compression testing and oedometer tests. The results demonstrated that cemented samples show initially much higher stiffness and strength than noncemented samples. But as the stress level increases a yielding state is encountered where interparticle bonding begins to break intensively. Before yielding (at low stresses), the strength is governed by the cement dosage and the one-dimensional compression is almost negligible. Beyond yielding (at high stresses) the strength is governed by the stress level just like for any frictional material. Under onedimensional compression, a clear collapse is observed; the compression lines tend towards the compression line of the noncemented clay with a gradient that lightly steepens with increasing cement dosage.

Determination of consolidation parameters of dredging and industrial waste sludge.

The paper presents the laboratory setup and some preliminary results of large strain consolidation testing on several types of dredging and industrial waste sludge. The tests are performed in the framework of a larger research project – funded within the Flemish Environmental Technology Platform (MIP) - investigating the impact of additives, specifically coagulants and flocculants, on the consolidation behaviour of mineral sludges.. Large strain consolidation parameters are determined using an adapted version of the well-known seepage consolidation test setup (Imai 1979). Interpretation of the test results is based on an iterative calculation using large strain consolidation theory as presented by Abu-Heyleh (1992). This procedure should allow an optimisation of type, concentration and application method of additives for a specific material.