Jan Boháč

Microstructures and physical properties of “backfill” clays: comparison of residual and sedimentary montmorillonite clays

Hydrophysical behaviour of a backfill material produced as a mixture of montmorillonite-rich clays (fine-grained fraction) and ballast material (coarse-grained fraction) is studied. Two different genetic types of clays are evaluated—bentonite, representing the residual weathering type, and sedimentary montmorillonite-rich clay. Hydrophysical properties (liquid and plastic limits, permeability and swelling pressure) were determined for mixtures of mixing ratios from 0% to 100% by weight. The microstructural study focused mainly on pore space and specific surface determination that were employed for the interpretation of the hydrophysical properties. The most favourable results were obtained for activated Ca-bentonite but sedimentary montmorillonite-rich clays yielded good quality backfilling mixtures as well. The transition from fine to coarse-fraction dominated behaviour was established for mixtures containing about 20% of clayey admixture.

Mechanisms controlling the behaviour of double-porosity clay fills; in situ and centrifuge study

Abstract Two instrumented embankments built on a double-porosity landfill of clayey lumps were monitored in situ and modelled in a geotechnical drum centrifuge. Field measurements of hydrostatic levelling, depth reference points and pore pressure gauges showed high and variable compressibility of the landfills. Introductory modelling in a mini-centrifuge, combined with oedometer testing, demonstrated the key mechanisms in the double-porosity fills: irreversible deformation at low stress ranges as a result of rearrangement of the lumps, and reversible deformation (swelling) at higher stresses, similar to the behaviour of reconstituted material. Placing fill under water resulted in high initial void ratios followed by large deformations while loading. Dry filling followed by saturation may be recommended for further development of the landfills in future. Data from the geotechnical centrifuge models confirmed that permeability was controlled by the complex structure of the clay fills. Similarly to the field measurements, there were significant initial settlements on loading as a result of compression of open macro-voids. The double-porosity structure in the fresh fill allowed excess pore pressures to dissipate quickly, which accelerated the consolidation process initially. Thereafter, dissipation was controlled by the permeability of the intragranular pores in the clay, once the intergranular pores had closed.

A simple method for air volume change measurement in triaxial tests

The paper presents a simple and inexpensive device to measure the volume change of the pore-air of dry soil specimens in the drained triaxial tests under atmospheric conditions. The principle of the method was described by Bishop and Henkel (Bishop and Henkel 1962). The modified apparatus makes use of low-cost photoelectric sensors and computer control. The applicability of the method is demonstrated by a comparison with test data from conventional triaxial tests on water saturated specimens.

Compression of collapsed loess: Studies on bonded and unbonded soils

Abstract Compression of bonded (collapsible) and debonded (collapsed) loess is studied in the laboratory using reconstituted samples of Sedlec loess. Oedometric and hydrostatic (triaxial) compression curves yield identical compression indices. Intrinsic (ICL) and semilogarithmic (LCL) compression lines are favourably compared with the test results of reconstituted samples. Owing to the aging of reconstituted samples, the bonded structure that develops seems to be qualitatively identical to that of natural collapsible loess. In the course of loading, both natural and aged samples undergo gradual destruction of the structural bonds. The only difference is in the initial porosity, which in the case of natural samples is much higher (these are metastable). Reconstituted samples, even if aged, and consequently bonded, cannot, therefore, be collapsible.

Membrane Penetration in Triaxial Tests

Volumetric deformations of triaxial specimens of sand measured with a burette are burdened with an error caused by a rubber membrane penetrating into the peripheral voids of the specimen. This effect is of the same order of magnitude as volumetric deformation of the soil skeleton and should be eliminated. To this end, a method has been proposed based on the K0-stress path. Membrane penetration for the particular material has been approximated by logarithmic and hyperbolic curves. The resulting correction considerably surpasses most published expressions because the latter usually do not consider the porosity of specimens.

Suction and Collapse of Lumpy Spoilheaps in Northwestern Bohemia

Large spoilheaps of overburden from open-cast coal mining in Northwestern Bohemia offer a space for development. However, they exhibit unfavourable features such as collapse potential caused by the lumpy structure with high overall void ratios. Suction oscillations contribute to the structure degradation by the intervoids closure. The paper presents in-situ suction monitoring and laboratory investigation of the collapse potential of spoilheaps.

Experimental Study of Small Strain Stiffness of Unsaturated Silty Clay

The small strain stiffness of saturated soils has been extensively studied in the past decades, and therefore the literature database of small strain stiffness of different soils is vast and contains the necessary information for development and calibration of numerical models. However, there is a lack of data of small strain stiffness in case of unsaturated soils. This paper presents the stiffness of unsaturated silty clay in both small and very small strain regions.

SHEAR STRENGTH OF SOILS FROM THE DOBKOVICKY LANDSLIDE IN THE CENTRAL BOHEMIAN UPLANDS DETERMINATED BY LABORATORY TESTS

The aim of this paper is to evaluate soil shear strength from the Dobkovicky landslide. The landslide was activated on June 6, 2013 after heavy rains and the D8 motorway, which was under construction at the time, was damaged. The laboratory tests were carried out on two types of soils, clay and tuff, both from the surface of the rupture. Critical and residual friction angles were evaluated on both types of soils.