The Drained Strength of Bentonite Enhanced Sand

INTRODUCTION Barriers with a low hydraulic conductivity are used as part of waste containment systems to prevent groundwater contamination by liquids from the waste. Commonly barriers are either a geomembrane (usually an HDPE sheet), a mineral layer or a combination of the two. Recently there has been increasing interest in the use of bentonite±sand mixtures as the mineral layer, in both land®ll liners and vertical cut-off walls, partly because they are less susceptible to frost damage and desiccation cracking than compacted clay (Dixon et al., 1985; Kraus et al., 1997). Currently there is uncertainty about the strength and bearing capacity of these materials. This note reports drained strength data for bentonite±sand mixtures and proposes that trends in these data are mainly the result of variations in the relative density of the sand

The strength of unsaturated bentonite-enhanced sand

A modification to Rowe’s stress-dilatancy equation is presented that extends its range of application to include unsaturated soil behaviour. The results of a programme of constant water content triaxial tests on unsaturated bentonite-enhanced sand (BES) are reported, together with those of a programme of saturated drained triaxial tests on the sand. It is shown that the variation in the rate of dilation at failure with the sand relative density is similar for the two materials. It is proposed that the packing and friction angle of the sand particles and the degree of saturation control the shear strength of unsaturated BES containing modest amounts of bentonite, and that the shear strength of the bentonite component can be ignored

The interpretation of CPT data from hydraulically placed pfa

Cone penetration test (CPT) results on an ~20 year old, hydraulically placed pulverised fuel ash (PFA) deposit are reported, along with the results of oedometer compression tests on ‘undisturbed’ specimens recovered during the site investigation and resedimented PFA samples. The latter showed that aged PFA can undergo significant secondary compression. The correlation between volume compressibility and cone resistance, mv=1/(αqc), is fitted to compressibility data from undisturbed samples to determine α. The resulting CPT compressibility profile shows good agreement with compressibility trends for aged PFA estimated from the tests on resedimented ash. It is therefore recommended that a value of α=11 should be used for normally consolidated, aged PFA. The danger of biased sampling in very loose non-cohesive materials and the need for depth profiling by in situ measurement are highlighted

Behaviour of a trial embankment on hydraulically placed pfa

The paper describes the performance of a 5.3-m trial embankment constructed on approximately 45 m of hydraulically placed pulverised fuel ash (pfa). It is planned to redevelop the 17-ha lagoon containing the pfa as a landfill. There is little variation in the particle size distribution of the uniformly graded silt sized pfa over the lagoon. However, the density of the pfa varies with depth with loose material underlying a denser surface layer, in a pattern that probably results from the water level in the lagoon during pfa deposition. Settlement under the trial embankment was apparently largely complete by the end of the construction period (17 days), with approximately 300 mm of settlement under the crest of the embankment. The embankment settlement is significantly affected by compression of the loose layers within the deposit. Analysis of the problem using the conventional one-dimensional settlement method, and an mv profile determined by CPT calibrated against laboratory tests gave a reasonable prediction of the embankment crest settlement

The factors controlling the engineering properties of bentonite-enhanced sand

This paper considers the engineering behaviour of bentonite-enhanced sand (BES) mixtures in relation to their performance as environmental barriers. Data on the swelling and hydraulic conductivity are presented. At low effective stresses the bentonite within BES mixtures swells sufficiently to separate the sand particles. In such states two factors affect the void ratio reached by the bentonite after swelling: the ionic concentration of the pore solution and the bentonite fabric after compaction. Bentonite swelling is very sensitive to the pore solution concentration because increasing concentration suppresses the diffuse double layer component of swelling. Remoulding during compaction can result in a slight reduction in bentonite swelling, probably because of disruption to the cluster-based fabric of bentonite. At high effective stresses the bentonite has insufficient swelling capacity to force the sand particles apart, and the sand pore volume thus limits swelling. A model to predict the swelling and hydraulic conductivity of BES in distilled water and various salt solutions is described. This model requires the swelling behaviour and hydraulic conductivity of the bentonite in the relevant solution, and the compressibility and porosity of the sand component as input parameters. Soil tortuosity is used as a fitting parameter, and is estimated from Archie’s equation. Application of this model to the swelling of compacted mixtures is shown to produce a good fit with the experimental data