The effect of organic fibres on the undrained shear strength of soil

Abstract

Accurate determination of soil’s undrained shear strength is essential to make informed decisions regarding construction or maintenance problems. Reduced scale modelling can achieve accurate predictions of in-situ field conditions without intensive field investigations. Geotechnical centrifuge modelling replicates in-situ conditions by accelerating a reduced 1/n scale model to a multiple n of earth’s gravity, thereby replicating the stress level of a 1:1 prototype. In this study bearing capacity testing and penetrometer profiling were carried out on normally and over consolidated kaolin clay models, with various fibre contents. Construction of fibrous models in the drum centrifuge ensured a homogenous soil and a random orientation of fibres during the consolidation process. Bearing capacity testing was carried out in flight with different size and shaped foundations. The undrained shear strength was back calculated from the foundation bearing capacity at two locations; at maximum resistance and at the resistance corresponding to depths of 0.5 times the width (B) or diameter (D) of the foundation; cone and piezoball penetrometers were also conducted in flight. Shear vane testing and soil characterisation was performed on samples retrieved from the centrifuge models. Analysis of the fibrous models determined that with increased fibre content the undrained shear strength increased; no optimum fibre content was determined in the testing undertaken. Undrained shear strength increased once 4 % and 2-3% fibre content was reached in normally and over consolidated models respectively. Maximum resistance of the models occurred within the initial 5-10 mm of the model surface. Determination of the undrained shear strengths at depths corresponding to 0.5 B/D of the foundation showed that shear strength increased with increasing depth in the majority of cases. Back calculations of undrained shear strength from bearing capacity testing in normally consolidated and over consolidated models corresponded well with that of the penetrometers; while the shear vane results produced a poor correlation. On average piezoball results gave higher undrained shear strength, with reduced scatter, compared to CPT results. Over consolidated models produced greater undrained shear strengths than normally consolidated models. Atterberg limits and plasticity index was found to increase with increased fibre content; models were seen to change from clays of intermediate plasticity at 0 % fibre content to clays of high plasticity at 4 % fibre content.