The effect of fines on the small to large strain behaviour of sand non-plastic fines mixtures

Abstract

Sand non-plastic fines mixtures are usually studied at low stress level and with low fines contents. In the presented work, large to small strain behaviour of sand non-plastic fines mixtures with fines content up to a high value has been studied using a series of laboratory tests at relatively high stress levels. Two kinds of sand were chosen as the host sand, one is Leighton Buzzard sand (LBS) which has a strong mineralogy of quartz and is commonly used in laboratory tests, the other is pumice sand which can break at stresses reachable in conventional apparatuses. Crushed silica silt was chosen as the non-plastic fines. One-dimensional compression tests were conducted in oedometer apparatuses reaching elevated stresses to determine the normal compression line (NCL). Drained, undrained, and constant p’ shearing tests were conducted in triaxial apparatuses and a largest axial strain around 30% was achieved to determine the critical state line (CSL). For the exploration of behaviour of mixtures at small strain level, tests with bender elements installed in the oedometer and triaxial apparatuses were carried out to determine the small-strain stiffness at different stress levels or shear strain levels during compression and shearing. The transitional fines content, FCt, is found to be 30-40% for LBS-silt mixtures and 50-60% for pumice-silt mixtures. The behaviour of mixtures is mainly controlled by the sand particles when the fines content is lower than FCt, and is mainly controlled by the fines particles when the fines content is greater than FCt. A unique NCL is found for clean LBS as well as for clean pumice sand in e-logp’ plane when particle breakage occurs, and as the fines content approaches to FCt less breakage occurs and the non-convergence of NCL becomes significant, the location of NCL moves down. After FCt is reached, the location of NCL turns to move up. Unique CSLs in e-logp’ plane are found for both clean sands and the mixtures with different fines content. The location of CSL moves in the same pattern as what the NCL shows. At low stress level the small-strain stiffness G0 under compression depends on the volumetric condition and the mean effective stress p’. It decreases significantly when the fines is added to the clean sand since the strong force chains are disturbed. When the NCL in e-logp’ plane is reached at a relatively high stress level, G0 can be written as a function only of p’. It increases as the fines content approaches to FCt since the strong force chains are strengthened. The small-strain stiffness G0 at critical state can be written as a function only of the mean effective stress p’, and is lower than it under compression at the same p’. It decreases when the fines is added to the clean sand at a relatively low stress level where no sand particle breakage occurs. At high stress levels where particle breakage may occur, it increases as the fines content approaches to FCt. The strain-introduced stiffness anisotropy is observed at critical state.