Behaviour of sand in monotonic and cyclic simple shear loading at low-stress level

Geotechnical engineers deal with sands of varying in-situ (initial) conditions that develop through the process of land formation in natural environments and geotechnical constructions, such as compaction. The formation and construction processes can give different sand particle arrangement (fabric), which could significantly influence the behaviour of sand in subsequent loading. Geotechnical properties of sand are generally obtained from laboratory tests on reconstituted samples because of difficulties in collecting undisturbed sand samples from the field. The sample is sheared typically in direct shear and triaxial conditions, and in simple shear mode in some advanced laboratories. Soil strength depends on effective stress. In the field, sometimes the effective stress could be very low, in the order of 10–30 kPa, such as the soil around buried pipelines and near the seabed surface; however, in many cases, such as the soil around shallow and pile foundations, medium to high effective stresses are encountered. In the present study, the monotonic and cyclic behaviour of sand is investigated for simple shear loading conditions with a particular focus on the behaviour at low-stress levels. The laboratory tests were conducted using a combined advanced dynamic cyclic simple shear apparatus with a high-precision feedback system for controlling and measuring the forces and displacements while maintaining a high level of accuracy, which is essential, especially for tests at low normal stresses. The direct simple shear (DSS) tests were conducted on dry loose to dense sand under constant normal stresses of 12.5 kPa–400 kPa to investigate the effects of confining pressure on stress–strain behaviour. The monotonic test results show an increased shear to normal stress ratio, and thereby the mobilized friction angle, at the low-stress level, compared to high normal stresses. The strain-controlled (constant strain amplitude) cyclic test results show that the stress–strain behaviour and cyclic compaction are governed by the normal stress and shear strain amplitude. The sand becomes densified when the applied shear strain amplitude is greater than a threshold value. DSS tests were also conducted with stress-controlled loading conditions by applying a constant stress amplitude cyclic loading. In strain-controlled tests, the lower the normal stress, the higher the compaction is; however, an opposite trend exists in stress-controlled (constant stress amplitude) cyclic tests. Multistage cyclic tests under a wide range of constant normal stresses were conducted on dry sand by applying stress- and strain-controlled cyclic loads. Test results show that the volumetric compaction depends on normal stress and shear strain amplitude. The cyclic shear modulus and damping ratio are not affected by the low-amplitude cyclic loading history. The cyclic stress–dilatancy relationship depends on stress level, number of cycles, and shear strain amplitude. After a few cycles, the stress–dilatancy can be expressed by two parallel lines for loading and unloading, except for the initial part of the loading and unloading paths. The DSS test results show a good agreement with the results of hollow cylinder torsional shear tests. The effects of confining pressure and reconstituted sand sample preparation method on the behaviour of sand in the direct simple shear and triaxial compression modes are also investigated. The DSS tests were conducted on sand samples prepared by four methods: funnel raining, multiple-sieve raining, dry tamping, and table tapping. In addition to the monotonic DSS tests under constant vertical stress, a set of undrained DSS tests was conducted by maintaining a constant height of the sample during shearing, using the advanced computer-controlled system. Based on stress–strain response, it is inferred that the table tapping could give a stronger fabric than that of dry tamping and multiple-sieve raining methods. In the undrained tests, the phase transformation and steady-state lines in the stress space change by 3°–4° due to specimen preparation method, relative density and the initial consolidation pressure.

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