A Multiaxial Constitutive Model for Fibre-reinforced Sand

Fibre orientation in fibre-reinforced sand (FRS) is highly anisotropic due to compaction during sample preparation or field construction. This makes the mechanical behaviour of FRS, such as strength and dilatancy, highly dependent on the strain increment direction. While constitutive models able to capture such anisotropic behaviour of FRS have been proposed for conventional triaxial compression and extension conditions only, this paper proposes for the first time a full anisotropic model for FRS formulated in the general multiaxial stress space. The new model is developed based on the assumption that the strain of FRS is dependent on the deformation of the sand skeleton. In turn, the fibre presence affects the void ratio and effective stress of the soil skeleton, which governs the elastic properties, dilatancy and plastic hardening of the FRS. The effect of anisotropic fibre orientation on the FRS behaviour is considered through an anisotropic variable which characterises the relative orientation between the loading direction tensor and fibre orientation tensor. The model does not require direct measurement of the stress-strain relationship of individual fibres. Though the model is for FRS under multiaxial loading conditions, the parameters associated with the fibre inclusion can be determined based on triaxial test results, provided that the orientation of fibres is known. The model has been used to predict the stress-strain relationship of fibre-reinforced Hostun RF (S28) sand under multiaxial loading conditions. Satisfactory agreement between the experimental data and model predictions is observed

Small strain stiffness evolution of reconstituted medium density chalk

The mechanical behaviour of reconstituted chalk deposited has been investigated in a comprehensive experimental campaign using measurement and dynamic testing in the form of bender elements. Characterisation of the small strain shear stiffness (G0) of this material has been performed over a range of the isotropic stress levels and over-consolidation ratios. A variation of (G0) with current stress and overconsolidation ratio is, for the first time, proposed for this material. Following the widespread idea that the process of remoulding chalk may release calcium carbonate, which provides a cementing agent for grain contact overgrowth, the G0 evolution at the constant stress state, over time, has been investigated for curing periods up to 23 days. Regardless of the previous stress path history prior to curing, an increase of the small strain shear stiffness with time was observed under a constant effective stress condition. This appeared to be partly associated with creep (or secondary) deformation but further increase was also observed when measurable creep deformations ceased

Monotonic and cyclic lateral load tests on driven piles in chalk

This paper describes the results of a pile-testing campaign on open-ended tubular steel instrumented piles driven into the Chalk formation in the UK. The testing campaign comprised the performance of both monotonic and one-way cyclic lateral load tests, performed at different times after pile installation. The tests were performed on five piles with uniform outer diameters of 762 mm and embedded lengths of 4 m and 10 m to investigate the difference in response between short and long piles. Lateral pile head load–displacement behaviour to failure has been analysed. The tangent stiffness evolution during monotonic loading has been evaluated at different times after pile installation and the chalk set-up has been found to have no effect on pile behaviour under lateral loading. The pile secant stiffness during cyclic lateral loading is also investigated. Accumulated pile head lateral displacements are discussed and their pattern is described by a logarithmic function that varies with number of cycles. The creation of a gap between the Chalk and the pile during cyclic lateral loading was observed, which influenced the shape of the load–displacement loops. The influence of the instrument protection system was taken into account in analysing the results. </jats:p

Effect of saturation conditions on thermal properties of sands

Soil thermal conductivity, diffusivity and heat capacity are critical parameters that govern the design of specific geotechnical infrastructure including energy piles, high-voltage cables and pipelines. The presence and flow of water are of particular importance as it dominates the conduction and convection hear transferring mechanisms, leading to a global increase in heat conduction ability by replacing air within the multi-phased soil system. This paper presents preliminary results from an experimental programme that aims to characterise the effects of saturation conditions on the thermal properties of a siliceous sand. Thermal conductivities were investigated for samples prepared to varied relative densities and degrees of saturation by two methods: (Ⅰ) sand-water mixed at specific water contents and moist-tamped in moulds to the target densities; (Ⅱ) cycles of wetting-draining applied to initially dry specimens through a customised test set-up. Thermal needles were employed that trigger transient heat source and monitor temperature variations. Significant differences were observed in the thermal conductivities between the Method Ⅰ specimens and the Method Ⅱ specimens in the first saturation stage. However, the differences diminished in the subsequent infiltration cycles. Overall, the results confirmed strong dependency of sand’s thermal conductivity on saturation conditions as the presence of water envelopes and bridges sand particles in heat conduction