S. E. Stallebrass

A Constitutive Model Combining the Microscopic and Macroscopic Behaviour of Sands in Shear and Volumetric Deformation

The paper describes how dissipation functions and yield surfaces derived by considering the microscopic mechanisms of particle deformation and particle rearrangement of sands (Chandler, 1985) can be combined with the volumetric constraints of the framework of Critical State Soil Mechanics (Schofield and Wroth, 1968) to give a model that provides a consistent link between the features of the shear deformation of sands at large strain and the volumetric state of the sand. The difficulties of relating microscopic parameters used to formulate yield surfaces and flow rules to conventional macroscopic critical state parameters are discussed. The potential of the approach is demonstrated by comparison with laboratory test results.

Physical modelling of lime stabilisation in soft soils around deep excavations

The availability of space above ground decreases as cities expand, causing a demand for very deep underground structures so developments must mitigate the risk of damaging adjacent buildings. This is especially critical in soft clays where ground movements are considerable and can extend far beyond the excavation site. This paper investigates the efficacy of a shallow lime stabilised clay layer on reducing heave and the settlement profile behind an embedded retaining wall. Centrifuge modelling at 160 g was used to observe surface and subsurface soil movements of a 12 m deep excavation (H) supported by a retaining wall of 8.8 m embedment at prototype scale. Since this research focussed on measures used to minimise heave the model comprised a high stiffness, fully supported ‘rigid wall’ to eliminate ground movements attributed to wall deformation. A direct comparison between a reference test, with no improvements and a test comprising H/2 thick 5% lime stabilised layer indicated that the lime treatment increased the excavation stability by a factor of three.

Remoulding of the Mercia Mudstone Group around CFA pile shafts

A field test has been undertaken to investigate changes to in situ Mercia Mudstone at the soil–pile interface after installation of four 5.5 m long 350 mm diameter continuous flight auger piles. The test investigated whether a remoulded zone exists, the extent of the zone, changes caused by remoulding and the effect of the installation method. The piles and surrounding soil were excavated after installation to a depth of just under 2 m. The sections of pile and surrounding soil were returned to the laboratory, where a variety of detailed observations at both micro- and macroscopic scales were undertaken, together with chemical and mineralogical analysis. It was found that a remoulded zone existed in all piles but that this varied in thickness both laterally and vertically around a pile. Across all piles the maximum thickness observed was 55 mm. The average thickness, fabric and texture were all affected by installation method. A distinctive vertically oriented fabric was observed in which up to four vertically oriented layers could be distinguished. There was evidence of changes in texture and fabric, structure, colour, mineralogy and chemistry.