Eroding foreshores endanger the floodplains of many estuaries, as such, effective and environmentally friendly interventions are sought to stabilise slopes and mitigate erosion. As a step in forestalling these losses, we developed laboratory microcosms to simulate tidal cycles and examined the mechanisms of erosion and failure on sandy foreshore slopes. As an experimental aim, we applied microbially induced calcite precipitation (MICP) to selected slopes and compared the effectiveness of this microbial geo-technological strategy to mitigate erosion and stabilise slopes. To assess shoreline stability, thirty cycles of slowly simulated tidal currents were applied to a sandy slope. Significant sediment detachment occurred as tides moved up the slope surface. For steeper slopes, one tidal event was sufficient to cause collapse of the slopes to the soil's angle of repose (similar to 35 degrees). Subsequent tidal cycles gradually eroded surface sediments further reducing slope angle (on an average 0.2 degrees per tidal event). These mechanisms were similar for all slopes irrespective of initial slope inclination. MICP was evaluated as a remedial measure by treating a steep slope of 53 degrees and an erosion-prone slope angle of 35 degrees with Sporosarcina pasteurii and cementation solution (0.7 M CaCl2 and urea) before tidal simulations. MICP produced 120 kg calcite per m(3) of soil, filling 9.9% of pore space. Cemented sand withstood up to 470 kPa unconfined compressive stress and showed significantly improved slope stability; both slopes showed negligible sediment erosion. With efforts towards optimisation for upscaling and further environmental considerations (including effect of slope saturation on MICP treatment, saline water and estuarine/coastal ecology amongst others), the MICP process demonstrates promise to protect foreshore slope sites. (C) 2015 Elsevier B.V. All rights reserved

The paper presents an experimental study on water retention behaviour of two reconstituted soils, a silty clay (Speswhite kaolin) and a sandy clayey silt (Barcelona silt) and two compacted soils, a silty clay (Speswhite kaolin) and a silty sandy gravel (Adige river embankment). These soils were used with the aim of encompassing a wide range of soil types and soil fabrics. Specimens were subjected to drying and wetting ‘hydraulic' paths by removing or adding water under zero total stress and to ‘mechanical' wetting paths by compressing specimens at constant water content. To investigate a broad range of void ratios, the reconstituted sandy clayey silt was initially reconstituted from slurry by applying different consolidation vertical stresses, whereas the compacted Speswhite kaolin and Adige river embankment were compacted to a broad range of water contents and compaction energy. On the basis of the experimental data, a modification to the van Genuchten's model is proposed to account for the effect of void ratio on ‘main drying' and ‘main wetting' behaviour. The model is based on parameters having clear physical meaning and can properly describe water retention at medium and low degrees of saturation. It is formulated in such a way that effect of void ratio on ‘scanning' behaviour can potentially be accounted for. The model applies to both the compacted and reconstituted soils investigated in this programme and describes water retention behaviour regardless of whether the degree of saturation is changed in a mechanical or hydraulic fashion.

This paper presents an experimental study of the compaction behaviour of non-active clay. One-dimensional static compaction tests were carried out at high and medium water content with matric suction monitoring using Trento high-capacity tensiometers. At lower water contents, a transistor psychrometer was used to measure post-compaction suction. Samples were compacted on the dry side of optimum to cover a wide range of compaction water contents and vertical stresses. Three water content regions were identified in the compaction plane depending on whether post-compaction suction increased, decreased or remained constant as the degree of saturation was increased at constant water content. Hydraulic paths of specimens subjected to loading–unloading cycles at constant water content have clearly shown that post-compaction suction may increase as the degree of saturation increases. This non-intuitive behaviour was demonstrated to be associated with the coupling between mechanical and water retention behaviour. To this end, a coupled mechanical water retention model was formulated. Irreversible one-dimensional mechanical paths were modelled by a boundary surface in the space average skeleton vertical stress, modified suction and void ratio. Irreversible hydraulic ‘wetting’ paths were modelled by a boundary surface in the space suction, degree of saturation, and void ratio. This study was completed by investigating the pore size distribution of compacted samples through MIP tests.