Piled embankments rely upon arching of the embankment material onto underlying piles, thus potentially significantly reducing load on the soft subsoil that more generally prevails beneath the embankment. Finite-element modelling of a piled embankment in plane strain has previously been reported; the ‘subsoil' was not explicitly modelled, but was represented by a vertical stress acting on the underside of the embankment. This technical note considers extension of this work to three dimensions. The results make particular reference to prediction of the stress on the subsoil at the point of ‘maximum arching' and the height of influence of arching within the embankment.
The British 'Code of practice for strengthened/reinforced soils and other fills' (BS 8006) was substantially revised in 2010, with a further 'Corrigendum' in 2012. Historically, BS 8006 considered arching in a piled embankment, based on an interpretation of the 'Marston' equation. The 2010 revision included an alternative method related to the analysis of arching in a piled embankment, which was proposed by Hewlett and Randolph in 1988, and later itself amended in the 2012 Corrigendum. This contribution considers BS 8006 predictions of reinforcement tension using these methods as a basis for embankment load on the reinforcement, for a wide range of piled embankment geometries. The predictions are compared with results from three-dimensional finite-element analysis, demonstrating encouraging correspondence with the Hewlett and Randolph approach (but noting that the 2010 revision overpredicts the data whereas the 2012 revision underpredicts it). Good predictions of maximum reinforcement sag are also achieved by slight modification of the BS 8006 method.
Discrete (spaced) pile rows are an established method of improving slope stability, or 'dowelling' an existing slip. The piles predominantly provide horizontal restraint to the potentially unstable mass of the slope. The method becomes more cost effective as the pile spacing increases, but there is also increasing risk that the soil will 'flow' through the gap between piles, rather than arching across it. Two-and three-dimensional numerical analyses of a generic slope with piles at various locations are undertaken. A simple model of the stabilising force required for a given increase in factor of safety of the slope is then combined with a model of limiting pile row interaction to allow direct estimation of the effect of piles at a particular spacing ratio in improving the factor of safety.
The ‘delayed collapse’ of cuttings in stiff high-plasticity clay is known to be significantly affected by long-term recovery of pore pressures and progressive failure resulting from brittle material behaviour. Limit equilibrium methods are not well equipped to model these effects, but the increasing use of numerical modelling offers new potential to improve understanding of these failure mechanisms. A series of numerical analyses incorporating brittle material behaviour has been undertaken, specifically considering deep-seated first-time failure of cuttings in weathered London Clay. Because of progressive failure, the average mobilised strength is between the peak bulk strength and the residual strength. In current practice, parameter selection is often based on Chandler and Skempton's back-analysis of several deep-seated failures in the ‘brown’ weathered London Clay. This paper considers variation of the critical (minimum) height for delayed deep-seated failure in cuttings in weathered London Clay with slope angle, and variation of mobilised strength with cutting height.
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