Three dimensional numerical modelling of rows of discrete piles used to stabilise large landslides.
University of Southampton, Engineering and the Environment,
A number of three dimensional finite difference analyses using FLAC3D have been carried out to investigate the complex pile-soil interaction effects of single isolated pile and groups of piles. The three-dimensional numerical models developed to carry out these analyses are able to model the full pile-soil interaction problem including three dimensional and surface effects which cannot be understood fully using two dimensional analyses. FLAC3D analyses are initially carried out to investigate the response of a single pile subjected to lateral soil movements. These analyses explore and verify the failure mechanisms for landslide stabilising piles categorised by Viggiani (1981). The effect of the strength of the slip plane interface and a sloping ground surface on the behaviour of the pile is then investigated. The initial numerical results from models with a rigid pile, a distinct plane of sliding and a horizontal ground surface, as assumed by Viggiani, agree well with his limit equilibrium solutions. The further analyses show that the strength of the slip plane interface has a considerable influence on the pile behaviour, and that the slope of the ground surface is only significant above certain angles. The behaviour of single and two pile rows with increasing pile spacing is analysed numerically. The FLAC3D analyses show that pile-soil interaction within a group of piles has a significant influence on performance. If the performance of a single pile row is compared with that of two pile rows, the single pile row installed at a spacing of 2 d (where d is the diameter of the pile) and the two piles rows installed at a spacing between piles in a row of 3 d provide a stabilising force equivalent to the force obtained from a solid retaining wall. For both cases, soil movement through the piles remains very small, as a passive wedge type failure mechanism forms in front of the piles. The behaviour of rows of piles used to stabilise a landslide near Ironbridge, Telford is back analysed. The numerical analyses show that the FLAC3D models can be used to back analyse complex pile stabilised landslides, in a way not possible by simple limit equilibrium analysis. However, detailed material properties, and the location of slip planes and their strength parameters, are very important input parameters if accurate outputs are to be obtained
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