Pile stabilisation of large landslides

Large scale land-sliding regularly causes damage to our infrastructure, including buildings and property, pipelines and cables, roads, railways and sea defences. The presence of infrastructure within or adjacent to the zone of sliding requires that the slides are stabilised. While the cost of this may be considerable, it is often less than the cost of continued damage and loss of use, or of re-routing the infrastructure concerned.Increasingly, discrete piles are used to increase the stability of landslides, and are often favoured because they can be directly targeted at the infrastructure to be stabilised, i.e. piles can be placed immediately upslope and downslope of infrastructures such as railway lines and roads crossing a slipping mass, providing direct support to the infrastructure. In heavily built up areas, piles can be fitted around existing buildings, or where redevelopment takes place they can act both as foundations for the new building and to stabilise lateral movements.The size and extent of the slipping mass often mean that the forces acting on the stabilising piles (for the required design increase in factor of safety) are very large. The large pile bending resistance needed can only be provided by large diameter piles, often installed in several rows. The economic design of piles on this scale requires a better understanding of the interaction between the multiple rows of piles, in particular how load transfers onto the different rows. Under large slope loads plastic hinges may form within the pile section, and these change the mechanism by which further load is then carried by the pile. The formation of the plastic hinges is a complex soil structure interaction problem, and it can be difficult to predict their exact location, particularly where there are large variations in soil stiffness over the pile depth.While simple elastic analyses are commonly used in design to predict pile bending behaviour, interaction between the failing mass and multiple rows of piles may be better analysed using simple (2D) finite element analyses of the slope. However, there is a need for more monitoring data of real pile behaviour, to instil confidence in the use of more sophisticated elastic or finite element analysis in design. Compared with other large scale geotechnical constructions (such as retaining walls and basements), there is very little high quality monitoring data to show how rows of discrete piles behave with ongoing slope movements.Instrumentation will be installed into a large landslide near Telford that is to be stabilised with discrete piles, with the aim of developing a comprehensive understanding of the interaction between the climate, slope movements, and the rows of installed piles. The instrumentation will include inclinometers to measure ground and pile displacements, strain gauges to measure pile bending, piezometers to measure pore water pressures and a weather station to record climate parameters such as rainfall, temperature, and solar radiation. The landslide and piles will be back analysed using elastic (beam on spring) and full two and three-dimensional finite difference analyses. The proposed research will also consider how simpler finite difference analyses could be used in the design of landslide stabilising piles. The measured data and numerical models will then be used in a critical appraisal of pile design, and how it may be improved.