Much of our work involves investigating landslides and working with developments on unstable slopes. Where possible this has involved stabilisation of the slope but sometimes it has been necessary to build on the slope in such a way as to prevent damage to the structure from ongoing movement.
The investigative work that we have done has also led to new understanding of the geology of unstable areas, particularly in Ventnor, Seaview and Cowes, Isle of Wight.
Work on slopes is far more involved than just applying a search routine to find the minimum factor of safety on a slope. The strength of clays, in particular, depends on their stress history and will reduce once their peak value has been reached. To obtain meaningful results one has to know the underlying geology and the geological history of the site in order to estimate the stresses in the ground and estimate its strength and the position of any past slips.
Then the factor of safety can be calculated. It is often helpful to produce a contour plot of the factors of safety on a site. The adjacent diagram shows a ground model followed by a fos plot where the fos varies from 1.0 (red)to 1.24 (blue). This plot agreed well with what could be seen on the ground but does extend a little further behind the existing rear scarp than was obvious, indicating the likelihood of further retreat.
The intention on this site was to build over the edge of the cliff. Therefore a staged programme of pile installation and excavation was devised to ensure that the ground remained stable at all times and the forces within and displacements to the piles and structural elements stayed within limits.
It is the ability to combine the geotechnical and structural design, in combination with a thorough knowledge of the plant available to carry out the work that makes building on such sites possible.