Interface friction test of a draped netting system in a chalk cutting

Ravelling failure is a common issue on cut slopes in chalk at slope angles over 45 degrees, as it can undermine other areas and precipitate larger failures. One of the common methods of mitigating the risks of ravelling type failures on chalk slopes adjacent infrastructure is the use of draped netting systems. This paper presents a case study of the design and installation of such a netting system on a chalk cutting on the London Underground Metropolitan line between Rickmansworth and Chorleywood stations. At this location, ravelling of chalk from frost shattering was generating toe debris and the serviceability of the slope was an issue. In addition, larger failures from root balls and shallow soil slips in head deposits were posing a risk to railway operations on the Metropolitan line. A draped netting system was identified as being the most suitable solution for the site and mitigating the risk of material reaching the tracks. On most draped netting systems, it is the installation of netting anchorages, which typically constitutes the main cost of the system. For this project, an innovative test procedure was used to assess the interface friction between the netting mesh and the slope crest. By quantifying this design parameter, it was possible to reduce the number of anchorages at the crest of the slope. This paper describes the methodology used and the resulting saving gained by reducing the number of anchorages installed on the project.