Stabilisation of a soft fill slope by compacted rockfill – an example of good practice

Remediation was designed for a marginally stable fill slope comprising up to 5 m loose cohesive fill in a residential area adjacent to a deep valley. Excessive ground settlements around a sewage pumping station (SPS) and a tension crack in the vicinity of residential houses in an area near Glasgow were linked to possible burst utilities and gradual washout of fines in the fill. Desk study and ground investigations were carried out to determine the site history and geological setting. Remedial options were considered. In light of confined site space and availability of materials, crushed compacted rock fill was considered the best option for replacement of the soft fill. Topsoil [150mm] was used to cover the rockfill and promote vegetation growth. The advantages of the rockfill over compacted cohesive fill include ease of construction, performance and suitability. Effective drainage including in-slope and contour channel drainage were designed to prevent build-up of pore water pressures in the slope beneath topsoil. Construction sequence including excavation and preparation of temporary cut, installation of drainage layer, deposition and compaction of rockfill material in layers, deposition and seeding of the topsoil layer, construction of permanent surface drainage system and maintenance access was also designed.