Slope Stability East Grinstead
Background
The site was a Victorian road cutting, approximately 8m deep into the weathered Ardingly Sandstone. The 50º - 70º side slopes were densely vegetated with numerous mature trees growing along the crest. The trees were removed in early 2008 and many of the roots grubbed out. Shortly after this work was undertaken, a shallow failure occurred on part of the eastern side slope, which was followed by sporadic spalling of loose debris onto the road. One lane of the road was closed and temporary traffic control measures put in place.
Investigation
Southern Testing were commissioned, on an urgent basis, to investigate the slope and provide outline remedial measures.
A series of dynamic probes were completed along the crest of the slope to prove the top of rockhead. Where deep weathering was identified, rotary drilled boreholes were formed using a mini-tracked rig.
In general 1-2m of loose soils were found over jointed rock but in one location over 9m of residual soils were encountered. Standpipes were placed in the boreholes to record water levels. A visual assessment of the exposed soil rock faces was made from road level.
Recommendations
Due to the urgency of the work, there was insufficient time to carry out geotechnical testing or a comprehensive slope stability analysis.
However based on our site findings and knowledge of the area, we were able to recommend the following;
- Reduction of slope angles in residual soils.
- Soil nailing of weathered material along cutting base
- Covering of the whole slope with a matting system to encourage re-vegetation and protect the exposed faces from significant weathering.
- Provision of drainage at the top of slope
- Erection of a small retaining wall or catch fence at the base to guard against any debris entering the carriageway.
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