Slope Stability Investigation and Remedial Design - Chafford Hundred
Background
The site is located at a former chalk pit, which was backfilled with Thanet Sand to form the eastern embankment of a nature reserve around 30 years ago. The embankment is approximately 30m high, which stands out about 38° in the lower part and was subsequently regraded to a reduced slope of 26° in the upper part.
A shallow rotational slip occurred over the lower section of the regraded slope in 2004, at a point where there was an absence of established vegetation. Slippage initially occurred over the unprotected area between the tree line and the Covamat, and subsequently regressed up the slope, creating a 17m wide scarp, approximately midway down the regraded slope.
Investigation
Southern Testing undertook the detailed site investigation, comprising boreholes in the slipped area and installation of slip indicators and data loggers for monitoring slope movement and groundwater condition. The soils encountered comprise firm to stiff brown very sandy silty clays altered with loose to medium dense brown silty clayey sand. From the field monitoring, there was movement at depths of 850mm and 2m. No groundwater table was detected, although there was increase in precipitation as the slope was slacker. Back analysis was carried out for the slipped slope to verify the soil parameters and groundwater condition, which were utilised for remedial designs.
Remedial Treatments
Integrated remedial treatments were proposed to ensure the embankment safe and sustainable in the natural reserve area, comprising:-
- 7m long soil nails with hollow self-drilling tendons at a spacing of 2m by 2m on slope surface over the slipped area.
- Coated mesh was adopted over the slipped area to retain the soils between nails and protect ground surface erosion.
- A tree planting scheme is proposed for notch planting of bare root stock over the slope including the remediated slip area with rabbit protection.
Southern Testing managed the project, undertook the site investigation, proposed the remedial method, and supervised the construction.
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