M25 Rooks Nest Farm Slope Stabilisation

Rooks Nest Farm sits near junction 6 of the M25.

For many years the site had caused problems with the road carriageway, with regular resurfacing works required due to the historic slip plane within the underlying Gault Clay. As any regular commuter using the M25 will attest to, any road works or lane closures can turn traffic on this already overcrowded motorway into a crawl and cause severe delays and unpredictable journey times for motorists. Something had to be done.

WJ Groundwater was called upon to investigate the cause of the recurrent slope movement and offer a viable remediation solution.

Following extensive on site investigation work, it was established that pore pressures within the clay – and in particular at the slip plane – was the major cause of slope movement. Reducing these pressures would be challenging, due to the low permeability strata of clay, but WJ met this challenge head on.

Working with French partners TP Geo, we proposed and implemented a comprehensive siphon drainage solution. This no-power system worked at very low flows and relied solely on the change in elevation of the slope to remove the water from the wells. The system consisted of 48 No. siphon wells, connected to several outfall cambers further down the slope.

During the systems operation, there was a huge reduction in the amount of carriageway remediation work and resurfacing required in this area. As a direct result, this section of the M25 saw less lane closures, allowing traffic to move more freely through the area.

• LOCATION: Godstone, Surrey
• CILENT: Highways Agency
• MAIN CONTRACTOR: Carillion