Retention Design, Thurstonland

Client: Martello Piling Ltd
Main Contractor: DCT Ltd

Permanent Repairs to Road Slip

A contiguous bored piled wall was designed by AGE as part of the remedial works to a slippage affecting the road. The geology of the site comprised weathered mudstone of the Lower Coal Measures, overlain by superficial deposits comprising completely weathered mudstone. The superficial deposits contained landslip debris. The mudstone was highly weathered at the top but became a weak mudstone or stiff clay below 2.5m depth. Groundwater levels within the slope lay close to ground level. It is understood the slope failure occurred following a protracted period of heavy rain.

In the absence of laboratory strength determinations, a slope stability back-analysis of the recent slope failure was carried out in order to derive strength parameters for the superficial deposits. The pile design assumed the piles are cantilevered due to the possible movement of the down-slope material away from the piles. HB loading was adopted as a surcharge right up to the pile line so as to represent the worst-case vertical loading induced if vehicles parked upon the footway.

The piles were designed as 600 mm diameter piles installed at 800 mm centres by 9 metres depth with full depth reinforcement. A capping beam of 800 mm width and 400 mm depth was also designed for the bored pile wall length. The piles were installed with Martello's novel piling rig, that allows large diameter conventional rotary bored piling within low headroom constraints with increased rig stability.