RSA Geotechnics were requested to carry out monitoring of a failed embankment side slope on the A130 in Essex. The slip occurred on the northbound side of the road to the north of Monument Bridge. Monument Bridge carries the A130 over the railway line that runs between Southend Victoria Station and London.
The A130 is maintained by County Route under the terms a DBFO Contract on behalf of Essex County Council. Ringway Infrastructure Services Limited are employed by County Route as the Managing Agent.
RSA Geotechnics was initially requested by Ringway to inspect the failed side slope. Following the inspection, a proposal was put to County Route for monitoring of the side slope to check for further movement.
Immediately north of Monument Bridge the embankment is approximately 9m high and gradually reduces in height down to the original ground level prior to construction of the road, approximately 0.5km to the north. The embankment side slope angle is approximately 1(vertical):3(horizontal), equivalent to an angle of 18˚ along this stretch.
The slope face is vegetated with grass, shrubs and young trees. An open drainage ditch run is present along the base (toe) of the embankment slope and a wooden boundary fence runs alongside the ditch. At the top of the slope there is an approximately 5m wide grass verge between the slope crest and the carriageway edge.
The slope failure had taken place on the embankment side and was approximately 100m in length. The southern end of the failed stretch was approximately 140m north of the bridge. The amount of slippage was variable along this stretch. Parts of the original embankment slope face had slipped downwards and formed a series of ridges and depressions of slip debris longitudinally along the embankment. The form of the slip debris gave the impression of shallow translational movement rather than deep seated failure.
The top of the slip was approximately 4 to 5m from the slope crest at its closest and slip debris had reached the bottom of the slope in places.
A topographical survey of the slope face was carried out. Permanent ground markers remote from the slope (survey stations) and a grid of survey points across the slope were installed. The levels and co-ordinates of the survey points were determined, and regular monitoring subsequently took place over a three month period.
The monitoring data generally showed no discernible signs of mass movement across the slope face over the monitoring period.