Installing monitoring equipment on a steep embankment next to the busy A30 in Devon needed a clever solution.
Keeping a vital traffic link between Devon and Cornwall open was the challenge faced by a Geotechnics team sent to investigate and monitor a steep and potentially unstable embankment next to the busy A30 outside Tedburn St Mary near Exeter.
Client Enterprise Mouchel was alerted to the potential threat after instruments previously installed by Geotechnics indicated very high groundwater pressures as well as movement identified by marker pegs, which the client had placed on the embankment.
More information was needed and the client decided that inclinometers were needed to allow lateral movements within the slope to be monitored. But before work could begin Geotechnics had to choose a rig capable of drilling through the embankment into natural, stable Carboniferous mudstones into which the inclinometer could be socketted.
Accessibility to the site was the main obstacle as the embankment itself is fairly heavily vegetated and slopes down at a gradient of around 1:2. It is locally steeper at the investigation area due to the features of the slip. Land ownership at the base of the embankment meant that access was only available from the top of the embankment, directly off the A30.
Geotechnics looked at two options. One involved a slope climbing rig which would travel down the slope to the top of the slip to allow drilling to proceed. However, to ensure that the rig and crew wouldn’t slide down as well, the rig would need to be tethered to anchors at the top of the embankment. This risked anchoring into the failing material itself and potentially helping to pull the embankment down.
The chosen option was a tracked excavator with a modified boom and a rotary drill attachment. Under a lane one closure of the A30, the rig, together with compressor, water bowser and support vehicles, was transported to the site and the rig tracked into position.
The boom reached over the crash barrier down to the proposed borehole position and, using rotary solid-stem augering techniques, drilling depths of between 13.5m and 15.5m were successfully achieved.
A 58mm outside diameter click-lock inclinometer access pipe was then placed into the open borehole and filled with water to counteract buoyancy caused by subsequent grouting, which was achieved using a mixture of bentonite powder and cement.
The installations were finished with raised lockable barrel covers cemented at the surface. This work was completed in two working days - slightly ahead of schedule - and all plant was removed from site and the traffic management removed the following morning, as planned.
A week later, three sets of base readings were taken at each borehole using a biaxial inclinometer probe.
Data was then taken back to the office and downloaded into In-Site inclinometer software and results shown as tabulated readings and graphs. The client is doing on-going monitoring of the instruments.