LOCATION: Newton Abbot, Devon

• Slope Stability Assessment, Cuttings, Embankments

• Material Classification, Earthworks

• Groundwater

• Road Pavement

South West Geotechnical Ltd, acting on behalf of the local Authority undertook a geotechnical investigation to assist with a new link road to the north of Newton Abbot. The new road bisects a deeply incised valley feature making access challenging. However, with SWGs in house tracked Comacchio 205 drilling rig and help from local contractor Marshalls Plant for the excavators, or drilling team and engineering geologists delivered a successful investigation.

SWG designed and completed the investigation, providing detailed ground conditions advice for the design of the road and related infrastructure.

The exploratory holes generally encountered a relatively thin mantle of topsoil overlying residual soil derived from the weathering of the Gurrington Slate bedrock. Alluvial soils were encountered at the base of the valley feature. 

Groundwater was encountered near surface in the vicinity of a proposed cut slope and was considered critical to the long term stability of the slope.  To manager the risks, the resulting cut slope requires horizontal drainage holes drilled into the toe to allow the groundwater table to be pushed back to an acceptable distance to achieve an appropriate Factor of Safety.  Localised herringbone drainage was also recommended to pick up any perched water.

Fill embankments were considered stable at the proposed angles and significant long term consolidation settlement of the embankments was variable but acceptable.  Appropriate earthworks were recommended to ensure a good key into the existing sloping ground profile to allow appropriate compaction of fill materials.

A cut and fill balance was projected with the re-use of site won fill as either Class 1B or 2C materials in accordance with Highways Agency Series 600 with materials generally close to optimum moisture content. 

In addition, design CBR values were provided.