As part of the redevelopment of the St Athan RAF airfield in the Vale of Glamorgan for commercial purposes, Alun Griffiths Contractors constructed a new access road from the B4265 to the northern access gate of the airfield. We designed and implemented an integrated geotechnical and geo-environmental ground investigation along the proposed length of the road (some 2.2km), and provided design support regarding ground and groundwater conditions and earthworks.
Our considerable knowledge of the geological conditions in the area allowed a reduction in the number of boreholes required to confirm the Ground Model, before excavation proceeded with no unforeseen ground conditions as a result of our geotechnical advice.
The new road included two significant earthwork structures along with a series of low embankments across side-long ground. In the west of the route, at the junction with the B4265, a cutting was constructed within the Porthkerry Formation bedrock to a depth of 5m.
To the east of the rock cutting, the route crosses the Llanmaes Brook on an embankment of around 4.5m height. Due to flooding constraints, the brook was diverted through the embankment via two concrete culverts set at different levels.
Due to our knowledge and geomorphological experience, we were able to predict and then locate a band of tufa (a soft chalk like soil made up of the remains of freshwater snails) and a partly backfilled stream valley, which had not been identified in any of the previous assessments for the route by others.
The tufa was very soft and would have proven a significant hazard to the new embankment and culvert structures, and the backfill materials contained significant quantities of materials which were unsuitable as an embankment formation and for re-use as construction fill on the scheme without treatment.
Early identification of the soils allowed remedial actions to be taken and construction to proceed with minimal programme delays or additional construction costs.
As well as an investigation report, a Geotechnical Design Report (in line with the Design Manual for Roads and Bridges, HD22/08) was prepared to assist the designers, including an assessment of the excavatability and re-use options of the excavated Porthkerry Formation from the cutting, and assessments of the stability of the embankment and culverts crossing the Llanmaes Brook.