Project background

The Havant Thicket Reservoir is a proposed 160 ha new reservoir site on the south coast of England and is the first reservoir to be built in the UK in over 30 years. It will be bound by a 3-km-long, 20-m-high soil embankment. RSK Geophysics was commissioned by Atkins and Portsmouth Water to investigate the geological succession to the top of the chalk, including the thickness of suitable material (e.g., London Clay Formation) for use as earthwork fill and waterproofing of the reservoir. The survey also sought to determine whether there were any unknown subsurface periglacial features that may cause water loss or embankment instability, such as pingos or dissolution features. The site of the reservoir is underlain by a varying stratum including Head deposits and London Clay, Harwich and White Chalk Subgroup Formations.

Geophysical survey

Geophysical surveys including electromagnetic (EM) ground conductivity mapping and electrical resistivity tomography (ERT) profiling was used as part of a larger phased ground investigation by RSK company Structural Soils, which included 24 deep boreholes and 21 trial pits to provide an accurate site model. EM mapping using a CMD Explorer was used to cover the lateral extents of the whole site to provide shallow conductivity data at depths of up to 6–7 m, showing variation in the shallow superficial deposits. ERT data along 11 profile lines covering a total 9400 m were collected to image the geological succession up to 65 m below ground level.

Results

The EM data successfully highlighted where London Clay and Head deposits crop in the centre of the site and more shallow sand deposits at the southwestern edge. The EM survey was most sensitive in the upper 2 m of ground. The ERT data provided imaging to much greater depths along discrete profiles. The thickness and levels of the London Clay and top of the Chalk were clearly defined in the ERT data, providing continuous thickness information along the profiles between borehole locations.

Added value

Both geophysics and ground investigation intrusive information were used to reduce uncertainty in the preliminary ground model. By deploying the geophysical survey with full coverage, the client was able to significantly reduce the risk of encountering subsurface periglacial features. This provided confidence to the client that they had chosen the right site for the project.

Fig 2 EM conductivity data Fig 2 conductivity scale

Figure 2: EM conductivity data showing high conductivity material (coloured orange/red) where London Clay present at the surface. Blue colours represent low conductivity sands. Circles represent strata confirmation with borehole logs.

resistivity data - RSK Geosciences project

Figure 3: ERT data showing low resistivity London Clay at shallow depths in the west with Reading beds and chalk bedrock at greater depth. The ERT data is supported by discrete boreholes acting as control points.