Taylor Wimpey Homes wanted to redevelop a greenfield site as a large residential development. On-site sustainable management of surface water was vitally important to the scheme. Previous testing undertaken by others was inconclusive, so we were commissioned to undertake supplementary hydrogeology assessment to assess the infiltration/permeability characteristics of the strata.
In accordance with BRE 365 and BS EN ISO 22282-1(&2) and the client’s evolving requirements, a phased investigation was undertaken consisting of a combination of trial pits and borehole soakaway testing within the Porthkerry Limestone bedrock. The most applicable test procedures being selected based on the relevant guidance (e.g. Figure 1 and Table 2, BS EN ISO 22282-1, 2012).
Tests were undertaken at specific locations and at varying depths to assess variability in the permeability of the bedrock with depth and rotary coring was undertaken to assess whether any variability recorded was associated with changes in the lithology, structure or fracture status of the bedrock. Testing indicated infiltration within the Porthkerry Limestone was mainly controlled by the presence of fissures and fractures and their infill status; these varied locally. Where fissures/fractures were filled with clay (e.g. in the upper zone), the infiltration rates were significantly lower.
To inform the long-term performance of the drainage design, seasonal groundwater monitoring was undertaken by our engineering geologists. In addition, continuous monitoring of groundwater levels was undertaken down-gradient of test boreholes to assess resultant short-term changes while infiltration was occurring. Based on the data, potential hydrogeological and geotechnical implications were identified and these were outlined to the client. This included potential effects to another residential development down-gradient and the stability of quarry faces. This included liaison and discussion with the local authority planning department to address concerns on bedrock dissolution.
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