The site presented unique challenges due to its complex history, including evidence of quarrying activities in the west portion, tramway encroachment, and the demolition of residential buildings in the east portion between the 1900s and 1960s. Published geological information suggested a ground model consisting of localised glacial deposits, overlying the Twrch Sandstone Formation (Millstone/Basal Grit). The presence of Carboniferous Limestone beneath the Twrch Sandstone, presented potential dissolution risks and extensive associated features (e.g. dolines) were noted in the area and documented in relevant literature (Thomas, 1954).
The site investigation required assessment of both contamination risks and complex geotechnical conditions to support the multi-faceted development design. It included trial pitting, windowless sample boreholes, soakaway infiltration testing, laboratory testing and gas and groundwater monitoring. Continual review of emerging ground conditions and Client communication allowed for adaptive scope modification to ensure sufficient ground information was obtained, particularly in areas where historic quarrying had created difficult drilling conditions.
Our intrusive investigation successfully validated the desk study predictions of historical quarrying with deeper bedrock depths and thicker, low strength backfill materials up to around 3.5m being encountered in the west where suspected. This contrasted with shallow bedrock (less than 1m) in the remainder of the site. Due to the variable ground model, we recommended different foundation approaches for different areas of the development:
- Areas with shallow bedrock: conventional spread foundations on competent Twrch Sandstone bedrock;
- Areas affected by quarrying: Alternative foundation solutions including deeper foundations (e.g. piles) or ground treatment options.
We recommend all foundations were reinforced and geogrids were incorporated into the new areas of car parking as a precautionary measure against potential dissolution risks.
The investigation confirmed the absence of significant soil contamination despite the industrial heritage. Ground gas assessment concluded that protective measures were to be incorporated into the building design to ensure occupant safety.
The sustainable drainage systems report revealed constraints due to ground conditions and site sensitivity, leading to recommendations for alternative solutions that balanced sustainability goals with site safety requirements.
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