The intrusive investigation comprised CPT probe holes, window sampling, trial pitting, soakaway testing and DCP testing. Soil samples were obtained and analysed for a broad range of contaminants of concern as well for geotechnical testing.

Window sampling for the installation of groundwater monitoring wells across the site was undertaken and subsequent groundwater monitoring was carried out to assess the groundwater levels across the site.

The CPT trial holes recorded the top of the chalk head at variable depths across the site between 1.50-3.00m bgl in the Windowless sampler holes and 2.20-5.80m bgl in the CPT holes with no chalk recorded in the CPT or WS holes on the north-eastern portion of the site. Superficial soils comprising clays and sands were recorded above. A general trend of increasingly competent chalk with depth was identified. Soil strength across the site varied significantly within the superficial soils with areas of soft spots where very low to low undrained shear strength was recorded and areas where high to very high undrained shear strength was recorded.

It was considered likely that the excavation would encounter groundwater or significant perched water within the Topsoil and clayey Sand/ Sand Clay of the River Terrace Deposits, especially after periods of prolonged rainfall or during winter, therefore dewatering was likely to be required.

Chemical laboratory testing of the Topsoil and River Terrace Deposits did not reveal any elevated levels of determinands above the guideline levels for a ‘Residential with Homegrown Produce’ land-use scenario.

 

Detailed Geotechnical modelling and analysis of the CPT probe logs and geotechnical testing results was required to better understand the ground conditions across the site. Due to the varying depths to chalk, the high groundwater levels during site works and the highly variable strength of the soils, modelling software was used to provided bearing capacities across the site. By undertaking the modelling, the site went from initially being proposed to be piled across all 300 plots, to the site being zoned into foundations at 1.00m, 2.00m and some plots being piled. This was discussed within meetings with the client and using detailed proposed development plans the site was delineated into the three foundation depth type zones. Further works in the form of cable percussion boreholes have since been proposed and these boreholes are targeting areas where soft spots, high ground water levels and deep roots were all recorded. Further discussions between the client, Ground & Water and the structural engineer have resulted in the project moving forward to a design stage with the results of the deep boreholes are being processed to provide pile loads.

Thanks to the level of detail this site investigation went to, moving from a fully piled foundation strategy to the three foundation depth zones will save our client a considerable amount of time and money.