Detailed Site Investigation Gets Sussex Housing Development Moving

3rd July 2024

Geotechnical • Specialist Services

Thanks to detailed Geotechnical modelling and analysis of the site of a large housing development in Climping (West Sussex); an initial proposal that foundations should be piled across all 300 plots due to a suspected high chalk dissolution risk and difficult ground conditions was reduced to the site being zoned into foundations at 1.00m, 2.00m and some plots being piled.

Our Client

The client is a housing developer, which Ground & Water has recently started working with on large scale residential housing developments.


The site comprised undeveloped agricultural land with dense woodland located along the western and southern boundaries. The site was underlain on the western portion of the site by the superficial River Terrace Deposits (Sand Silt and Clay) with the eastern portion of the site by the Raised Beach Deposits, overlying the Lewes Nodular Chalk, Seaford Chalk, Newhaven Chalk, Culver Chalk and Portsdown Chalk formations. It was immediately clear that an accurate Chalk Dissolution Risk Assessment was required.

As a result of the desk-based review of the site, the geology showed a moderately low risk. Based on collective information to date and the relatively competent ground encountered to a moderate depth, the risk was considered to be generally low, subject to further testing. However, the Lidar data did record a circular feature on the northern portion of the site that should be investigated further. Several dynamic probes undertaken across the site to confirm the strength of the soils were recommended.




Our Challenge

Our challenge was to investigate the potential risk factors pertaining to the site with reference to a chalk dissolution assessment and foundation design for a proposed large residential housing scheme on an existing agricultural field. The works comprised an initial desk-based chalk dissolution risk assessment followed by intrusive site works and Phase II investigation report.

The Ground & Water Approach

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.



The Outcome

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.

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