The Initial Investigation

The initial site investigation was undertaken in September 2022 and comprised the drilling of six Windowless Sampler Boreholes to depths of between 1.23 – 5.45m bgl with Standard Penetration Tests at 1.00m intervals. Super Heavy Dynamic Probes were undertaken through the base of boreholes to final depths of 3.00 – 5.00m bgl.

The ground model for the site after the initial set of site works indicated the superficial Head Deposits to be deeper in the north-western part of the site (2.40 – >5.45m bgl), becoming shallower towards the east (1.70 – 1.80m bgl) and south. These were generally absent in the southern part of the site, which was in accordance with the anticipated geology of the area.

The Additional Investigation – Design and Implementation

Additional site works were undertaken and comprised the drilling of 145 Super Heavy Dynamic Probes across the site to depths of between 1.60m – 12.00m bgl, in order to carry out a more precise geotechnical risk assessment. The drilling locations within the north-western and north-eastern grids were determined using a 10m x 10m format whilst the southern grid used a 13m x 13m format. Prior to the commencement of site works, the probing locations were geo-marked.

The results of the intrusive investigation / chalk dissolution risk zones and geological conditions were compared against NHBC tree ring requirements and a full foundation assessment was carried out for different parts of the site. The most difficult technical issues applied in the north-western grid, where several plots were selected to be piled at depth within competent chalk. Special foundation solution (cantilevering / cruciform) was also proposed for a corner of a block in the eastern part of the site, spanning over a localised problem area.

In broad terms, it was recommended the rest of the plots in the northern section could use strip foundations at specific depths, based on tree requirements within superficial clay. This took into account the results of the volume change potential of the soils, and the noted soft zones; with proposed allowable bearing pressures and the use of mesh to allow for differential settlements. In the southern section, traditional strip footings at shallow depths were applicable, as the non – volume change potential chalk bedrock was shallow, consistent and without particular soft zones.

This site investigation brought home the risks associated with Chalk Dissolution and the benefits of carrying out an extensive intrusive investigation as described here. Failure properly identify the specific risk areas can lead to significant headaches and costs for the developer if properties are affected by subsidence – or worse – at some point in the future.