Chalk forms the bedrock geology of great swathes of the south and east of England, extending from the Isle of Wight in the south to Yorkshire in the north. Chalk is a soft limestone formed by the compression of microscopic plankton that settled to the sea floor between about 60 and 100 million years ago.
Like other limestones, it is prone to dissolution resulting in a voided or karstic topography. Dissolution of chalk occurs as a result of the migration of water through unsaturated chalk. This water movement causes the chalk to dissolve leaving behind a void. Over time, any overlying soils settle into the void left by the dissolution of the chalk. This settlement of overlying soil propagates upwards and can manifest at ground surface as a sinkhole.
Dissolution of the chalk can occur due to man-made influences too. In developed areas, dissolution features can occur where a new source of water ingress is introduced to a dissolution-prone geology. Examples include leaking or damaged underground water pipes discharging higher volumes of water into the ground, or soakaway drainage chambers which can channel water collected over a large catchment area into a smaller, more concentrated areas. In these examples, dissolution of the chalk will occur at an increased rate local to the water ingress point and potentially cause collapse.
Chalk dissolution is a geotechnical hazard that the CampbellReith geotechnical team are no strangers to. The presence of dissolution features below ground (voids, caves and zones of soft or loose infill) may not be immediately obvious at surface level but by undertaking a desk study that considers the ground conditions and groundwater regime at a site, our team can identify whether they may pose a risk.
Man has also exploited the chalk since the Stone Age, excavating it for flints, to obtain lime for farming or building purposes, or as a building material itself. More potential cavities! Again, a desk study can quickly identify whether this hazard may exist at any given site.
The presence of chalk dissolution or other cavities doesn’t mean a site can’t be developed, however they highlight the importance of communicating the potential for geotechnical hazards at a site early on in the design process, undertaking appropriate staged investigations and refining risk assessments so that safe, economic and sustainable solutions can be developed. If you have any concerns about chalk, speak to us!
