Complex Geology Creates Challenges in Plymouth
The Plymouth area is challenging in both contamination and geotechnical aspects. Plymouth’s naval and industrial history, infilling of tidal creeks with waste and wartime destruction all make for a range of contamination challenges. Geotechnically too, layers of man-made ground and alluvial/tidal silt and peat hide a complex bedrock geology with block faulting exposing different strata over a short distance, including the sinkhole- and dissolution-prone Plymouth Limestone Formation. Many of the sea inlets, including Millbay harbour, were originally formed by the collapse of cavern systems.
For the past year Ground Consultants Limited have been investigating the ground on a residential development site in the west of Plymouth. Aware of Plymouth’s complex geology, we carried out a detailed review of reports on nearby sites, and the BGS geological mapping and memoir for Plymouth, to develop the original ground investigation scope.
The mapping indicated that the site was on the Torpoint Formation, usually comprising mudstone and siltstone, and not on the Plymouth Limestone Formation. However, the geological memoir stated that massive limestone beds also occur within the Torpoint Formation in the Millbay area, which are associated with significant cavities.
We therefore scoped five deep rotary boreholes along with the normal shallower investigations. This turned out to have been prudent, as three of the five boreholes discovered solution features in limestone beneath the site, creating a potential sinkhole risk.
A further phase of investigation was necessary to find out the full extent of the solution features. This required a grid of deep boreholes across the site with a total drilling length of around half a kilometre. We proved that a significant amount of the limestone had been dissolved into cavities, and we found evidence of their interconnection across the site by observing water from drilling upwelling in other boreholes in different parts of the site.
A foundation solution involving deep piles for the larger buildings on the site is envisaged. It’s sobering to think that a typical ground investigation scope for this type of development, focusing on proving the top levels of the bedrock, would have completely missed the deeper solution features. A detailed scoping exercise, preferably a full desk study, is essential before carrying out ground investigation fieldworks, especially in the complex geology of Plymouth.
