The Ground & Water Approach – Phase I of the site investigation involved a Desk Study. The aims were to determine the underlying geology; any potential contamination risks from historic maps and any hydrogeology and hydrology influences.
The geological map for the area indicated that the site was underlain superficial storm beach deposits and bedrock deposits of the Undifferentiated Earnley Sand Formation, plus other deposits of sand, silt and clay.
The historic maps identified a potential risk from construction and demolition activities on-site associated with the West Battery. The gun emplacements and associated infrastructure were previously located immediately to the south of the site. While there were no records of buried storage tanks on the site, it was noted there were records of nine unspecified historical storage tanks within a 250m radius of the site.
A risk assessment determined that demolition and construction activities between 1931 – 1968 are likely to have created a capping of Made Ground containing demolition debris. Building materials used in the late 19th and early 20th Century may have included asbestos. Lead, cadmium and other heavy metal-based paints may also have been used. Fires may have taken place on the site as part of any demolition processes to remove waste. Potential pollutants such as carcinogenic PAHs in the tars produced by combustion of timber and other materials may be present in the near surface soils. Oil spills may have also occurred.
The Desk Study revealed the site was located on a secondary aquifer. The nearest major water feature was the Solent, 90m to the southwest. Groundwater was anticipated to be shallow, possibly tidal, and would flow towards the Solent. The site was within a Flood Zone 3 and did not benefit from flood defences.
Given the known risk of UXOs, the two-day Phase II site investigation works were supervised by a UXO specialist. Phase II of the site investigation comprised a single deep Cable Percussion Borehole for a potential pile design for the platform, a single Windowless Sampler Borehole and two Trial Pit/Foundation Exposures adjacent to the historic wall located on site. Also the site investigation comprised installing two groundwater monitoring wells. Monitoring visits were conducted over a three-month period at low and high tide times.
The Outcome – The ground conditions encountered within the trial holes constructed on the site generally conformed to those anticipated from the Desk Study. A capping of Made Ground was noted to overlie superficial deposits. Chemical testing revealed no elevated level of harmful substances within the samples of Made Ground tested. A groundwater strike was noted during the site investigation at a depth of 1.70m bgl in one monitoring well. No other groundwater was observed including in the trial pit and foundation exposures.
In terms of design for the platform for LCT 7074, engineering specifications were supplied for both a reinforced concrete raft foundation and piled raft foundation. These were based on findings from analysis of the underlying geology obtained from samples taken during the site investigation.
The danger of encountering buried storage tanks or UXO was still present. Ground & Water provided guidance for the construction contractor.
Following a chequered post-war career as a floating nightclub in Birkenhead, LCT 7074 eventually sunk at her moorings. She was salvaged and moved to Portsmouth in 2014. The national Museum of the Royal Navy is currently raising funds to restore her, so she can be displayed at the D-Day Museum. Ground & Water feels privileged to have played its small part in preserving this important part of our naval history for future generations to enjoy.