Published: 26/02/2025 12:00
Richter was approached by Envolve Infrastructure Limited, the main contractor for Wessex Water’s £30 million capital investment to upgrade the Holdenhurst Water Treatment Works. The primary goal was to increase stormwater capacity while keeping the existing treatment facilities operational throughout the construction period. This presented a unique challenge: how to expand the site’s capacity without increasing its footprint and ensure the new infrastructure integrated seamlessly with the existing system.
Richter quickly identified that the large-scale cofferdam required for the stormwater holding tanks posed significant geotechnical challenges. The high-water table and the presence of running sands in the trial cofferdam demanded a specialised design to ensure safe and effective excavation. In response, Richter’s Groundwater team was engaged to devise a tailored groundwater control solution for the deep excavation.
A deep well groundwater control system was chosen after an in-depth design analysis of existing ground investigation (GI) data and pumping tests supervised by Richter. Due to the depth of cofferdam, well-pointing was not suitable due to cavitation caused by the partial vacuum and instead, the use of deep wells was proposed to pump groundwater at a controlled rate, allowing excavation to proceed safely. This system was designed with wells located inside the sheet pile pans with a target groundwater level just below the formation level to ensure unsaturated ground conditions while excavating and eliminate the risk of failure of the base of the excavation. Throughout the excavation process, a series of monitoring methods were deployed including inclinometers and survey monitoring to measure movement of the cofferdam due to dewatering and the cofferdam construction sequence. The groundwater was also closely monitored for turbidity before being discharged into the treatment works' regulated outfall.
The design stages, led by Richter’s multi-disciplinary team, included a conceptual groundwater control design, supervised pumping tests, and detailed assessments of the groundwater control systems, which included modelling software such as SEEP/W at preliminary design stages before refining the design following the pumping tests. Following this, the Richter Temporary Works team moved on to provide support for the temporary works associated with the cofferdam. Richter provided CAT III independent checks, ensuring the project adhered to the highest safety and regulatory standards. WALLAP was used to simulate the phased installation and removal of the cofferdam and shoring systems with consideration for the dynamic groundwater conditions caused by the dewatering and allowance for possible failure of the dewatering system. Richter also assessed a set of spun-iron syphon pipes that were 3m from one corner of the cofferdam using Oasys Pdisp and Oasys Xdisp. This analysis was to check that the stress and strains experienced by the syphon pipes due to movement of the cofferdam and groundwater related settlement would not exceed allowable limits and damage the pipeline.
As the project continues toward its expected completion in March 2025, Richter continues to provide support for the project. Richter's expertise in cross-discipline design—encompassing hydrogeological, geotechnical, and structural elements—ensured the seamless integration of temporary works into the overall construction sequence. This holistic and sustainable approach minimised risk and delays, delivering a temporary works design for large-scale infrastructure projects that will significantly improve the area by reducing overflows by nearly a third.
