Dewatering effects on the Richmond South Trunk Main – geotechnical considerations for design and construction

The Richmond South bulk water main construction project comprised installation of approximately 1km of DN450 PE pipe and associated infrastructure, including rammed steel conduits beneath state highways, and stream crossings in Richmond, Tasman, New Zealand. The conventional cut and cover pipe installation methodology required dewatering, and subsequent discharge and treatment of the water after extraction from the coarse grained Holocene alluvium. A dewatering assessment for an assumed pipe ram launch pit was undertaken in the design stage, and information added to the contract documents.

The construction works began in June 2021 and encountered lithology preventing deeper well pointing, higher groundwater level, and inflows of water into excavations in excess of that anticipated by the design. The higher groundwater level, the different hydraulic conductivity of the soils than those initially assumed in the design, and the timing of the works in the calendar year are identified as key causes of the excessive inflow. The ability of the Contractor to adapt their methodology, then extract and discharge large volumes of sediment laden water in an urban environment became critical for the completion of the works, incurring additional time, cost, and engineering effort to address.

This paper discusses the adequacy of geotechnical investigations and detail of dewatering assessment including sensitivity to input criteria. The implications of unforeseen geotechnical and groundwater conditions, primarily their effect on the practicality of pipe installation works, cost, and contractual arrangements are also discussed.