A collaborative approach to high quality ground investigation required for the design of five deep Thames Tideway Tunnel shafts

The Eastern Contract of the Thames Tideway Tunnel is to be constructed wholly within the White Chalk Subgroup – predominantly the Seaford Chalk with five diaphragm wall shafts extending over 73m below ground. Whilst the geological sequence in London is well documented through the superficial deposits, London Clay Formation, Lambeth Group and into the Thanet Formation, historical ground investigation (GI) data rarely penetrates deep into the chalk. Tideway provided ground investigation data for the shaft works sites which, in general, characterised the underlying geology. Upon review by the Principal Contractor (a Joint Venture between Costain, Vinci Construction Grands Projets and Bachy Soletanche) there was a need to supplement this data with site specific boreholes, to carry out specialist in-situ and laboratory testing, particularly in the chalk, and address potential geological risks. With design underway, the Principal Contractor, Designer (Mott MacDonald) and GI contractor (SOCOTEC) were challenged to deliver a GI to budget that not just met the requirements of the specification, but also was able to be amended based on information received during the works. Information captured during the construction of the boreholes was fed back to the designer with sufficient time to allow alterations to borehole locations and testing to ensure that ground risks could be fully investigated during the GI in a single phase rather than a traditional multi-phased approach. Early stage discussions between all parties ensured a suitably targeted, cost effective approach of ground investigation works.The fundamental requirement for the GI was to obtain the highest quality data about the chalk, so triple tube wireline rotary core drilling with on-site logging and sample preparation was the main method of investigation. High Pressure Dilatometer (HPD) testing, water injection (single packer) testing, pumping tests and a comprehensive suite of downhole geophysical surveying were carried out with the test depth selection refined on site in response to contemporaneous core logging. With the changing landscape of active construction sites, refinement of the GI programme was possible only through the involvement of all parties during the fieldwork with continual monitoring and review. This facilitated the quick provision of high quality ground data, which was invaluable to the tight design programme.