Site investigation for HS2 across the Colne Valley and Chiltern Hills

The proposed civil engineering works for the UK’s new high speed railway, High Speed Two (HS2), across the Colne Valley and Chiltern Hills, includes a 3300 m viaduct, earthworks, 16k m of large diameter twin bore tunnel with cross passages and associated portals and shafts. This area has until now been poorly investigated. Following preparation of a desk study and preliminary geomorphological assessment, a three-component ground investigation strategy was adopted: use of advanced techniques at key locations to define detailed behaviour of the ground; targeted high quality investigation to determine parameters for geotechnical design, and their expected variability; and ground profiling to allow detailed stratigraphic and geotechnical correlation between investigation locations, using rapid field and laboratory techniques. Significant effort, including the use of logging masterclasses, ensured that consistent, detailed logs were produced identifying marker horizons and key features of engineering interest. Environmental and other factors imposed very strict constraints and also influenced the timing and sequencing of the investigation.