ROCK QUALITY assessments of steep chalk faces and determining dewatering characteristics for tunnels and deep excavations are just some of the aspects of site investigation contractor Foundation and Exploration Services (FES)'s involvement in CTRL.
FES carried out 18 contracts along the route during the six ground investigation phases from 1994 to 1999. In several areas of north Kent, including the line of the North Downs Tunnel, chalk quality was investigated by rotary coring to 120m depth. FES engineers also worked from hydraulic access platforms to scanline log up to 17m high chalk faces, recording structure and stratigraphy and mapping discontinuities and joints. Results were used in statistical analysis to establish fracture/joint patterns before construction.
At Stratford, pumping tests and specialist groundwater abstraction procedures were piloted to determine the dewatering requirements of the ground before tunnelling. FES used acidisation techniques 80m into the chalk aquifer to improve well yields sevenfold.
As well as on several geoenvironmental investigations along the route, FES chemists were also needed at the contaminated Stratford site to ensure that well drilling would not cause any onward migration of contaminants into the aquifers.
More conventional geotechnical, geoenvironmental and hydrogeological investigations involved up to eight types of rig to suit a variety of logistical problems - from heavy, truck-mounted units for large diameter rotary wireline coring to smaller, highly-manoeuvrable, compact track-mounted rigs.
The firm carried out trackside work next to live railways at Cheriton, foundation studies for the new bridge over the River Medway, involving 40m rotary and cable percussion boreholes at pier locations and worked off purpose-built platforms to drill on inclines and embankments.
FES also assessed groundwater on many sections, including a 19km stretch in open country between Boxley and Lenham Heath and a 15km route from Sevington to the Cheriton terminal.