AFTER A number of false starts, construction proper is under way on the first part of the Channel Tunnel Rail Link. Geotechnics is playing a crucial role in construction and ground characterisation is key to success.
Site investigation contractor Foundation & Exploration Services carried out a number of different site investigations with a total value of £750,000 over three years for client Rail Link Engineering, responsible for design and project management of the high speed rail link. The first package comprised 147 boreholes on contracts 330 (the Thames tunnel to Medway Valley) and 410 (the North Downs tunnel) for preliminary design.
The second package, awarded in 1997, was to obtain data for foundation and earthworks design and to assess potentially contaminated land. This covered contracts 330, 350 (the Medway crossing) and 410 and involved 43 cable percussion boreholes, seven large diameter wireline rotary cored boreholes, 30 window samples and 114 trial pits.
The firm also carried out a programme of insitu tests, including pump and infiltration tests in the area of the Thames tunnel portal in Swanscombe marshes. Further testing was carried out for drainage design.
Rotary boreholes up to 120m deep assessed chalk quality along the North Downs tunnel. This is 3.2km long, reaching depths of 100m. It is being constructed as a single bore, just over 14m wide and 13m high through predominantly hard, stable chalk.
Backhoe excavators excavate the tunnel, which has a preliminary sprayed concrete lining. Rock bolting provides support where chalk strength is low or where solution features are present. Breakthrough is expected next May with tunnel completion due at the end of 2001.
FES also carried out scanline logging on four exposures along the North Kent line and Galley Hill road. Structure and stratigraphy were recorded and discontinuities and joints mapped for statistical analysis to establish fracture/joint patterns.
The third package of work in 1998 included rotary and cable percussion drilling to depths of 40m alongside the M2 motorway where it crosses the River Medway. Twelve boreholes were put down over a 500m section at proposed bridge support locations along the west bank. Further work included an investigation of the east bank to finalise bridge design and additional boreholes for foundation design along the route.
The Medway crossing comprises two reinforced concrete approach viaducts: one is 400m long over nine piers, the other 600m long over 13 piers. A 150m long centre span bridges the navigable channel. The piers are founded on piles up to 1.8m diameter. The bridge is due for completion at the end of 2001.