Table 1.

Ground investigation strategy (Lawrence et al. 2018)

Structure typeBoreholeSpacing (m)Depth
Closed face TBM tunnelsCable percussion (LCP) start to 20 m (maximum) with rotary follow-on.* Deep borehole to base of current stratum100–150 m and distributed either side of the tunnel. Every 500 mMinimum of 10 m (c. 1.5 tunnel diameter) below invert of deeper tunnel
ShaftsOne deep rotary borehole to provide continuous core for logging and advanced effective stress testing. One deep borehole for SBP/HPD/SBP to provide stiffness and in situ stress profile. One LCP borehole to identify water strikesAt least 3 equidistant around the shaft2 of 3 to at least 15 m below base of shaft
PortalMore than 2 rotary boreholes to provide continuous core for logging and advanced effective stress testing. More than 2 deep boreholes for SBP/HPD/SBP to provide stiffness and in situ stress profile100 m maximum and distributed either side of portalMinimum of 10 m below deepest part of structure (i.e. below base of retaining walls)
Cut and cover box stationsMore than 4 deep rotary boreholes to provide continuous core for logging and advanced effective stress testing. More than 2 deep boreholes for SBP/HPD/SBP to provide stiffness and in situ stress profile100–150 m around perimeter of station box10–15 m below base of deepest part of structure (i.e. below base of retaining walls)

*Cable percussion boreholes were drilled through the superficial deposits with rotary techniques through the London Clay Formation, the tunnel horizons and below. Additionally, use of rotary techniques below 20 m was found to reduce the risk of CP casing jamming in the boreholes. Rotary techniques were always specified where the borehole penetrated more than 3 m into the Chalk Group, to allow good sampling of the chalk. LCP, light cable percussion; HPD, high-pressure dilatometer; SBP, self-boring pressuremeter.