Table 1.

Crossrail Ground Investigation Strategy for structures

Structure typeBoreholeSpacingDepth
Closed face TBM tunnelsLight cable percussion (CP) 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 times tunnel diameter) below invert of deeper tunnel
ShaftsOne deep borehole should be rotary to provide continuous core for logging and advanced effective stress testing. Second deep borehole for self-boring pressuremeter/high-pressure dilatometer/self-boring permeameter  depending upon ground conditions to provide stiffness and in situ stress profile. Where possible cable percussion borehole to below base of shaft to identify water strikes, which are difficult to identify in rotary holes drilled with flushAt least three equidistant around the shaftTwo of three to at least 15 m below base of shaft
PortalAt least two boreholes should be rotary to provide continuous core for logging and advanced effective stress testing. At least two boreholes for self-boring pressuremeter/high-pressure dilatometer/self-boring permeameter depending upon ground conditions 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 stationsMinimum of four deeper holes by rotary techniques to provide continuous core for logging and advanced effective stress testing. At least two boreholes for self-boring pressuremeter/high-pressure dilatometer/self-boring permeameter depending upon ground conditions 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.

Downhole and cross-hole seismic testing should be considered for shaft and station sites where there is ground variability (e.g. Lambeth Group sand channels).

Self-boring pressuremeters and high-pressure dilatometers were equipped with internal ‘feeler arms’ to accurately measure the cavity expansion at several points in the test pocket to give accurate estimates of in situ stiffness. The pressuremeters/dilatometers also gave measurements of in situ stress. In situ stress and stiffness are key parameters in the design of retaining structures for shafts, portals and station boxes.