Ground Deformations Resulting from Shield Tunnelling in London Clay

Abstract

The paper describes the results of a field measurement program aimed at determining the ground movements created by the hand excavation of a 4.146 m diameter shield-driven tunnel at a depth of 29.3 m in stiff, fissured, heavily overconsolidated London clay. Approximately half of the recorded maximum surface settlement of 6.1 mm is both measured and calculated to have taken place during passage of the shield, the remainder being attributed to post-shield deformations. Calculations are based on a field and laboratory-determined average radial yield rate for the clay at the tunnel of 0.0055 mm/min. It is concluded that a normal probability surface settlement profile is developed over the shield and tail.Post-shield inward decompressions (K0 ≈ 1.65) and possible recompression as a clay–grout interaction effect could then be responsible for a slight reconsolidation of the clay at soffit and for distortion of the final settlement profile out of concordance with an error curve.