Measurement of ground-lining pressure distribution around two tunnels in stiff stony clay

Abstract

This thesis describes the design, construction and calibration of a hybrid hydraulic/electrical type of earth pressure cell and the subsequent use of these cells for the measurement of radial ground pressures bearing on concrete segmental tunnel linings at two locations in mixed clayey ground in north-east England. Also described is an accompanying programme of lining distortion measurement and a preliminary discussion is directed towards theoretical aspects of ground/lining interaction mechanics. The tunnels studied were 3.20 m diameter and at depths of 11.77 m and 12.39 m to the crown. Lining/soil radial interaction pressures were found to be almost uniformly distributed about the tunnel, these recorded pressures being almost one-half the maximum possible overburden pressure calculated on a ɤ z basis. Furthermore, these ultimate pressures were achieved after a period of only 7 to 8 days following lining erection and grouting. Ultimate measured lining ring distortions were also realized after this 7 day period. This relatively rapid stabilisation of ground pressure contrasts with a much more protracted, on-going distortion reported by other workers in other materials, but is consistent with contractual experience which suggests that tunnel secondary linings could be safely erected, with little risk of brittle fracture, much earlier following primary lining construction than has hitherto been considered prudent.