Field measurement by fully grouted vibrating wire piezometers

Abstract

Subsurface instrumentation was installed at a field monitoring site in Hyde Park and bordering Bayswater Road for measuring the ground responses to Crossrail tunnelling near Lancaster Gate. Prior to tunnel construction, pore-water pressures were measured, both in the ‘greenfield' ground and the ground in the vicinity of existing London Underground running tunnels, by three multi-level vibrating-wire piezometer boreholes fully backfilled with cement–bentonite grout. The pore-water pressures in the ‘greenfield' ground were at the same time measured by conventional standpipe piezometers and pushed-in spade cells with built-in vibrating-wire piezometers. This paper investigates the performance of the multi-level vibrating-wire piezometers by comparing their post-installation and steady-state pore-water pressure measurements with those from the other piezometer types. Generally they performed well, providing an efficient means of determining pore-water pressures at several depths within one borehole. One of the ‘greenfield' multi-level vibrating-wire piezometers indicated underdrainage within the London Clay while the other was influenced by inter-connectivity between individual piezometers within the borehole and also the presence of a claystone horizon. One borehole close to the existing tunnel indicated drainage of groundwater into it; this effect is compared with predictions made using a simplistic finite-difference model.