Field measurement by fully grouted vibrating wire piezometers


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 the passage of the tunnel boring machines, 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 (MLVW) piezometers installed in 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 four pushed-in pressure spade cells with built-in VW piezometers. This paper investigates the performance of the MLVW piezometers by comparing the post-installation and steady-state pore water pressure measurements made using these devices with those from the other piezometer types. Results from falling head permeability tests in the standpipe piezometers are presented and discussed. The measurements indicate drainage of groundwater into the existing century-old segmental cast iron lined tunnels. This measured drainage effect is compared with predictions made using a simplified finite difference model. It is concluded that the MLVW piezometers performed well and provide an efficient means of measuring pore water pressures at several depths within one borehole.