Inlet Pipe Failure

In April 2006, a 2.5m diameter raw water tunnel (sitting at a depth of ~27.00m b.g.l.) dramatically burst, creating a fifteen metre wide crater and ejected Terrace Gravel and London Clay deposits to a distance of up to twenty five metres. Fugro Engineering Services were invited to create a programme of works to determine the causative factors and to assist the Thames Water Project Team with the remediation using a wide range of specialist in-house techniques, including cable percussive boring, rotary drilling, pushed thin wall sampling, CPT, geophysical surveys and geotechnical testing. The project was high profile and time critical.

Cable percussive boring adjacent to the 15m diameter collapse crater

Phase 1: Initial Geophysical Survey
The initial works were geophysical surveys using GPR, microgravity and resistivity techniques to ascertain where any voids had been created.

Phase 2: Cable Percussion - Determining ground conditions
FESL determined a safe working distance from the edge of the crater using slope stability calculations. Cable percussive boring techniques were employed to establish the limit of disturbed ground and to provide data for the design of the proposed remediation. Piezometers and standpipes were installed in boreholes to monitor groundwater conditions.

Phase 3: Cable Percussion and Rotary - Cross Hole Geophysical Investigation
Cable percussion boring and rotary drilling continued for further geologic and geophysical investigations (using the Geobore-S wireline system to obtain high quality continuous core samples). The core was logged in detail and sub sampled immediately on site for geotechnical and chemical testing. A cross-hole geophysical survey was designed and implemented using the boreholes installed to show a clearer picture of the subsurface conditions around the tunnel.

Phase 4: CPT - Increasing lateral resolution
In order to pinpoint the location of pipe failure and areas of disturbance an intensive regime of sixty Cone Penetration Tests (CPTs) using sleeve friction, tip pressure and excess pore water pressure data was conducted. This successfully located a ~2.5m diameter 'chimney' through which the water had burst.

The fifteen metre diameter collapse crater and gravel blown out of the ground

Phase 5: Cable Percussion - Shaft design and safety
Two shafts were designed to rescue and reline the tunnel and FESL conducted further cable percussive boreholes to aid with design and also to install inclinometer and extensometer monitoring points. These were monitored daily throughout shaft excavation to ascertain whether slope stability was at risk on the reservoir wall. A standpipe was installed in one borehole for use as a dewatering well to relieve pressure from the base of the shaft.

Forensic Phase - Rotary with Cable Percussion
A variety of in situ testing was employed including pushed thin wall sampling, self-boring pressuremeter and permeameter testing, downhole geophysical logging and an additional crosshole geophysical survey.