Project Details

Anglian Water’s new foul water sewer in Cambridge will be constructed in tunnel. With an estimated value of £3.7 million the scheme extends for 2 km beneath the city streets, and comprises a 1600 m section of 1.2 m diameter pipeline and a 400 m section of 600 mm diameter. Work is due to start in July/August 2000 with one of Anglian Water’s partner constructors, Barhale, appointed to the task.

Ground Investigation

A ground investigation comprising a detailed desk-study, including a full appraisal of the archaeology and history of the area, together with twenty nine cable percussion boreholes and laboratory testing, was carried out in 1999 by A F Howland Associates of Norwich. Howlands have the Term Tender for carrying out all of Anglian Water’s ground investigation work. This provides a broad based geotechnical, environmental and archaeological consultancy service used by Anglian for scheme evaluation right through to construction.

Geology

Desk-study work had predicted that the area would be underlain by Gault Clay which caps the Lower Greensand aquifer, with the Gault covered by River Terrace Deposits at the eastern end of the tunnel route. However, the borehole findings suggest the superficial sequence to be more complex, with made ground largely replacing the River Terrace Deposits, and the thicknesses of cohesive and granular deposits appearing elsewhere probably representing unmapped Head deposits or channel infill deposits associated with former tributaries of the River Cam. However, no evidence of the deep buried channels which are known to exist in the area was found at the borehole positions.

Geotechnics

It is expected that tunnelling will take place though the Gault Clay and a comprehensive analysis of the parameters of the deposit was performed. This was based on a programme of extensive laboratory testing consisting mainly of classification tests, undrained shear strength, effective stress and chemical tests. The results show the Gault to be a clay of very high plasticity that varies from firm to very stiff (PI around 40 to 50% and Cu 60 to 260 kPa). Subtle variations in its character and parameters with depth and lateral extent may reflect slight differences in lithology, weathering, mineralogy or stress relief effects. Assessment of palaeo-geomorphology showed a valley-side setting for some of the tunnel route and this may contribute to the pre-existing stress relief pattern and its influence on the nature of the Gault.

Assuming tunnelling takes place entirely in the Gault Clay, analysis by Howlands suggests that settlement along the centre line of the tunnel would be less than 10 mm, based on conservative calculation, but decreasing to under 5 mm when adopting a less cautious approach. Instrumentation and monitoring is proposed due to the presence of many listed buildings and this needs to be commenced prior to construction in order to isolate the time when tunnelling takes place.

Archaeology

Almost continual occupation from the Iron Age to the present day in the Cambridge area has resulted in multi-layered development over successive periods of time, which includes evidence of Roman, Saxon and Medieval settlement. The history of the area was unravelled by Howlands archaeologist to help identify any areas which might constitute a hazard to tunnelling operations (e.g. obstructions from old buried foundations and walls, disused wells, contamination from past industrial sites, backfilled coprolite workings). The features have been presented on a series of archaeological ‘hazard’ maps.

Investigation Strategy

This represents the most current of a series of full geotechnical and environmental appraisals by Howlands for Anglian Water. It follows a philosophy of geotechnical involvement from scheme inception through the tender process and into the construction phase. The benefit of this has been fully recognised by Anglian Water on major schemes such as Project Orwell and Project Clearwater.