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A conventional site investigation was designed utilising shell and auger boreholes, trial pitting, rotary coring (with wireline recovery), and slit trenches to locate
services. Geophysics was also carried out along the dock road tunnel alignment to determine rockhead.
The regional geology comprises river alluvium and boulder clay, overlying very strong Carboniferous
Limestone. The majority of the Investigation was in soft ground with an erratic rockhead often occurring within the proposed sewer horizon. Rockhead on the dock road tunnel alignment was near the surface in the city but
dropped rapidly to at or near invert level for the final 600m to the pumping station.
The soft ground section was in Glacial moraines and recent alluvium, with a horizon of large limestone boulders at rockhead. It was not
possible to penetrate this layer with conventional shell and auger rigs, and recovery from rotary coring was less than 50%. Consequently a 600mm dia tripod piling rig was used to penetrate this layer, for sampling and permeability
testing.
In the overlying soft alluvium, cone penetration testing and insitu shear vane testing was carried out.
At the pumping station rockhead was found to rise to within 6.0m of the surface. A large scale pumping
test was carried out from a trial pit excavated down to rockhead.
Rotary coring using wireline recovery produced excellent results, with all core geotechnically logged to SubTerra Engineering's specification with full
discontinuity logging. Fracture Index was calculated for runs of similar spacing, and a graphic log included to allow visual assessment of fracture spacing.
Samples of core were selected for strength testing (PLI & UCS),
and linear cutting/abrasion tests to provide an indication of wear on steel components for machine excavation.
To obtain structural geology data three rotary core holes were angled to allow the use of a core
orientation device. This allowed the dip and dip direction of the major discontinuities to be measured.
Stereographic projections were produced to show the variation in discontinuity orientation along the tunnel alignment.
The results were correlated with plots produced from field mapping data from various outcrops along the route.
35 slit trenches were also carried out in the City to locate services, three of which were of archaeological
importance as the foundations of the old city wall and the abutments of a medieval bridge were found.
The scheme was tendered as 10 individual contracts in 2000, with construction commencing in 2000/2001.
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