Faced with driving a 400m length of the sewer tunnel by pipe jack at its Kelvin Valley site, contractor Edmund Nuttall appointed geotechnical specialist Donaldson Associates to assess ground conditions. A full face slurry boring machine had been chosen for the tunnel which passed through unstable, water bearing silts of low strength.

Site investigation combined rapid assessment of the ground conditions using piezocone testing, high quality MOSTAP sampling and conventional drilling methods with piston sampling and traditional undisturbed and bulk sampling. The investigation revealed that the tunnel alignment lay in a substantial deposit of thinly laminated silt, at, near or above its liquid limit.

TBM steerage is controlled by the machine gaining a reaction from both the installed tunnel lining and the surrounding soil. It was considered that the effective control of the TBM could be achieved by reducing pore water pressures and consequently increasing the effective stress conditions of the soil.

Assessment of mass permeability of the deposit took account of site investigation data on particle size.

With the efficiency of a particular groundwater control method dependent on the permeability and soil structure, conventional practice would dictate that the soil at Kelvin Valley was close to the threshold of permeability that can be readily dewatered, even with the use of vacuum well systems.

In reaching a decision, Nuttall took into account the CIRIA report Control of groundwater for temporary works and the Funders report on Groundwater control which discuss suitability of control methods with respect to particle size and permeability. These data sets have been combined, and areas of overlap have been identified. The soil at Kelvin Valley was found to be at the lower limit of soils where vacuum wells are efficient and drainage becomes impractical.

The answer was a corridor of wells along the tunnel alignment, so tunnelling could continue without disturbance by the wells. A 0.3mm to 0.5mm clean sand well screen was installed over the full depth of the deposit to a level 5m below tunnel invert level, which prevented the ingress of silt.

The success of this project demonstrates that in particular conditions vacuum wells can be used to dewater very fine soils.

Related Files
Particle size and permeability data.