Severn Trent Water is currently building a major storm water tank at Willenhall, near Birmingham to capture 20,000 cubic metres of storm water as part of a wider storm water management programme.

In the event of a major rainstorm, the storm tank is designed to provide a buffer capacity to remove the threat of combined storm water and sewerage from over-spilling into the river.

The construction of an underground structure of this magnitude has posed substantial challenges to Main Contractor North Midland Construction PLC. The challenges are made more difficult due to the variable geology across the 170m length of excavation, together with a high water table and sub-artesian pressures in the region. With the deepest excavation at 14m below the original ground level, a substantial dewatering and artesian pressure relief operation is required to lower the water table and to prevent ground heave by reducing the underlying sub-artesian head.

To overcome the identified difficulties in advance of the commencement of the excavation, North Midland Construction (NMC) followed a policy of early contractor involvement so that a strong technical team was assembled six months before work started. This team comprised North Midland Construction (Main Contractor), Eastwood and Partners (Structural Engineers to NMC), Ivor King (Sheet Pile Wall Contractor) and OGI Groundwater Specialists Ltd (Dewatering Contractors).

With the team assembled, the design of the temporary works structure, together with the ground stabilisation, could be refined taking into account the individual operations. In particular, the number of cross-bracing structures required was a critical factor: too many and the access to the excavation base using a long reach excavator was impractical; too few and there would be insufficient support of the sides of the structure.

OGI was engaged to overcome and manage three major groundwater problems. The first was to lower the water table within the excavation, so providing a safe and stable ground environment within which the earth moving contractor could excavate the ground and specialist contractors could install the tension piles.

Secondly, high artesian groundwater pressure existed in a sub-stratum just below the excavation formation. OGI was required to reduce this sub-artesian pressure, and maintain this reduction throughout the excavation construction until sufficient weight of the structure was achieved.
Thirdly, due to the multi-layer hydrogeology at the site, OGI designed and installed a robust pressure reduction system to lower the perched water table on the outside of the sheet pile wall, thereby reducing the high level active earth pressure.

OGI worked closely with Ivor King to ensure that the expected reduction in the groundwater pressure was taken into account in the calculations of total active horizontal pressure on the sheet pile wall. To ensure the continuous lowering of groundwater pressure is maintained, OGI designed a combined active and passive dewatering system for maintaining reduced the groundwater pressures.

Active pumping boreholes were constructed both inside and outside the structure to ensure that under working conditions the water level was maintained sufficiently low to provide a safe and stable excavation.

In addition, a series of passive boreholes were constructed to provide two key requirements. The first requirement was to ensure that if the power failed for any reason, the groundwater would bleed into the excavation through the passive wells without failure of the ground beneath the formation. The second requirement of the passive system was to allow the bleeding of the groundwater around the sheet pile wall through engineered pathways, so maintaining a reduced water pressure on the outside of the sheet pile structure.

Early contractor involvement resulted in benefits to both the economics and safety of the project. NMC accepted OGI’s recommendations to undertake a series of test boreholes in advance of the commencement of construction from which the hydrogeological characteristics and critical groundwater properties were calculated from pumping and monitoring tests. This information allowed OGI to develop a mathematical model of the site (see contour plan and section simulations) which was calibrated against the test results.

Having a calibrated model in advance of the commencement of the construction programme was beneficial to the overall programme. OGI was able to optimise the design of the dewatering and artesian pressure system and to predict horizontal active pressures on the sheet piles.
The dewatering operation commenced in April 2010 with OGI’s experienced site team drilling and installing further dewatering and artesian pressure relief boreholes within NMC’s rigorous safety management plan.

The dewatering operation started at the deeper end of the structure, working north towards the shallower end. With each borehole being pumped before being connected to the discharge system, the drawdown and flow rates were relayed to OGI’s technical support team in Durham where the groundwater model was refined. This enabled a more detailed picture of the site hydrogeology to be built up from which the site team could be directed to ensure the most effective and efficient borehole depths and locations.

In conclusion, the NMC management of the Willenhall storm tank construction recognised the benefits of integrating the key specialist contractors at an early stage. OGI’s early involvement gave time to apply their unique skills in real-time mathematical modelling in combination with experienced design, dewatering system installation, monitoring and testing.

The benefits of this approach resulted in the OGI site team installing much of the groundwater management system ahead of the main commencement of the construction works. The remainder of the system was installed to the construction programme working in conjunction with the other key contractors to provide efficient progress within a safe and stable construction environment.