This £9.3 million scheme was designed to deliver environmental improvements for the highly urbanised Black Country area of the West Midlands. The project required construction of a rectangular reinforced concrete storm water tank measuring 170m x 15m internally to a depth of approximately 14m below ground level (bgl).
The challenges facing the main Contractor were considerable, 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 was required to lower the water table and to prevent ground heave by reducing the underlying sub-artesian head.
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 designed a combined active and passive dewatering system for maintaining reduced 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. OGI’s Client 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 which was calibrated against the test results.
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.
The project was successfully completed in July 2011