A bespoke ??underground river?? has delivered a unique solution to a £750m redevelopment of one of the UK??s top universities, dramatically reducing the risk of flooding and helping to manage surface water run-off.
York University??s Heslington East Campus is a 65 hectare site and, when completed, will house academic and start up businesses as well as new student residential accommodation. Due to the both the poor ground conditions and environmental restrictions on the site, a typical sustainable urban drainage infiltration system wouldn??t work. The heavy, clay soil would retain water rather than dissipating naturally, and restrictions dictated that the surface water run-off from these pavements be managed as part of a SUDS and should be no greater than that of the original green-field site. To combat these challenges a revised solution was required.
Using the topography of the site, multi-disciplinary civil engineering consultancy, AECOM Ltd, were able to work with the lightly contoured location, discovering a shallow depression ideally suited to create a 64,000 sq m artificial lake, which would form a vital part of the surface water drainage scheme due to its position on a natural low point and being out-falling through a beck in the river. AECOM also specified a concrete block permeable paving system from specialist manufacturer, Tobermore, which would deliver the capacity and flexibility needed for the bold new system.
AECOM Technical Advisor Martin Crabtree explained: ?With such a large and linear network of pavements the only cost effective solution was permeable paving. We looked at conventional point drainage and kerb-drain systems but none gave us the capacity or flexibility we needed within the cost framework??
The construction team was able to adapt the SUDS design principles to create a subterranean water course that ran underneath the 5km of paved roadways. Instead of surface run off being collected in a conventional impermeable attenuation reservoirs before being allowed to dissipate naturally into the underlying sub strata, the underground river captures surface run-off through the block paving and channels it by force of gravity, to the artificial lake which is, in places, almost a kilometre away.
Tobermore??s geotechnical consultants, Geoman, said: ?As natural infiltration was not possible, we took the decision to install an impermeable liner beneath the base and sidewalls of the whole pavement construction so that all captured run-off was completely managed.??
Due to the frequent and heavy vehicular traffic expected on the site – up to 12 buses per hour and a cumulative passage of 15 million standard [8000kg] axle movements over the pavement??s 25 year design life ?? the permeable paving had to perform. Tobermore??s heavy-duty Hydropave was installed in accordance with BS7533-13 2009, with the sub-grade strengthened to a minimum 5% California Bearing Ratio (CBR) due to the poor nature of the ground conditions.
The excavation was then lined with 2000 gauge waterproof polyethylene sheeting and covered with a 150mm deep layer of 20-4mm coarse graded aggregate. Over this was placed a 300mm deep layer of cement stabilised 20-4mm coarse aggregate and a laying course of 6-3mmm clean grit, 50mm thick. The heavy duty Hydropave ML block paving was then mechanically placed using a Probst powered laying machine, approximately 1sq m per drop, and vibration-compacted in the usual way.
The Hydropave system is unique in that it has a wider than usual inter-block joint and integral spacer nibs which maintain a consistent 6mm joint width. This wider joint also gives the Hydropave System permeability rates of 1800 litres/sec/hectare, providing a significant safety factor on the required minimum rates of 180 litres/sec/hectare under BS7533-13. David Henderson, MD of Tobermore, said: ?This is one of the largest permeable paving projects in Europe and required a robust product in order to ensure longevity of both performance and quality, which is why Tobermore??s world class products were chosen for the job.??
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