Happy campus

If you have ever been to the University of York, you cannot have failed to notice the lake that is central to the campus landscape. So you might be forgiven for thinking that the lake at the new campus - Heslington East - is just an effort to link the design of the two campuses.

The reality is that the lake at Heslington - along with the sustainable urban drainage system formed by a permeable pavement and swales - was the deciding factor when it came to planning approval. The university would only be granted consent for the 65ha campus if the roads and pathways created the same run off as the existing farm land.

The result - a collaboration between consultant Aecom and paving specialist Tobermore - will result in one of the largest, longest term permeable pavements in Europe with a 25-year design life. The current titleholder of the largest area is located at the London Olympics site, but that pavement only has a design life of two years.

“The cost difference between this approach and a flexible asphalt paving is not significant but the visual impact of the conventional alternative would be massive.”

Martin Crabtree, Aecom

The work at York’s new campus is phased and 60,000m2 of paving has been laid since the work started in 2008 with the area set to double by the time the project is completed later this year.

“The University of York started work on a planning proposal the Heslington East campus in 2004 and a public enquiry in 2007 approved the proposal but with some conditions,” explains Aecom senior transportation consultant Martin Crabtree. “One of the conditions was for the network of footpaths and roadways to create a run-off no greater than the existing greenfield site. This condition was a significant challenge given the size of the paved areas.”

Before development, the site was sloping farmland underlain by heavy clay and run off from the site drained into the Germany Beck - a tributary of the River Ouse, the waters of which are infamous for causing major flooding in the city of York.

“The development was a key part of the university’s plan to grow student numbers by 10,000, staff by 3,000 and to provide space for start-up companies employing another 5,000 people - a significant expansion,” says Crabtree.

The aim was for the campus to be car free, with car parks around the periphery and a “movement spine” to carry buses and service vehicles through the development, along with a network of pavements for pedestrians. It was the heavy loading and turning forces, combined with the long service life, that has separated the design of this permeable pavement from previous designs. “The permeable pavements have been designed to withstand15 million standard axles,” says Crabtree.

Aecom first contacted Tobermore to find a solution in 2008 after Tobermore specification sales manager Robert Davenhall met Aecom’s Paul Turton at a conference.

The solution being used at the York site uses Tobermore’s Hydropave 240 brick paving system but there are a number of structural differences from other block paving systems that made this ideal for the university development.

“The cost difference between this approach and a flexible asphalt paving is not significant but the visual impact of the conventional alternative would be massive,” says Crabtree. “An asphalt solution would have needed a high number of culverts, gullies and attenuation ponds.”

“The block measures 120 by 240mm and is what we’d call an oversize block as standard systems use 100 by 200mm blocks,” adds Tobermore’s Davenhall. “The larger size offers better interlock and is more heavy duty.

“Design is critical to the success of permeable pavements. There are no geotechnical limitations to the use of the technique, just the need for the right design.”

Robert Davenhall, Tobermore

“There is no standard design for permeable paving but it is generally only used in car parks. The design for York does vary but 70% of the road is formed from the pavers underlain by a 50mm thick layer of 6mm aggregate that also fills the voids between the block.” Below this there is 300mm of cement stabilised coarse graded aggregate that provides the durability of the road and has a 24% void ratio. The lower level of the pavement is formed from 150mm of 4mm to 20mm coarse graded aggregate with a 32% void ratio.

Permeability retained

The use of the 6mm aggregate is one of the key differences of the design and, according to Davenhall, this allows the pavement to retain its permeability even between the recommended five-year cleaning intervals. This concept is being put to the test at York where newly installed sections of Hydropave are being used
by construction traffic for access to areas still under development.

“Some of the paved areas are covered by mud but our tests show it can still perform under these conditions and can be cleaned by jet washing,” says Davenhall. “If necessary the grit between the blocks can be replaced.”

Flow from the paved areas is prevented from flooding nearby ground by an impermeable membrane in the side and base of the road. Pipework diverts runoff into a number of swales - grass-lined drainage channels - that guide the water down the lake, designed to mirror the lake at the main campus and to provide attenuation.

The flow of water through the foundation layers of the road is controlled by baffles to direct a fixed volume of water - determined from the maximum rainfall expected from a 10-hour storm - into each swale. “The swales slow the flow of the water and prevent areas becoming overloaded and causing surface flooding,” says Davenhall.

Nonetheless, the site team faces a challenge when it comes to constructing the next phase as thetopography of the site means that the road passes through some “low points” where the groundwater will be high. Work on this section of the scheme started in March and is expected to take six months.

Davenhall believes that use of permeable paving at York will help to improve the image of such sustainable urban drainage systems due to the scale of the project and the loading capacity.

New sustainable urban drainage guidelines are due to be published at the end of the year - a move welcomed by Davenhall. “Design is critical to the success of permeable pavements,” he says. “There are no geotechnical limitations to the use of the technique, just the need for the right design.”

Davenhall believes that acceptance of the approach has been held back by a number of highprofile failures and points to the use of permeable paving at a bus depot where use of the wrong sand to fill the gaps between blocks resulted in liquefaction under load.

“The design work we have done for the York project shows that permeable paving can deliver a durable solution for more than just paths and lightly loaded areas,” says Davenhall. “The next big hurdle for the solution is to get local authorities to accept the capabilities of the system and adopt roads formed in this way.”