It’s little more than an underground box, so this is a fair question for our subterranean immersion correspondent Angus Young to address.
When you sink a road several metres underground in Hull it’s the obvious question to ask. As anyone who has driven under the section of Chanterlands Avenue which dips under a railway bridge will know, heavy rainfall usually creates a big puddle pretty quickly. So will the same thing happen on a much bigger scale in the underpass currently being built on the A63 at Castle Street in the city centre?
According to National Highways’ senior project director Fran Oliver, the answer is no.
Speaking at a recent city council scrutiny meeting, she gave details of the a new pumping station being constructed next the junction with Commercial Road which has been designed to keep traffic flowing once the new-look dual carriageway is finally finished in Spring next year, She said: “The pumping equipment in the building will serve the underpass in times when there is excess water from run-off from the road. It will have a huge capacity and it’s a really significant part of the overall scheme. Just to be clear, as well as dealing with normal run-off it is also designed for a flooding-type incident on the road. We are conscious it’s a big dip in the road and we know what water is like so we don’t want it to collect there. The pumping station is designed to deal with flooding although it’s more than likely to be triggered by excess rainfall on a more regular basis”
She said sensors inside the underpass will pass on real-time information on the build-up of any water. If large volumes need pumping out, water will initially be held in storage tanks before being gradually released into the city’s main drainage system.
“The pumping station will be connected to our regional control centre so they will know when to switch it on in the event of a problem if they need to override the automatic systems,” said Ms Oliver. “There’s a full emergency plan in place and National Highways also has mobile pumps which, in exceptional circumstances, can be brought here pretty quickly. Inside the station itself, there is a pump and a back-up pump as well as a power back-up.”
She said she had no worries about the underpass itself being able to withstand extra pressure from any flooding, however small. “The structure is incredibly robust. The concrete walls are capable of bearing an enormous amount of pressure and they go 27 metres deep into the ground. They are absolutely huge concrete walls and, together with the base slab, they are why it has taken us a little longer to install. We have tried to ensure that all the elements work together. For example, when we finish the base slab and take the props out, we won’t do anything for a week or so because we have got so much monitoring instrumentation to make sure everything is absolutely belt and braces.”
Because of the high water table in the immediate area, she said National Highways had opted to use concrete for the walls of the underpass instead of sheet piling which is more commonly used in similar construction projects elsewhere. “A lot of the design was influenced by the high water table. Huge volumes of steel and concrete have been put in, essentially to weigh it down because the water pressure is so great. Other underpasses might be constructed with sheet piles but they just wouldn’t be appropriate here because they wouldn’t be able to stand up to the strength of the water.”
When completed and fully open, the new underpass has been designed to have a 120-year lifespan.