Severe water ingress has forced engineers constructing a cable tunnel beneath London's Olympic Park to delay the breakthrough of the fourth and final bore, it emerged this week.
Difficulties in de-watering a 10m diameter, 26m deep shaft which will be used to connect power cables into an electricity substation in Hackney, means that contractors must build a secant piled wall around the shaft to prevent water flooding the tunnel.'The last shaft has been particularly difficult to de-water,' said Olympic Delivery Authority project director Terry McDonald. 'We dug down to 16m when we hit a very high pressure aquifer. So for the remaining 10m we are building a secant piled wall. We will then excavate and line the shaft, before then cutting out an eye for the tunnel.'This work is likely to take up to seven weeks. Even though the tunnel boring machine (TBM) is expected to reach the shaft in two to three weeks, it will be another three to four weeks before it can break through.Two 4.15m diameter TBMs moving in opposite directions underneath Stratford, east London have been boring the tunnel to take National Grid power lines underground. Two other 2.82m diameter TBMs have been doing the same on a second tunnel for EDF Energy power lines.This will allow 52 electricity pylons to be demolished so that construction work on the Olympic park can start next summer.