SEVERE WATER ingress has forced engineers constructing a cable tunnel beneath London's Olympic Park to delay the breakthrough of the fourth and -nal bore, it emerged this week.
Dif- ulties 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 ooding the tunnel.
fiThe last shaft has been particularly dif- lt to dewater, fl said Olympic Delivery Authority project director Terry McDonald.
fiWe 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. fl 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.
A series of shafts have also been dug to connect the underground cables to electricity sub stations.
McDonald said the extra work will not delay the overall project as mechanical and electrical contractors could start their work in the rest of the tunnel.
Contractor on the £80M project is Murphy. Faber Maunsell is consulting engineer and Arup the Olympic Delivery Authority's (ODA) checking engineer.
Speaking at a press brie-ng to mark the - st two breakthroughs Murphy managing director James O'Callaghan explained the tunnels encounter some challenging ground conditions. fiThe geology has been extremely variable, fl said O'Callaghan.
fiIt quickly moves from hard clays to very very soft silts and muds, and we have been working under three bar pressure for most of the drive. fl This variability forced the contractor to constantly change the additives it used to condition and stabilise the ground.