A massive pumping operation is being planned along 10km of the Channel Tunnel Rail Link in East London to lower rapidly rising groundwater and assist tunnel construction. Nina Lovelace investigates.
In Stratford it's been coming up at 1m a year, ' says Channel Tunnel Rail Link hydrologist David Whitaker. He is speaking of east London's rising groundwater levels. Sitting in his site office at Stratford, Whitaker proffers a graph to illustrate his point. The graph line shows a steep slope - revealing that in some areas groundwater is now only 7m below ground.
Once it was over 30m.
Groundwater levels in London have been rising steadily since heavy industry began to move out of the capital last century. This allowed groundwater resources to start rising.
But this creates construction and safety risks for Rail Link Engineering (RLE) designers.
For the second phase of the CTRL they are planning to build double bore 9m diameter tunnels 30m below ground level. To carry the new high speed line between Kent and north London they must tunnel through ground with water pressures reaching 25 bar.
As a result east London will get one of the biggest temporary dewatering schemes the country has ever seen, using pumps each capable of shifting a massive 500litres/second sustained over three years. Construction of a 10km long 600mm trunk main is needed to dispose of these massive volumes.
'It had been discovered on the Jubilee Line Extension that dewatering greatly benefited ventilation shaft construction, ' says Whitaker. Removing water reduced soil and water pressures around the shaft during construction and the risk of uplift and collapse during base and wall construction, he says.
With three shafts on the CTRL in ground with a high water table, dewatering was decided on. Much of the expertise needed was already on board at the scheme's new Stratford station, which will be permanently dewatered to ensure the structure's stability. Tunnelling Contracts 220 and 240 also need dewatering and were within the area of influence of the permanent system, making the dewatering job easier.
So within two years East London will have permanent and temporary dewatering systems working in tandem to create a network of drilled well clusters up to 30m deep dotted around the shafts and station, and four wells every 400m along 10km of Contracts 220 and 240 tunnel trace.
A pump will be sunk into each well down to the chalk beds below the Thanet Sands which hold the rising lower aquifer. A separate upper aquifer overlies the impervious London clay above the Thanets, but this will remain largely unaffected.
Shaft construction was not the only deciding factor for dewatering, Whitaker explains.
Without dewatering pressures would be greater than atmospheric, meaning that workers at the cutting head at the face of the tunnel boring machines (TBMs) would have to work under pressure. Dewatering lowers the health risks and saves time by allowing workers and TBMs spend maximum time in service.
Dewatering also aids the digging of emergency and maintenance cross passages between bores - construction of which is programme critical, says Whitaker. 'Cross passages have to be hand excavated, which is much more difficult under high water pressures, ' he adds.
Under pressurised conditions, the surrounding ground would need to be frozen, adding to cost and time.
Dewatering has an added advantage because it lowers the moisture content of the resulting tunnelling spoil, easing handling and treatment.
Whitaker is keen to point out however that despite these benefits the team is not relying on dewatering to save time on its tight tunnelling programme.
'Tunnelling against lower pressures may reduce TBM wear and speed up the tunnelling process, but we're not banking on it, ' he says.
To determine how many pumps would be needed along the route, extensive pumping trials were carried out during site investigation by project dewatering contractors; WJ Groundwater on Contract 220 and Bachy Soletanche on Contract 240.
Data was then fed into a groundwater model, which not only worked out the numbers of pumps and how quickly they could remove the water, but also gave RLE an idea of what the overall environmental effects the groundwatering procedure would be.
Overhead rivers and marshes would not be drained as they will still be fed by the upper aquifer that lies above the London Clay, although up to 1km away borehole users abstracting water may see their yields drop.
RLE's main concern was how much overhead settlement would occur as the water was sucked out of the porous Thanet Sands.
But the model showed that as the abstraction affects such a wide area, the surrounding ground will settle uniformly and properties and services with it. 'Significant differential settlement isn't an issue, ' says Whitaker.
The disposal of the huge volumes of water was a concern, however. Water in the lower aquifer is close to potable standards and initially the water yielded was to be directed straight into the sewers, following some settlement treatment in tanks. Some water will flow into sewers, but London's combined flow system will be unable to take the volumes as they rise without risk of flooding.
As a result a new main will be dug alongside the 10km tunnel trace to remove the water and deposit it instead into the adjacent Lea and Roding rivers. At its biggest bore the main is expected to be in excess of 600mm diameter to cope with high initial flows. Dewatering works are essential to the overall programme, adds Whitaker.
Temporary dewatering must lower groundwater levels enough to allow tunnelling to begin in 18 months' time. Ventilation shaft sites will receive up to a dozen wells because construction at these is due to start sooner, so water levels must be reduced more quickly.
Eleven permanent dewatering pumps are already operation at the Stratford station box, excavation for which began in July.
Temporary pumps are being drilled into areas where the lowest tunnel inverts lie. The pumps will stay down there for three years until the end of tunnel construction, but Whitaker does not expect them to need much maintenance due to the high quality aquifer water. Flow rates will be carefully monitored to check their operation.