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Taking the soft approach


Construction of the approach structures for the Thames Tunnels is complicated by soft ground, high groundwater and strict environmental constraints. Colin Warren and Paul Watson report.

The £130M Thames Tunnel contract will carry the CTRL beneath the Thames estuary at depths of up to 40m. The section, being built by main contractor Hochtief/Murphy joint venture (HMJV), comprises twin, 2.5km long, bored tunnels and their approach structures.

The tunnels will be built under the Thames from Swanscombe marshes on the south side to West Thurrock marshes to the north side. From the south bank, the tunnels will pass beneath old Blue Circle Industries (BCI) cement kiln dust tips as they head out under the River Thames at 20m below river bed level. On the north, they will pass beneath a site of special scientific interest (SSSI) natural wildlife site before emerging next to the Van Ommeren Vopak petrochemical storage facility.

Ground level across the marshes is flat and low lying, but on the north side of the river, land raising has built up the ground level by as much as 3m. On the south bank, the deposition of cementkiln dust from the Northfleet Cement Works has raised the ground level by as much as 13m.

There are also flood defence works immediately next to the river. At 6m high, they are designed to protect low lying areas in Kent and Essex against a one in 1,000 year flood event.

The main site is on the Swanscombe marshes. Although previously used for - among other things - quarrying and heavy industrial processes, the drained marshland forms one of the UK's few safe habitats for water voles.

This meant that extensive mitigation had to be carried out before main work began, to provide a new habitat for the rare and protected species.

Geology typically consists of 10m to 12m of alluvium (very soft clays and peats) over up to 6m of terrace gravels (dense flint gravels) resting on low-medium density Upper Chalk of the Seaford and Lewes Chalks.

The alluvium and much of the gravel is absent in the river.

Groundwater levels in both the gravels and chalk are close to ground level and are tidal close to the river. The chalk is highly to moderately weathered in the top 10m (CIRIA Grades D and C), becoming blocky (CIRIA Grades A and B) with depth.

Typical permeabilities in the gravel range between 10 -3 m/s and 10 -4 m/s and in the chalk from 10 -4 m/s to 10 -6 m/s. The chalk contains many flint beds, with flints typically 0.2m in diameter and beds spaced 0.5m to 1m apart.

While no major faults were identified, several minor faults and open fissures could be met based on evidence from nearby quarries.

Major engineering challenges associated with this contract include:

lDewatering the gravels and chalk to allow construction of the approach structures on each side, while controlling external drawdown in the gravels and potential settlement of the overlying alluvium in the surrounding area;

lDealing with the highly abrasive flints within the chalk which have given problems on previous tunnel contracts in the UK, most recently Brighton's stormwater sewer;

lPumping and treating the gravel and chalk slurry to obtain engineered fill that can be used to infill an old chalk quarry situated next to Swanscombe marshes between the North Kent Railway Line and London Road which runs between Northfleet and Bluewater;

lConstructing major approach structures through very soft alluvium and peat layers in ground with a high water table.

This is the first time that a major chalk tunnel in the UK has been driven using a slurry TBM.

As a result, large-scale trials of both TBM and proposed treatment plant were needed to demonstrate that tunnelling and spoil handling would work.

The finished bored tunnels will be 2,515m long and have an internal diameter of 7.15m (cut diameter 8m). Following the tunnel alignment, the river channel is around 1.2km wide and 18m deep.

At its lowest point, the tunnel will be subjected to water pressures of up to 4bar. The tunnel will have four, 3.5m diameter cr oss passages every 665m, three allowing access between the tunnels, while the fourth will be built at the low point of the tunnel to accommodate the drainage sump.

Construction of the cross passages will involve ground treatment and hand excavation.

To reduce the construction programme and to minimise risk, the tunnels will be driven using separate TBMs. However, as construction of the tunnels is sequential, both TBMs will share the same back-up. Tunnelling equipment has been built by German firm Herrenknecht. The slurry TBMs will also be able to use compressed air at the face and incorporate onboard oxygen decompression facilities.

The concrete segments are being cast on site. These are reinforced with steel-fibre to give handling durability and, by adding polypropylene fibres, the lining performance will also be improved in the event of a fire. Each 1.5m wide ring will comprise nine segments plus a key. Watertightness of the tunnel lining will be provided by a dual gasket system using both EPDM and hydrophilic gaskets.

A total of 400,000m 3of spoil will be produced during tunnelling, 89% of which will be chalk, 18% gravel and 2% soft alluvial clay and peat. This will be crushed at the face and pumped through a high pressure slurry main to the surface before being sent to a treatment plant.

The gravels and crushed flints will be screened off and the remaining slurry passed through a series of hydrocyclones and centrifuges to remove the water.

Chalk residue will be transported by closed conveyor to Craylands Lane Pit, an old chalk quarry next to the site. At the pit, lime will be added to the chalk spoil before it is compacted in thin layers as part of a land raise for future development.

The tunnel approach structures comprise 300m long cut and cover sections followed by 150m of retained cut. A further 295m of open cut will be built to the south as the alignment passes through the chalk spine of Galley Hill Road.

The cut and cover and retained cut sections will be formed using diaphragm walls installed by Amec-Spie. The 1.2m thick walls will typically be built in 6.8m long panels up to 30m deep (up to 12m into the chalk). The panels are reinforced using three unlinked steel reinforcing cages fabricated using 50mm reinforcing bar and one in 10 has been sonically logged. The excavations are sealed against water ingress using 150mm wide water bars between the panels.

Extensive dewatering by WJ Groundwater was carried out before and during construction of the approach structures.

Slurry cross-walls were installed to divide each approach structure into three hydraulically partitioned chambers. Forty four dewatering wells were installed between the diaphragm walls on the south side and 50 were put down on the northern side.

Another 60 recharge wells will be installed outside the northern box to limit settlement of industrial buildings next to the excavation.

Outflow from the wells on both sides is expected to reach 600 litres/s. At the moment, 250 litres/s is being pumped out on the south side, giving 15m to 20m drawdown in the highly permeable gravels and chalk beneath the box.

Settlement points, inclinometers, strain gauges and piezometers are regularly monitored to provide feedback on settlement, wall deflection, prop loads, and water levels within the boxes and up to 500m from dewatering operations. Recorded water levels and plots are available for inspection on a daily basis on the intranet.

Dewatering is rigorously monitored, with stringent performance criteria specified for maximum and minimum drawdown requirements at each stage of the excavation. The superposition effects of differing dewatering regimes within adjacent excavation cells has provided a considerable challenge.

Groundwater will be allowed to recover once work is finished.

Any uplift pressures will be resisted on both sides by up to 150, 750mm diameter tension piles up to 18m below formation level.

Amec-Spie JV is installing the piles in advance of approach structure excavation. To limit interaction effects between diaphragm walling and piling, the contractor is using bored piling with bentonite support.

However, this has led to some bore failures in the gravels, and trials are under way using casing support through the gravels.

The cut and cover and retained cut sections of the approach structures will be built predominantly using bottom-up construction techniques.

However, two shor t sections of top down construction - one on the south and one on the north - are needed to allow utility and road diversions over the box.

Excavation is being carried out by contractor Land and Water, using long-reach excavators. Three levels of tubular steel props installed by Linkweld will provide initial excavation support.

Propping was designed by HMJV and is verified by detailed strain gauge instrumentation of the props and inclinometer and total position monitoring of the diaphragm walls.

As with all CTRL contracts, safety remains the number one priority and already the Thames Tunnel contract has completed over 500,000 hours worked without a reportable lost time accident.

Work began in July 2001 and is well under way on the southern approach, with over 30,000m 2ofdiaphragm walling installed and more than 110,000 spoil excavated.

TBM launch is on schedule for July 2002.

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