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Thames Tunnel spoil and Ebbsfleet quarries

Construction of the £130M Thames Tunnel (CTRL Contract 320) is already well advanced and is expected to be completed by August 2004. The project comprises twin 2.5km long bored tunnels (8.15m external diameter) running underneath the River Thames and two 300m cut and cover tunnels, one on either side of the river (Figure 5).

The contract also involves construction of two retained cut approach structures on soft marshes, one 160m long on the northern bank and the other 440m long on the southern side.

On the northern approach the cut and cover tunnel (Thurrock site) is a maximum of 23m wide and 17m deep whereas the one on the southern side (Swanscombe site) is up to 36m wide and 19m deep.

The two bored tunnels, both now finished, run primarily through chalk, where sharp and abrasive flints posed a major concern. Andrew Lord emphasised the importance of detailed studies of the chalk during the planning stage.

Close inspection of that part of the stratigraphic column of the chalk indicated regular flint bands on the tunnel alignment.

The size of the flints and the frequency of these bands were then assessed by correlation with exposures of chalk nearby at the Swanscombe quarry (Figure 6). This allowed the efficient planning of the tunnel alignment as well as optimisation of the slurry pressurised TBM being used. A special TBM crusher was designed to crush the larger flints, allowing them to be safely pumped to the surface together with the chalk slurry.

One of the main challenges associated with tunnelling was the disposal of the excavated chalk. The spoil, in the form of a slurry with a high water content, was initially passed through screens to remove the flint and subsequently through a series of centrifuges to reduce the water content. Although the moisture content of the spoil was reduced, it was still over 30%, making it too high for placing as fill.

Additionally, the criteria for sustainable disposal dictated that the final mix should have an undrained shear strength of at least 50kPa within three months of being deposited.

Adding lime reduced the water content but did not provide the required strength. This was achieved by adding just 3% to 5% of cement to make the spoil suitable for disposal in an old chalk quarry, Craylands Lane Pit (Figure 5).

Lord then described the £120M Ebbsfleet Valley and associated A2 road widening (CTRL Contract 342), due to be completed in June 2005.

The 3.5km long contract links the southern portal of the Thames Tunnel at Swanscombe with the already completed Section One at Southfleet in Kent and includes a new International Station at Ebbsfleet (Figure 7).

Additionally, elevated embankments and a ramped viaduct connect the existing North Kent Line with the CTRL.

Widening of the A2 is associated with construction of a cut and cover tunnel for the CTRL under the road at Pepper Hill with the provision of slip and approach roads for access to and from the new station and its extensive car parking facilities.

The contract also includes construction of two bridges to carry road and rail traffic where the CTRL alignment passes through chalk spines at Galley Hill Road and the North Kent Line, together with a bridge over the River Ebbsfleet. The past industrial use of much of the site as chalk quarries for cement production, together with the presence of several electrical sub-stations in the vicinity has meant considerable diversions of utilities.

Of the challenges during the planning and construction stages of this contract, the presence of potentially contaminated sites in the extensive former chalk quarries and the discovery of an AngloSaxon watermill of archaeological importance during construction were mentioned.

Possibly the most exciting aspect of this contract was the placement of two bridges to carry the North Kent Line over the new CTRL. The two bridges had to be slid into place in just 72 hours to minimise disruption to the railway network.

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