Sub-surface dewatering for an inclined SCL tunnel

Crossrail is a major new east-west rail link through central London. The main running tunnels have been driven by TBM with most of the stations and link tunnels constructed using sprayed concrete lining (SCL) techniques.

This paper is concerned with the dewatering and depressurisation measures required for VD4 which comprises a 37m long by 6m diameter inclined ventilation tunnel for the Crossrail Liverpool St Station at Moorgate in the City of London. As shown in Figure 1 and 2, VD4 links from the Eastbound Launching Chamber (EB) down below the Westbound Launching Chamber (WB) to an existing shaft. The TBM running tunnels were not constructed at the time of the VD4 works. The shaft was constructed for Crossrail in 2004/5 in the basement of an office development known as Moorhouse. In order to avoid the need to handle potentially large volumes of slurry, the shaft was constructed in ‘dry’ conditions by underpinning. The design and construction of the shaft has been described in some detail in previous publications [1] [2]. The method of shaft construction required depressurisation measures to be adopted to allow excavation through several water bearing horizons including non-cohesive sands (Lambeth Group Laminated Beds and Lower Shelly Beds) present at the level of the connection to VD4. The depressurisation measures required for the shaft construction were successfully achieved using an array of ejector wells installed from surface around the shaft. The wells were operated from the second basement level. The shaft is 8.2m diameter and sufficiently deep (38m below basement level and 50mbgl) that depressurisation wells were also required in the Lower Aquifer (Lambeth Group Upnor Formation and Thanet Sand). Investigations at the start of the VD4 works confirmed that the shaft dewatering wells had been grouted up sometime after the shaft works had been completed.