BTS MEETING | DEWATERING STRATEGIES FOR LONDON


7 Chalk wells


0 RTD -20 London Clay -40 Channel sands -60 Thanet Sand


6 external Thanet wells


Shaft


30m dia. 44m deep


6 internal Thanet wells


Pre-pumping GWL


Sand involves lower flows with localised impact but limited drawdown. The Crossrail Limmo Main Drive Shaft was 30m


in diameter with an excavation depth to 44m. Side support was by diaphragm walls which provided full cut-off through the River Terrace Deposits and Lambeth Group Channel Sands. Pressure relief in the Thanet Sand was needed to construct the base slab with a reduced requirement during the remainder of the tunnel drive construction period. The required drawdown in the Thanet Sand was achieved using the well array illustrated in figure 6. Following excavation and construction of the


-80 Chalk Figure 6:


Dewatering strategy during the excavation of Limmo main drive shaft (Crossrail)


● Due to the depth, ejectors or submersible pumps are required.


● Well installations are unlikely to be in artesian conditions.


This contrasts with in-tunnel / in-shaft dewatering


systems as follows: ● Better access to target horizons (from pilot tunnel or shaft perimeter).


● Systematic probe drilling can be carried out to identify the presence and extent of any sand.


● It is cost effective to install wells at close spacing. ● Well installations may be relatively short. ● Simple and robust vacuum wellpoint systems can be used if drawdown is less than 5m.


● Well installations may be into artesian conditions which require a stuffing box.


A combined surface and in-tunnel/shaft approach,


particularly where pore pressures are high, is often the most attractive strategy. Over the past 30 years, there have been some


significant improvements in dewatering equipment for use in tunnels and shafts. Initially, it was common to use surface equipment below ground but now, more compact specialist equipment for in-tunnelling and shaft dewatering is available. This has now come full circle with in-tunnel equipment and techniques being applied to various surface installations at congested urban sites .


DEWATERING STRATEGY IN THE THANET SAND The Thanet Sand and chalk below are typically in hydraulic connection so that groundwater lowering can be achieved either by pumping from the chalk and under-draining the Thanet Sand or by pumping directly from the Thanet Sand. Pumping from the chalk may generate high flows and a large distance of influence but can be necessary where a significant drawdown is required. Conversely, pumping directly from the Thanet


16 | November 2021


primary base slab, pumping from the chalk and external Thanet Sand wells stopped, and only the internal Thanet Sand wells were operated passively to provide sufficient pressure relief. After completion of the tunnelling drives, the secondary base slab, lining walls and roof were completed and then the internal Thanet wells were decommissioned. Total flow during the eight-month shaft excavation and primary base-slab construction period was 160 lit/ sec, achieving a drawdown of approximately 30m in the Thanet Sand, with most of the flow derived from the chalk. For the remaining two-year tunnelling period, the flow was reduced to 10 lit/sec from the Thanet Sand only, achieving approximately 10m drawdown in the Thanet Sand.


DEWATERING STRATEGY IN CHALK For the chalk in London below the Thanet Sand, much of the significant jointing is near horizontal so that the local vertical permeability can be two or three times lower than the horizontal permeability. At depth, the overburden stress appears to close many of the fissures and joints resulting in a significant reduction in permeability with depth. Chalk is also soluble so that water flow can enlarge joints and enhance flow over geological time. This setting has led to the development of the


following strategy for deep shafts which reach into water-bearing chalk: 1. Prior to shaft construction, install a chalk well within the footprint of the shaft and undertake a pump test and geophysical logging.


2. Use the data from the pumping test to review the depth of the diaphragm wall or other cut-off planned for the works. Numerical modelling may be undertaken to assess inflows.


3. Install the diaphragm wall or other cut-offs. 4. Install a second chalk well within the footprint of the shaft and repeat the pump test to assess inflows. Note that at least two installations are required, so that one can be pumped, and the groundwater level monitored in the other. Where there is a hard limit on abstraction flow


rates or external drawdown, this strategy requires contingency measures, such as a grout curtain or base grouting to be available if flows or drawdowns risk exceeding any constraints.


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