TREATMENT OF CHALK SPOILS FROM THE LEE and CROSSRAIL C 310 THAMES TUNNELS
Clients: Vinci Construction Grands Projets Bachy Soletanche Joint Venture / Hochtief Murphy Joint Venture
The Thames Water Lee tunnel, with its 8.88m diameter, 6,904m length and its geology of more than 98% in chalk, is a real challenge in terms of spoil treatment. The total volume of the chalk slurry after separation of the flints (5 to 25%, average 15%) represents more than 1,200,000m3 of liquid slurry. In such geology, where 95% of the solids could be transformed into extra-fines, well below the cut off point for cyclones or centrifuges, several method have been tried in the past to facilitate the disposal of the spoils, but none met the requirements of the contractor, both in terms of costs and final result. This presentation will describe the methods and results achieved by the Morgan Sindall Vinci Construction Grands Projets Bachy Soletanche Joint Venture MVB in reaching a consistency of more than 75% solids by weight, and thus drastically reducing the volume of spoils and easing their disposal.
The presentation will also give an overview of the results obtained on another tunnel site in similar geology, the Crossrail Thames Tunnel, by Hochtief Murphy Joint Venture HMJV. After their success in the award winning CTRL 320 tunnel, where the discharge of the centrifuges was mixed with cement before placement, HMJV has accepted to reconsider its excess mud treatment method, as they has been convinced by the arguments in favour of filter-presses. This enable them to experience a different approach to the slurry management.
PRESENTATION OF THE PROJECTS
The Lee Tunnel is part of a scheme to store and send to treatment scheme to store and send to treatment the sewage and rainwater from London, which presently overflows into Thames River. The 6.9 km long Lee Tunnel runs underwater and underground through East London, to carry sewage from London's largest combined sewer overflow from the Abbey Mills Pumping Station in Stratford to the Beckton Sewage Treatment Works (STW) in Newham. Tunnel Work started in February 2012, and the entire project is expected to be completed in 2015.
CROSSRAIL C 310 :
The 2.6 km long twin tubes Thames Tunnel is the only Crossrail route which crosses the river Thames. It is also the only slurry shield TBM of this project, because of the chalk and Thanet sand, elsewhere EPB machines run in London clay, sand and gravel. Boring started in October 2012, and is due to be completed early 2014.
The Lee tunnel runs predominantly in Seaford Chalk formation, with a small section of overlaying fine clayey Thanet sands. This Chalk consists of white, sometimes light grey, low to medium dense, weak to moderately weak chalk, with layers of flints ranging from 5 to 25%, with an expected average value of 15%. A bulk density of 2 kN/m3 has been assumed for the design.
At the time of the sizing of the separation plant, we pointed out that there were two questions which were not fully answered in the GIBR and which were of major importance for the sizing of the Separation Plant : one was the ability of the chalk to go into suspension, and the other was the filterability of the resulting slurry. We have therefore received 2 tons of samples that we tested in our laboratory.
The Crossrail Thames Tunnel is in the same geology : only the proportion of Thanet sand in the drive is slightly greater :
PRELIMINARY WORK AND LABORATORY TESTS
The "Volume & Mass Balance" calculation sheets, or "VMB", have been developed by MS to help in the sizing of a Slurry Treatment Plant with regards to the geology of one particular tunnel project, and to the technical choices of the contractor in terms of instant boring speed and daily progress target. It also helps to study the consequences of any modification of a parameter, such as density, viscosity, or instant and daily boring progress target in terms of sizing of the components or in terms of consumables
It includes the following sections :
- a summary of the geological profile of the tunnel,
- the VMB calculations for each geology,
- a summary showing the critical values given by the VMB in these different geologies - values which will become sizing criteria,
- a calculation of the spoils volumes and of the consumables for the entire tunnel.