OSPE - The Voice - Winter 2019 - 34

COVER

from around the word to work on the project, including from
Spain and the UK. WJ Groundwater has pioneered many novel
dewatering techniques in tunnel construction, beginning with
the Dockland Light Rail Project in East London, the London
Underground Jubilee Line and, most recently for several stations
and cross passages for the crossrail project which runs east-west
under the city of London (soon to be opened as the renamed
Elizabeth Line). WJ's experience and success in dewatering
stations, shafts, portals and cross passages resulted in their
position as the dewatering vendor of choice for the contractors
building Crossrail, one of which was Dragados.
Dragados, coming to Canada as a member of the successful
joint venture proponent on the ECLRT, knew that they needed
to have world class experts. The stakes are high when mining
below the standing ground water level - any short comings in
the dewatering systems can lead to significant delays, and far
worse, instability of the mining face can result in loss of material,
subsurface instability and settlement. Dewatering needs to
be done right. This involves a combination of engineering
design, know-how and a can-do attitude in the field under any
conditions the subsurface presents. CTS needed a partner that
would work together with them through to the end, and invited
WJ to bid on their project. WJ was selected as the dewatering
contractor for the three mined stations and would go on to win
six other stations along the alignment after opening their first
Canadian office on Eglinton Avenue.
WJ began in earnest with 3-D finite element analysis
of the groundwater drawdown. From the beginning, the
groundwater drawdown models highlighted that the Oakwood
Main Entrance underground access, also called the cross-cut,

Final lining works and station waterproofing

THE VOICE

Winter 2020

was the most challenging aspect of the project. This was the
deepest excavation and the ground conditions featured a lowpermeability saturated silty sand, with a soil matrix that fines
downwards. The base of the crosscut was founded just above a
sand/clay interface, which is especially challenging - to halve the
depth water above the interface requires having the well spacing,
so doubling the quantity of wells. The aquifer held 18 metres of
water above the clay interface, 17 metres of which needed to
be drained by the dewatering system! As the excavation depth
approaches the clay interface, the well count risks growing
exponentially. Proposing ejector wells and deep wells from the
road surface, WJ designed the dewatering system to maximise
WJ dewatering equipment on site

OSPE - The Voice - Winter 2019

Table of Contents for the Digital Edition of OSPE - The Voice - Winter 2019