Examples of projects carried out by WJ include:
Castlehaven Coastal Protection Scheme, Isle of Wight
TJ Brent Limited
2004–5
The site located on the southern side of the island is situated within an area prone to landslips and coastal erosion. The project involved the installation of a specialised groundwater control system to reduce the pore pressures within the ground, and extensive instrumentation to monitor its effects. The instrumentation included observation wells containing downhole dataloggers, 42 vibrating wire transducers installed in wells, inclinometers to monitor ground movement and open channel flowmeters to monitor the discharge flows. WJ also installed a fully automated weather station to monitor the local conditions relevant to the system operation. A total of five multi-channel dataloggers were installed to continuously monitor the water levels. WJ carried out long-term monitoring on the site for a period of approximately 15 months. As part of this work WJ managed the data collection, processing and presentation, issuing a fortnightly report.
Moorhouse, Crossrail Draught Relief Shaft, London
Skanska UK Building
2003–4
The redevelopment of the site near Moorgate, London, included the construction of a ventilation shaft 8.5 m in diameter and some 50 m deep from street level. WJ was responsible for the design and installation of a dewatering system to allow the shaft to be constructed in stable ground. This consisted of six deepwells to reduce the water levels in the lower aquifer and 11 ejector wells to relieve pore pressures within the Lambeth group. To monitor the performance of the dewatering system and the construction of the shaft, WJ installed an array of piezometers with triple response zones to monitor water levels, and inclinometers and extensometers to monitor ground movements. Extensometers were also installed in adjacent piles to monitor movement during shaft construction. The monitoring of water levels and the pile extensometers were automated with the use of a datalogger. The information was made available in near real time in graphical form on site via a colour LCD screen incorporated into the datalogger. This was used by our site operatives and engineers to check the operation of the system. The data was also uploaded to the internet daily for use by the many different organisations involved in the project. This easy accessibility of the monitoring information promoted a cooperative approach and allowed the construction to proceed with confidence.
Lapps Quay, Cork, Ireland
PJ Hegarty
2003–4
This mixed development site adjacent to the River Lee is 100m by 60m in plan. The scheme had a requirement for a double basement car park with a formation level at 7.5 m depth. Ground conditions comprised coarse gravels with tidal standing groundwater level reaching virtually up to ground level. Sheet pile side support was installed to 17 m depth, not sufficient to reach the base of the gravels. WJ designed and installed a deepwell dewatering system comprising 24 wells installed to 25 m depth. To monitor the performance of the dewatering system 10 piezometers were installed inside and outside the sheet-piles as well as within the adjacent river to monitor tidal response. Vibrating wire transducers were installed within the piezometers and connected to a datalogger. The extraction flows of approximately 200 l/s were monitored using four 150 mm electro-magnetic flowmeters also connected to the datalogger. The data was uploaded to the internet on a daily basis. This allowed the client’s and consultant’s staff to view the data in Ireland as well as allowing our own staff easy access to the data in the UK.
Dubai International Airport, Terminal 3, Dubai, UAE
Al Naboodah
2002 to 2007
The main contract works involved the excavation of an area approximately 700 m by 500 m to a depth of up to 25 m. Side support was by diaphragm wall. Ground conditions comprised sand and weak sandstone with standing groundwater level within 3 m of ground level. WJ carried out the design, installation and operation of a well dewatering system comprising over two hundred 38 m deepwells. Installation was phased which necessitated the use of a time-dependent 3D computer model to support the design assumptions. Maintenance of the drawdown was critical for the stability of the diaphragm wall. WJ supplied and installed an automated datalogger monitoring system, which provided a real time call-out alarm system and access to the data via WJ’s monitoring web site. Discharge flows were of the order of 180 l/s.
Dublin Port Tunnel, Ireland
Nishimatsu and Mowlem
2002 to 2006
WJ were responsible for the design, installation and operation of deepwell dewatering schemes for a shaft 50 m diameter by 30 m deep; an approach structure 300 m long, 22 m wide by 15 to 20 m deep; and an area 100 m by 40 m for a railway underpass constructed using a pipe arch system. Ground conditions below formation level comprised boulder clay with sand bands over limestone. Dewatering was carried out using approximately 50 wells installed up to 40 m depth.
CTRL 250 Ripple Lane, East London
Nutall Wayss & Freytag Kier JV
2002–3
The main portal for the Channel Tunnel Rail Link London tunnels at Ripple Lane comprised a cut and cover tunnel and open cut section of mainline railway some 440 m in length and from 15 to 38 m wide. Sheet-piles were used for side support to allow excavation to formation level, which sloped from 7 to 15 m depth. The London clay is virtually absent and pressures in the underlying Lambeth beds and Thanet sand were within 1 m of ground level. WJ carried out the design, installation and operation of a dewatering scheme. This included the installation of piezometers and vibrating wire transducers to monitor the water levels. These were connected to a datalogger for automatic collection, which also provided an alarm system, notifying our site operatives should water levels rise above certain targets or pumps fail.
CTRL 320 Thames Tunnel, Dartford
Hochtief Murphy JV
2001–4
WJ was responsible for the design, installation and operation of dewatering schemes for two cut and cover/open cut approach structures for twin railway tunnels under the Thames. The approach structures were each approximately 400 m long and 25 m wide. Side support was by diaphragm wall and excavation formation level varied from -4 to -18 mOD. Ground level was at 1 mOD and standing groundwater level was at ground level. Ground conditions comprised alluvial clay/peat over river gravels over weathered chalk. The structures were generally founded in the gravels and the diaphragm wall was installed to toe into the chalk. On the south side dewatering was carried out with 53 internal chalk wells at a total discharge flow of over 600 l/s. On the north side dewatering was achieved with 14 wells and a discharge flow of 150 l/s. Pumping trials and studies on the north side had shown that dewatering would lead to external drawdown in the gravels and the potential for underdrainage and consolidation of the alluvial soils. As a result WJ installed an array of 50 recharge wells around an adjacent sensitive facility. Abstracted water was recharged to the gravels to maintain groundwater levels and avoid the settlement risk. The abstraction wells were installed inside the diaphragm wall and were sealed on completion.
CTRL 430 Ashford Tunnel, Kent
Skanska Construction
2000–1
The main contract works involved the construction of 1.3 km of high-speed mainline cut and cover railway tunnel and approach retained cuts. The structure and construction sequence was complex, with two chords splitting off and rising up to an adjacent station. In addition the main tunnel passed beneath a mainline railway that could not be disrupted. Side support was by contiguous piles and excavation depth was 10 to 18 m. Ground conditions comprised Atherfield and Weald clays, which are stiff fissured clays with pore pressures 2 to 3 m below ground level. Control of pore pressures in the Weald clay to below formation level would allow significant saving by reducing the extent of temporary propping required during the construction of the works. WJ carried out pumping trials to demonstrate that vacuum-assisted drainage using an array of ejector wells would be effective in controlling pore pressures in the Weald clay. Subsequently WJ installed an array of approximately 200 ejector wells on both sides of the tunnel at a spacing of 10 to 15 m and up to 30 m deep. Discharge flows were only approximately 2 to 5 l/min/well but this was sufficient to achieve the 10 to 15 m reduction in pore pressures required to reduce the need for temporary propping for the contiguous piles.
Canary Wharf Phase 2, Isle of Dogs, London
Canary Wharf Limited
1998–2004
This was one of the largest building projects in Europe comprising more than 15 major buildings up to 45 storeys high in an area 400 m by 400 m. WJ installed a deepwell scheme comprising 15 wells 70 m deep designed to lower the groundwater by up to 25 m to allow the piles for the development to be installed into dry ground. A telemetry system was also installed by WJ to monitor groundwater levels and discharge flows and to provide an alarm warning in the event of a system stoppage.