London Reservoir Sheet Pile Cut Off Wall
Sheet Piling (UK) Ltd have recently commenced works at William Girling Reservoir, North London where we are installing a sheet pile cut off wall through the existing reservoir clay earthwork embankment.
Sheet Piling are working on the project for Costain Atkins Black & Veatch (CABV) JV and Thames Water in order to provide an impermeable sheet pile cut off wall in an area where leakage through the reservoir core is currently ongoing. If the current situation remained without any remedial works being undertaken, there is a significant risk of potential for instability of the reservoir embankment which would have catastrophic consequences.
William Girling Reservoir is part of the Lee Valley Reservoir Chain that supplies drinking water to London. It is named after William Girling OBE, a Chairman of the Metropolitan Water Board (MWB). The reservoir and the nearby King George V Reservoir are known collectively as the Chingford Reservoirs. The storage reservoir, which is owned by Thames Water, covers 334 acres with a perimeter of 3.5miles.
Construction of the reservoir commenced in 1936 by John Mowlem using mechanical scrapers and bulldozers for the first time in British dam construction and therefore, allowed rapid progress. Part way through construction, a major slip occurred when the embankment had reached 7.0m in height. Two independent soils mechanic’s expert’s Dr Herbert Chatley and Professor Karl Terzaghi both investigated the slip recommending modifications to the design. Subsequent investigations into this landslip can be regards as the birth of modern soil mechanics in Britain.
Construction was further delayed by the outbreak of the Second World War and the reservoir was not finally completed until 1951.
The main obstacle and hazard to installing 19.5m long Arcelor PU32 sheet piles through the existing central puddle clay core which is an integral part of the continuous earth embankment that enclosed the Basin; was the presence of 400kV high voltage overhead cables running across part of the embankment where the seepage was occurring.
With that in mind, Sheet Piling (UK) Ltd proposed an innovative solution to overcome the additional problem posed by the overhead cables; on a project which was already problematical to install sheet piles though a puddle clay core in an operational reservoir.
The solution involved pre-cutting the sheet piles into pre-defined lengths to allow them to be welded in-situ along the length of the puddle clay core whilst working until HV cables.
Access to the extents of leakage through the clay core was also limited and as such, as solution involving lifting all plant and equipment onto the embankment using a Liebherr LR1600 600 Tonne Crawler Crane was developed. The maximum lift undertaken was at a 75m radius.
Initially full length 19.5m long sheet piles were installed using a Giken F201 ‘Silent Vibrationless’ Pile Press with this methodology being chosen to avoid any damage to the reservoir embankment clay core during the works.
Once the exclusion zone was reached for working under the HV cables, then piles were pitched in 6.5m lengths and welded in-situ prior to driving using the Giken Pile Press.
Further along the pile line where the cables sag between pylons, the available head height is even less meaning the piles were pitched in 5.0m lengths.
In order to facilitate safe and effective movement along the remote and narrow embankment crest along with handling and pitching of the sheet piles under the HV cables exclusion zone; sheet Piling modified a 13Te excavator with a side grip articulating clamping system. The excavator was also fitted together with height and slew restrictors. This provided a safe and novel solution to the problem of distributing and pitching of sheet piles.
All welding working was undertaken in accordance with Sheet Piling’s Welding and Inspection Quality Assurance Procedures including Weld Procedures, Welder Qualifications, Independent Weld Testing Protocols and Acceptance Criteria.
A further innovation on the project was the implementation of a bespoke instrumentation device which allowed the inclination of the installed sheet pile to be measured upon completion to verify the specified installation tolerances had been achieved. This was of utmost importance on the project since the puddle clay core is only 3.7m wide at the base and tapers to 1.5m wide at the crest.
Ensuring the sheet piles were installed through the clay core at the base and achieve a minimum penetration of 1.5m into the underlaying London Clay to provide an effective cut off was critical.
In total an overall plan length of cut off wall of 96.6L.m. was required on the project consisting of 12.0L.m. of full-length piles, 57.6L.m. 1/3 length piles and 27.0L.m. of ¼ piles.
The works commenced in May 2020 and are currently on programme to be completed by October 2020.
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Sheet Piling (UK) Limited is one of the UK's leading piling contractors. Owning and operating a comprehensive range of specialist piling equipment Sheet Piling (UK) Ltd also carry extensive stocks of new and used piles enabling rapid reaction to any urgent requirements or emergency situations.