Case Study: Permanent Sheet Pile Retaining Wall, Heathrow EBR

Permanent Sheet Pile Retaining Wall, Heathrow EBR

Contract No:C1168
Client:Morgan Sindall
Location:Heathrow Eastern Balancing Reservoir
Value:Approximately £2,100,000.00
Summary:Design, supply, installation of approximately 1800 tonnes of sheet piles to form two sheet pile walls running the entire length of the reservoir as well as a number of cofferdams to accommodate penstocks and a Temporary Access Platform to be extracted at a later date.

Sheet Piling (UK) Ltd were employed by Morgan Sindall Ltd to undertake the ‘Design, Supply and Installation’ of sheet piles to form two permanent water retaining walls, with the allowance to install smaller temporary cofferdams to enable the penstocks to be installed.

The Eastern Balancing Reservoir (EBR) site was formerly a single shallow gravel pit. However it now comprises three ponds in a series receiving all the surface water drainage from the eastern catchment of Heathrow airport. Including drainage from roofs, roads and car park. This catchment consists of 469 hectares with around 52 kilometres of pipework, which is transferred to the EBR’s Middle and Lower Ponds. The total water volume is estimated to be around 189,500m3 of which approximately 57,000m3 can be controlled by water level management.

The EBR is designed to;

  • Act as the storage lagoon for fire-fighting water across the Airport
  • Receive and balance water flows from the Eastern catchment area of the Airport and attenuate discharge rate to the River Crane.
  • Separate ‘clean’ and ‘dirty’ quality water prior to clean water discharge to the River Crane as well as store and treat ‘dirty’ water before discharge to the River Crane.

The sheet piling was being installed in order to alleviate the butyl curtain that had previously been installed to form a barrier between the ‘clean’ and ‘dirty’ water, unfortunately this butyl curtain had failed so a new long term solution was required the ensure the quality of the water being discharged into the River Crane.

Working closely with both the Consulting Engineers (Atkins), and the Main Contractor, Sheet Piling (UK) Ltd proposed a solution involving the construction of the retaining walls utilising GU21 and GU23 sheet piles supplied by Arcelor Mittal together with EZ18-630 sheet piles from our extensive stock of Emirates Steel Sheet Piles.

Permanent Sheet Pile Retaining Wall Heathrow

The nature of the works were complex on this project due to the following restrictions and requirements:-

  • Installing the pile walls whilst working over water meaning extensive floating plant was required
  • The requirement to minimise silt disturbance as much as possible whilst working on the middle pond as to not reduce the water quality in the lower pond
  • Install all piles so as to not require any installation of propping
  • As the materials storage area was at a raised level from the water, all the materials had to be transported down an access ramp to the access platform and loaded onto floating plant.

The plant selected for this scheme consisted of Sheet Piling (UK) Ltd newly purchased  Kowan ZU100 silent pile press, which enabled the installation of both Z section sheet piles and U section sheet piles in addition to a externally hired ZU100 to allow both pile walls to be installed in conjunction with one another.

The use of a ZU100 pile press was selected as they don’t require a piling platform to be adjacent to the pile wall unlike all other methods of installation, so in order to utilise the ZU100 pile press, the power pack was positioned on the main barge on which the crawler crane supplying piles to the pile press was also positioned and the cables connecting to the giken were floated using buoys to ensure they didn’t snag on anything below water level.

Due to the sheet pile installation being required over water, suitable floating plant was required on which a crawler crane could be located. This was constructed using a series of VolkerBrookes Linkfloats all interlinked to form a barge that could hold the crane, the pile presses power pack, the stores container and suitable welfare facilities for the Installation gang.

In order to transport the piles to the storage barge docked on the access platform, the piles had to be transported down an access ramp, due to the gradient of the ramp, a crane could not be used to travel the piles down the ramp, so the suitable altenative was to use a 13Te telehandler with an extended folk attachment. This allowed a steady flow of piles from the material storage area to the access platform.

Due to the location of the works and the changing ground conditions and silt levels, the pile line was probed prior to installation using a ZU100 pile press in order to monitor the depth of silt and clays ensuring the toe level of the piles was sufficient, fortunately the clay level remained fairly constant opposed to the silt level which constantly fluctuated across the pond.

Once the Piling works were completed, the EBR site was back to being fully functional for the displacement of substances generated in the drainage of Heathrow Airport these substances are from the maintenance and de-icing of the aeroplanes of which there are on average 1293 flights per day.

This project in total involved the installation of 1800Te of mainly Emirates Steel Sheet Piles between September 2016 and April 2017.