Part of the MLM Group

National Grid: Chelsea Harbour

  • Project Cost £33 million
  • Architect Duggan Morris Architects
  • Client Marcol Group Plc
  • Images Fluid Structures

National Grid are currently designing a 3.5m diameter tunnel that will run under several parts of London. The Wimbledon to Kensal Green tunnel will run under the CHDL site (i.e. a vacant site that Chelsea Harbour Limited own) and it will also run under Harbour Avenue adjacent to the CHL site (A group of buildings that Chelsea Harbour also own at a depth of approx 30 to 35m.

Fluid were asked to advise Chelsea Harbour (CH) on the impact of the tunnel and assist with the drafting of legal agreements for both sites. Before the tunnel is constructed legal agreements need to be prepared and any additional costs that arise due to the tunnel need to be covered and agreed by CH and National Grid. Also the impact on any future development that CH wish to carry out needs to be assessed.

When the tunnel is formed it causes a settlement trough (currently estimated by National Grid to be 5mm based on a 1.5% volume loss). The tunnel will place restrictions on both sites and can potentially limit CH’s ability to develop their sites in the future.

Risks

  • Tunneling work causes excessive settlement damaging Chelsea Harbour buildings.
  • Presence of tunnel prevents Chelsea Harbour from developing site to its’ full potential.
  • Tunneling work damages other utilities in the area leading to disruption and nuisance.
  • Tunnel collapses causing significant damage to buildings.
  • Tunneling work causes vibration that disrupts tenants and leads to loss of revenue.
  • Tunnel reduces capacity of existing foundations.

Design Procedures

  • Negotiations with National Grid: – The most important thing going forward is that both sites have a consistent approach in negotiations with National Grid.
  • Scheme Drawings: – We have prepared scheme drawings for the CHDL site showing a 2 storey basement carpark with a 20 storey reinforced concrete frame building over. We have also developed a section showing how the tunnel relates to the basement.
  • Identify Main Site Constraint: – The National Grid Tunnel.
  • Identify Easements Strips: – There are easement strips for the railway and the underground gas main and electricity cables. It is likely that some form of agreement will be required with National Grid for planning and to allow construction to proceed. (Also working adjacent to the railway will impose constraints).
  • Locate Thames Water Sewers: – The Eel Brook sewer runs under the site and is currently owned by Thames Water. It is likely that a building over agreement will be required.
  • Identify Gas Main Locations :- There is an easement strip for a 30inch gas main and there will need to be a risk analysis carried out for working close to the 30 inch diameter high pressure gas main during construction and also the possible effects of a gas explosion on the building in the future will need to be considered. Even though the risk of an explosion is very remote the consequences could be very significant if a key element of the structure that supports the 20 storey building is affected. This issue will need to be addressed as part of the design and risk management process. Is it possible to relocate the gas main?
  • Establish Right of Support: – The right of support is also an issue that will need to be dealt with through the legal agreements with National Grid.
  • Agree Proposed Basement depths and Future Flexibility: – Although we have currently shown a 2 storey deep basement for the CHDL site, our client may still want to negotiate future flexibility with National Grid so that they can maximise the potential of the site. For example, this may involve an allowance for a 4 storey basement at some stage in the future. A suitably flexible tunnel design will need to be negotiated with National Grid to help Chelsea Harbour maximise the potential of the site as much as possible.
  • Agree Easement Zones and Future Flexibility: – The scheme drawings that we have produced so far show the footprint of the building within the easement zones. Although it may be unlikely, Chelsea Harbour may also want to explore the possibility of relocating some of the services so that the easements could also be relocated to release more area on the site. In addition, the possibility of removing or building over the railway easement strip should also be explored and several scenarios will need to be considered so that any agreement with National Grid gives Chelsea Harbour as much flexibility on the site as possible now and in the future.
  • Other Utilities: – What assurances do CH have that the utilities that cross the site will not be affected by tunnelling works? What is an acceptable level of movement for the 30 inch diameter gas main (owned by National Grid)? Archive information notes that the 30 inch high pressure main is under nitrogen. Have gas board/pipe owner agreed figures? Has condition survey been carried out/requested? Has risk analysis been carried out? Is the gas main in good condition? Is it the same construction over the full length of the site? Is it all steel or are some sections cast iron (unlikely but good to check)? Will the gas board/pipe owner carry out a condition survey post construction? What is acceptable level of movement for Large diameter TW sewers?Have TW agreed? Have condition surveys been carried out? Are there any other services/utilities crossing the site that are likely to be affected?
  • Detailed As-Built Drawings:- It is essential that Chelsea Harbour receive detailed as-built drawings of the tunnel so that any development decisions by Chelsea Harbour can be made in an informed manner. The position of the tunnel also needs to be related to physical “markers” (locations to be agreed) at ground level so that the position of the tunnel can always be accurately plotted from the surface whenever any future work is undertaken.
  • Geotechnical Site Investigation: – A detailed site investigation (with a desktop study, contamination testing, interpretative report, etc) with boreholes taken to a suitable depth will need to be commissioned.
  • Costs incurred by CH due to presence of Tunnel: – A way of agreeing the extra cost incurred on the CHD and CHDL site due to the presence of the tunnel needs to be agreed.