Settlement is the technical term given to the way the ground moves around a hole after it has been dug out. Digging tunnels, shafts and basements always causes small movements in the ground.
How will Crossrail minimise settlement?
We know a lot about how settlement works in London from projects such as the Heathrow Express Railway, the Jubilee Line extension, tunnels for the Docklands Light Railway, the Channel Tunnel Rail Link and the Thames Water Ring Main. We have also learned a lot about how best to minimise it.
We also know how to limit the effect of this movement on buildings. In most cases you would not be able to see the effects of settlement but in some cases there may be small cracks in plaster, and in a few cases doors or windows may stick. Very rarely, the settlement can affect the structure of the building. We try to create as little settlement as we can. We do this firstly by controlling the way that we carry out the tunnelling and then, if necessary, by treating the ground so that it is less likely to move.
The most effective way of treating the ground is by using ‘compensation grouting’.
What is compensation grouting?
Compensation grouting is a well established technique employed on tunnelling projects around the world to minimise settlement. It works by injecting a cement-like substance, called grout, into the ground to firm up the area where settlement is expected to occur.
The grout is injected via small-diameter underground pipes which spread out from a central ‘grout shaft’. This method can be employed very precisely and is an effective way of minimising settlement damage to buildings.
What are grout shafts?
Grout shafts are holes in the ground from which grout can be pumped. They are operated by small teams (usually between five and ten people) from worksites approximately 30m x 6m in size. When they are not operational these worksites can sometimes be reduced in size.
The worksites are surrounded by hoardings to limit dust and noise emissions. In addition, much of the equipment is located within the shafts themselves, further limiting noise and dust.
Thorough assessment before the Crossrail works and continuous monitoring during them mean that we can accurately determine where grout shafts are required and when grouting needs to be done.
Most commonly they are set up in advance of excavation and underground work, at a distance of between 20 and 50 metres from the elements of work that need compensation grouting.
Some shafts are required for only one phase of work, some for several phases; some will need to be in place for a period beyond completion of the works to control minor long-term settlement. When they are no longer required they will be removed, the shafts backfilled and the ground reinstated.
The first grout shafts for Crossrail were completed in the summer of 2011. This timelapse video shows the construction of a grout shaft in Dering Yard for the Crossrail Bond Street eastern ticket hall site. The construction shown here was carried out over a 2 month period.
What is Crossrail going to do about Settlement?
Crossrail Information Paper D12 - Ground Settlement - explains the arrangements for assessing, monitoring and mitigating the effects of ground settlement arising from Crossrail construction on individual buildings ("the settlement policy").
The Information Paper includes information on the Crossrail Settlement Deed, including the Deed itself and the qualifying criteria. This is available as a download here.
Where does the Crossrail Settlement Policy apply?
The Crossrail Settlement Policy applies to all buildings that may be affected by our tunnelling. But the policy does not apply to new buildings which:
- were granted planning permission after 1 March 2008; or
- are not substantially complete ten months before we plan to start work that will affect the buildings.
Who can have a Settlement Deed?
If the settlement policy applies to a property that you own, you can also ask us for a Settlement Deed. This is a formal legal undertaking but you don’t have to enter into a deed to benefit from the settlement policy.
You can have a Settlement Deed if:
- your building is 30 metres or less from our tunnels or our other excavations;
- you let us know you want a deed at least ten months before we start work that may cause settlement beneath your building; and
- you sign and return a copy of the deed to us within 21 days.
The procedure for entering into a settlement deed, which also contains a copy of the application form, is available for download here, or by application to the Crossrail Helpdesk at: email@example.com.
Please read the Information Paper, qualifying criteria and procedure carefully before making any application. It is not necessary to enter into the deed to benefit from the settlement policy.
As part of the commitments given in Parliament, a guide has been produced for affected property owners, setting out in clear language how the Settlement Policy is applied. This was distributed to all properties close to the proposed tunnels in 2009.
In addition to this we have produced a guide to ground treatment which is also available to download.