Introduction
Compensation grouting is undertaken where there is a risk of ground settlement in the vicinity of underground excavations and tunnelling, particularly in areas of sprayed concrete lining (SCL) tunnelling. These mitigation works are constructed prior to work starting and are reinstated at the cessation of construction. However, the shafts and their expansive grout pipe (‘Tubes a Manchette’ or TaM) networks could be repurposed to provide the basic infrastructure for ground source heat collection as they have a significant contact area with the earth several metres below ground level and can consist of many kilometres of pipe. Trials undertaken at Whitechapel have demonstrated that viable heat can be extracted from these networks, and a supporting document provides details of the heat output. This could have beneficial implications for the longer term viability of treating these structures as heat networks with a temporary use for controlling settlement during construction. As part of Crossrail’s Innovate18 platform, the opportunity was also taken to identify other potential end uses for grout shafts. These other uses could include storage solutions for bikes, an option investigated by Crossrail in association with TfL but found not to be cost effective as the existing proprietary systems would have required a bespoke solution to be accommodated within the grout shafts. Crossrail also facilitated work by Urben Ltd to study the potential to utilise grout shafts for alternative storage solutions and in one novel example a habitable space. Further details can be found in the Underground Urbanism Report by Urben: The potential re-use of Crossrail grout shafts, contained within this learning legacy.
What is the benefit to future projects?
The experience on Crossrail was that there were a number of significant hurdles to overcome in repurposing grout shafts arising from their temporary nature and associated planning.
- The shafts are constructed as temporary structures and therefore would not necessarily pass the rigour of acceptance procedures for longer term or permanent structures
- The diameter of the shaft may be sub-optimal for a longer term use such as bike storage or habitable space.
- The location of the shafts may be sub-optimal for longer term use
- TaM diameter may be suboptimal for use as ground source heat collectors
- Planning permission is typically sought and approved on the basis of these being temporary structures only.
For future projects, Crossrail would advocate early consideration of how these structures could be used on a more permanent basis post their temporary use for compensation grouting. Decision making for this should therefore be an integral part of project development and included as part of the scope, and in doing so take the opportunity to look at secondary benefits from the construction of infrastructure. Discussions could be held with local community and local government stakeholders regarding the longer term value of these structures for either ground source heat or other repurposing uses (Some examples are assessed in the supporting documents). These options are not mutually exclusive and could be combined.
In addressing the construction of grout shafts as permanent infrastructure, the constraints identified above should be considered at the start and designed and planned accordingly, including potentially reaching early agreements with land and property owners or developers so that there is an agreed market for the heating/cooling once compensation grouting has ceased.
There may be considerable planning benefits and ease of the “licence to construct” for the primary infrastructure if this alternative approach is adopted.
