Question to the Department for Transport:

To ask the Secretary of State for Transport, whether the nominated undertakers are (a) aware of the unique foundations of the Alexandra and Ainsworth estate in South Hampstead, (b) taking steps to survey the estate individually and (c) planning for mitigation measures including but not limited to (i) compensation grouting, (ii) underpinning and (iii) jacking to ensure the stability of the foundations of the estate as set out in Phase 3 Clause 6 of HS2 Information Paper C3.

(a) Yes. HS2 Ltd has identified that part of the Alexandra and Ainsworth Estate is built on piled foundations. As the planned tunnel is approximately 40m deep at this location, there is no risk of the piles intersecting the tunnel. The significant depth also means the ground movement and differential settlement at the surface is likely to be small. A Phase 1 green field ground movement assessment and a Phase 2 ground movement has been undertaken along the route. As these buildings are listed, there will also be a phase 3 assessment, which will take account of the unique features of the building and its foundations.

HS2 Ltd has received no representations on these issues.

(b) Yes, HS2 Ltd has identified that part of the Alexandra and Ainsworth estate is built on piled foundations. The building will be subject to a survey in accordance with Information Paper C3 which forms part of the High Speed Rail (London to West Midlands) Act 2017.

(c) The ground movement at the building, using conservative volume loss assumptions, is between 1 and 7mm. In practice the experience of driving tunnels on Crossrail and HS1 in similar ground conditions demonstrates that the tunnels will be driven to a lower volume loss resulting in less movement. The tunnel boring machines specified for HS2 are high performance machines designed to minimise ground movement at source. This means there should be no need for the mitigations referred to.