The River Misbourne

The River Misbourne is a Chalk Stream, one of only 210 in the world. The Chiltern Tunnels pass under the Misbourne at two locations – Chalfont St Giles, and Shardeloes Lake.

During the select committee hearings, concern was expressed that only 6m of solid chalk would separate the tunnel from the structureless chalk rubble beneath the bed of the Misbourne –

Petitioners warned that vibrations from the Tunnel Boring Machines (TBMs) could disrupt the chalk rubble above, and open gaps in the river bed, allowing the watercourse to find a lower path below the surface.

Detailed ground investigations in this area were not completed until early 2017. These revealed a situation which was far worse –

Boreholes ML035-RO002a and CR003 (nearest the crossing point) found that the chalk rubble depth was in fact 22m, on top of a layer of weak chalk. As the tunnel crown is 20m below the surface, the TBM cutters would break through the chalk, into the rubble above.

Giving evidence (for HS2) at the Select Committee, Mr Smart stated

Now, as has already been said, we are about to embark on some very extensive
geotechnical investigation across the whole route, which informs the decision that we’ve
made and, in the event of some really difficult ground, it may be that we will just lower
the alignment slightly, by a metre or something if we had to.

In an earlier exchange, Mr Strachan (QC for HS2) quoted from a letter from the Environment Agency –

‘Before we can approve applications in line with the protected provisions within the Hybrid Bill, we’ll need to be satisfied that all potential risks to the river and surrounding environment have been mitigated, and this will need to be supported by evidence from your ground investigation programme and we’ll continue to provide advice to ensure the proposed mitigation will be acceptable and approvals can be issued.’
So, Mr Smart, in general terms you’re referring to the protected provisions, but the EA is identifying here that they will continue to provide an approvals process before anything occurs, to ensure the protection of the River Misbourne and the water supply in the area, is that how it would work ?

Despite the ground investigation revealing that the tunnels would not be contained within solid chalk, the EA have not imposed any conditions (such as requiring that the tunnel be lowered) which would restore the situation to that which was described to the Hybrid Bill select committee. Our legal proceedings will provide an opportunity to rectify this ommission.