HS2 | Two more sinkholes form on Chilterns tunnelling route

This post was originally published on this site

The two sinkholes have appeared in a field adjacent to Frith Hill on the outskirts of South Heath, Buckinghamshire. HS2’s working assumption is that a combination of disturbance from tunnelling, and potentially recent rainfall, weakened pre-existing weak spots which then collapsed to form the sinkholes. The first hole appeared on Wednesday 7 February with the second appeared in the same field on Monday 12 February.

Tunnelling operations are currently ongoing, HS2 confirmed.

A local resident’s photo of the first sinkhole that formed, surrounded by fences

This section of route was originally due to be at surface level surface before the tunnel was extended when the bill was in Parliament in 2015.

The Chiltern Hills Area of Outstanding Natural Beauty is made up of chalk bedrock which is naturally vulnerable to sinkholes. Water erodes the chalk over millions of years creating weak spots which can collapse suddenly. Heavy rain, or disturbance linked to construction, can sometimes accelerate the process, causing weak points to collapse creating a sinkhole.

Last May, a sinkhole collapsed above the Chiltern tunnel near Shardeloes Lake in Little Missenden. This has now been repaired by HS2. A similar one occurred in November near Hyde Heath and has also now been repaired. Water monitoring conducted showed no significant impact on water quality.

A spokesperson for HS2 Ltd said: “We have always recognised that the geology of this part of the route is particularly challenging – with poorer quality, less stable, chalk susceptible to ground movement in the final approach to the North Portal. We are working closely with the landowner and tenant, temporary fencing has been installed, and extra monitoring put in place.

“Throughout the excavation of the Chiltern tunnels we have been engaging with all key stakeholders including Buckinghamshire Council, Affinity Water and the Environment Agency, and we will keep them updated as we go forward. Safety remains our top priority.”

The two tunnel boring machines (TBMs) digging HS2’s Chiltern tunnels are operated by the Align JV (Bouygues Travaux Publics, McAlpine, VolkerFitzpatrick). In December, it was announced they had reached the Chesham Road intervention shaft, on track to complete their 16km journey under the Chilterns this year.

Like what you’ve read? To receive New Civil Engineer’s daily and weekly newsletters click here.