Two more sinkholes above HS2 Chiltern tunnel

The two slumps, in a field adjacent to Frith Hill on the outskirts of Small Heath Buckinghamshire, were discovered five days apart. The first feature was found during routine monitoring work above the tunnel on February 7.

A second, larger feature was found in the same field on February 12 by HS2 surveyors.

TBM Florence in the Chiltern tunnel

Both sites have been fenced off. The field is privately owned and there is no public right of way across it.

HS2 said its “working assumption” was that a combination of disturbance from tunnelling, and recent rainfall, weakened pre-existing weak spots which collapsed to form the sink holes.

Tunnelling operations are continuing. The sinkholes will be filled once HS2 is confident that the ground has settled.

Two sinkholes also developed above the Chiltern tunnel last year – the first at Little Missenden in May, and the second near Hyde Heath in November. Both have been repaired.

“All the sinkholes have occurred in areas where the tunnel is particularly shallow. Water monitoring showed no significant impact on water quality,” HS2 said.

“The Chiltern hills are made 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.”

A spokesperson for HS2 Ltd said the company had always recognised that the geology of this part of the route was 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,” the spokesperson said.

“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. Safety remains our top priority.”

Stretching from the South Portal near the M25 to South Heath in Buckinghamshire, the 16km twin-bore Chiltern tunnel is the longest on the project. It is being excavated by two TBMs, Florence and Cecilia, which were launched in summer 2021.