CONTRACTOR NUTTALL this week had to stop work on a 4km tunnel it is building in Portsmouth after water started to flood through cracks in the lining.
The 2.85m diameter Lovat earth pressure balance tunnel boring machine ran into trouble as ground conditions deteriorated just 200m from completion of its run at Eastney pumping station in Portsmouth.
No one was hurt in the incident and, as there were only allotments above, there is no risk of property damage.
However, as water poured into the tunnel from the ground above, miners were evacuated after it became unsafe to continue with repairs. Compressed air is being used as a temporary support for the tunnel.
Ground movement in the surrounding silty sand and clay stratum is thought to have caused the cracking in the concrete segment lining. But project manager Damon Elliott stated it was 'too early to tell' the precise cause and said he 'did not fully understand' the reasons for the cracking as yet.
It is expected that repairs will be carried out after the ground around the tunnel has been frozen. Work will be carried out from allotments above but only after agreement has been negotiated with allotment holders who are very concerned about the affect of the proposed work on their crops.
Elliott explained: 'Further work is needed to ensure we complete this section safely and quickly while also protecting the miners working below ground.'
The tunnel is part of Southern Water's £100M water improvement scheme for Portsmouth and will transfer treated wastewater from Budds Farm treatment works to the Eastney pumping station.