An interesting article from the tunnelling industry experts.
At a meeting lecture of the British Tunnelling Society last month (October 2014), Professor Rory Mortimore of Brighton University, as a leading geologist with specialist knowledge of chalk geology and Managing Director of ChalkRock Ltd, provided further insightful information for the spiralling costs above the initial construction estimates.
According to Mortimore, the preliminary design and cost estimate of the tunnel in the late 1990s was established before a detailed investigation of the geology had been carried out. The assumption, he reported to the BTS audience, was that the chalk geology of the Stonehenge area was same as other familiar chalk deposits in the UK; but in reality, as revealed by the detailed site and ground investigation studies, it is significantly different and complex.
“The big surprise,” said Mortimore, “was discovery that the geology on the tunnel route contains a large deposit of phosphatic chalks which contain weak and poorly banded sand and silt layers and a high register of radon radiation. Such a large deposit of phosphatic chalks were unknown in Wiltshire and, indeed, in Europe and their impact on the proposed tunnel project were profound.”
When the site investigation programme began, the prime objective was to know how extensive is the 15m thick layer of phosphatic chalk, and to investigate how it was formed. Following that, the impact of the engineering properties and behaviours of the deposits had to be incorporated into the project estimates.
“As a tunnelling medium, the weak chalk, with its poorly banded layers of sand and silt presented the potential of loose running ground in the face, which was not favourable for the assumed open face NATM or sequential face excavation method, similar to the now completed twin-tube highway tunnel for the A3 route at Hindhead in Surrey that was being developed at the same time,” said Mortimore. “A tunnelling method that provided positive face support would be required to reduce construction risk and this increased the cost of the tunnelling operation.”
Another aspect that increased the estimated cost of the project was the special handling required for the radon-contaminated tunnel material and the potential of working in the material to cause phosphate contamination of the groundwater. “Disposing of radon contaminated phosphatic chalk in a landfill presented major concerns and special handling of groundwater and construction wastewater added to the tunnelling and construction cost estimates,” said Mortimore.
More at http://www.tunneltalk.com/UK-21Nov2014-Stonehenge-TBM-bored-road-traffic-tunnel-revived.php
Thanks to Nettie for the link.