...However, the engineering team experienced a number of challenges once they got into the tunnel where they planned to lower the line by up to 200mm.
“This is where we had to deal with the natural elements. The ground conditions weren’t exactly as we’d expected it in a lot of places,” said [Morgan Sindall’s rail director, Neil] Barnes. “We were going through variable ground. So we had areas of two types of rock. There was dolerite, which is a hard rock, and interspersed within that areas of mudstone.”
He added that over a long period of time the area in and around the tunnel had been heavily quarried, with a naturally-formed lagoon quite close to the site.
“At times that tunnel becomes a river, so we knew it was going to be wet and we knew we were going to have to cut through rock,” said Barnes. The biggest challenge was the variability of the mudstone, which became weaker when water came into contact with it.
“Due to this, in some places we had to over dig to get through poor mudstone to a satisfactory formation, we then had to refill this with additional concrete,” Barnes added.
RTM was told the team excavated 50% more material than expected, and over 2,000 tonnes of foundation concrete was poured during the blockade. Steel dowels were also fixed into the base rock to secure the concrete base slabs of the track bed to the formation.
The team also put a new ‘upsized’ drainage system in the tunnel, installed at invert level next to the base slabs. “The key element is that we have got the water to fall from one end to the other,” said Barnes. “It didn’t before.”
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