the rail engineer • February 2014
39
(Top) Rib break-out. Note the protected catenary. (Bottom left) Diamond saws cut grooves for the ribs. (Bottom right) Concrete spraying.
L-shaped reinforcing bars were embedded 450mm into the side walls and secured with resin.
New ribs The widening of the tunnel at the haunches had the potential to seriously weaken the structure, so the work was sequenced to allow construction of reinforced concrete ribs at 2.5 metre centres followed by removal of the brickwork between the ribs. Freyssinet cut the 350-500mm deep recesses for the ribs with a 1200mm diameter diamond saw, arm-mounted on a vehicle. Run-off water from the sawing operation was collected, de-silted and cleaned before disposal. The brickwork was broken out using a pecker on a mini-excavator, the reinforcement cage was inserted and the rib was concreted by spraying to a depth of 350mm. Once the rib concrete had cured, alternate panels of brickwork between the ribs could be broken out first, followed by the others. Brickwork was broken back sufficiently far that at least 80mm of concrete could be applied without fouling the dynamic envelope. The precut mesh sheets were fixed to the wall and the concrete sprayed on. All sprayed concrete was delivered using the dry process where the pre-mixed but dry concrete constituents (cement, sand and small aggregate) are fed to the nozzle by hose before being mixed with
water at the point of spraying. It tends to give higher quality and greater strength than the wetmix alternative. Complementary works included stabilising the brickwork of the tunnel entrance wing walls using stainless steel pins, after which 100mm thick sprayed concrete was applied, again with reinforcing mesh.
At Connaught, the solution was to inject fast-cure polyurethane resin into the joints. This technique was first verified on a five metre stretch of tunnel to establish the depth, angle and spacing of injection holes as well as the likely consumption of resin. Following its total success, the method was extended to the 500 metre long approach ramps with the surface of the tunnel invert being made good with a locked-in cementitious chase along the joint line once the water flow had ceased.
Lessons learned in Glasgow This was not the first such problem that Freyssinet had faced. Its expertise had been gained on earlier contracts such as Glasgow Underground where over 60 tonnes of cementitious grout and 3,000 litres of polyurethane resin were injected
to fill voids and seal leaks in a 100 metre long stretch of tunnel close to Buchanan Street Station. The contract involved extensive monitoring of the tunnel to ensure the integrity of the linings while the injection was carried out during nightshift possessions of the track. Careful coordination of plant and materials was essential to ensure the tunnel and station could reopen at the end of every shift. As infrastructure gets older, and is being asked to carry more and heavier traffic, problems stemming from old brickwork, water ingress, inadequate clearances and failing supports will only get worse. That should keep specialist contractors such as Freyssinet busy for some time. Kevin Bennett is sales and technical director for Freyssinet UK
Enlarging the bore In the case of Harfleur, it was possible to restore the dynamic envelope by fairly moderate alteration of the tunnel section. But what happens when major enlargement of the bore is required? One solution is to drop the floor but that can bring its own complications. A recent example occurred in Connaught Tunnel, an abandoned tunnel dating from 1878 which runs beneath the Royal Docks and is now being altered to permit the passage of Crossrail trains (issue 100, February 2013). The invert of the tunnel had to be lowered by approximately one metre, halving the original thickness of the concrete floor. However, significant amounts of water began to enter into the tunnel through construction joints and cracks which had been exposed by the cutting operation. Freyssinet has a range of sprayon waterproof coatings, sheet linings and leak sealing injection services on offer to tunnellers.
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