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Tunnel trials

Two 700m cut and cover tunnels provide vehicle acess below the southern runway, both built by Downer-Paul Y JV in a HK$650M contract.

Much of the eastern access tunnel was slotted into specially placed rockfill to ease excavation and control groundwater.

The tunnel was designed by Hyder as a three box structure with two vehicle tunnels and a utilities cell. Typical external box dimensions are 26.5m by 8.3m. Extensive dewatering was needed to allow safe construction of temporary side slopes in the granite rockfill.

The most challenging part of construction was where three ramps lead from the airport platform level to a circular underground roundabout and then into the main tunnel before crossing under the southern runway.The roundabout had to be built within a 5m retained height free-standing sheetpile cofferdam. But at its south western corner it extended into mixed rock and granite fill and a grout curtain had to be installed to restrict groundwater inflow enough to make it controllable by pumping.

Two stages of grouting were carried out via 900mm grout holes at 500mm spacing. Grout was injected until either volume reached 150 litres or the grout pressure hit 600kPa.

But leakage was still too high for excavation and eventually the problem was solved by driving sheet piles down 40m into the alluvium.

The western vehicle access tunnel was originally intended to be built in soft fill material, but was repositioned so that a section could lie in a trench blasted through bedrock. However as the ends still extended into rockfill reclamation, some form of groundwater cut off was needed.

Temporary works consisted of anchored cofferdam and sheet piled earthdams to form a cut off at either end. Somewhat surprisingly, given that the fill was predominantly rock boulders, the temporary works contractor opted to form the cofferdam using sheet piles.

Perhaps more surprisingly the contractor was successful. Sheet pile positions were predrilled to break the boulders. In places a heavy section of H-steel pile was needed. Paul Wheeler

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