Surprise peat pocket puts bridge opening date back

Transit NZ faces a delay in opening a $6 million motorway overbridge in Mt Roskill, after running into more peat than its designers estimated.

The agency has confirmed that contractors on the $169 million motorway extension from Hillsborough to New Windsor are digging out an embankment already largely constructed on the new bridge in May Rd.

Regional manager Peter Spies could not predict how much the setback would cost, but said it would not affect the 254m traffic bridge's central concrete beams, as these were laid over piers on piles sunk deep underground. But he said ground on the southern approach to the bridge had turned out to be weaker than around the rest of the structure, meaning earth fill would probably have to be replaced with lighter material.

He said the ground itself would also be strengthened.

Mr Spies said there had always been a risk of finding pockets of peat between test bores in an area also containing large volumes of basalt from the Mt Roskill lava field.

The bridge's opening is now likely to be delayed until the end of December, giving contractors less opportunity in the new construction season to drive haulage trucks under it with stockpiled earth needed for the western end of the motorway.

Traffic will continue to use a 300m temporary bypass along May Rd to get past the incomplete bridge, which will eventually be one of six crossings of the 4km motorway extension.

A 150m pedestrian and cycle bridge costing $1.6 million is expected to be complete by the end of this month at Keith Hay Park, and a second traffic detour is due to open next month in Dominion Rd, for construction of a motorway interchange.