A 51,000-year-old piece of totara log recovered during drilling under Tauranga Harbour has foreshadowed a series of inexplicable setbacks that threatened to delay completion of the $99 million Southern Pipeline.

Carbon dating revealed it was a relic of an ancient forest, with the shuddering discovery of the log one of the reasons why drilling from Memorial Park to Matapihi was running behind schedule.

The harbour crossing was the trickiest bit of the project, involving a specialist workforce from 14 countries.

Project manager Steve Wiggill said difficulties had been encountered during the phase of the job called reaming - the widening of the initial 750mm diameter pilot hole drilled under the harbour.

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The reamer struck an obstruction late August when it was about 300m from reaching Matapihi.

"The obstruction has been removed and identified as a piece of totara wood that is approximately 51,000 years old. This is now in the care of Tauranga Museum."

The reamer was damaged by a pneumatic hammer used to dislodge the wood and it was possible that debris from the reamer had compromised subsequent drill runs.

''Since then, it has also become apparent that the material we are reaming through is generally more challenging than the contractor thought. To address these issues, we have recruited international experts and brought in additional machinery.''

Two further setbacks happened when a second bigger reamer was introduced to take the line out to the maximum size needed to thread in the final sewer pipe.

It reached 500m when a drilling rod snapped. Wiggill said they recovered the broken rod, threaded in new rods, reintroduced the reamer and started again.

They were about 400m from the end when a second new rod snapped. Wiggill said it was not particularly uncommon for rods to break but it was uncommon to have them snapping one after the other.

Torque and thrust limits on the drilling rig meant that, theoretically, the rods should not break. ''It is an inexplicable fracture at this moment.''

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Clearing the obstruction caused by the totara and the two breakages had added seven weeks of repairs to the job. Wiggill said it was likely that some work would need to continue in January.

Construction of the pipeline to take waste from the city's booming southern suburbs, was in its final phase - seven years after construction began at Greerton. The cost of the setbacks were being absorbed in the original harbour-crossing contract.

The Matapihi section of the pipeline had already been built and connected to the Te Maunga treatment works. Matapihi Maori hapu originally opposed construction of a pipeline that carried human waste through their rohe (tribal areas).


Harbour crossing phase of Southern Pipeline
How much: $21 million
Distance: Nearly 1.6km
Maximum depth: 35m under harbour floor