Digging down to survey soil strength

June 22, 2017

Hamilton-based Beca geotechnical engineer, Madeleine Prebble and Jordan Young, an undergraduate in mechanical engineering.

A geotech study into the land surrounding the Taranaki Cathedral was successfully completed this month and early results are promising, says Jenny Goddard.

“Part of the reason is the potential for liquefaction and seismic-induced settlement of the site when there’s an earthquake,” Jenny explains. “We are cautiously optimistic.”

The work is essential in order to determine how stable the soil is prior to the $15 million earthquake strengthening and addition of a light-filled atrium on the far side of the church. A major campaign to raise the funds and enhance the historic cathedral, named The Cathedral Project – a Taranaki Taonga, is well under way.

“At the moment the extra foundation work is particularly at the eastern part of the church,” Jenny says. An extra line of foundation work at this point would of course cause disturbance in the area and would likely raise concerns especially as there are graves in the vicinity, she says. It would also increase the cost of the work.

At this point it appears there were no surprises and that the soil was similar to other areas in New Plymouth.

The information from the geotech investigation will be passed on to engineers who need to know the exact nature of the soil, Jenny says. The information, which would reveal what could be expected in an earthquake, will then be used in the design model for the foundations.

Hamilton-based Beca geotechnical engineer, Madeleine Prebble and Jordan Young, an undergraduate in mechanical engineering (and son of New Plymouth MP Jonathan Young), travelled down to carry out their part of the survey. This involved using hand augers to remove a plug of soil down to a two-metre depth, then conducting a vane shear test. The testing measures the resistance of the soil inside the hole.

“Shear vane testing tells us about the soil strength, for example if it is cohesive and binds together, unlike sand,” Madeleine said on site. “If we found anything unusual we might take a sample.” However, any such contamination was unlikely, she said. They are at pains to ensure the soil is returned and there’s no sign of disturbance. “After we’ve finished, you can’t see where we’ve been.”

At the same time locally-based Opus staff carried out two deeper cone penetrations at the front and the back of the church. A probe is forced into the soil to a depth of 12-14m. “We’re expecting no volcanic ash and that they will get into gravels and sandstone,” Jenny says.

The sites of the probes were carefully chosen to ensure nothing significant, such as graves, were disturbed.

“We hope the report will be finished by early July. It’s a good start in the process,” Jenny says. The report will be handed on to Homes Consultants who will carry out the structural assessment for the seismic upgrade.

Different soil samples from around the cathedral are laid out.

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