Geotech investigations

UNIQUE SOIL CONDITIONS

Cheal Consultants were engaged to provide geotechnical reporting for the Sir Howard Morrison Performing Arts Centre.

Due to the unique nature of the pumice soils at the site, Cheal identified significant problems with traditional methods of geotechnical strength testing and numerical analysis, such as settlement and liquefaction analysis.

Their report used a wider, more holistic approach in order to assess how the soils would behave under seismic conditions. Site specific testing was also supplemented by a large amount of existing data, including geological information and reports from nearby buildings.

GEOLOGICAL HISTORY

The geological sequence across the wider Rotorua area is relatively uniform. ‘Bedrock’ can be considered a thick sequence of fine volcanic ash and pumice-rich lake sediments that started accumulating about 250,000 years ago while the lake was much larger. Over the last 60,000 years the lake level gradually lowered leaving a series of terraces deposits comprising mainly pumiceous sands and gravels.

The geotechnical investigations extended through the terrace deposits and into the top of the lake sediments, which at the Sir Howard Morrison Centre occurs at a depth of around 6m to 7m. Cheal noted a distinct increase in strength at a depth of around 15m.

LIQUEFACTION?

Cheal also assessed that liquefaction is unlikely to occur, even in a large earthquake, but if it does it will be limited to a few randomly distributed, relatively small areas of silty material, with a minor amount of localised settlement.

SITE SOIL SUBCLASS

The site subsoil class for the Detailed Seismic Assessment has been based on soil class D.