Earth buildings surveyed following Canterbury earthquake

Issue 23 – December 2010

The Earth Building Association of New Zealand organised a reconnaissance survey by three engineers of earth buildings in the Canterbury area following the Canterbury earthquake. The survey concludes that only minor damage occurred to the houses that were designed to New Zealand's earth buildings Standards.

On 4 September 2010 a magnitude 7.1 earthquake occurred in Canterbury, approximately 40 km west of Christchurch, at a depth of approximately 10 km near the town of Darfield. Modified Mercalli intensities of up to MMI 8 and greater were experienced at some sites.

The MMI scale, a scale used in New Zealand, is a 12-step ranking, hence Modified Mercalli (MM) which measures the felt intensity of an earthquake from MMI 1(Imperceptible) through MMI 8 (Heavily damaging) to MMI 12 (Total destruction).

Some historic and older earth buildings were damaged during the earthquake and some more recent earth buildings suffered some minor cracking damage.

The Earth Building Association of New Zealand organised three engineers – Hugh Morris, Richard Walker, and Thijs Drupsteen – to do a reconnaissance survey of Canterbury earth buildings following the earthquake. The three investigating engineers are members Standards New Zealand's earth buildings Standards committee.

Fifteen earth buildings were visited during the survey including several historic or reconstructed historic and older unreinforced earth buildings, and several more recent reinforced earth buildings, and two timber-framed buildings with external pressed earth brick veneer walls.

Conclusions from the reconnaissance survey

Reinforced earth houses constructed since the 1990s performed well provided the overall wall bracing was adequate and detailing of the reinforcement and connections was generally in accordance with the New Zealand earth building Standards.

Several unreinforced earth buildings constructed before 1940 or reconstructed historic buildings suffered significant structural damage and will require reconstruction or substantial repair.

Unreinforced rammed earth buildings, and reinforced cinva ram brick buildings, constructed between 1940 and 1990, with reinforced concrete foundations and bond beams and adequate overall wall bracing generally performed moderately well given the level of shaking they experienced.

Minor cracking was observed in all but one of the modern houses. Performance was good where reinforcement and construction complied with the New Zealand Standards. Where buildings did not comply, the damage could have been prevented by following the Standards.

Some limited cracking can be expected in most earth buildings during major earthquake events, particularly adjacent to windows and door openings. This cracking is generally of no structural significance if the buildings are provided with vertical and horizontal reinforcing and the overall wall bracing provided in the building is adequate and in accordance with the requirements of the New Zealand earth building Standards. Cracking would be more widespread in older unreinforced earth buildings with possibly greater structural significance.

The New Zealand earth building Standards deal well with key types of failure observed but need to be applied with a good understanding of the earth material characteristics.

Adequate anchoring of the top plate and full height continuous reinforcement and good intersection details are critical in all forms of new earth construction. Minimum bracing wall lengths must be used as specified in the New Zealand earth building Standards. Earth brick veneers with soil cement mortar require positive connection to the ties.

Related Standards

Published in building.