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Lifting earthquake-affected buildings in Christchurch

A significant number of buildings will need to be lifted to allow their foundations to be replaced following the earthquakes in Christchurch.

The on-going seismic activity in Christchurch has created hazards that would not normally be expected by contractors undertaking foundation replacement in other parts of New Zealand.

The New Zealand Heavy Haulage Association’s Best Practice Guide to Building Relocation in New Zealand details industry best practice when lifting buildings.

This fact sheet has been written as a supplement to the guide. It aims to improve awareness of the hazards involved in lifting buildings in Christchurch.  It also provides industry recommendations for controlling these hazards.

What hazards could be encountered when lifting earthquake-affected buildings?

Ground instability

The Christchurch earthquakes have caused varying degrees of ground uplift, slumping and liquefaction.  A geotechnical engineer’s advice should be sought prior to work commencing on unstable ground.  Operators need to make site specific assessments of ground conditions to ensure that buildings can be jacked and temporarily supported safely without their supports sinking.  Where necessary, additional dunnage should be placed under jacks or supports to ensure safe bearing.  

Structure instability

Buildings to be lifted for foundation replacement will have suffered varying degrees of structural movement during the earthquakes. Operators need to carry out a thorough visual inspection of each structure to assess what additional bracing, propping, and repairs may be required before lifting.

The earthquakes may have caused significant damage to the foundations of a building, and in some instances the building may not be entirely supported by the foundations. There is an increased risk to workers while they are disconnecting the building from the foundations, particularly with the likelihood of aftershocks.  Therefore, jacks and/or temporary supports should be installed to stabilise the building before it is disconnected from the foundations.

Building collapse from aftershock

Aftershocks are a significant hazard for all buildings that are being lifted, or have been lifted for foundation remediation. This hazard must be controlled through the use of safe working practices to ensure the safety of the building and the people working under or in close proximity to it.

Reinforcing the basics of good jacking procedure is essential:

Contaminated silt and soil

The earthquakes have caused widespread deposits of liquefied silts which may be contaminated with sewage and storm water from ruptured pipelines.  Contaminated soils can be a breeding ground for bacteria, viruses, and parasites. Personnel working under buildings can be at risk of infection and should be provided with protective clothing and equipment to minimise this hazard.  

Good personal hygiene standards are important when conducting work with contaminated silt and soil.  Workers should clean their hands with soap and water, and dry their hands prior to eating or smoking, and when leaving the worksite.  Smoking while working is not recommended, as this is a common source of infection.

Further information on how to manage this hazard can be obtained from the Department of Labour factsheet - Advice for working with sewage contaminated silt and soil.

Demolition

Some buildings being lifted for foundation replacement or removal will have attached structures requiring demolition.  These structures may have sustained damage in the earthquakes and demolition by hand may not be able to be performed safely.  Demolition by excavator may be a safer option.

Other hazards

The Department of Labour factsheet Working in earthquake-affected buildingsidentifies a number of hazards specific to Christchurch and provides useful information on how to control them.

Methods to support and stabilise lifted buildings

There are a number of methods that house movers use to support buildings once they have been lifted.  The method they use depends on a range of factors including: size of building, topography of site, exposure to wind, and complexity of the new foundations.

Note: lifting and stabilising work conducted on TC2[1] and TC3[2] land should be informed by geological advice.  Contact the engineer responsible for the foundation repair/rebuild proposals to see if there are any particular concerns that need to be accounted for.

Details of each method and industry recommendations on how they should be used to minimise the risk to a building in an aftershock are as follows:

Timber cribbing or stys

Timber stys are typically made from stacked layers of 150mm x 125mm x 900mm  pinus radiata timber.   Timber house piles may be used, but a minimum length of 900mm is recommended.  A number of stacks or stys will be used to support each building, and it is best practice to install them underneath the external and internal load-bearing walls of a building.  If this is not possible, steel cross beams should be placed on top of the stys to transfer the load off the floor.  Typically stys will be constructed level and true with two blocks of timber per course.  Due to the risk of aftershocks in Christchurch, it is recommended that the number of blocks per course be increased to three or four as is normal practice for buildings of heavy construction.

Tripod pipe stands

Tripod stands are manufactured from welded steel pipe with a one metre-square base and height adjustable. They have a safe working load of three tonnes and can be clamp-fixed to the bearers of a building.  It is recommended that they are placed on level timber dunnage so they do not sink in soft soils.

Jacks

High-lift jacks are the most common means of support used by house movers when re-piling buildings or placing new foundations for relocated buildings.  They provide stable support, even in strong wind conditions. However it is best practice to provide additional support using either timber stys or tripod stands when personnel are working with machinery under a building or on exposed sites.  Additional supports are now recommended when working in Christchurch.

Supplementary methods to brace lifted buildings

The violent horizontal and vertical shaking experienced in a significant earthquake or aftershock has the potential to dislodge lifted buildings off their temporary supports.

For this reason, the industry believes it is essential in terms of safety to provide a supplementary bracing system for all lifted buildings that are being worked under for foundation replacement, or have the potential to injure should they fall off their supports in an aftershock.

The following systems - reviewed by consulting engineers - are satisfactory for reducing the amount of lateral movement in an aftershock.

Buried concrete block method

This system involves burying a number of 1000kg concrete blocks around the perimeter of a lifted building and fixing them to the building using acrow-props.

Screw pile method

This system involves winding in a number of double-helix screw piles around the perimeter of a lifted building and securing them to the building using chains and shackles.

For full technical information and engineering calculations for each method, please contact the New Zealand Heavy Haulage Association.

Note: lifting and stabilising work should be co-ordinated with the foundation repair work so that houses are not left on temporary supports for any longer than is necessary (bearing in mind resource and labour constraints).

References

Revised Guidance on Repairing and Rebuilding Houses Affected by the Canterbury Earthquake Sequence - Department of Building and Housing (now the Ministry of Business, Innovation and Employment – Building and Housing Group)

Best Practice Guide to Building Relocation in New Zealand:  available from the New Zealand Heavy Haulage Association

Working in earthquake-affected buildings – Department of Labour

Advice for working with sewage-contaminated silt and soil – Department of Labour

Acknowledgements

The New Zealand Heavy Haulage Association wishes to thank the following member companies for their contribution to this document:

Special acknowledgement must be made to Carl Baker of Hastings House Removals Ltd for compiling this document.

Footnotes

[1] Technical Category Two land - minor to moderate land damage from liquefaction is possible in future large earthquakes.

[2] Technical Category Three land - moderate to significant land damage from liquefaction is possible in future large earthquakes.