Since the Canterbury Earthquake Sequence we have partnered with many insurance companies to provide designs for the rebuild of dwellings damaged beyond economic repair. This work is in addition to new residential building work.
With the partitioning of residential land into three distinct categories there has been a greater need for specific engineering design of residential foundations, especially in the most liquefaction prone sites (TC3 zoned land) where the foundations need to be capable of withstanding considerable differential settlement and still fulfil amenity requirements after a moderate earthquake (SLS event), and protect life safety during a major earthquake (ULS event).
The need to design more robust foundation systems can be further complicated by other site specific issues such as poor static bearing conditions or the need to raise the Finished Floor Level of the dwelling due to flooding risk.
Foundation System Examples
The foundation systems described below are some of the more common systems that we have designed.
RE-LEVELLABLE CONCRETE RAFT
A re-levellable concrete raft consists of a grillage of beams and ribs throughout the slab creating a rigid raft that is able to undergo loss of support of up to 2.0m externally and 4.0m internally to the footprint of the dwelling without undergoing excessive settlement or damage.
By constructing the concrete raft over a mass concrete underslab and embedding mechanical jacks into the concrete raft, this foundation system can be re-levelled after earthquake induced differential settlement.
TIMBER SURFACE STRUCTURES
Timber surface structures consist of a typical timber subfloor embedded in a reinforced concrete slab which is constructed over a geogrid reinforced gravel pad. This foundation system is capable of withstanding predicted SLS settlements of up to 200mm and the timber subfloor provides an easy access for re-levelling of the foundations by ‘jacking and packing’ of the bearers over the timber subfloor piles.
Where an attached concrete floored garage is desirable, the concrete slab of the foundation system can be extended out to form the floor of the garage.
A concrete raft foundation supported on piles installed into a dense non-liquefiable bearing layer is one of the most robust foundation systems for limiting settlement and, when designed correctly, should result in negligible settlement from both SLS and ULS earthquake events.
The three most common piling methods we specify in are:
Driven concrete or timber piles
Vibro-installed hollow timber piles
Steel Screw Piles
Driven concrete timber piles are generally the most economic option for piling of a residential dwelling, however, there can be issues with driving vibrations affecting neighbouring sites and dwellings.
In cases where low impact methods are required it can then be preferable to implement either vibro-installed timber piles which vibrate the individual piles into the ground or steel screw piles both of which should not result in vibration damage to neighbouring sites. Steel screw piles can also be used where new foundations are being installed under existing dwellings as the ability to site-weld on new lengths of pile as they are being installed can result in relatively little head room being required for installation.
HYBRID FOUNDATION SYSTEMS
Many sites across Christchurch can be affected not just by liquefaction related settlement but also static settlement due to either poor static bearing or poor subsoil conditions such as peat and historic fill.
In these situations we are able to design Hybrid Foundation systems that consist of a re-levellable concrete raft of timber floored surface structure supported on piles installed for static loads only. While the piles and dwelling are expected to settle during an earthquake the foundation system above the piles can still be re-levelled after an event.