In November 2019 a change was made to Acceptable Solution B1/AS1 of the Building Code. The change prevents B1/AS from being used on liquefaction-prone ground and takes effect from 29th November 2021. The change to the Building Code was made as a result of the experience of the Canterbury earthquakes, and subsequent recommendations made by the Royal Commission of Inquiry.
The change to B1/AS1 helps those building on liquefaction-prone ground select foundations for residential homes. The change also revokes the use of a 'deemed to comply' pathway for foundations unless the ground has been assessed and/or categorised as not being liquefaction-prone – i.e. 'good ground'.
To ensure readiness for this change, councils have been advised to map their jurisdictions to undertake hazard mapping and identity liquefaction-prone areas. The Marlborough District Council commissioned the University of Auckland to undertake a study to address this requirement. The name of this report is the Liquefaction Vulnerability study: Lower Wairau Plains.
The report summarises the development of liquefaction vulnerability maps for the Lower Wairau Plains in Marlborough based on the ‘Planning and engineering guidance for potentially liquefaction-prone land’ (MBIE/MfE/EQC 2017). The methodology and datasets that were used are summarised in this report and a suite of maps of liquefaction vulnerability categories presented.
Although the ‘Liquefaction Vulnerability study: Lower Wairau Plains’ identifies potentially liquefiable areas, it does not provide any guidance for stakeholders on how to determine the theoretical liquefaction triggering potential of the site soils, to predict the likely associated ground deformation, or how to design foundation solutions for sites on potentially liquefiable ground, which will likely meet the performance standards of the Building Code. To assist stakeholders in the industry the Marlborough District Council has produced the Marlborough District Council - Liquefaction Assessment Guidelines. The objectives of the document are:
(1) Promote consistency of approach to assessing liquefaction risk in the whole Marlborough region,
(2) Provide sound guidelines for the determination of the theoretical liquefaction triggering potential of soils, due to seismic loading, to support rational foundation design, which are informed by the latest research and the MBIE Guidelines Modules (1 to 6),
(3) Provide suitable foundation design solutions that:
(i) Take into account the likely future performance of the ground, under seismic loading,
(ii) Provides MDC with ‘reasonable grounds’ to be satisfied that the minimum performance standards of the Building Code (Clause B1) are satisfied, which will enable them to grant consent for foundation solutions, sited on potentially liquefiable ground.