Liquefaction

Liquefaction is the process by where soils that are subject to ground shaking stress, such as that experienced in an earthquake loose strength and take a liquid form. It can cause major damage to structures, such as buildings and roads, that are located within soils that are prone to liquefaction by causing them to settle into the weaker liquid soils.

Liquefaction is most likely to occur in areas that have recently deposited, saturated, uniformly graded fine grain sands and coarse silts. How it occurs in any particular site is difficult to predict, as relatively small variations in soil density and grading can alter liquefaction susceptibility. The nature of a particular earthquake, the induced ground acceleration and length of sustained shaking, will also determine whether an area will sustain any liquefaction induced settlement or ground damage.

Liquefaction-induced lateral spread is another natural hazard that is commonly assessed. This occurs predominately in the vicinity of watercourses where liquefied soil can laterally displace towards the watercourse. The extent of the ground displacement is dependent on the nature of the earthquake and the geology is similar to those in liquefiable areas. The effects of lateral spread generally diminish with distance from the river watercourse.

Liquefaction and lateral spread are identified as natural hazards in both the Resource Management Act and the Building Act.  Provision to identify and protect against these natural hazards must accompany development applications, whether it be for a subdivision or a building structure.

Whether a particular site is prone to liquefaction and lateral spread is identified in a geotechnical assessment. Initially, a series of shallow ground tests and a review of existing area data is carried out to determine whether a particular site is prone to liquefaction and lateral spread. If it is determined to be a risk then the level of risk in terms of potential settlement amounts is quantified (how far a structure would be expected to settle into the liquified soils). By quantifying this, engineering design can be completed to strengthen structures constructed within these soils.

If there is some risk, but it is a relatively low risk site, then it may be possible to use comparative data to quantify the potential amount of settlement that a design will have to account for. In higher risk sites, more sophisticated testing may have to occur to more accurately quantify this.

Cone Penetrometer Testing (CPT) has become a common tool for quantifying liquefaction potential, including liquefaction induced lateral spread, within a particular sites subsoils. CPT’s are a drilling rig that identifies soil stratigraphy and the relative engineering properties of these soil layers.

The data acquired in a geotechnical assessment is put into a report format that is suitable for supporting building and/or resource consent applications. The data is also picked up by our structural and/or civil engineering teams and used to carry out a suitable design for any structures that are proposed to be constructed within the site.

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