Liquefaction (Groundwater Trigger)

Primary reference(s)

USGS, no date. Science Explorer: Liquefaction. United States Geological Survey (USGS). Accessed 12 October 2020.

Additional scientific description

Soil propensity to liquefaction has been related to grading, uniformity of grain size and relative density or voids ratio. A uniformly graded soil is more susceptible to soil liquefaction than a well-graded soil because the resistance to volumetric strain of a well-graded soil decreases the amount of excess pore pressure that can develop under undrained conditions. Historically, sands were considered to be the only type of soil susceptible to liquefaction. Yet, liquefaction also occurs in gravel and silt (Seed et al., 2003). ‘Running sand’ or ‘boiling sand’ is a product of the liquefaction process that can also occur in peat.

Liquefaction susceptibility is also influenced by particle shape; soil deposits with rounded particles being more susceptible to liquefaction than soils with angular particles. Structureless anthropogenic soils, such as those placed during land reclamation are susceptible to liquefaction. During construction, liquefaction occurs when the groundwater conditions reduce the effective stress of the soil to zero. At this point, the seepage pressure can disturb the soil structure and mobilise the sediment as quick, running or boiling sand (BRANZ Seismic Resilience, no date).

Liquefaction, as a secondary hazard associated with earthquakes, can also manifest via surface ruptures and fissures, as seen in Christchurch, New Zealand in 2011 (Cubrinovski, 2013).

The liquefaction associated with the Christchurch earthquakes caused significant disruption to transport infrastructure, and to storm- and wastewater networks, and posed physical and mental health hazards for the exposed community and clean-up (Villemure et al., 2012). From a human health perspective, the liquefaction material posed several hazards. Due to the extensive damage to the sewage disposal networks from lateral spreading and differential settlement, there was a risk that much of the liquefaction ejecta had been contaminated with raw sewage creating a long-term health risk to the population. During hot and windy conditions, the dry finer portions of silt were mobilised by the wind creating a possible respiratory health hazard. Many volunteers were involved in the clean-up operations. Indeed, the much-celebrated Student-Army was successfully used to coordinate the work around the city (Villemure et al., 2012).

Metrics and numeric limits

Areas that are most prone to earthquakes tend to undertake earthquake hazard susceptibility mapping, which usually embraces zones that are prone to liquefaction (USGS, no date). These commonly include more recently deposited lithologies such as Alluvium or Quaternary deposits.

Examples of drivers, outcomes and risk management

The consequences to structures and infrastructure of liquefaction include: differential settlement of structures often resulting in cracking; loss of bearing support; flotation of buried structures such as sewer lines, tanks, and pipes; strong lateral forces against retaining structures such as seawalls; lateral spreading (limited lateral movement); and lateral flows (extensive lateral movement), particularly impacting on slopes or valley sides (e.g., Cubrinovski, 2013).

The primary mitigation measure is to use planning to avoid development over liquefiable soils. Other types of mitigation are incorporated in building design (NZGS and MB IE, 2017). During construction, controlling both the rate of excavation and the head, or increasing seepage flow paths to reduce seepage forces are the key methods used to minimise liquefaction (Pane et al., 2015).

Health impacts are associated with primary consequences of liquefaction material both when wet and when material is dry and dusty; as well as secondary impacts from damage to infrastructure such as water and sewage pipes and health care facilities (Cubrinovski, 2013).