Soil profile inversion in earthquake-induced liquefaction-affected soils and the potential effects on urban trees

- Earthquakes caused liquefaction resulting in sandy ejecta deposition above topsoil
- 40 cm deep ejecta: mainly fine and medium sand with low organic and fines content
- Ejecta had poor ability to store water relative to undisturbed soil
- Macronutrient and exchangeable base concentration was very low in ejecta
- Tree growth is likely to be affected by earthquake-induced ejecta deposition

A sequence of three large earthquakes in 2012/2011 caused liquefaction ground damage to over a third of Christchurch, New Zealand. This paper presents results of a characterisation of liquefaction-affected soil (LAS) and, from these results, infers its suitability for supporting existing trees and future plantings. The study site was intensively sampled and an analysis of physical and chemical soil properties was undertaken. Results indicate that during the liquefaction process, the subsoil was ejected upwards onto the existing soil surface. The ejecta comprised 66% fine sand (100-250 μm) and 20% medium sand (250-500 μm) and had a maximum depth of 40 cm. Relative to undisturbed soil the coarse texture of LAS resulted in rapid water drainage and low water holding capacity. Total available water by volume for LAS was only 7.6%, whereas plant readily available water was only 7%. Liquefaction-affected soil also had significantly lower organic matter content (0.03%) and was nutrient deficient, having very low concentrations of macronutrients (N, P, K) and exchangeable bases (Ca, Mg, Na). A discussion of liquefaction-affected soils' suitability for supporting existing and newly planted trees is presented in the context of acceptable thresholds for the measured physical and chemical properties.