The changing frequency of La Niña cycles and their effect on footing design in expansive soils

Expansive soils are affected by moisture content changes, which can result in significant shrinkage or swelling and subsequent ground movement. The extent of ground movement is dependent on the soil's physical properties (reactivity) as well as the local climate that largely controls the availability of soil moisture. The El Niño and La Niña cycles, also collectively known as the El Niño-Southern Oscillation, significantly influence weather patterns in Australia. A change in the intensity and/or the frequency of these cycles can affect soil shrink-swell behaviour and thus long-term ground movement patterns. If not properly accounted for, shallow-depth lightweight structures such as water mains, pavements, residential building footings and other utilities may face significant additional stresses when founded on expansive soils. In recent decades, La Niña events of higher frequency and longer durations have been experienced, and this has been attributed to climate change. Using a numerical model, this paper assesses the effect of potential changes in La Niña frequency on the soil moisture and related suction profile and how this affects residential footing behaviour and design in Australia. Data from a monitored research site were used to validate the numerical model. The analyses indicated that the considered changes in La Niña cycles, in combination with various drought scenarios, are likely to significantly affect the suction profiles and thus long-term ground movements. The changes in such movements will affect the requirements of geo-structures constructed on/in expansive soils. This is illustrated through a footing design exercise, which demonstrates how structures may fail prematurely or show significant serviceability issues if ground movements are not accounted for in the design.