Mass Movement (Landslips and Slumps)
Mass movement involves the downward movement of soil and rock under the assistance of gravity.
There are many forms of mass movement including soil creep, earthflow, slumps, landslips, landslides and rock avalanches. Although relatively infrequent in Tasmania, large mass movement events are dramatic, resulting in permanent loss of houses, roads and agricultural land.
Forms of mass movement include:
- Earthflow - slabs of soil become liquid and flow downhill
- Slumps and landslips - usually caused by rotational failure of subsoil layer
- Rock avalanches - collapse of rock structures
- Soil creep - slow downhill creeping and rolling of soil particles
- Terracetting - minor form of mass movement caused by livestock traversing hillsides
Landslips, slides and earth flows occur on steep slopes with high clay content.
- Triggered by over-saturation of soils from excessive rainfall
- Rainfall increases soil weight, and lubricates layers within the soil
- Slips usually occur along a plane of weakness or highly lubricated soil layer
- Landslips, slides and earth flows may also be triggered by erosion at the base of the slope.
Prevention and Control
Landslip prevention and control usually requires a combination of vegetative measures and engineering structures. Where landslips have occurred after tree clearing, revegetation of cleared areas with densely planted tree species above the slip zone is required to de-water and stabilise soils and remove soil water by evapotranspiration.
- Dense tree planting above susceptible areas
- Surface drains within and above affected area
- Sub-surface drains.
In some cases engineering structures such as soil pins, retaining walls, rock filling and slope grooming may also be required to prevent or control landslips. Seek geotechnical advice.