Landslip or Landslide
The textbook definition of a landslide or a landslip is the movement of a mass of rock, debris or earth down a slope. The term landslide is not limited to the strict definition of the words components “land” and “slide” but instead the meaning of the word use has a much more extensive implication. Landslides can be falls, slides, spreads or flows or rock, soil, earth and debris.
Hazard
In relation to landslip risk assessments, a hazard is anything relating to slope instability that may pose a risk to either people or property both on the subject site and on adjacent or nearby properties. This may include any landslide type, any credible mode of failure of the ground or slope, erosion or even ground subsidence and collapse. Examples of possible hazards may include:
- A rockfall from exposed rock outcrop up slope of the proposed development.
- Failure of a steep slope and subsequent debris flow down slope.
- An earth slump or slide in proximity to the proposed development.
- Failure of a retaining wall or unsupported cut resulting from earth or debris siding.
- Failure of poorly constructed fill embankments.
- Development of tunnel or gully erosion resulting from susceptible soils and poorly designed drainage.
- Ground subsidence and sink holes resulting in collapse.
Geotechnical Assessment
A Geotechnical Assessment usually precedes the more detailed Landslip Risk Assessment (in a two tiered assessment system). Some local governments prescribe a two tiered assessment system (level 1 and level 2) in their relevant planning overlay where as others do not and require a full risk assessment. A Geotechnical Assessment involves fieldwork (mapping, drilling etc) and collection of base data. The collection of base data is a desk top study (or research) that looks at the natural and man-made factors which contribute to a site being more or less susceptible to the risk of landslide. Such factors include rainfall, aspect, slope, elevation, geology and landform. The desk top study also looks at the areas’ stability history to see if previous landslides have ever been identified on or near site. This collection of base data aims to provide a detailed understanding of a site’s existing conditions including any aspects of slope stability. A Geotechnical Assessment will then identify and lists all credible potential hazards that may exist in relation to the site and the intended development. If the data and on-site evidence suggests that all of the potential hazards are of an Acceptable Risk level (low risk) then the assessment may be able to make a statement to that effect and a Landslip Risk Assessment may not be required. If any identified hazards cannot easily be assessed as “low risk” then a Land Risk Assessment will have to be undertaken in addition to the Geotechnical Assessment.
Landslip Risk Assessment
A Landslip Risk Assessment (LRA) includes the same information contained within a Geotechnical Assessment but in addition it also includes a full quantitative risk assessment in accordance with the requirements of the Australian Geomechanics Society (2007) guidelines “Practice Note Guidelines for Landslide Risk Management”. An LRA assesses the risk of landslide to people and property both on-site and surrounding the site both in descriptive terms (low, medium, high) and with actual numbers to provide a risk probability. An LRA must be able to make a statement declaring that the subject site and the proposed development can meet Tolerable Risk criteria and that the site is suitable for the proposed development.
Slope Stability Analysis
A slope stability analysis is a separate geotechnical analysis used to assess the stability of a given slope. It is not on its own equivalent to a Landslip Risk Assessment but it may be used as part of a Geotechnical or Landslip Risk Assessment to assist in identifying credible hazards or determining a level of likelihood for a given hazard. Stability analyses are typically conducted by Geotechnical Engineers applying a more engineering approach to slope stability using computer modelling software. This involves measuring and entering the slopes soil strength parameters into the software and modelling potential failures given a specific set of criteria and site conditions. To perform a slope stability analysis soil samples may have to be collected and analysed in a geotechnical laboratory.
Acceptable Risk
An Acceptable Risk level is defined by the local government authority for which the relevant planning overlay applies, but it is generally expected to correspond with the risk levels recommended in the AGS Landslide Risk Management Guidelines (AGS, 2007c and 2007d). Typically for a qualitative or descriptive assessment such as a Geotechnical Assessment the Acceptable Risk level is LOW for property and infrastructure. For quantitative or numerical assessments such as a full Landslip Risk Assessment, the Acceptable Risk level to life and development is 1 x 10-5 for existing slopes and development and 1 x 10-6 for newly created slopes and development.
Tolerable Risk
A Tolerable Risk level is defined by the local government authority for which the relevant planning overlay applies, but it is generally expected to correspond with the risk levels recommended in the AGS Landslide Risk Management Guidelines (AGS, 2007c and 2007d). Typically for a qualitative or descriptive assessment the Tolerable Risk level to property and infrastructure is Moderate. For quantitative or numerical assessments such as a full Landslip Risk Assessment, the Tolerable Risk level to life and development is 1 x 10-4 for existing slopes and development and 1 x 10-5 for newly created slopes and development.
EMO
EMO stands for Erosion Management Overly. This is a local government planning overlay that may apply to certain areas in relation to planning applications where there is a perceived risk of landslide. Not all local government areas have an EMO and the requirements prescribed by the EMO can be quite different from council to council. In some local governments an EMO may not apply but similar requirements may be triggered under another planning overlay. To check if your proposed development or site location is affected by an EMO, you will need to contact the Council that the site is situated in.
