Managing The Risks Of Landslides

Published in the Engineers Australia Magazine January 2010 Edition.

Landslide is common in most populated areas in Australia, but public awareness increased greatly after the Thredbo Landslide in 1997 where about 3500t of debris crashed down a ski slope, killing 18 people.

Landslides occur when the weight of the soil on a slope exceeds the strength of the soil to hold it in place on that slope. Loss of soil strength can occur following prolonged rainfall.

Landslide includes rock fall, deep seated slides, debris flows and shallow landslides. However, the general public generally do not understand the risks.

I remember a case where a resident complained about 100m3 of red-brown soil had moved onto his building pad. After it was explained to him that a landslide had occurred, he responded by saying that his property had never experienced landslide in the past. He was amazed when he was informed that his property had experienced many landslides in the past, evidenced by numerous s-shaped trees.

After his neighbour refused ownership or to pay for removal of the soil, the owner sold the soil to a landscaping company after which the neighbor decided he deserved some of the proceeds.

Mostly, the consequences of landslide can be far-reaching including loss of property, loss of life, stress and disruption in many forms. The cost of landslides is not covered by most insurance policies with the costs having to be borne by individuals and government.

Australia-wide, after every wet spell, there will be multiple rock falls and landslides along narrow roads leading to the mountain resorts, mostly not recorded.

Total costs to society in Australia, including loss of property and rectification could be at least of the order of $1 billion dollars in the last 100 years.

Recently, we were requested to identify safe building locations on a certain rural residential property. Certain safe locations were identified but one location which was identified as an old landslip zone, slipped again fairly dramatically following rains in 2009 as per the photograph below.

Landslide is common in most populated areas in Australia, but public awareness increased greatly after the Thredbo Landslide in 1997 where about 3500t of debris crashed down a ski slope, killing 18 people.

Landslides occur when the weight of the soil on a slope exceeds the strength of the soil to hold it in place on that slope. Loss of soil strength can occur following prolonged rainfall.

Landslide includes rock fall, deep seated slides, debris flows and shallow landslides. However, the general public generally do not understand the risks.

I remember a case where a resident complained about 100m3 of red-brown soil had moved onto his building pad. After it was explained to him that a landslide had occurred, he responded by saying that his property had never experienced landslide in the past. He was amazed when he was informed that his property had experienced many landslides in the past, evidenced by numerous s-shaped trees.

After his neighbour refused ownership or to pay for removal of the soil, the owner sold the soil to a landscaping company after which the neighbor decided he deserved some of the proceeds.

Mostly, the consequences of landslide can be far-reaching including loss of property, loss of life, stress and disruption in many forms. The cost of landslides is not covered by most insurance policies with the costs having to be borne by individuals and government.

Australia-wide, after every wet spell, there will be multiple rock falls and landslides along narrow roads leading to the mountain resorts, mostly not recorded.

Total costs to society in Australia, including loss of property and rectification could be at least of the order of $1 billion dollars in the last 100 years.

Recently, we were requested to identify safe building locations on a certain rural residential property. Certain safe locations were identified but one location which was identified as an old landslip zone, slipped again fairly dramatically following rains in 2009 as per the photograph below.

Builders also request landslip reports on moderate to steep slope sites. This is because the slope risk report completes their duty of care, limits their liability and provides information concerning geotechnical concerns of the area.

Slope risk reports are relatively inexpensive for house lots. Once it is explained that the necessity arises from outcomes from the Thredbo landslide disaster, there seems to be general acceptance and even enthusiasm for receipt of a positive report about their block.

The task is ongoing for engineering professionals involved in the geosciences is to continue ongoing research into landslide, to devise ever more accurate methods to understand landslide, to increase our knowledge of the geology of Australia and to mentor and set up an information passage to pass this knowledge through to the up-coming generation of geotechnical engineers. Legislators will have the task of ensuring the expertise makes it way to the public.

For the future, I expect greater use of geophysical methods to scan the subsurface for information of the soils and rock. This method is also less intrusive and explores to deeper depths, than conventional soil testing equipment.

Local authorities have always had a concern for the consequences of landslide, but since the Thredbo landslide, concern was highlighted. As well as duty of care and public safety, councils are concerned with their liability if they approve a development for construction, that may have landslide susceptibility.

This is an edited version of an article by James Tayler, a chartered structural and geotechnical engineer and manager of engineering consultancy Earthsolve.

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