Common safety misconceptions

Common safety misconceptions
Malay Mail 15/12/2004

THE Malay Mail is doing a series on hillslope developments in Selangor. Today's segment focuses on some common misconceptions about such projects and the risks involved when they are abandoned.

SOIL tests alone will not tell whether a slope is safe, and poorly designed retaining walls can actually cause slope failures.

Geo-technician Dr. Gue See Sew said the public tend to get confused over articles that appear in the media on slope safety.

"Some might think about the environmentally-friendly idea that natural hillslopes should be left untouched by development. However, this could actually result in more landslides," said Gue.

Gue debunked some of the misconceptions about slope safety and gave his explanations below.

1) Soil tests showed that the slope is safe.

Soil tests are reports of the soil properties at the location but they do not tell whether a slope is safe. Rather, an engineer needs to study the overall slope and carry out analyses of the slope using the soil tests' results and slope geometry to determine the factor of safety (FOS).

A slope's geology and composition can vary significantly over a short distance. Geological features, soil types and properties have significant influence on slope stability.

2) Heavy rain causes slope failure.

Although it triggers landslips, increased rainfall raises the ground water table and decreases the FOS of the slope.

The minimum FOS ranges from 1.2 to 1.4, depending on the risk of life and economical ramifications. The threshold value at failure is unity. A simple analogy of FOS can be illustrated using example of weight lifting. Suppose the maximum weight a person could lift is 50kg, and when the person is given 50kg, then the FOS at failure or threshold is 1.0 (50 divided by 50). If the person is given 40kg, then the FOS is 1.25 (50 divided by 40),

3) Erosion will not cause slope failure

Erosion can propagate a slip further and cause a bigger landslide. The small and localised erosion is further eroded by rainfall and water flow, causing more soil mass to fail. This is repeated until the whole slope is not stable and slides.

4) Retaining walls always prevent slope failure

Structural solution like retaining wall usually can retain soil mass of the slope. However, poorly constructed wall can cause slope failure.

5) Slopes are maintenance free

Slopes are not always maintenance free as the maintenance works, such as clearing of clogged drains and patching up localised erosion spots, are required.
Poorly maintained slopes will lead to slope failures, while clogging increases water pressure build-up through seepage and localised erosion can propagate landslides.

6) The slope has been standing for more than 10 years! So it is safe

Natural slopes can give way suddenly without warning even though it has been standing for years. Natural slopes may be standing up without signs of failure but the factor of safety could be low and near the threshold.

7) Environmental Impact Assessment (EIA) report ensures slope stability.

An EIA report is a study of the environmental impact of a proposed development in the area and its surroundings. It doesn't examine the engineering of the slopes in detail to determine if a slope is safe and the required stabilisation measures, if any. Detailed investigation, analysis and design will only be carried out after the approval of EIA report, and before the approval of earthwork plans.

8) Geological report shows that the slope is safe

Geological report covers the history of the soil and the underlying bedrock to explain the geological features, types of rock present, soil stratification, weathering grade and minerals present. It does not cover the engineering and design of the slopes. Only an engineer's report or a geo-technical report with interpretation of field and laboratory tests and detailed analyses, will show whether a slope is safe.

Risks in abandoned hillslope projects

HILLSLOPE projects may be abandoned because of financial difficulties or any other reasons. Partly developed hillslope is usually left as it is, and this is a risk to the public.

The dangers:

1) Incomplete earthwork

Hillslope developments mostly involve substantial earthwork to prepare the necessary platforms for building construction.

Earthwork involves re-grading the existing slopes and transporting its fill to form the required slopes. However, in an abandoned hillslope project, the earthwork is not completed and the cut and fill slopes are not fully graded to the design and safe gradient.

In addition, soil erosion takes place and gullies formed could de-stabilise the slopes. Hence, slopes in abandoned projects are often unstable in the long term and susceptible to continued erosion and ingression from rainfall.

2) Incomplete slope strengthening works

In addition to earthwork, there are some earth retaining structures or soil reinforcement which were originally designed to stabilise and retain the slopes. However, if these slope strengthening work is not completed, it may not fully retain or strengthen the soil slopes originally designed for. Hence, the stability of the slopes is in doubt.

3) Incomplete drainage works

Similarly, incomplete drainage work reduces the stability of the slopes as it affects the ground water table. Incomplete drainage work may cause water pressure build-up by the incomplete channeling of water flow to the main drainage outlets.

Subsequently, the build-up water will seep into the hillslopes, raising the ground water table profile and therefore increasing the risk of slope instability.

4) No slope maintenance.

Most abandoned projects will be left as it is without further maintenance. As a result, drainage paths get blocked or silted by the accumulation of decayed vegetation and soil.