Slope Failure Due to Erosion

This slope is located near the roadside from Bintulu to Samalaju.  The hill is low, less than 6m at the frontage and the slope provided is about average 1:1 near the road and then gently slopes towards the oil palm plantation.

The failure is localised, more of vertical collapse due to erosion than collapse through large lateral movements such as circular slippage.   Why?

The types of soils/rocks and the drainage, were the probable causes. On the top of the slope, a deep eroded channel was found, probably formed from broken shallow drain along the slope after collecting rain water. With time, due to the silty and sandy nature of the residual soil, the erosion and seepage of rainwater changed this shallow drain to deep channel, cutting into the completely weathered siltstone laminated with Mudstone, Shale and Sandstone, forming drainage holes coming out from the slope. The slope collapsed inwards as well as downwards leading the multiple cracking of slopes as well as the collapses of concrete benches and shallow drains.

Concrete drain provided must be continuous and be able to discharge effectively to the lower ground. It shall not be leaking or seeping through cracks or drain back on silty and sandy soils especially if the rainfall catchment area is large. Apparently the backgound oil palm plantation also drains towards the slope.

As the slope is far away from the road pavement, it does not seem to pose any danger to the road users.