You may wonder why the road fails so badly near the hill area. It looks like the hill has been cut down and forms the base of the road and hence the road should be stable.
But if you look at the type of soil and the water retained in the drain, then you will understand why.
The soil is whitish and if you squeeze on your hand, it is not as sticky. It is also difficult to roll into a thread. The soil is silt which loses strength easily when infiltrated by water. Silt ranks very low in the Engineering Chart as filling materials.
The blocked drain at the other side of the road was the main source of trouble, water. This road foundation was formed by excavating the hill, but the other far side is filled up from the excavated materials to form wider road. The circular slip failure is therefore on the filled side which is sloping towards the hill toe.
Water is the chief enemy of engineering projects and this happens especially during raining season.
Ensure that all the drains are properly maintained and water flows away quickly, then you will reduce most of this type of failure.
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