A bridge too steep

It is certainly not passable for traffic, but pedestrians still can climb through. Why the bridge failed?

It is located at a river tributary on the coastal plain of Batang Saribas. The underlying soils are soft, deep and sensitive in many areas.

The bridge has three span deck supported by two abutments on the river bank and two piers in the river.

One of the piers fell, bringing down the decks onto the river bank.

A few causes may result such failures:

(a)  river bank moves during piling. Piling generates vibration, vibration weakens soil strengths, this is particular true for senstive soils. In many areas, sensitivity can reach 4-6, thus river bank stability which has about 1.2-1.5 safety can easily drop below 1.0. When the river bank moves, the piles move too, bending the piles as well breaking/weakening the piles.
Besides piling, using heavy equipments or building temporary earth platform on the river bank for piling, earthworks and slope protection works also cause the bank to move.

(b) long piles require a number of jointings, if welding cannot be done properly, it becomes weak points, in particularly slanting piles. The joint strength must be as strong as the piles, whether under stress or bending strength, otherwise filures may occur at these points, 

(c) defective piles, whether due to manufacturing, transporting or during driving, have to be rejected and replaced.

Cause(a) was the major reason of failure, while (b) and (c) often aggravated the situations.

Prevention: River bank shall be monitored for movements, this can be done by setting up monitoring points in lines at various locations of the river bank during driving and construction. If the movements still persist after stoppage of construction activities, then the river bank has to be released of stress such as removing all the unnecessary loading or may even have to  excavate the filled ground on the river bank.
Solid RC piles and heavy dynamic piling shall be avoided.  Steel pipe piles and drop hammer driving are preferred with close monitoring.