Failure of a Bridge Pier

The Bridge Pier of the 36m main span and flanking 12m span bridge tilted backwards to the river bank during launching of the bridge deck steel trusses. The pier was made up of eight driven steel pipe piles and then welded with steel pipe braces to form a high supporting structure.
The Contractor reported that the Pier failed suddenly with the Resident Engineer agreeing initially. The welding of the steel joints was found to be of poor quality, but should not be the main cause of failure. The river bank showed serious slip which indicated slope failure towards the river. 
There was no earthquake, no piling, no rain and no torrential  river flow or erosion of the river bank. Then why did the Pier fail?  

If the river bank was unstable, it would have failed during pile driving which generated a lot of vibration, or when filling up the road behind the bridge abutment wall, which exerted the most load. 

Why it failed after two months of the completion of sub-structure works? The steel trusses were light and should not generate much loading to the bank!

Upon questions, queries and interviews by the design Engineer and Client on the sequence of events happening prior to failure, the frightened Resident Engineer finally admitted that he was threatened not to tell the truth. The installation Contractor actually filled up the river bank with earth from neighbouring hill, between the pier and abutment, so that he did not need to construct working platforms for the launching of the steel trusses. When the pier failed, he quickly removed all the earth and asked him to shout up. He said he did not know that filling would cause failure. He thought that this was normal way of construction practised locally. In fact, this Resident Engineer claimed that he had 15 years of working experience in Saudi Arabia, that was why the Client engaged him to supervise the construction work!

Thus, the causes were immediately determined. The additional fill, 2.5m thick at abutment to 6m thick at the pier on the river bank was a substantial load behind the pier, triggered the soft soil slope to fail and thereby pushing the pier towards the river. The poor welding of the joints caused the vertical members of the pier to break at the joints, resulting the pier falling backwards.

Solution: 
As the piles moved by more than 1.5m, they were all bent and would not be able to take the required bridge loading, the pier was abandoned. The 24m span and 12m span bridge deck was changed to 36m span bridge deck. The relevant substructures were strengthened and modified, some costs were shared between the substructure contractor (for shoddy welding) and the installation Contractor(for causing the failure using improper construction method).  The Client offered to use the fabricated 24m and 12m trusses for other bridge projects. The Design Engineer offered to re-design the rectification works. The work was finally completed in another three months time.