Deep River Bank Failure

This section of the Batang Layar is more than 1km wide and the slope is gentle with an average slope of 1:7 to 1:9. The underlying soils are predominantly river and marine deposits about 40m thickness. The clay/silt soils are very soft at top to firm/stiff at he bottom. The sensitivity is high with about 4 to 6. A wharf was supposed to be built next to this part of the river bank.

Slope Analysis using Modified Bishop Method indicated the stability was about 1.2 to 1.3. This is generally the safety of straight natural formed large river bank after years of flood, deposition, erosion, failure and stabilisation.
Thus any construction or additional filling up of river bank have to be done carefully and slowly, which has to be specified clearly. In particular, the piling method which creates vibration and may cause loss of soil strength. The filling up of the river bank have to be done in stages.

Then the slope next door failed during construction of the wharf. An excavator was seen sinking into the ground. The affected semi-circular failed zone is more than 50m and deep into the river. Why it failed?

ANSWER: Another Contractor used the river bank as his sand stockyard for another project. I requested the RE to warn him not to store the materials near the river bank and he retorted back that the land was outside our wharf project and asked us to mind our own business.
According to the RE, this Contractor used the barge to berth at the river bank during high tide and stocked up the sand to more than 4m height and within few days, the whole river bank failed. The Contractor then panicked and tried to salvage as much as possible the sand from being washed away by flowing river using the excavator. Unfortunately, he salvaged too much and his excavator sunk into the soft soil.
I did another geotechnical analysis using additional load of 40KPa, the safety factor went below 1.0, signifying the failure of the slope.
This slip failure went deep into the river and near to our wharf front, bulging out near the shipping path, which made the channel shallower, but did not affect the wharf structure. 
We have a lot of big-headed and ignorant people who knew little about engineering and yet acted as if they know better than the engineers.