Slope Failure Due To Liquefaction

The slope is 1:2 with a height of 11m. The whitish soil is Sandy Silt/Clay, in fact about 25% clay, 25% Silt and the remaining is fine sand. The average natural moisture content is about 24% and the Liquid limit is about 30%. This existing Slope was stable for more than 20 years.
Then a new owner acquired the land and intended to re-build  new house and swimming pool near and on the slope. During installation of concrete piles, the slope failed and the pile driving machine fell down to the toe of the slope. The soil was seen flowing down the slope after immediate failure. There was raining in the previous night.

A sample of this soil was sent to the laboratory for testing. The moisture content was found to be about 22%, there was no obvious increase of water content in the soil.

Why did the slope fail?

Piling caused vibration. The installation of piles brought shock waves into the slope and liquefied the silty and sandy soils which actually have low liquid limits. That was why the soil flowed like lava, with little change to the moisture content, but after a few hours, it became "solid" again.