A test pit was dug to 4.2 m. From top to bottom, it was 1.6 m silty clayey sand, 0.9 m peat/organic silt, and 1.7 m mineral silt. The bottom of pit was examined immediately after excavation and it was stiff silt but after half an hour the silt materials spalled and caved in. GWL was at top of silt layer. The silt on the wall appeared to be firm. A photo is attached.
What does the spalling of wall suggest to an engineer. Could the silt be soft or.. Thanks.