sewer installation

while digging the trenches for the 100mm sewer pipes, the excavator found an area with bad soil. The soil was quicksand like and the soil was caving in from the sides as we tried to dig some more. The soil report states that the site is M class but my guess is that there are patches where its P class (not good)
This was solved by using 10bags of quick set concrete mix to hold the sides of the trench walls up.