Expanded clay is used trenches in various situations to construct drainage, particularly for stabilising slopes that consist of silty clay material.
If the drainage trench follows the line of maximum slope, this decreases the water pressure in the slope, consequently increasing the safety coefficient.
In these cases if expanded clay is used to construct the upstream part of the trench, its low density provides additional mechanical stabilisation by reducing the weight of the trench at a point where this might otherwise become a factor leading to collapse.