The retaining board, 1 m. square, was coated with sand, so that
for damp sand. Hence, for a horizontal thrust on
, the plane of rupture (which bisects the angle between the vertical and the natural slope) makes an angle of
with the vertical. The board,
, was set at this angle to the vertical, sand was filled in level with the top, and it was found that the structure was at the limit of stability when
