Note Return to text — EXPERIMENTS ON RETAINING WALLS AND PRESSURES ON TUNNELS.

In the next experiments, however, referring to a retaining structure consisting of two boards, hinged at the top, Fig. 8, and backed by sand level at the top, the Rankine theory is applicable when the board, , is placed either at or below the plane of rupture, on the left of . The thrust on is then assumed to act horizontally, at above , and is combined with the weight of the sand, , to find the resultant on the board. If the board is at the plane of rupture, this resultant will make the angle below the normal to ; hence, if one assumes a less thrust on , especially if inclined downward, the new resultant on will make an angle greater than with the normal to , which is inconsistent with stability.[Footnote 2] The same reasoning applies when lies below the plane of rupture.[Footnote 3]