, then support can be derived from three sides, and
. Thus if we assume the horizontal cross-section of the prism to be 15 by 7.5 ft.,
lb. per sq. ft., where
lb. per cu. ft. Thus, such a vertical prism of any height, at the heading, can be sustained by a cohesion of 338 lb. per sq. ft. acting on three of its sides. This refers to a tunnel 15 ft. wide. For the completed tunnel 15 ft. wide, the part vertically over the tunnel can be supported if the two sides can furnish double this unit cohesion. The term
will appear in an equation mentioned later. It is seen now that when this term is zero, cohesion alone can sustain the prism, and the pressure reduces to zero. When this term is negative, it indicates that that too large a value of
