, for the saturated material, but with a weight per cubic foot equal to that of the earth in air less the buoyant effect of the water. Thus, if a cubic foot of the porous earth, in air weighed 90 lb., and if the voids were 40%, then 1 cu. ft. of earth contains 0.6 cu. ft. of solids and the buoyant effect of the water is the weight of an equal volume of water or
lb. Hence, the weight per cubic foot of earth in water is
lb.
Similarly, for the pressures,
and
, at the top of a tunnel,