A tunnel of lenght L=150 m, height H=7.2 m, and width W=5.8 m (with a flat roof ) . With a distance D= 60 m beneath the ground. The roof of the tunnel is to be supported entirely by square steel columns, each with a cross-sectional area of 960 cm2.assof1.0 cm3 of the ground material is 2.8 g. (a) What is the total weight of the ground material the columns must support? (b) How many columns are need to keep the compressive stress on each column at 1/2 it ultimate strenght?