An arched (semi-circle) rectangular tunnel 28 ft wide and 28 ft high is driven in dry, moderately blocky, and seamy ground.

(a) Select a steel rib suitable for these conditions; specify flange width, web depth, weight per foot, and set spacing.

(b) Suppose yieldable steel arches where used to support the arched back on the same set spacing under the same rock load (“head”). What thrust in a yieldable set is indicated?

(c) If large, long bolts were used instead, estimate the bolting pressure that is indicated assuming the same spacing and thrust capacity provided by the yieldable arches.