(2) The bulging or rising of the bottom of the tunnel may usually be considered as a consequence of the squeezing together of the side walls. It usually occurs in very loose soils, and is chiefly important from the fact that the reconstruction of the side walls is made necessary. The sinking of the tunnel bottom is a more serious occurrence. It seldom happens unless there is a cavity beneath the floor, due either to natural causes or to the fact that mining operations have gone on in the hill or mountain penetrated by the tunnel. When the bottom of the tunnel sinks, three cases may be considered: (_a_) when the sinking is limited to the middle of the tunnel floor; (_b_) when only a portion of the foundation masonry is affected; and, (_c_) when the entire lining is disturbed. In the first case repairs are easily made by filling in the cavity with new material. In the second case the unimpaired portion of the masonry is temporarily supported by shoring while the injured portion is removed and rebuilt on a firm foundation. The remaining cavity is then filled. In the case of the complete failure of the lining, the method of repairing employed when the roof falls, and described below, is usually adopted. (3) The most dangerous of all failures is the falling of the tunnel roof. In such casualties two cases may be considered: (_a_) When the falling mass completely fills the tunnel section, and (_b_) when it fills only a portion of the section. [Illustration: FIG. 152.--Tunneling through Caved Material by Heading.]