=Prevention of Collapse.=--Tunnels very seldom collapse without giving some previous warning of the possible failure, and also of the manner in which the failure is likely to occur. From these indications the engineer is often able to foresee the nature of the danger and take steps to check it. The danger may occur either during excavation or after the lining is built. During excavation the danger of collapse is indicated beforehand by the partial crushing or deflection of the strutting timbers. If the timbers are too light or the bearing surfaces are too small, crushing takes place where the pressures are the greatest, and the timbers bend, burst, or crack in places, and the joints open in other places. The remedy in such cases is to insert additional timbers to strengthen the weak points, or it may be necessary to construct a double strutting throughout. When the distance spanned by the roof timbers is too great, failure is generally indicated by the excessive deflection of these timbers, and this may often be remedied by inserting intermediate struts or props. In some respects the best remedy under any of these conditions is to construct the masonry as soon as possible. When collapse is likely to occur after the masonry is completed, its probability is generally indicated by the cracking and distortion of the lining. A study of the cause is quite likely to show that it is the percolation of water through the material surrounding the lining which causes cavities behind the lining in some places, and an increase of the pressures in other places. When it is certain that this water comes from the surface streams above, these streams may often be diverted or have their beds lined with concrete to prevent further percolation. When percolating water is not the cause of the trouble, a usually efficient remedy is to sink a shaft over the weak point, and refill it with material of more stable character. These, and the remedies previously suggested, are designed to prevent failure without resorting to reconstruction. When they or similar means prove insufficient, reconstruction or repairs have to be resorted to. =Repairing Failures.=--Tunnels may collapse in several ways: (1) The front and sides of the excavation may cave in; (2) the floor or bottom may bulge or sink; (3) the roof may fall in; (4) the material above the entrances may slide and fill them up. (1) One of the most common accidents is the caving of the front and sides of the excavation. This may often be prevented by taking care that the face of the excavation follows the natural slope of the material instead of being more or less nearly vertical. When, however, caving does occur it may usually be repaired by removing the fallen material, strongly shoring the cavity, and filling in behind with stone, timber, or fascines.