=Accidents and Repairs.=--One of the most frequent accidents in the Belgian method of tunneling is the sinking of the roof arch owing to its unstable foundation on the unexcavated soil of the lower portion of the section. The amount of settlement may vary from a few inches in firm soil to over 2 ft. in loose soils. To counteract the effect of this settlement it is the general practice to build the arch some inches higher than its normal position. When the settlement is great enough to infringe seriously upon the tunnel section, repairs have to be made; and the only way of accomplishing them is to demolish the arch and rebuild it from the side walls. It is usually considered best not to demolish the arch until the invert has been placed, so that no further disturbance is likely to occur once the lining is completed anew. The rotation of the arch about its keystone, or the opening of the arch at the crown, by the squeezing inward of the haunches by the lateral pressures, is another characteristic accident. Fig. 74 shows the nature of the distortion produced; the segments of the arch move toward each other by revolving on the intradosal edges of the keystone, which are broken away and crushed together with the operation, while the extradosal edges are opened. It is to prevent this occurrence that the horizontal struts shown in Fig. 71 are employed. The manner of repairing this accident differs, depending upon the extent of the injury. When the intradosal edges of the keystone are but slightly crushed, the repairing is done as directed by Fig. 75. When the keystone is completely crushed, however, the indications are that the material of the keystone, usually brick, is not strong enough to resist the pressures coming upon it, and it is advisable to substitute a stronger material in the repairs, and a stone keystone is constructed as shown by Fig. 75. The middle stone of this keystone extends through the depth of the arch ring, and the two side stones only half-way through, their purpose being merely to resist the crushing forces which are greatest at the intrados. Sometimes, when the pressures are unsymmetrical, the arch ring breaks at the haunches as well as the crown, as shown by Fig. 75, which also indicates the mode of repairing. This consists in demolishing the original arch, and rebuilding it with stone voussoirs inserted in place of the brick in which the rupture occurred. [Illustration: FIG. 75.--Sketches Showing Methods of Repairing Roof Arch Failures.] CHAPTER XIII. THE GERMAN METHOD--EXCAVATING TUNNELS THROUGH SOFT GROUND (Continued); BALTIMORE BELT LINE TUNNEL.