When the whole section is filled by the fallen material, the problem may be considered as the excavation of a new tunnel of short length inside the old tunnel, and under rather more difficult conditions. The first task, particularly if men have been imprisoned behind the fallen material, is to open communication through it between the two uninjured portions of the tunnel. It is advisable to do this even when there is no danger to life because of imprisoned workmen, since it enables the work of repairing to be conducted from both directions. The excavation of a passageway through the fallen material is rendered difficult, both because the fallen material is of an unstable character, and also because it is usually filled with the lining masonry, timbering, etc. When, therefore, the accident has happened before the full section of the original material has been removed, the first heading or drift is driven through this original material rather than through the fallen débris. Any of the regular soft-ground methods of tunneling may be employed, but it is usually better to select one which allows the masonry to be built with as little excavation as possible at first. For this reason the German method of tunneling is particularly suited to repair work of this nature. The Belgian method may also be used to advantage, particularly when the caving extends to the surface of the ground above, and the upper portion of the débris is, therefore, practically the same material as that through which the original tunnel was driven. The greatest defect of the Belgian method for making repairs is that the roof arch is supported by a rather unstable mass of mingled earth, stone, and timber, which constitutes the bottom layer of the fallen material. The method of strutting the work when the German or Belgian method is used is shown by [Fig. 152]. It sometimes happens that the fallen débris is so unstable that it will not carry safely the arch masonry in the Belgian method or the strutting in the German method, and in these cases one of the full-section methods of excavation is usually adopted. The nature of the strutting employed is shown by [Fig. 153]. When the section has been opened and the new masonry built, great care should be taken to fill the cavity behind the masonry with timber or stone; and should the disturbance reach to the ground surface it is often a good plan to sink a shaft through the disturbed material, and fill it with more stable material.
Fig. 153.—Tunneling through Caved Material by Drifts.
When the fallen débris fills only a part of the section, the first thing to provide against is the occurrence of any further caving; and this is usually done by building a protecting roof above the line of the future roof masonry. [Figs. 154] and [155] show two methods of constructing this temporary roof, which it will be noticed is filled above with cordwood packing. As soon as the temporary roof is completed, the lining masonry is constructed.
Figs. 154 and 155.—Filling in Roof Cavity Formed by Falling Material.
Fig. 156.—Timbering to Prevent Landslides at Portal.
(4) Landslides which close the tunnel entrance are repaired in a variety of ways. [Fig. 156] shows a common method of preventing the extension of a landslide which has been started by the excavation for the entrance masonry. [Fig. 157] shows a method often adopted when the slope is quite flat and the amount of sliding material is small. It consists essentially of removing the fallen material and building a new portal farther back; that is, the open cut is extended and the tunnel is shortened. When the amount of the sliding material is very large, the contrary practice of lengthening the tunnel and shortening the open cut, as shown by [Fig. 158], may be adopted.
