TUXXKLS THROUGH HARD KOCK 115 



where the full top section has been excavated by the removal 

 >t parts N<>. These two tracks are again combined ami form 

 a single track along the top of p:irt No. 5, which has been left 

 widiM- than part No. 4 for this particular purpose. When purt 

 No. 3 is t- xravatrd a standard-gauge track is laid on its floor; 

 and as the full section of the tunnel is completed by taking out 

 parts Nos. 4 and o, this single track is replaced by two standard- 

 gauge tracks, into which it switches. Spoil is transferred from 

 the narrow-gauge tracks on the upper level, to the standard- 

 gauge tracks on the tunnel floor, by means of chutes, and build- 

 ing material is transferred in the opposite direction by means of 

 hoisting apparatus. 



When the excavation is made by a single wide heading, and 

 a single other cut for removing the bench, which is the method 

 preferred by American engineers, the work begins by removing 

 a top heading the full width of the section. This heading is 

 usually made 7 ft. or 8 ft. high, and is excavated by the center cut 

 method. The method of strutting usually employed, is to erect 

 successive three- or five-segment timber arches, whose feet rest 

 on the top of the bench ; when the bench is removed, posts an 

 inserted under the feet of each arch. These arches are covered 

 with a lagging of plank. In America it has often been the 

 practice to let this strutting serve as a temporary lining, and to 

 replace it only after some time, often after years, with a perma- 

 nent lining of masonry. In a succeeding chapter, some of the 

 nit-tin xls adopted in relining timler-lined arches with masonry 

 are drsrriiVd. The hauling is done by a narrow-gauge track 

 l.vid on tin- bottom of tin- heading, and by either narrow or 

 lir<>ad gauge tracks laid on the floor of the completed section 

 Ix-lttw. A device called a lieiich carriage is often employed to 

 cnal ile the cars running on the heading tracks to dump their 

 loads into the cars below, without interfering with the work on 

 the bench front. This device consists of a wide platform 

 carried on trucks, running on rails at the sides of the tunnel 

 floor, so that it is level with the floor of the heading. The 



