170 TUNNELING 



pressures parallel to the axis of the tunnel. In the construc- 

 tion shown by Fig. 95, special provision is made to resist 

 pressures normal to the plane of the center or twisting pres- 

 sures, by the strength of the transverse bracing extending hori- 

 zontally across the center. 



Masonry. The construction of the masonry lining begins 

 with the invert, as indicated by Fig. 93, and is carried up to the 

 roof of parts No. 2, as already indicated, and is then discon- 

 tinued until the upper parts Nos. 4, 5, and 6 are excavated. 

 The next step is to sink side trenches at the ends of part No. 7, 

 which reach to the top of the completed side walls. This 

 .operation leaves the way clear to finish the side walls and to 

 construct the roof arch in the ordinary manner of such work in 



tunneling. Since this method of 

 tunneling is used only in very soft 

 ground which yields under load, the 

 usual practice is to construct the in- 

 vert and side walls on a continuous 

 no. %. -sketch showing invert foundation course of concrete as in- 



and Foundation Masonry, Italian dicated b\ r Fig. 96. The lining IS 

 Method. u -U ' 



usually built in successive rings, and 



the usual precautions are taken with respect to filling in the 

 voids behind the lining. The thickness of the lining is based 

 upon the figures for laminated clay of the third variety given 

 in Table II. 



Hauling The system of hauling adopted with this method 



of tunneling is very simple, since the excavation of the various 

 parts is driven only from 6 ft. to 10 ft. ahead, and the work pro- 

 gresses slowly to allow for the construction of the heavy strutting 

 required. To take away the material from the center bottom 

 drift, narrow-gauge tracks carried by cross-beams between the 

 side posts above the floor line are employed. This same 

 narrow-gauge line is employed to take away a portion of parts 

 No. 2, the remaining portion being left and used for the refill- 

 ing after the bottom portion of the lining has been built, as 



