140 TUNNELING 



by the illustration. Since the roof arch rests for some time 

 supported directly by the unexcavated earth below, settle- 

 ment is liable, particularly in working through soft ground. 

 This fact may not be very important so long as the settle- 

 ment is uniform, and is not enough to encroach on the space 

 necessary for the safe passage of travel. To pi-event the 

 latter possibility the centers are placed from 9 ins. to 15 ins. 

 higher than their true positions, depending upon the nature of 

 the soil, so that considerable settlement is possible without any 

 danger of the necessary cross-section being infringed upon. 

 In conclusion it may be noted that the lining may be con- 

 structed in a series of consecutive rings, or as a single cylin- 

 drical mass. 



Hauling. Since in this method of tunneling the upper part 

 of the section is excavated and lined before the excavation of 

 the lower part is begun, the upper portion is always more ad- 

 vanced than the lower. To carry away the earth excavated at 

 the front, therefore, an elevation has to be surmounted ; and 

 this is usually done by constructing an inclined plane rising 

 from the floor of the tunnel to the floor of the heading, as shown 

 by Fig. 66. This inclined plane has, of course, to be moved ahead 

 as the work advances, and to permit of this movement with as 

 little interruption of the other w r ork as possible, two planes are 

 employed. One is erected at the right-hand side of the section, 

 and serves to carry the traffic while the left-hand side of the 

 lower section is being removed some distance ahead and the 

 other plane is being erected. The inclination given to these 

 planes depends upon the size of the loads to be hauled, but they 

 should always have as slight a grade as practicable. Narrow- 

 gauge tracks are laid on these planes and along the floor of the 

 upper part of the section passing through the center opening 

 mentioned before as being left in the centers and strutting. 



In excavating the top center heading there is, of course, an- 

 other rise to its floor from the floor of the upper part of the 

 section. "Where, as is usually the case in soft soils, this top 



