86 TUNNELING 



a contact with the surface of the excavation at such points 

 as are to be determined. If the only information desired is 

 whether or not the excavation is sufficient or beyond the es- 

 tablished lines, the rod is set to the proper radius, and if it 

 swings clear the fact is determined. If a true copy of the 

 actual cross-section is desired, the rod is brought into contact 

 with the significant points in the cross-section, and the angles 

 and distances are recorded. 



The general method of directing the excavation in enlarging 

 the profile by referring all points of the profile to the vertical 

 axis is the one usually employed in tunneling, and gives good 

 results. It is considered better in actual practice to have the 

 \ation exceed the profile somewhat than to have it fall 

 short of it, since the voids can be more easily filled in with 

 riprap than the encroaching rock can be excavated during the 

 building of the masonry. In tunnels where strutting is neces- 

 sary, the excavation must be made enough larger than the 

 finished section to provide the space for it. In soft-ground 

 tunnels it is also usual to enlarge the excavation to allow for 

 the probable slight sinking of the masonry. The proper allow- 

 ance for strutting is usually left to the judgment of the fore- 

 man of excavation, but the allowance for settlement must be 

 fixed by the engineer. 



SHAFTS. 



Shafts are vertical walls or passages sunk along the line of 

 the tunnel at one or more points between the entrances, to 

 permit the tunnel excavation to be attacked at several different 

 points at once, thus greatly reducing the time required for 

 excavation. Shafts may be located directly over the center 

 of the tunnel or to one side of it, and, while usually vertical, 

 are sometimes inclined. During the construction of the tunnel 

 the shafts serve the same purpose as the entrances ; hence they 

 must afford a passageway for the excavated materials, which 



