KMAN Mill MOD 149 



the tracks on the bottoms of the side drifts. Generally, how- 

 ever, the separate soffit car line is not used unless the material 

 is of such a firm character that the headings and drifts can be 

 carried a great distance ahead of the masonry work. With the 

 center top heading plan of beginning the excavation, the car 

 tnu-k has, of course, to be laid on the top of the center core. 

 The center core itself is removed by means of car tracks along 

 the floor of the completed tunnel. 



Advantages and Disadvantages. Like the Belgian method 

 of tunneling, the German method has its advantages and dis- 

 advantages. Since the excavation consists at first of a narrow 

 annular gallery only, the equilibrium of the earth is not greatly 

 disturbed, and the strutting does not need to be so heavy as in 

 methods where the opening is much larger. The undisturl>ed 

 -enter core also furnishes an excellent support for the strutting, 

 and for the centers upon which the roof arches are built 

 Another important advantage of the method is that the con- 

 struction of the masonry lining is begun logically at the bottom, 

 and progresses upward, and a more homogeneous and stable 

 eonstnietion is possible. The great disadvantage of the method 

 is the small space in which the hauling has to be done. The 

 spoil cars practically fill the narrow drifts in passing to and from 

 tin- fnmt, and interfere greatly with the \\<rk of the carpenters ^ 

 and masons. Another objection to the method is that the 

 invert is the very hist portion of the lining to l>e built. This 

 may not IK? a serious objection in reasonably com jmct and stable 

 materials, but in very loose soils there is always the danger of 

 ;ie walls l>eing squeezed together before the invert masonry 

 i> in position to hold them apart. Altogether the difficulties 

 are of a rharaeter which tend to increase the expense of the 

 mr t hod, and this is the reason why to-day it is seldom used 

 even in the eountry where it was first developed, and for some 

 extensively employed. For repairing accidents, sueh aa 

 the caving in of completed tunnels, the German method of tun- 

 neling is frequently used, localise of the ease with which the 



