4li TUNNELIM! 



setting on a sill laid across the bottom of the heading. These 

 frames are spaced at close intervals, and carry longitudinal 

 planks or poling-boards. The sill of the frame is sometimes 

 omitted when the soil is stable enough to permit it, and in its 

 place wooden footing blocks are substituted to carry the side 

 (Mists. In soils where the pressures are great enough to bend 

 the crown bar, a secondary frame is employed, as shown by 

 Fig. 23, the two inclined roof members, or rafters, of which 

 sii|>i>ort the crown bar at the center. 



It is the more common practice in driving headings through 

 soft soils to use inclined poling-boards to support the roof. 



FIG. 24. Longitudinal Pollng-Board Sys- 

 tem of Roof Strutting. 



FIG. 25. Transverse Poling-Board System 

 of Roof Strutting. 



Fig. 24 shows one method of doing this. The method of 

 operation is as follows : Assuming the poling-boards a and b 

 to be in place, and supported by the frames A, B C, as shown, 

 the first step in continuation of the work is to insert the 

 poling-board c over the crown bar of frame C, and under the 

 block m. Excavation is then begun at the top, and as fast as 

 the soil is removed ahead of it the poling-board c is driven 

 .ahead until its rear end only slightly overhangs the crown bar 

 of frame C. The remainder of the face of the heading is then 

 excavated nearly to the front end of the poling-board <?, and 

 another frame is set up. By a succession of these operations 



