272 



TUNNELING 



the disturbance reach to the ground surface it is often a good 

 plan to sink a shaft through the disturbed material, and fill it 

 with more stable material. 



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f^ 1 \=^p=T Vf 

 FIG. 135. Tunneling through Caved Material by Drifts. 



When the fallen debris fills only a part of the section, the 

 first thing to provide against is the occurrence of any further 

 caving ; and this is usually done by building a protecting roof 

 above the line of the future roof masonry. Figs. 136 and 137 



FIGS. 136 and 137. Filling in Roof Cavity Formed by Falling Material. 



show two methods of constructing this temporary roof, which 

 it will be noticed is filled above with cord wood packing. As 

 soon as the temporary roof is completed, the lining masonry is 

 constructed. 



