IT XX ELS THROUGH SOFT GROUND 



139 



supported by alternate lengths or pillars of unexcavated earth 

 and completed side wall. The next step is to remove the 

 remaining sections of earth between the section* of aide wall, 

 and fill in the space with masonry. 

 Fig. 65 is a cross-section, showing 

 the masonry completed for one-half 

 and the inclined props in position 

 for the other half; and Fig. 67 is 

 a longitudinal section showing the 

 pillars of unexcavated earth be- 

 tween the consecutive sets of in- 

 clined struts and several other 

 details of the lining, strutting, and 

 excavating work. 



The invert masonry is built after 

 the side walls are completed. This 

 is regarded as a defect of this method of tunneling, since the 

 lateral pressures may squeeze the side walls together and dis- 

 tort the arch before the invert is in place to brace them apart 



Fio. 66. Sketch Showing Method of 

 I'nderpining Roof Arch with the 

 Side Wall MMonry. 



To prevent as much as possible 

 the distortion of the arch after 

 the centers are removed, it is 

 considered go<xl practice to 

 shore the masonry with hori- 

 zontal beams having their ends 

 abutting against plank, as shown by Fifc. 65. These hori- 

 zontal lieams should lie placed at close intervals, and be 

 supported at intermediate points by vertical pontn, as shown 



Km. 06. Illicit inlinul Hwtlon Showing 

 Conntructlon by the* Belgian Method. 



