176 



TUNNELING 



FIG. 104. Sketch Showing Construc- 

 tion of Masonry Lining, Quicksand 

 Method. 



bars braced by short struts against the inclined props employed 

 to carry the roof arch when the material on which they origi- 

 nally rested is removed. This side 

 strutting is shown at the right 

 hand of Fig. 104. 



Masonry. As soon as the upper 

 part of the section has been opened 

 the roof arch is built with its feet 

 resting on planks laid on the unex- 

 cavated material below. This arch 

 is built exactly as in the regular 

 Belgian method previously de- 

 scribed, using the same forms of 

 centers and the same methods 

 throughout, except that the poling- 

 boards of the strutting are usually left remaining above the 

 arch masonry. To prevent the possibility of water percolating 

 through the arch masonry, many engineers also advise the 

 plastering of the extrados of the arch with a layer of cement 

 mortar. This plastering is designed to lead the water along 

 the haunches of the arch and down behind the side walls. In 

 constructing the masonry below the roof arch the invert is 

 built first, contrary to the regular Belgian method, and the 

 side walls are carried up on each side from the invert ma- 

 sonry. Seepage holes are left in the invert masonry, and also 

 in the side walls just above the intrados of the invert. At the 

 center of the invert a culvert or drain is constructed, as shown 

 by Fig. 104, inside the invert masonry. This culvert is com- 

 monly made with an elliptical section with its major axis hori- 

 zontal, and having openings at frequent intervals at its top. 

 The thickness of the lining masonry required in quicksand is 

 shown by Table II. 



Removing the Seepage Water. After the tunnel is completed 

 the water which seeps in through the weep-holes left in the ma- 

 sonry passes out of the tunnel, following the direction of the 



