SPKCIAL TKKACHKKors (JKnfNI) MKTHOD 



175 



Fin. 108. Sketch Showing 



I'rrlimiiiiin MniinageOal- 

 leriec. Quicksand Method. 



by driving a top center heading, as shown by Fig. 102. This 

 heading is deepened and widened after the manner usual to the 

 Belgian method, until the top of the sec- 

 tion is open down to the springing lines 

 of the roof arch. To collect the seepage 

 water from the center top heading it is 

 provided with a center bottom drain con- 

 structed like the drain in the bottom 

 drift, as shown by Fig. 102. When the 

 top heading is deepened to the level of 

 the springing lines of the roof arch, its 

 bottom drain is reconstructed at the new 

 level, and serves to drain the full top 

 section opened for the construction of the 

 arch. This top drain is usually con- 

 structed to empty into the drain in the bottom drift 



Strutting. The method of strutting the bottom drift has 

 already been described. For the remainder of the excavation 

 the regular Belgian method of radial roof strutting-frames is 



employed, as shown by Fig. 103. 

 Contrary to what might l>e expected, 

 the numl>er of radial struts required 

 is not usually greater than would be 

 used in many other soils besides 

 quicksand. Single-track rail way tun- 

 nels have been constructed through 

 quicksand in several instances where 

 the number of radial props required 

 on each side of the center did not 

 exceed four or five. It is necessary, 

 however, to place the poling-lxianls 

 close together, and to JKK k the joints U-tween them to 

 prevent the inflow of the line sand. In strutting the lower 

 part of the section it is also necessary to support the sides with 

 tight planking. This is usually held in place by longitudinal 



Pin. 103. Sketch Showing Con- 



-trii.-il.iii iif It.H.f Strutting 

 Qufcknand Method. 



