OPEN-CUT TfNNKLINC MKTIlnDS 



181 



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Pio. 106. Diagram Showing 8o> 

 quence of Conatruction in Open- 

 Cut Tunnelti. 



construction of the lining in a single piece from the bottom up, 

 thus enabling better workmanship and stronger construction 

 than when the separate parts are built at different times. The 

 great objection to the method when B 



it is used for building subways 

 under city streets is, that it occupies 

 so much room that the street usually 

 has to be closed to regular traffic. 

 For this reason the single longi- 

 tudinal trench method is seldom 

 employed, except in those portions 

 of city subways which pass under 

 public squares or parks where room 

 is plenty. 



Parallel Longitudinal Trenches. 

 The parallel longitudinal trench method of open-cut tunneling 

 consists in excavating two narrow parallel trenches for the side 

 walls, leaving the center core to be removed after the side 

 walls have been built The diagram, Fig. 106, shows the 



sequence of opera- 

 tions in this method. 

 The two trenches No. 

 1 are first excavated 

 a little wider than the 

 side wall masonry, 

 and strutted as shown 

 by Fig. 107. At the 

 bottoms of these 

 trenches a foundation 

 course of concrete is 

 laid, as shown by Fig. 

 108, if the ground is 



soft; or the masonry is started directly on the natural material, 

 if it is rock. From the foundations the walls are carried up t<: 

 the level of the springing lines of the roof arch, if an arch is 



Pio. 107. Hkntrh Showing Method of Timbering Open- 

 Cut Tunnel*, Double Parallel Tn-ii.li M.-th.Ml. 



