SPECIAL TREACHEROUS GROUND METHOD 173 



Advantages and Disadvantages. The great advantage claimed 

 for the Italian method of tunneling is that it is built in two 

 separate parts, each of which is separately excavated, strutted, 

 and lined, and thus can be employed successfully in very 

 treacherous soils. Its chief disadvantage is its excessive cost, 

 which limits its use to tunnels through treacherous soils where 

 other methods of timbering cannot be used. 



QUICKSAND TUNNELING. 



When an underground stream of water passes with force 

 through a bed of sand it produces the phenomenon known as 

 quicksand. This phenomenon is due to the fineness of the 

 particles of sand and to the force of the water, and its activity- 

 is directly proportional to them. When sand is confined it 

 furnishes a good foundation bed, since it is practically incom- 

 pressible. To work successfully in quicksand, therefore, it is 

 necessary to drain it and to confine the particles of sand so 

 that they cannot flow away with the water. This observation 

 suggests the mode of procedure adopted in excavating tunnels 

 through quicksand, which is to drain the tunnel section by 

 opening a gallery at its bottom to collect and carry away the 

 water, and to prevent the movement or flowing of the sand by 

 strutting the sides of the excavation with a tight planking. 



The sand having to be drained and confined as described, the 

 ordinary methods of soft-ground tunneling must l>e employed, 

 with the following modifications : 



(1) The first work to be performed is to open a bottom 

 gallery to drain the tunnel. This gallery should be lined with 

 boards laid close and braced sufficiently by interior frames to 

 prevent distortion of the lining. The interstices or seams be- 

 tween the lining boards should !> packed with straw so as to 

 permit the percolation of water and yet prevent the movement 

 of the sand. 



(2) As fast as the excavation progresses its walls should 



