CHOICK HKTWKKN A TfNM.I. A M > A\ <>IT.\ CUT 5 



divide hard rocks into tun sulM-lasses ; vi/., hard rocks un- 

 affected by tii.- atmosphere, and those affected by it. This 

 distinction is chiefly important in tunneling as determining 

 whether or not a lining will l>e required. 



Soft rocks, as the term implies, are those in whieli the force 

 of cohesion is less than in hard rocks, and which in consequence 

 offer less resistance to attacks tending to break down their 

 original structure. They are always affected by the atmosphere. 

 Sandstones, laminated clay shales, mica-schists, and all schisto>e 

 st. mes, chalk and some volcanic rocks, can In- classified in this 

 group. Soft rocks require t . le supported by timbering during 

 excavation, and need to be protected by a strong lining to 

 exclude the air, and to support the vertical pressures, and 

 prevent the fall of fragments. 



Soft soils are composed of detrital materials, having so little 

 cohesion that they may be excavated without the use of 

 explosives. Tunnels excavated through these soils must be 

 strongly timbered during excavation to support the verti- 

 cal pressure and prevent caving : and they also always require 

 a strong lining. Gravel, sand, shale, clay, quicksand, and peat 

 are the soft soils generally encountered in the excavation of 

 tunnels, (iravels and dry sand are the strongest and firmest; 

 shales are very firm, but they possess the great defect of being 

 liable to swell in the presence of w;iter or merely by exposure 

 to the air. to such an extent that they have Wn known to 

 crush the timbering built to support them. Quicksand and 

 peat are proverbially treacherous materials. Clays are s 

 times firm and tenacious, but when laminated and in the 

 piv-fiice of water are amon^ the most treacherous soils. 

 Laminated clays may be described as ordinary clays altered 

 by chemical and mechanical agencies, and several modifications 

 of the same structure are often found in the same locality. 

 They are composed of lamina- of lenticular form separated by 

 smooth surfaces and easily detached from each other. Lami- 

 nated clays generally have a dark color, red, ocher or greenish 



