CHOH'K 11KTNVKKN A Tl NNKL A M AN PKM (1 I 7 



whose strata are inclined at -".0 t tin- horizon, for instance, 

 the tuniH'l will cut tliese strata at an angle of 30. By tho 

 excavation tlie natural equilibrium of the soil is disturbed, 

 and while the earth tends to fall and settle on both sides 

 at an angle depending upon the friction and cohesion of tin? 

 material, this angle will l>e much greater on one side than mi 

 the other Ix'cause of the inclination of the strata; and hem c 

 the prism of falling earth on one side is greater than on tin? 

 other, and consequently the pressures are different, or in 

 other words, they are unsymmetrieal. These unsyimnetriea! 

 pressures are usually easily taken care of as far as the lining 

 is concerned, but they may cause serious cave-ins and Uidly 

 distort the strutting. Caving-in during excavation may be 

 prevented by cutting the materials according to their natural 

 slope; but the distortion of the strutting is a more serious 

 problem to handle, and one which oftentimes requires tint 

 utmost vigilance and care to prevent serious trouble. 



Presence of Water. An idea of the likelihood of finding- 

 water in the tunnel may be obtained by studying the hydro- 

 graphic basin of the locality. From it the source and direction 

 of the springs, creeks, ravines, etc., can be traced, and from 

 the geological map it can be seen where the strata In-aiing 

 these waters meet the center line. Not only ought the surface 

 water to be attentively studied, but underground springs, which 

 are frequently encountered in the excavation of tunnels, re- 

 quire careful attention. Both the surface and underground 

 waters follow the pervious strata, and are diverted by im- 

 pervious strata. Roeks generally may be classed as im- 

 pervious; but they contain crevices and faults, \\hich often 

 allow water to pass through them; and it is, therefore, not 

 uncommon to encounter large quantities of water in excavating 

 tunnels through rock. As a riile, water will IK; found under 

 high mountains, which comes from the melted ice and snow 

 '.ating through the rock crevices. 



Some detrital soils, like gravel and sand, are pervious, and 



