6 TUNNELING 



blue, and are very often found alternating with strata of 

 stiatites or calcareous material. For purposes of construction 

 they have been divided into three varieties. 



Laminated clays of the first variety are those which alter- 

 nate with calcareous strata and are not so greatly altered as 

 to lose their original stratification. Laminated clays of the 

 second variety are those in which the calcareous strata are 

 broken and reduced to small pieces, but in which the former 

 structure is not completely destroyed ; the clay is not reduced 

 to a humid state. Laminated clays of the third variety 

 are those in which the clay by the force of continued disturb- 

 ance, and in the presence of water, has become plastic. 

 Laminated clays are very treacherous soils ; quicksand and 

 peat may be classed, as regards their treacherous nature, 

 among the laminated clays of the third variety. 



Inclination of Strata Knowing the inclination of the 



strata, or the angle which they make with the horizon, it is 

 easy to determine where they intersect the vertical plane of the 

 tunnel passing through the center line, thus giving to a certain 

 extent a knowledge of the different strata which will be met 

 in the excavation* On the inclination of the strata depend : 

 (1) The cost of the excavation ; the blasting, for instance, will 

 be more efficient if the rocks are attacked perpendicular to the 

 stratification ; (2) The character of the timbering or strut- 

 ting ; the tendency of the rock to fall is greater if the strata 

 are horizontal than if they are vertical ; (3) The character and 

 thickness of the lining; horizontal strata are in the weakest 

 position to resist the vertical pressure from the load above 

 when deprived of the supporting rock below, while vertical 

 strata, when penetrated, act as a sort of arch to support the 

 pressure of the load above. The foregoing remarks apply 

 only to hard or soft rock materials. 



In detrital formations the inclination of the strata is an 

 important consideration, because of the unsymmetrical pres- 

 sures developed. In excavating a tunnel through soft soil 



