TINNKLIXG - n;> 



the other end rising to land like the water supply tunnels of 

 Chicago, Milwaukee, and Cleveland, the longitudinal profile is 

 commonly level, or else descends from the shore to a level 

 position reaching out under the water. 



The drainag-j of submarine tunnels during construction is 

 one of the most serious problems with which the engineer has 

 to deal in such works. This arises from the fact that, since the 

 entrances of the tunnel are higher than the other parts, all of 

 the seepage water remains in the tunnel unless pumped out, and 

 from the possibility of encountering faults or permeable strata, 

 which reach to the stream bed and give access to water in 

 greater or less quantities. Generally, therefore, the excavation 

 is conducted in such a manner that the inflowing water is led 

 directly to sumps. To drain these sumps pumping stations 

 are necessary at the shore shafts, and they should have ample 

 capacity to handle the ordinary amount of seepage, and enough 

 surplus capacity to meet probable increases in the inflow. For 

 extraordinary emergencies this plant may" have to be greatly 

 enlarged, but it is not usual to provide for these at the outset 

 unless their likelihood is obvious from the start. The character 

 and si/.e of the pumping plants used in constructing a numlx-i- 

 of \\cll-kno\vn tunnels are described in Chapter XII. 



In this hook submarine tunnels will be classified as follows: 

 (1) Tunnels in rock or very compact soils, which are driven by 

 any of the ordinary methods of tunneling similar materials on 

 land; (2) tunnels in loose soils, whifli may lie driven, (") by 

 compressed air, (4) by shields, or (c) by shields and compressed 

 air combined; (8) tunnels on the river bed, which are con- 

 structed, (a) by cofferdams, or (i) by caissons; (4) tunnels 

 partly in firm soil and partly in treacherous soils, which are 

 driven partly by one of the firm-soil methods, and partly by one 

 of the soft-soil methods. To illustrate tunnels of the first class, 

 the River Severn tunnel in Kngland lias been selected; to 

 illustrate those of the second class, the several tunnels disciis-,, ,1 

 in the chapter .n the shield system of tunneling and the Mil- 



