I9I0.] DAVIES— TUNNEL CONSTRUCTION. 183 



position outside the lining. The penetrating power of grout under 

 high pressure is remarkable, and it will follow the easiest channel. 

 It is not desirable to use grout where the exterior soil is silt, as the 

 pressure of water softens the silt into mud, preventing the setting of 

 cement and demoralizing the silt in proximity to the lining. Fur- 

 ther, it is not required in that character of soil to fill the voids, as 

 clay will flow sufficiently to fill all voids itself. In sandy soil, either 

 with metal or concrete lining, grout is always used, and usually in 

 the case of tunnel built in rock. Along the streets of New York 

 we have repeatedly traced dead connections with sewers by finding 

 grout follow open pipes into houses and fill the cellars. Lifting the 

 tracks of railroads or lifting asphalt pavements of streets sixty feet 

 overhead of the tunnel, have not by any means been unknown occur- 

 rences, and the bodily raising of a standing building has been done 

 on one occasion. By filling all voids promptly as the shield 

 progresses, when tunnelling under land, the settlement of soil can 

 be reduced to an extremely small amount, so much so that short 

 lengths of tunnel have been driven under occupied dwellings without 

 the tenants being aware of the fact. 



The subject would hardly be complete without a brief reference 

 to the construction of portions of the tunnel work by caisson 

 methods. The general vmderlying principles of caisson construction 

 are similar to those of shield tunnel work — progress being made 

 vertically instead of horizontally — but for a great many years the 

 use of caissons has been almost exclusively for the construction of 

 piers for bridges, or where solid support is necessary for building 

 foundations. 



In locating the tunnels for the Hudson and Manhattan system, 

 where the north and south line connecting the steam railway ter- 

 minals on the New Jersey side intersect the up-town pair of river 

 tubes, a complicated problem existed as to how to connect these 

 lines, so as to enable the trains crossing the river to run southerly 

 to the Pennsylvania and Erie Railroad terminals, northerly to the 

 Lackawanna terminal, and from the latter point to the down-town 

 river tubes. To enable these train movements to be executed 

 necessitated what in railway parlance is known as a " Y " junction. 

 In case the tracks for trains moving in opposite directions had been 



