182 DAVIES— TUNNEL CONSTRUCTION. [February i8, 



two blow-outs occurred, in neither of which cases was the tunnel 

 lost, and each of these was due to striking fissures in the rock 

 formation close in shore. It is not unusual to provide against the 

 chances of " blow outs " by artificially increasing the cover over a 

 tunnel by dumping good stiff clay as a thick blanket over the tube 

 location before the work is executed, and ultimately dredging and 

 removing this clay. This method was carried out, to a very large 

 extent, in the East River tunnels of the Pennsylvania Railroad. 



One important item of work in connection with subaqueous 

 tunnelling is that of '' grouting." The purpose of this is two fold: 

 (i) To fill the small annular space due to the shield's being slightly 

 larger than the tunnel lining, as well as any other voids or cavities 

 in the rear of the working face, and render the exterior soil solid, 

 thereby giving more perfect support to the lining; (2) to water- 

 proof and protect the material of the lining itself. Grouting con- 

 sists in mixing hydraulic cement with an excessive quantity of 

 water, so as to make it almost liquid, and then pumping it, either 

 with a force pump or grout machine, through holes provided in the 

 lining, into any spaces or interstices in the surrounding material. 

 Cement mixed neat or in a very rich mortar with sand, and partic- 

 ularly if it has originally been mixed with an excess of water, when 

 set up and thoroughly crystallized, makes an exceedingly impervious 

 material. In the iron lining plates, there are usually provided one 

 hole to each segment of plate, screw-tapped to permit the insertion 

 of a piece of iron pipe having a hose attachment connected to the 

 grout machine. This machine was invented by Greathead, and 

 consists of a small tank in which blades or paddles are rapidly 

 revolved on a shaft by an air engine or motor. To the tank is fitted 

 an extension box with a flap door opening inwards, through which 

 the change of cement (usually one barrel at a time) with the 

 water needed to make slurry of it, can be admitted. The only other 

 attachments are a large pipe for admission of air at high pressure, 

 and a discharge pipe fitted wdth a cock. The operation is simple. 

 Having put into the tank the cement and water, the paddles are run 

 until they are thoroughly mixed, the flap door closed, and air ad- 

 mitted at high pressure into the tank. As soon as the discharge 

 valve is opened, the mixed grout is forced through the hose to its 



