RAILWAY CONSTRUCTION. 181 



of the sliding cap was filled in with cement grouting by means 

 of an ingenious machine designed for the purpose. 



Large tunnels under rivers or tidal estuaries must each be 

 dealt with according to the particular circumstances of depth 

 below stream-bed, material to be cut through, length of tunnel, 

 and gradient. The chief obstacle to be contended against in so 

 much of the river tunnel-work is the large volume of water 

 which pours into the workings through fissures in rock or seams 

 of gravel and sand, necessitating the constant use of most 

 powerful pumps. In ordinary land tunnels the gradients are 

 generally laid out to fall towards one or both entrances, and any 

 water finding its way into the excavations may be led away to 

 the entrances by drains or pipes. On the other hand, in a river 

 tunnel the gradients generally fall away from the entrances 

 down towards the centre of the river, and all water coming in 

 must be pumped out and raised up to at least the level of the 

 river. In places where the water comes streaming in from 

 many points, any failure or stoppage of the pumps would place 

 the lives of the miners, and the security of the work itself, in 

 great jeopardy. Iron shields, or protection chambers for the 

 miners advancing the excavation, have been used with great 

 success in carrying on work through loose wet strata which 

 appeared to defy all other means of progress. Solid rock, chalk, 

 or compact clay, may present no difficulty so far as they go, but 

 a continued dip in the gradient, or a line of fault, may suddenly 

 change the entire course of operations, and require the imme- 

 diate use of the most powerful pumping machinery and protective 

 appliances. The special features of each case will demand special 

 precautions, and the judgment and inventive powers of the 

 engineer will be severely tested in coping with the difficulties 

 with which he is surrounded. 



