RAILWAY CONSTRUCTION. 165 



strong timbering may be used, and be placed closer together, the 

 logs and planks are frequently bulged out and broken by the 

 action of the clay. Specially strong supports are required for this 

 description of clay, and extra thickness of material in the perma- 

 nent work of side walls and arching. 



Solid unbroken beds of chalk are not difficult to cut through : 

 the material is easy to work, and the excavation will stand with 

 ordinary timbering; but where the chalk is broken and inter- 

 sected with deep pockets of gravel and sand, the operations are 

 very much impeded. The loose material, once set free by cutting 

 through the confining barrier of chalk, will quickly fall into and 

 fill up the excavation if not held back by strong timbering. 

 Side walls and arching are generally necessary for tunnels 

 through chalk. 



Soft wet clay, quicksands, or other strata having springs of 

 water percolating through them, are serious obstacles in the way 

 of expeditious tunnelling. No sooner is one cube yard of this 

 soft material removed than another slides down, or is washed 

 down, to take its place. When once the excavation taps the 

 water-bearing strata, large volumes of water will find their way 

 into the workings, and must be conveyed away to the mouth of 

 the tunnel, or pumped up through the nearest shaft. The timber- 

 ing of the sides and roof through this description of working is 

 very tedious, and attended also with a considerable amount of 

 risk. The absence of really solid ground on which to place or 

 shore up the supports, taxes the skill of the excavators, and very 

 often, when a short length has been made apparently secure, it 

 will come down with a run, compelling all hands to beat a hasty 

 retreat. The permanent lining through such treacherous material 

 should follow the excavation very closely, and special care should 

 be exercised in building the walls, arching and invert. 



In the excavation through stratified rocks it is necessary to 

 note carefully the lie of the strata, whether horizontal, vertical, 

 or shelving, as with each one the excavators are exposed to risks, 

 against which every precaution should be taken. A large hori- 

 zontal slab of solid-looking rock will suddenly break and fall 

 down without any warning. A heavy mass from a vertical layer, 

 perhaps unkeyed, or loosened, by an adjacent blasting operation, 

 drops down when least expected ; and pieces from the high side 

 of the shelving layers detach themselves and slide into the work- 

 ing in a most unaccountable manner. 



