1 62 RAILWAY CONSTRUCTION. 



filling and through the wall to carry away all ordinary or flood 

 water coming down from the cliffs and hills above. Where a 

 retaining wall is built along the margin of a river, the lower 

 portion, which will be in contact with the water when the river 

 is full, should be constructed of selected large heavy stones to 

 withstand the scouring action of the water, and any brushwood 

 or floating timber which may be brought down by flood water. 



Where retaining walls are built to support wet clay, or in 

 embanked places on wet side-lying ground, the efficiency of the 

 work will be much increased by constructing a layer, two or 

 three feet in thickness, of dry, flat, bedded stones carefully hand- 

 laid, from the foundation to the top of the wall, as shown in 

 Fig. 199. 



These dry stones form a continuous vertical drain to take 

 away water from any part of the earthwork down to the outlets 

 left in the lower portion of the wall. 



The building of retaining walls entirely of dry stone is very 

 questionable economy, and entails a constant expenditure in 

 maintenance and renewal. The working out of one stone loosens 

 the surrounding portion of the wall, and if not at once repaired, 

 a length of the wall will fall down, bringing with it a large 

 quantity of the earthwork. 



If readily obtained, large heavy stones should be selected for 

 the coping of retaining walls, so as to minimize as much as 

 possible the chance of their disturbance or displacement. Where 

 lighter stones have to be used, or bricks laid on edge, they should 

 be bedded and pointed in cement. 



In many places it is necessary to form wide and massive 

 foundations of concrete on which to build the retaining wall ; 

 and in some cases of soft, treacherous ground, timber piling may 

 be necessary. 



Tunnels. It would be difficult to assign a date to the first 

 examples of subterranean works constructed for utilitarian 

 purposes. Nature had furnished so many grand specimens of 

 caves, grottoes, and underground passages formed in the solid 

 rock, that man soon grasped the principle, and essayed to carry 

 out similar works on his own account. The early attempts 

 would probably be limited to forming places of shelter, storage 

 or security. Advantage would be taken of those rocks which 

 from their locality, accessibility, and compactness of material, 

 promised favourable results. The appliances being few and 



