RAILWAY CONSTRUCTION. 127 



effected by suitable dredges or scoops worked from the surface 

 or top of brickwork. Should trees or other obstructive masses 

 be met with embedded in the strata, it will be necessary to 

 employ divers to remove them piecemeal out of the way of 

 the curb. 



When the brick well has been lowered down to the full 

 depth, and is thoroughly bedded in a stratum of strong material, 

 the test weights should be left on for some time to ascertain if 

 there is any further sinking. After all the weights have been 

 removed the bottom of the well can be dredged out clean, and 

 the interior filled in with concrete to such height as may be 

 considered necessary. 



Brick wells must be watched carefully to ensure that they 

 sink down in a perfectly vertical position. Any inclination 

 away from the perpendicular must be corrected at once by means 

 of guys and struts, the same as in sinking iron cylinders. The 

 principal difficulty will be with the first 20 or 25 feet. 



The diameter of the well will depend upon the weight it has 

 to carry, and its height from river bed to under side of girders. 

 The wells may be either circular or polygonal in section, and 

 built singly or in pairs, as shown in sketches (Fig. 121). 



Many piers and abutments of bridges in shallow or moder- 

 ately deep water are built by means of coffer-dams of timber and 

 clay puddle. The coffer-dam forms a water-tight wall round the 

 site of the foundation, from which the water is pumped out, and 

 the excavation carried down to the depth required. In very 

 shallow water it is sometimes sufficient to drive only a single 

 row of piles, and form a bank of good clay puddle on the outside, 

 as shown in Fig. 122. In deep water it is necessary to drive a 

 double row of piles, 3 or 4 or more feet apart, and fill in the 

 space between with clay puddle, as shown in Fig. 123. The 

 piles for coffer-dam work should be carefully selected, of good 

 timber straight, and correctly sawn on the contact faces. Guide- 

 piles are first driven in proper line and position round the 

 intended foundation. To these strong horizontal double waling 

 pieces are securely bolted, one on each side of the guide-pile, one 

 pair near the top, and the other pair as low down as can be 

 "placed. The sheeting piles, which are lowered down between 

 the horizontal waling or guiding pieces, are driven as close to 

 one another as possible, being assisted in doing so by the sheet- 

 pile shoe, shown on Fig. 124, which is made not with a point like 



