128 RAILWAY CONSTRUCTION. 



an ordinary pile shoe (Fig. 125), but with a cutting edge slightly 

 inclined, so that in driving the tendency of the pile is to drift 

 towards the pile previously driven. Sometimes the outer row 

 of piles consists of whole balks, and the inner row of half balks ; 

 the size of the piles must, however, be regulated by the depth 

 and current of the water. When both rows of piles have been 

 completed, the space between should be dredged out, and then 

 filled with carefully prepared clay puddle. To enable the puddle 

 to adapt itself thoroughly to the wooden sides, it is desirable to 

 remove the inside walings after all the piles are driven, as any 

 internal projections interfere with the proper punning and 

 settling of the puddle. The swelling of the puddled clay has a 

 tendency to force apart the two rows of piles, and to counteract 

 this as much as possible, iron tie-rods should be passed through 

 from side to side every few feet, and screwed up against large 

 washers placed on the outside of the outer walings. Strong 

 struts or cross-bracing of timber must be placed from side to 

 side inside the coffer-dam to resist the pressure of the water 

 in the river. This cross-bracing can be removed gradually 

 as the work of building progresses upwards, and be replaced 

 with short struts wedged in against the sides of the finished 

 courses. 



In cases where the ground is soft, and when it is not con- 

 sidered prudent to excavate the foundations deeper for fear 

 of disturbing the stability of the coffer-dam piles, rows of large, 

 square bearing-piles may be driven in the floor of the foundation, 

 as shown in Fig. 111. The tops of these bearing-piles must all 

 be sawn off to the same level, and a platform of strong double 

 planking securely fixed to the piles to receive the foundation 

 course of concrete, masonry, or brickwork. The spaces around 

 the tops of the piles and the under side of the timber platform 

 should be filled in with good cement concrete. 



The interior of the coffer-dam is kept dry by constant 

 pumping, either by hand pumps or steam pumps, according to 

 the volume of water finding its way into the foundations. 

 When the finished pier or abutment has been carried up above 

 the river water-level, the coffer-dam is no longer required, and 

 may be removed. Sometimes, to save the timber, the piles are" 

 drawn by means of strong tackle fitted up for the purpose ; but 

 in doing this there is considerable risk of disturbance to the 

 foundations, and it is better to leave the piles in the ground 



