ENCLOSING DAMS. 39 



is deposited is always a difficult operation, on account of the 

 tendency of the clay to close round the auger. This difficulty 

 may be met by boring the hole through the inside pile a little 

 larger than the intended bolt, and then driving a sharp-edged 

 tube through the clay until it reaches the outer pile, into which 

 the sharp end of the tube is driven to get a hold ; the tube 

 is then cleared of clay, and the hole bored through the outer pile 

 to the proper size. The tube can be withdrawn after the bolt is 

 placed in position. 



The vibration caused by pumps, particularly those running at 

 a high speed, working on a dam, or on a stage connected with a 

 dam, has frequently an injurious effect. It is therefore advis- 

 able, whenever possible, to work the pumps from a stage or 

 position independent of the dam, or, if this is impossible, to 

 adopt a type of pump working at a low rate of speed, so as to 

 reduce the vibration to a minimum. 



When leakage appears, in consequence of a settlement of the 

 puddle causing a vacuity beneath the bolt, the defect may be 

 remedied by boring a hole through the inner pile, just below 

 the bolt where the leakage appears. Through this hole small 

 cylinders or pellets of tempered clay are forced, by means of a 

 hard- wood plug (made very slightly slack to the hole) and maul, 

 until the vacuity under the through-bolt is filled up. 



A leak may be reached from the top of a clay dam by boring 

 vertically, but in such a case, it will be necessary to drive a tube 

 down through the clay to the leak, so as to keep the hole open. 

 Plugs of prepared clay are then dropped in and rammed down 

 until the leak is stopped. 



Single rows of piles have been successfully adopted in 

 the construction of dams, and are particularly applicable where 

 the working space is limited, and in deep water where there is 

 little or no fall of the tide, conditions which render it difficult to 

 clear out the mud and silt from between two rows of piles, and 

 still more difficult to get in through-bolts. In constructing a 

 dam in this way, very careful work is necessary to ensure the 

 piles being close jointed. The advantages are that much less 

 space is required ; any leaks that do occur are at once seen, 

 and can be stopped without great difficulty or loss of time ; less 

 timber and iron is required ; and the work can be carried on 

 much more expeditiously than is the case with double rows of 

 piles and clay puddle. 



