ENCLOSING DAMS. 37 



through soft mud with greater facility, but under any circum- 

 stances it is desirable and wisest to dredge a trench down to the 

 more solid bed before the material to form the bank is deposited. 

 When chalk or rubble stone is used to form the embankment, 

 the outer or water face must be sealed with an impervious 

 material, such as clay or stiff mud. 



Where the head of water is considerable and the working 

 space limited, as is generally the case in dock construction, 

 the use of timber piles, either in single or double rows with 

 clay puddle between, or iron caissons, are resorted to. Dams 

 so constructed are less liable to failure than earthwork 

 dams. 



The leakage, and danger of breaches, either in the dam itself 

 or through faulty places in the bottom, increases greatly as the 

 head of water increases. 



In all dams of whatever construction ample means should 

 be provided by suitable valves, or, where the area is large, 

 by sluices, to ensure the free escape of the enclosed water at as 

 nearly as possible the same rate as the falling tide. Such 

 valves and sluices are best placed at extreme low water, but in 

 no case should the sill of the valves or sluice be placed more 

 than two feet above the lowest water level. 



The practice of driving whole-timber gauge-piles and filling 

 in with half-timber sheeting is not always judicious. Especially 

 in deep dams, by using half timbers, inefficient work is frequently 

 the result ; there is greater difficulty in keeping the dam tight, 

 and consequently more water has to be contended with; by 

 the use of whole timbers, as a rule less timber is destroyed and 

 the dam more easily made. 



After dam piles have passed the lower walings and entered 

 the ground, they are more or less beyond control ; any slight 

 obstruction may give them an inward or outward cant or 

 tendency, so that at the foot they may be very irregular on 

 plan, and, if driven very deep into the ground, may separate 

 entirely. In the case, therefore, of all long piles, they should 

 be tongued and grooved either by working the solid timber, 

 or by spiking 3-inch fillets to the sides of the piles. Another 

 method sometimes adopted is a continuous flat iron tongue, 

 about 3 inches by J, let into grooves cut in the centre of the 

 piles ; the wooden tongue and groove is, however, to be preferred 

 as being stiffer and less liable to displacement in driving. This 



