11 



possible lifting power. The "propagation" method may 

 be used where the soil is very wet. By this method one 

 cap is used to set off the first stick and the others 

 are exploded by the shock of the first charge. If the 

 soil is not wet, a cap should be used for each charge. 

 The time needed to construct a ditch in this way is 

 less than that required by other methods. 



By the time the previously ' described work is 

 completed, the. soil types available should be known 

 with some degree of accuracy. Only first-class clay 

 or clay soils should be used for filling the trench 

 which has been excavated. This cut-off wall or core 

 should be built up several feet through the center of 

 the dam. The better, more impervious soils that are 

 available for construction should be used directly under 

 the center of the dam and on the upstream side. Less 

 desirable or lower-qualit,y earths, such as sand, stone, 

 gravel, and so forth, may be used in the fill, but they 

 should be incorporated only on the downstream side of 

 the highest point in the dam and extending to the toe; 

 so placed, these poorer earths provide weight to prevent 

 s-lipping and are at the maximum distance from the 

 saturation point of the water. 



In building small dams or where there is first- 

 class clay soil, it is unnecessary to construct a core 

 through the dam. The same result may be effected, after 

 the whole area of the base is plowed, by rem.oving the 

 topsoil down to solid earth on the upstream half of the 

 dam and properly placing it on the lower half. The 

 solid earth should then be plowed so it will bind with 

 new earth moved in. Thereafter, the builder should 

 follow the same construction procedure recommended for 

 a dam in which a core is made. 



After the base has been properly prepared, the 

 drain line should be laid. A shallow excavation to 

 solid earth should be dug from the proposed pond outlet 

 to a point below the dam. Earth, the best clay avail- 

 able, should be firmly packed around the pipe, particu- 

 larly on the under side. Addition of water to the 

 soil to form a stiff mud will facilitate a firmer pack 

 in the tamping process. At some central point in the 

 dam, a concrete collar should be poured around the pipe. 

 This prevents water from following the pipe and devel- 

 oping leaks; it also prevents burrowing animals, such 

 as crayfish, from following the pipe. After the pipe- 

 line has been set up, construction of the dam may go 

 forward. 



