38 



corners where the floor and sides come together. The concrete work follows 

 shortly after the earth forms are taken off when the banks are still moist. 

 The concrete forms are placed in position in the finished earth ditch, but 

 instead of placing them continuously as the earth forms, only every alter- 

 nate form is put in place (Pig. 30) ; then the concrete, which is mixed wet, 

 is placed between the form and the earth and well- stirred or cut with thin 

 bars. To protect the earth slope when pouring the concrete mixture, it is 

 well to cover the earth slope with thin galvanized iron sheets, which are 

 pulled up as the concrete is poured in. The sides and bottom are put in 

 at the same time. This gives a good connection at the corners which is very 

 desirable. To do this it is necessary to block the forms above the ground 

 by 3 inches (the thickness of the lining). To hold the concrete at the 

 ends of the sides and also to hold the form the right distance away from the 

 earth side, 2 inch by 3 inch pieces are placed edgewise between the earth 

 slope and the wooden forms. When the sections have hardened the forms 

 are removed and moved ahead to the adjacent section. In order that the 

 ends of the form will rest on the two adjacent completed sections the forms 

 should bea little longer than 6 feet (the length of a section), preferably 6 

 feet 6 inches. After the removal of the forms the concrete must be pre- 

 vented from drying out too quickly, this may be done by protecting it with 

 burlap kept wet by sprinkling or by letting water in the completed section 

 as soon as possible. It is preferable to keep the concrete moist for several 

 days after the removal of the forms. 



The proper handling of the forms especially on rough side hill work 

 will materially affect the cost. When the lining is started from the upper 

 end of a canal and the work progresses downstream, probably the most 

 economical manner is to place the forms in position for a length of canal 

 which can be lined in one day and begin the concrete work at the down- 

 stream end and extend it upstream. The concrete at the downstream end 

 hardens first and this allows the removal of the downstream forms which 

 are carried downstream in the ditch and placed in position at a distance 

 from their previous position equal to the length of canal lined in one set- 

 ting of the forms. This procedure allows continuous work and does away 

 with the necessity for carrying the forms around the side hill. 

 Joints. 



The lining is done in strips in order that all contraction cracks will occur 

 at the joints which are places of weakness. To separate the sections more 

 distinctly the edges of the sections may be painted with oil or a strip of 

 tarred paper may be used. By using short sections the contraction cracks 

 are very small and the seepage through them is negligib.e. It is probable 

 that the. cracks in most cases will silt up. 

 Expansion joints. 



Ordinarily expansion joints are not necessary but there are some classes 

 of soil in which any seepage through cracks will cause the settlement of the 

 soil and destroy the lining, in which case it may be desirable to do away 

 with all open joints to prevent seepage by using some form of expansion 

 joint. This, however, is quite uncommon but occasionally occurs where the 

 soil has probably been formed from the coarser material carried by the 

 flood waters of a heavy cloudburst and deposited in small fan shaped val- 

 leys or benches. The lack of rainfall and of the occurrence of further 

 cloudburst, has left the soil in an unsettled condition and any seepage 

 water passing through the concrete lining may carry off the finer soil par- 

 ticles into the subsoil below and cause a settlement. 



