DEAII^rAGE OF lERIGATED LAND. 25 



The spoil should be placed well away frOm the channel and may be 

 deposited on either or both sides. Openings in the spoil banks should 

 be left wherever lateral or waste ditches are to enter. The contractor 

 should be required to give the canal as true a form as possible, as 

 irregularities are likely to become more pronounced. It is well to 

 excavate a little below grade to allow for silting and spaUing. If it 

 is found impossible to give the banks the proper slope, owing to the 

 bad condition of the soil, it is well to excavate in terraces and leave 

 the final form to the action of the elements. As a rule it is not advis- 

 able to attempt to cut out a canal by means of water, but this has 

 sometimes proved effective where the fall was sufficient. 



Little clearing for right of way is required in the irrigated section 

 and Uttle rock is encountered. Rock and frost should be broken by 

 means of dynamite before an attempt is made to use a bucket. 



COVERED DRAINS. 



In installing covered drains either hand labor or trenching ma- 

 chinery may be used. On small projects hand trenching is frequently 

 cheaper, but on larger projects the machine generally can do the work 

 more rapidly and economicall}^. In either case methods and devices 

 adapted to the nature of the soil and to other local conditions must 

 be employed. 



If hand labor is used it is necessary to operate with small gangs, 

 never more than a half dozen men to the line, as the trench must be 

 opened from top to bottom as rapidly as possible and the tile laid and 

 blinded before caving takes place. The men must work as closely 

 together as is practicable; and it is generally advisable to do rapid, 

 systematic work for a short time or until a given length of drain is 

 completed, and then to rest for a few minutes and bo prepared for 

 another vigorous attack. Each man should remove a spading and 

 move backward. The man removing the last spading should also 

 grade the trench bottom. He should not step on the finished bottom, 

 and no one should stand near the edge of the trench. The tile should 

 be laid at once and should bo blinded by means of a few inches of 

 earth caved from the edges of the trench. If the banks tend to cave 

 off in larg.'^ fhuriks or slabs it will be necessary to brace them apart 

 with planks separated by stout crosspieces or by trench jacks. 



A very troublesome condition is that in which the presence of a 

 wet, pervious stratum near the bottom of the trench causes a lateral 

 and upward movement of the soil in the bottom of the trench. In 

 such a case it is necessary to provide a tight cribbing to shut out the 

 oozing material. A d(!sign for such a cribbing is shown in figure 20. 

 It consists of two heavy timbers, luild apart by means of trench jacks, 

 behind which is driv<!n lumber sheiitirig properly matcluul and bev- 

 eled at the lower ends to irisure a tiglit fit. The sheeting juay bo 



