DRAINAGE OF IRRIGATED 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 spalling. 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 little 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 economically. 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 be 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 be blinded by means of a few inches of 
earth caved from the edges of the trench. If the banks tend to cave 
off in large chunks 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 design for such a cribbing is shown in figure 20. 
It consists of two heavy timbers, held apart by means of trench jacks, 
behind which is driven lumber sheeting properly matched and bev- 
eled at the lower ends to insure a tight fit. The sheeting may be 
