DRAINAGE OF IRRIGATED SHALE LAND. 21 
vertical lumber sheeting. The boards should fit tightly; they may 
be driven with a heavy maul and removed with a light derrick or 
grabhook. Where the sheeting is driven and pulled by hand, planed 
2 by 6 inch planks have been found the most satisfactory. Sizes 
larger than this are driven and pulled with difficulty. The bottom 
end of each of these planks should have a long bevel on one side, so 
that it will drive readily and straight. The tops of the planks should 
be beveled slightly, or a cap used to prevent “brooming” while 
being driven, and the planks should be long enough to extend below 
the grade line. As the material is excavated, two more heavy planks 
should be placed near the bottom and held apart by trench jacks 
to prevent the weight of the material from displacing the bottom of 
the sheeting. These can be used also for the workmen to stand on 
when the bottom of the trench becomes too soft. In the latter event 
it becomes necessary to place boards under the tile in order to hold 
them on the grade and prevent them from sinking. When large-sized 
tile are used a cradle must be placed under them. This cradle re- 
sembles a ladder, the strips running lengthwise being of 2-inch mate- 
rial and spaced so that the sides of the tile will rest against them 
as well as-on the crosspieces of broad 1-inch lumber. The tile should 
be covered by some material such as cinders, gravel, or broken shale. 
Hay and straw have been used with success. The tile should be well 
blinded and weighted down before removing the sheeting, which 
should be pulled slowly and carefully to prevent the soft material 
that sloughs in from the sides from pushing the tile off grade. 
Trenching machines especially built with shields for soft material 
would handle some of these soils satisfactorily. 
The excavation of ditches should begin at their outlets or at their 
junctions with other drains and proceed toward the upper end. 
Trenching should be done as neatly as possible and should follow 
closely the line of stakes; where the drain changes direction the turn 
should be made by a neat curve. If possible, the top soil should be 
thrown out on one side of the ditch and the shale on the other, and 
in back-filling the shale should be put in first. 
No attempt should be made to grade the tile by the water in the 
ditch. Grades for the drains always should be established by sur- 
veys, and the ditch should be dug accurately to the depths specified ; 
these depths should be measured from the grade stakes set for that 
purpose, and the ditch graded evenly on the bottom by means of the 
“line and gage” method or by any other equally accurate device for 
obtaining an even and true bottom upon which to lay the tile. 
The tile should be laid as close as possible, beginning at the lower 
end and proceeding upstream. They should be turned about until 
their upper edges close. If there is silt or other fine material that is 
likely to run into the tile, the lower edges must be laid close and the 
