Page 337 - Moving the Earth_ The Workbook of Excavation

LANDSCAPING AND AGRICULTURAL GRADING

LANDSCAPING AND AGRICULTURAL GRADING 7.31

The tile lines which do most of the work of draining are called laterals, and the larger pipe into
which they empty is called the baseline. Four- or 5-inch laterals and 6- or 8-inch baselines are
usual. Sizes are ordinarily much smaller in proportion to acreage than in nonirrigated fields, but
layouts are similar.
If the problem is water leaking from an adjoining canal, an intercepting drain should be placed
parallel to the canal, at a distance of 50 to 70 feet. Water may leak under it if it is too close or too
shallow.
It is best practice to lay all drainage tile on gravel, and under tar paper and gravel, as described
in Chap. 5, as the effective life of the system will be many times that of plain tile. Since tiling is
generally done in saturated land, a tile box should be used to avoid danger of cave-ins.

Moles. A mole or mole-ball, Fig. 7.21, is a ripper accessory that will open a drainage tunnel in
a plastic soil. The type illustrated is a torpedo-shaped piece of iron, attached to the heel of the
standard. As it is pulled through the ground, the mole presses the soil outward with great force,
leaving an open tunnel with a firm lining. Seepage from the surface is aided by soil breakage.

FIGURE 7.21 Mole ball. (Courtesy of John
Deere.)

Gradient is determined by surface slope and the route taken across it. It should be between 6
inches and 2 feet to 1,000 feet, and in no case should be steep enough to permit erosion in the tunnel.
A piece of tile should be placed in each outlet to protect it from erosion or stoppage.
This device is effective only in soils which are damp and plastic enough to be molded, and not
soft or loose enough to flow or cave into the passage. An uneven surface or stones in the soil make
it difficult or impossible to run it at an even gradient.
Mole drains may not work at all, may stop up in a couple of weeks, or may give satisfactory
service for years. Occasionally their good effects are accomplished as much or more by the inci-
dental breaking of an impervious hardpan than by the drains themselves. The work is much more
economical than tiling.
Leaching. After a field has been subdrained, alkali can be leached out of it by repeated soaking
with irrigation water. This dissolves the chemicals and removes them through the tile lines.