Figure 2.8. Cross Drain Installation With Pipe Drain in Subgrade.

2.15.

SUPPLEMENTARY DRAINAGE

A cross drain is mentioned in the discussion of the classic solution to the water-pocket

problem in paragraph 2.12. Actually, a cross, or lateral, drain is only one type of supplementary

drainage, that is, the drainage needed to dry up water pockets and to divert water before it

reaches and endangers the track. Other remedies used to solve the drainage problem are

intercepting ditches, cinder trenches, grouting, and French drains.

a. Cross or lateral drain. The simplest form of cross drain is the removal of ballast

from several cribs, that is, from the spaces between several ties. Sometimes pipes are driven at a

slant into the roadbed from the side, as shown in figure 2.8. Cross drains tap any water tending

to form water pockets and lead it to the main side ditch or to a pipe drain. Lateral pipe drains

should be about 4 inches in diameter. If the drainage problem is severe, it may be necessary to

locate cross drains every one-half rail length, that is, at about 20-foot intervals.

Figure 2.8. Cross Drain Installation With Pipe Drain in Subgrade.

A procedure often used with pipe drains is to have them run on top of the

subgrade directly below the ballast and drain into the side ditch, as shown in figure 2.9. This

arrangement aids the flow of water across the subgrade, preventing it from seeping into the

subsoil, if the soil is spongy and soft rather than solid and impenetrable. In addition, these

drains lead off spring water if springs occur in the roadbed, as they frequently do in wet cuts.

Perforated pipes 2 to 3 inches in diameter are used. Although vitrified pipe is specified for

drainage, corrugated metal pipe is frequently used, especially for short-term military use. It

should be perforated to allow water to seep through, and galvanized or asphalt-coated to