10 January 2002
h. Vents and drains. Piping vents and drains will be located at all high and low points, respectively
\ (see figure 4-19) /1/. Piping drains will be provided in valve manholes only where access can be achieved
and where system drainage is provided. Piping vents may be located anywhere in the trench piping system
with an access cover provided in the trench top, as detailed in figure 4-18 or with an aluminum access door
shown in figure 3-19.
4-3. GENERAL CONSIDERATIONS. The designer will address these key areas to ensure a satisfactory
shallow trench design.
a. The grading design will ensure ground water will not pond or sit over the trench. The trench will
not be routed through existing flood plains, swales, or in areas where seasonal water accumulates. Another
distribution system, preferably an Aboveground Heat Distribution System (chap 7), should be used in these
areas. In areas where seasonal ground water will cause a trench flotation problem, the design will include a
subdrainage system along the trench if thickening of system walls and floor slabs to offset the buoyancy
effect is not practical.
b. The trench floors will slope a minimum of 1 inch in 20 feet toward valve manholes and the piping
will parallel the trench floor. This will allow the trench to drain off all water that may enter. Drainage will
then be provided from the valve manholes as described in chapter 3 of this manual.
c. Valve manholes should be spaced to minimize the depth of the trench system. Additional
manholes may be less expensive than excessive trench depths to accommodate the slope while still
keeping the trench tops at grade.