PART D - COVER, BEDDING, AND FILL FOR CMP
The function of the cover (figures 5-1, page 5-2 and figure 5-6) over culverts is to prevent crushing of the pipe by
distributing the live loads over a larger area. The depth of the cover required is a function of the live loads and the
culvert diameter and material. CMP will be the only type of culvert discussed in this lesson.
Cover requirements for CMP can be divided into two categories. The minimum amount of cover required to
protect the culvert against crushing by vehicular or aircraft loads is--
For vehicles, one-half the diameter of the culvert or 12 inches minimum, whichever is greater (figure
For aircraft, the depth is variable and must be specifically designed for each type of aircraft and culvert
diameter and gauge. Table 5-1 gives load groupings for the type of aircraft that will use the airfield.
Table 5-1. Characteristics of US aircraft
There is a maximum depth of cover for a particular pipe diameter and gauge. This requirement is necessary, since
the weight of a large soil mass could crush or distort the culvert.