BACKFILL (1995)
a. Proper backfill material selection, placement and compaction is essential to the satisfactory performance of
pipe culverts. Railroad and highway engineering departments should have established criteria for
construction of fill embankments. While such fill requirements may be adequate for pipe culvert
installations in general, specific requirements may vary depending on pipe culvert material, size and shape;
fill heights and in situ conditions.
b. Generally, the selected culvert backfill material should be a well graded granular material, although
cohesive materials can be used if careful attention is given to compaction and optimum moisture content.
Use of excavated, on site material may be possible provided this material possesses the required engineering
properties. The selected fill material must be free from large clods, frozen lumps, rocks, debris and organic
material. Extremely fine granular fill may not be suitable due to the possibility of infiltration and piping
action. However, use of a filter fabric separator between fill materials of dissimilar size may solve this
problem.
c. Special care must be taken to provide proper bedding and haunch support for the pipe. See Figure 1-4-22. A
relatively loose layer of bedding (generally several inches thick) should be placed under the pipe. However,
the fill in the haunch areas is to be carefully compacted to provide support in these critical areas. This is
especially important for pipe-arch shapes.
d. The selected backfill material should be placed in layers not exceeding 6 inches to 8 inches thickness and
compacted to specified density at or near optimum moisture content. Care must be taken to maintain
balanced loading on the pipe culvert by keeping the fill level approximately equal on both sides of the pipe at
all times. Generally, a compaction density conforming to 90% of ASTM D698 (Standard Proctor) should be
adequate. However, individual project specifications will control.