Untitled - Future Pipe Industries

Figure 21: Backfilling pipe haunches
Pipes jointed in the trenches should not be left for long periods without backfilling as some joints may open as a result of the
daily expansion and contraction of the pipes due to daily night/day temperature fluctuation. Pipes should be backfilled as
soon as the joint assembly is made in the trench.
The Contractor should note that the compaction of clean and mixed sand is best achieved when the material is at its optimum
moisture content. While the wetting of sand is recommended prior to compaction, trench flooding should be avoided to prevent
pipes from floating.
Following the first two layers where the backfill has been sufficiently placed, compaction should proceed from the sides of
the trench towards the pipe. The pipe zone backfill should proceed in 6” to 12” lifts depending on backfill type (see section
4.1). The pipe zone backfilling and compaction should continue until the backfill reaches at least 6” above the pipe
crown. For pipes larger than 36” in diameter, backfilling the pipe zone should continue to 12” above the pipe crown. The
use of plate vibrators is highly recommended to achieve the required proctor density.
After completion of backfilling in the pipe zone, native material excavated from the trench may be used to complete backfilling
to final grade. No compaction is required in these final backfilling layers except where specified by the Engineer, or in
the case of traffic or other high external loads over the pipe are present, and where settlement of the native backfill is to be
avoided.
Caution: Sand layers of more than 12” cannot be compacted properly and may result in loss or reduced support for the
pipes. The best compaction results are achieved with wet sand near its optimum moisture content. Flooding of the trench must
be avoided as pipe floatation may occur. A minimum of 0.8 pipe diameter of compacted granular backfill above the pipe
crown is normally required to prevent pipes from floating.
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