SECTION P-306  TRENCHING, EXCAVATION AND BACKFILL
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P-306.2 Trenching and bedding.
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P-306.2.3 Soft load-bearing materials. If soft materials of poor load-bearing quality are found at the bottom of the trench, stabilization shall be achieved by overexcavating not less than two pipe diameters and backfilling to the installation level of the bottom of the pipe with fine gravel, crushed stone or a concrete foundation. The concrete foundation shall be bedded with sand tamped into place so as to provide uniform load-bearing support for the pipe between joints. When drainage is installed in filled or unstable ground, pipe shall be supported on concrete piers, with spacing consistent with Table 308.5, or a constant bed of concrete or clevis hangers attached to reinforcing rods in the concrete floor above the piping.
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P-306.4 Tunneling. Where pipe is to be installed by tunneling, jacking or a combination of both, the pipe shall be protected from damage during installation and from subsequent uneven loading. Where earth tunnels are used, adequate supporting structures shall be provided to prevent future settling or caving. The length of tunneling shall be limited to only that required to clear the obstacle above.
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