Simplified Trench Guidelines
Relatively simple installation guidelines may be applicable to many rural transmission and distribution water lines, force main sewer lines, and many process water lines. These lines typically represent pressure pipes installed at relatively shallow depths which are sufficiently stiff to resist the minimal earth load, especially small diameter lines. In some cases, a pipeline may contain sections that require specific engineering analysis such as a shallow (less than 3 ft deep) section that crosses a road. In general, PE pipe applications that fall within the AWWA "Design Window" (see Open Cut Design information) do not require a specific installation specification.
The following simplified guidelines apply under the following conditions:
- Pipe Diameter ≤ 24-inch
- DR ≤ 21
- Depth of Cover between 2.5 feet and 25 feet, except where a vehicle traffic is present, the minimum cover is 3.0 ft.
- Groundwater elevation at least 2 feet below the surface
- Stable soil
- Trenching. Trench collapses can occur in any soil and account for a large number of worker deaths each year. In unbraced or unsupported excavations, the trench wall must be sloped to a safe angle. All trench shoring and bracing must be kept above the pipe. The length of open trench required for fused pipe sections should be such that bending and lowering the pipe into the ditch does not exceed the manufacturer's minimum recommended bend radius and result in kinking. The trench width at pipe grade should be equal to the pipe outer diameter plus 12 inches.
- Dewatering. For safe and proper construction the groundwater level in the trench should be kept below the pipe invert. This can be accomplished by deep wells, well points or sump pumps placed in the trench.
- Bedding. Where the trench bottom soil can be cut and graded without difficulty, pressure pipe may be installed directly on the prepared trench bottom. For pressure pipe, the trench bottom may undulate, but must support the pipe smoothly and be free of ridges, hollows, and lumps. In gravity flow situations, the trench bottom may be padded with 4 to 6 inches of tamped bedding material, as described below.
- Placing pipe in trench. PE pressure pipe up to approximately 8-in. diameter and 6 lbs per ft can usually be placed in the trench manually. Heavier, larger diameter pipe will require handling equipment to lift, move, and carefully lower the pipe into the trench.
- Pipe embedment. The embedment material should be a coarse grained soil, such as gravel or sand, or a coarse grained soil containing fines, such as a silty sand or clayey sand. The particle size should not exceed one-half inch for 2 to 4-inch pipe, three-quarter inch for 6 to 8-inch pipe and one inch for all other sizes. The embedment should be placed in layers, or lifts, not exceeding 6 inches in thickness, followed by mechanical tamping to a level of at least 85% percent Standard Proctor Density, with a level of 95% under streets and roads (see ASTM International D698, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort).
- Leak testing. If a leak test is required prior to trench backfill, it should be conducted after the embedment material is placed. (Information on pressure testing of installed PE pipe systems is provided in Chapter 2 of the Plastic Pipe Institute (PPI) Handbook of Polyethylene Pipe.
- Trench backfill. The final backfill may consist of the excavated material, provided it is free from unsuitable matter such as large lumps of clay, organic material, boulders or stones larger than 8 inches, or construction debris. Where the pipe is located beneath a road, the final backfill should be placed in lifts, not exceeding 6 inches in thickness, and compacted to 95% Standard Proctor Density.
Additional design and construction details are provided in Chapter 7 of the Plastic Pipe Institute (PPI) Handbook of Polyethylene Pipe.