What precautions should be taken during trench excavation and pipeline layout for steel mesh skeleton PE pipes?

Trench excavation and pipeline layout for steel mesh skeleton PE composite pipes require careful attention to several key aspects to ensure structural integrity and long-term performance.

Trench Excavation

The trench bottom width should generally not be less than the pipe outer diameter plus 300mm. When connections are made underground, the width should be at least the pipe outer diameter plus 500mm, with a minimum total width of 700mm. For double-pipe installations in the same trench with connections made at the trench edge, the distance between the trench wall and the second pipe should be at least 0.3m.

The trench should be excavated according to the designed alignment and elevation. When excavating manually without groundwater, a reserve layer of 0.05–0.10m should be left at the bottom for manual finishing. When using machinery or when groundwater is present, the reserve should be no less than 0.15m before manual cleanup to the design elevation. The trench bottom must be flat, compact, and free of hard materials. The bottom may have undulations, but it must smoothly support the pipe without ridges, pits, or clods.

When the trench bottom encounters old structures, hard stones, wood, garbage, or other debris, these must be removed and replaced with a layer of sand or plain soil no less than 0.15m thick, leveled and compacted to the design elevation. In collapsible loess areas, trenching during the rainy season should be avoided. If unavoidable, standing water must be removed and a reserve layer of 0.03–0.06m should be left for compaction treatment. Soft soil foundations and soils with special corrosive properties must be treated according to design requirements.

Joint working pits should be excavated before pipe laying, based on the pipe length (typically 12m per section), to allow space for installation, welding, and pressure testing. After the pipe is placed, any discrepancies in the pits should be corrected to prevent damage to the pipe.

In areas with high groundwater levels or during rainy season construction, dewatering measures must be taken and standing water in the trench must be promptly removed.

Pipeline Laying

Before laying, the trench bottom elevation and foundation quality must be inspected and approved. Pipes should be inspected one by one for appearance defects before being lowered into the trench. Any pipe that does not meet standards must be rejected.

Pipes should be handled with care — lifted and placed gently. Dragging, rolling, or using forklifts, tractors, or other machinery to pull pipes along the trench bottom is strictly prohibited. When mechanical lifting is used for large-diameter pipes, metal slings or metal materials must not be used to bundle or hoist the pipe, and care must be taken to prevent scratching, twisting, or excessive stretching and bending.

The pipe can be laid in a gently curving pattern following the natural terrain to reduce thermal stress, but the bending radius must comply with the pipe manufacturer's design requirements. Under no circumstances should mechanical force or heating be used to bend the pipe. When the pipeline needs to change direction, elastic bending or elbows should be used according to design specifications. When both horizontal and vertical direction changes occur simultaneously, elastic bending should be avoided.

The minimum burial depth (from ground surface to pipe top) should comply with design specifications. Generally, under roadways it should not be less than 1m, under non-road areas not less than 0.6m, and under paddy fields not less than 0.8m. The pipe should not pass beneath building foundations or below other utility structures such as drainage or heating pipes. When crossing other utilities, a minimum clearance of 200mm should be maintained.

When crossing roads or encountering other obstacles, a steel or reinforced concrete sleeve should be installed. The sleeve inner diameter should be at least 150mm larger than the pipe outer diameter. The sleeve should extend at least 1m beyond each side of the road shoulder or drainage ditch, and the top of the sleeve should be at least 0.5m below the road surface. Joints within the sleeve section should be avoided if possible.

A warning tape should be buried along the pipeline route, typically at a distance of no less than 300mm above the pipe top. For pipes with a diameter greater than 400mm, two parallel warning tapes should be laid directly above the pipe with a horizontal spacing of 100–200mm. The tape should be made of polyethylene or other non-decomposable material, colored yellow, and printed with clear warning text and contact information.

Backfilling

Backfilling should be done promptly after pipe installation, but joint areas should remain exposed until pressure testing is completed. The trench must be free of water during backfilling. All bricks, stones, wood, frozen soil blocks, organic matter, and other debris must be removed from the trench before backfilling.

From the trench bottom to 300mm above the pipe top, backfill material should be uniformly placed and must not be dumped or thrown directly onto the pipe. Within this zone, hard particles should be smaller than 10mm. From 300mm to 700mm above the pipe top, hard particle size should be less than 50mm. Mechanical equipment should not be used for backfilling within 500mm above the pipe top, and when the覆土 (cover) is less than 750mm, large or medium compaction equipment must not be used and no machinery should pass over the trench.

The backfill on both sides of the pipe should be compacted symmetrically and simultaneously, with the height difference on either side not exceeding 300mm. Each layer should be compacted to at least 85% of the original soil's relative compaction degree.