Correct trenching, bedding, and backfill are what keep a double-wall underground composite fuel pipe reliably sealed for decades. This guide covers the on-site preparation steps contractors should follow when installing Ai Yuan HDPE/EVOH composite pipes at service stations, from trench geometry to compaction testing.
Why Bedding Quality Determines Pipe Life
An underground fuel line fails not because the pipe itself wears out, but because uneven soil support causes movement, point loading, and stress on electrofusion joints. The Ai Yuan double-layer composite pipe — an HDPE outer layer, an EVOH barrier layer, conductive masterbatch, and imported adhesive combined in an extruder — is flexible enough to ride out minor soil movement, but it relies on the surrounding bed being uniform. A poorly prepared trench undoes the value of a well-made pipe.
The job of each layer under load
When backfill is even, the HDPE outer wall carries the pipe’s structural load and the EVOH barrier keeps hydrocarbon vapors and liquid from migrating through the wall. In balanced soil, neither layer is overstressed. When one side of the trench is softer than the other, the pipe flexes unevenly and the joint area — where two pipe ends are fused — takes the brunt. Keep the bed level and you protect the joint.
Trench Geometry and Dimensions
Start with the right trench shape before any pipe is lowered in.
Width and clearance
For the 6-meter straight Ai Yuan composite pipes (models 125/110, 110, 90, and the 75/63 and 65/54 lines), allow working clearance on both sides. A practical guide is 300 mm of clear space on each side of the pipe so installers can align the ends and operate the electrofusion machine without forcing the pipe. For coiled double-wall pipe (75/63 and 65/54 in 100 m, 75 m, and 50 m coils), allow a bit more width where the coil is straightened and laid out.
Depth
Depth is set by site drawings and frost line, but as a rule the top of the pipe should sit below expected surface loads. Keep the trench bottom continuous and level; a hard rock point under one section is a stress raiser.
Bedding Material and Placement
The sand or fine gravel you place under and around the pipe does the actual supporting.
Choosing the bedding media
Use clean, fine-grained sand or crushed stone that is free of sharp edges, large lumps, and frozen material. Oversized aggregate can concentrate load on a small contact point and dent a thermoplastic wall. For the Ai Yuan conductive composite pipe, avoid bedding that could be driven into the wall during compaction.
Laying the initial bed
Spread the bed material to a uniform depth, then level it carefully with a straightedge. Never drop the pipe onto a rough or uneven bed. A consistent bed depth of roughly 100 to 150 mm under the pipe is a common starting point, adjusted to the pipe diameter.
Aligning and Jointing Before Backfill
Do not backfill around the pipe until the joints are made and checked.
Dry preparation
Wipe the pipe ends clean and keep the joint area dry before electrofusion. The Ai Yuan fittings — made from imported PE raw material and all electrofusion type — need clean, moisture-free contact surfaces for a reliable fusion. Any water or grit in the joint zone can leave a weak bond.
Support both sides of the joint
Support the pipe on either side of each fusion joint so the two ends stay aligned while the fitting heats and cools. For the smaller coiled diameters this matters most, because the natural coil curvature tries to pull the ends out of line.
Backfilling and Compaction
Backfill in layers rather than dumping soil in one pass.
First layer over the pipe
Place the initial backfill in small layers over the pipe, hand-placing material under the haunches — the curved area beneath the pipe — so no void is left. Voids under a composite pipe become settlement points later.
Compaction control
Compact each layer with a tool appropriate to the depth. Around the pipe itself, use a light vibratory plate or hand tamper to avoid over-compacting and denting the thermoplastic wall. Above the pipe, once there is enough cover, heavier equipment can be used. The goal is uniform compaction, not maximum compaction at one spot.
Avoiding thermal damage
Because the pipe is a bonded composite (HDPE plus EVOH barrier and conductive layer), keep hot work — torches, welding of other utilities, hot asphalt — clear of the trench until the pipe is adequately covered. Thermoplastic walls soften under sustained heat.
Conductivity and Grounding Considerations
The Ai Yuan pipe includes a conductive masterbatch in its construction, which supports static dissipation and leak-detection monitoring in double-wall systems. When bedding and backfilling, do not place metal scrap, rebar offcuts, or conductive debris in the trench that could create unintended contact points. Where the project calls for electrical continuity checks or tightness monitoring, complete those measurements before final backfill so any fix is easy.
Testing Before Final Cover
Run the pressure and tightness checks while the trench is still partially open.
Why test before full backfill
If a joint leaks or a wall is damaged, it is far cheaper to find it while the pipe is exposed. Fill the system, hold pressure, and inspect every electrofusion joint for weeping. For the double-wall models (125/110, 75/63, 65/54), also verify the annular space and monitoring points respond correctly before burying them.
Record the results
Log the test pressures, hold times, and findings in the site record. A complete test log is part of good workmanship and helps with future inspection and maintenance.
Backfill Completion and Surface Restoration
After testing passes, finish the backfill to the required cover, compacting in layers to match the surrounding ground so the surface does not settle. Restore any paved or paved-over surface promptly to keep water out of the trench and protect the pipe path from foot and vehicle traffic.
Field Checklist for the Installation Crew
- Confirm trench width, depth, and a continuous level bed before lowering pipe.
- Use clean, uniform bedding media; no sharp aggregate against the wall.
- Hand-place and level the initial bed; do not drop pipe onto rough ground.
- Keep joints dry and clean; support both sides during electrofusion.
- Backfill in thin layers; compact under the haunches; avoid voids.
- Match compaction force to cover depth; protect the thermoplastic wall.
- Test and record pressure, tightness, and double-wall monitoring before final cover.
Taking these steps turns a sound Ai Yuan double-wall composite pipe into a long-lived, leak-tight underground fuel line. When in doubt on any bedding or jointing detail, the installing contractor should confirm the method with the pipe supplier before proceeding.