How to design a soil cover (ridge ?) for HDPE pipeline 1

You did not mention pipe wall thickness, Standard Dimensional Ratio (SDR or just DR). Usually, soil friction will keep buried HDPE pipe with typical wall thickness from changing length due to temperature variation, even when fused joints are used. Burying all of the pipe is an advantage. I designed and performed construction management for a 1 mile long, 20" thin-walled (SDR 32.5) low-pressure, heated water (120[sup]o[/sup] F) HDPE pipeline for one of our electric generating stations:

Other than pipeline entrance and exit, all of it was direct buried.

We had to cross a swamp - polyethylene pipe floats, even when filled with water. For high water table, buried pipe has to be anchored to prevent flotation. Uplift for an empty large HDPE pipe is very high. The pipe material is "weak", anchors have to be spaced close together or uplift force will "bend" the pipeline.

Pipe bedding / backfill is not critical, unless there will be significant surface loading (like vehicle traffic). We had to cross a public highway - address bedding / backfill carefully in surface loaded locations.

Fittings (including elbows) for HDPE pipe of that size are relatively weak (tees are weaker than elbows), thrust blocks may be needed. Fittings are fabricated from short, miter-cut pieces of HDPE pipe that are fused together. Buried fittings get support from the soil, a big advantage. We had buried fittings on our pipeline and had to use thrust blocks, too.

For ease of construction, we used flanged joints for the fittings - instead of fusing elbows into long lengths of fused pipe.

Fused joints were used for "straight" runs of pipe. BTW, surprisingly tight "sweeping" turns can be made with fused "straight" HDPE pipe, reducing the number of elbows need. On our project, this allowed a significant reduction in the number of elbows.

See "Second Edition, Handbook of PE Pipe", by the Plastic Pipe Institute.

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