After the installation of directly buried insulated pipes, backfilling is the final step, yet it's often the most easily overlooked. Many construction companies, rushing to meet deadlines, simply shovel the soil excavated from the trench back in, ignoring even the stones, let alone compacting it in layers. However, the quality of the backfill directly determines how long the insulated pipes can operate stably.
If the backfill contains stones, broken bricks, or construction waste, the outer protective pipe will be the first to suffer. While high-density polyethylene outer protective pipes have some impact resistance, they cannot withstand the repeated compression from sharp stones during backfilling and compaction. Once the outer protective pipe is punctured or dented, groundwater will slowly seep in through these damaged points. Polyurethane foam is most vulnerable to water-its thermal conductivity increases exponentially after water seeps in, significantly reducing its insulation effect.
Furthermore, the foam will slowly hydrolyze and pulverize in humid environments, potentially becoming completely ineffective within a few years. After three to five years of operation, if heat loss becomes significantly higher or even leaks occur, digging it up often reveals that the foam layer has softened from water, and the inner wall of the outer protective pipe is covered in water stains. At that point, repairs are difficult.
Besides damaging the outer protective pipe, poor backfill quality leads to another problem: insufficient pipe restraint. According to regulations, a layer of fine sand, generally at least 200 mm thick, should be laid on both sides and above the pipe to fill the gaps around it and provide uniform lateral support. If clay or soil containing gravel is used directly for backfilling, the gaps around the pipe are difficult to fill, and the pipe will not receive sufficient friction to restrain axial displacement during thermal expansion and contraction. As a result, the pipe arches upwards when heated, cracking the road surface and even causing joint detachment and leaks. Especially at stress concentration points such as elbows and tees, insufficient restraint greatly increases the risk of pipe failure.
Layered compaction of the backfill is equally important. Regulations require that each backfill layer not exceed 300 mm in thickness and be compacted layer by layer. Some construction sites, to save time, simply dump soil into the trench and roughly compact it with machinery, considering the job done. The consequence of this is insufficient soil compaction beneath the pipeline, leading to uneven settlement after a period of operation. This can cause sections of the pipeline to be suspended or experience concentrated stress, damaging welds and the outer protective casing.
Backfilling may seem insignificant and inexpensive, but its impact on the pipeline's lifespan is long-term. Once the insulated pipes are buried, digging them up for inspection and repair is much more costly. Rather than waiting years for problems to surface, it's better to pay more attention to detail during backfilling-removing stones, laying a layer of fine sand, and compacting it in layers. Proper attention to these details ensures the pipeline's 30-year design lifespan.