Regarding the issue of pipeline backfilling

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Regarding the issue of pipeline backfilling, a project carried out recently exposed a problem that is very important yet often easily overlooked: namely, the issue related to the backfill soil used for pipelines and its degree of compaction. I work in the field of urban gas pipelines. According to the CJJ33-2005 standards, the area on both sides of the pipes and within 0.5 meters above the pipe surface is to be filled with soil; this soil must not contain debris such as gravel or bricks, and lime-soil mixtures shall not be used for filling. In the backfill soil above 0.5M above the pipe top, stones shall not exceed 10% of the total volume, shall have a diameter not greater than 0.1M, and shall be evenly distributed. The density below 0.5 M above the pipe top must be no less than 90%, while the density above 0.5 M above the pipe top shall meet the requirements for density of the corresponding ground surface. ============================================================================ The following is a reply from a user on Zhulong Web: During the backfilling of pipes, it is difficult to ensure uniform density in the soil on both sides of the pipe. The friend who is on the 4th floor suggested using medium-grained sand or gravel for backfilling, which can basically ensure proper density. Based on actual construction experience in the Tianjin area, it is not allowed to use coarse sand and gravel for backfilling, mainly because these two methods may damage the external anti-corrosion layer of the pipes. Backfilling with medium sand is acceptable, but it must be compacted using water hammering. I believe that pipeline backfilling can be considered from the following three aspects: Pipeline backfilling includes backfilling at the corners of the pipes and backfilling along the sides of the pipes, and due to the differences in these locations, different requirements apply. (1) Filling at the corner areas: Filling at the corner areas refers to the filling that takes place within the range of the designed support angles. This area is part of the pipe foundation, and its main requirements are to bond closely with the pipes, to be able to accommodate the deformation of the pipes, and to have a sufficient degree of compactness. Therefore, medium-coarse sand is the best material for filling this area. This region is usually located in the lower half of the pipe (corresponding to the 4–8 o’clock positions on a clock). Filling in this area is difficult and it’s not easy to achieve a tight bond with the pipes; thus, careful construction is necessary to ensure quality and prevent any voids from forming. In fact, many cases of pipe deformation are caused by improper construction in these areas. (2) Backfilling on the pipe side: The quality of backfilling on the pipe side directly affects the deformation modulus of the soil surrounding the pipe, which in turn has a significant impact on the degree of pipe deformation. Therefore, great attention must be paid to the backfilling in this area. Based on studies on the degree of deformation of soil, at the same density, the degree of deformation is related to the particle size of the soil; the larger the particle size, the higher the modulus of deformation. Therefore, where possible, it is advisable to use soil with larger particle sizes, such as small gravel, crushed stone, medium-coarse sand, or native soil that meets the required standards. For ease of construction, the maximum particle diameter should not exceed 40 mm. The backfill soil on the pipe side should be thoroughly compacted, with a density of over 95%, to ensure it has sufficient modulus of deformation. When selecting backfill material, attention should also be paid to the interaction between the backfill and the original soil. The so-called migration refers to a phenomenon that occurs between granular soil masses; this phenomenon leads to a decrease in the density of the natural soil, which in turn affects the overall deformation modulus of the soil surrounding the pipeline, resulting in increased deformation of the pipeline during operation. To prevent this phenomenon from occurring, research has shown that it is sufficient for the particle size of the backfill soil to meet any one of the following criteria in order to avoid it. a: D15/d85