# Numerical Simulation on Backfilling of Buried Pipes Using Controlled Low Strength Materials
## 1 Introduction  
The backfill compaction quality of pipelines has an important influence on the safe operation of pipelines [1; 2]. At present, existing water supply and drainage pipelines are mostly backfilled with solid particles. When groove backfill is conducted, the bottom area of the pipeline is narrow, and there is a backfill dead angle in the effective support foot area of the pipeline. Due to the difficulty of using compaction equipment, the compaction quality is difficult to guarantee. Pipelines may be damaged after long term operation. Once the pipeline leaks, the surrounding soil will gradually lose, which leads to frequent road cracking, settlement and collapse accidents. Controlled low strength materials (CLSM) is a self-compacting and high fluidity low strength material defined by ACI 229 [3]. It can effectively backfill the area that cannot be compacted by large compaction equipment. Different from the conventional backfill method, it only needs to be filled with fluidization, which greatly saves resources and costs. At present, CLSM has been extensively studied in China and abroad. In general, CLSM is mainly composed of cement, fly ash, sand and cement. In recent years, many scholars have studied the feasibility of using green and sustainable materials as aggregates to prepare CLSM. [PERSON] et al. [4] and [PERSON] et al. [5] successfully prepared CLSM using waste soil instead of aggregates. [PERSON] et al. [6] used construction waste to prepare CLSM using only 110 kg of water per cubic meter. The same acceptable CLSM performance was obtained as in the control group. [PERSON] et al. [7] successfully prepared CLSM based on the volcanic ash reaction of waste glass powder and hydrated lime, which provided a way for waste glass to be used in the constructionindustry. [PERSON] et al. [8] studied slag preparation CLSM and found that an increase in slag content can improve processability and reduce strength. [PERSON], et al. [9] used recycled tire rubber-sand mixtures as lightweight backfill and found that the effective use of waste tires can reduce environmental pollution. [PERSON] et al. [10] studied the comprehensive use of industrial waste incineration bottom ash to prepare CLSM and found that excessive use of bottom ash will have the problem of water secretion. In addition, some scholars have studied the application of CLSM to construction engineering. [PERSON] et al. [11] applied CLSM to tunnel pavement for laboratory and field test studies. It can be paved as a pavement but must meet certain acceptance criteria. [PERSON] et al. [12] used marine dredged soil to make CLSM for geothermal systems, which could reduce the total cost by up to 37%. [PERSON] et al. [13] conducted small-scale indoor experiments on controlled low strength material from regenerated soils and concluded that CLSM may be a potential option for stabilizing underground piping systems. [PERSON] et al. [14] proposed a method to optimize CLSM design that could be applied to the backfilling of narrow trenches. [PERSON] et al. [15] use CLSM mixtures as pipe backfills. The final backfill took less than a day and the piping performance was satisfactory. At present, there are many studies on the preparation of CLSM using various types of aggregates, but there are few studies on the application of CLSM to buried pipeline backfilling. The force deformation characteristics of the pipe are still unclear. For buried pipes, it is quite necessary to study the arching effect between them and the surrounding soil. Figure 1 shows a schematic diagram of the arching effect of rigid and flexible pipes in soil. Buried pipes share surface loads with surrounding soils. For rigid pipes, negative earth arches are formed above the pipe, while for flexible pipes, positive earth arches are formed above the pipe [16; 17], reducing the force above the pipe. For the backfilling of buried pipelines using CLSM, the \"pipe soil action\" between it and the pipeline is not yet known.