تقييم تأثير التسرب في انابيب المجاري على البنى التحتية

EVALUATION THE IMPACT OF LEAKAGE SEWER ON INFRASTRUCTURE

  Sinkholes occurrences are repeatedly reported all over the world, and soil erosion process near defective sewer pipe is observed to be one of the conceivable reasons of these occurrences. Such accidents caused significant economic losses and presented a serious threat to human life. When water infiltrates into sewer pipes through the openings and cracks and other defects, the soil particles can be migrated into the sewer pipe with water leading to form cavities and eventually to a sinkhole. In addition, water exfiltration through the defects into the soil lead to the dilution of soil near the sewer pipe, which in turn leads to the process of soil erosion. The present study focused on ‘the mechanism of soil erosion by water exfiltration/infiltration cycle through pipe defect’ and using dimensional analysis to develope a dimentionless model that predicts soil erosion of local soil.

    Experiments were conducted using model tests “to simulate” soil erosion through a defected sewer pipe, where the experimental model involved soil exposed to cyclic water flow through leak located at crown of the pipe. The model tests were performed under varying matrix of influencing factors that affect the erosion procces such as:  1 leak with  2  soil particle size distribution  3 dry density of the soil  4 initial water content  5 the height of water level in soil  6 the flow rate through the leak  7 the number of cycle. Where the eroded soil is collected, dried, weighted and sieved for each cycle, cavity formation process was observed and evaluated during the tests under the different soil conditions. The present study investigated the ground settlement induced by erosion and the ‘susceptibility’ of sewer pipe bedding material to erosion process. Where ground displacement of each flow cycle is tracked by image correlation using particle image velocimetry  PIV .

    Results indicated that the soil particle size and the leak width was the most influential factors affecting the soil erosion induced by defected sewer pipe among the other factors. Where the amount of collected eroded soil is inversely proportional to the ratio of soil particle size to leak size. Results of the experiments and data analysis showed that the soil drains through the leak into the pipe easily and continuously when the ratio of soil particle size to leak size is less than 0.17, particle sizes less than 0.42mm are more prone to erosion and larger sizes are more resistant. Soil with 5% of initial water content has 7% more amount of eroded soil mass, while soil with 10% of initial water content has 16% more amount of eroded soil mass. In the study, pipe embedment material  subbase type  D  according to the Iraqi specification  was studied, and compared to the local sandy soil, subbase type  D showed more resistance to erosion at rate of  50-90%  for different leakage sizes, where the local sandy soil were more susceptible to erosion under cyclic water flow. The dimensional analysis prediction model provide effective approaches “to predict the behavior of soil erosion of” local sandy “soil”.