# Experimental Study of Ground Subsidence Mechanism Caused by Sewer Pipe Cracks
###### Abstract  
Simultaneous leakage of soil particles and groundwater into underground vacant spaces caused by Sewer pipe defects and cracks can cause underground cavities and even ground collapse on the surface. Numerical studies of this problem are limited, and the mechanism of ground subsidence caused by leakage of soil and groundwater into construction sites has yet to be verified. In this study, the factors affecting the mechanism of ground subsidence are verified via physical modelling of a sewerp pipe defect. Through visual inspection and particle image velocimetry analysis, it has been found that the groundwater flow direction, hydraulic gradient around the leakage point, and strength of the ground to support itself are the main factors that dominate the mechanism of ground subsidence. These factors can affect ground deformation and cavity expansion direction, ground subsidence development and collapse speed, and occurrence of sudden ground collapse, respectively. It was also found that pore pressure fluctuates when cavity cavern collapse and cavity expansion occur.  
There are many causes of ground subsidence. In Seoul, sewer and water supply pipeline defects, groundwater level drawdown, and excavation activity are the main causes of ground subsidence. Around 85% of all ground subsidence cases are caused by sewer pipe defects, while 14% are caused by excavation activities during construction (Seoul Metropolitan Government [7]). For example, soil particle loss along with groundwater leakage through a defect in an excavation wall triggered a sudden ground sinking into which two pedestrians fell near a construction site at Yongsan Station, Seoul, Korea [8]. In addition, a huge ground sinking around a tunnel construction site in Fukuoka, Japan was caused by groundwater leakage into the tunnel [9; 10; 11]. Although many cases are related to groundwater-related problems, the mechanism of ground subsidence caused by groundwater is not yet clearly understood.  
Currently, ground subsidence is not well studied, and as previously mentioned, it can have many causes and factors. The present study focuses specifically on ground subsidence induced by sewer pipe leakage. Tests simulated cracks occurring in sewer pipes and the succession of flooding and drainage during rainy seasons.  
A small-scale model test method was used to understand the different mechanisms of ground subsidence according to different groundwater flow modes. Vertical drainage tests were performed to simulate groundwater leakage from sewer pipe cracks, and water supply and drainage tests were performed to simulate the various possible underground soil responses. Various methods of observation were used to monitor the mechanisms of specimen failure. A camera was used to record the tests in their entirety. Pore pressure and suction measurements were used to monitor variations within the specimen that could not directly be observed. Settlement was measured through laser sensors, as well as collapse time. Test results were then discussed and summarized to estimate the effect of groundwater flow mode on the ground subsidence mechanism.