Recently, there have been several occurrences of ground subsidence in major cities in Korea. One of the major causes of ground subsidence has been attributed to underground cavities. Underground cavities are generated by soil loss due to sewage damage, poor compaction of the soil around pipes, and inadequate measures for underground water because of excavations [1
]. In particular, according to a study by the Korea Institute of Geoscience and Mineral Resources [4
], approximately 85% of 3200 cases of ground subsidence that occurred in Seoul from 2010 to the first half of 2015 can be attributed to sewage damage. Bae, et al. [5
] conducted research on underground cavities, which are the major cause of ground subsidence. They divided the cavities into area with damaged sewage and area near an excavation. From this, they determined that approximately 82% of the cases were caused by damaged sewerage. Most of the ground subsidence cases in urban areas in Korea occur as the surrounding soil flows into the damaged sewage, thus generating an underground cavity. Therefore, technology for minimizing sewage damage is needed to prevent the occurrence of ground subsidence.
The mechanism of generating underground cavities due to damaged sewage must be investigated in order to prevent the sewage damage [6
]. Several studies have been conducted in Korea using experiments and numerical analyses to examine the mechanism of generating underground cavities. Kim and Umm [7
] analyzed the effects of discharged underground soil that had been disturbed by the flow of underground water during the expansion of underground cavities. Additionally, Lee, et al. [8
] used a discrete element method to investigate how the settlement of the ground surface increased as the relaxation zone of the surrounding soil and the cavity generated due to the damaged sewage expanded. Moreover, Kim, et al. [9
] simulated the relaxation zone of the surrounding soil and the underground cavity using a discrete element method based on the results of a laboratory model test. Furthermore, Lee, et al. [10
] evaluated the applicability of a numerical simulation for the phenomenon of ground subsidence based on a numerical analysis using a large displacement analysis method, while analyzing the effects of a reduction in the unsaturated soil strength due to a rise in the groundwater level. You, et al. [6
] identified the distribution characteristics of the void ratio of the soil around a cavity due to soil loss based on finite element analysis. Additionally, You, et al. also examined how to determine the boundary of the relaxation zone by analyzing the distribution characteristics of the shear stress reduction ratio of soil around a cavity. Moreover, Lee, et al. [11
] inspected the correlation according to the load condition that was applied to the asphalt pavement layer by considering the thickness of the asphalt pavement layer, the soil depth, and the width and height of the cavity as influential factors. These factors are the criteria employed for the underground cavity grade system in Korea (Seoul). Jeong, et al. [12
] experimentally evaluated the relaxation of the ground and the scale of an underground cavity according to the mixing ratio of sand and clay. Furthermore, Jeong, et al. [12
] discovered that the scale of ground subsidence can be reduced when the clay content is decreased. In addition to this, Cooper [13
] analyzed the ground subsidence phenomenon that occurred over an extended period of time. They [13
] reported that ground subsidence was caused by an underground cavity that was found in a gypsum layer. A disaster prediction map was produced for areas with a high risk of ground subsidence. Tharp [14
] conducted a study on the mechanism of sinkholes and suggested that abrupt changes in the pore pressure around an underground cavity affect the occurrence of sinkholes. Moreover, Drumm, et al. [15
] proposed a stability chart based on a stability evaluation of ground subsidence in areas where karst is formed. They [15
] did this while performing a two-dimensional finite element method (FEM) numerical analysis in which the shear stress reduction method was applied. From this, they [15
] suggested a safety factor calculation method for ground where cavities are found. Additionally, Suchowerska, et al. [16
] analyzed the correlation between ground settlement and destruction in the upper part of a cavity due to an underground cavity by applying the ground condition, cavity shape, and presence of an underground anomaly zone as influential factors. Furthermore, Kuwano, et al. [17
] analyzed the effect of the underground water level such that it generates cavities, and this was achieved by performing a laboratory model test. From these results, it is known that a relaxation zone occurs in the area around underground cavities.
A relaxation zone refers to ground that is in a stress release state or a relaxed state due to a reduction in the rigidity or compactness around the excavation area when the excavation is performed. This must be taken into consideration for ensuring the stability of the excavated ground. Specifically, the size of underground cavity may expand as the area of the relaxation zone increases. Therefore, the expansion of the cavity can be prevented if the expansion of a relaxation zone is avoided after a cavity is generated due to damage of the buried pipes.
Meanwhile, the restoration method of underground cavity that causes ground subsidence can be divided into an open cut method and a trenchless method. One of the examples of the trenchless method of restoring underground cavities is the grouting method, which has the advantage of filling the voids in the ground; however, it causes environmental pollution. Specifically, a cement-based filler and a liquid solution are used together, and the applied area cannot be controlled depending on the injection method. Furthermore, there is a possibility of leakage of heavy metals. An open-cut method for restoring the ground subsidence typically involves controlling the traffic around the road where the underground cavity has occurred, excavating the road and filling the cavity, compacting the soil, and then reinstalling the road structures. This method may induce strength degradation of the ground due to a disturbance in the ground from the excavation, segregation, or poor compaction. The restored soil may be lost again, which may regenerate the underground cavity. Despite these drawbacks, using an open cut method is inevitable in areas where ground subsidence has occurred when replacing the pipe lines and the group collapses due to the deteriorated buried facilities or damaged pipe lines.
In this study, the reinforcement effect was evaluated by using a concrete mat that was developed to prevent ground subsidence by suppressing the expansion of the relaxation zone and cavity, which may occur while buried pipes are in service. This was performed when an open cut method was applied to the ground subsidence that had occurred in the area with buried pipes. Specifically, a laboratory test was conducted to analyze the characteristics of the stress occurrence according to the concrete mat reinforcement. Moreover, a numerical analysis was performed to estimate the range of underground reinforcement of a concrete mat that is installed on top of the buried pipes. The results were analyzed to examine the stress reduction in the ground.