# Reinforcement Effect of a Concrete Mat to Prevent Ground Collapses Due to Buried Pipe Damage
## 1 Introduction
[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, [PERSON], 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, [PERSON], 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.