Tunnel construction opportunities involving shallow overburdens under difficult (e.g., soft, unconsolidated) grounds have been increasing in Japan. Various auxiliary methods for excavating mountain tunnels have been developed and can satisfy stringent construction requirements. The ground improvement method, which is one of the auxiliary methods for shallow overburden tunnels, has demonstrated its ability to effectively control the amount of settlement under soft ground. However, the mechanism of the ground improvement method has not been clarified, nor has a suitable design code been established for it. Therefore, because the strength of the improved ground and the suitable length and width of the improved area have not been fully understood, an empirical design has been applied in every case. In this paper, the mechanical behavior during the excavation, including that of the stabilized ground, is evaluated through trapdoor experiments and numerical analyses. In addition, the enhancement of tunnel stability resulting from the application of the ground improvement method is discussed.