As underground space development has recently increased, ground subsidence accidents that occur during underground development and the use and management of underground facilities are also increasing. The most significant cause of ground subsidence caused by underground space development is the fine loss by groundwater. Accordingly, the "Special Act on underground safety management" was enacted, but only general deformation analysis was performed without considering changes in groundwater level.
Thus, in this paper, the soil material behavior due to fine loss is investigated, and the governing equation is defined. In addition, a hydraulic-mechanical constitutive model is developed through the lab-scale experiment on fine loss. The numerical analysis program was developed by applying the constitutive model. Based on this, an example interpretation of the stability analysis on fine loss in underground space development was performed.
The mechanism of fine loss in the soil material is almost the same as the internal erosion phenomenon in the embankment. This paper proposes the critical hydraulic gradient equation that causes fine loss by modifying the existing constriction pore model. The analysis was performed on soil materials with various grain size distributions. As a result of the analysis, in the case of linearly graded, some sections (i>0.5) showed a higher possibility of fine loss than gap graded.
By classifying the soil material into coarse particles with relatively large particles and fine particles with small particles, the phase relationship is defined for the soil material, and the governing equation was derived for the soil material containing fine particles. In addition, a hydraulic model was developed for the attachment and detachment of particles by flow rate, and a mechanical constitutive model was developed for the changes in the critical state line due to fine contents based on the existing modified Cam Clay Model.
The consolidation test and consolidation undrained test were carried out depending on changes in the fine contents. As result of the test, it was found that as the fine content increased, the soil material was more contractive, and the permeability of the soil material decreased. The results of the consolidation undrained test showed that the compressibility of soil material increased as fine content increased. This tendency became more apparent as the confining stress increased. Through the test results, the critical state line was figured out according to the fine content, and as the fine content increased, the critical state line was found to change in the downward direction (to decrease the void ratio).
It was developed for a numerical analysis program applying the constitutive model proposed in this study and a Pre & Post module to improve usability. An example interpretation of fine loss during excavation was carried out by using the developed program and the lab-scale experiment results. As a result of the example interpretation, although the amount of subsidence increased in the vertical direction of the back surface due to fine loss, there was no significant change in the strut axial force and moment of wall. it is judged that this result was caused by the clogging in the vicinity of the wall.
It is expected that the fine loss evaluation method and the developed program proposed in this study will contribute to predicting and evaluating the fine loss in underground space development.