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In this study, two caverns viz. Powerhouse Cavern (PHC) and Transformer Cavern (TC) forming major part of hydro-electric project in eastern Himalayas are considered for analysing convergence of cavern components and efficiency of support system with different pillar widths. Detailed comparative analysis of stability of caverns was carried out for unsupported, supported caverns with pillar widths of 30 m and 40 m between PHC and TC. Trend of cavern convergence, maximum principal stress together with deformations, axial, shear forces, bending moment in cavern support components like rock bolts, cable anchors, liner are examined. Pattern of distribution and location of points of concern enable identifying critical locations to resist collapse of cavern system. Evaluation of adequacy of support components were carried out to check the adequacy. Analysis revealed a strength factor greater than 1 for locations along periphery of both caverns and supports provided to the cavern system were adequate. Results establish site applicability of the support systems as well as adequacy of pillar width of 40 m to ensure stability of the entire system. The study concludes that a pillar width of 40 m ensured more stable conditions with minimal shear strain and no overlap of plastic zone between caverns. [ABSTRACT FROM AUTHOR]

This paper is based on the construction project of Wuhan Metro line 6 from Qianjincun Station to Mayinglu Station. In view of the geological condition of overlying sand and underlying limestone, by using FLAC3D5.0 software and considering the fluid–solid coupling effect, the influences of distance between karst cave roof and bedrock surface d, cave diameter D, width w and filling degree ω on the deformation and force of subway tunnel segment after excavation are analyzed. The results show that: (1) The vertical settlements and the axial forces at vault and arch bottom are negatively correlated with d and ω. The axial force at right hance and bending moments at each control point are positively correlated with d and ω. (2) With the increase of D and w, the vertical displacements and axial forces at vault and arch bottom tend to increase. The axial force at right hance and bending moments at each control point have a negative correlation with D and w. (3) Compared with other control points, the axial force and bending moment of arch bottom are the largest and the bending moment of right hance is the smallest. (4) The greater d, the smaller the amplitude of variation of the displacements and internal forces of the segment. The greater D, w and ω, the greater the amplitude of variation of the displacements and internal forces of the segment. The research results can provide guidance for similar engineering. [ABSTRACT FROM AUTHOR]

In this study, based on the elastic foundation beam theory, a local elastic foundation model has been developed. Using the model, the mechanical characteristics of the effect of tunnel-filled karst on the lining structure has been investigated. Incorporating the characteristic of the karst area, the formulas for the displacement, bending moment, and shear force of an equivalent beam have been derived. By applying the formulas to a beam under different working conditions, such as different cavern sizes and different compression modulus ratios of the filling media, analyses of the mechanical states have been carried out. The results show that if the compression modulus ratio of different filling media is close to unity, the effect of the cave on the lining structure is very small. On the other hand, if the ratio is much less than unity, the effect is more significant. The effect on the stability of the lining structure is greater for the larger caves, and smaller for the smaller caves. For the tunnel bottom equivalent beam, its bending point is at the point where the cave is in contact with the surrounding rock. Further, the maximum shear force is at the contact surface. For a beam that is closer to the karst, the displacement is larger. Under this situation, the beam section is in an adverse force state and prone to cracks. The results from this study can be used as a significant guide for the design, maintenance and construction of a tunnel which has a cavern at the base of the tunnel. [ABSTRACT FROM AUTHOR]