Numerical Investigation of the Influence of Geometrical and Mechanical Parameters of the Intersecting Joint Sets on the Area of Failure Zones around Tunnel

Summary: One of the important problems in designing underground structures is geometrical and mechanical parameters of joint sets. Therefore designing underground structures should be done according to the structural conditions of the rock mass. In this regard, it is important to consider joint condition and spatial position of joints toward the underground excavation. In this study, using UDEC software, the structural stability of a tunnel with a horseshoe cross section in an intersecting jointed rock mass is investigated. Rock mass was containing two joint sets with different joint spacing. In the numerical models, joint shear stiffness, joint dip, joint opening, joint spacing, the ratio of joint spacing to width tunnel (l/b) and in situ stress were considered as variables. The area of failure zone was calculated separately by changing abovementioned parameters and the best condition concerning the least failure zone was determined. According to results, the largest failure area will created when dip difference between joint sets is lower than 40 degree, and further increasing of dip difference between joint sets causes reduction of area of failure zone. Also, increasing the ratio of joint spacing to tunnel width, leading to increase of the area of failure zone and consequently increase of instabilities. Instability exist in the range of this ratio from 0.11 to 0.33, and increasing of the ratio more than 0.33 provides improvement of stability condition (i.e., instability approaches zero) due to increasing block size and impossibility of block caving. The safest situation regarding minimum and maximum area of failure zone will occur for l/b=0.11 and 0.33, respectively.
In this research, the effective parameters of jointed rock mass, which affect the tunnel stability and the extent of the failure zone, were examined. The final shape of failure zone has presented in a table. Also the safest situation of intersecting joints has introduced.
Methodology and Approaches: UDEC software has the ability of modeling jointed rock masses by considering the characteristics of joints. Therefore, using this software, the extent of the failure zone were calculated.
Results and In this research, the area and the shape of failure zone of a jointed-rock are analyzed and the effect of main parameters on tunnel stability are discussed.