Risk of shear failure and extensional failure around over-stressed excavations in brittle rock
The authors investigate failure modes surrounding over-stressed tunnels in rock. Three lines of investigation are employed: failure in over-stressed 3D models of tunnels bored under 3D-stress, failure modes in 2D numerical simulations of 1000m and 2000m deep tunnels using FRACOD, both in intact rock and in rock masses with one or two joint sets, and finally, observations in TBM tunnels in hard and medium hard massive rock. The reason for ‘stressinduced’ failure to initiate when the assumed maximum tangential stress is approximately 0.4 to 0.5 × UCS in massive rock, is known to be due to exceedance of a critical extensional strain which is generated by a Poisson ratio effect. However, because similar ‘stress/strength’ failure limits are found in mining, in nuclear waste research excavations, and in deep road tunnels in Norway, one is easily mislead into thinking of compressive stress induced failure. Because of this, the empirical SRF (stress reduction factor in Q) is set to accelerate as the estimated ratio σθ max/UCS >>0.4. In mining similar ‘stress/strength’ ratios are used to suggest depth of breakout. The reality behind the fracture initiation stress/strength ratio ‘0.4’ is actually because of combinations of familiar tensile and compressive strength ratios (such as 10) with Poisson’s ratio (say 0.25). We exceed the extensional strain limits and start to see AE when tangential stress reaches ≈ 0.4 UCS, due to simple arithmetic. The combination of 2D theoretical FRACOD models and actual tunnelling suggests frequent initiation of failure by ‘stable’ extensional strain fracturing, but propagation in ‘unstable’ and therefore dynamic shear. In the case of very deep tunnels (and 3D physical simulations) compressive stresses may be too high for extension strain fracturing, and shearing will dominate, both ahead of the face and following the face. When shallower, extensional strain initiation but propagation in shear are suggested. The various failure modes are richly illustrated, and the inability of conventional continuum modelling are emphasized, unless cohesion weakening and friction mobilization at different strain levels are used to reach a pseudo state of yield, but still considering a continuum.
N. Barton B. Shen
NB&A, Oslo, Norway and CSIRO Energy, Brisbane, Australia
国际会议
西安
英文
72-96
2016-05-25（万方平台首次上网日期，不代表论文的发表时间）