Rock Fall/Sliding Hazards In Roads In Afghanistan And Proposed Sustainable Mitigation Measures

Mujahid, Noor (Civil Engineering Department, Engineering College, Kabul University) | Hamayoon, Kheradi (Civil Engineering Department, Engineering College, Kabul University)


ABSTRACT The elasto-plastic computational models commonly in use predict that lining loading increases with increasing ground strength under certain conditions. This is contrary to the behavior that might be expected on the basis of intuition or tunneling experience. The present paper investigates the conditions under which this paradox occurs and shows that it can be traced back to a combination of large deformations ahead of the face and small deformations of the support system. Additionally, the paper investigates why such behavior does not occur in reality. It shows that the decisive simplifying modeling assumptions are, alone or in combination with each other, (i) that ground behavior is time-independent (whereas in reality overstressed ground generally creeps) and, (ii) that the support operates with full stiffness close to the face (which is not feasible in most cases due to the nature of the excavation and support installation procedures). When these two effects are taken into account in the design computations, the paradoxical model behavior is eliminated. 1. INTRODUCTION Under certain conditions which are frequently encountered in tunnel design, the computational models commonly in use predict that poor-quality ground will be more favorable for tunnel construction than high-quality ground. More specifically, the models suggest that ground of higher strength develops a greater load upon the lining than the load developed by low-strength ground (all of the other parameters being equal). This is clearly contrary to the behavior that might be expected both intuitively and on the basis of tunneling experience, which is that overstressing of the lining or severe convergences are associated with ground of poor quality [1]. The model behavior deserves to be called a paradox, i.e. “a seemingly absurd or contradictory statement or proposition which when investigated may prove to be well founded or true” (Oxford Dictionary). The paradox has been mentioned in passing in a number of older works dealing with the elasto-plastic analysis of tunnels. (A listing of the works can be found in [2]). More recently, it has been noted by Boldini et al. [3] and Graziani et al. [4], who obtained “unforeseen results” from axisymmetric elasto-plastic numerical analyses of advancing tunnels, and explained them by means of the convergence–confinement method (“The decrease in the loading in the plastic case is caused by the increased convergence before the installation of the lining, which overshadows the negative effect of the flattening of the convergence curve in the plastic range”). Also, Mair [5] drew basically the same conclusion when discussing the results of plane strain analyses (“This is because the weaker ground leads to higher deformations occurring ahead of the face prior to installation of the lining; the consequence of more ground deformation before installation is a smaller pressure induced on the lining”). Although the paradox has been noted by a number of authors, it is, interestingly, neither widely appreciated nor well understood in the broader engineering and scientific community. It may therefore perplex the tunnel engineer and raise doubts as to the predictive power of standard tunnel design calculations, and this makes it deserving of closer investigation.

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