Dynamic Mechanism Research on the Tectonic Activation of Anninghe Rift

Anninghe rift is located on the western edge of Yangtze Block next to Tibetan Plateau, along the axis of a continental paleorift zone, Panxi paleorift. Recent studies have found that an upward mantle convection system existed since the late Pliocene in the deep lithosphere of a long and narrow area controlled by Anninghe fault. Lithospheric temperature distribution in the area has characteristics similar to that in Baikal and other modern rifts. A mantle upwelling area was in a constant state of “pull-subsidence.” Brittle rock mass of the shallow crust cracked into the new secondary subsidence blocks. A thick lacustrine sedimentary sequence of continental subsidence type developed. These all indicate that Anninghe rift is in an obvious tectonic activation state. It is believed that the tectonic activation of Anninghe rift has been produced by both horizontal squeeze from a plastic flow of the upper crust and expansion from mantle uplift. The pressure from the plastic flow of the upper crust is slightly greater than the expansion stress from the uplifting of lithosphere. Under this specific geodynamic environment, whether the tectonic activation of Anninghe rift can continue depends on the thermal motion rate of deep mantle materials and the eastward migration of the crustal materials of Tibetan Plateau.

A Novel Closed-Form Solution for Circular Openings in Generalized Hoek-Brown Media

A novel closed-form solution is presented in this paper for the estimation of displacements around circular openings in a brittle rock mass subject to a hydrostatic stress field. The rock mass is assumed to be elastic-brittle-plastic media governed by the generalized Hoek-Brown yield criterion. The present closed-form solution was validated by employing the existing analytical solutions. Results of several example cases are analyzed to show that, with the simplified assumption, a novel closed-form solution is derived and found to be in an excellent agreement with those obtained by using the exact integration method with mathematical software. Parametric sensitivity analysis is carried out and the parameterartends to be the sensitive factor. As a closed-form solution that does not require transformation technique and the use of any numerical method, this work can provide a better choice in the preliminary design for circular opening.

Analysis of Features and Causation of Rock Burst of Qirehataer Diversion Tunnel

Rock burst will happened when the underground engineering is excavated in the hard and brittle rock mass because of high in situ stress or concentrated stress. The length of Qirehataer diversion tunnel is 15.639km,and the maximum over-depth is 1720m. With excavation of tunnel, lightly to moderate rock burst had taken place in 600m length about of tunnel, and moderate rock burst locally. The mainly lithology of tunnel is gneiss granite and the percent is 64.5%, uniaxial compressive strength of which is 63.5MPa so it should be classified hard rock. The maximum principle stress is 30MPa in the place of rock bust. This paper analysis engineering geology conditions of this area and the characters of rock burst, author concludes that posteriority and continuity for time and Traceability and continuity for place are different from others rock burst examples. The reason is that course of stress adjustment is continually and repetitive.

Rock Burst Analyse in Qirehataer Diversion Tunnel

Rock burst is a geological hazard which occurs when hard and brittle rock mass is excavated under high in-situ stresses or high stress concentration. The Qirehataer diversion tunnel has 15.66 km long at a maximum depth of 1720 m. About 64.5% of the tunnel is located in gneissic granite. During excavation of the tunnel, light to moderate rock burst occurred in about 300 m length of the tunnel. This paper provides the analysis of the characteristics and stress conditions in the rock burst site using the stress conditions around the excavation, and provides an explanation on the reasons why rock burst occurred from the beginning to middle of the vault on the right side of the tunnel section, and the most serious situation is located at 60° with respect to horizontal. Stress release hole and bolt or bolt and net are used to prevent and manage rock burst which have been shown to be effective in this case.

Hoek-Brown parameters for predicting the depth of brittle failure around tunnels

A review of underground openings, excavated in varying rock masstypes and conditions, indicates that the initiation of brittlefailure occurs when the damage index, Di, expressed as theratio of the maximum tangential boundary stress to the laboratoryunconfined compressive strength exceeds approx0.4. When thedamage index exceeds this value, the depth of brittle failure around a tunnel can be estimated by using a strengthenvelope based solely on cohesion, which in terms of theHoek-Brown parameters implies that m = 0. It is proposed that inthe brittle failure process peak cohesion and friction are notmobilized together, and that around underground openings thebrittle failure process is dominated by a loss of the intrinsiccohesion of the rock mass such that the frictional strengthcomponent can be ignored for estimating the depth of brittlefailure, an essential component in designing support for theopening. Case histories were analyzed using the Hoek-Brownfailure criterion, with traditional frictional parameters, and withthe proposed brittle rock mass parameters: m = 0 and s = 0.11. Theanalyses show that use of a rock mass failure criteria withfrictional parameters (m > 0) significantly underpredicts thedepth of brittle failure while use of the brittle parametersprovides good agreement with field observations. Analyses usingthe brittle parameters also show that in intermediate stressenvironments, where stress-induced brittle failure is localized, atunnel with a flat roof is more stable than a tunnel with anarched roof. This is consistent with field observations. Hence,the Hoek-Brown brittle parameters can be used to estimate thedepth of brittle failure around tunnels, the support demand-loadscaused by stress-induced failure, and the optimum geometry of theopening.Key words: spalling, depth of failure, rock mass strength, brittle failure criterion, cohesion loss, Hoek-Brown brittle parameters