2007 Vol. 13, No. 3

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CENOZOIC EVOLUTION AND TECTONIC RECONSTRUCTION OF THE QAIDAM BASIN: EVIDENCE FROM SEISMIC PROFILES
YIN An, DANG Yu-qi, CHEN Xuan-hua, WANG Li-qun, JIANG Wu-ming, JIANG Rong-bao, WANG Xiao-feng, ZHOU Su-ping, LIU Ming-de, MA Li-xie
2007, 13(3): 193-211.
Abstract (397) HTML (138) PDF (938KB)(31)
Abstract:
The Qaidam basin is the largest topographic depression inside the Tibetan plateau. Understanding the tectonic origin of the Qaidam basin has important implications for unraveling the formation mechanism and growth history of the Tibetan plateau.In this paper, the authors analyze regional seismic-reflection profiles across the basin.The first-order structure of the basin is a broad Cenozoic synclinorium, with an amplitude ranging from > 16 km in the west to < 4 km in the east.The fold axis propagated from the western Qaidam basin against the Altyn Tagh fault at 65-50.5Ma to the eastern basin at 23.3Ma; its half-wavelength changes from ~170 km in the west to ~50 km in the east. The formation of the synclinorium was induced by an older thrust system initiated at ~65-50.5Ma at the northern margin and a younger thrust system initiated at 35.5-23.3Ma at the southern margin.Cenozoic upper-crustal shortening decreases across the basin from > 48% in the west to < 1% in the east, suggesting a progressive shift in crustal thickening mechanisms across the Qaidam basin, from dominantly upper-crustal shortening in the west to dominantly lower-crustal shortening in the east.
RHEOLOGIC IMPLICATIONS OF CONJUGATE SHEAR ANGLES
ZHANG Yi-kun
2007, 13(3): 212-219.
Abstract (178) HTML (80) PDF (353KB)(9)
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A rheologic theory on conjugate shear angles is proposed based on the maximum lateral displacement rate (MLDR)hypothesis, which states that when compressive or tensile stress is applied to a solid, shear bands are formed in the solid in the orientations that give rise to the maximum lateral displacement rates of the blocks divided by the shear bands.In other words, it is postulated that the sheared blocks are laterally extruded from or fed into the deformation domain at the greatest possible velocity.The merit of this theory is:the sense of shear displacement is reversible.Hence, the same shear bands can be activated in both compressive and tensile stress regimes.In an isotropic model, the angle θ between the driving stress and the shear bands formed is determined by the equation, tan θ=, where n is the power-law index of flow.
EFFECTS OF POISSON'S RATIO ON FAILURE MODE AND OVERALL DEFORMATIONAL CHARACTERISTICS OF A ROCK SPECIMEN
WANG Xue-bin
2007, 13(3): 220-226.
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Abstract:
Using FISH functions written in FLAC to calculate the overall deformational characteristics, the effects of Poisson's ratio in the elastic stage on the failure mode and overall deformational characteristics were modeled for a rock specimen with a material imperfection in plane strain compression. In the elastic stage, the adopted constitutive relation of rock was linear elastic; and in the strainsoftening stage, a composite Mohr-Coulomb criterion with tension cut-off was used and the post-peak constitutive relation was also linear.The increased Poisson's ratio in the elastic stage leads to a transition of failure mode from single to complex shear fractures and a larger failed area is expected.The predicted shear band inclination is lower at a higher Poisson's ratio in the elastic stage, which cannot be explained by Coulomb, Roscoe and Arthur theories.Prior to the peak stress, the calculated slopes of the linear stress-axial strain curves, stress-lateral strain curves, lateral strain-axial strain curves and volumetric strain-axial strain curves at different Poisson's ratios in the elastic stage are consistent with the linear elastic solutions in plane strain compression.The calculated Poisson's ratio in plane strain compression is higher than 0.5 if the adopted Poisson's ratio in the elastic stage exceeds 1/3, as is confirmed by the present numerical simulation.At a higher Poisson's ratio in the elastic stage, the post-peak lateral strain-axial strain curve, volumetric strain-axial strain curve and calculated Poisson's ratio-axial strain curve become less steep; whereas the post-peak stress-lateral strain curve becomes steep and the precursor to failure is less apparent.
PREDICTION OF CRUSTAL STRESS OF DEEP-BURIED TUNNELS BASED ON BP ARTIFICIAL NEURAL NETWORK
SUN Wei-feng, TAN Cheng-xuan, WANG Zhi-ming, ZHANG Chun-shan, WU Shu-ren
2007, 13(3): 227-232.
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The measurement of the crustal stress at depth is a difficult problem in geological engineering projects.The crustal stress is hard to determine or measuring data are not ideal because of the limitation of unduly simple research means and measuring techniques.On the other hand, satisfying results can be achieved by artificial neural network (ANN)even though the data have deficiencies such as data noise, partial absence, lack of cognition because of the native advantages:self-learning, self-adaptability, robust, error tolerance and generalization.Based on the BP artificial neural network method, this paper provides a prediction model for the crustal stress values using 6 factors:depth, field density, elastic modulus, triaxial compressive strength (10 MPa confining pressure), acoustic emission stress measurements and fissure density.The authors made hydrofracturing stress measurements in the Qinling deep-buried long tunnel by using the BP artificial neural network model, perpormed a fitting analysis of the measured data and predicted the crustal stress at depth.The main conclusion is that the BP artificial network model is feasible in the prediction of the crustal stress value of deep-buried tunnels.
NUMERICAL SIMULATION OF THE TECTONIC STRESS FIELD AND FORECASTING OF FRACTURES BASED ON MSC MARC
LI Zhi-yong, ZENG Zuo-xun, HUANG Zheng, LIU Li-lin, WEI Zhong-yuan, ZHANG Kun
2007, 13(3): 233-238, 232.
Abstract (209) HTML (104) PDF (298KB)(6)
Abstract:
MSC MARC is a nonlinear FEA program.Its ability of highly nonlinear finite element analysis and good secondary development interface enable us to construct a complicated 3D geological numerical model and perform 3D tectonic stress modeling and nonlinear analysis, and complex constitutive equations and fracture critera of rocks can be established by secondary open subroutines.Based on the finite element analysis software MSC MARC.a 3D finite elemant model of the Baobei anticline the Yanqi basin, Xinjiang, is established, and the simulation analysis of the tectonic stress field and fracture forecasting are performed.The predicted regions with high-density fractures are in agreement to the real case.
SIMULATION STUDY OF THE TECTONIC STRESS FIELD IN MINES———A CASE STUDY OF THE BAODIAN COAL MINE
WANG Ji-lin, JIANG Bo, WANG Chao-yong
2007, 13(3): 239-246.
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By physical experiments and numerical simulation, the ruptural deformation process and failure morphological features of rock masses were analysed.The rule in the course of rock mass failure deformation was summarized.The rock mass with homogeneous lithologic character can be treated as isotropy on a macroscopic scale. The defects existing in a geologic body naturally such as holes, fissures and bedding planes make the stress in a geologic body distributed unevenly. Rupture first takes place in a high stress difference area of the geologic body when the stress difference reaches or exceeds the ultimate strength. The failure deformation will continue with increasing tectonic stress and the strike of failure planes is approximatively parallel to the high-stress difference isolines. Based on an analysis of mine geologic conditions and tectonite fabrics, according to the rule in the course of rock mass failure deformation, the mode and direction of action of external force for the Yanshanian structures in the mines are analyzed by taking the Baodian coal mine for example.By comparative analysis, the sequence of formation of structures in the mine is validated, and the evolution of mine structure is revealed.
MECHANIC RESPONSE OF A SHALLOW-EMBEDDED AND DOUBLE-ARCH TUNNEL UNDER PARTIAL PRESSURES DURING CONSTRUCTION
XIANG An-tian, ZHU He-hua, DING Wen-qi, JIA Ming-hui
2007, 13(3): 247-254.
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Taking an expressway in Jiangxi province as the engineering background, the behavior of a shallow-embedded and double-arch tunnel with partial-pressure during construction is simulated.Several aspects, such as stress concentration and transfer, plastic zone distribution, vault displacement and change of vertical stress, (σy)of the mid-leading wall with construction are studied systemtically. The study indicates that :1) asymmetrical excavation is the key cause of partial pressure in the mid-leading wall and the construction of the first and second lining can not ameliorate the conditions greatly; 2) under conditions of shallow embedment, vault displacement increases with embedment depth and displacement mainly happens during excavation of tunnel before lining construction; 3)after excavation of the up-bench tunnel, pulling stress concentrates around the top arch and lining should be constructed promptly or beforehand; 4)as a result of partial pressure, when plastic zones around the side-wall and mid-leading wall heel, which are near the mountain ridge, are better developed; and 5) when construction finished, compressive stresses at tunnel side-walls disappear and are transferred to the inverse arch and second lining, making them concentrated by shear stress and normal stress and improving the stability of the tunnel.
NUMERICAL SIMULATION OF SEISMIC RESPONSE AT THE ENTRANCE OF THE UNSYMMETRICAL LOADING TUNNEL
XIONG Liang-xiao, LI Tian-bin, LIU Yong
2007, 13(3): 255-260.
Abstract (233) HTML (91) PDF (268KB)(10)
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In the mathematic modeling of the seismic response of a tunnel, the lateral calculation range and artificial boundary have great influence on the result of calculation.With the Huangcaoping tunnel as the study objective, its seismic response is simulated using the finite difference procedure FLAC3D.The 5, 6, 7, 8, 9 and 10 times the diameter of the entrance of the tunnel are taken as the lateral calculation range respectively, and the cutting boundary, free-field boundary and viscous boundary in FLAC3D are used for the calculation.The results of the study indicate that it is relatively reasonable to take 7 ~ 8 times the diameter of the entrance of the tunnel as the lateral calculaiton range and the free field boundary or viscous boundary as the artificial boundary when the seismic waves are P-waves.
ENGINEERING PROPERTIES OF EARLY HOLOCENE SOFT CLAY ON THE EAST BANK OF ERHAI LAKE
WANG Xian-li, QU Yong-xin, JIANG Liang-wen, ZHANG Yong-shuang
2007, 13(3): 261-269.
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Early Holocene thick soft clay is distributed around Erhai Lake.The results of engineering property tests indicate that the soft clay on the east bank of Erhai Lake has such engineering properties as a high water content, a high porosity ratio, high compressibility, low strength and high plasticity.The phenomenon of over-consolidation is very remarkable.The statistic analysis of physical-mechanical index parameters of soft clay indicates that :the variability coefficient of the mechanical index is high while the variability coefficient of the physical index is low, and the correlations between various physical indices and between the physical index and mechanical index are apparent.Knowledge of the relation between all kinds of indices of various engineering properties of soft clay is not only helpful to revealing the engineering properties and formation mechanism of soft clay, but also instructs us to choose relative measures to solve the engineering problems of soft clay.
RELATION BETWEEN THE GROUND STRESS STATE AND GEOLOGICAL HAZARDS IN THE UPPER REACHES OF THE YELLOW RIVER
ZHANG Chun-shan, NAN Qing-min, LIAO Chun-ting, TAN Cheng-xuan, WU Man-lu, MA Yin-sheng
2007, 13(3): 270-277, 269.
Abstract (189) HTML (99) PDF (779KB)(11)
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On the basis of the data collected from the study region extensively, this paper simulates the ground stress state of the region and discusses the relation between the stress state and geological hazards according to the measured values of the ground stress. The conclusion is as follows : earthquake activity is the result of release of ground stress when it is accumulated to a certain extent; strong earthquake activity generally occurs at a site where the direction of the maximum principal stress makes an acute angle with the fault strike; earthquake activity usually took place in a local high-stress region. The distribution of collapses, landslides and mud flows has a good corresponding relation with the high-value region of the maximum differential stress but no distinct corresponding relation with the high-value region of the maximum principal stress.
ASSESSMENT OF CRUSTAL STABILITY IN THE QINLING-DABA REGION
HAN Jin-liang, WU Shu-ren, TAN Cheng-xuan, LI Dong-lin, SHI Ling
2007, 13(3): 278-288.
Abstract (140) HTML (62) PDF (589KB)(3)
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The Three Gorges reservoir water-diversion project is a new line of the south-north water diversion project.There are six major factors affecting the crustal stability in the region :crustal structure, present tectonic stress fields, tectonic framework, fault activity, earthquake activity and rock properties. The authors select dominant conditions affecting regional crust stability as evaluating factors to construct a fuzzy evaluation model.Based on the above-mentioned six factors and by using the fuzzy evaluation model, the authors evaluate the crustal stability of the Qinling-Daba region.The results show that the Qinling-Daba region has a higher degree of crustal stability and is and suitable for the large water diversion project.