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2008, 27 (4): 0-0 doi:

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STATE，ISSUES AND RELEVANT RECOMMENDATIONS FOR SECURITY RISK MANAGEMENT OF CHINA¢S UNDERGROUND ENGINEERING

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2008, 27 (4): 649-649 doi:

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Underground project is a high-risk and complex project with vast construction investment，long-term construction，numerous subprojects，complexity of construction technology，and unpredictability of risk factors and social characteristics of the environmental impact. The establishment of a risk management system，detailed risk assessment for construction of metro project，and risk control，are necessary and should be extended to the overall underground construction areas. This paper mainly describes the present status and issues existing in risk management for safety operation in underground projects，which include：(1) the lack of standardized security risk management system；(2) unreasonable safety risk management，and insufficient funding for security risk management；(3) inadequate or nonprofessional teams or uneven-developed professional level；(4) risk management-related technical specifications，standards are not consistent with the current development of underground works；and (5) the lack of appropriate security risk management information platform. Related to issues existing in risk management in China，four suggestions are proposed as follows：(1) to strengthen codification of risk management for safety in underground projects；(2) to promote risk management as an indispensable part of project management；(3) to set up an aid system for warning decision making based on informationization technique；and (4) to strengthen study on major accident prediction and prevention technique for risk management.

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THERMO-HYDRO-MECHANO-CHEMICAL COUPLING STUDIES ON EXCAVATION DAMAGE ZONE IN CRYSTALLINE ROCKS

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2008, 27 (4): 656-656 doi:

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This paper presents the development of thermo-hydro-mechano-chemical(THMC) studies on excavation damage zone(EDZ) in crystalline rocks，which is an important part of international cooperative project DECOVALEX-THMC，denoted TASK B. The basic idea of THMC coupling studies of EDZ in crystalline rock is introduced. The mechanism of EDZ formation and evolution can be characterized and analyzed based on typical experiments carried out in underground laboratory(for example，Äspö HRL). Then the THMC models of elasticity，elastoplasticity and visco-elastoplasticity and so on can be built and the associated codes can be developed. The behaviors of THMC coupling in EDZ of crystalline rock will be analyzed by using these codes. Comparisons of the results between each research group in different countries and technical evaluation on the models that each group used will be conducted by experts or scientists majored in this field. By doing so，the model and method that each group developed will be updated according to the evaluation results and the THMC model in EDZ of crystalline rock will be obtained appropriately. The guidance document of THMC studies in EDZ of crystalline rock can be compiled based on the work described above；and it can provide technical supports for the studies of underground disposal for high-level nuclear waste disposal.

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VALIDATION FOR ROCK BLOCK STABILITY AND ITS APPLICATION TO ROCK SLOPE STABILITY EVALUATION USING DDA METHOD

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2008, 27 (4): 664-664 doi:

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The process in calculation of the contact forces between blocks is firstly introduces，and a method about the factor of safety calculation in discontinuous deformation analysis(DDA) is then presented. The stability validation with a rock block sliding along a incline has been carried out. Two cases，one with the friction angle j，and the other with the friction angle j and cohesion c together，are considered. At last，according to the stability and reinforcement issues of a weak fault controlled rock slope，which is formed by right dam abutment excavation of a hydro-project，DDA method has been used to analyze the stability of the fault controlled rocky slope with the slices used in limit equilibrium methods；and its actual stability states，separately with/without the utilization of rock bolt reinforcement，have been evaluated. It is shown that，for the calculation of the factor of safety along a given sliding surface with DDA method，as the contact forces are based on the rigorous equilibrium equations，where the discontinuous deformation of block system is allowed，the improper assumption for the utilization of shear strength between the sliding slices with a constant ratio is not necessary；and the stability calculation results by DDA method may reflect a more reasonable stability situation of an actual rock slope. In addition，the numerical simulation of rock bolts to the reinforcement of rock slope in DDA shows an effective role in promoting the stability of rock slope.

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NUMERICAL SIMULATION ON ASYMMETRIC DEFORMATION OF DEEP SOFT ROCK ROADWAY IN KONGZHUANG COAL MINE

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2008, 27 (4): 673-673 doi:

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With the further deep mining and the complexity of the geological and mechanical conditions of roadway become more complicated compared with those in shallow roadway. Due to the special rock strata structure and the engineering geological conditions of high geostress，high geothermal temperature，high risk of water inrush as well as the disturbance of mining activity(three-high and one-disturbance action)，roadways at great depth have the complex mechanical behaviors both in deformability and stress distribution. Consequently，the nonlinear mechanical phenomena occur with high frequency，which bring about great difficulties for deep engineering support during excavation and mining. Because of structure stress effect，high content of clay mineral and influence of the goaf on the top，it had taken on asymmetric deformation characteristic in Haulage Tunnel(level －785 m underground) at Kongzhuang Coal Mine. Based on the in-situ investigation，experimental test，analysis of engineering geological conditions，geomechanical evaluation and numerical simulation，the displacement distribution and stress distribution of the surrounding rock mass in Haulage Tunnel(level －785 m underground) at Kongzhuang Coal Mine are considered. The numerical results indicate that centralized pressure on the upper goaf may be the main reason for asymmetric deformation in Haulage Tunnel(level －785 m underground) at Kongzhuang Coal Mine；and clay mineral swelling when interaction with water and in-situ stress will further deteriorate of the roadway rock structure. It provided a preparatory basis for decision-making of asymmetric deformation in Haulage Tunnel(level －785 m underground) at Kongzhuang Coal Mine. The result of in-situ test shows that the design can ultimately guarantee the stability of deep coal gateway，and the success supporting effect can be obtained.

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NEW COMPUTATION METHOD FOR SOIL FOUNDATION SETTLEMENTS

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2008, 27 (4): 679-679 doi:

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Based on in-situ plate loading test P-S curve，a new settlement computation of the foundation is proposed. Supposing that the in-situ plate loading test P-S curve is governed by a hyperbola equation，a hyperbola model is constructed，and the tangent modulus and secant modulus of the soil can be obtained from this hyperbola equation，which could be used in the layer-wise summation method to compute the foundation settlements，and to form the hyperbola tangent modulus method and hyperbola secant modulus method for natural soil. Under the assumption of the in-situ plate loading test P-S curve as a hyperbola equation，it only needs three parameters for the method，that is initial tangent modulus Es0，internal friction angle j and cohesion c. For any other in-situ plate loading test P-S curve，the relationship between soil tangent modulus and loading level is constructed；and the general curvilinear tangent modulus method is proposed which is applied to the layer-wise summation method. As the tangent modulus or secant modulus change nonlinearly with the loading level changing，the new method can calculate the settlement when the load approaches to the ultimate bearing capacity of foundation. Based on the undisturbed soil tangent modulus and secant modulus，the new method of the foundation nonlinear settlement is presented，which can be used to calculate the complete process of the nonlinear settlement of the foundation. Because the soil deformation parameters are determined from the in-situ plate loading test，the intactness and nonlinearity for the soil can be better considered；the foundation nonlinear settlement can be calculated more correctly. Moreover，few parameters are needed. The effectiveness and practicability of this new method are examined in the pile foundation，raft foundation，complex embankment foundation and composite foundation. This may be a great progress for settlement computation of soil foundation.

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STABILITY ANALYSIS OF ROCKY SLOPE BASED ON SLOPE STRUCTURES

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2008, 27 (4): 687-687 doi:

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It is of great importance to obtain the reasonable results of stability analysis of rocky slope by using the engineering geological information to establish the correct calculation model. The model is performed based on the slope structures，which are established from geological information. There are many differences between slope structures and rock masses because rock mass of slope will be encountered with the epigenetic reformation and natural forces. The main difference is that rock mass of slope has the special engineering geological properties，such as traces of epigenetic reformation，the surface weathering，rock mass stress relaxation，erosion of surface water and so on. The rocky slope structure now only contains rock masses and its geological tectonic surfaces，and it will necessarily be enriched by these properties. The reasonable model of stability analysis for rocky slope may be obtained by the slope structure. Rocky slope structures will be first analyzed；and the differences can be obtained. Some key issues for the rocky slope structure，i.e. characteristics of the epigenetic reformation，the action scope of natural factors on slope and the boundary to establish geological mechanical model，are discussed. Many special engineering geological characteristics of slope rock mass，rock-soil slope examples，discontinuities，faults，groundwater，key slide surfaces，calculation parameters and so on can be achieved by the slope structures. The 2D or 3D reasonable results of the stability analysis of slope structures are verified.

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ANALYSIS OF GROUND SETTLEMENTS OF OVERLENGTH PIPE AND BOX CULVERT ADVANCING UNDER AIRPORT TAXIWAY

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2008, 27 (4): 696-696 doi:

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The vehicle tunnels adopt box culvert advancing scheme under overlength pipe shed，which is the first case to underpass the west L-taxiway of Beijing Capital International Airport without suspending air service during construction in China. The ground settlement should be strictly controlled. For the case of safety，the numerical simulation of construction has been carried out. The characteristics of ground settlement have been analyzed according to the construction of long pipe shed，excavation of pilot drift and the advancing of box culvert. The simulation result shows that the maximum ground settlement is 24 mm at normal condition. In actual practice，the settlement has been well controlled within 30 mm，which meets the limitation of allowable value. It indicates that the simulation models and results are effective and rational；and the box culvert advancing scheme under overlength pipe shed is feasible.

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STABILITY ANALYSIS OF YUANMO HIGHWAY SLOPE UNDER INTENSIVE RAINFALL

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2008, 27 (4): 704-704 doi:

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Based on the finite element method of saturated and unsaturated rainfall infiltrations and the strength theory of unsaturated soils，a strength reduction method of slope stability analysis was presented. The method could consider the influence of strength variation of unsaturated zone and seepage load in saturated zone. It was applied to the deformation calculation and stability evaluation for the engineering slope K261 of Yuanmo highway under the influence of precipitation. It showed that the range of unsaturated zone would decrease；the pressure water head would increase and the transient saturated zone would appear because of the strong rainfall. The factor of safety of engineering slope would obviously decrease due to the unsaturated zone strength decreasing and the load changing of saturated zone seepage. The engineering slope would be unsteady under the extreme condition of constant rainfall accompanied with seismic loading. Preventing rainfall infiltration from slope and draining of slope could improve the stability of engineering slope effectively. At the same time，some resisting slip piles are suggested to for the 6-grade engineering slope.

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EXPERIMENTAL STUDY ON DAMPING CHARACTERISTICS OF ROCK
UNDER CYCLIC LOADING

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2008, 27 (4): 712-712 doi:

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The four-grade uniaxial cyclic loading and unloading tests for two groups of red argillaceous siltstones were carried out on MTS815 Flex Test GT rock mechanics test system. The sine wave is adopted in loading test，and its frequency was 3 Hz. The maximum cyclic stress was less than the value of the average intensity of compression；and the cyclic number of each level cyclic stress was 30. Through the test，the relationships of dynamic moduli of elasticity and damping ratio to dynamic strain for the two groups of rocks were gained. The test results showed that the curves of strain and stress for loading and unloading were not superposition under cyclic loading，but hysteresis loops. The strain wave location was always later than stress wave location；and the hysteresis loops were not ellipse-shaped，but tine-lobation when the cyclic loading was reversed. The dynamic strain increased with the increase of oscillation amplitude；the dynamic modules of elasticity was linearly decreasing with the dynamic strain increasing，but the damping ratio was reversed. The four relationship equations were gained，and the correlation coefficient R2 was over 0.98. Moreover，the irreversible plastic deformation of rock increased，and at the same time，the fracture deformation of rock gradually increased，which was produced by cyclic load.

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ON PROBABILITY MODEL OF PHYSICO-MECHANICAL PARAMETERS OF QUASI-BRITTLE MATERIALS AND ASSOCIATED MECHANICAL FAILURE BEHAVIORS

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2008, 27 (4): 718-718 doi:

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Based on the statistical theory and elastic damage theory，constitutive relation of quasi-brittle material model was discussed. Heterogeneity was introduced into the numerical model assigning mechanical parameters throughout the numerical specimens by following Weibull distribution function，normal distribution function，even distribution function or logarithmic normal distribution function. Firstly，heterogeneity index was defined in the models with different distribution functions. By using a numerical code RFPA3D，fracture process of quasi-brittle material was investigated by different statistical distribution functions；and the complete stress-strain curves，peak strength and AE serials were all obtained to analyze the mechanical failure behaviors. The mechanical behaviors obtained in the heterogeneous numerical model implemented by using logarithmic normal distribution function were similar to that in Weibull distribution function. The nonlinear and the brittle failure behaviors become more obvious in more homogeneous models；and the peak strength increases with the homogeneity. Numerical results compared with laboratory test results；and analytical results show both Weibull distribution and logarithmic normal distribution functions are more reasonable and reliable for elastic damage model. Weibull distribution function is the most simple and convenient one among these related distribution functions. Compared with damage model based on statistical theory；numerical modeling is an efficient and convenient method to consider the heterogeneity in many scales and complicated boundary conditions in quasi-brittle materials. The combination of damage theory and computing method is a promising approach to resolve the stochastic problems due to the heterogeneities in quasi-brittle materials.

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STUDY ON ELASTOPLASTIC DAMAGE MODEL OF UNSATURATED ROCK MASS AND ITS APPLICATION

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2008, 27 (4): 728-728 doi:

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Considering rock as porous media，the relation between rock porosity and volumetric strain is established. Then the rock permeability coefficient，damage，saturation¢s evolution equations are defined；and the unsaturated elastoplastic damage coupling model is proposed based on the effective stress law of porous media. The proposed model is implemented with user subroutines into the nonlinear FE software ABAQUS；then the stability evaluation of rock mass for the diversion tunnel of Jinping Hydropower Station is conducted. The numerical results show that the proposed model is reasonable and it can explain the rock mass¢s saturated- unsaturated-resaturated process.

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PARALLEL NUMERICAL SIMULATION OF SEEPAGE IN FRACTURED ROCK MASS BASED ON STATISTIC MODEL

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2008, 27 (4): 736-736 doi:

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To simulate the fractures in natural rock，the fracture network has been formed with Matlab based on statistical parameters of fracture shapes and random distribution of fractures. Then finite element mesh has been generated with fracture network. Considering enormous time steps and huge workload in the numerical analysis，the element-by-element(EBE) finite element parallel computation method is adopted for numerical simulation. This algorithm does not generate global stiffness matrix and only stores element stiffness matrices，which can reduced the number of iteration in preconditioning conjugate gradient method and improve the convergence of calculation. Corresponding parallel program for numerical analysis is developed and then implemented on cluster of workstation. Numerical analysis of seepage is conducted for two examples，which include separately one set fracture sets and two different strikes of fractures. Computational results show that the numerical analysis model is effective for the fractured rock mass.

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KRIGING INTERPOLATION MESHLESS METHOD BASED ON FINITE COVERS AND ITS APPLICATION TO CRACK PROPAGATION

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2008, 27 (4): 743-743 doi:

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The meshless methods have a relative simple pretreatment process. The Kriging interpolation procedure is one of the meshless methods. Numerical manifold method can solve both continuous and discontinuous deformation problems in a unified mathematical formulation. The finite cover is the essential technique in this method. Both the finite cover technique and Kriging interpolation method are integrated to develop a Kriging interpolation procedure based on finite covers which take advantages of these two types of numerical methods. The merit of the proposed method is that the shape functions constructed using this method have the properties of Kronecker d-function，which will make the essential boundary conditions be easily implemented. The fundamental theory of this procedure is illustrated and numerical analyses of examples show that the proposed procedure is an effective and simple method for singular and discontinuous problems.

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RESEARCH ON PIPING MECHANISM IN SANDY SOILS BASED ON DISCRETE ELEMENT THEORY

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2008, 27 (4): 749-749 doi:

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Using particle flow code(PFC) based on discrete element theory and combining with the small-scale model tests，the piping in sandy soils is simulated with fluid-solid coupling. The porosity of sample，fluid velocity，contact number of particles，pathway of movable particles and loss fraction of eroded particles and so on are traced and recorded in the analytical model. The results show that the movable particles are eroded gradually；and the fluid velocity and porosity of sample increase gradually with the increase of water head；the permeability of sample changes，which will be influenced greatly by the response of fluid velocity and water head. The results reveal that the geometric and hydraulic features of system change dynamically and nonlinearly，which disclose their interactive behaviors during piping. The results also show that the loss fraction of eroded particles decreases along the seepage flow path；the small particles remove randomly；and the leak passages of piping are formed randomly when piping，which are the outcome of the complicated interactions between water and soil. The violent fluctuation of contact number of movable particles shows the violent motion of small particles during piping；and the stress field of soils is not influenced by particle erosion fundamentally. The availability and rationality of the proposed numerical method is verified by comparing the numerical solutions with the results of relevant physical model tests. The results are valuable to further probing into the piping mechanism in sandy soils.

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MODEL EXPERIMENT STUDY ON INFLUENCES OF REINFORCEMENT ON INTERVALS OF ROCK BOLTS IN SURROUNDING ROCK UNDER EXPLOSIVE WAVES

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2008, 27 (4): 757-757 doi:

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For the purpose of the comparison of the influences of rock bolts¢ intervals on reinforcement effects and anti-detonating characteristics of surrounding rock around two underground openings，the different reinforcement effects of two kinds of rock bolts，which have the same length and the different intervals in surrounding rock of two underground openings，are studied by model experiments of geomechanics under the effect of explosive stress waves produced by focus charging. Because of the differences of the intervals of the rock bolts，the distinctions of stress in surrounding rock，surface strain of two underground openings，the roof-to-floor relative displacements of two underground openings，accelerations of roof，floor，sidewall and macroscopic damage forms are compared and analyzed. By comparing the caverns reinforced by the large intervals of rock bolts，the results show that the wall rock¢s stress is relative larger in surrounding rock of the underground opening reinforced by the small intervals of rock bolts. However，the surface strain of the underground opening，the relative roof-to-floor displacements and the accelerations of roof and floor of the underground opening reinforced by the small intervals of rock bolts are relative smaller；and there is no cracks in the reinforcement zone. These conditions indicate that the small interval of rock bolts should be used to reinforce the surrounding rock of underground openings. The corresponding numerical analysis of model experiment is carried out. Some parts of the model test data are compared with the results of numerical analysis. The peak values of the vault stress and the roof-to-floor displacements of model test are consistent with numerical analysis. The experimental result is reliable，and this can provide references to the design of protective projects and the reinforcement of the wall rock and tunnels.

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INFLUENTIAL FACTORS OF ACOUSTIC EMISSION LOCATION EXPERIMENT OF ROCK UNDER UNIAXIAL COMPRESSION

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2008, 27 (4): 765-765 doi:

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Acoustic emission(AE) technique can be used to monitor the microcracks development in the rock test samples continuously in real-time；it is better than other methods. AE location technique is employed to study crack initiation and propagation process of Chongqing sandstone. Also，the crack spatial evolution mode with loading time has been observed and the influential factors of AE location experiment of the sandstone have been discussed. The experimental results are displayed as follows. (1) The attrition of the tip of the sample has remarkable influence on AE characteristic in the rock failure process；and the mixture of the stearic acid and the vaseline(weight ratio 1∶1) is used as attrition-reduced agent to the AE location experiment. No AE event is found when the initial crack is closed；it will obtain a better effect. (2) The AE location of the Chongqing sandstone test sample mostly is distributed in central-section and assumes dispersion. No void space is shown in the central-section of the sample. Contrasted with the relevant studies，it is shown that the rock type，the structure and the level of uniformity are the major influential factors. (3) The load mode and the load control mode can directly affect the rock failure process so as to affect the AE events. (4) AE location experiment of the Chongqing sandstone test sample under fatigue load shows that the load history affects the AE time series and the AE events. The experimental and analytical results can afford useful help for the program design of AE location experiment of rocks.

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RESEARCH ON VIBRATION INDUCED BY DYNAMIC UNLOADING OF IN-SITU STRESS IN ASYMMETRICAL GEOSTRESS FIELD

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2008, 27 (4): 773-773 doi:

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The characteristics of vibration induced by dynamic unloading of in-situ stress(VIDU) on the excavation contour during blasting is studied with combined method of numerical simulation by dynamic FEM and verification with monitored in-situ vibration data. The VIDU is a superimposure of the vibration induced by dynamic unloading of radial in-situ stress(VIDUR) and shearing in-situ stress(VIDUS). It is founded that the distribution of VIDU，VIDUR and VIDUS are all determined by the distribution of unloading in-situ stress(IS) respectively. The IS depends on the coefficient of lateral surrounding rock pressure l and the comparatively width of blasting layer b. In the maximum principal geostress direction，the VIDUR and VIDUS have the same direction and the summation is VIDU. The situation is conversed at the minor principal geostress direction；the VIDUR and VIDUS have different directions and the VIDU equals the difference of them. The distribution of VIDU in the surrounding rock of circular tunnel beyond the excavating contour is ellipse-shaped；and the major axis of which is always at the direction of the maximum principal in-situ stress direction. The larger the l is，the larger the major axis of the ellipse is. The vibration induced by excavation of the underground powerhouse with method of drilling and blasting at Pubugou Hydropower Project is presented as an example of the transient unloading of in-situ stress of rock mass. The arriving moment of the VIDU in the field vibration curves is identified employing the maximum module method of wavelet transform，which has the characteristic of making the abrupt change of signal. The VIDU is separated from the total vibration to a certain extent；and the numerical simulation results are then verified.

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REALIZATION OF PAN JIAZHENG¢S EXTREMUM PRINCIPLE WITH OPTIMIZATION METHODS

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2008, 27 (4): 782-782 doi:

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Slope stability analysis based on the limit equilibrium method is statically indeterminate，which requires various assumptions to evaluate the factor of safety. The interslice force function f (x) is a major assumption that is important but is not adequately considered in the past. The authors propose that the factor of safety and the interslice force function can be determined in accordance with the Pan Jiazheng¢s extremum principle from an optimization analysis. For any prescribed failure surface，the interslice force function is taken as the control variable；and the maximum factor of safety will be computed by tuning function with the simulated annealing method，which is actually equivalent to a lower bound approach. For locating the critical failure surface，the particle swarm or the harmony method is used，which is an upper bound approach. In addition，the problem of convergence in evaluating the factor of safety is investigated；and it is demonstrated that interslice force function f (x) cannot be arbitrarily assigned if consistent and acceptable internal forces are required. The presented approach has a further advantage that convergence during the stability analysis is greatly reduced which is not possible with all the other existing“rigorous”methods，and an interslice force function is not required in the analysis.

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WAVE-INDUCED DYNAMIC RESPONSE OF SEABED AROUND SUBMARINE PIPELINE

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2008, 27 (4): 789-789 doi:

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The evaluation of wave-induced excess pore water pressure and effective stresses are recognized as an important factor in the design of submarine pipelines. In most previous investigations for the seabed-pipeline interaction，the effect of pipeline on dynamic response of seabed has been usually ignored；and nonslip condition is mainly assumed at the interface between pipeline and seabed soils. The effect of full interaction between seabed and pipeline has not been considered in the analyses of stresses and deformations of the seabed and submarine pipeline. Furthermore，the inertia effects of both accelerations of seafloor soils and pipeline on response of seabed have not been taken into account in most previous investigations. The governing equations of the seabed and pipeline are formulated respectively based on the Biot¢s theory of dynamic consolidation of two-phase media and principle of elastic dynamics. The seabed-pipeline interaction problem is investigated by using the friction contact theory；and numerical formulations based on FEM are established. Based on the computed results，it is found that the soil-pipeline interaction plays an important role in effect stresses of the seabed around pipeline；and the wave-induced excess pore water pressure along the pipeline surface is significantly affected by the shape of the trench layer in the vicinity of the bottom of the pipeline.

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EXPERIMENTAL STUDY ON MECHANO-HYDRO-CHEMICAL COUPLING PROCESS IN CRACKED ROCKS

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2008, 27 (4): 796-796 doi:

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Rock specimens of red sandstone rocks taken from Longyou Grottoes in China¢s Zhejiang Province are obtained that contain one or two preexisting flaws. Erosive effects of water chemistry of cracked rocks are studied under different chemical solutions and the same velocity. Variations of the mass and wave velocity of the specimens and pH values of solutions with sandstone being soaked are obtained；and the erosion characteristics are analyzed. Based on the results of uniaxial compressive test under different chemical solutions and velocities，the effects of strength and deformation and main modes of crack coalescence are analyzed according to uniaxial compressive stress-strain curves for different cracked rocks under chemical corrosion. The studies indicate that the specimen quality variations have not relations with chemical solutions. The pH value in the chemical solutions ultimately tends to neutral pH，and the elastic wave velocity ultimately almost does not change with time. Preexisting crack arrangement，the pH value and velocity of the chemical solutions all have great influences on the sandstone samples¢ uniaxial compressive strength. Cracked red sandstone sample are tension destruction under uniaxial compression；and the crack propagation direction are consistent with the loading direction. The modes of crack coalescence for specimens containing 1- or 2-flaw are tension and tension-shear modes. Immersion velocity and the chemical solution have not influence on the modes of crack coalescence. The research result provides understanding of the fundamental nature of rock failure and damage mechanics models for simulating nonlinear evolution of tested rock under coupled mechano-hydro-chemical processes.

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DAMAGE MODEL FOR UNSATURATED STRUCTURAL LOESS UNDER CYCLIC LOADING

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2008, 27 (4): 805-805 doi:

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Based on the boundary surface elastoplastic model，the cemented damage theory and the basic Barcelona model for unsaturated loess，an elastoplastic damage model for unsaturated loess under cyclic loading is proposed. Firstly，the law for bonds degradation is given，where the structure damage and the accumulation of absolute values of strain increments are combined. Then，the water-retention curve is employed to consider the coupling of suctions and stresses under constant water condition. Finally，triaxial tests under cyclic loading are carried out；the comparison between experimental results and model simulations shows that the proposed model is capable of describing mechanical behaviors of unsaturated loess under cyclic loading.

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PHYSICAL SIMULATION STUDY ON DEFORMATION AND FAILURE RESPONSE OF AN ANTI-INCLINED SLOPE DURING EXCAVATION

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2008, 27 (4): 811-811 doi:

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Based on geologic structure model and the base contact friction experiment method，the deformation and failure response of the anti-inclined slope during excavation with different gradients are studied. The results reveal the depth of the unloading zones of slope and further development with the slope excavation. Furthermore，to verify the reliability of the physical simulation results，numerical simulation is conducted for comparison. The results show that during excavation，the depth of the slope intensively unloading zones is 30–40 m；and the depth of the relaxation belt of base is 20–25 m. It is found that the fracture intensity and plastic zone distribution are decreased with the increase of slope angle under the specific situation of original intensity relaxed slope. These achievements are helpful for the slope mode optimization and design of the slope reinforcement.

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RESEARCH ON NONLINEAR DYNAMIC WARNING FOR HIGH SLOPES BASED ON PROGRESSIVE EVOLUTION PROCESS

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2008, 27 (4): 818-818 doi:

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Aiming at the warning issue of high slopes，the changing characteristics between the slices through the shear stress relaxation of rocks and soils on the sliding surface are performed considering the inhomogeneity of geological body and the progressive evolution features of slopes，which are based on the improved Bishop slicing model. According to the friction law of a single variable，the single slice bifurcation evaluating model by combining the friction law with the simplified slicing dynamic model of the slope，which is defined by shearing stress of the sliding surface，sliding velocity and the displacement of the slope，is built up. Introducing the adiabatic approximation hypothesis from synergetics，the bifurcation model of slope displacement prognosis is constructed. By introducing the extreme condition of the velocity changing of slopes，the nonlinear evolution formulae to describe the process of unstable failure of slopes are presented；and then，the way from a single variable analysis of the systematical prognosis of slopes is proposed. Meanwhile，the chaotic dynamic evolution and early warning criterion of slopes are both put forward according to the condition of target point. By applying the prognosis model and early warning criterion to a certain high slope of Meihe expressway in China¢s Guangdong Province， the reliability of the models is testified；and the conclusion that the prognosis accuracy near tension crack points is higher than those of other locations is obtained. The achieved results can provide a new way for early warning of slope safety.

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STUDY ON DYNAMIC RESPONSE OF PILE BASED ON COMPLEX STIFFNESS TRANSFER MODEL OF RADIAL MULTIZONE PLANE STRAIN

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2008, 27 (4): 825-825 doi:

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Most of the studies on the dynamic response of pile assumed that the soil surrounding the pile are homogenous in radial direction in the past decades，and fewer researchers took the inhomogeneity into account. Moreover，the inhomogeneous model of soil used in these studies may not be suitable for the fact according to some special researches. A new model for the analysis of pile axial response to harmonic loading is presented. Firstly，the soil is assumed to be viscoelastic and radially inhomogeneous. The surrounding soil is divided into two regions：the outer region is homogenous and extend far infinitely，the inner region is divided multizone of which parameters can be considered linearly or nonlinearly. By utilizing the plane strain model and combining the boundary condition，complex stiffness at the interface of pile and soil is derived by complex stiffness transfer method from outer zone to inner zone. Then，the pile is modeled one-dimensional elastic theory valid for long straight bar. Through mathematical derivations，the expression of the impedance function and frequency response function at pile top are derived. Subsequently，the relevant responses in time domain are obtained by the convolution theorem and the inverse Fourier transform. Finally，the selected numerical results are obtained to investigate the influences of the changes of the soil parameters caused by construction on the dynamic responses of pile in frequency and time domain；and it is proven that the new soil model is reasonable. Moreover，the soil model considering the soil inhomogeneity is fitting for the engineering practice and can provide new theoretical support for the aseismic design and pile integrality testing.

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STUDY ON NONLINEAR VISCOELASTO-PLASTIC CREEP MODEL OF ROCK AND ITS PARAMETER IDENTIFICATION

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2008, 27 (4): 832-832 doi:

[PDF] 247 KB (2806)

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Based on the mechanical behaviors of rock at the accelerating creep stage，a nonlinear coefficient of viscosity component is proposed. By using this proposed component，a new nonlinear visicoelasto-plastic creep model of rock is put forward；and the model can describe the three stages of rock nonlinear creep. Under certain condition，the model can be disintegrated into Burgers model or Nishihara model. The creep equation of rock at the constant stress is deduced；and the nonlinear creep properties of rock are studied in accordance with the proposed nonlinear creep constitutive model. Creep experiments are carried out for sandstone and marble specimens from left bank high slope of Jinping First Stage Hydropower Project by using the rock servo-controlled rheology testing machine；and the typical triaxial acceleration creep curves of rock are obtained. A new BFGS-LSM algorithm is brought forward by combining the quasi-Newton algorithm(BFGS) with the least square method(LSM). Throughout this algorithm，the exact solution can be fast approximated，which is not easily converged to the local minimum. The algorithm is adopted to investigate the triaxial creep tests of rock；and the related creep parameters of rock specimens using the proposed model are identified. The result shows that the new nonlinear viscoelasto- plastic creep model of rock accords better with the creep test curves.

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EXPERIMENTAL STUDY ON RHEOLOGICAL BEHAVIORS OF WANZHOU RED SANDSTONE IN THREE GORGES RESERVOIR AREA

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2008, 27 (4): 840-840 doi:

[PDF] 444 KB (1788)

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Many approximately horizontal slip surface landslides are found in Jurassic red strata in Wanzhou of the Three Gorges Reservoir area. However，the formation mechanism of these landslides is not clear. In order to discover the causes of these landslides，experiments on rheological behaviors were performed on the sandstone of Erchengyan landslide bedrock in Wanzhou with triaxial creep test machine RLM–2000. Before the specimen failure，there is an accelerated creep stage，which lasts for about 8 h；this course is similar to the failure of deformation of some approximately horizontal slip surface landslides. Tautochrone of test data indicates this kind of rock is close to linear viscoelasto-plastic materials. Burgers model is used to fit the curves；and the three-dimensional constitutive relationship of Burgers model and its parameters are obtained. The results show that the proposed rheological Burgers model can exactly express the rheological characteristics of Jurassic red strata in Wanzhou. It can also describe the mechanism of the approximately horizontal slip surface landslides occurring in Jurassic red strata in Wanzhou. There is a nearly linear relationship between the creep strain rate and time in the beginning of accelerative creep stage. And the creep strain rate behaves closely to exponential growth with time in the last stage of accelerative creep. The accelerative creep shows highly nonlinear characteristics；and the creep strain rate displays a discontinuous form，which occurs in jumps change. Furthermore，the experimental results reveal the long-term strength of the rock is only 44 percent of the peak strength in triaxial tests. This conclusion is very helpful for the research on the mechanism and prevention of the horizontal slip surface landslides.

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STUDY ON FORMATION MECHANISM AND STABILITY EVALUATION OF BADASHAN LANDSLIDE GROUP IN QINGHAI PROVINCE

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2008, 27 (4): 848-848 doi:

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Based on field investigation，indoor test and related studies of Badashan landslide group，the basic characteristics and formation mechanism are elaborated. According to different presupposed working conditions，the stability is calculated and evaluated by adopting limit equilibrium analysis method and numerical simulation. At the same time，landslides H3，H4 and H5，which are closely related with tower ground stability of 750 kV transmission line from Guanting to Lanzhou are mainly considered. The results show that coefficients of integral stability of landslides H3，H4 and H5 are all greater than 1.10，and therefore these landslides are under steady state in natural state. As a result，stabilities of towers No.20，21 and 22 can be satisfied. Under rainstorm state，stability coefficient of landslide H3 is slightly less than 1.05 and therefore it is under metastable state. In addition，landslides H4 and H5 are under stable state. As a result，stabilities of towers No.21，22 except for 20 can be satisfied. Under seismic loading with magnitude VIII，stability coefficients of landslides H3，H4 and H5 are all less than 1.0 and therefore they are unstable. As a result，stabilities of towers No.20，21 and 22 can not be satisfied. What¢s more，sensibility analysis of stability influencing parameters on landslides H3，H4 and H5 is carried out. The analytical result shows that internal friction angle of landslide mass is the most sensitive factor that influences the landslide stability. Finally，relevant engineering strengthening measures are provided based on formation mechanism；and quantitative evaluation results of the landslides group to guarantee safe operation of the transmission line are presented. Therefore，the results are of great significance in practice.

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INFLUENCE OF SEEPAGE ON STABILITY OF EARTH-ROCK DAM IN PLAIN AREA UNDER SEISMIC LOADING

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2008, 27 (4): 858-858 doi:

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The soil layers with liquefaction characteristics can be often found in the foundation of earth-rock dam in plain area. The soil liquefaction under seismic loading and its influence on the stability of the dam are important issues. The Erwangzhuang reservoir dam，a typical earth-rock dam in plain area，is chosen as an example to study the problem. Based on the indoor and in-situ tests，the liquefaction possibility of the soil layers，the dynamic properties of soil and the change of seepage field in the soil under earthquake are all concerned. The in-situ test results show that under seismic loading with magnitude 7，some in-situ soil layers could be liquefied. The results of indoor model tests show that the seepage field in the soil is changed under vibration；but the change of seepage field is not obvious. Utilizing the soil parameters obtained by indoor tests，the dynamic numerical analysis is performed with/without considering seepage. The numerical results show that the depth of liquefied zone agrees with the results of tests. The size of liquefaction region in the foundation is affected by the distribution of soil layers remarkably；yet the seepage influence is not obvious. When considering seepage，the factor of safety of upstream increases and the factor of safety of downstream will decrease. The influence of the seepage on the horizontal displacement can also be found at the same time.