Weiya Xu

Experimental Researches on Hydro-Mechanical Properties of Altered Rock Under Confining Pressures

Altered rock, as the abutment materials of Xiaowan Hydropower Station in China, is a kind of geological defective rock mass. It is loosely structured and its strength is low, with some development of pores and cavities. Research on the hydro-mechanical coupling of the altered rock are of important significance to hydropower projects. In this study, the advanced fully automatic triaxial fluid flow-rheological test servo system is employed to study the hydro-mechanical coupling characteristics of the altered rock, and the water pressures and confining pressures in the laboratory tests are set to simulate the conditions of excavation and impoundment of Xiaowan Hydropower Station. Based on the test results, the stress–strain laws of the rock specimens under the effect of complete hydro-mechanical coupling, as well as the lateral strain and volumetric strain characteristics, are studied. The fluid flow laws of the rock specimens and the effects of the confining pressures on the fluid flow are analyzed. The fluid flow failure characteristic under the effect of the complete hydro-mechanical coupling is discussed. The research achievements show that with the change of the stress states, the permeability of the rock also changes, and the permeability evolution shows the phase characteristic during the process of stress and strain. The impacts of the confining pressures on the strength and deformation and permeability of the altered rock are obvious. The failure behaviours of the rock specimens under the effect of coupling relates to the confining pressures, including two kinds of splitting failure and shear failure. The fluid flow failure characteristic of the rock specimens depend upon the initiation, growth and coalesce of micro-cracks, heterogeneity, confining pressures and properties of the rock.

Prediction of landslide displacement based on GA-LSSVM with multiple factors

This paper presents a new model for predicting the displacement of a landslide based on the least-squares support vector machine (LSSVM) with multiple factors and a genetic algorithm (GA) is used to optimize the parameters of the LSSVM model. First, based on original monitoring displacement data, single factor GA-LSSVM models are established with and without wavelet decomposition. Second, from the analysis of the basic characteristics of a landslide, the main influencing factors of landslide displacement are identified according to their correlation coefficients. A multifactor GA-LSSVM model is then established for the prediction of landslide displacement. A case study of a landslide reveals that wavelet decomposition can efficiently improve the prediction accuracy of the GA-LSSVM model. In addition, the multifactor GA-LSSVM model performs consistently better than the single factor models for the same measurements.

Time-Dependent Behavior of Cataclastic Rocks in a Multi-Loading Triaxial Creep Test

Weak cataclastic rocks are widely used as the host rock medium in various applications of hydraulic engineering, such as dam foundation stability, tunnel support design, and hydraulic fracturing (Habimana et al. 2002; Tsai et al. 2008; Zhang et al. 2013). Cataclastic rocks are a type of sedimentary rock with several fault zones (Elodie and Christopher 2010; Lori and James 2012). They are characterized by loose textural structure, high moisture content, and poorly cemented contact surface (Wong et al. 1997; Burgi et al. 1999; Zhang et al. 2015). A thorough understanding of the time-dependent behavior of cataclastic rocks is necessary to guarantee the safety of a structure during its lifetime (Maranini and Brignoli 1999; Fabre and Pellet 2006; Zhang et al. 2012; Brantut et al. 2013). However, because of the difficulties in collecting undisturbed samples and in performing laboratory tests, numerous phenomena of the time-dependent behavior of such rocks have not been deeply studied by researchers, such as (1) the existence of critical deviatoric stress that corresponds to the change in time-dependent strain, (2) the influence of confining pressure on the time-dependent behavior, and (3) the relationships between long-term strength and steady strain rate. The tectonically fractured zones encountered during preliminary investigations of a hydropower project in southwest China have presented a good opportunity to acquire a thorough understanding of the time-dependent behavior of cataclastic rocks. The influences of both confining pressure and deviatoric stress on the time-dependent behavior of rocks can be incorporated simultaneously in a multi-loading triaxial creep test (Li and Xia 2000); thus, a series of multi-loading triaxial creep tests with cataclastic rocks is performed in this study. The relationships among steady strain rate, deviatoric stress and confining pressure are proposed on the basis of the evolution analysis of timedependent behavior. Long-term strength is defined as the interaction point at which expansion rate is greater than compression rate.

A NONLINEAR VISCOELASTIC-PLASTIC RHEOLOGICAL MODEL FOR ROCKS BASED ON FRACTIONAL DERIVATIVE THEORY

The soft-matter element between the ideal solid and the ideal liquid is established and is described based on the definition of the fractional derivatives. By replacing a component in the generalized Kelvin model with the soft-matter component and connecting it in series with a nonlinear visco-plastic body, a nonlinear viscoelasto-plastic rheological model is proposed based on the fractional derivatives in order to describe the rheological behaviors of rocks. The data obtained from the triaxial creep tests of a typical rock are simulated with this model and the fitting result is good. The model can describe well three rheological stages of the rock during the triaxial creep tests. The validity of this model is then discussed. In this model, the fractional order β controls creep strain rate in the stable creep stage under the condition of low stress; while the creep index n controls creep rate of the accelerated rheological stage under the condition of high stress. Few parameters and good simulation results manifest the outstanding performance of the model. The model also adopts the damage theory to describe the progressive deterioration of rock viscous coefficient of the accelerated creep stage. The model can also give an excellent description of the three rheological stages of rocks, especially the accelerated creep stage.

Experimental investigation and constitutive modelling of creep-damage behaviours in monzogranite

In this study, the creep-damage behaviour of monzogranite is first investigated through triaxial creep tests under different confining pressures. Creep deformation and its rate in the whole creep process are compared and analysed on the basis of the experimental results. It is seen that the rate of creep strains is closely related to applied stress and can be described by an exponential function in terms of deviatoric stress. A discrete viscoplastic damage constitutive model is then formulated in the discrete thermodynamic framework and with micro-mechanical consideration. Comparisons between experimental data and numerical prediction show that the model can well describe the typical phenomena of creep behaviour.

Comprehensive assessment and global stabilisation measures of a large landslide in hydropower engineering

From 20 October to 5 November 2008, Gu-shui hydropower station of south-west China experienced a continuous intense rainfall. A landslide was reactivated on the Zheng-gang deposit body in the dam site area. Based on the comprehensive geological field investigations, the evolution process, formation mechanism, deformation mechanism and instability mode of the slide were analysed. The landslide is mainly composed of bedrock, slips zone soil and unconsolidated accumulation, and the slip zone soil is the transfixion slipping surface. The subsequent rainfalls increased the tension crack connectivity and the tensile strength gradually decreased. Then, the landslide is in a limit state and exhibit creep deformation. Prompt remedial works are needed due to the possibility of triggers and several stabilisation measures are evaluated. After comparison and optimisation, the most appropriate global remedial measure was proposed. Drainage measures are also considered and the landslide will be safer.

Risk analysis of building damage induced by landslide impact disaster

Assessing building damage in a geological landslide is a dynamic process in which the event can be regarded as the combined effect of the landslide impact and the building’s response. In conventional models, a landslide mass is typically assumed to be rigid, and standard energy principles are used to calculate equivalent impact loads. Because the disintegration phenomenon of a landslide mass is being ignored, the conventional method often leads to a larger impact force than observed in reality. Based on a case study of a critical landslide, this paper applies a granular discrete element method to investigate landslide processes. First, macroscopic and meso-structural parameters are calibrated through biaxial simulation tests. Second, energy conversion laws between the impact force, geotechnical strength parameters and the outrun distance of the landslide are studied, providing a more realistic load curve of the landslide impact. Finally, a distressed building model is established through a loading curve analysis to study the structural damage state with different parameters. Numerical results show that building damage can be characterised through an equivalent impact force and that the precise calculation of dissipated energy in a landslide is greatly improved by taking into account both the energy dissipation at the slipping surface and the internal friction in the rock mass. The proposed evaluation factor of the damage degree of buildings shows great promise in analysing landslide motion and assessing the safety of building structures.

Experimental analysis and modeling of the mechanical behavior of breccia lava in the dam foundation of the Baihetan Hydropower Project

As a type of pyroclastic rock, the breccia lava in the dam foundation of the Baihetan Hydropower Project is characterized by relatively low density, high natural moisture content and porosity, and lower ultrasonic velocity. When it is used as a bearing rock, its mechanical behavior will be critical for the safety and stability of the world’s second largest hydropower station. Therefore, uniaxial and triaxial compression tests were performed to study the mechanical behavior of the breccia lava and scanning electron microscope (SEM) tests were carried out to reveal the microscopic failure modes of this rock. The experimental results indicated that all critical stresses, including the crack initiation stress (σci), crack damage stress (σcd), and peak stress (σp), exhibit strong dependence on the confining pressure. Experiential functions were used to describe the evolution of the elastic modulus and Poisson’s ratio with confining pressure. Grain crushing and the growth and frictional sliding of microcracks were determined to cause the failure of the specimens. Based on the experimental results, a coupled elastoplastic damage model was proposed within a thermodynamic framework. In this model, two separate loading functions were employed to describe the damage and plasticity behavior of the breccia lava. A computational integration algorithm with high numerical accuracy and efficiency was developed to deal with the material under three different loading conditions: plasticity, damage, and coupled elastoplastic damage. The model was validated through comparison with the experimental data, and the good agreement between the two datasets confirms that the model can provide a good representation of mechanical behavior, particularly the post-peak behavior of the breccia lava.

Dependency of hydromechanical properties of monzonitic granite on confining pressure and fluid pressure under compression

Monzonitic granite is a low-permeability rock. Monzonitic granite formations are ideal for underground storage of oil due to their low permeability and high mechanical strength. In this study, a series of coupled hydromechanical triaxial tests are carried out using monzonitic granite specimens. The influence of confining and fluid pressures on stress, strain, and permeability is investigated. Failure characteristics under different confining and fluid pressures are discussed based on the analysis of macro fracture planes and micro scanning electron microscopy (SEM). The test results show that the change of permeability with stress and strain reflects the deformation stages of compaction, compression, crack propagation, coalesce, and failure of cracks. Due to the low porosity, the change of permeability is small in the initial phases of compaction and compression, whereas there is a significant increase in permeability when new cracks start to develop and coalesce. Confining pressures have a significant impact on the strength and permeability, particularly the crack damage stress of the rock. Compared with confining pressure, the effect of fluid pressure on rock strength and crack damage stress is small. For the monzonitic granite specimens tested, changing the confining pressure results in different failure modes, whereas the fluid pressure has a relatively small effect on the failure modes.

Multiscale Study of the Nonlinear Behavior of Heterogeneous Clayey Rocks Based on the FFT Method

A multiscale model based on the fast Fourier transform (FFT) is applied to study the nonlinear mechanical behavior of Callovo-Oxfordian (COx) argillite, a typical heterogeneous clayey rocks. COx argillite is modeled as a three-phase composite with a clay matrix and two types of mineral inclusions. The macroscopic mechanical behavior of argillite samples with different mineralogical compositions are satisfactorily predicted by unified local constitutive models and material parameters. Moreover, the numerical implementation of the FFT-based nonlinear homogenization is easier than direct homogenization, such as the FEM-based homogenization, because it automatically satisfies the periodic boundary condition.