Papers by Author: Xia Ting Feng

Abstract: Tunnel collapse causes serious casualties and economic losses. One typical case analysis of a collapse in deep-buried tunnel based on microseismic monitoring is presented. The results show that the number of microseismic event keeps increasing and the distribution of microseismic events becomes concentrated in space domain gradually during collapse nucleation process. And average distance squared decreases gradually during the imminent period time just before the collapse. The failure evolution mechanism of the collapse is analyzed by moment tensor method. It is noted that the failure mechanism between this kind of collapse and immediate strain-structure rockburst is similar. However, the proportion of shear and mixed fracture for collapse is higher than immediate strain-structure rockburst. It due to the rich structure planes in collapse zone.

Abstract: it is already confirmed that the rock mechanical characteristics under unloading condition differ from those under load condition, and the excavation process should be considered to be a typical kind of unloading mechanical behavior. As thus, a series of representative unloading tests and shear strength tests about unloading cracks are performed on hard granite samples from the underground workshop excavation zone in Dagangshan hydropower station in western China, where the surrounding rock is affected by high geo-stress. Based on the tests results, some significant theory research is made.

Abstract: At present, waste rubber tires cause a significant environmental and human health problem. An innovative way to recycle used tires is that the waste tire rubber is used in Portland cement concrete as a portion of aggregates. In this paper, the crumbed tire rubber particles were used to replace fine aggregates, which includes two grain sizes, from 3 to 4 mm and 50-mesh.. The replacement levels were 0, 5, 10, 15, and 20 % by volume of the fine aggregates. Rubber concrete mixtures were produced according to mixture proportions design. The slump of fresh concrete and the unit weight of the nine mixes were examined. The mechanical properties investigation of harden concrete included determining the 28 days compressive strength and dynamic modulus of elasticity. The study demonstrated that 5 and 15 vol% replacements with rubber powder can meet fundamental engineering demands, although substituting part of fine aggregate with fine rubber powder deteriorated mechanical properties of concrete.

Abstract: Pavement straw composite fibers (PSCF), a new material, were independently invented and developed by blending agriculture waste straw and modified bentonite, as showed in this paper. The chemical corrosion properties of PSCF in different pH values were test in microscale, and digital imaging processing technology was adopted to analyzing quantitatively the physical and mechanical properties, including particle area, pores connectivity rate and compressive strength, before and after PSCF were corroded. Research results show that the particle area of acidified PSCF is smaller than that of alkalized PSCF, however, the fractal dimension of acidified PSCF is bigger than that of alkalized PSCF. The change of PSCF microstructure has significant effect on its macromechanics properties. The compressibility of acidified PSCF was obviously bigger than that of alkalized PSCF, so the strength of acidified PSCF decreased significantly with the acidifying time increasing. The main reason was gradually decreasing of the particle area, decreasing of the fractal dimension and the loose tendency of fibers particle distribution. The interaction between mineral composition of PSCF and hydrochemistry solution was the basic factors determining straw structural characteristics, and the physical and mechanical properties of PSCF were affected by its structural characteristics. The main process was that mineral compositions of PSCF were cemented by the corrosive action of hydrochemistry solution, or some compositions were dissolved and precipitated, the porosity and particle area of PSCF, the arrangement pattern of soil particles as well as other microstructures changed. And deformation properties of PSCF changed correspondingly with the change of PSCF structure shape. Moreover, the constitutive model of PSCF with effect of acid and alkali was presented according to test data of digital image. It provided reliable techniques to quantitatively evaluate the pavement performance of PSCF in micro and macro scales. Furthermore, the chemical corrosion theory was improved, and the reference for constitutive model research was provided.

Abstract: Surrounding rock mass stability is one of the key technical problems in the design of the diversion tunnels in Jinping II Hydropower Plant Project. The major difficulties lie in three facts: (1) high stress induced by the great depth; (2) the brittle failure characters of marble; (3) the interaction of these four tunnels. A systemic procedure is introduced in this paper. Firstly, the integration of the code FLAC3D and the multivariate linear regression method is adopted to back analyze the in situ stress field. Then the mechanical parameters of the surrounding rock mass are back analyzed based on the PSO (Particle Swarm Optimization) algorithm and the code FLAC3D. The stress release method is adopted in the numerical analysis of the excavating and supporting process of tunnels by the code FLAC3D. Finally, the multi indexes integration method is presented to analyze and evaluate the stability of the tunnels and to validate the rationality of the design scheme.

Abstract: This paper proposes two concepts in order to estimate the stability of surrounding rockmass of deep tunnel, one is YAI (yielding approach index) which is used to denote the stability difference among the zones in surrounding rockmass whose stress states are close to yielding, the other is FD (failure degree) which focuses on the estimation of failure degree of the zones in surrounding rockmass and can be calculated by the equivalent plastic strain when failure occurs. Finally, a numerical analysis on the excavating process of a deep tunnel is performed, and YAI and FD are used to estimate the stability states of rockmass. The accurate estimation on plastic field and the proper evaluation on stability state indicate that these two concepts are rational and of perfectly practical use.

Abstract: Multiple-cable single borehole anchorages are widely used in China, accepted and regarded as a very effective method for slope protection, landslide remediation and foundation ditch support. This paper presents a new method for testing tensional stresses of individual steel strands within anchor cables in single boreholes. A rig has been designed to permit testing of cables within a single borehole, including two special features: a metal plate that distributes tension between strands of a cable and a sensing device that measures the stress in each individual strand. The feasibility of the method has been demonstrated by an in situ tensional stress test on anchor cables at a landslide remediation site that successfully measured the forces on individual steel strands fastened to different anchorage segments within a borehole. The steel cable strands displayed nonlinear extension characteristics; and the length of anchorage segments and grout strength met design criteria. The results confirm that this design of anchorage is satisfactory and can provide valuable data for future design and construction.

Abstract: In this paper, the experimental study on the mechanical property of limestone under triaxial compression with different hydrochemical environments is conducted and the non-linear characteristics of complete stress-strain process of limestone subjected to chemical corrosion, are analyzed. The behaviors of deformation and strength of limestone eroded by different chemical solutions are obtained. It is known from the experimental and analytical results that different chemical environments such as chemical composition, pH value etc can affect the mechanical property of rock differently. How to establish a multifactor characteristic constitutive model which can reflect different chemical environments is the key problem to study the coupling corrosion effect of stress and chemistry of rock. The constitutive model of evolutionary neural network for rock under chemical corrosive environments is put forward, and the neural network constitutive relationship under stress-chemistry coupling corrosion is established by applying the theory of evolutionary computation and neural network. The neural network constitutive model established in this presented paper mainly takes into account the following three aspects: chemical environments of rock specimens, content of main mineral compositions that are liable to being eroded by chemical solution, and mechanical environments. The results are in good agreement with the experimental data.

Abstract: In this paper, a new concept, YAI (yielding approach index), is firstly proposed to estimate the safety differences of rock and rockmass whose stress states are close to yielding. Secondly, in order to describe the intrinsic heterogeneity of strength of the same rock, the safety parameter ω, which is the phase complementary parameter of YAI, is supposed to be a stochastic variable in this paper and conform to the Weibull’s distribution. By analyzing the triaxial test data of five types of rocks, it can be found that ω as a stochastic variable evidently conforms to the Weibull’s distribution. Results in this paper give an important and useful reference to the safety analysis for geotechnical engineering.

Abstract: A series of numerical experiments are conducted by the Elasto-Plastic CA model, which was successfully used to simulate the rock failure process under uniaxial compression in previous work by the authors, to obtain the failure processes of heterogeneous rocks as well as the stress-strain relation and strain-AE relation under tensile loading at meso level. The model can consider the heterogeneity of the materials conveniently, and has the advantages of localization, parallelization etc. By constructing some local simple rules, the model can perfectly simulate the self-organization process of rock failure process. In this paper, the domain is discretized into the system composed of cell elements which are assumed to conform to the constitutive laws of elasto-brittle-plasticity. The Weibull’s stochastic distribution is introduced to represent the heterogeneity of rock materials, and Mohr-Columb criterion with tension cut-off is considered as the yield criterion. The process of crack initiation, propagation and coalescence is well simulated and the results obtained reproduce the main features known of rock behavior, both at meso and overall stress-strain levels.