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2010, 29 (05): -0 doi:
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DESORPTION CHARACTERISTICS OF ADSORBED GAS IN COAL SAMPLES UNDER COUPLING TEMPERATURE AND UNIAXIAL COMPRESSION

HE Manchao1，2，WANG Chunguang1，2，LI Dejian1，2，LIU Jing1，2，ZHANG Xiaohu2

2010, 29 (05): -872 doi:
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[PDF 468 KB] (1361)

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The desorption process of adsorbed gas in coal from Nanshan coal mine is studied using an experimental system with temperature-pressure coupling in particular to treat the multiphysical effect on deep coal sample. This experimental system has been designed to apply triaxial pressure and temperature to coal sample，and reproduce the desorption-releasing process of absorbed gas as a result of combination of pressure and temperature. Uniaxial compression tests are carried out on coal samples under constant temperature and increasing temperature，respectively. After failure，the deformed samples were subjected to confining pressures. In test，the releasing gas pressure and flow are real-time monitored. The mixed gases is sampled and detected by using a gas chromatography. The results show that a spontaneous negative value observed in gas pressure is consistent with the refluence of gas determined from the volumetric dilatancy development. A large amount of gas is expelled during the closure of fracture due to the present of the confining pressure. Correspondingly，the concentrations of methane，carbon dioxide and ethane are increased sharply. Temperature change is one of desorption-induced factors for adsorbed gas. The existence of a large number of connected cracks in coal matrix may play an important role in gas transport. The mechanism of gas emission is also preliminarily discussed.

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EXPONENTIAL LAWS OF CRITICAL DISPLACEMENT EVOLUTION
FOR LANDSLIDES AND AVALANCHES

QIN Siqing，WANG Yuanyuan，MA Ping

2010, 29 (05): -880 doi:
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[PDF 321 KB] (1542)

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Two universal exponential laws for critical displacement evolution of landslides and avalanches are put forward according to the locked patch concept and renormalization group theory in a new viewpoint. It is found that the critical instability displacement of slopes has a dependency on the displacement at the onset point of accelerating creep and the number of locked patches. The first law is suitable for the analysis of brittle failure of slopes，such as rock avalanches，rock-falls and rock toppling failures. The second law can be applied to the prediction of creep failure，such as rockslides，colluvial-deposit landslides and loess landslides as well as clay landslides with locked patches. The two laws have wide suitability and applicability by analyzing a few typical examples of landslides and avalanches. The suggested method could be hopefully applied to the medium-term，short-term and critical-sliding prediction of landslides and avalanches in the future.

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ANALYTICAL ALGORITHM TO ESTIMATE TENSILE MODULUS IN BRAZILIAN DISK SPLITTING TESTS

GONG Fengqiang1，2，LI Xibing1，ZHAO J2

2010, 29 (05): -891 doi:
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[PDF 541 KB] (2214)

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An analytical algorithm to estimate the tensile modulus of Brazilian disk splitting tests is presented. Combined with the theoretical elastic solution of disk on cardiac force and the physical parameters obtained in actual measure of experiments，the total displacement of deformation on the direction of the vertical load direction can be obtained by the integral of strain of every point on this direction based on the principle of calculus；and the quantitative relationship between tensile modulus of rock specimen center and the total displacement of deformation on the direction of the vertical load is derived. Five physical parameters，including the loading power，sample diameter，sample thickness，rock Poisson's ratio and the total displacement of deformation on the direction of vertical load direction，are used in this quantitative relationship. This quantitative relationship has a clear meaning and easy to use；and it can provide a new approach to estimate the tensile modulus in Brazilian disk splitting tests.

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EXPERIMENTAL RESEARCH ON MECHANICAL PROPERTIES OF WATER-COOLED GRANITE UNDER HIGH TEMPERATURES WITHIN 600 ℃

XI Baoping1，2，ZHAO Yangsheng1，2

2010, 29 (05): -898 doi:
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[PDF 352 KB] (1508)

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According to the experimental research on mechanical properties of water-cooled granite samples under high temperatures within 600 ℃，the degradation mechanism of thermal cracking of water-cooled granite is discussed. The results show that during water cooling process under high temperature，thermal cracking and thermal shock are found in granite samples due to drastic changes of temperature；and the mechanical properties of rock mass are degraded when temperature decreases gradually，such as supersonic wave，uniaxial compressive strength，tensile strength and elastic modulus. The following results are also presented：(1) when granite is cooled from high temperature，the supersonic wave is of negative exponential law with temperature；(2) the peak stress，the peak strain and the uniaxial compression strength of water-cooled granite are greatly affected；(3) high temperature with water cooling treatment has great influence on the tensile strength of granite；and (4) the elastic modulus with increasing temperature has negative logarithmic law for water-cooled granite samples.

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COMPARATIVE RESEARCH ON PERMEABILITY CHARACTERISTICS IN COMPLETE STRESS-STRAIN PROCESS OF BRIQUETTES AND COAL SAMPLES

CAO Shugang，LI Yong，GUO Ping，BAI Yanjie，LIU Yanbao

2010, 29 (05): -906 doi:
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[PDF 239 KB] (1384)

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Triaxial seepage tests of briquettes and row coal specimens are implemented by self-developed triaxial seepage test system. The complete stress-strain curves under different confining pressures are obtained. Seepage velocity and transverse deformation in the whole test process are automatically collected by flowmeter and ring extensormeter. The different destructional forms of two types of samples are analyzed by microscopic damage mechanics theory；and the relation of seepage velocity and the process of deformation and destroy is presented. The sensitivity of seepage velocity is effected by the external variables；and the coal and gas outbursts are also discussed. The results show that the complete stress-strain curves of two kinds of samples could be divided into five stages. It has a good relationship with the curve of seepage velocity-axial strain. Due to different structural characteristics，the two types of coal specimens have different damage mechanisms under axial pressure. Seepage velocity-axial strain curves have great difference，especially in the destructional phase. The deformation of briquettes will affect seepage velocity in the first two stages，but it will have certain effects on coal in the whole compression process. The seepage velocity of briquettes is sensitive to axial pressure and axial strain，but it is sensitive only to transverse strain and volume strain for row coal. The five stages in the compression stress-strain- seepage curves of row coal could be explained by four stages(preparation，initialization，development，and completion) of coal and gas outbursts processes. The variety of seepage velocity in coal bed could be used for forecasting coal and gas outburst indirectly. The results have a certain referential value for exploring the real laws of gas migration.

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DEVELOPMENT AND APPLICATION OF TRIAXIAL SERVO- CONTROLLED SEEPAGE EQUIPMENT FOR THERMO-FLUID-SOLID COUPLING OF COAL CONTAINING METHANE

XU Jiang1，PENG Shoujian1，YIN Guangzhi1，TAO Yunqi2，YANG Hongwei1，WANG Weizhong1

2010, 29 (05): -914 doi:
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[PDF 440 KB] (1556)

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A self-developed triaxial servo-controlled seepage equipment for thermo-fluid-solid coupling of coal containing methane is introduced. It is composed of servo-controlled loading system，triaxial compression chamber，water bath with thermostatic control system，interstitial pressure control system，data measuring system and supporting system. The maximum axial pressure is 100 MPa；and the maximum confining pressure is 10 MPa. The heating temperature can reach as high as 100 ℃；and the dimensions of tested cylinder samples are f50 mm×100 mm. There are some advantages of the seepage equipment：(1) Tests performed on this equipment can show the comprehensive influence of geostress，gas pressure and temperature up on permeability of coal containing methane. (2) Many kinds of load forms can be completed on this servo-controlled system. (3) Plane-charging can be carried out for the coal sample，which reflects the actual gas source situation of coal bed. (4) An orientation system is designed to insure the accuracy of the data achieved in the tests. (5) By using the sensors，the data are more sensitive and accurate. (6) A large-scale water bath with thermostat-controlled system is designed；and a water circulating pump is also installed. (7) It can be used to study permeability and deformation characteristics of coal containing methane. Experiments on coal containing methane with different confining pressures and different temperatures in the seepage equipment demonstrate that the deformation of coal containing methane has four stages；and the compressive strength increases with the increase of confining pressure. Moreover，the seepage discharge is different in each stage and declines with the increasing of temperature in general. The seepage equipment can be used to study certain scientific law of thermo-fluid-solid coupling seepage of coal containing methane，which can provide fundamental basis for studying gas movement in underground coal seam and gas suction technology.

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MICROSEISMIC MONITORING SYSTEM ESTABLISHMENT AND ITS ENGINEERING APPLICATIONS TO LEFT BANK SLOPE OF JINPING I HYDROPOWER STATION

XU Nuwen1，2，TANG Chun¢an1，2，SHA Chun3，LIANG Zhengzhao1，YANG Juying4，ZOU Yanyan3

2010, 29 (05): -925 doi:
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[PDF 1188 KB] (1992)

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Many deep cracks and faults，such as f2，f5 and lamprophyres，which may result in huge potential instable blocks，are the most significant factors that influence the stability of the left bank slope of Jinping I Hydropower Station. In order to real-time monitor and analyze microseismicity in deep rock mass of the left slope，an ESG microseismic monitoring system manufactured in Canada was installed in June，2009. It is the first time to introduce microseismic monitoring technique into high steep rock slope engineering in China. On the basis of setting up three-dimensional geological model of the left slope and optimizing the sensor emplacement，the positioning accuracy of the system has been adjusted according to artificial fixed blasting tests. The results show that seismic source location error is less than 12 m in the scope of the sensor array，which demonstrates microseismic monitoring system has a high positioning accuracy. The waveforms analysis and clustering are investigated and different types of waveforms have been distinguished. Moreover，the tempo-spatial distribution regularities of microseismic events are also analyzed. In addition，a preliminary delineation of deep rock mass deformation zone induced by microseismicity in left bank slope is provided. Furthermore，the stress field and potential slip surface of left slope are simulated by RFPA，which is corresponding to the results of site exploration and microseismic monitoring. This research indicates that the design and implementation of microseismic monitoring system can meet the global monitoring of deep rock mass deformation，which could identify the potential rock damage regions and sliding surface. It will provide a reference for grouting and reinforcement of the left bank slope. A new idea on stability analysis of high rock slopes can thus be obtained.

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RESEARCH ON CUSHIONING MECHANISM OF NEW-TYPED ENERGY DISSIPATIVE ROCK SHED

HE Siming1，WU Yong1，2

2010, 29 (05): -932 doi:
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[PDF 431 KB] (1362)

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As a kind of serious hazard in mountains，rock-falls are hard to be treated accurately in active ways for their abruptness and randomness. Currently，the most popular method is to set cushion on shed tunnel to buffer the impact passively. However，the effect is not perfect as expected and the cost may be great if adopting thick cushion. Based on flexible retaining theory，a new type of shed tunnel is proposed，which can dissipate impact energy easily by installing shock absorber. Then by modeling the new structure as a mass spring system，the cushioning mechanism and protection effect of new structure are studied with energy theory. Finally，an example is given to check the presented approach. The result shows that the rock impact can be dissipated well by energy dissipative devices，and the maximum impact force on the new structure is smaller. More important，the investment of construction is reduced greatly.

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RESEARCH ON RIVER-BLOCKING MECHANISM OF XIAOJIAQIAO LANDSLIDE AND DISASTERS OF CHAIN EFFECTS

XU Wenjie1，2，CHEN Zuyu2，HE Bingshun2，SUN Tao3

2010, 29 (05): -942 doi:
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[PDF 685 KB] (1421)

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Earthquake is considered to be one of the main factors of the generation of a landslide. Especially in high mountain gorge areas，the huge landslide body inrushes into the river and barriers the river，which may further expand the disaster areas and form a disaster chain. Taking Xiaojiaoqiao landslide for example，the three-dimensional(3D) instability process of the landslide is reconstructed by using dynamic finite element method based on detailed geologic investigation results of the landslide area. With the analytical results，the forming process and river-blocking mechanism of Xiaojiaqiao landslide can be divided into four phases：(1) earthquake triggering and progressive failure of the rock mass；(2) landslide body failure and sliding with high speed；(3) impacting the opposite slope，being disintegrated，and blocking the river and forming the landslide dam；and (4) density under the impacting. Based on the geological structure characteristics of the landslide dam and the stability analytical results under different cases，the stability of the slope of landslide dam is good under natural state，but there may induce local collapses with strong earthquake；and the most possible failure model may be the gradual erosion damage of topping. As for the internal rock structure with general stability，there is little possibility for piping failure and dam-breaking as a whole.

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SHEAR WAVE VELOCITY-BASED APPROACH FOR EVALUATING GRAVEL SOILS LIQUEFACTION

CAO Zhenzhong，YUAN Xiaoming

2010, 29 (05): -951 doi:
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[PDF 523 KB] (1368)

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Although the in-situ shear wave velocity tests have been used commonly as engineering testing techniques and shear wave velocity Vs has gradually become a basic index for soil liquefaction evaluation，the in-situ Vs-based liquefaction assessment methods for gravel soils are not available so far. The significant phenomena of gravelly soil liquefaction in the 2008 Wenchuan 8.0 Earthquake are investigated and the in-situ Vs structures for 45 sites in the event are obtained. In terms of the investigation data，the Vs-based approach for liquefaction evaluation of gravel soils is presented and the existing Vs-based methods for evaluating sandy soil liquefaction are inspected. The liquefaction discrimination of gravel soils can be divided into the two steps：the preliminary evaluation and further evaluation. In the preliminary step the geological ages，the buried condition of gravel soil layer and gravel contents of gravel soils are considered and five parameters including seismic intensity，reference values of Vs with depth of gravel soils，groundwater table and gravel content of gravel soils are concerned in the further step. The reference values of Vs as well as the influential coefficients of gravel soil depths and groundwater levels are deduced by normalization method and optimization method respectively. As the type of gravel soils differs from the sandy soils，the densities of two type soils even for the same Vs value are different. If the existing liquefaction assessment procedures for sandy soils are employed，the liquefaction resistance of gravel soils will be significantly overestimated，which means that the methods of liquefaction evaluation for sandy soils are not suitable for gravel soils. The proposed method is noticed with clear expression and high success ratio of regression discrimination；and it can easily be used in engineering practice and code revision.

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MODEL TEST RESEARCH ON GLOBAL STABILITY OF LEFT BANK HIGH SLOPE OF JINPING I HYDROPOWER STATION

LIU Li，CHEN Rui，QIAO Gaoqian

2010, 29 (05): -959 doi:
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[PDF 451 KB] (1397)

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The model test method employed in Jinping I Hydropower Station on the whole high slope is presented to evaluate the actual effects. A new upside load with bottom dynamic load test method for the slope modeling is also proposed to simulate and analyze the stability of high slope under upwards normal load，the dynamic impact effect under blasting wave；and the failure process and the safety degree of slope under overload are also given. The test results show that the deformation of high slope engineering is small without slope failure under the normal load；and there is no crack and thus the slope will not fail. In addition，with reinforced structures under bottom with dynamic blasting load at construction site，the slope will also not fail，but slope cracks and possible failure are traced mostly outside the reinforced zone of slope under increasing overload ultimately. The complete processes of deformation and failure are verified by the test. The stress distribution，deformation characteristics and failure trend of the high slope are achieved. The deformation and failure caused by the maximum principal stress in slope are also simulated. The geostress effect and mechanical mechanism under unloading are studied. The results show that reinforcements adopted in the slope are effective and acceptable，and the global high slope is stable during construction. The further techniques for slope safety are therein suggested：anchorage structures should be highly effective，the slope rock damage should be avoided，and field monitoring and information feedback should be implemented perennially in later period for slope engineering. It can be found that the model test is also effective for other high slope projects.

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EXPERIMENTAL INVESTIGATION ON CREEP BEHAVIORS OF HARD ROCK IN DAM FOUNDATION AND ITS SEEPAGE LAWS DURING COMPLETE PROCESS OF ROCK CREEP

WANG Rubin1，2，XU Weiya1，2，WANG Wei1，2，ZHANG Zhiliang1，2，ZHANG Yu1，2

2010, 29 (05): -969 doi:
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[PDF 358 KB] (1350)

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Rheological experiments of hard metamorphic-volcanic breccia specimens in dam foundation under triaxial compression with seepage pressure are carried out on the advanced rock servo-controlled triaxial rheology testing equipment. Based on experimental results of hard rocks，the creep properties of metamorphic-volcanic breccia specimens under different confining pressures are studied；and the variation of seepage rate with time in the stage of completed process of hard rock creep is analyzed. The results show that，when the applied stress level is considerably less than fracture stress，the creep deformations of metamorphic-volcanic breccia specimens are not obvious，and the main creep form is named steady state creep. On the other hand，when the applied stress level is greater than or less than but close to fracture stress，the creep deformations and accelerative creep characteristics become more obvious. Meanwhile，cracks of metamorphic-volcanic breccia specimens appear and spread. The metamorphic-volcanic breccia specimens show significant volume expansion phenomenon under triaxial compression；and circumferential creep deformations are obviously higher than those of axial creep deformations. The seepage rate of metamorphic- volcanic breccia in stage of steady state creep is not changed with time significantly. But in the stage of creep acceleration，the seepage rate increases evidently；and the seepage acceleration at confining pressure of 2 MPa is greater than those at confining pressure of 6 MPa. The results can provide dependable reference for the establishment of rock rheological model and parameter identification.

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A ROCK DILATION ANGLE MODEL AND ITS VERIFICATION

ZHAO Xingguang1，2，CAI Ming3，CAI Meifeng1

2010, 29 (05): -981 doi:
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[PDF 310 KB] (3155)

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Conventional Mohr-Coulomb model and strain-softening based Mohr-Coulomb model often take an assumption of constant dilation angle，which can not be supported by experimental data，and it is observed that the approach is not successful in characterizing the nonlinear deformation behaviors of rocks. Based on published data acquired from modified triaxial compressive tests with volumetric strain measurement performed on seven types of rocks，a mobilized dilation angle model which considers both confining stress and plastic shear strain is developed. Based on the model response and in combination with the grain size description and the uniaxial compressive strength of the seven types of rocks，the model is generalized for four rock types：coarse-grained hard rock，medium-grained hard rock，fine-medium-grained soft rock，and fine-grained soft rock. According to the principle of nonassociated flow rule in the strain-softening models in FLAC，the relationship between the plastic shear strain in the proposed dilation angle model and characteristic plastic parameter in strain-softening model is deduced；and the proposed dilation angle model is implemented in FLAC using FISH language. The dilation angle model is used to predict the volumetric-axial strain relationships of Moura coal；and the results are found to be in good agreement with experimental results. Finally，the importance of using the confinement and plastic shear strain dependent dilation angle model for rock engineering applications is discussed.

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DYNAMIC FINITE ELEMENT ANALYSIS OF LARGE-SCALE COMPLEX UNDERGROUND CAVERNS WITH THREE-DIMENSIONAL ELASTOPLASTIC DAMAGE MODEL

ZHANG Zhiguo，XIAO Ming，ZHANG Yuting，ZUO Shuangying

2010, 29 (05): -989 doi:
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[PDF 405 KB] (1295)

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The seismic response characteristics of underground caverns are studied by employing dynamic time-history method. Based on the dynamic equilibrium equations of kinemato-dynamic time-history method and Newmark direct integration method，the computational codes are programmed by adopting three-dimensional elastoplastic finite element theory. The concepts of additional plastic load and additional damage load are considered to simulate the plasticity and damage of elements. The damage state of rock mass within each time step is inherited by subsequent time steps. Therefore，the irreversible trend of rock damage can be reflected. The incremental variable plastic stiffness iteration method is employed to solve dynamic equations. The calculation results show that the proposed iteration method has good convergence and fast computational speed. The iteration of each step can attain convergence within 3 steps. The way to setup boundary condition and the way to input dynamic loads are expounded by performing dynamic time-history analysis of large-scale complex underground caverns of Yuzixi hydropower station. The calculation results show that the computational speed of the proposed method in the codes is faster than that of common software. The calculation results are reasonable as well，which indicate that the proposed methods are rational and can be used to carry out dynamic time-history analysis of underground engineering.

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EXPERIMENTAL RESEARCH ON CREEP OF FRACTURED SANDSTONE

GUO Chenye1，XIAN Xuefu1，JIANG Yongdong1，TANG Hongmei1，2，YAO Weijing1

2010, 29 (05): -995 doi:
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[PDF 276 KB] (1178)

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In order to understand the creep characteristics of fractured sandstone，a series of creep tests of fractured sandstone from T3xj6 sandstone in Yongchuan mine are carried out by a MTS815 rock testing system；and the whole process of fractured sandstone is described with modified Nishihara model. The results show that creep law of fractured sandstone is similar to those of the general creep characteristics of coal and rock. Fractured sandstone also has long-term strength，which is determined according to the stress-strain relationship obtained by theoretical analysis. The whole process of fractured sandstone can be described with modified Nishihara model. When load stress is greater than long-term strength of fractured sandstone，unknown parameters of modified Nishihara model could be achieved with least squares method. The homogeneity，load control and leak tightness of rock samples can influence the successful creep of fractured sandstone. These results have great significance to revealing time lag of underground engineering disasters；and supporting parameters of surrounding rock mass in excavation roadway can also be determined.

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MICROSEISMIC SOURCES LOCATION METHODS BASED ON DIFFERENT INITIAL VALUES

LIN Feng，LI Shulin，XUE Yunliang，XU Hongbin

2010, 29 (05): -1002 doi:
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[PDF 317 KB] (1679)

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The characteristics of the linear location and the Geiger¢s location method are analyzed；and a new location method by combining the two location methods is given. With the multi-channel digital microseismic monitoring system in Shizhuyuan mine，the blasting with three location methods is proposed；and the results by making a blasting test at the assigned location are compared. Some conclusions are drawn as follows：the combination of the linear location and the Geiger¢s location method can solve the problem of low accuracy for the linear location；and the choice of initial value for the conventional Geiger¢s location method can be optimized. In addition，the efficiency of the iterative solution can be improved；and the singular result with an inaccurate initial location value when singular matrix happens in the locating process with the Geiger¢s method can be achieved.

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SPLIT HOPKINSON PRESSURE BAR(SHPB) EXPERIMENTS ON DYNAMIC STRENGTH OF WATER-SATURATED SANDSTONE

WANG Bin1，2，LI Xibing1，YIN Tubing1，MA Chunde 1，YIN Zhiqiang1，LI Zhiguo1

2010, 29 (05): -1009 doi:
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[PDF 299 KB] (1681)

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Impact compressive experiments on air-dried and water-saturated sandstone samples taken from Kaiyang Phosphate Mine with a constant length-diameter ratio of 0.5 are conducted with the modified f 75 mm split Hopkinson pressure bar(SHPB) apparatus. Compared with the static mechanical results tested by INSTRON material testing machine，the SHBP test results show that stress-strain relation of water-saturated sandstone under impact loads with intermediate strain rate is different from that under static loads；and its dynamic strength is close to that of air-dried sandstone，which is opposite to the decreased strength under static load condition. For air-dried sandstone，its dynamic yield stress under impact loads with intermediate strain rate is similar to its static yield stress. However，for water-saturated sandstone，the dynamic yield stress increases by two times compared with its static yield stress，which indicates that the sensitivity of water-saturated sandstone dynamic strength dependent on strain rate is more notable than that of air-dried sandstone. Water can influence the dynamic damage patterns of sandstone，thus the impact damage of air-dried sandstone becomes more serious than that of water-saturated sandstone. So the water-free viscosity and Stefan effect of water-saturated sandstone should be considered under impact loads with intermediate strain rate.

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TIME-DELAY ESTIMATION OF ACOUSTIC EMISSION SIGNALS OF ROCK USING TIME-FREQUENCY ENERGY ANALYSIS BASED ON WAVELET TRANSFORM

KANG Yumei1，ZHU Wancheng1，BAI Quan2，XIE Linmao1，TANG Chun¢an3

2010, 29 (05): -1016 doi:
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[PDF 259 KB] (1492)

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Since acoustic emission(AE) source location is based on the time delays of AE signals received at each sensor，the accuracy of AE source location is directly affected by the time-delay estimation of AE signals. First，based on the recognition of the waveform and frequency of AE signals using wavelet transform；the waveforms with a single(or very narrow band) frequency，which are related to AE wave with different damages，are extracted and used to estimate the time delays of the AE signals among sensors；thus a more scientific method for AE source location based on wavelet transform is established. Then，the AE signals during the failure process of rock specimen under uniaxial compression are monitored by using Hyperion AE system，and the AE sources are located by using the common arithmetic provided by this AE system. Finally，the wavelet transform-based method is employed to locate the AE source based on the time delays of AE signals estimated according to the time-frequency energy analyses of AE signals. Comparisons between this new AE source location results with those obtained from Hyperion AE system indicate that the errors of AE source location can be effectively diminished；thus it can enhance the precision for damage detection in rocks.

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FREQUENCY-SPECTRUM CHARACTERS OF MICROSEISMIC SIGNALS INDUCED BY ROOF STRATUM FRACTURE

LU Caiping1，2，DOU Linming1，2，GUO Xiaoqiang2，LIU Biao2，LU Zhenyu2，FAN Jun2

2010, 29 (05): -1022 doi:
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[PDF 390 KB] (1116)

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To evaluate and monitor the danger and intensity of roof dynamic fracture by the evolution rules of frequency-spectrum，the research on microseismic(MS) activity rules in the process of roof fracture is presented. With SOS MS monitoring system，the hard and weak roof fracture processes are measured in field. The achieved results are as follows. (1) Before the roof fracture，the amplitude of MS signals in the low-frequency band increases gradually；then it reaches the maximum value at roof fracture and pressurization；after that，the amplitude begins to decrease quickly. Simultaneously，the correlation between precursory signal main frequency and the pressurization intensity is negative. (2) Before the weak roof fracture，the main frequency of MS signals begins to move to the low-frequency band，but the main frequency of precursory signals is larger than 20 Hz. After roof pressurization，the main frequency will move to the high-frequency band. (3) The process of hard roof fracture is transient，and the amplitude of main shock signals is higher，but the main frequency is lower than 5 Hz. Amplitude in the low-frequency band before and after fracturing is lower，and the precursory effect is not obvious. With above rules，the process and intensity of roof fracture and pressurization can be evaluated and predicted effectively.

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GEOMECHANICAL MODEL EXPERIMENT ON QIANJIANGPING LANDSLIDE IN THREE GORGES RESERVOIR AREA

XIAO Shirong1，LIU Defu1，JIANG Fuxing2，JIANG Xiaolan3

2010, 29 (05): -1030 doi:
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[PDF 367 KB] (1710)

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A geomechanical model experiment on Qianjiangping landslide in the Three Gorges Reservoir area is conducted. Taking microseism and centigrade meters as the main experiment gauging methods，the mechanism of the landslide is studied and the feasibility of landslide is forecast by micro-seismic monitoring. The experimental results show that the strong rainfall before the initial reservoir impoundment period has little influence on the landslide stability；and the uplifting pressure due to the reservoir water will only induce the creeping of landslide，while the shear strength reduction of slippery zone by attenuation of water immersion is the true failure cause of the landslide. The microseismic events achieved from the experiment display the distortion-breakage-destruction process of landslide clearly，revealing the slippery zone destruction process that is from the toe to the middle of landslide；and the feasibility of landslide forecast by microseismic monitoring is verified.

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ELASTOPLASTIC ANALYSIS OF CIRCULAR OPENINGS IN STRAIN-SOFTENING ROCK MASSES

ZHANG Qiang，WANG Shuilin，GE Xiurun

2010, 29 (05): -1035 doi:
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[PDF 231 KB] (1715)

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Geotechnical materials are generally performed with strain-softening mechanical behaviors. Considering post-strength surface shrinkage characters during strain softening process，it is assumed that the rock mass material parameters keep unchangeable in sufficiently small region. In this way，the rock mass is divided into multi-plastic rings after failure. Based on the plane strain condition and Mohr-Coulomb criterion，the quasi-strain softening elastoplastic closed-form expressions are obtained. Considering the rock mass strength evolution law，the multi-plastic radii can also be obtained using rock mass rupture conditions on the adjacent plastic rings. It is generally known that the ground reaction curve(GRC) is a very important basis for the underground lining design. The rock mass deformation parameters are also deteriorated with the increase of plastic deformation. So，the influences of deteriorated degree of shear modulus on rock mass deformation，softening radius and residual radius are analyzed. Finally，the analytical method is validated by an example；and the results can provide meaningful theoretical foundations for underground engineering optimization design and its stability analysis.

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EXPERIMENTAL RESEARCH ON DAMPING PARAMETERS OF ROCK UNDER CYCLIC LOADING

LIU Jianfeng1，2，XU Jin1，2，LI Qingsong1，2，LI Ganglin1，2

2010, 29 (05): -1041 doi:
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[PDF 266 KB] (1603)

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Theoretical analysis of damping parameters of rocks is carried out；and the uniaxial cyclic loading tests for fine sandstone and silty mudstone are realized on MTS815 Flex Test GT rock mechanics test system. The sine wave is achieved in cyclic loading tests，and its frequency is 3 Hz. The cyclic number is 30 under dynamic stresses from 1.0 to 6.5 MPa. The method for testing damping parameters of rock by cyclic loading is provided；and some change characters between rock density and plastic deformation under cyclic loading and unloading are obtained together with the areas of hysteresis loops，damping ratios，damping coefficients，etc.. Under the same test conditions，it is found that the higher the rock density is，the smaller the plastic deformation between the hysteresis loops under cyclic loading and unloading tests is，and the denser the space of hysteresis loops is；conversely，the less the rock density is，the sparser the space of hysteresis loops is. With the rock density increasing，the area of hysteresis loops decreases，and the energy dissipation of rock is less；meanwhile，the area of hysteresis loops and the energy dissipation are larger. The results show that the damping ratio decreases，and the damping coefficient approximately increases with rock density increasing. Thus，it is possible to preliminarily estimate damping parameters according to rock density.

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DISPLACEMENT CONTROL EFFECTS AND PARAMETER ANALYSIS OF PASSIVE ZONE IMPROVEMENT OF FOUNDATION PITS

ZHENG Junjie1，ZHANG Rongjun1，DING Lieyun2，CHEN Qiang1，ZHAO Dongan1

2010, 29 (05): -1051 doi:
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Based on the deep foundation pit of Jiyuqiao Station of Wuhan Metro Line 2，a three-dimensional elastoplastic numerical simulation model is established. In this model，the groundwater seepage and soil consolidation effect are both considered. The ground settlement and lateral displacement of diaphragm wall are calculated；and the basic deformation characteristics of the foundation pit are studied. Then，the calculated results are compared with field data to verify the rationality and accuracy of the numerical model. Finally，the main factors and arrangement pattern of passive zone improvement，which both have influences on the magnitude and distribution of the foundation pit deformation，are analyzed. The results show that the displacement control effects of passive zone improvement are significant. Preferably，the width and depth of skirt border improvement should not be too large. When the width(or depth) exceeds a certain value，the increasing rate of improving effect will be very little. The optimum improving effect can be obtained when the width and depth of skirt border improvement are approximately equal. On the assumption that the parameters of skirt border improvement are unchanged，the improving effect is mainly determined by the replacement ratio of the strip improvement；and it is basically irrelevant to the arrangement pattern of passive zone improvement.

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ANALYSIS OF CHARACTERISTIC PARAMETERS OF SOIL-WATER CHARACTERISTIC CURVE(SWCC) AND UNSATURATED SHEAR STRENGTH PREDICTION OF JINGMEN EXPANSIVE SOIL

ZHOU Baochun1，2，KONG Lingwei1，CHEN Wei1，BAI Hao1，LI Xiongwei1

2010, 29 (05): -1059 doi:
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In order to discuss the relationship between the unsaturated shear strength and the soil-water characteristic curve(SWCC) of Jingmen intact，compacted and lime-treated expansive soils，nonlinear curve fittings are carried out to fit the experimental data of SWCC based on Fredlund-Xing model，and the correlation analysis is presented for the shear strength results of the suction-controlled consolidated-drained triaxial compression tests based on the Fredlund equation(1996) for predicting shear strength with respect to SWCC. The results show that the experimental data of SWCC for the three types of expansive soils can be well fitted by the Fredlund-Xing model. The air entry value of Jingmen intact，compacted and lime-treated expansive soils is 210，68 and 18 kPa respectively. For the water stability，the lime-treated expansive soil is the best，followed by the compacted one，and the intact one is the worst；each of the residual suction of the three types of expansive soils can be taken as 3 000 kPa. For the three types of expansive soils，neither the effective angle of shearing resistance(j¢) nor the angle of shearing resistance with respect to matric suction(jb) is constant，where j¢ is influenced by the matric suction，and jb is influenced by matric suction and stress state，which means the Mohr-Coulomb failure surface is bidirectional bending. The unsaturated shear strength of the three types of expansive soils can be well predicted by the Fredlund equation over a wide matric suction；and the soil parameter κ of Jingmen intact，compacted and lime-treated expansive soils is 2.4，2.7，3.4，respectively. The main causation of the difference between the predicted shear strength；and the test results show that the Fredlund equation does not consider that jb is influenced by stress state.

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FIELD MEASUREMENT AND NUMERICAL SIMULATION OF ESTUARINE DEPOSIT SETTLEMENTS UNDER EMBANKMENT LOADS

JIANG Min1，2，BIAN Xuecheng1，2，WU Jianguo3，HUANG Jianwu3，CHEN Yunmin1，2，CHEN Renpeng1，2

2010, 29 (05): -1067 doi:
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Settlement deformations of soft estuarine deposit under applied loads are essential to embankment construction and land reclamation in coastal areas. Based on the laboratory tests，a soft soil model derived from the modified Cam-clay model is proposed to considering the inherent structure characteristics of soft estuarine soil. Using this soil model，a finite element numerical model with realistic geometric sizes and physical properties from an embankment engineering is established to investigate ground settlement and excess pore water pressure dissipation procedure of soft estuarine deposit. Two load conditions are taken into account in the same analysis model，one is large area uniform surcharge loads，and the other is newly-built sea embankment. The numerical model incorporates both effect of the prefabricated vertical drains(PVDs) installed in the underlying soil deposit and the stage construction process of the embankment in the analysis. Good resemblances are found for the in-situ monitoring data and the simulation results，both for vertical and lateral ground deformations. At the same time，the accumulation and dissipation processes of excess pore water pressure at different depths beneath embankment and backfill area are accurately simulated by the proposed numerical model.

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LARGE-SCALE TESTS ON UPLIFT ULTIMATE BEARING CAPACITIES OF ENLARGED BASE PILES IN SATURATED AND UNSATURATED SILTY SOILS

CHEN Renpeng1，2，ZHANG Geqiang1，2，KONG Linggang1，2，CHEN Yunmin1，2，XING Yuelong3，YING Jianguo3

2010, 29 (05): -1074 doi:
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Two groups of tests have been conducted in a large-scale model chamber，and each group contains four uplifting tests with embedment ratios(embedment depth to enlarged base diameter) of 1，2，3 and 5，respectively. One group of tests was conducted in unsaturated slit with degree of saturation of 49% and the others in saturated silt soil. The effects of soil saturation and embedment ratios on the ultimate bearing capacity and the failure patterns of the enlarged base piles are discussed. It is found that the uplift ultimate bearing capacities of the enlarged base piles decrease 30%–50% while the degree of saturation of silty soils increases from 49% to 100%，but 8 and 12 times larger while embedment ratios increase from 1 to 5 in unsaturated slity soil and saturated slity soil，respectively. The failure pattern of the soil around the pile changed from inverted truncated cone slip-surface to local failure around the enlarged base with the increase of embedment ratio.

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MODEL EXPERIMENTAL RESEARCH ON WETTING DAMAGE MECHANISM OF WIDENING LOESS ROADBED

WENG Xiaolin1，2，LI Lintao3，ZHANG Liujun2

2010, 29 (05): -1080 doi:
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To study the wetting mechanism of widening loess roadbed and its failure modes，the deformation measurement system is installed on geotechnical centrifuge. To achieve the actual stress with the loess embankment broadening，the Xi¢an—Tongguan Expressway is adopted. The results show that the widening embankment slope toe with high water content will influence loess roadbed stability in the zone with shallow water level. During the heavy rainfall or the sudden jump of the water level under the embankment load，the local loess soil instability will greatly threaten the safety of the widened embankment，and the flooding-damaged formation of slip surface is a progressive failure process. Due to local swampy ground and low shear strength，initial shear failure of foundation will occur on the old road slope once the foot of the wetting slope causes excessive distortion generated by the global body from top to bottom of the slip slope. The sliding surface of loess roadbed is approximately circular shape，and the method for stability evaluation of the foundation under widening load is proposed using fissures arching. The results also show that the new and old roadbeds with reinforced materials in soils attached to the role can form an effective soil arching effect，they can make the best of embankment rigidity to adjust deformation of the foundation settlement.

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编委与学者讨论

2010, 29 (05): -1080 doi:
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