Journal of Changjiang River Scientific Research Institute

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Unloading Strength Characteristics and Damage Characteristics of Sandstone under Freeze-thaw Cycles

ZHU Zhen-de, Semerjan Memetyusup, FANG Ruo-jin, NI Xiao-hui

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 1-5. https://doi.org/10.11988/ckyyb.20171094

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Geotechnical projects in cold regions suffer from both cyclic freezing-thawing and excavation unloading, giving rise to rock degradation. In this article, the strength characteristics and damage characteristics of sandstone subjected to both cyclic freezing-thawing and excavation unloading are analyzed systematically by freeze-thaw cycle test and triaxial unloading test. The results are concluded as follows: in freeze-thaw cycle test, the sandstone experienced a stage from tension failure gradually to shear failure as confining pressure rises; as the freeze-thaw cycle proceeds, the sandstone witnessed an apparent lateral expansion in the middle, with different levels of tension cracks, circular cracks, rock powder and rock fragments; while in triaxial unloading test, as freezing-thawing proceeds, the peak strength of sandstone reduces, and dilatancy stress declines in an exponential relation; destructive confining pressure is in a quadratic functional relation with cycle number; moreover, the freeze-thaw damage value increases linearly with the number of freeze-thaw cycles, indicating that the effect of cyclic freezing and thawing intensifies gradually.

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Peak Shear Strength Criteria for Frozen Rock Joint Surface

SHEN Shi-wei, HAN Ya-lu, XU Yan, LI Huo-hua

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 6-12. https://doi.org/10.11988/ckyyb.20171136

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In permafrost region, rock joint as a mechanical weakness has great influence on the safety and durability of engineering projects. Studying the mechanical properties of frozen rock is of great significance for engineering construction in permafrost regions. To investigate the shear mechanical performance of frozen rock joints, four sets of natural joint surfaces of tuff collected from G214 National Highway in Qinghai Province were selected for direct shear test after being frozen at given temperature. Results show that the peak strength of the frozen rock joint is proportional to normal stress, and is inversely proportional to opening degree and roughness. When the opening of joint surface is greater than the maximum fluctuation of joint, the joint surface is dominated by disconnection failure mode, and the peak shear strength is controlled by adhesive force between ice and contact surface; when the opening of joint surface is smaller than the maximum fluctuation, ice shearing failure is the dominant failure mode under small normal stress; while under large normal stress, shearing failure of ice and rock both exist, and the peak shear strength is determined by the shear strength of both ice and rock joint. Furthermore, the peak shear strength criteria for frozen rock joints with various failure modes were derived.

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Characteristics of Rock Core Discing under High Geostress

HU Wei, WU Ai-qing, CHEN Sheng-hong, LIU Yuan-kun, LI Yong-song

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 13-20. https://doi.org/10.11988/ckyyb.20171104

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The rock drilling core discing in Jinping Hydro-power Station under high geostress environment is investigated. On the basis of summarizing the existing research results, the macroscopic failure characteristics, discing quantity, thickness and characteristics of borehole were described. The core discing mechanism and the field stress condition of the core discing were summarized through numerical simulation.Results show that: 1) during the drilling process, the tensile stress zone crossing rock core is the dominant factor of core discing crack, and the morphology of rock disk is affected by the tensile stress in the core; 2) sufficient field stress is the primary condition of core discing, and higher tensile stress is induced in the core with higher field stress,and also the condition of core discing crack initiation and expansion is more adequate, and the rock disk’s thickness gets smaller; 3) the increase of radial field stress of borehole has a positive effect on the tensile stress induced in the drilling core, and the axial field stress is opposite; 4) the average value of the maximum tensile stress cross line in the core is defined by comparing with the tensile strength of rock, and the formula of field stress in the presence of drilling core discing is deduced.According to the calculation,the critical condition of Jinping marble rock(T_2b) discing crack is that the radial field stress of borehole should be beyond 34.44 MPa.

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Estimation of Void Space of Joint under Initial Contact State: Method and Application

GUI Yang, XIA Cai-chu, QIAN Xin, YU Qiang-feng, ZHUANG Xiao-qing

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 21-25. https://doi.org/10.11988/ckyyb.20171100

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The initial contact of the two surfaces of a joint has important effect on its shear and coupled shear-flow properties. TJXW-3D portable rock surface topography scanner developed by Tongji University is able to realize special reference points. In this article, the scanner was first used to digitize joints in initial closed state and joint with two surfaces, respectively. With permanent reference points attached to the sides of the joint samples, the subsequent measurements were automatically transferred into the coordinate system of the first measurement, thus the void space of the joint in initial contact can be obtained. The aperture distributions of void spaces of three joints with different surface roughness under initial contact state were analyzed, and the results were in accordance with prior studies. The aperture of joints ranged from 0.31 mm to 0.41 mm, following Gaussian distribution. Furthermore, the Brown joint closure model was used to demonstrate the application of the void space calculation method, and the result was compared with that of normal compression test. The Brown model result was larger than test result as Brown model does not consider the interaction between upheaves. The result lays a foundation for the numerical research on the coupled shear-flow property of joint.

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Influence of Flaw Parameters on the Failure Mode and Strength of Single-flawed Rock Specimens

YANG Heng-tao, LIN Hang

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 26-33. https://doi.org/10.11988/ckyyb.20171081

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In tests of rock specimens containing single flaw, researching the crack types and failure modes are conducive to predicting the initiation and propagation of cracks; yet in engineering design, the bearing capacity, namely damage strength, should receive more attention. In order to investigate into the relation between crack and rock strength, we built numerical models for corresponding test specimens in FLAC^3D, and furthermore researched theinfluence of single crack’s geometric parameters (inclusive of inclination angle, length, and thickness) on the damage strength of rock specimens by adjusting the parameters in numerical model. Results revealed that the peak strength of specimen declined continuously as crack length increased; while when inclination angle increased, peak strength displayed a trend of increasing in general, increasing slowly when inclination angle was smaller than 60°and rapidly when inclination angle was larger than 60°. Within the test range, peak strength remained unchanged as flaw thickness increased.

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Correlation Between Point Loading Index and Uniaxial Compressive Strength of Rock-like Material Based on Size Effect

LIN Jun, SHA Peng, WU Fa-quan, CHANG Jin-yuan

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 34-44. https://doi.org/10.11988/ckyyb.20171091

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The point load intensity index is considered as the optimum method for measuring and estimating uniaxial compressive strength as it is of unique advantage by indirectly determining the uniaxial compressive strength of rock. By summing up a large number of literatures in China and abroad, we found that current research results are limited by the anisotropy of rock materials, fuzzy correspondence of data, heavy equipment and lack of experimental verification. In this paper, we researched the correlation between point load index and uniaxial compressive strength of rock-like isotropic material of different sizes and strengths by using a portable instrument for point load test. According to test results we conclude that the test data follows the normal distribution law with obvious central tendency; size effect of rock-like material on point load index is apparent; according to P-D²_e curve, the highest correlation coefficient is 0.97, indicating that the point load index of irregular samples can be rectified to that of standard equivalent size. Furthermore, the relational expression between point load index and uniaxial compression strength is established, with a correlation coefficient up to 0.994, which suggest that the rectified point load index well reflects the correlation and improves calculation accuracy. Finally, through point load test of irregular specimens, the error of the proposed relational expression is verified within 15%.

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Thermal Conductivity Characteristics of Beishan Granite after Thermal Treatment

GUO Zheng, ZHAO Xing-guang, LI Peng-fei, XIE Jing-li, LIU Yue-miao

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 45-51. https://doi.org/10.11988/ckyyb.20171109

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In deep geological disposal of high-level radioactive waste (HLW), the thermal conductivity of host rock is a key factor in design because it has a direct impact on the repository layout and the optimization of distance between disposal elements. This study aims at investigating the thermal conductivity characteristics of Beishan granite treated with different temperatures ranging between 200 ℃ and 800 ℃. The influence of thermal treatment on thethermal conductivity of Beishan granite is analyzed, and the relations between thermal conductivity and other conventional physical parameters are also discussed. The effect of water saturation on the thermal conductivity is revealed as follows: 1) the thermal conductivity of Beishan granite specimens decays as treatment temperature rises, and the decay rate reaches peak value in the temperature range between 550 ℃ and 650 ℃; 2) as treatment temperature rises, the mass,dry density and P-wave velocity of specimens increase in general while the volume and porosity of specimens present an increasing trend; according to the relations between thermal conductivity and P-wave velocity, porosity, and dry density, respectively,models are established to predict the thermal conductivity of thermal-treated specimens; 3) before thermal treatment, the thermal conductivity values of saturated specimens increase by 9.7%-12.1% compared with those of dry ones,and the thermal conductivity of saturated specimens increases in an approximately linear trend with the increase of thermal conductivity of dry specimens; 4) on the other hand, when the thermal-treated specimens were saturated with water, the thermal conductivity shows a slightly decreasing trend as treatment temperature rises. We also found that the effect of water saturation on thermal conductivity increases with the increase of rock porosity, and this behavior can be described reasonably using a linear equation. The research achievements provide a theoretical basis for the design and optimization of deep geological disposal engineering.

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Quantifying the Process of Karst Collapse by a Physical Model

WU Qing-hua, ZHANG Wei, LIU Yu, CUI Hao-dong

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 52-58. https://doi.org/10.11988/ckyyb.20171079

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At present, there is no unified, systematic cognition regarding the mechanism of karst collapse as research achievements are mainly concentrated on the causes and influential factors of karst collapse in qualitative sense rather than quantitative approach. In this paper, the process of karst collapse covered with sand layer was investigated quantitatively, and the barrier effect of clay layer covering the karst was studied by sampling (keeping the pressure head unchanged during the process of sampling) and pressure monitoring on a physical model. Results showed that: 1) in the structure of rock covered with sand, three times of karst collapse happened during the steady decline of groundwater level, with the critical seepage-breakage gradients rcaching 0.525, 2.500 and 3.400, respectively; the maximum collapse rate gradually decreased, yet the durations and degrees of collapse gradually increased; moreover, the process of the first karst collapse was more complicated than that of the second and the third collapse; 2) the seepage theory of liquefaction has been verified by the results of karst collapse of rock covered with sand layer; 3) groundwater level fluctuation exacerbates the formation of karst collapse; 4) the seepage-breakage gradient increased from 0.525 to 48.300 in the presence of clay layer, indicating that clay layer could significantly improve the safety of karst overburden layer and effectively inhibit karst collapse. The results of this study could improve the understanding of karst collapse mechanism in a quantitative sense.

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Application of Nuclear Magnetic Resonance Technology in the Study of Unsaturated Hydraulic Conductivity

CHEN Bao, ZHANG Kang, HUANG Yi-yi, PENG Xiang

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 59-64. https://doi.org/10.11988/ckyyb.20171096

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It is difficult to determinate the transient water content (suction) distribution in geomaterials rapidly and accurately during the permeability test of unsaturated soils. Ordinary water content measurement methods are not only time-consuming but also cumbersome with the traditional relative large probes bonded with the specimen, likely posing destructive effects on the test sample. In this paper, the water content in white Portland cement specimen was measured swiftly and non-destructively by employing the nuclear magnetic resonance (NMR) technology. The unsaturated permeability coefficients of white Portland cement were obtained by the instantaneous profile method. Results showed that the unsaturated hydraulic conductivity of white Portland cement varied greatly with suction, ranging from 4.61×10^-12 to 7.59×10^-8 m/s; when suction was higher than 1 MPa, hydraulic conductivity increased slowly with suction decreasing; while as suction reduced below 1 MPa, hydraulic conductivity showed a steep increase by several orders of magnitude. Compared with traditional technology, NMR technology is more applicable in measuring the water content distribution in permeability test of non-saturated soil.

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Failure and Coal Bumps Characteristics of Hard Roof-coal Structure

WANG Ning, JIANG Yao-dong, ZHU Deng-yuan, YAN Zheng-yu

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 65-69. https://doi.org/10.11988/ckyyb.20171103

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Uniaxial compression tests with different loading rates were conducted on standard samples of combination samples of “roof-coal” structure caused by coal bump. By analyzing the stress and strain development process and
acoustic emission response characteristics under different loading conditions, the basic damage modes and the possible conditions of abrupt change were obtained. The deformation and failure of the samples experienced obvious development stages with the increase of loading rate, namely progressive shear failure, splitting failure and structural failure. Sudden holistic instability eventually occurred with the weakening of internal bearing structure. Through analyzing the relative slip characteristics of composite structure and the geological conditions and mining effects of typical mines, the failure mode of coal bumps under the condition of compound coal-rock is clarified. The results offer reference for targeted failure control.

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A Method of Measuring the Strain of Geotextile

CHEN Ling-wei, ZHOU Xiao-wen, PENG Wei-ping, LI Bo, TONG Jun

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 70-74. https://doi.org/10.11988/ckyyb.20171072

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Despite wide usage in reinforced soil projects, geotextile is featured with big challenge and low precision in strain measurement as it is a flexible material of low elastic modulus. The role of geotextile in engineering as well as the strain of geotextile are still uncertain. In this article, we proposed a method of accurately measuring the strain of geotextile by controlling the shape and thickness of a flexible binder to paste strain gauges, which, hence deforming in coordination with geotextile. Moreover, we also developed an equipment for calibrating the stress-strain of geotextile, by which the observed value of strain gauge can be transformed to the actual deformation of geotextile. The equipment’s applicability was verified by centrifugal model test. The research achievement offers technical support for further researches on geotextile’s reinforce mechanism and influence on mold bag sand cofferdam stability.

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Degradation Behavior of Soft Rock Filling Material for Dam Construction

HUANG Ze-an, ZHOU Yue-feng, HE Xiao-min, ZHANG Ting, HUANG Bin

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 75-78. https://doi.org/10.11988/ckyyb.20171440

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In this article, the mechanical behavior of soft rock filling material for dam construction before and after degradation is investigated through laboratory tests at different consolidation pressures and compaction degrees. The degradation in strength and deformation characteristics of the rockfill material is analyzed. The relationship between degree of compaction and normalized strength, and the relationship between degree of compaction and normalized deformation modulus are built respectively. The controlling degree of compaction is also given for construction. Furthermore, according to results from underwater test of repose angle, the nonlinear strength, in particular, the strength at low stress state is studied. Research findings conclude that the degradation leads to the decline of volume strain,and poses larger impact on deformation modulus than strength;a 96% degree of compaction is reasonable for
soft rock filling material as it ensures a normalized strength larger than 0.9 and normalized deformation modulus larger than 0.8, and guarantees the compaction efficiency as well. Moreover, the strength parameters of soft rock filling material can be acquired from underwater repose angle test on compacted samples; and the nonlinear strength can be obtained through triaxial test. The research findings can be utilized to evaluating the stress-strain rules of rockfill dam built in soft rock and to providing guidance for dam construction.

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Current In-situ Stress State and Stability of Active Fault Zone in Piedmont of Altai Mountains

ZHOU Chun-hua, LI Yun-an, YIN Jian-min, WANG Yang, AI Kai, TANG Qian

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 79-84. https://doi.org/10.11988/ckyyb.20171080

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There is an active tectonic zone in large scale in Altai region of the Asian continent, and historical earthquake data shows that the Altai Mountain is also active in seismicity. Analyzing the current stress state is of greatsignificance to the assessment of earthquake activity risk. In this research, the distribution law of current in-situstress state in the active fault zone is revealed by hydraulic fracturing measurement at two boreholes (nearly 700 m in depth) in the piedmont of Altai Mountains. Results show that in buried depth of about 320 m, the spatial principal stress state changes from thrust type to strike-slip type, indicating a stress partition near the active faults; the orientation of maximum horizontal principal stress of the two boreholes is mostly in NW, which, together with the focal mechanism, verifies the right-lateral strike-slip characteristics of NWW active faults. Finally, according to the measured stress data, mechanical analysis on the stability of the active faults is carried out based on Coulomb faulting criterion and the law of Byerlee. Results conclude that up until now, the active faults in the piedmont of the Altai Mountains are relatively stable, with no mechanical condition brewing instability. The research results offer basic geomechanics data for studies on the regional active faults’ current activity, and provide scientific reference for selecting the location of engineering project near the active faults.

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Method of Geostress Relieving and Measuring by Borehole Slots: Numerical Analysis and Engineering Application

LI Yong-song, PENG Qian, YIN Jian-min, AI Kai

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 85-91. https://doi.org/10.11988/ckyyb.20171135

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To overcome the difficulty of geostress measurement in high geostress environment, we discussed the optimization of narrow slot depth and slot layout by numerical simulation, and applied the method of geostress relieving by borehole slots to the stress measurement of Jinping projects’s underground laboratory. Numerical simulation suggest that when initial stress field is dominated by stress component parallel to the slot direction, the required depth of slot for complete release of stress on both sides of the slot reaches the maximum 16 mm. In field test, only one slot is arranged at a given depth in the borehole to improving the reliability of experiment and avoiding the mutual influence between the narrow slots. Meanwhile, the slot spacing is generally greater than 200 mm. The measured result shows that the rock mass at test site is in high geostress state, which is in accordance with the core disking in the process of drilling.

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Characteristics of Ground Penetrating Radar Profile in Advanced Geological Prediction for Tunnels: Analysis and Application

ZHOU Li-ming, FU Dai-guang, XIAO Guo-qiang, ZHANG Yang

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 92-96. https://doi.org/10.11988/ckyyb.20171127

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To improve the accuracy of advanced geological prediction for tunnels by Ground Penetrating Radar (GPR), the GPR profiles of bad geological bodies (including horizontal fracture zone, inclined fracture zone, cavity and karst cave) were extracted by forward simulation in two-dimensional finite difference time-domain models. Case study on three karst tunnels of Anjiang Highway demonstrated the effectiveness of the forward simulation result. The result revealed that GPR is of obvious efficiency in identifying the location and horizontal development of fracture zone and karst cave, but has some limitations for vertical development and tilt angle of bad geological bodies. In conclusion, satisfactory result of advanced prediction can be obtained by analyzing measurement data through GPR profile based on detailed geological sketch.

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Analysis and Evaluation of Hidden Diseases in Old Tunnel with GPR Images

LI Ning, LIU Zhen-dong, GUO Xiu-jun, WANG Ying-ying

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 97-103. https://doi.org/10.11988/ckyyb.20171088

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The worsening of hidden diseases of old tunnel, which are of variety and complexity, would cause harm to tunnel structure and traffic safety. Although ground penetrating radar (GPR) technology has been widely applied to the investigation of hidden diseases, no uniform understanding or systematic conclusion of anomaly features in GPR images has been reached. In this article we sum up the types and causes of old tunnel diseases, and set up corresponding earth-electricity models to identify anomaly features through forward modeling and comparing the GPR images with field measurements. Moreover, we establish an evaluation system for old tunnel disease by introducing the concept of membership degree in fuzzy mathematics. Results show that the dielectric constants of lining crack and cavity are significantly different due to different filling mediums, reflected by event dislocation and local improvement of diffraction patterns. The non-compacted and impervious layer gradually develops into a mixed structure involving water, concrete, and residues with multiple reflection interfaces, and the corresponding GPR image is cluttered with local strong reflection. Application practice prove that the presented model could well assess the safety levels of old tunnels based on GPR detection of potential disease areas.

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A Method for Determining Empty Area Profile Based on Borehole Ultrasonic Scanning Technique

WANG Jin-chao, WANG Chuan-ying, TANG Xin-jian

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 104-109. https://doi.org/10.11988/ckyyb.20171086

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Due to the extremely complex detection environment of empty area in deep underground engineering, the detection data is often limited, thus we can only obtain rough contours by conventional fitting method. In view of this, we propose a method for determining the 3-D profile of empty area by using borehole ultrasonic scanning technique. Firstly we present a preprocessing method for the effective screening of ultrasound scanning data by determining valid data points and smoothing non-effective detection points; secondly, by introducing the second-order Bezier curve, we present a curve fitting method which could cover every single effective detection point, favorable for the fitting characteristics of horizontal sections of the empty area; after horizontal contour fitting, we accomplishedvertical fitting by linear interpolation based on depth information, and acquired computation result and 3-D images by MatLab software. Subsequently we compared the results with those obtained from other fitting methods, and the comparison suggest that: 1) the borehole ultrasonic scanning system is able to provide effective data for the modeling of empty area; 2) the improved Bezier curve is feasible and suitable for the fitting of empty area profile; 3) the presented contour fitting method based on ultrasound scanning is proved feasible and accurate.

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Research on Dielectric Property and Interface Reflection Mechanism of Unsaturated Loess by Ground Penetrating Radar

LÜ Gao, YANG Jie, LI Ning, HU De-xiu

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 110-115. https://doi.org/10.11988/ckyyb.20171083

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The dielectric property of loess and the reflection mechanism and law of ground penetrating radar (GPR) wave are studied in this article. Through tests on the relative permittivity and conductivity of unsaturated loess, the inner relation between relative permittivity and three phases (gas, liquid, and solid) is analyzed. Moreover, the mathematical-physical relations of relative permittivity versus compaction degree and moisture content are obtained. The influence of GPR antenna frequency on the moisture content-permittivity relation is also analyzed. Results suggest that relative permittivity of unsaturated loess is positively related with moisture content and compaction degree; given the same loess sample, relative permittivity is positively related with antenna frequency. In addition, the interface reflection numerical model of relative permittivity versus moisture content is built, indicating that echo intensity of GPR grows with the increase of moisture content. The above conclusions are of guiding significance for GPR detection of unsaturated loess engineering.

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An Underwater Settlement Monitoring Technique for Deep-sea Immersed Tunnel Foundation

WANG Yan-ning, JIANG Bin-song, ZHANG Qiang, CHEN Yun-tao

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 116-121. https://doi.org/10.11988/ckyyb.20171076

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The island-tunnel segment of Hong Kong-Zhuhai-Macau Bridge is featured with deep soft soil under the immersed tube foundation. Sand compaction pile together with preloading was adopted to eliminate the differential settlement between joint parts. In this paper, the technique of monitoring tube foundation settlement by liquid pressure settlement sensor is introduced in details. Satisfactory settlement data was obtained. A variety of technical approaches, including the timing of operations, protection of wire connectors and wire, were employed to improve the success rate of equipment burying. Test results showed that the technique could perform continuous stable wireless monitoring as it is of high stability, low error rate, and is less susceptible to wind and waves and construction interferences. Therefore it could realize the continuous and stable wireless monitoring of the foundation. The study is expected to provide reference for the monitoring of composite foundation under deep water conditions.

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Deformation Monitoring of Shaft Lining in Weakly Consolidated Strata

LÜ Xian-zhou, WANG Wei-ming, JIA Hai-bin, PAN Ge-lin, ZHAO Zeng-hui

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 122-128. https://doi.org/10.11988/ckyyb.20171234

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Shaft lining in thick weakly consolidated strata is featured with large deformation and repeated damage after repair. Based on the concrete strength failure criterion under biaxial compression and analytical solution for spatially axisymmetric problem of a thick-walled cylinder, we obtained the limit value of damage of the shaft lining in Xinjulong coal mine. We selected three formations to carry out monitoring by a multi-layer automatic deformation monitoring system for the auxiliary shaft in Xinjulong coal mine, and evaluated the status of the lining. Results show that the deformation of the shaft lining under the condition of ultimate failure is 2.369 mm. The shaft lining located in the border of the bottom aquifer and the bedrock subjects to the most severe deformation, about 82.5% of the deformation limit. The shaft lining deformation increment fluctuates in a certain range belonging to elastic deformation. Finally, we inversed the stress state according the deformation value of the shaft lining. The obtained value of additional stress is lower than the ultimate compressive strength. Long-term project practice shows that the deformation monitoring results could reflect the real stress condition of the shaft lining. Moreover, the monitoring system could realize real-time dynamic evaluation of the shaft lining status.

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Method of Measuring Compaction Degree of Loess-filled Ground Based on Simultaneous Measurement of Soil Penetration Resistance and Water Content

YU Yong-tang, ZHENG Jian-guo, LIU Zheng-hong, ZHANG Ji-wen, DU Wei-fei

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 129-134. https://doi.org/10.11988/ckyyb.20171073

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When Proctor Penetrometer is used to measure the compaction degree of filled soil in field, the variation of soil water content would bring about large error to the measurement results. In this article, a new method for compaction measurement integrating Proctor Penetrometer and soil moisture sensor is proposed and the soil models are built based on dual-sensor data fusion to reduce the influence of water content on the compaction test. Compacted loess samples with different water content and dry density are prepared. The penetration resistance measured by Proctor Penetrometer and the output voltage measured by soil moisture sensor are compared with the dry density determined accurately by sand replacement method and the water content determined accurately by oven-drying method. Binary regression analysis method is applied to conduct data fusion and set up a mathematical model involving
dry density, penetration resistance and output voltage. Test result indicates that after fusing the penetration resistance with the output voltage, the dry density can be measured directly and less influenced by water content and closer to the true soil dry density value, thus the influence of water content on the measurement results is greatly reduced.

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Field Comparative Test of Compressive and Uplift Behavior of Post-grouting Pile

ZHANG Shi-min, WANG Hong-bo, ZHANG Si-feng, JING Feng-wei, LI Zong-shuai

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 135-138. https://doi.org/10.11988/ckyyb.20171078

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The demand for high bearing capacity posed by compressive load and uplift load on foundation pile can
be addressed by pressure grouting. By conducting compressive test and uplift test on post-grouting piles, we studied the mechanism of load transfer on post-grouting piles, and compared the differences in side friction and bearing capacity of piles between compressive test and uplift test. We also identified the load transfer paths and the approaches of compressive force and uplift force in the failure process. Research result indicates that pressure grouting technology improves the bearing capacity of pile and the friction between pile and soil; moreover, a mechanical interaction is produced by pressure grouting, which also enhances the bearing capacity of pile, in particular, the uplift pile. The results offer reference for researches on improving the bearing capacity of uplift pile without increasing the pile length.

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Mechanism of Fracturing Fluid Viscosity in Response to Pulse Frequency

NI Zhun-lin, CAO Han, PENG Can-wei, SUN Ping-he

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 139-143. https://doi.org/10.11988/ckyyb.20171089

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To investigate the changes of rheology of pulse hydraulic fracturing fluid in response to frequency in shale gas exploitation, we conducted pulse shear tests on guar gum of two different viscosities (30 mPa·s and 10 mPa·s) under five different frequencies (0 Hz, 1 Hz, 3 Hz, 5 Hz and 7 Hz). Test results show that the response of fracturing fluids to pulse frequency varies with the change of viscosity. For fracturing fluid of relatively high viscosity, as frequency rises, the structural viscosity and plastic viscosity both present downward trend in general; whereas apparent viscosity firstly reduces by shear thinning, and then changes slightly when frequency reaches 5 Hz and even more; the flow index of fracturing fluid declines after increasing to a maximum value at frequency 5 Hz. For fracturing fluid of relatively low viscosity, as frequency rises, viscosity changes more slowly than plastic viscosity does, which is favorable for proppant migration during proppant stage, playing a significant role in stabilizing fractures at the later stage of fracturing. In conclusion, the response of rheological properties of highly-viscous fracturing fluid to pulse frequency is less obvious than that of lowly-viscous fluid.

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Stability of the Left Bank High Slope in Strong Unloading Area of Baihetan Hydropower Station

CHEN Yuan, ZHANG Lin, YANG Bao-quan, YANG Jin-wang, LI Si-ying

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 144-150. https://doi.org/10.11988/ckyyb.20171107

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A comprehensive model test method is proposed to study the stability of rock slope. The comprehensive method involves the strength reduction method based on temperature-dependent similar material and the overloading method by inclined lifting. Corresponding safety evaluation method for slope stability is also established. The left bank high slope in strong unloading area at Baihetan Hydropower Station is taken as a case study. Through two model tests of a typical section and the whole slope in strong unloading area, the deformation features and failure mechanism after reducing the strength of bottom sliding surfaces are obtained. Results reveal that the gently-inclined structural planes are dominant factors of slope stability; the failure mode is block sliding with sliding-tension fracturing. Moreover, results also suggest that concrete replacement plugs in the front area of bottom sliding surface is an effective measure of enhancing the shear capacity of bottom sliding surface, reducing slope deformation and preventing blocks from sliding. The safety factor of reinforced slope is between 1.42 and 1.58. The research results provide an important scientific basis for the slope safety evaluation and the reinforcement treatment of Baihetan Hydropower Station, and also offer reference for similar slope projects.

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Correction Factor in Calculating Rock Mass Quality Index by BQ Method in Slope Engineering

OU Zhe, WANG Tie, YANG Jia-fu, ZOU Ming

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 151-158. https://doi.org/10.11988/ckyyb.20171099

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The calculation formula of rock quality index in slope engineering (hereafter referred to as BQ (basic quality)), together with the valuing of correction factor in the formula, is given in Standard of Engineering Rock Mass Classification(GB/T 50218-2014) for the rock quality rating and self-stability evaluation of slope engineering in China. With other commonly used rating methods as reference, we pointed out that the evaluation index of correction term and correction factor in the BQ formula needs to be improved. On the basis of the current BQ formula, we propose the correction factor of slope height influence and the correction factor of stress state influence, and give the corresponding calculation formulas and valuing criteria. We also refined the evaluation indexes and valuing criterion of the correction factor of structural plane influence for high and steep slope in complex geological conditions. Finally, we give the optimized BQ formula and the corresponding stability evaluation system for BQ method. The research results are conducive for a correct rock quality rating, and offer reference for slope stability evaluation and reinforcement design work.

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Failure Mechanism of Surrounding Rock of Deep Tunnel in Consideration of Varying Geometric Parameters of Structural Planes

WANG Ke-zhong, WU Hui, MA Fei

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 159-163. https://doi.org/10.11988/ckyyb.20171172

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The strength of rock mass is controlled by structural plane. On the basis of theoretical analysis on stress distribution and failure mechanism of single structural plane in surrounding rock, the influences of dip angle of structural plane and distance from structural plane to tunnel center on the failure mechanism of jointed rock mass are researched. The results are concluded as follows: 1) discontinuities are more likely to occur when structural plane is closer to the excavation contour line; 2) the position where structural plane first destructs is affected by the angle of structural plane. Furthermore, the deformation of 18 groups of rock masses with different combinations of discontinuity distribution, rock strength and discontinuity strength are calculated and compared in three-dimensional element code (3DEC), and the failure mechanisms of surrounding rock with various discontinuities distribution are analyzed. The contribution of structural plane’s mechanical properties to rock failure is also studied: the distribution of structural plane determines the location of the first failure; and the failure of structural plane does not necessarily lead to the failure of surrounding rock.

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Analytic Solutions of Stress and Displacement for a Non-circular Pressurized Tunnel in Consideration of Support Delay

TAO Jun-ye, LU Ai-zhong, YIN Chong-lin

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 164-170. https://doi.org/10.11988/ckyyb.20171119

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In association with the conformal transformation method of plane elastic complex function,the analyticalsolutions of stress and displacement for a non-circular lined tunnel subjected to in-situ stress and internal water pressure are derived. The support delay and the full contact between surrounding rock-mass and lining are taken into consideration. The power series method is used to simplify the calculation process. With a horseshoe tunnel as an example,the analytical solutions are verified by numerical results. Furthermore,the tangential stresses along the excavation boundary and the inner and outer boundaries of the lining,as well as the contact stresses along the interface of the lining-surrounding rock mass are calculated. Besides,the stress distributions of the surrounding rock mass and the lining under different lateral pressure coefficients,displacement release coefficients and internal water pressures are discussed. The results show that the analytical solution is in good agreement with the numerical solution,and the displacement release coefficient and internal water pressure have significant influence on the stress distributions of the lining and surrounding rock mass. Great values of displacement release coefficient and internal water pressure will give rise to tensile stress along the inner boundary of the lining.

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Transverse Vibrations of Variable Cross-sectional Timoshenko Beam on Dual-parameter Elastic Foundation

GAO Dai-heng, ZHENG Hong

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 171-175. https://doi.org/10.11988/ckyyb.20171085

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To study the influence of cross sectional shear deformation and moment of inertia on transverse vibration of elastic foundation beam, the finite element analysis method for variable cross-sectional Timoshenko beam on Pasternak elastic foundation model was developed and improved, and the modal analysis method for arbitrary cross-section of Timoshenko beam was established. In specific, the motion differential equations of foundation beam were derived based on Timoshenko beam theory and Newton’s second law; in the light of piecewise equivalent idea, the variable cross-sections were discretized into many simple uniform beam sections, which corresponded respectively with each finite element with four degrees of freedom. Furthermore, with beam of simply supported boundary which is fully supported on elastic foundation as an example, the characteristics of free vibration were analyzed and compared with those obtained from reliable semi-analytical approach. The results showed that the present method is effective and accuracy and therefore a new insight is provided to the analysis of variable cross-sectional foundation-beam.

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Load Sharing Ratio of Gravity Anchor Pier Foundation and Anchor Block of Xiushan Bridge

BAI Yu-dong, WANG Shuai, ZHANG Yi-hu

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 176-179. https://doi.org/10.11988/ckyyb.20171121

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The load sharing ratio of gravity anchor pier foundation of Xiushan bridge is complicated as the foundation is separated from anchor block. In this article, a 3-D finite difference model of the interaction among anchorage, pile foundation of support pier, and ground is established to research the load sharing ratios and failure modes of different parts of the anchorage system of Xiushan bridge. The simulation load increases by 0.5P (P is the design cable load) until limit state appears. Results reveal that when cable load is between 1P and 2P, the load sharing ratio of anchor block bottom, rock mass in front of anchor block, support pier foundation, and anchor block side is 64%, 23%, 6%, 7%, respectively; when cable load reaches 2.5P, tensile failure of rock on the bottom of anchor block appears due to non-uniform shear force on the interface between anchor and rock. The limit cable load of support pier foundation is 3.5P, approximated to uniaxial compressive failure. As load increases continuously, the contact surface of anchor block bottom gradually detaches, and the horizontal load moves toward the rock mass in front
of anchor block. The research achievements offer reference for the structural design, foundation reinforcement, and limit bearing capacity of gravity anchorage system.

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Viscoelastic Model of Surface Subsidence above Tunnels

KONG Qing-cong, LI Yin-ping, LI Shuo, YANG Bo-jin, LI Jin-long

Journal of Changjiang River Scientific Research Institute. 2018, 35(3): 180-186. https://doi.org/10.11988/ckyyb.20171074

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Surface subsidence will be induced by rock-soil mass being disturbed in the process of excavation and long-term operation of tunnels. At present, there is no mature analytical model based on the theory of mechanics. In this research, the surface subsidence above tunnel is approximated to boundary deformation in the elastic semi-infinite space containing a horizontally cylindrical cavern with shrinkage force. By solving the symmetrically plan-strain problem in elastic infinite space, the elastic analytical solution of the surface subsidence in integral form is derived with the principle of superposition. Subsequently, the viscoelastic analytical solution in integral form in space-time domain is obtained by Laplace transformation of elastic analytical solution with volume deformation regarded as elasticity and distortion as Maxwell viscoelasticity. The proposed model reveals that the width coefficient of settlement troughs above tunnels is 0.6, in good agreement with the monitoring data in many other regions. The method in this article offers a theoretical approach for surface subsidence prediction in the process of excavation and long-term operation of tunnels.

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01 March 2018, Volume 35 Issue 3

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01 March 2018, Volume 35 Issue 3