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  --2021, 40 (4)   Published: 01 April 2021
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 2021, 40 (4): 0-0
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Stiffness design theory for tunnel-support system Hot!

ZHANG Dingli,FANG Huangcheng,CHEN Liping,SUN Zhenyu
 2021, 40 (4): 649-662 doi: 10.13722/j.cnki.jrme.2020.0822
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Surrounding rock deformation is the source of the supporting load and most tunnel accidents,so tunnel engineering design should focus on the deformation control. In general,the surrounding rock deformation is essentially caused by the unbalanced force,and its amount is the result of the interaction between the surrounding rock load and the supporting resistance. In this paper,a dynamic analysis model for surrounding rock deformation is established using Newton¢s second law and an explicit expression of surrounding rock deformation is thus obtained,which can be used to predict and evaluate the deformation process of the surrounding rock. Generally,the support stiffness determines the surrounding rock deformation under given stratum conditions. Therefore,the distribution of the support stiffness should be calculated according to the requirements of deformation control,which is the core content of the proposed support stiffness design theory. Since the internal force of the supporting structure increases with increasing the stiffness,a structural strength checking method is given to dynamically adjust the support stiffness and deformation control objectives. Based on the above basic principles,a theoretical system of “surrounding rock deformation control as the goal,support stiffness design as the core and strength checking as the guarantee” is formed. Finally,the collaborative control theory and design method of the surrounding rock-supporting structure system are given to overcome the difficulties of large deformation and serious advanced failure in complex tunnels.

Experimental study on shear characteristics of bolted rock joints under constant normal stiffness boundary conditions Hot!

JIANG Yujing1,2,3,ZHANG Sunhao1,2,LUAN Hengjie1,2,WANG Changsheng1,2,3,WEN Zhijie1,2,WANG Dong1,2,HAN Wei3
 2021, 40 (4): 663-675 doi: 10.13722/j.cnki.jrme.2020.0871
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Understanding the shear behavior of bolted rock joints is crucial to designing of geotechnical projects that include discontinuities. The effects of constant normal stiffness(CNS) boundary conditions on shear characteristics of bolted rock joints were investigated through shear tests on bolted rock joints and non-bolted rock joints under different CNS boundary conditions. The results show that,at the pre peak stage,the effect of the normal stiffness on the first peak value of the shear stress(Tp) is not obvious. At the post peak stage, the shear stress-shear displacement curve respectively shows displacement softening characteristic,stability characteristic,and displacement hardening characteristic when the normal stiffness is less than 3 GPa/m,equal to 3 GPa/m and equal to 5 GPa/m, and the shear stress of bolt broken(Tf) increases significantly with increasing the normal stiffness. At the bolt broken stage,the shear stress-shear displacement shows step subsidence. The Tp and Tf of bolted joints are both higher than those of non-bolted joints. When the normal stiffness exceeds 3 GPa/m,the shear strength loss caused by joint wear can be counteracted by bolting. The first peak value increment of the shear stress(T1) increases with increasing the normal stiffness and JRC,while the shear stress increment of bolt broken(T2) decreases with increasing the normal stiffness and increases with increasing JRC. The failure range of the joint surface increases with increasing the normal stiffness,and the elliptical failure range of the bolt hole wall decreases with increasing the normal stiffness. The deformation of the bolt shows a shape of“Z”after shearing,and the failure mode of the bolt changes from tensile failure to bending failure and then to shear failure as the normal stiffness increases. The breaking shear displacement of the bolt tends to increase as the normal stiffness ranging from 0 to 1 GPa/m and decreases continuously as the normal stiffness varying from 1 to 5 GPa/m.

Shaking table test of dynamic responses of a layered complex rock slope under earthquake

LIU Hanxiang,ZHOU Yifei,LI Xin
 2021, 40 (4): 676-689 doi: 10.13722/j.cnki.jrme.2020.0772
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A layered complex rock slope always consists of complex lithology and rock structure,and its seismic responses and landslide formation mechanisms are not completely understood. In the present study,a small-scale shaking table test was carried out on a layered complex rock slope model,taking a typical landslide triggered by the 2008“5·12”Wenchuan earthquake as the prototype slope,simulating the original topography and rock strata and using the accelerations recorded at the seismic station adjacent to the landslide area as input waves. Based on the sensor measurements,the natural frequencies and acceleration responses of the model slope were analyzed,and the horizontal and vertical responses were compared. In addition,the acceleration and displacement responses of the weak layer in the model slope,namely carbonaceous slate,were analyzed to emphasize the role of the layer in the whole slope¢s responses. The main results show that the model slope demonstrates different vibration modes under horizontal and vertical excitations,and the natural frequencies for both directions undergo three-phase decreasing process which indicates a gradual deterioration of the slope structure. The topographic amplification effect of the vertical acceleration of the model slope is weaker than that of the horizontal acceleration. However,the model slope shows a more significant modification effect on the vertical input waves,and the carbonaceous slate plays a weakening effect of the vertical acceleration. Bulging deformation was observed on the surface of the carbonaceous slate under a high level of excitation. At the stage of the destructive test,the peak horizontal displacement in the carbonaceous slate is larger than that in the overlying phyllite slate. However,the incompatible deformation among different strata does not cause the model slope to slide along the carbonaceous slate,which is inconsistent with the actual location of the sliding surface of the prototype landslide. Nonetheless,the above results,especially the differential responses between the horizontal and vertical accelerations and between different strata,can still help to explain the failure mechanism of a layered complex rock slope triggered by an earthquake.

Three-point-bending test of crack propagation and fracture parameters of coal specimens

WANG Xiaoran1,2,WANG Enyuan1,2,LIU Xiaofei1,2,LI Nan3,ZHOU Xin1,2
 2021, 40 (4): 690-702 doi: 10.13722/j.cnki.jrme.2020.0736
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Fracture of quasi-brittle materials such as coal involves a distributed microcracking region around the crack tip,called fracture process zone(FPZ). This non-linear FPZ influences both the strength and the stability of a coal structure,and often makes the linear elastic fracture mechanics(LEFM) fail to describe the fracture process of coal samples. To better understand the non-linear phenomena of coal fractures,three-point bending tests of coal beams with a center notch were performed. The integrated acoustic emission(AE) and digital image correlation(DIC) techniques were used to track the crack propagation during pre-peak and post-peak stages visually. Based on the equivalent linear elastic fracture mechanics(ELEFM),the fracture parameters were calculated,and the fracture criterion and the crack propagation speed were also analyzed and discussed. Additionally,comparison between theory and test results was performed. The results indicate that,at the pre-peak regime,the FPZ of coal is gradually developing with a fully developed FPZ length of 7.5 mm and a critical opening displacement of 38.4 μm from three separate tests of the same material. During the crack propagation in the post-peak regime,the FPZ length and the critical opening displacement keep constant,while the traction-free crack length increases with CMOD,leading the increment of the effective crack length. Most of the AE events are located within or surrounding the FPZ,which indicates that the AE are mainly from the microcracking activities in the FPZ. Using ELEFM,the average calculated fracture toughness KIC is 0.316 MPa·,and the critical energy release rate Gc by Irwin¢s relation is in the range of 39.6–40.4 N/m which is very close to the fracture energy Gf = 36.2–39.6 N/m determined following a RILEM recommendation. The post-peak response during crack propagation follows the fracture criterion of KI = KIC if FPZ is considered,from which the theory CMOD-load curve agrees well with the experiment result. Both theory and experimental results show that the crack speed decreases as the crack length increases.

Initiation pressure model for liquid CO2 fracturing through upward penetrating boreholes and its engineering verification

FAN Shixing1,WEN Hu1,JIN Yongfei1,CHEN Jian2,TONG Xiaozhang1,2,CHENG Xiaojiao1,YU Zhijin1
 2021, 40 (4): 703-712 doi: 10.13722/j.cnki.jrme.2020.0228
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Determining the initiation pressure of coalseam in liquid CO2(L-CO2) fracturing technology is a fundamental scientific issue. According to the stress superposition principle and maximum tensile failure criterion and combined with the weakening effect of low viscosity of L-CO2 on the tensile strength of coal,an initiation pressure model for L-CO2 fracturing was established by superimposing the horizontal principal stress,the fracturing fluid pressure and the circumferential stress caused by the percolation effect,and the upper and lower limits of the initiation pressure model were determined by the limit assumption of fracturing fluid viscosity. A set of high pressure(30 MPa) L-CO2 fracturing equipment with a three-stage L-CO2 injection process was firstly developed to be implemented in underground coal mine,and the first permeability enhancement field test through L-CO2 fracturing was carried out in Pansan coal mine in Huainan. The initiation pressure calculated by the theoretical model ranges from 15.4 to 21.4 MPa,which is not only compatible with the four measured values of two fracturing boreholes(22.9 MPa,16.2 MPa and 21.3 MPa,20.1 MPa,respectively) but also consistent with 65%–70%(15.6–23.2 MPa) of the initiation pressure using hydraulic fracturing,showing the correctness of the initiation pressure theoretical model. The research results have a certain guidance meaning for the application of L-CO2 fracturing.

Study on shear mechanical properties of mudstone with weak intercalation

ZHANG Zelin1,WANG Tao2,WU Shuren2,PENG Hongtao1
 2021, 40 (4): 713-724 doi: 10.13722/j.cnki.jrme.2020.0725
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Weak intercalation has great influence on slope and surrounding rock stability. The study on shear mechanical properties and failure modes of weak intercalation has practical engineering significance. In this study,mudstone with weak intercalation in Tianshui,Gansu province was studied. Based on triaxial undrained shear test and numerical analysis,shear mechanical properties of remodeling weak intercalated layer samples with different dip angles were investigated. The stress-strain relationship of the weak intercalation was analyzed,and the shear failure characteristics were discussed. The results show that the experimental and theoretical analysis results are basically consistent with the numerical results and that the shear failure occurs inside the weak layer. With increasing the thickness and the dip angle of the weak layer,the shear strength of the sample decreases gradually in the range of 0°–60° of the dip angle. The weak layer with a low dip angle shows crushed and inflated failure while occurs shear slip along the weak layer as the dip angle is greater than 15°. When the weak layer is thin,the sample is sheared along the structural plane. While the weak layer is thick,extrusion occurs inside the weak layer,or shear slip occurs on the contact surface of the weak layer. What is inconsistent is that,when the dip angle of the weak layer is 0°–15°,the experimental and theoretical analysis results indicate that failure occurs by cutting through the weak layer,while the numerical study shows that there is no obvious shear failure in the weak layer and the failure reflects extrusion and swelling within the weak layer.

Simulation of rock failure by Voronoi-based discontinuous deformation analysis

ZHANG Kaiyu1,2,XIA Kaiwen1,2,LIU Feng1,2
 2021, 40 (4): 725-738 doi: 10.13722/j.cnki.jrme.2020.0745
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Discontinuous deformation analysis(DDA) is an implicit discrete element method for simulating deformable discrete block systems. Since the meso-grain structure of rocks is similar to Voronoi polygons,this paper will study the rock failure simulation with DDA using Voronoi discretizations,and compare its performance with DDA using triangle discretizations. The influence of the sub-block size on the elastic parameters of the material is studied,and the numbers of sub-blocks and the contact pairs using these two discretizations are compared. By simulating the uniaxial compression test,Brazilian test,Brazilian test with an eccentric circular hole and Kalthoff- Winkler impact experiment,the influences of these two types of discretizations on rock macroscopic parameters,failure modes and crack propagation paths are analyzed. Meanwhile,in order to reduce mesh-dependence,the influence of the randomness of the discretizations on the simulation results is discussed. The results show that an overly regular discretization will lead to an unreasonable failure pattern,while an overly random discretisation may lead to more disperse results and also worse convergence. That is to say,a proper degree of randomness is necessary for such simulations. It is also shown that DDA with Voronoi discretizations can effectively simulate the failure process of rocks,and that, in comparison with the triangle model,the Voronoi model leads to greater computational efficiency,higher elastic modulus,higher compression strength,higher tensile strength and more realistic compression-tensile ratio,and the crack propagation path is similar to the inter-granular fracture of the rock.

Research on deformation characteristics and instability mechanisms of large monoclinal layered bedrock landslides:a case study of the Longjing landslide in Shizhu county,Chongqing

ZHU Sainan1,WEI Yingjuan2,WANG Ping3,ZHANG Zhihua3,WU Xiaobin3,WANG Wenpei1,YANG Liu3
 2021, 40 (4): 739-750 doi: 10.13722/j.cnki.jrme.2020.0653
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Taking Longjing landslide as an example,the deformation characteristics,genetic mechanisms and three-dimensional stability of the landslide are analyzed in detail by means of field survey and mapping,satellite remote sensing interpretation,UAV survey,engineering geological drilling,deep displacement monitoring,and field and laboratory tests. The landslide about 142.83×104 m3 is a large monoclinal layered rock landslide with complex failure characteristics. The elevation difference between the front and the rear margins is about 50 m. The leading edge shear outlet is near a precipice,which provides potential energy for the high shear of the landslide. Two groups of dominant structural fractures and weak interlayers in the slope cut the medium-thick bedrock into fragmentation blocks,which makes the rock mass has good dispersion and provides favorable conditions for the long run-out landslide. After long-term evolutionary creep,the shear strength of the soft interlayer decreases gradually,forming a sliding zone,which provides the mechanical conditions for the landslide. The three-dimensional static analysis is carried out by using the method of mechanical vector solution. When the saturated residual shear strength is adopted,the safety factor of the landslide is 0.93. The slip body will creep along the true trend direction of the soft interlayer along the strata,and will slip in the SE direction at a small angle after being blocked by the stable mountain in the west. In this paper,the deformation characteristics and instability mechanisms of Longjing landslide are analyzed comprehensively by various means,which can provide useful reference for the early identification and instability mode judgment of wedge landslides with blocked steering.

Shaking table test study on seismic optimization comparisons of multi-anchor piles for strengthening soil slopes under earthquake

PAI Lifang1,2,WU Honggang2,3,4,MA Huimin5
 2021, 40 (4): 751-765 doi: 10.13722/j.cnki.jrme.2020.0634
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To study the distribution characteristics and spatial variations of the acceleration and dynamic soil pressure responses of slopes strengthened by multi-anchor piles under earthquake action,the optimal seismic performances of multi-anchor piles were discussed. In this paper,the regional spatial distribution characteristics along the pile elevation were obtained through the preliminary analysis of time-domain characteristics of the stability,the acceleration and the dynamic earth pressure of a multi-anchor pile slope model by large shaking table test. Then,the correlation of the damage level of the slope strengthened by multi-anchor piles was obtained by using Fourier change and statistical probability scatter matrix operation. Finally,regional differences of seismic deformation Sd were calculated by SPECTR with or without optimization of multi-anchored pile slope model. The results show that,under the action of different earthquake intensities,the slope model shows a continuous spatial deformation effect of regional damage and failure,and the spatial distributions of the acceleration and the dynamic earth pressure show an outstanding response to the unoptimized lateral amplitude of the pile structure. The acceleration lag difference along the elevation is mainly caused by the propagation stage after the main earthquake,the correlation between the seismic earth pressure and the acceleration response of each group before the pile in the same earthquake area is very weak,and the ground motion characteristic of the foreshock is not simply repeated superposition of the ground motion sequence of all levels. The shock absorbing layer with polystyrene foam(EPS plate) and multi-anchor piles with energy dissipation spring as self-coordination device of the anchor head play a buffering and energy dissipation role on the deformation of slope body under earthquake action,and the optimization effect is related to the position of the shock absorbing layer. Under the actions of low and medium strength earthquakes,the telescopic deformation of the energy-dissipating spring device of the anchor head improves the deformation coordination of the multi-anchor pile along the elevation and the seismic wave propagation characteristics of the pile. Under the action of a high-strength earthquake,the sliding surface of the pile is highly sensitive to the earthquake and EPS has plastic deformation,which increases the relative displacement value on the optimized side and is easy to cause“bulging”failure or shear failure on the sliding surface of the multi-anchor pile easily forming the seismic weak link of the multi-anchor pile. These results are helpful to provide theoretical basis for the optimal seismic design of multi-anchor piles.

Experimental study on size effect of compressive strength of jointed rock mass based on representative sampling

LIU Dan1,2,HUANG Man1,2,HONG Chenjie1,2,CHEN Xuannan1,2,DU Shigui1,2
 2021, 40 (4): 766-776 doi: 10.13722/j.cnki.jrme.2020.0664
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Representative sampling of different-size samples has a greater influence on the size effect law of jointed rock mass strength. Based on the proposed representative sampling method,series of jointed rock mass models with different sizes are constructed in this paper,and numerical simulation of the size effect on the compressive strength is carried out. The results show that the failure patterns and the stress-strain curves of rock mass samples of different sizes are quite different,and the failure mode and mechanical properties of different size rock masses are reflected by the test results obtained through representative samples,which verifies the feasibility of the representative sampling method. The size effect law of the compressive strength of jointed rock masses with different trace lengths and dip angles are analyzed,showing that the peak compressive strength gradually decreases and tends to be stable as the size increases and that different trace lengths and dip angles have different degrees of reduction in the strength. Further analysis indicates that the dispersion of the compressive strength of small-sized samples is larger while that of large-sized samples is smaller. The representative sampling method has a more obvious effect on the sampling of small-sized samples and can integrally analyze the mechanical properties of jointed rock masses of different sizes. The research work provides an experimental method for the study on size effect.

Influence of rainfall preponderance infiltration path on reactivation of ancient landslides

ZHANG Yongshuang1,2,WU Ruian3,REN Sanshao1,2
 2021, 40 (4): 777-789 doi: 10.13722/j.cnki.jrme.2020.0755
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Water is one of important factors leading to the reactivation of ancient landslides,and cracks can intensify rainfall infiltration. The coupling effect of water and cracks is the essential reason to promote the reactivation of ancient landslides. Based on studying lots of ancient landslide reactivation cases,the centrifuge physical model tests were carried out. The experiment results show that the permeability of ancient landslides is so weak that it is difficult for regular rainfall to infiltrate into the deep site of landslides,which cannot easily induce the reactivation of large-scale landslides. When cracks appear on the landslide surface and gradually expand downward,the reactivation possibility of ancient landslides in a large scale increases significantly. Furthermore,numerical simulation is performed to analyze the influence of the depth,location and number of cracks on the reactivation of ancient landslides under rainfall conditions. The results indicate that with increasing the crack depth,the influence range of seepage is larger and larger,and the time for rainwater to reach the sliding zone is shorter and shorter. When the crack is located at the rear of the landslide,the landslide is easy to reactivate with a push type along the ancient sliding zone. When the crack is located at the front of the landslide,it is easy to promote the occurrence of progressive reactivation from the front edge to the rear edge,and the required rainfall time is shorter. With increasing the number of cracks,the influence range of the seepage field in the landslide body is enlarged and the saturation time is shortened obviously,which aggravates the reactivation of the ancient landslide. It is suggested that timely treatment of the cracks in the ancient landslides is the key to effectively prevent the reactivation of ancient landslides.

A prediction model of dynamic pore water pressure for MICP-treated calcareous sand

LIU Hanlong1,2,ZHANG Yu1,2,GUO Wei3,XIAO Peng1,4,HUANG Ming5,CHU Jian6,XIAO Yang1,2
 2021, 40 (4): 790-801 doi: 10.13722/j.cnki.jrme.2020.0581
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The calcareous sand foundation may be liquefied to cause structural damages under dynamic loadings such as earthquakes and waves. The microbially induced calcite precipitation(MICP) can enhance the strength of the calcareous sand foundation on a large scale and improve its liquefaction resistance. In this paper,the dynamic pore pressure development of MICP-treated calcareous sand was studied through a series of cyclic triaxial tests,and the effects of the effective confining pressure,the dynamic stress ratio,the relative density and the biocementation level on the development of the dynamic pore water pressure of MICP-treated calcareous sand were investigated. It is found that the pore water pressure development of MICP-treated calcareous sand is greatly affected by the dynamic stress ratio and the biocementation level. Based on the dynamic stress ratio and the biocementation level,MICP-treated calcareous sand presents three different pore water pressure development patterns. The pore water pressure curve gradually transitions from S-type to hyperbolic type with increasing the dynamic stress ratio or the biocementation level. Based on the development of the dynamic pore water pressure,an uniform pore water pressure model was proposed for MICP-treated calcareous sand. The advantage of this model over other models in predicting the development of the dynamic pore water pressure in MICP-treated calcareous sand was demonstrated through comparisons.

Experimental study on mechanical and deformation performances of geogrids- reinforced soil retaining walls under cyclic loading

XIAO Chengzhi,LI Haiqian,GAO Shan,WANG Zihan
 2021, 40 (4): 802-813 doi: 10.13722/j.cnki.jrme.2020.0707
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Offset distance is one of the key factors affecting the mechanical and deformation performances of multi-tiered geogrids-reinforced soil retaining walls(GRSRW). Firstly,the ultimate bearing capacity test of the single-tiered GRSRW was carried out in door to determine the magnitude of the cyclic load. Furthermore,the displacement,the earth pressure and the reinforcement strain in multi-tiered geogrids-reinforced soil retaining walls(MGRSRW) and potential sliding surface were analyzed under consideration of variation of the offset distance D. The test results show that the maximum displacement of the panel and the peak of the additional horizontal earth pressure appear at 0.85 height of the wall. With increasing the offset distance,the horizontal displacement and the additional horizontal earth pressure of the lower retaining wall as well as the additional vertical pressure near the bottom of the panel decrease,and the strain of the geogrid at the bottom also decreases. The foundation settlement and the horizontal displacement of the retaining wall face increase obviously at the beginning of the test,while the increase amplitude slows down and converges as the number of cycles N is greater than 5 000. The potential sliding surfaces of the upper retaining wall mainly pass through the side of the loading plate near the facing panels and 0.6 height of the upper wall,or through the side of the loading plate away from the facing panels and 0.3–0.4 height of the facing panel. The results will enrich experimental researches and provide reference for practical applications.

Effect of the strength of rock blocks on the shear characteristics of soil-rock mixtures

YANG Zhongping1,2,3,ZHAO Yalong1,2,3,HU Yuanxin4,LI Shiqi1,2,3,LEI Xiaodan1,5,LI Xuyong1,2,3
 2021, 40 (4): 814-827 doi: 10.13722/j.cnki.jrme.2020.0488
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The insufficient strength of rock blocks is one of the most important reasons for the instability of large soil-rock mixture(S-RM) filling bodies. In order to deeply explore the influence of the strength of rock blocks on the shear characteristics of the S-RMs and to make up for the deficiencies in the current research field,systematic research was conducted utilizing indoor large-scale direct shear test and discrete element numerical simulation(PFC) based on a great filling project in Chongqing. The macroscopic performances and microcosmic mechanisms of the block strength affecting the shear deformation,the peak shear strength,the crack extension process and the way of the energy response of the S-RM were revealed finally. The results show that,under a high axial pressure,the shear characteristics of the S-RMs with different distinct block strengths are remarkably different. The stress-strain relationships for hard and soft rock mixtures are respectively characterized by strain hardening and strain softening. The shearing process is basically consistent with the linear Mohr-coulomb strength theory,and with increasing the block strength,the internal friction angle and the peak shear strength of the mixture increase while the cohesion shows a trend of increasing first and then decreasing. The greater the block strength,the more complete the fracture development and the greater the fracture density. The hard rock shows a strong remolding ability which makes the mixture tend to form a shear plane with a complex cross-section,while the shear plane of the soft rock mixture is almost the same as that of plain soils. In the shearing process,the hard rock mixture dissipates the energy mainly in the way of friction,while the soft rock mixture absorbs and stores deformation energy mainly through the plastic deformation and the adjustment of spatial positions between rock blocks. Finally,PFC numerical simulation software was adopted to further expand the analysis,and a shear strength prediction model of S-RMs was proposed.

Plastic deformation and critical dynamic stress of fine-grained soils under intermittent loading of trains

NIE Rusong1,2,LI Yafeng1,LENG Wuming1,2,SUN Baoli1,LIU Xing1,CHEN Mengfan1
 2021, 40 (4): 828-841
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The dynamic loading of the trains on the subgrade includes the cyclic loading stage and the intermittent stage,that is,an intermittent dynamic loading. To study the plastic deformation behavior and the critical dynamic stress of the subgrade filler under the intermittent loading of trains,a series of continuous and continuous-stopping repeated load triaxial tests under different water contents,confining pressures and dynamic stress levels were conducted,and the influence of the loading intermittence on the deformation characteristics of the silt filler and the plastic deformation behavior under intermittent loading were analyzed. The test results indicate that due to the unloading and drainage in the intermittent stage,the excess pore water pressure accumulated in the loading stage dissipates in the intermittent stage and the particles and the structure of the soil were also adjusted,reducing the accumulation of the plastic strain in subsequent loading stages,and increasing the critical dynamic stress of the sample. The plastic deformation behavior of the silt filler under the intermittent loading can be classified as plastic shakedown,plastic creep and incremental collapse. Based on the plastic strain rate,a criterion for classifying the plastic deformation behaviors of silt under intermittent loading was established. By introducing the static strength,an empirical formula of the critical dynamic stress of silt under intermittent loading,considering the water content and the confining pressure,was proposed. The research results are of great significance for understanding the deformation behavior and dynamic stability of the subgrade under the actual railway operation conditions.

Analysis of unsaturated seepage siltation of groundwater source heat pump recharge

PAN Ding1,TANG Hong1,LIU Jun2
 2021, 40 (4): 842-850 doi: 10.13722/j.cnki.jrme.2020.0727
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At present,analysis of recharge clogging focuses on saturated seepage,rather than unsaturated seepage which is discussed in this paper. In view of the difficult problem of water source heat pump pressure recharge in Wuhan area,using the unsaturated soil VG model and combined with relevant pumping recharge examples,the saturation of soils under different pressures is evaluated,and the change of the amount of pumped r is analyzed. It is pointed out that the pressure recharge will increase the involvement of gas phase factors and,to a certain extent,results in unsaturated seepage. Gas phase blockage maybe further causes unsaturated blockage. In addition,numerical simulation using FLAC3D is carried out,the pore pressure fields of pressure recharge and unsaturated seepage are compared. The change of the pore pressure field during the formation of unsaturated blockage is analyzed,and the area of unsaturated seepage around the back irrigation well is derived. Finally,different measures for the removal of siltation are proposed in the front and later stages,and new ideas are put forward for the study of segment blockage.

Performance analysis of a 30.2 m deep-large excavation in Hangzhou soft clay

CHENG Kang1,2,3,XU Riqing1,2,YING Hongwei1,2,4,LI Binghe5,GAN Xiaolu1,2,QIU Zhijian6,ZHAN Xiaobo7,QIN Jianshe6
 2021, 40 (4): 851-863 doi: 10.13722/j.cnki.jrme.2020.0636
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Taking a 30.2 m deep and large basement excavation in Hangzhou soft clay as the research object,the excavation performances,including the stress of diaphragm walls,the displacement of columns,the axial force,the earth pressure and the settlements of ground surface,are thoroughly investigated based on 16 excavation cases collected in Hangzhou and the data published in literature about similar excavation in Shanghai. Following conclusions are obtained:(1) The location of the maximum wall deflection is between He-5 and He + 2.5. Benefiting from the excellent integrity of the retaining system,the maximum wall deflection is well controlled with an average value of 0.28%He. (2) Due to the“corner effect”,the deflection of the walls located at the excavation center is 3.5 times of that located at the corners,suggesting that the“corner effect”is significant to restrict the deflection for the excavation without adopting the zoned excavation technique. (3) Both the diaphragm walls and the ground surface produce a maximum displacement increment in stage 6 due to the“creep effect”of Hangzhou soft clay and the “depth effect” of deep excavation. The excavation of deep soil can release more stress and induce greater displacement than shallow soil. (4) The horizontal struts mainly carry the load due to the removal of the nearby soil and are slightly influenced by the distant excavation. The transferring of the excavation-induced load to the struts is completed in 1–2 months after strut casting. (5) Different from trapezoid or AEP triangle envelope distribution mode,the horizontal earth pressure behind the diaphragm wall of the considered project is linearly distributed along the depth. The ground adjacent to the excavation surface is more likely in an active state and generates smaller horizontal earth pressure. Based on the 30.2 m deep excavation and the collected 16 similar excavation cases in Hangzhou,an empirical relation between the excavation area and the maximum wall deflection is proposed.

Study on bolt grouting mechanism and control technology of broken surrounding rock in deep roadway

PAN Rui1,2,3
 2021, 40 (4): 864-864 doi: 10.13722/j.cnki.jrme.2020.1039
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