Analysis of strength characteristics of loess before and after freezing using a hollow cylinder torsional shear apparatus

This paper aims to comprehensively analyze the influence of the principal stress angle rotation and intermediate principal stress on loess's strength and deformation characteristics. A hollow cylinder torsional shear apparatus was utilized to conduct tests on remolded samples under both normal and frozen conditions to investigate the mechanical properties and deformation behavior of loess under complex stress conditions. The results indicate significant differences in the internal changes of soil particles, unfrozen water, and relative positions in soil samples under normal and frozen conditions, leading to noticeable variations in strength and strain development.In frozen state, loess experiences primarily compressive failure with a slow growth of cracks, while at normal temperature, it predominantly exhibits shear failure. With the increase in the principal stress angle, the deformation patterns of the soil samples under different conditions become essentially consistent, gradually transitioning from compression to extension, accompanied by a reduction in axial strength. The gradual increase in the principal stress axis angle(α) reduces the strength of the generalized shear stress and shear strain curves.Under an increasing α, frozen soil exhibits strain-hardening characteristics, with the maximum shear strength occurring at α = 45°. The intermediate principal stress coefficient(b) also significantly impacts the strength of frozen soil, with an increasing b resulting in a gradual decrease in generalized shear stress strength. This study provides a reference for comprehensively exploring the mechanical properties of soil under traffic load and a reliable theoretical basis for the design and maintenance of roadbeds.

A novel specimen preparation method for TJ-1 lunar soil simulant in hollow cylinder apparatus

Conventional methods for hollow cylinder apparatus （HCA） specimen preparation are not applicable for T J-1 lunar soil simulant due to its wide particle size distribution. A novel method to prepare uniform T J-1 specimen for HCA tests is put forward. The method is a combination of the multi-layering dry-rodding method and a new under-compaction criterion in the multi-layer with under-compaction method （UCM）. In the novel method, the specimen is prepared with 5 layers by dry-rodding and the UCM is used to determine the height after each layer is compacted. The density uniformity of specimen is evaluated by the freezing method to find out the best under-compaction criterion. Two HCA specimens with the same target density are prepared by the novel method and examined in the tests of pure rotation of the principal stresses. Their conformable mechanical behaviors ascertain the effectiveness of the method to produce uniform and reproducible HCA specimens. Four groups of HCA tests are carried out to investigate the anisotropic and non-coaxial behaviors of TJ-I lunar soil simulant. The results indicate that the principal stress direction, the deviator stress ratio, the stress level and the coefficient of the intermediate principal stress significantly influence the strength and deformation properties of T J-1 lunar soil simulant.

空心圆柱仪(HCA)性能指标分析及同类产品比较

从量程控制、频率匹配、传感器种类和精度的选择等三个方面分析了甄选HCA仪的主要要素。同时对目前国际上具有代表性的一些空心圆柱仪在试样尺寸、加载能力和测量精度以及工作频率等方面的性能进行了比较,并着重强调了其在研究主应力轴旋转问题中的工作能力,这些分析对比工作对研究者根据自身的试验要求来选购和开发同类仪器具有参考价值。

Rock strength weakening subject to principal stress rotation:Experimental and numerical investigations

During the construction and operation of gas storage reservoirs,changes in the principal stress direction can induce fracture propagation under conditions of lower differential stress,potentially leading to failure in the surrounding rock.However,the weakening of strength due to pure stress rotation has not yet been investigated.Based on fracture mechanics,an enhanced Mohr-Coulomb strength criterion considering stress rotation is proposed and verified with experimental and numerical simulations.The micro-damage state and the evolution of the rock under the pure stress-rotation condition are analyzed.The findings indicate that differential stress exceeding the crack initiation stress is a prerequisite for stress rotation to promote the development of rock damage.As the differential stress increases,stress rotation is more likely to induce rock damage,leading to a transition from brittle to plastic failure,characterized by wider fractures and a more complex fracture network.Overall,a negative exponential relationship exists between the stress rotation angle required for rock failure and the differential stress.The feasibility of applying the enhanced criterion to practical engineering is discussed using monitoring data obtained from a mine-by tunnel.This study introduces new concepts for understanding the damage evolution of the surrounding rock under complex stress paths and offers a new theoretical basis for predicting the damage of gas storage reservoirs.

空心圆柱仪模拟列车荷载下土中应力路径能力分析

土体作为各向异性散粒体材料,其动力特性依赖于所处的应力路径。列车荷载下地基土在最大剪应力空间呈心形线旋转应力状态,但目前还未见室内试验中模拟心形线应力路径的相关研究。首先提出了土体动力响应解的简化拟合形式,并通过典型列车荷载作用下地基土所受应力特征分析验证了该简化形式的合理性。针对目前国际上已有的3类空心圆柱仪(HCA),推导出模拟列车荷载下地基土应力特征的加载方式:(1)双向振动HCA可通过确定轴力和扭矩的应力形式来实现心形线应力路径加载,此时球应力p的变化规律与轴力的加载波形一致,中主应力系数b在内、外压相等时呈余弦变化规律;(2)三向振动HCA在模拟列车荷载下的应力特征时,可以保持p不变或b恒为0.5;(3)四向振动HCA能够通过控制轴力、扭矩和内、外压的共同变化来较全面地模拟列车荷载下土单元体的应力特征,并可实现b为任意合理值且保持不变的前提下p、q之间保持恒定线性关系。并推导了3类HCA的加载参数在正应力均以压应力为主及中主应力方向维持在直径方向时的约束条件。

考虑主应力轴旋转的砂土变形特性试验研究

采用空心圆柱扭剪仪(HCA)进行一系列排水试验,研究主应力轴旋转条件下砂土的变形特性。试验采用2种类型的加载路径,第一种保持偏应力不变旋转主应力轴,第二种在增加偏应力的同时旋转主应力轴。通过改变偏应力的应力水平,研究砂土的变形以及非共轴度的变化规律。同时,对同等试验条件、材料和加载路径下不同密实度试样的试验结果分析,研究密实度对于非共轴度的影响。另外,采用2种材料进行对比试验(Portaway砂与LB砂),研究砂土初始各向异性与非共轴性之间的关系。试验研究结果表明:砂土的变形随着主应力轴的旋转而累加,并体现出显著的非共轴特性。砂土的非共轴度与材料所处的应力水平、密实度及初始各向异性均存在一定相关性。

四向振动空心圆柱扭剪仪模拟主应力轴旋转应力路径能力分析

主应力轴旋转对土体的动力特性有着显著影响。利用GDS动态空心圆柱扭剪仪(HCA)能够实现内围压、外围压、轴力和扭矩四向动态加载的功能,提出了3种主应力方向变化的应力路径加载方式,分析了3种应力路径在实际工程中的应用,并进行了试验验证。当四向动载根据推导的波形函数加载时,可以实现:1平均主应力p、中主应力系数b和偏应力q保持恒定,主应力轴循环旋转的应力路径;2平均应力p和中主应力系数b保持不变,偏应力q循环变化,而主应力方向角?=90°突变的应力路径;3平均主应力p、中主应力系数b保持不变,偏应力q、主应力方向角α连续变化的应力路径。应力路径1为模拟海洋波浪荷载作用下的应力路径试验提供了理论依据,应力路径2可用于模拟不同方向地震荷载的作用,应力路径3可以模拟波浪荷载作用下桩周土体的应力路径以及交通荷载作用下的应力路径。

浙江大学5Hz空心圆柱仪试样固结变形研究

目前,国际上研究主应力轴旋转对土体性状影响的较理想仪器主要是空心圆柱仪,浙江大学岩土工程研究所与英国GDS仪器设备有限公司共同研发的5Hz空心圆柱仪,可进行静态试验及中高频率下轴力、扭矩共同变载试验。准确测定空心圆柱试样的变形是后续应力控制试验的前提及应变测量的基准,而试样在等压固结以后,由于装样操作及气泡的影响很难直接精确地测量。通过对试样内压腔体变、反压腔体变及轴向应变的分析,利用迭代法,推导空心圆柱试样等压固结后内外径计算公式,修正Tastuoka关于试样内外径变化的假设,并与局部传感器实测值进行对比,说明由体变和轴向位移变化计算外径的方法是可行的,用修正后的内径计算结果重新进行试验可以大大提高试验的测试精度和应力控制的可靠性。

空心圆柱仪模拟波浪荷载下主应力轴旋转应力路径能力探讨

针对波浪荷载作用下土体单元复杂特殊的应力状态可利用空心圆柱仪模拟其主应力轴旋转应力路径。以波浪荷载下土体应力路径形状为出发点，推导出两向～四向空心圆柱仪（HCA仪）动力加载参数波形公式，经过理论分析验证了该表达方式的合理性，即：（1）两向振动HCA模拟只要满足内外围压差的绝对值与动偏应力q的不等式关系就可成功实现圆形应力路径的加载；（2）三向振动HCA可以模拟中主应力系数b或平均主应力p恒定不变的情况，当b值不变时，p值随主应力轴转角单调变化：（3）四向振动HCA能够较真实全面地模拟波浪荷载下土体的应力特征，同时还可实现主应力轴循环旋转过程中b、p、q三值均不变的应力状态。

Macro-micro investigation of granular materials in torsional shear test

A three-dimensional numerical torsion shear test is presented on hollow cylinder specimen which is performed on a spherical assemblage with fixed principal stress axes using the discrete element code PFC3D.Stack wall technique boundary conditions are employed and optimized to reasonably capture the microstructure evolution.Parametric studies are conducted in terms of the ratio κ,normal and shear stiffness of particles,wall stiffness and friction coefficients.Afterwards,in comparison with physical test,numerical results for a fixed principal stress angle(α=45°) are presented.The results show that the numerical test could capture the macro-micro mechanical behavior of the spherical particle assembly.The evolution of the coordination number demonstrates that particles in shear banding undergo remarkable decrease.The effects of localization on specimens illustrate that global stress and strain recorded from a hollow cylinder apparatus could not represent the localized response.The shearing band initiation and evolution from porosity and shear rate are visualized by contour lines in different shear strains.

Undrained monotonic and cyclic behavior of sand-ground rubber mixtures

In this study the stress–strain characteristics of sand-ground rubber mixtures are investigated in the sandlike zone,at different confining pressures,using hollow cylinder specimens subjected to torsional monotonic and cyclic loading.Under monotonic loading a mixture of sand-ground rubber with 10% and 25% rubber content show more contraction behaviour than that observed in a pure sand specimen.Phase transformation point in these mixtures are located on a larger shear strain.As expected,the shear strength of specimens decreases with increase of ground rubber content.However,with increasing of effective confining pressure,the loss in shear strength of the mixture is decreased.In addition,a mixture with 25% ground rubber shows a smaller loss in shear strength compared to a mixture with 10% ground rubber mixture.Under cyclic loading mixtures with 10% and 25% ground rubber have similar liquefaction resistance,especially at confining pressures of 110 k Pa and 260 k Pa.Therefore,by using of the mixture with 25% ground rubber,a larger volume of scrap tires could be recycled.The addition of ground rubber to sand would affect the shear strain variation and excess pore water pressure trends,and this effect was further intensified with increasing ground rubber percentage.

Definition of failure criterion for frozen soil under directional shear-stress path

A series of directional shear tests on remolded frozen soil was carried out at 10°C by using a hollow cylinder apparatus to study failure criterion under a directional shear-stress path.Directional shear tests were conducted at five shear rates(10,20,30,40,and 50 kPa/min)and five intermediate principal stress coefficients(b=0,0.25,0.5,0.75,and 1),with the mean principal stress(p=4.5 MPa)kept constant.The results show that the torsional strength and the generalized strength both increase with the increase of the shear rates.According to the failure modes of frozen soil under different shear rates,the specimens present obvious plastic failure and shear band;and the torsional shear component dominates the failure modes of hollow cylindrical specimens.A shear rate of 30 kPa/min is chosen as the loading rate in the directional shear tests of frozen soil.The shape of the failure curve in theπplane is dependent on the directional anglesαof the major prin cipal stress.It is reasonable to use the strain-hardening curves to define the deviatoric stress value atγg=15%(generalized shear strain)as the failure criterion of frozen soil under a directional shear-stress path.

Effect of Initial Stress Conditions on the Threshold Shear Strain of Nanjing's Saturated Fine Sand

By using GDS dynamic hollow cylinder torsional apparatus, a series of cyclic torsional triaxial tests under complex initial consolidation condition are performed on Nanjing saturated fine sand. The effects of the initial principal stress direction αo, the initial ratio of deviatoric stress η0, the initial average effective principal stress Po and the initial intermediate principal stress parameter b0 on the threshold shear strain γt of Nanjing saturated fine sand are then systematically investigated. The results show that γt increases as η0,p0 and b0 increase respectively, while the other three parameters remain constant. ao has a great influence on γt, which is reduced when ao increases from 0° to 45°and increased when α0 increases from 45° to 90°. The effect of α0 on γt, plays a leading role and the effect of η0 will weaken when ao is approximately 45°.

基于HCA的细尾矿砂破坏准则试验研究

为研究细尾矿砂破坏准则的具体形式,通过对空心圆柱扭剪仪基本公式推导的方法得出了试验应力路径所需的计算公式;在自行研发的辅助仪器配合下,利用空心圆柱扭剪仪试验的方法测试了细尾矿砂子午平面、π平面上的破坏曲线和不同静水压力下中主应力对各个主应变的影响。结果表明:在子午平面内随着静水压力的增加,破坏点广义剪应力增加;静水压力较小状态π平面内破坏曲线表现为Mohr-Coulomb准则,较大状态表现为Drucker-Prager准则,两种状态静水压力分界点应位于200～300kPa内,破坏准则是否与中主应力有关取决于中主应力对整体应变的影响水平。细尾矿砂的破坏准则应为Mohr-Coulomb与Drucker-Prager准则的组合。

主应力轴旋转下原状软黏土的变形及强度特性

为研究主应力轴旋转情况下饱和软黏土的变形及强度特征,采用先进的空心圆柱扭剪仪(HCA)对软黏土进行了一系列不排水剪切试验。试验采用等压固结模式对软黏土空心薄壁试样进行固结,并在3种不同中主应力系数情况下,对试样进行不同主应力轴旋转角度下的不排水剪切试验;剪切过程中保持平均应力、中主应力系数及主应力轴旋转角度不变,只增加偏应力,研究不同中主应力系数情况下主应力轴旋转角度对软黏土应力应变特征以及强度特性的影响。研究结果表明:不同主应力轴旋转角度下天然软黏土的变形及强度特征存在明显的差异,在中主应力系数b=0.5的非轴对称加载情况下,不排水剪切强度随着主应力轴旋转角度的增大呈勺形变化;在b=0的轴对称压缩条件及b=1的轴对称拉伸条件下,随主应力轴旋转角度的增加,不排水剪切强度均呈递减趋势。