MODEL FOR PREDICTING SHEAR STRENGTH OF SATURATED SOFT CLAY AFTER IMPACT LOADING

Previous studies showed that the samples of saturated soft clay undergoing several stages of impact loading and reconsolidation show obvious quasi-overconsolidation state, and have an increased shear strength. In this paper, the definition of quasi-overconsolidation ratio is given, and furthermore a model for predicting the shear strength of saturated soft clay after impact loading is suggested. In this model, the conditions of partial drainage and stages of impact are taken into account. The calculation results by the proposed model are in good accordance with test data. Besides, the effect of disturbance consolidation on the undrained shear strength by this model is discussed.     

Revisiting the Bjerrum's correction factor:Use of the liquidity index for assessing the effect of soil plasticity on undrained shear strength

The undrained shear strength(s_u) of fine-grained soils that can be measured in situ and in laboratory is one of the key geotechnical parameters.The unconfined compression test(UCT) is widely used in laboratory to measure this parameter due to its simplicity;however,it is severely affected by sample disturbance.The vane shear test(VST) technique that is less sensitive to sample disturbance involves a correction factor against the soil plasticity,commonly known as the Bjerrum's correction factor,μ.This study aims to reevaluate the Bjerrum's correction factor in consideration of a different approach and a relatively new method of testing.Atterberg limits test,miniature VST,and reverse extrusion test(RET)were conducted on 120 remolded samples.The effect of soil plasticity on undrained shear strength was examined using the liquidity index instead of Bjerrum's correction factor.In comparison with the result obatined using the Bjerrum's correction factor,the lindrained shear strength was better represented when s_u values were correlated with the liquidity index.The results were validated by the RET,which was proven to take into account soil plasticity with a reliable degree of accuracy.This study also shows that the RET has strong promise as a new tool for testing undrained shear strength of fine-grained soils.     

Laboratory study on the mechanical behaviors of an anisotropic shale rock

Shale gas is becoming an important energy source worldwide.The geomechanical properties of shale rocks can have a major impact on the ef fi ciency of shale gas exploration.This paper studied the mineralogical and mechanical characteristics of a typical gas shale in Ohio,USA.Scanning electron microscope(SEM)with energy dispersive X-ray(EDX)analyses was employed to measure the microstructure and material composition of the shale rock.The anisotropic behaviors of shale rock,including compressive and tensile strengths,were experimentally measured.The characteristics of shale rock were also studied by nondestructive wave speed measurements.The shale demonstrated strong anisotropic behaviors with the tensile strengths perpendicular to the bedding plane around 300e360 times of that parallel to bedding plane.Results of ultrasonic tests indicated that both compression and shear wave velocities show strong anisotropic patterns.The compression wave speed was the smallest in the direction perpendicular to the bedding plane;while the shear wave speed was the smallest in the direction parallel to the bedding plane.The ratio of wave speed anisotropy is around 1.3e1.4 for compression wave;the ratio of shear wave speed anisotropy is larger and more diverse compared with the compression wave anisotropy.This might be related to the larger variability in the frictional adhesive strength along bedding plane than the compressive adhesive strength.     

ANALYSIS OF SIZE EFFECT, SHEARDEFORMATION AND DILATION IN DIRECT SHEAR TEST BASED ON GRADIENTDEPENDENT PLASTICITY

The paper concerns the issue of size law, localized deformation and dilation or compaction due to shearlocalization. It is assumed that the shear localization initiates at the peak shear stress in the form of single shear band, andbased on gradient-dependent plasticity, an analytical solution on size effect or snap-back is obtained. The results show thatthe post peak response becomes steeper and even exhibits snap-back with increasing of length. For small specimen, therelative shear displacement when specimen failure occurs is lower than that of larger specimen and the shearstress-relative displacement curve becomes steeper. The theoretical solution on non-uniformity of strains in shear band isobtained and evolution of the relative shear displacement is represented. By resorting to the linear relation between localplastic shear strain and local plastic volumetric strain, the dilation and compaction within shear band are analyzed.Relation between apparent shear strain and apparent normal strain and relation between shear displacement and verticaldisplacement are established.     

Design and verification of portable direct shear tester with application to remolded colluvium geomaterials

The mechanical properties of colluvium strongly govern the stability of colluvial slopes, and they are essential for the related analysis and design. Nevertheless, their measurement is not easy on account of heterogeneity in property and difficulty of sampling. This study attempted to evaluate the shear strength of remolded colluvium by means of a simple direct shear test in the field. A portable direct shear tester was designed to overcome the inconvenience and expensiveness of the conventional large-scale in-situ direct shear test. It can be easily assembled and applied for the simple field direct shear test. For calibration, the results of the portable direct shear tester were compared with the results of the laboratory direct shear tester for four different types of soil samples, i.e. standard sand, slate debris, arenaceous shale debris and mixture of gravel and sand. Correlation formulas were established based on the calibration, enabling the portable direct shear tester to measure and estimate the shear strength of remolded colluvium in field. This study primarily focuses on the colluvium in Central Taiwan, including the lateritic Dadu Terrace and the arenaceous shale of Taiping-Wufeng mounts. The portable direct shear tester was applied to sites selected in these areas, and the results were further analyzed and discussed.     

Human-induced landslide on a high cut slope: a case of repeated failures due to multi-excavation

The paper attempts to represent a case of repeated failures on a high cut slope due to multi-excavation. The characteristics of each failure induced by excavation are analyzed through geological investigation, and then a geological model at different failure stages is proposed. The geological analysis shows that the excavation-induced repeated failures are related to the exposure of the weak bedding plane and the toe unloading of the cut slope. Numerical modeling is conducted based on a sequential method, taking into account the main failure stages of cut slope. The simulation results fairly coincide with the practical phenomena observed in field. It is shown that the decrease in normal stress of displaced mass on cut slope will induce the increase in shear stress in bedding planes and that at the toe of the cut slope. The released stress leads to repeated gravitational instabilities of cut slope due to the decrease in normal stress and the increase in shear stress along the bedding planes of mudstone.     

Shear strength of an unsaturated weakly expansive soil

To study the weakly expansive clay obtained from a slope along Wuhan—Shiyan expressway in Hubei Province,soil-water property tests and some unsaturated triaxial tests with suction control were conducted,and the soil-water retention curve（SWRC） and unsaturated shear strength of this soil were obtained.Results show that the air-entry suction and the residual degree of saturation of the tested soil are 106 kPa and 8%,respectively.The boundary effect zone and the transition zone can be identified on the desorption curve,but the residual zone is not so obvious.The unsaturated shear strength increases as suction increases within the range of controlled suction in the test,and friction angle,b,in the triaxial shear test is 17.6°.Based on the results,constitutive models for predicting the unsaturated shear strength using the SWRC were evaluated,and comparisons between prediction and measurement were made.It is concluded that for engineering purpose,the constitutive model should be carefully selected based on soil properties when predicting the unsaturated shear strength using the SWRC.     

Pseudo-Dynamic Testing for Seismic Performance Assessment of Buildings with Seismic Isolation System Using Scrap Tire Rubber Pad Isolators

Investigation of seismic performance of buildings with STRP （scrap tire rubber pad） seismic isolators by means of pseudo-dynamic tests and numerical simulation is presented. The isolated building is numerically modeled, while the base isolation layer is considered as the experimental substructure in the pseudo-dynamic tests. The test result verifies that the STRP isolator shows acceptable shear deformation performance predicted by the design methods, and demonstrated that seismic isolation using STRP works as a protective measure to provide enhanced seismic performance of the building indicated by the reduction of top floor absolute acceleration, drift and base shear as designated.     

Behaviour of layered sandstone under Brazilian test conditions:Layer orientation and shape effects

The experimental study in this paper focuses on the effects of the layer orientation and sample shape on failure strength and fracture pattern of samples tested under Brazilian test conditions(i.e.diametrical loading of cylindrical discs)for one particular layered sandstone which is from Modave in the south of Belgium.The variations of the strength in combination with the failure patterns are examined as a function of the inclination angle between the layer plane and the loading direction.The experimental,results clearly show that the induced fracture patterns are a combination of tensile and/or shear fractures.In shape effect experiments the layer thickness and the number of layer boundaries are investigated.Different blocks of Modave sandstone are used to prepare samples.The layer thickness is different among the various blocks,but the layer thickness in each studied rock block can be considered to be constant;hence,the number of layer boundaries changes according to the sample diameter for samples of the same block.The experimental study shows that the layer thickness plays a more important role than the number of layer boundaries per sample.     

A critical state based thermo-elasto-plastic constitutive model for structured clays

In this paper, a critical state based thermo-elasto-plastic constitutive model is developed for destructured, naturally structured and artificially structured saturated clays. The model is an extension of the previously developed thermo-mechanical model by the authors for saturated clays, considering the effects of structure on the mechanical behaviors of the soil. It is based on change in the position of normal consolidation line(NCL) in a compression plane(e-ln p′) due to the soil's structure and variation of temperature. The present model is able to simulate the mechanical behavior of structured saturated clays in a triaxial plane at elevated temperatures lower than the boiling point of water. An attempt has been made to use the lowest possible number of parameters compared with that of Came Clay model and to ensure that these new parameters have clear physical interpretations. The sufficiency of the model was verified by the test results on artificially and naturally structured soils using thermal triaxial tests.     

基于梯度塑性理论的岩样单轴压缩扩容分析

采用梯度塑性理论，对岩样剪应变局部化引起的扩容进行了理论分析。假设岩石的剪切本构关系为弹性-应变软化双线性，局部化启动于应力峰值强度，利用局部塑性剪应变与局部塑性体积应变的线性关系，得到了局部塑性体积应变、局部塑性体积增量及剪胀引起的剪切带总塑性体积增量的解析式，这体现了该理论在研究剪胀问题时的优越性。另外，还得到了弹性阶段及应变软化阶段的轴向应力-体积应变曲线的理论关系。塑性体积应变是专指由剪切带剪胀而引起的，因而，轴向应力．体积应变不具有尺寸效应，与局部化带的尺寸无关，但扩容角、剪切降模量及泊松比却对该曲线有重要影响。在弹性阶段及应变软化阶段轴向应力-体积应变均呈线性。在相同的应力水平下，扩容角越大则剪胀程度越大；剪切降模量越大，剪胀程度越小。在应变软化阶段，泊松比不影响塑性体积应变。     

准脆性材料试件应变软化尺度效应理论研究

研究了由于剪切局部化而引起的试件长度的尺寸效应，基于可以考虑微结构相互作用的非局部理论，得到了非局部塑性剪应变与局部塑性剪应变及其二阶应变梯度的关系，通过获得剪切带内部的塑性剪切应变，得到了岩样轴向的平均应变与位移的理论关系，研究结果表明，这一关系具有尺寸效应，随着试件高度的增加，应力-应变曲线变陡，当试件高度非常大时，发生Ⅱ类变形行为，而且，随着剪切带倾角的增加，应力-应变曲线也变陡，将理论结果与前人的试验结果进行了比较，结果表明吻合良好。     

断层岩爆是应变局部化导致的系统失稳回跳

讨论了应变局部化、岩爆及Ⅱ类变形行为的关系，并利用梯度塑性理论得到了4种等效的回跳准则。局部化是岩爆的前兆之一，也是出现Ⅱ类变形行为的原因。利用最小势能原理及梯度塑性理论，可得到系统的失稳判据；利用位移法，可以得到系统的Ⅱ类变形行为。若非弹性剪切位移(或平均塑性剪切应变)的增加快于弹性剪切位移(或平均弹性剪切应变)的降低，将出现Ⅰ类变形行为；反之，将出现Ⅱ类变形行为。弹性区与剪切带宽度的比率越大，或剪切弹性模量与剪切降模量的比率越小，系统越容易失稳。     

平面应变压缩岩样侧向变形特征数值模拟

在平面应变压缩条件下,采用拉格朗日元法对岩样的应变局部化启动、传播、形成及侧向位移特征进行了数值模拟。在数值计算中,采用了莫尔-库仑剪破坏与拉破坏复合的破坏准则,峰后岩石的本构关系为线性应变软化。研究发现,试样的侧向位移具有非均匀性。由于端面约束的作用,接近试样上、下端面的节点的侧向位移远小于试样中部节点的侧向位移。在试样的上、下端面附近,由于弹性应变的恢复快于塑性应变的增加,因而,轴向压缩应力-侧向位移曲线出现了快速回跳现象。通常,可以通过检测岩样不同位置的侧向位移-时步曲线识别出变形局部化启动的时刻。相同高度的侧向位移-时步曲线最先发生分离的时刻,即为局部化启动的时刻。局部化启动之后,试样两侧的侧向位移具有非对称性,侧向位移变化具有非同步性。     

岩样失稳回跳与直剪试验机-岩样系统失稳回跳关系研究

研究了岩样及直剪试验机-岩样系统的弹性回跳不稳定性。将试验机简化为具有一定高度和剪切模量的钢块，利用梯度塑性理论得到了直剪试验机．岩样系统的剪应力．剪应变的理论关系。当不考虑钢块的高度时，这一关系便蜕化为岩样的剪应力-平均剪应变的理论关系。如果试验机-岩样系统回跳，那么岩样可能失稳回跳，也可能不失稳回跳；当钢块高度及剪切模量比值越大，试验机-岩样系统越容易发生失稳回跳。当这一比值较大时，纵然岩样不回跳，系统也会回跳：当这一比值较小时，只有当岩样回跳，系统才会回跳；当系统不回跳时，试样必不回跳。试样回跳，必然导致系统回跳：如果试样不回跳，则系统可能回跳，也可能不回跳。理论结果可解释若干常见的实验现象。     

岩样直剪应力-应变全程曲线研究

基于直剪试验机–岩样系统加载过程中各组成部件不同受力特点,运用功能原理,建立联合作用下的一维剪切实测应力–应变曲线全过程的参数方程,推导岩样失稳破坏判据和系统回跳条件,指出试验机–岩样系统弹性区段的存在是造成实测剪应力–剪应变曲线与岩样真实直剪本构曲线不一致和产生岩样失稳破坏、系统回跳现象的主要原因。通过算例分析剪切带宽度和试件高度对实测直剪应力–应变全程曲线的影响关系。分析结果表明,加载系统弹性受力区段越长,局部化剪切带宽度越小,实测软化段曲线就越陡,加载系统就越不稳定,越容易出现系统回跳现象,且在加载过程中岩样就会越早发生脆性破坏。     

岩样单轴压缩塑性变形及断裂能研究

首先研究了应变软化阶段岩石试件轴向塑性变形。假设局部化开始于峰值强度而轴向塑性位移根源于局部化的剪切位移。剪切带的相对塑性剪切位移与应力水平及剪切带宽度有关，剪切带宽度由梯度塑性理论确定。剪切带的相对塑性剪切位移在轴向的分量为轴向塑性压缩位移。研究结果表明：剪切带倾角对相对应力-塑性变形曲线斜率有一定的影响；若剪切带倾角存在尺寸效应，不同高宽比试件的相对应力-塑性变形曲线不是一条严格直线，而是一个狭窄的区域，类似“马尾”。但是，剪切带倾角对相对应力-塑性变形曲线斜率的影响是有限的，峰后应力-塑性变形曲线的斜率基本上是常量，这与前人的一些试验现象相符。然后，研究了单轴压缩条件下岩石试件全部断裂能的尺度律。全部断裂能由峰前断裂能及峰后断裂能两部分构成。在峰值强度前，采用Scott模型描述了材料的非线性弹性特征，得到了峰前断裂能的解析解。结果表明：峰前断裂能与试件的高度有关。在峰值强度后，材料的剪切应力-塑性剪切应变的本构关系为线性应变软化，采用梯度塑性理论计算了由于剪切带塑性剪切变形而消耗的断裂能。目前提出的关于全部断裂能尺寸效应的解析解的正确性被前人的试验结果的线性回归结果验证。增加试件高度，全部断裂能增加。增加弹性模量，全部断裂能降低。若不考虑剪切带倾角及抗压强度的尺寸效应，全部断裂能存在尺寸效应的原因是：峰前的均匀塑性变形。     

考虑应变率及应变梯度效应的断层岩爆分析

从理论上研究了考虑应变率效应及微小结构相互影响的断层带．弹性岩石系统的剪切不稳定性。在经典弹-塑性理论的屈服函数中引入应变梯度及考虑应变率效应的函数。将断层岩爆简化为一维动态剪切问题，利用：局部塑性剪应变的对称性；在断层带的边界上，其应变值为零；断层带的宽度由塑性剪切应变取极值来确定等条件，得到了断层带内部的局部塑性剪切应变及局部塑性剪切位移，二者都随应变率的增加而趋于明显增加。同时，还得到了应变软化阶段系统结构响应的理论表达式。令峰后刚度为无穷大，则可得到岩爆发生的临界加载应变率，该参数不仅与岩石材料的本构参数有关，而且还与岩石结构的几何尺寸(即弹性区域的尺寸)有关。应变率增加使断层．弹性岩石系统容易发生失稳回跳。此外，若不考虑应变率效应，则本文的理论结果可以退化为对断层岩爆的静力分析结果。     

ANALYSIS ON INCLINATION ANGLE OF SHEARBAND UNDER LOW CONFINING PRESSUREBASED ON GRADIENT-DEPENDENT PLASTICITY

The inclination angle ofsbear band is analyzed considering heterogeneity of rock material when a single shear hand is formed in the center of specimen under triaxial compression. The analytical solution of post-peak axial stress-axial strain curve is deduced using the assumption that the total post-peak deformation is composed of entire uniform elastic deformation and localized shear plastic deformation dependent on the thickness of shear band. The obtained solution shows that the post-peak stiffness is related to the inclination angle of shear band, confining pressure, thickness of shear hand and elastic modulus, etc. Using the solution, the expression for the inclination angle of shear hand can be presented easily and it is dependent on constitutive parameters of rock material and geometry parameters of rock specimen. Larger dilation angle or loading rate leads to increment of the inclination angle. In addition, the inclination angle increases with the thickness of the shear band, which cannot be explained or forecasted by other existing solutions, such as Coulomb inclination, Roscoe inclination and Arthur inclination, etc.     

平面应变条件下砂土局部化剪切带的有限元模拟

 根据饱和砂土的排水平面应变压缩试验的应变场,分析研究砂土的应变局部化现象以及剪切带的形成。在平面应变压缩条件下,砂土在峰值应力状态附近出现应变局部化现象,在残余状态最终形成一条V型剪切带。剪切带的形成是一个渐进过程,砂土呈现出渐进性破坏特性。这种由软化特性引起的应变局部化剪切带是砂土材料非常重要的变形和强度特性之一。基于砂土的三要素弹黏塑性本构模型和动态松弛有限元求解技术,对砂土的应变局部化现象和剪切带的形成进行有限元模拟。其中,砂土本构模型中引入应变局部化参数S来表示砂土峰值以后的软化和剪切带效果,剪切带在单元内未考虑其方向性,而是假设剪切带方向与最大剪切应变的方向一致。分析结果表明,动态松弛有限元法及砂土三要素模型能合理地模拟砂土的应变局部化现象,剪切带附近的最大剪应变值也非常接近,从而实现对砂土材料从硬化→峰值→软化→残余的全过程模拟以及对砂土应变局部化剪切带的定量化分析。