The Effect of Relative Density and Confining Stress on Shear Properties of Sands with Varying Grading

The research investigated the influence of grain size distribution on the shear behavior of sand specimen in loose, medium and dense states. The investigation aimed at understanding the extent or degree at which static shear strength of soil is affected by its density. A standard indicator (coefficient of uniformity) was used in constituting four (4) different sand specimens—narrowly graded (NAG), intermediately graded (ING), well graded (WG) and gap graded (GAG). Different normal stress values were applied during the testing program. The specimens were sheared using a ring shear apparatus until a residual value of shear stress was obtained. Result obtained from the analysis shows two remarkable stress paths. Medium dense to dense specimens show dilative stress path while loose (less dense) specimens show contractive stress path. At loose state, WG specimens show higher peak and residual shear strengths than poorly graded ones. Of importance was the fact that all the NAG specimens underwent complete liquefaction but WG specimens did not undergo complete liquefaction. This was interpreted as strong inter-particle contacts/forces binding the grains which enhanced high shear resistance. The research also showed that increase in relative density leads to higher shear strength, particle size distribution notwithstanding. Also, result of the analysis showed that WG specimens in medium-and dense states have higher peak but lower residual strength than ING and NAG specimens-this phenomenon was termed reverse behavior by the author(s). GAG specimens when sheared have the lowest peak and residual strength at any given value of relative density.     

Experimental investigation of the effect of grading characteristics on the liquefaction resistance of various graded sands

The liquefaction susceptibility of various graded fine to medium saturated sands are evaluated by stress controlled cyclic triaxial laboratory tests. Cyclic triaxial tests are performed on reconstituted specimens having global relative density of 60%. In all cyclic triaxial tests; loading pattern is selected as a sinusoidal wave form with 1.0 Hz frequency, and effective consolidation pressure is chosen to be 100 kPa. Liquefaction resistance is defined as the required cyclic stress ratio which caused initial liquefaction in 10 cycles during the cyclic triaxial test. The results are used to draw relationship between grading characteristics (e.g. coefficient of uniformity and coefficient of curvature) and the liquefaction resistance of various graded sands. It is found that a relationship between cyclic resistance and any of the size (i.e. D₁₀, D₃₀ or D₆₀) would be more realistic than to build a relation between grading characteristics and the cyclic resistance.     

Experimental study of the grading characteristic effect on the liquefaction resistance of various graded sands and gravelly sands

The liquefaction vulnerability of fine- to coarse-graded saturated sand and gravelly sand is evaluated by stress control cyclic triaxial laboratory tests. Cyclic triaxial test on reconstituted specimens is carried out with relative density of 45 %. Cyclic triaxial tests performed by sinusoidal wave form with 1-Hz frequency and with the effective consolidation pressure equal to 100 kPa. Cyclic stress ratio which caused initial liquefaction in 15 cycles recognized as the liquefaction resistance in cyclic triaxial test of present study. The result had been used to find a relationship between liquefaction resistance and the grading characteristics (e.g., coefficient of curvature and coefficient of uniformity) of various graded sands and gravelly sands. Then, it is realized that a relationship between liquefaction resistance and any of the sizes (i.e., D ₁₀, D ₃₀, D ₅₀, D ₆₀) appears to be more practical.     

Laboratory determination of mechanical and hydraulic properties of chemically grouted sands

The purpose of this paper is to investigate mechanical and hydraulic properties of sands treated with mineral-based grouts through the results of a laboratory test programme consisting of unconfined compression tests (UCS), triaxial bender element tests (BeT) and constant flow permeability tests in triaxial apparatus. An improved apparatus was set up for obtaining high quality, multiple grouted specimens from a single column. Two selected natural sands having different grain sizes were grouted with two mineral-based silica grouts, resulting in different levels of improvement. The behaviour of the sands treated by mineral grouts, in terms of strength, initial stiffness and permeability, was compared with that exhibited by more traditional silicate grouts. The results of this study indicate that sands treated with mineral grouts result in higher strengths, higher initial shear modulus and lower permeability values than the sands treated with the silicate solution. The effect of grout type, effective confining pressure, and sand particle-size on small-strain shear modulus of grouted sand specimens was evaluated. Based on test results, the small strain shear modulus increment from treated to untreated specimens has been correlated with the unconfined compressive strength, obtaining a unique relationship regardless of grout type and grain-size of tested sands.     

钙质砂抗剪强度特性的环剪试验

珊瑚礁沉积的钙质砂与石英砂的物理力学性质有较大差别。对取自南海岛礁的钙质砂进行了单次往返环剪试验以分析钙质砂的抗剪强度特性,试验中考虑了相对密实度和竖向应力对结果的影响,并与相同级配和试验条件下的石英砂进行对比分析。结果表明：钙质砂正向剪切时应力-位移曲线为软化型,具有明显的残余强度特性,而反向剪切时则表现为硬化型,正向和反向剪切强度基本一致;石英砂正向剪切和反向剪切均表现为软化型。钙质砂正向剪切和反向剪切残余强度与峰值强度的比值在0.75～0.93之间;石英砂正向剪切和反向剪切残余强度与对应峰值强度的比值在0.89～0.96之间。相同级配和试验条件下,钙质砂残余强度均大于石英砂,且强度比值基本保持在1.05～1.3之间。在100、200 kPa竖向荷载作用下,钙质砂0.5～2.0 mm的颗粒发生了破碎,破碎率分别为4%和6%。     

Effect of grading characteristics on the undrained shear strength of sand: review with new evidences

In this paper, the shear strength of saturated pure sand and sand–silt mixture is evaluated by monotonic undrained triaxial tests that were carried out on reconstituted specimens at same relative densities and a constant confining pressure (σ ₃?=?300 kPa). The test results were used to conclude on the effect of low non-plastic contents (0–20 %) and grading characteristics on the liquefaction resistance of the sand. The test results indicate that the undrained residual strength reduced with the increase of non-plastic fine content. Also, shear strength of gap-graded sand mixed with low non-plastic fine content increases with decrease in effective size (D ₅₀). In other words, in this state, we can use the D ₅₀ as a parameter to control of silty sand’s undrained resistance. Besides, the undrained residual strength of pure sand specimens with same effective size increases due to increase of coefficient of uniformity (C _u).     

Shear strength and pore-water pressure characteristics of sandy soil mixed with plastic fine

Recent earthquake case histories have revealed the liquefaction of mixtures of sand and fine particles during earthquakes. Different from earlier studies which placed an emphasis on characterisation of liquefaction in terms of the induced shear stress required to cause liquefaction, this study adopted a strain approach because excess pore-water pressure generation is controlled mainly by the level of induced shear strains. The current study includes the results of a set of laboratory tests carried out on sand specimens with the same relative densities and variation in the plastic fines (kaolinite or bentonite) contents ranging from 0 to either 30 % and consolidated at mean confining pressure of 100, 200 and 300 kPa using static triaxial test apparatus, in order to study the influence of fine content and other parameters on the undrained shear strength and liquefaction potential of clayey sand specimens; also, pore-water pressures in the specimens are discussed. Results of tests show that the peak strength decreases as the fines (kaolinite or bentonite) content increases up to a threshold content of fines (FC_th) after which, increases in plastic fine content lead to improve the peak shear strength of specimens, and also the ultimate steady-state strength has been improved due to the increased in plastic fines content. Also, pore pressure build-up in clayey sands is generally slower than that observed in pure sand.     

Triaxial shear testing of polished slickensided surfaces

A series of consolidated-drained triaxial tests were performed on precut and polished clay specimens to measure their drained residual strength. Two soils were tested during this research program: Rancho Solano Clay and San Francisco Bay Mud. Specimens were tested using a specially modified triaxial compression device which was developed to minimize the effects of end-platen restraint on the measured strengths. Special attention was paid to the influence of changing specimen area and membrane effects during the test. Triaxial test results were compared with baseline measurements of drained residual shear strength that were made for each of the clay soils using Bromhead ring shear tests and polished-specimen direct shear tests. Residual strength values measured in the triaxial device were higher than the drained residual strengths measured in the Bromhead ring shear apparatus and the direct shear device, indicating that this test approach is more challenging than the use of direct shear tests conducted on polished slickensided surfaces. Comparison of single stage and multistage triaxial test data indicates that multistage triaxial testing may work well for specimens that fail along a well-defined plane, provided that careful attention is given to the effects of end platen restraint, membrane restraint, and changes in specimen area during shear.     

Direct shear testing of polished slickensided surfaces

A series of ring shear and direct shear tests were performed to measure the drained residual strength of three clay soils. For each of the soils, slickensided direct shear specimens were prepared by wire-cutting intact specimens, and polishing the resulting shear plane on a variety of surfaces to align the clay particles in the direction of shear. Drained direct shear tests were then conducted on each of the polished specimens. The resulting shear strengths were compared with the residual strengths measured in the ring shear device to evaluate the effectiveness of the different polishing techniques for creating slickensided surfaces. Test results indicated that the measurement of residual strengths along preformed slickensided surfaces is extremely sensitive to both the soil type and the slickenside preparation technique that is used. Consequently, this approach does not appear to be a viable alternative to conventional repeated direct shear or ring shear tests to measure residual shear strengths.     

Coupling pore-water pressure with distinct element method and steady state strengths in numerical triaxial compression tests under undrained conditions

The undrained shear behaviour of sands has been a key topic after the devastating geo-disasters during the 1964 Niigata Earthquake in Japan. Extensive geo-technical soil tests, especially undrained triaxial compression tests, have revealed that the liquefaction phenomenon was the major cause for the disaster expansions. To numerically reproduce the liquefaction phenomenon, the pore-water pressure was coupled with a distinct element method. In this model, the dynamic changes in pore-water pressure were taken into consideration by the changes in volumetric strain and modulus of compressibility of water in the respective measurement spheres. Fluid-flows among the measurement spheres were controlled by Darcy’s law. The effective stress paths and steady state strengths in undrained triaxial compression tests associated with the wide ranges of initial void ratio were investigated. The effective mean stresses of medium-dense to dense numerical specimens at the steady state were negatively proportional to the initial void ratio. Loose numerical specimens reproduced quasi-liquefaction with the effective mean stresses that were less than 25% of the initial value. The medium-dense numerical specimens reproduced the phase transformation that was a typical characteristic of granular materials. The rolling restraints did not much influence of the effective angle of internal friction but strongly affected pore-water pressure behaviour within a certain range of initial void ratio.     

颗粒级配对粒状材料不排水力学特性的影响

为了研究颗粒级配对粒状材料不排水力学特性的影响,采用两种形状自相似度很高的材料（玻璃球和Hostun砂）进行了一系列传统三轴不排水压缩试验。对于每种材料,保持相近的相对密实度制作了6个不同颗粒级配（不均匀系数 1.1～20）的试样,然后进行剪切。试验结果表明,颗粒级配对粒状材料的不排水力学性能存在明显影响：两种材料试样的偏应力水平随着不均匀系数的增加而降低,抗剪强度随着材料的不均匀系数 的增加而降低,并在 5后呈现出趋稳势态。基于不排水剪切试验中颗粒集合体二阶功的演化规律,对材料受荷载过程中出现不稳定性的条件进行分析。结果表明,在相同密实度下,粒状材料的静态液化潜能随着材料不均匀系数 的提高而增强,高不均匀系数的材料变得不稳定。     

定轴排水剪切试验中各向异性砂土的力学响应

南京云母砂主要由片状颗粒组成,具有较为明显的各向异性特性。采用香港科技大学的自动控制空心圆柱扭剪仪进行了应力主轴方向角分别为10°、30°、60°、80°的密实云母砂定轴剪切试验,研究云母砂的初始各向异性规律。为研究中主应力的影响,又对同一密度的试样在中主应力系数b＝0.1、0.5、1.0三种不同情况下进行定轴剪切试验。试验结果表明,在大主应力方向角从0°～90°的变化过程中,强度先减小后增大,且随着b值的增加强度增大;变形表现出明显的各向异性,随着b值的增加体变增大     

Observation of shear zone development in ring-shear apparatus with a transparent shear box

Using a new ring-shear apparatus with a transparent shear box and video image analysis system, drained and undrained speed-controlled tests were conducted on coarse-grained silica sands to study the shear-zone formation process in granular materials. Velocity distribution profiles of grains under shear at various stages in the ring shear tests were observed through processing the video image by the Particle Image Velocimetry (PIV) program. Shear-zone thickness and type of shear mode (slide-like or flow-like) during shear were observed. Before reaching peak strength in low-speed and drained condition test, a comparatively major part of the sample in the upper shear box showed a velocity distribution profile of structural deformation and dilatancy behavior. After peak strength, the velocity profile changed into a slide-like mode and thereafter showed almost no change. In higher speed tests with drained and undrained conditions, an almost slide-like mode was observed, compared to low-speed test. Apparent shear-zone thicknesses of high-speed tests are thinner than low-speed tests. Unexpectedly, almost no difference was observed in the shear-zone thickness and mode of shear (slide or flow-like) between drained and undrained tests. This study was conducted as part of the International Programme on Landslides (IPL) M101 “Areal prediction of earthquake and rain induced rapid and long-traveling flow phenomena (APERITIF)” of the International Consortium on Landslides (ICL). These results will contribute to understanding the mechanism of shear-zone development in granular materials as a basic knowledge for disaster risk mitigation of rapid long run-out landslides.     

砂土充填大理岩节理的剪切强度经验公式

为研究砂土充填对节理抗剪强度的影响,利用GCTS（RDS?200型）岩石剪切测试系统对4种不同摩擦系数砂土充填的粉晶大理岩节理进行了直剪试验。结果表明：相同法向应力下,未充填节理的峰值剪切应力大于充填节理的峰值剪切应力,说明砂土的存在降低了节理的剪切强度;由节理面三维形貌参数最大谷深 表征节理面粗糙程度,得到了未充填节理的峰值剪切强度公式,公式预测值与试验值基本吻合;得到了用充填砂土摩擦系数表达的充填节理峰值剪切强度公式,公式预测值与试验值较为吻合。研究结论对充填节理岩体稳定性评价具有一定指导意义。     

Importance of Tensile Strength on the Shear Behavior of Discontinuities

In this study, the shear behavior of discontinuities possessing two different rock wall types with distinct separate compressive strengths was investigated. The designed profiles consisted of regular artificial joints molded by five types of plaster mortars, each representing a distinct uniaxial compressive strength. The compressive strengths of plaster specimens ranged from 5.9 to 19.5 MPa. These specimens were molded considering a regular triangular asperity profile and were designed so as to achieve joint walls with different strength material combinations. The results showed that the shear behavior of discontinuities possessing different joint wall compressive strengths (DDJCS) tested under constant normal load (CNL) conditions is the same as those possessing identical joint wall strengths, but the shear strength of DDJCS is governed by minor joint wall compressive strength. In addition, it was measured that the predicted values obtained by Barton’s empirical criterion are greater than the experimental results. The finding indicates that there is a correlation between the joint roughness coefficient (JRC), normal stress, and mechanical strength. It was observed that the mode of failure of asperities is either pure tensile, pure shear, or a combination of both. Therefore, Barton’s strength criterion, which considers the compressive strength of joint walls, was modified by substituting the compressive strength with the tensile strength. The validity of the modified criterion was examined by the comparison of the predicted shear values with the laboratory shear test results reported by Grasselli (Ph.D. thesis n.2404, Civil Engineering Department, EPFL, Lausanne, Switzerland, 2001). These comparisons infer that the modified criterion can predict the shear strength of joints more precisely.     

Prediction of cyclic resistance ratio for silty sands and its applications in the simplified liquefaction analysis

Liquefaction resistance of granular soils is commonly characterized by the cyclic resistance ratio (CRR) in the simplified shear stress procedure of liquefaction potential assessment. This parameter is commonly estimated by cyclic tests on reconstituted samples or empirical correlations between liquefied/non-liquefied case histories. The current study employs results of cyclic triaxial tests on reconstituted soil specimens and presents a predictive equation for cyclic resistance ratio (CRR) of clean and silty sands. The CRR equation is a function of relative density, effective mean confining pressure, non-plastic fines content, number of harmonic cycles for liquefaction onset, and some other basic soil properties. It is demonstrated that the developed relationship obtains reasonable accuracy in the prediction of laboratory-based CRR. Based on the developed CRR model, new relationships are then presented for the coefficient of effective overburden pressure (K_σ) and magnitude scaling factor (MSF), two important modification factors in the simplified shear stress procedure. These new modification factors are then compared with those recommended by previous researchers. Finally, the possible application of the proposed CRR model in field condition is shown for a specific case. This study provides a preliminary insight into the liquefaction resistance of silty sands prior to the complementary laboratory studies.     

安哥拉Quelo砂抗剪强度特性试验研究

基于安哥拉Quelo（Muceque）砂原状土和重塑土的直剪试验,探讨了结构性、含水率、干密度对Quelo砂抗剪强度的影响。试验结果表明,结构性对Quelo砂的抗剪强度影响较为复杂,原状土与重塑土的抗剪力学指标具有明显差异;Quelo砂遇水软化特性显著,原状土和重塑土的黏聚力和内摩擦角随含水率的增大近似呈对数衰减;Quelo砂级配良好,压实后抗剪强度高,适宜用作回填材料,重塑土的黏聚力随干密度的增大近似呈线性增大,内摩擦角随干密度的增大分为稳定段和增大段,呈分段函数的特征。     

Characterization of mesoscale instabilities in localized granular shear using digital image correlation

Within shear bands in sands, deformation is largely non-affine, stemming primarily from buckling of well-known force chains and also from vortex-like structures. In the spirit of current trends toward multiscale modeling, understanding the links between these mesoscale deformational entities and corresponding macroscale response will form the basis for the next generation of sand behavioral models and may also aid in efforts to understand jamming–unjamming transitions in dense granular flows in general. Experimental methods to quantify and characterize such subscale kinematics, in particular in real sands, will play critical roles in these efforts. Digital Image Correlation (DIC) is a fast growing experimental technique to nondestructively measure surface displacements from digital images. Here, DIC has been employed to identify and characterize the development of vortex structures inside shear bands formed in dense sands during plane strain compression. A rigorous assessment of the DIC method has been performed, in particular for subscale behavioral characterization in unbonded granular solids, and guidelines are offered for accurate implementation. While DIC systematically overestimates shear band thickness, a methodology has been devised to compensate for this overestimation. Shear band thickness for four different uniform sands were found to range between 6 and 9 grain diameters, and for a well-graded sand between 8 and 9.5 grain diameters. These determinations agree with visual inspections of grain kinematics from the image data, as well as recent theoretical predictions.     

K0固结条件下砂土的循环剪切特性试验研究

利用土工静力-动力液压三轴-扭转多功能剪切仪,针对福建标准砂Dr = 30 %,在K0固结条件下,进行不同围压下的循环三轴试验,探讨了K0固结过程和K0固结条件下砂土的孔隙水压力特性与强度特性,并与原有均等固结条件和非均等固结条件下的循环三轴试验进行对比分析。通过分析和比较表明,砂土的K0固结试验过程可以分为两个阶段,第1个阶段是初期的缓慢变形阶段,第2个阶段是后期的稳定发展阶段,围压对静止土压力系数K0的测定有一定影响。不同固结过程对砂土动孔隙水压力的发展有很大的影响,K0固结条件下试样出现明显的剪胀现象。另外,K0固结条件下砂土动强度高于非均等固结条件（Kc = 1.67）下砂土动强度,低于非均等固结条件（Kc =2.94）下砂土动强度。     

Modeling of the simple shear deformation of sand: effects of principal stress rotation

The paper presents a simple constitutive model for the behavior of sands during monotonic simple shear loading. The model is developed specifically to account for the effects of principal stress rotation on the simple shear response of sands. The main feature of the model is the incorporation of two important effects of principal stress on stress–strain response: anisotropy and non-coaxiality. In particular, an anisotropic failure criterion, cross-anisotropic elasticity, and a plastic flow rule and a stress–dilatancy relationship that incorporate the effects of non-coaxiality are adopted in the model. Simulations of published experimental results from direct simple shear and hollow cylindrical torsional simple shear tests on sands show the satisfactory performance of the model. It is envisioned that the model can be valuable in modeling in situ simple shear response of sands and in interpreting simple shear test results.