Brief note on the influence of shape and percentage of gravel on the shear strength of sand and gravel mixtures

The paper records the influence of the shape and the percentage of gravel on the shear strength/frictional angle of sand and gravel mixtures using direct shear tests. The shear strength is mainly derived from the frictional forces developed due to sliding and interlock; they depend on the maximum particle size and shape, the uniformity coefficient, density and the effective normal stress. As the size of material in a mixture is variable, the shear strength also depends upon the ratio of the specimen diameter to the maximum particle size. In this study, two different shapes of limestone were used, angular and rounded, and the maximum gravel size was 6.3 mm in diameter. Air-dried samples were used in the tests. It is concluded that the shape and percentage of gravel have an important influence on the shear strength properties. Electronic Publication     

Microscopic investigation into liquefaction resistance of pre-sheared sand: Effects of particle shape and initial anisotropy

Liquefaction resistance of sand can be either increased or reduced due to an undrained cyclic pre-shearing depending on the degree of pre-shearing, which hinders a better prediction of liquefaction potential to be established. The mechanism of such changes in liquefaction resistance has been poorly understood. This contribution aims to gain micromechanical insights into pre-shearing effects on liquefaction resistance of sand using discrete element method (DEM) simulations. In particular, effects of particle shape and initial anisotropy on liquefaction resistance are investigated. The simulation results from samples consisting of non-spherical particles with an initial anisotropy can qualitatively capture the mechanical responses observed in equivalent laboratory experiments. The samples which yielded a qualitative agreement with the laboratory results are further analyzed micromechanically, and the relationships between liquefaction resistance and some microscopic parameters before cyclic loading are discussed. Microscopic analyses reveal that mean mechanical coordination number is well correlated with liquefaction resistance, whereas liquefaction resistance is less sensitive to anisotropy in particle orientation induced by pre-shearing.     

地质条件与施工工艺对基桩低应变时域反射波形的影响分析

从反射波法检测的基本原理入手,通过工程实例论述了场地条件,如土质、施工工艺等对低应变时域反射波曲线的影响,总结了土阻力及不同的成桩工艺对反射波形的影响规律,有利于指导工程实践。     

Experimental investigation of inter-particle contact evolution of sheared granular materials using X-ray micro-tomography

The inter-particle contact evolution of two sheared granular materials, i.e., a spherical glass bead (GB) specimen and an angular Leighton Buzzard Sand (LBS) specimen, is investigated non-destructively using X-ray micro-tomography. A miniature triaxial apparatus is developed for the testing with in-situ scanning. Full-field X-ray CT images of the two specimens are obtained at different shearing stages. A series of image processing and analysis techniques in combination with a particle-tracking approach is developed to detect the inter-particle contacts and to determine the contact gain, the contact loss, and the contact movement during each shear increment. It is found that the average coordination number (CN) experiences a strong change in the pre-peak shearing stage, and tends to reach a steady value after the peak. As the shear progresses, the average CN of the particles with different sizes follows the same trend as the overall average CN. Additionally, as the shear progresses, the branch vectors of the specimens, which, prior to shearing, are nearly isotropically distributed for the rounded GB and concentrated along the horizontal direction for the angular LBS, are found to show a directional preference towards the loading direction. The contact gain and the contact loss, which contribute to this directional preference, and the contact movement, which leads to the attenuation of the directional preference, are shown to be the two competing factors determining the evolution of the fabric anisotropy of granular materials. The higher degree of fabric anisotropy in the shear bands is shown to be mainly attributed to the higher percentages of contact gain and contact loss when compared to that of the entire samples.     

Influence of bedding structure on stress-induced elastic wave anisotropy in tight sandstones

To understand the evolution of stress-induced elastic wave anisotropy,three triaxial experiments were performed on sandstone specimens with bedding orientations parallel,perpendicular,and oblique to the maximum principal stress.P-wave velocities along 64 different directions on each specimen were monitored frequently to understand the anisotropy change at various stress levels by fitting Thomsen’s anisotropy equation.The results show that the elastic wave anisotropy is very sensitive to mechanical loading.Under hydrostatic loading,the magnitude of anisotropy is reduced in all three specimens.However,under deviatoric stress loading,the evolution of anisotropic characteristics(magnitude and orientation of the symmetry axis)is bedding orientation dependent.Anisotropy reversal occurs in specimens with bedding normal/oblique to the maximum principal stress.P-wave anisotropyε0 is linearly related to volumetric strain Sv and dilatancy,indicating that stress-induced redistribution of microcracks has a significant effect on P-wave velocity anisotropy.The closure of initial cracks and pores aligned in the bedding direction contributes to the decrease of the anisotropy.However,opening of new cracks,aligned in the maximum principal direction,accounts for the increase of the anisotropy.The experimental results provide some insights into the microstructural behavior under loading and provide an experimental basis for seismic data interpretation and parameter selection in engineering applications.     

Mechanics of thin-walled curved beams made of composite materials, allowing for shear deformability

In this paper, a new theoretical model is developed for the generalized linear analysis of composite thin-walled curved beams with open and closed cross-sections. In the present model two important concepts concerning to composite thin-walled curved beams are addressed. The first one is the incorporation in the model of what is called full shear deformability, i.e. shear flexibility due to both bending and non-uniform warping is considered. The second feature is connected with the constitutive aspects, and it contemplates the use of different hypotheses that can be adopted in the formulation. These topics are treated in a straightforward way by means of the Linearized Principle of Virtual Work. In order to obtain the motion equations of the model a non-linear displacement field, whose rotations are formulated by means of the rule of semitangential transformation, is employed. This model allows the study of many problems of statics, free and forced vibrations with arbitrary initial stresses and linear stability of composite thin-walled curved beams with general cross-sections. A discussion about the constitutive equations is performed in order to explain characteristic features of the effects included in the theory. This paper presents the theoretical formulation together with finite element procedures that are developed to obtain the numerical approximations to the general equations of thin-walled shear-deformable composite curved beams. For this kind of structural member, iso-parametric finite elements are introduced. Numerical examples are carried out in several topics of statics, dynamics and buckling problems, focusing attention in the validation of the theory with respect to experimental data and with 2D and 3D computational approaches. Also, new parametric studies are performed in order to show the influence of shear deformability on the mechanics of the thin-walled composite curved-beams with open and closed cross-sections as well as to illustrate the utility of the model.     

Assessment of anisotropic elastic parameters of saturated clay measured in triaxial apparatus: Appraisal of techniques and derivation procedures

Recent developments in laboratory techniques adopting highly precise local instrumentation have made possible the determination of all five independent elastic parameters necessary for describing the small-strain stiffness in cross-anisotropic soils. However, the techniques and the derivation procedures are not necessarily straightforward, and different processes sometimes lead to apparently inconsistent sets of parameters, revealing their complex and sensitive nature. This paper firstly reports a new fully-instrumented triaxial system that was optimised to test Japanese standard-sized (∅70–75 mm) clay samples. The testing techniques and procedures to obtain the five cross-anisotropic elastic parameters, defined in this paper for recoverable strain smaller than 0.001%, are reviewed. Some updates, including notes on how to deal with creep in soft clays and the optimisation of drained probe rates, are described. A simplified procedure is proposed for completing the parameter determination, without actually measuring the radial displacement, by inverting the relationship between the undrained Young׳s modulus and the drained elastic parameters in saturated soils. The validity of this approach is demonstrated by comparing the parameters of sedimentary clays obtained with and without radial measurements. By eliminating the need for complex radial instrumentation, this approach will make the quantification of stiffness anisotropy more accessible in less-equipped laboratories.     

Assessment of deformation modulus of weak rock masses from pressuremeter tests and seismic surveys

The deformation modulus of intact rock can be determined through standardized laboratory tests for heavily jointed rock masses but this is very difficult, while in situ tests are time-consuming and expensive. In this study, the deformation modulus of selected heavily jointed, sheared and/or blocky, weathered, weak greywacke, andesite and claystone were assessed, based on pressuremeter tests, geo-engineering characterization and seismic surveys. Empirical equations based on GSI and RMR values are proposed to indirectly estimate the deformation modulus of the greywackes. For the andesites, the spacing of the discontinuities is greater than the length of the pressuremeter probe hence the intact rather than rock mass deformation modulus is obtained. The pressuremeter test results from the claystones could not be correlated with the field data; the relationship between the ratio of rock mass modulus to intact rock modulus and RQD appears to give a better estimation of the deformation modulus.      

Particle breakage of granular materials during sample preparation

Particle breakage is commonly observed in granular materials when subjected to external loads. It was found that particle breakage would occur during both sample preparation and loading stages. However, main attention was usually paid to the particle breakage behaviour of samples during loading stage. This study attempts to explore the breakage behaviour of granular materials during sample preparation. Triaxial samples of rockfill aggregates are prepared by layered compaction method to achieve different relative densities. Extents of particle breakage based on the gradings before and after test are presented and analysed. It is found that particle breakage during sample preparation cannot be ignored. Gradings after test are observed to shift away from the initial grading. Aggregates with larger size that appear to break are more than the smaller-sized ones. Irrespective of the initial gradings, an increase in the extent of particle breakage with the increasing relative density is observed during sample preparation.     

Assessment of K0 correlation to strength for granular materials

The coefficient of lateral stress at rest K₀ is a state soil variable, yet is well correlated to strength. As different types of friction angles can be defined, it is important to examine the applicability of K₀-strength correlation and further clarify the mechanism of K₀-stress state. In this study, the values of K₀ were experimentally investigated for different types of granular materials, focusing on its correlation to material strength and the effect of particle shape and surface roughness. For this purpose, laboratory tests were conducted to directly measure K₀ of natural sand, spherically shaped glass bead, and surface-etched glass bead packings under various stress and soil conditions. Triaxial and other basic property tests were also conducted to characterize the test granular materials. It was revealed that the effect of material density on K₀ differed depending on the stress history whereas the effect of particle surface roughness, within the range considered in this study, was relatively small. Test results highlight that the values of the friction angle employed into Jaky's K₀ equation to match measured K₀ values are not unique, showing a state-dependent aspect. Inter-particle stress analysis was introduced to assess the correlation of K₀ to the friction angle as postulated by Jaky's K₀ equation.     

Particle breakage of sand subjected to friction and collision in drum tests

This paper presents a laboratory experimental study on particle breakage of sand subjected to friction and collision,by a number of drum tests on granular materials(silica sand No.3 and ceramic balls)to investigate the characteristics of particle breakage and its effect on the characteristics of grain size distribution of sand.Particle breakage increased in up convexity with increasing duration of drum tests,but increased linearly with increasing number of balls.Particle breakage showed an increase,followed by a decrease while increasing the amount of sand.There may be existence of a characteristic amount of sand causing a maximum particle breakage.Friction tests caused much less particle breakage than collision tests did.Friction and collision resulted in different mechanisms of particle breakage,mainly by abrasion for friction and by splitting for collision.The fines content increased with increasing relative breakage.Particle breakage in the friction tests(abrasion)resulted in a sharper increase but with a smaller total amount of fines content in comparison with that in the collision tests(splitting).For the collision tests,the fines content showed a decrease followed by an increase as the amount of sand increased,whereas it increased in up convexity with increasing number of balls.The characteristic grain sizes D₁₀ and D₃₀ decreased in down convexity with increasing relative breakage,which could be described by a natural exponential function.However,the characteristic grain sizes D50 and D60 decreased linearly while increasing the relative breakage.In addition,the coefficients of uniformity and curvature of sand showed an increase followed by a decrease while increasing the relative breakage.     

Evaluation of depth-dependent porosity and bulk modulus of a shear using permeability–depth trends

Both porosity and deformation properties of porous media are dependent on stress. For geological media, the stress-dependent porosity and deformation properties can be shown to be depth related. Unfortunately, both in situ porosity and in situ deformation properties are difficult to measure. In the current study, a method is developed to evaluate the changes in porosity and bulk modulus with depth by using in situ measurements of permeability, which can be easily obtained by employing packer tests. In the case study, the change in permeability of a shear in the Baihetan hydropower station in Southwest China is estimated. Then, the permeability–depth correlation is utilized to calculate the porosity and the bulk modulus at different depths. The results clearly show that the porosity decreases with depth while the bulk modulus increases with depth. Based on the proposed model, the bulk modulus is calculated to increase from 4–6 MPa on the ground surface to 18–30 MPa at the depth of 160 m. The Young's modulus of the shear estimated with this depth-dependent bulk modulus is consistent with in situ measurement.     

Stress-dependent shear wave splitting and permeability in fractured porous rock

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to 55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient (JMC). Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel (V_S1) and perpendicular (V_S2) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased. Simple models for the stress- and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength (JCS), JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.     

Assessment of clay stiffness and strength parameters using index properties

A new approach is developed to determine the shear wave velocity in saturated soft to firm clays using measurements of the liquid limit, plastic limit, and natural water content with depth. The shear wave velocity is assessed using the site-specific variation of the natural water content with the effective mean stress. Subsequently, an iterative process is envisaged to obtain the clay stiffness and strength parameters. The at-rest earth pressure coefficient, as well as bearing capacity factor and rigidity index related to the cone penetration test, is also acquired from the analyses. Comparisons are presented between the measured clay parameters and the results of corresponding analyses in five different case studies. It is demonstrated that the presented approach can provide acceptable estimates of saturated clay stiffness and strength parameters. One of the main privileges of the presented methodology is the site-specific procedure developed based on the relationships between clay strength and stiffness parameters, rather than adopting direct correlations. Despite of the utilized iterative processes, the presented approach can be easily implemented using a simple spreadsheet, benefiting both geotechnical researchers and practitioners.     

沟槽式埋管在杨东河水电站工程的应用

根据厂房、调压井的位置、压力管道沿线及厂区地形地质条件和施工、运行条件等边界条件,对杨东河(渡口)水电站工程压力管道进行合理选型和计算,最终确定沟槽式埋管作为推荐方案。     

Small strain stiffness and stiffness degradation curve of Bangkok Clays

The small strain stiffness and the stiffness degradation curve of soils are required in advanced numerical analyses of geotechnical engineering problems. The shear modulus at small strain (G_max) and the reference shear strain parameter (γ_0.7) are, for instance, two of the input parameters in a finite element analysis with the hardening soil model with small strain stiffness. The stiffness and strength parameters for the hardening soil model of soft and stiff Bangkok Clays has recently been published (Surarak et al., 2012). This paper is a continuation on the previous study on the stiffness of Bangkok Clay, and focuses on the small strain characteristics. The data are from the Bangkok MRT Blue line project as well as comprehensive studies at Chulalongkorn University and the Asian Institute of Technology. Based on these laboratory and field testing data, the parameters G_max and γ_0.7 can be determined using well-known empirical correlations and the concept of threshold shear strain. Finally, a comparison between the measured data and predictions is made.