Monotonic and Cyclic Behavior of Chiang Mai Sand Under Simple Shear Mode

Record of damages and casualties caused by earthquakes in Thailand reveals that Chiang Mai, the second largest province of Thailand, has faced the great threat among other provinces. Triaxial tests, standard penetration tests, and multichannel analysis of surface wave have been recently performed to understand the dynamic response of Chiang Mai ground. However, the cyclic simple shear test, which could convincingly represent the real seismic ground shaking under repeated horizontal shear force, has not been conducted yet. Therefore, this paper aims to characterize the behaviors of Chiang Mai sand under monotonic and cyclic loadings using a direct simple shear apparatus. Sand specimens taken from the Ping River were prepared by dry deposition technique. The mobilized friction angle at the critical state measured by the direct simple shear test is equal to 37.3° which is 9 % less than that measured by direct shear box test. This paper also provides the appropriate dynamic properties of Chiang Mai sand with the shear strain level <2 % for any practical purpose. Test results showed that the normalized shear modulus agrees well with other investigators while the damping ratio is noticeably smaller than the lower-bound range for sand reported in the past studies.     

落锥法确定粉土液限和塑限的试验研究

针对部分细粒土进行了室内落锥法确定液限和塑限的试验研究,试验结果表明：对于一些细粒土在双对数坐标上圆锥下沉深度与含水率之间的关系并非线性关系,无法依据线性关系确定液、塑限;采用规程建议的3个含水率范围有时根本无法获得有效试验数据,采用规程建议的3个含水率范围确定得到粉土的液、塑限试验结果的可靠性有待商榷;采用直线段试验数据确定的液、塑限进行分类可能出现相互矛盾甚至错误的结果。进一步试验结果表明：对于黏粒含量高于13 %的粉土,由于其抗剪强度以凝聚分量为主,落锥法适用,所测塑性指数能够反映其可塑性;对于部分黏粒含量低于13 %的粉土及外观接近粉土的粉砂,由于其抗剪强度以颗粒间摩擦分量为主,落锥法不再适用,所测塑性指数不能反映其可塑性。     

Soil/Geotextile Interface Behaviour in Direct Shear and Pullout Movements

This paper deals with soil/reinforced geotextile interface behaviour in direct shear and pullout movements. A soil/geosynthetic direct shear apparatus, developed in accordance to EN ISO 12957-1 (2004) and ASTM D5321-92 (1992) standards, is presented. Some details of the test (like: specimen fixation, influence of the vertical stress on the registered horizontal force, type of test and measurement of the vertical displacement) are discussed and modifications in the test procedures are adopted. Then, the reinforced geotextile and the residual soil of granite used in the research are described. The behaviour of soil/geosynthetic interface in direct shear is characterized based on modified direct shear tests and maximum interface shear stress is determined at peak and residual for a confining stress of 50 kPa. The modification in the pullout test apparatus described by Lopes and Ladeira (1996a; 1996b) and by Lopes and Lopes (1999) are noticed. The behaviour of soil/geosynthetic interface in pullout is characterized based on pullout tests performed, in accordance EN 13738 (2004), and interface shear stress at maximum pullout force is defined for a confining stress of 50 kPa. Finally, the values of interface coefficient at soil/geosynthetic interface are obtained in direct shear and in pullout and then compared. The main conclusions that can be outlined from the present study are the following: modifications should be made to EN ISO 12957-1 (2004) standard, namely in what concerns the dependence of the measured horizontal force from the vertical stress, the difficulties to perform constant area direct shear test with the lower half box filled with soil and on the measurement of the vertical displacement of the load plate; on the contrary to which is normally accepted the characteristics of the behaviour of soil/geosynthetic interface in pullout, when the geosynthetic has a full plane contact area with the soil, are not able to be obtained based on results of direct shear tests, as in this type of tests the contribution of the geosynthetic deformation on the characteristics of the interface in pullout is not considered.     

Analysis of mobilized stress ratio of gap-graded granular materials in direct shear state considering coarse fraction effect

Weathered rockfill materials, characterized by a mixture of soil matrix and rock aggregates, are widely distributed in mountainous areas. These soils are frequently used for subgrade or riprap in engineering practice, and the mobilized shear strength is crucial for analyzing the displacement and stability of these geo-structures. A series of direct shear tests are performed on a gap-graded soil with a full range of coarse fraction. The behavior of gap-graded soils is analyzed, and a simple model is proposed for the evolution of mobilized stress ratio during direct shearing process based on mixture theory. The change of inter-aggregate configuration is incorporated by introducing a structure variable which increases with coarse fraction and decreases approximately linearly with the overall horizontal shear strain in double logarithmic plot. It reasonably reflects a gradually transformation from a matrix-sustained structure into an aggregate-sustained one with the increase of coarse fraction. The model has four parameters, and at least two direct shear tests need to be done for the calibration. Validation of the model is done by using the test data in this work and those from the literature.

     

Shear behaviour of cemented pastefill-rock interfaces

The shear stress–strain behaviour and shear strength parameters of the interface between cemented paste backfill (CPB) and rock are of practical importance in the optimal and safe design of CPB structures. An understanding of the shear behaviour and properties at this interface is also required to develop comprehensive interface models for CPB-rock analyses, interface design methods for the static and dynamic stability analysis of CPB structures, and building high performance CPB structures. In this study, direct shear tests were conducted to investigate the interface shear strength behaviour between CPB and rock. All tests were carried out in a standard direct shear test apparatus for a range of curing ages of 1 to 28 days for the CPB. The procedures of the laboratory tests will be described. Results will be presented for interface shear behaviour, including stress–strain curves, vertical deformation and shear strength parameters. The test results show that the shear strength parameters and behaviour of the CPB-rock interface are time-dependent and significantly influenced by the normal load.     

Unidirectional cyclic resistance of Ticino and Toyoura sands from centrifuge cone penetration tests

The evaluation of the undrained cyclic resistance of sandy deposits is required to forecast the soil behaviour during an earthquake (liquefaction, cyclic mobility); due to the difficulties in obtaining undisturbed samples of most liquefiable soils, it is usually deduced from field test results such as cone penetration tests. This paper proposes a methodology to evaluate the undrained cyclic resistance from normalised cone resistance of two well-studied silica sands (Ticino and Toyoura), with different mineralogy, one mainly composed of feldspar, the other of quartz. The determination of the cyclic resistance of Ticino and Toyoura sands was achieved through undrained cyclic triaxial tests on reconstituted specimens. The tip resistance was deduced from CPTs performed in centrifuge with a miniaturised piezocone on homogeneous reconstituted models. Both the undrained cyclic and tip resistances were correlated with the state parameter ψ. Results of centrifuge and triaxial tests were combined through ψ to deduce the cyclic resistance ratio CRR directly from the normalised cone resistance. The shape of the curve relating CRR to the normalised cone resistance resulted unusual respect to all the recognised curves widespread in the geotechnical literature. The aim of the proposed correlations is to provide a useful instrument to improve the actual knowledge on liquefaction and to give a contribution based on the critical state soil mechanics framework to the development of refined correlations between the cyclic resistance of a sand and the results of cone penetration tests.     

Shear strengths of sandstone fractures under true triaxial stresses

True triaxial shear tests have been performed to determine the peak shear strengths of tension-induced fractures in three Thai sandstones. A polyaxial load frame is used to apply mutually perpendicular lateral stresses (σ_p and σ_o) to the 76 × 76 × 126 mm rectangular block specimens. The normal to the fracture plane makes an angle of 59.1° with the axial (major principal) stress. Results indicate that the lateral stress that is parallel to the fracture plane (σ_p) can significantly reduce the peak shear strength of the fractures. Under the same normal stress (σ_n) the fractures under high σ_p dilate more than those under low σ_p. According to the Coulomb criterion, the friction angle decreases exponentially with increasing σ_p/σ_o ratio and the cohesion decreases with increasing σ_p. The lateral stress σ_p has insignificant effect on the basic friction angle of the smooth saw-cut surfaces. The fracture shear strengths under σ_p = 0 correlate well with those obtained from the direct shear tests. It is postulated that when the fractures are confined laterally by σ_p, their asperities are strained into the aperture, and are sheared off more easily compared to those under unconfined condition.     

Laboratory Tests on Shotcrete-Rock Joints in Direct Shear, Tension and Compression

Summary. A series of laboratory tests was performed on cemented shotcrete-rock joints to investigate the strength and stiffness of the interfaces, while simulating field conditions as close as possible. The direct shear test formed the core of the experimental work, while the tension and compression tests were complementary. To simulate loading conditions experienced in practical cases the direct shear tests were performed under fairly low normal stresses. In most practical cases when shotcrete is used with rock bolts, the normal load on shotcrete lining seldom exceeds 0.2 to 0.5 MPa. The direct shear test results show that, for such normal load range the shear strength is determined by the bond strength for genuinely bonded shotcrete-rock interfaces. For higher normal stresses (σ_n > 1.0 MPa), which rarely exist at the shotcrete-rock interface, the shear strength is largely influenced by friction resulting in the cohesive strength being less significant. Assessment of the shear surface revealed that the steel fibres in the shotcrete appeared to contribute significantly to the frictional component. The shear and normal stiffnesses of the interface were also determined, which were in principal the stiffnesses of the bond. An interesting observation was the complex interaction at the interface and the mechanisms that controlled the peak shear strength which depended on the surface roughness, the existence of natural flaws and the normal load.     

砂土与钢材接触面剪切特性试验对比研究

为了研究砂土与钢材接触界面剪切特性,对原有的及改装后的DSJ-2型电动四联等应变直剪仪上分别进行了砂土同钢材界面的单剪试验和直剪试验,并做了对比分析。研究结果表明：1）单剪试验下在法向应力50kPa、100kPa时,砂土与钢材界面的剪应力-剪切位移关系曲线呈非线性弹性理想塑性关系,在法向应力200kPa、400kPa时呈刚塑性关系。直剪试验下在法向应力50kPa、100kPa时,砂土与钢材界面的剪应力-剪切位移关系曲线呈非线性关系,在法向应力200kPa、400kPa时呈双曲线关系;2）提出了界面抗剪强度指标的经验换算公式（直剪试验下得到的界面抗剪强度指标乘以一个小于1的系数）,以修正应用于设计的直剪试验结果。     

Undrained Shear Strength in Cohesive Soils Estimated by Directional Modes of In-Situ Shear Wave Velocity

The estimated undrained shear strength (s_u) is often not a unique value because it can be evaluated by various test types and/or procedures, such as different failure modes, shear strain rates, and boundary conditions. This study explores (1) the relationship between reference undrained shear strength and in situ shear wave velocity in terms of the effective overburden stress, and (2) the independent relationships to evaluate the undrained shear strength with special consideration of different directional and polarization modes (VH, HV, HH shear waves), which has not been reported. This evaluation is done via a worldwide database compiled from 43 well-documented geotechnical test sites associated with soft ground. Finally, new correlation models are proposed to estimate the undrained shear strength based on the in situ shear wave velocity as well as the plasticity index or the overconsolidation ratio. The application of the shear wave velocity–undrained shear strength relation is illustrated through two independent case studies. The proposed relationships are expected to contribute to reasonable estimates of undrained shear strength as well as offer practical guidance on even extrapolation beyond the data that is available to geotechnical engineers.     

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.     

Comparison between Empirical Estimation by JRC-JCS Model and Direct Shear Test for Joint Shear Strength

In order to study the reliability of the empirical estimation of joint shear strength by the JRC(joint roughness coefficient)-JCS(joint compressive strength) model,natural rock joints of dif-ferent lithologic characteristics and different sizes were selected as samples,and their shear strengths under dry and saturated conditions were measured by direct shear test and compared to those esti-mated by the JRC-JCS model.Comparison results show that for natural rock joints with joint surfaces closely matched,the...     

南海北部区域黏土循环动力特性试验研究

采用南海北部区域油田场址钻孔原状土样,开展黏土循环动力特性试验研究。结合共振柱与应变控制的循环单剪试验,获取动剪切模量和阻尼比随循环剪应变的变化规律,将最大动剪切模量与原位静力触探的计算结果进行对比。开展应力控制的循环单剪试验,采用对称加载方式获取土样动强度阈值,基于该阈值,通过单、双向加载方式获取土样动强度包络线,并将试验结果与Drammen黏土及世界范围内其余区域黏土进行对比。该研究初步获取了南海北部区域性黏土的循环特性及动力学参数,为南海北部区域油气田结构设计和地质风险评估提供基础资料,掌握了循环动力特性试验方法,为岩土工程动力学试验研究及数据分析提供技术参考。     

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.     

Shear Load Transfer Characteristics of Drilled Shafts Socketed in Rocks

This paper presents a shear load transfer function and an analytical method for estimating the load transfer characteristics of rock-socketed drilled shafts subjected to axial loads. A shear load transfer (f–w) function of rock-socketed drilled shafts is proposed based on the constant normal stiffness (CNS) direct shear tests. It is presented in terms of the borehole roughness and the geological strength index (GSI) so that the structural discontinuities and the surface conditions of the rock mass can be considered. An analytical method that takes into account the coupled soil resistance effects is proposed using a modified Mindlin’s point load solution. Through comparisons with load test results, the proposed methodology is in good agreement with the general trend observed in in situ measurements and represents an improvement in the prediction of the shear behavior of rock-socketed drilled shafts.     

Shear strength analysis of grouted coal waste materials to improve mine outslope stability: a laboratory analysis

Summary Slope stability problems exist within the Appalachian Basin as a result of the emplacement of coal mine waste materials on mine outslopes. Prevention or elimination of slope instability problems can be costly. In an attempt to test alternative methods of slope stabilization, the United States Bureau of Mines determined some physical characteristics and shear strengths of ungrouted and polyurethenegrouted samples of coal refuse, coal spoil and natural soils collected at a number of mine outslope sites in Pennsylvania and West Virginia. The particle size distribution, per cent field porosity, per cent field water content and shear strengths of the materials were determined in the laboratory. Fifty drained direct shear strength tests were performed with the sample materials using a 0.06 m³ shear box. Tests were done at field moisture, 100% saturation and grout-infused conditions. Normal loads of 103 kN m^–2, 206 kN m^–2 and 416 kN m^–2 were used. The grout-infused tests generally showed strength increases. An infinite slope model was used to demonstrate the potential effectiveness ofin situ grouting for a variety of field slope conditions. This modelling suggests thatin situ grouting has the potential of stabilizing slopes of up to 35° at depths of 14 m for refuse material and 30° at depths of 5 m for spoil and soil materials. The validity of these increases in material strength by grout injection will require field testing for confirmation.     

近海粘土设计参数与指标特性之间的关系分析

展示了近海粘土指标特性与设计参数之间相关关系的研究结果。该项研究的一个重要目的是建立一个按统一方法测定的高质量的土性测试指标数据库。该数据库由10个场地组成，包括大西洋、挪威北海、挪威海、里海和2个沿海陆上地点。数据库分析表明，前期固结压力pc?与液限指数IL之间存在良好的相关关系，但当液限指数小于0.5时，数据比较分散。同时还发现，前期固结压力与由三轴压缩、三轴拉伸及直剪试验测定的不排水抗剪强度之间均有很好的线性相关关系。由此，以pc?与IL之间的关系为基础，提出了使用这些线性关系式估算近海粘土原位不排水强度su的方法及建议。     

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.     

Application of the Dynamic Cone Penetrometer (DCP) for determination of the engineering parameters of sandy soils

Determination of the in situ engineering properties of foundation materials has always been a challenge for geotechnical engineers and, thus, several methods have been developed so far. Dynamic Cone Penetration (DCP) test is one of the most versatile amongst them. In the present research, a light weight simple DCP device was developed and used for evaluation of the engineering properties of sandy soils in laboratory conditions. The device consisted of an 8-kg hammer that drops over a height of 575 mm, and drives a 60° cone tip with 20 mm base diameter into the ground. To control the validation of the results, laboratory direct shear and plate load tests were used as reference tests. The soil sample was a poorly graded sandy soil (SP) taken from alluvial deposits of the Tehran plain. All DCP tests and PLTs were undertaken on compacted soil in a mould with 700 mm diameter and 700 mm height. Based on the results of the experiments, the relationships between Dynamic Penetration Index (DPI), relative density (D_r), modulus of elasticity (E), shear modulus (G), modulus of subgrade reaction (K_S), and the friction angle of the soil were obtained with a high coefficient of determination (> 90%). The repeatability of the test results was also evaluated by calculating the coefficient of variations (C_v), which was less than 30% for all tests.     

Shear strength characterization of municipal solid waste at the Suzhou landfill, China

The current practice of slope stability analysis for a municipal solid waste (MSW) landfill usually overlooks the dependence of waste properties on the fill age or embedment depth. Changes in shear strength of MSW as a function of fill age were investigated by performing field and laboratory studies on the Suzhou landfill in China. The field study included sampling from five boreholes advanced to the bottom of the landfill, cone penetration tests and monitoring of pore fluid pressures. Twenty-six borehole samples representative of different fill ages (0 to 13 years) were used to perform drained triaxial compression tests. The field and laboratory study showed that the waste body in the landfill can be sub-divided into several strata corresponding to different ranges of fill age. Each of the waste strata has individual composition and shear strength characteristics. The triaxial test results showed that the MSW samples exhibited a strain-hardening and contractive behavior. As the fill age of the waste increased from 1.7 years to 11 years, the cohesion mobilized at a strain level of 10% was found to decrease from 23.3 kPa to 0 kPa, and the mobilized friction angle at the same strain level increasing from 9.9° to 26°. For a confinement stress level greater than 50 kPa, the shear strength of the recently-placed MSW seemed to be lower than that of the older MSW. This behavior was consistent with the cone penetration test results. The field measurement of pore pressures revealed a perched leachate mound above an intermediate cover of soils and a substantial leachate mound near the bottom of the landfill. The measurements of shear strength properties and pore pressures were utilized to assess the slope stability of the Suzhou landfill.