Using Piezocone Dissipation Test to Estimate the Undrained Shear Strength (<i>s_u</i>), <i>N_kt</i>and <i>N_Δu</i>Factors in Cohesive Soils

The current practice of geotechnical engineering commonly uses a combination of theoretical and empirical correlations to estimate the soil undrained shear strength in clays from the piezocone test. In order to complement the use of such correlations, the application of a method to estimate the soil undrained shear strength, using measures of the excess pore pressure in dissipation tests of piezocone is presented. In cohesive soils, excess pore pressure and undrained shear strength are dependent on the same variables (stress state, stress history, soil stiffness), which allows them to be related by the theoretical cavity expansion-critical state framework. This paper mentions the mathematical formulation that supports the theoretical framework used, its relationship with the Nkt and NΔu factors and their estimation in a case studied. The results obtained are consistent within the dispersion found in the international literature and encourage the use of the method in engineering practice.

Influence of moisture content on shearing strength of unsaturated undisturbed quaternary system middle pleistocene

The unsaturated undisturbed quaternary system middle pleistocene loess,a typical unsaturated soil,often occurs in the implementation of western development strategy.To obtain the shearing strength characteristics of this unsaturated undisturbed loess,based on the analysis of mineral composition,the triaxial shear test of undisturbed quaternary system middle pleistocene loess under different moisture contents is conducted with the specialized triaxial instrument for unsaturated soil.The test results show that the mainly mineral composition of undisturbed quaternary system middle pleistocene loess is quartz and albite.Under the same confining pressure,the matric suction increases with the decrease of moisture content.The smaller the moisture content,the larger the matric suction;the higher the moisture content,the lower the matric suction.Under the same moisture content,the matric suction increases with the confining pressure and reaches a maximum when the confining pressure is 100 kPa,and then decreases with the increase of confining pressure.This phenomenon is closely related to the grain contact tightness of soil mass under high confining pressure.According to the triaxial test of loess,the sample of loess experiences 4 stages from loading to failure:1) compaction stage;2) compression stage;3) microcrack developing stage;4) shear failure stage.The test sample is of brittle failure(weak softening)under low moisture content and confining pressure.With the decrease of matric suction and the increase of consolidated confining pressure,the stress-strain curve changes from softening type to ideal plastic type.In the shearing strength parameters of unsaturated undisturbed loess,the influence of moisture content on internal friction angle is small,but that on cohesive force is obvious.Therefore,the shearing strength of unsaturated undisturbed loess is higher than that of saturated undisturbed loess and varies with the moisture content.

High-Speed Ring Shear Tests to Study the Motion and Acceleration Processes of the Yingong Landslide

In this paper, the motion and acceleration process, as well as the mechanism of a high speed and long run landslide are investigated by adopting high speed ring shear test and taking the landslide occurred at Yigong River in Bomi, Tibet on April 9, 2000 as the background. According to the motion characteristics of high-speed and long distance motion landside, the mechanism is studied under different conditions such as shear speed, consolidated drained and consolidated undrained status. Results show that high speed shearing process hinders and delays the dissipation of pore pressure, and drives pore water migrating to shear zone slowly. Both of water content and fine particle content at shear zone are obviously higher than those in other layers; and soil liquefaction occurs at shear zone in the saturated consolidated undrained ring shear tests. The effective internal friction angle of the consolidated undrained soil is much lower than that of the consolidated drained soil under ring shearing. The results also indicate that the shearing speed affecting the strength of soil to some extent. The higher the ring shearing speed is, the lower the strength of soil is. This investigation provides a preliminary interpretation of the mechanism of the motion and acceleration process of the Yigong landslide, occurred in Tibet in 2000.

Application of in situ direct shear device to shear strength measurement of rockfill materials

A simplified in situ direct shear test （DST） was developed for measuring the shear strength of soils in fields. In this test, a latticed sheafing frame replaces the upper half of the shear box used in the conventional direct shear box test. The latticed shearing frame is directly embedded in the ground to be tested after a construction process and is pulled with a flexible chain while a constant dead load is applied to the sample in the sheafing frame. This simplified in situ DST has been validated by comparing its results with those of triaxial tests on samples with parallel gradations under normal stresses less than 100 kPa. In this study, the DST was further validated by carrying out tests on samples with the same gradations, rather than on samples with parallel gradations, under normal stresses up to 880 kPa. In addition, the DST was performed inside fills in two applications.

Estimation of Axial Pile Bearing Capacity According to Shear Strength Parameters of Soil

At pesent, it is very popular to estimate pile bearing capacity by use of empirical formula and physical indexes of soil provided in the design codes for civil construction in China. This paper attempts to apply mechanical indexes of soil and semi-empirical formulas, which are based on soil mechanical theories and were summarized and presented by Meyerhof in 1976, to calculate the axial pile bearing capacity. Loading test results of 24 single piles in Tianjin area have been collected and compared with the proposed calulation approach.

Shear Strength Behavior of Two Landfill Clay Liners

Direct shear tests were conducted to obtain both the shear strength ofcompacted clay liners (CCLs) specimens and the interface shear strength between compacted clay linerand base soil. These experiments were conducted under the conditions of five different watercontents. The experimental results show that shear strength of both CCLs and CCLs/base interfacedecreases with the increase in the water content of CCLs and base soil. In addition, the considerateconcentration of NaCl in leachate has no deteriorating effect on the shear strength of liners.Triaxial shear tests were also conducted on clay liner specimens to obtain total and effective shearstrength under a fast compression. The shear strength parameters with total stress are φ=18. 5°and c=30 kPa for clay-bentonite, and φ=48. 5° and c=90 kPa for sand-bentonite and those witheffective stress are φ'= 27. 2° and c'=25 kPa for clay-bentonite, and φ'=35° and c'=100 kPa forsand-bentonite, respectively. These results indicate that the compacted clay-bentonite shows normalconsolidation, but that the compacted sand-bentonite exhibits over-consolidation.

Undrained shear strength evaluation for hydrate-bearing sediment overlying strata in the Shenhu area, northern South China Sea

The undrained shear strength of shallow strata is a critical parameter for safety design in deep-water operations.In situ piezocone penetration tests(CPTU) and laboratory experiments are performed at Site W18-19 in the Shenhu area, northern South China Sea, where China's first marine hydrate exploitation operation is due to be located. The validation of the undrained shear strength prediction model based on CPTU parameters. Different laboratory tests, including pocket penetrometer, torvane, miniature vane and unconsolidated undrained triaxial tests, are employed to solve empirical cone coefficients by statistical and mathematical methods. Finally, an optimized model is proposed to describe the longitudinal distribution of undrained shear strength in calcareous clay strata in the Shenhu area. Research results reveal that average empirical cone coefficients based on total cone resistance, effective resistance, and excess-pore pressure are 13.8, 4.2 and 14.4, respectively. The undrained shear strength prediction model shows a good fit with the laboratory results only within specific intervals based on their compaction degree and gas-bearing conditions. The optimized prediction model in piecewise function format can be used to describe the longitudinal distribution of the undrained shear strength for calcareous clay within all depth intervals from the mud-line to the upper boundary of hydrate-bearing sediments(HBS). The optimized prediction result indicates that the effective cone resistance model is suitable for very soft to firm calcareous clays,the excess-pore pressure model can depict the undrained shear strength for firm to very stiff but gas-free clays,while the total cone resistance model is advantageous for evaluating the undrained shear strength for very stiff and gassy clays. The optimized model in piecewise function format can considerably improve the adaptability of empirical models for calcareous clay in the Shenhu area. These results are significant for safety evaluations of proposed hydrate exploitation projects.

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 （su） of fine-grained soils that can be measured in situ and in laboratory isone of the key geotechnical parameters. The unconfined compression test （UCT） is widely used in laboratoryto measure this parameter due to its simplicity; however, it is severely affected by sampledisturbance. The vane shear test （VST） technique that is less sensitive to sample disturbance involves acorrection factor against the soil plasticity, commonly known as the Bjerrum＇s correction factor, m. Thisstudy aims to reevaluate the Bjerrum＇s correction factor in consideration of a different approach and arelatively 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 wasexamined using the liquidity index instead of Bjerrum＇s correction factor. In comparison with the resultobatined using the Bjerrum＇s correction factor, the undrained shear strength was better representedwhen su values were correlated with the liquidity index. The results were validated by the RET, whichwas proven to take into account soil plasticity with a reliable degree of accuracy. This study also showsthat the RET has strong promise as a new tool for testing undrained shear strength of fine-grained soils.

Evaluation of the Shear Strength of Perfobond Rib Connectors in Ultra High Performance Concrete

Since the previous strength prediction models for the perfobond rib connector were proposed based upon the results of push-out tests conducted on concretes with compressive strength below 50 MPa, push-out test is performed on perfobond shear connectors applying ultra high performance concretes with compressive strength higher than 80 MPa to evaluate their shear resistance. The test variables are chosen to be the diameter and number of dowel holes and, the change in the shear strength of the perfobond rib connector is examined with respect to the strength of two types of UHPC: steel fiber-reinforced concrete with compressive strength of 180 MPa and concrete without steel fiber with compressive strength of 80 MPa. The test results reveal that higher concrete strength and larger number of holes increased the shear strength, and that higher increase rate in the shear strength was achieved by the dowel action. The comparison with the predictions obtained by the previous models shows that the experimental results are close to the values given by the model proposed by Oguejiofor and Hosain [1].

Rock borehole shear tests in dam foundation of Xiangjiaba hydropower station

Xiangjiaba hydropowcr station is one of the complicated geological conditions of its dam foundation, parameters of rock masses are very important issues. To cascade power stations on the Jinsha River, China. Due to the evaluating the rock mass quality and determining the mechanical address these issues, several groups of rock borehole shear tests （RBSTs） were conducted on the black mudstone in the dam foundation of Xiangjiaba hydropower station in the second construction phase. Forty three groups of shear strengths of black mudstone samples were obtained from RBSTs, and the shear strength parameters （c and f） were calculated using the least squares method. In addition, the limitations and merits of RBST employed in the Xiangjiaba hydropower station were discussed. Test results indicate that the shear strength parameters obtained from RBST have a good correlation with the results from sotmd wave test in borehole. It is believed that RBST has a good adaptability and applicability in geotechnical engineering.

A Study on the Shear Strength Characteristic of Unsaturated Red Clay

The red clay in Chenzhou, Hunan province is mostly in unsaturated state. Simply applying the mechanical properties that derived from classic saturated soil mechanics often leads to slope failures in this region. In order to study the shear strength characteristic of unsaturated red clay in Chenzhou and to explore a shear strength equation that can be easily applied in engineering practice, a series of triaxial tests of saturated and unsaturated red clay samples were performed using the regular triaxial testing apparatus. The testing results show that the peak strength of red clay drops slightly before the moisture content of 30% but decreases sharply after that. The friction angle of red clay under unsaturated state is basically equal to the effective friction angle under saturated state, while the cohesion of unsaturated red clay is far much bigger than that of saturated one, which indicates that the matric suction makes a great contribution to the cohesion. By fitting the testing results with appropriate curves, the relationships between total strength parameters and with moisture content were obtained. The total increases logarithmically before the moisture content of 35% then decreases linearly, while decreases cubically with increasing moisture content.

Direct shear tests of coarse-grained fillings from high-speed railway subgrade in cold regions

In order to study the shear behavior of coarse-grained fillings taken from the subgrade bottom layer of a cold region high-speed railway,large scale direct shear tests were conducted with different normal pressures,water contents and temperatures.The results indicate that the relationship between shear displacement and shear stress changed from strain-softening at lower normal pressures to strain-hardening at higher normal pressures,in both unfrozen and frozen states.This phenomenon was mainly due to the shear dilatation deformation effect.The shear displacement-shear stress curves show similar stages.Besides,the shear stress rapidly increased and there was not an increment in the shear displacement during the initial stage of the shear process in the frozen state.In both the unfrozen or frozen states at the same water contents,the shear strength increased with increasing normal pressure.

Using Piezocone Tests for Analysis of Phreatic Water Conditions and Prediction of Soil Behavior in Tailing Dams

In this work the possibility of identifying two important aspects in the process of adopting soil parameters for calculating stability analysis models in tailing dams is discussed. The use of commercial computer programs for stability calculations allows obtaining numerically exact results. Its representativeness, however, will be linked to the correct definition of the phreatic regime and to the prediction of volumetric soil behavior during shearing (contractile vs. dilating materials). The theoretical principles for the selection of soils parameters for different failure models are briefly presented. Also, how the incorrect assumptions regarding material behavior can significantly affect the estimation of tailing dams’ stability. The results of CPTu tests for the diagnosis of the phreatic and mechanical condition of the materials are discussed and two examples are presented to remark on the care that should be taken to avoid incorrect soils parameters adoption.

钻孔剪切力学参数表征修正方法及其实验研究

钻孔剪切法是现场快速实测岩石抗剪强度的重要方法,但因测量误差较大导致其应用受限。通过对钻孔剪切原理及研究成果分析,发现了不完全剪切是岩石钻孔剪切试验误差大的原因,提出了基于压、剪不等面积的钻孔剪切法修正方法。设计实验装置开展钻孔剪切模拟试验,研究了不同剪切板参数下砂岩剪切力学参数表征规律,证明了所提方法的有效性和修正效果,最后通过文献数据验证了该方法的适用性。具体结果如下:①岩石钻孔剪切试验误差大的根本原因是受压面积和剪切面积选取失真且不相等,实际剪切破坏面积小于剪切齿齿间面积,且不同剪切齿长度情况下修正受压面积和剪切面积均随法向力的增大线性增大。基于剪切齿参数、受压面积和剪切面积的关系式,提出了钻孔剪切法修正方法。②岩石钻孔剪切的剪应力−剪切位移曲线可分为压密阶段、似弹性变形阶段、塑性变形阶段和峰后阶段。随法向力增大,不同剪切板参数下正应力、剪应力和剪切刚度均增大,达到剪应力峰值时的位移减小。钻孔剪切参数与剪切力学参数表征值之间存在明显规律。随剪切齿齿长的增加,黏聚力线性增大,内摩擦角呈二次函数增大。随剪切板面积的增加,黏聚力线性增大,内摩擦角呈二次函数减小。③剪切齿齿长和剪切板面积增大可在一定程度上减少测量偏差。剪切齿齿长从1.0 mm增大到2.5 mm,黏聚力和内摩擦角测量值的总偏差从62.96%降低到19.91%,总修正提升值从30.8%提高到74.1%;剪切板面积从400 mm^(2)增大到520 mm^(2),黏聚力和内摩擦角测量值的总偏差从64.07%降低到37.69%,总修正提升值从39.56%提高到56.05%。实验和文献数据验证均表明,本修正方法可以使钻孔剪切法测得的黏聚力和内摩擦角准确度分别提升约50%和6%。

Interfacial Shear Strength Measurements of SiC Fiber-Reinforced Titanium Composites

A continuous loading push-out test technique was used to measure the interfacial shear strength of SiC fiber reinforced Ti matrix composites. The interfacial shear strength of samples as-fabricated and after heat exposed at 700 and 800℃ for 50 h was successfully determined. It has been found that the interfacial shear strength of the specimen exposed at 700℃ was higher than that of as-prepared and exposed samples at 800℃. The shear strength of the as-prepared samples was about 112.45 MPa, and increased to about 153.77 MPa after heat-treating at 700℃ for 50 h, but decreased to 133.11 MPa after treating at 800℃ for 50 h. Scanning electron microscope （SEM） was used to investigate the interfacial morphology of the samples. The brittle phase was the main products of interface for samples exposed at 800℃, and the interface was easily peeled off.

固–液相变含水率区间松散土石体中细粒组分抗剪特性试验

松散土石体在泥水作用下易引发地质灾害,其中细粒组分在高含水率下的抗剪性能骤然劣化是触发相变失稳的重要因素之一。基于环剪试验,对三峡库区某滑坡松散土石体试样设计试验方案,研究细组构由软塑~流塑发展的固-液相变全过程抗剪特性演化规律。结果表明:松散土石体试样为粗细组分二元特征较强的级配不良土体介质,其中,细粒组分在原状干密度条件下的饱和含水率为23.6%,液塑限分别为27.2和18.1,据此选定含水率23.6%~29.0%范围中的5个取值制定试验方案,可完整覆盖细粒组分的软塑~流塑相变;含水率为25.0%、26.0%和27.0%时,试样处软塑状态,在低法向压力下均可顺利开展环剪试验;其中,含水率25.0%试样恰进入软塑态,较含水率23.6%试样的峰值和残余内摩擦角分别降低23.5%和18.6%,峰值和残余黏聚力均降低超过80%。含水率为28.0%和29.0%时,试样已处于流塑状态,仅在无法向压力时可顺利开展环剪试验,且抗剪强度极低,其物理意义贴近于高黏稠泥浆的黏滞性;根据试样的黏聚力c在进入软塑状态时基本丧失,而内摩擦角φ表现为在软塑区间随含水率增加而逐步降低,进而触及液限骤然丧失的特性分析,c和φ劣化过程异步;经单元体受力模式和概化分析认为,相变前软塑状态试样静止侧压力系数K0的激增缓解了φ值的大幅骤降。

粒度分布对红层土石混合体强度及颗粒破碎的影响

红层土石混合体已广泛应用于川渝地区公路、铁路的路基填筑,研究其强度及颗粒破碎特征可为该地区路基快速安全施工提供理论支撑。针对现有粒度分布评价方法对土石混合体适用性的不足,提出中间粒径块石相对含量C_(m)指标并拟定4个水平,选择四川盆地红层土石混合体为材料,比选叠环剪和大型直剪试验后,结合筛分试验探讨粒度分布对红层土石混合体强度及颗粒破碎的影响规律。试验结果表明:叠环剪试验中剪切面非人为固定,剪切带影响域明显大于大型直剪试验,相同试验条件下土石混合体强度低于大型直剪试验,且法向应力越大,强度差值越大。C_(m)对红层土石混合体的强度及颗粒破碎特征影响显著。随C_(m)的增大,黏聚力先减小后增大,内摩擦角先增大后减小,抗剪强度先增大后减小,均在C_(m)=0.543时有最值,此时,颗粒间的摩擦和重定向排列对强度的增强效应明显大于咬合减弱的负面效应,剪应力有二次攀升。叠环剪试验后,大粒径、中间粒径块石含量降低,土体含量增高,小粒径块石含量在C_(m)≤0.343时降低,C_(m)≥0.343时增高;颗粒相对破碎率随法向应力和C_(m)的增大,均呈先降低后升高的规律,C_(m)=0.343时颗粒相对破碎率最低。

埋深和水土含量对松散岩堆抗剪强度的影响

针对松散岩堆剪切力弱会引起滑坡、崩塌等严重自然灾害的问题,运用相似理论,以石英砂为基本岩块、黏性土为填充土,配置相似材料,分别控制不同填充土含量和含水率并进行三轴试验,研究不同埋深下填充土的含量和含水率对松散岩堆抗剪强度的影响。结果表明:填充土的含量和含水率是松散岩堆咬合力和内摩擦角的关键影响因素,咬合力受影响程度更大;填土中的水对于松散岩堆的剪切力起着削弱作用;岩堆剪切力随着埋深的增加而增加,不同埋深处岩堆有相同的最佳填充土含量和含水率;在恒定岩堆土重度情况下,抗剪强度和埋深呈线性关系。

粉煤灰改良红层泥岩填料静三轴试验研究

红层泥岩的吸水软化和体积膨胀特性易造成路基翻浆冒泥现象及路基不均匀沉降等病害,在作为公路路基填料时应对其进行改良。本文选用粉煤灰作为改良剂,通过室内X射线衍射试验、化学成分测试和易溶盐等试验,分析红层泥岩和粉煤灰的成分组成及离子含量;通过无侧限抗压强度试验、室内变水头渗透试验和固结等试验分析了试样改良前后物理力学特性;通过常规三轴UU试验,研究了不同粉煤灰掺量对改良试样剪切应力-应变及抗剪强度的影响;分析了粉煤灰掺量为10%时,养护龄期对改良试验抗剪强度参数的影响。结果表明:该地区红层泥岩属低液限黏土,其易溶盐溶液中的SO42-离子含量最高,pH值呈碱性;三轴剪切试验中,改良和未改良试样均呈塑性破坏,试样粉煤灰掺量和龄期均对试样抗剪强度提高具有促进作用,且黏聚力增长幅度远大于内摩擦角;粉煤灰掺量为10%龄期28 d时,试样黏聚力较未改良未养护试样和10%掺量未养护试样分别提升37.9%和6.9%,内摩擦角分别增大了1.2°和0.7°。试样的静力学试验均验证了粉煤灰改良的可靠性。

基于扭转剪切试验的沥青混合料抗剪强度分析

沥青路面的高温抗变形能力与沥青混合料的抗剪强度密切相关.根据路面荷载作用下沥青混合料中矿料颗粒的受力特点及转动状态,自主研发了扭转剪切试验装置.针对不同级配沥青混合料,在不同条件下进行扭转剪切试验;分析了骨架稳定性、沥青用量和温度对扭转剪切特性的影响,建立了考虑骨架稳定性指数、沥青用量和温度的抗剪强度预估模型.结果表明:最大扭转剪切力可以用于评价沥青混合料的抗剪强度;最大扭转剪切力随温度升高呈指数形式减小,随沥青用量增加呈二次函数关系变化,随骨架稳定性指数增大呈幂函数关系增大.构建的抗剪强度预估模型能够较为准确地表征各因素耦合作用下沥青混合料的抗剪强度,可以为沥青混合料优化设计及路面养护决策提供参考.