scholarly journals Determination of the Shear Strength of Rockfill from Small-Scale Laboratory Shear Tests: A Critical Review

2020 ◽  
Vol 2020 ◽  
pp. 1-18
Author(s):  
Akram Deiminiat ◽  
Li Li ◽  
Feitao Zeng ◽  
Thomas Pabst ◽  
Paul Chiasson ◽  
...  
Keyword(s):  
Shear Strength ◽  
Specimen Size ◽  
Experimental Results ◽  
Small Scale ◽  
Shear Tests ◽  
Direct Shear Tests ◽  
Specimen Width ◽  
Field Samples ◽  
Parallel Replacement ◽  
Scaling Down

Determining the shear strength of rockfill is a key task for the design and stability analysis of rockfill structures. When direct shear tests are performed, the well-established ASTM standard requires that specimen width and thickness must be at least 10 and 6 times the maximum particle size (dmax), respectively. When the value of dmax is very large, performing such tests in laboratory with field rockfill becomes difficult or impossible. Four scaling-down techniques were proposed in the past to obtain a modeled sample excluding oversize particles: scalping, parallel, replacement, and quadratic. It remains unclear which of the four scaling-down techniques yields reliable shear strength of field rockfill. In this paper, an extensive review is presented on existing experimental results to analyze the capacity of each scaling-down technique to determine the field rockfill shear strength. The analyses show that previous researches followed an inappropriate methodology to validate or invalidate a scaling-down technique through a direct comparison between the shear strengths of modeled and field samples. None of the four scaling-down techniques was shown to be able or unable to predict the field rockfill shear strength by extrapolation. The analyses further show that the minimum ratios of specimen size to dmax dictated by well-established standards are largely used but are too small to eliminate the specimen size effect. In most cases, this practice results in shear strength overestimation. The validity or invalidity of scaling-down techniques based on experimental results obtained by using the minimum ratios is uncertain. Recommendations are given for future studies.

scholarly journals Experimental Study on the Reliability of Scaling Down Techniques Used in Direct Shear Tests to Determine the Shear Strength of Rockfill and Waste Rocks

CivilEng ◽  
2022 ◽  
Vol 3 (1) ◽  
pp. 35-50
Author(s):  
Akram Deiminiat ◽  
Li Li
Keyword(s):  
Experimental Study ◽  
Particle Size ◽  
Shear Strength ◽  
Friction Angle ◽  
Direct Shear ◽  
Shear Tests ◽  
Direct Shear Tests ◽  
Field Samples ◽  
Waste Rocks ◽  
Scaling Down

The determination of shear strength parameters for coarse granular materials such as rockfill and waste rocks is challenging due to their oversized particles and the minimum required ratio of 10 between the specimen width (W) and the maximum particle size (dmax) of tested samples for direct shear tests. To overcome this problem, a common practice is to prepare test samples by excluding the oversized particles. This method is called the scalping scaling down technique. Making further modifications on scalped samples to achieve a specific particle size distribution curve (PSDC) leads to other scaling down techniques. Until now, the parallel scaling down technique has been the most popular and most commonly applied, generally because it produces a PSDC parallel and similar to that of field material. Recently, a critical literature review performed by the authors revealed that the methodology used by previous researchers to validate or invalidate the scaling down techniques in estimating the shear strength of field materials is inappropriate. The validity of scaling down techniques remains unknown. In addition, the minimum required W/dmax ratio of 10, stipulated in ASTM D3080/D3080M-11 for direct shear tests, is not large enough to eliminate the specimen size effect (SSE). The authors’ recent experimental study showed that a minimum W/dmax ratio of 60 is necessary to avoid any SSE in direct shear tests. In this study, a series of direct shear tests were performed on samples with different dmax values, prepared by applying scalping and parallel scaling down techniques. All tested specimens had a W/dmax ratio equal to or larger than 60. The test results of the scaled down samples with dmax values smaller than those of field samples were used to establish a predictive equation between the effective internal friction angle (hereafter named “friction angle”) and dmax, which was then used to predict the friction angles of the field samples. Comparisons between the measured and predicted friction angles of field samples demonstrated that the equations based on scalping scaling down technique correctly predicted the friction angles of field samples, whereas the equations based on parallel scaling down technique failed to correctly predict the friction angles of field samples. The scalping down technique has been validated, whereas the parallel scaling down technique has been invalidated by the experimental results presented in this study.

scholarly journals Using Post-Harvest Waste to Improve Shearing Behaviour of Loess and Its Validation by Multiscale Direct Shear Tests

2019 ◽  
Vol 9 (23) ◽  
pp. 5206 ◽  
Author(s):  
Wen-Chieh Cheng ◽  
Zhong-Fei Xue ◽  
Lin Wang ◽  
Jian Xu
Keyword(s):  
Shear Stress ◽  
Shear Strength ◽  
Strain Hardening ◽  
Gradual Increase ◽  
Small Scale ◽  
Optimal Dosage ◽  
Direct Shear ◽  
Shear Tests ◽  
Post Harvest ◽  
Direct Shear Tests

Loess and PHW (post-harvest waste) are easily accessible in the Chinese Loess Plateau and have been widely applied to construction of residential houses that have been inhabited for decades under the effect of freeze-thaw cycles. Although many researchers have recognised that the addition of fibers to loess soil is effective in preventing soil erosion and stabilising slopes, a consensus on this claim has not been reached yet. This study investigates the shearing behaviour of the loess-PHW mixture using small-scale and large-scale direct shear (SSDS and LSDS) tests. Four typical shear stress versus horizontal displacement curves from the multiscale direct shear tests are recognised where one is featured with strain-softening shape and the other three with a strain-hardening shape. Two out of the three curves with strain-hardening shape show a gradual increase in the shear stress at additional and larger displacements, respectively, in which some factor starts to have an influence on the shearing behaviour. Comparisons of the shear strength measured in SSDS and LSDS are made, indicating that there are differences between SSDS and LSDS. The effect of PHW addition on shear strength is assessed in order to determine the optimal dosage. The improvement of shear strength is attributed to the effect of particle inter-locking, resulting from the addition of PHW to loess specimens, and takes effect as the water content surpassed a threshold, i.e., >14%, that facilitates particle rearrangement. Particle-box interaction behaviour is assessed at the same time, and the findings satisfactorily address the main cause of the gradual increase in shear stress following the curve inflection point. The improved shearing behaviour proves the ability of the loess-PHW mixture to resist the seepage force and consequently stratum erosion.

Shear Strength of Sand–Clay Interfaces Through Large-Scale Direct Shear Tests

2020 ◽  
Vol 45 (5) ◽  
pp. 4343-4357
Author(s):  
Zhong-Liang Zhang ◽  
Zhen-Dong Cui ◽  
Ling-Zi Zhao
Keyword(s):  
Shear Strength ◽  
Large Scale ◽  
Direct Shear ◽  
Shear Tests ◽  
Direct Shear Tests

Effect of fibre content and structure on anisotropic elastic stiffness and shear strength of peat

2012 ◽  
Vol 49 (4) ◽  
pp. 403-415 ◽  
Author(s):  
Michael T. Hendry ◽  
Jitendra S. Sharma ◽  
C. Derek Martin ◽  
S. Lee Barbour
Keyword(s):  
Shear Strength ◽  
Confining Pressure ◽  
Deviatoric Stress ◽  
Direct Shear ◽  
Shear Tests ◽  
Direct Shear Tests ◽  
A Value ◽  
Frictional Strength ◽  
Fibrous Peat ◽  
Interparticle Friction

This paper presents the results of a laboratory testing program involving consolidated undrained triaxial tests and direct shear tests on remoulded peat, remoulded peat fibre, and Shelby specimens of peat obtained from a field site located in the Edson subdivision of the Canadian National railway in Alberta, Canada. These results were analyzed within the frameworks of elastic behaviour of cross-anisotropic materials and shear strength of fibre-reinforced soil. Shelby specimens were found to be inherently cross-anisotropic, whereas the remoulded peat and peat fibre specimens showed a transition from isotropic to cross-anisotropic with increasing vertical strain and effective confining pressure. The horizontal stiffness of Shelby specimens was found to be 2.6 to 2.9 times their vertical stiffness. The shear strength of intact peat is made up of interparticle friction as well as tension in the peat fibres. A novel procedure for estimating the interparticle frictional strength of fibrous peat from CU triaxial test results is proposed. It involves extrapolating the linear strain-hardening portion of the stress–strain curve to obtain the deviatoric stress at zero axial strain and plotting the deviatoric stress values thus obtained against initial mean effective confining pressure to obtain the frictional strength. Using this procedure, a value of 31° was obtained for the interparticle friction, which compares favourably with a value of 31° obtained from direct shear tests. It is recommended that further studies be undertaken to assess if interparticle frictional strength is an appropriate strength parameter for evaluation of the stability of structures founded on fibrous peat.

Determination of the shear strength parameters of an unsaturated soil using the direct shear test

1988 ◽  
Vol 25 (3) ◽  
pp. 500-510 ◽  
Author(s):  
J. K. M. Gan ◽  
D. G. Fredlund ◽  
H. Rahardjo
Keyword(s):  
Shear Strength ◽  
Unsaturated Soils ◽  
Testing Procedure ◽  
Direct Shear ◽  
Shear Tests ◽  
Direct Shear Tests ◽  
Wide Range ◽  
Practical Engineering ◽  
Direct Shear Apparatus

Multistage direct shear tests have been performed on saturated and unsaturated specimens of a compacted glacial till. A conventional direct shear apparatus was modified in order to use the axis-translation technique for direct shear tests on unsaturated soils. The soil can be subjected to a wide range of matric suctions. The testing procedure and some typical results are presented. Nonlinearity in the failure envelope with respect to matric suction was observed. Suggestions are made as to how best to handle the nonlinearity from a practical engineering standpoint. Key words: shear strength, unsaturated soils, negative pore-water pressures, soil suction, direct shear.

scholarly journals Comparison of Mechanical Properties of Fiber-Reinforced Sand under Triaxial Compression and Direct Shear

2015 ◽  
Vol 7 (1) ◽  
Author(s):  
Reza Noorzad ◽  
Seyed Taher Ghoreyshi Zarinkolaei
Keyword(s):  
Shear Strength ◽  
Axial Strain ◽  
Triaxial Compression ◽  
Polypropylene Fiber ◽  
Triaxial Tests ◽  
Direct Shear ◽  
Test Results ◽  
Peak Reduction ◽  
Shear Tests ◽  
Direct Shear Tests

AbstractThis research investigates the behavior of sand reinforced with polypropylene fiber. To do this, 40 direct shear tests and 40 triaxial tests were performed on the coastal beaches of Babolsar, a city in the North of Iran. The effect of parameters such as fiber content, length of fiber and normal or confining pressure on the behavior of Babolsar sand have been studied. In this study, four various fiber contents (0, 0.25, 0.5 and 1 percent), three different lengths of fiber (6, 12 and 18 mm) and four normal or confining pressures (50, 100, 200 and 400 kPa) have been employed. The test results show that fiber inclusion has a significant effect on the behavior of sand. In both direct shear and triaxial tests, the addition of fibers improved shear strength parameters (C, '), increased peak shear strength and axial strain at failure, and also limited the amount of post-peak reduction in shear resistance. The comparison of the test results revealed that due to better fiber orientation toward the direction of principal tensile strain in triaxial test as compared to direct shear tests, the fiber efficiency and its effect on soil behavior is much more significant in triaxial specimens.

scholarly journals Influence of particle size and gradation on shear strength–dilation relation of granular materials

2019 ◽  
Vol 56 (2) ◽  
pp. 208-227 ◽  
Author(s):  
Samaneh Amirpour Harehdasht ◽  
Mahmoud N. Hussien ◽  
Mourad Karray ◽  
Varvara Roubtsova ◽  
Mohamed Chekired
Keyword(s):  
Particle Size ◽  
Grain Size ◽  
Shear Strength ◽  
Granular Materials ◽  
Plane Strain ◽  
Computer Code ◽  
Direct Shear ◽  
Shear Tests ◽  
Empirical Formulas ◽  
Direct Shear Tests

Upon close scrutiny of data reported in the literature, taking into account particle-scale characteristics to optimize the precision of the well-known empirical Bolton’s equations and imposing particle-size limits on them is recommended. The present paper examines the potential influence of particle size and grading on the shear strength–dilation relation of granular materials from the results of 276 symmetrical direct shear tests. The applicability of physical symmetrical direct shear tests to interpret the plane strain frictional shearing resistance of granular materials has been widely discussed using the discrete element method (DEM) computer code SiGran. Sixteen different grain-size distribution curves of three different materials were tested at different normal pressures and initial relative densities. It is demonstrated that while the contribution of dilatancy to shear strength is not influenced by the variation in the coefficient of uniformity, Cu, in the investigated range, it significantly decreases with increasing mean particle size, D50. The coefficients of Bolton’s equations have been, therefore, adjusted to account for D50. A comparison of the predictions by the proposed empirical formulas with plane strain friction angle, [Formula: see text], and dilation angle, ψ, data from the literature shows that accounting for the grain size yields more accurate results.

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