• The Journal of Engineering Geology
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• v.9 no.1
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• pp.69-82
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• 1999

Direct shear tests are perfromed for artifical joint models made of cement with 25 types of different strength and roughness. The tests consist of the multi-stage test which is a common test method for a single joint plane and the test method suggested by ISRM. Then, not only the differences of friction angles between the two test methods are compared, but is the effectiveness of the multi-stage test investigated. The average of friction angles measured from the multi-stage test is $6.4^{\circ}$ lower than that from the ISRM test. Although the strength and roughness of samples vary, the differences of friction angles between the two test methods are constant. The relationship between the shear stress and the normal stress measured from the multi-stage test is well correlate with the Patton's equation. Whereas, the Barton's equation is best fitted with those measured from ISRM test.

• Journal of Korean Society of Coastal and Ocean Engineers
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• v.29 no.2
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• pp.67-76
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• 2017

In this study, the effect of caisson heel on the active earth pressure is investigated. Using limit analysis method, inclinations of slip surface developed above the heel with different lengths are analyzed. The shorter the heel length, the larger those of inside slip surface, however those of outside slip surface are not changed. According to the relative heel length, relationships of internal friction angle of backfill material - wall friction angle between caisson structure and backfill - friction angle acting on virtual section at the end of heel are presented. Earth pressures acting against caisson structure with relatively short heel are smaller than Rankine earth pressure but always greater than Coulomb earth pressure which does not consider the heel length.

• Journal of the Korean Geotechnical Society
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• v.36 no.2
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• pp.17-28
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• 2020

With respect to the tensile strength of volcanic rocks in Jeju Island, a comparative study was conducted using the existing research results and the test results performed in this study. In addition, the characteristics and effectiveness of the cohesion and internal friction angle estimated from the Brazilian tensile strength and unconfined compressive strength of Jeju volcanic rocks were investigated. As results, the Brazilian tensile strength of Jeju volcanic rocks was closely related to absorption, and decreased exponentially as the absorption increased. It was confirmed that the internal friction angle was closely related to the ratio of unconfined compressive strength to Brazilian tensile strength (σ_c / σ_t), and increased logarithmically as the ratio of σ_c / σ_t increased. In addition, the ratios of σ_c / σ_t of Jeju volcanic rocks were in the range of 5~20 depending on the magnitude of internal friction angle. In the case of cohesion, it was closely related to the absorption and Brazilian tensile strength. The cohesion exponentially decreased as the absorption increased, such as the relation between the Brazilian tensile strength and absorption. It was confirmed that there was a linear relation between the cohesion and Brazilian tensile strength.

• Tunnel and Underground Space
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• v.24 no.1
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• pp.32-45
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• 2014

Basic frictional angle is a parameter that can estimate shear strength of rock, and is a design parameter employed in slope stability analysis. Basic frictional angle generates various results in accordance with mineral composition, apart from rock surface roughness itself. This paper describes the correlation of basic frictional angle and mineral composition. The basic frictional angle is measured with the aid of the modified tilt testing apparatus, and its reliability is improved by the statistical method. Also, mineral composition is identified through the photographic analysis on rock specimen, and verified through the thin section analysis.

• Journal of the Korean Geosynthetics Society
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• v.17 no.2
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• pp.33-40
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• 2018

Characteristics of interface friction between cohesionless soils and geotechnical structure surfaces play an important role in the analysis of earth load and resistance on the structure. In general, geotechnical structures are mainly composed of either steel or concrete, and their surface roughnesses with respect to soil particle sizes influence the interface characteristics between soils and the structures. Accurate assessment of the interface friction characteristics between soils and structures is important to ensure the safety of geotechnical structures, such as mechanically stabilized earth walls reinforced with inextensible reinforcements, piles embedded into soils, retaining wall backfilled with soils. In this study, based on the database of high quality interface friction tests between frictional soils and solid surfaces from literature, equation representing peak interface friction angle is proposed. The influential factors of the peak interface friction angle are relative roughness between soil and solid surface, relative density of frictional soil, and residual (constant volume) interface friction angle. Futhermore, for the developed equation of the interface friction angle, its uncertainty was assessed statistically based on Goodness-of-fit test results.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.36 no.3
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• pp.463-470
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• 2016

A vane shear test (VST) is a simple testing method for determining an undrained shear strength of cohesive soils by minimizing soil disturbance. In this study, the VST was used to determine a shear strength of sand. Dry Nakdong River sand was prepared for loose and dense conditions in a cell and then pressurized with 25, 50, 75 or 100 kPa from the surface of sand. A vane (5 cm in diameter and 10 cm in height) was rotated and a torque was measured within sand. When a torque moment by vane and friction resistance moment by sand is assumed to be equalized, a friction angle can be obtained. When a vane rotates within clay, a uniform undrained shear strength is assumed to be acting on cylindrical failure surface. On the other hand, when it is applied for sand, the failure shape can be assumed to be an octagonal or square column. The relationship between measured torque and resistant force along assumed failure shapes due to friction of sand was derived and the internal friction angle of sand was determined for loose and dense conditions. For the same soil condition, a series of direct shear test was carried out and compared with VST result. The friction angle from VST was between 24-42 degrees for loose sand and 33-53 degrees for dense sand. This is similar to those of direct shear tests.

• Tunnel and Underground Space
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• v.25 no.3
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• pp.284-292
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• 2015

The generalized Hoek-Brown failure criterion, the strength parameters of which are determined by using the GSI index, is an empirical nonlinear failure criterion of rock mass and has been widely employed in various rock engineering practices. Many rock engineering practitioners, however, are still familiar with the description of the strength of rock mass in terms of friction angle and cohesion. In addition, almost all rock mechanics softwares incorporate the simple linear Mohr-Coulomb function. Therefore, it is necessary to provide a tool to implement the Hoek-Brown function in the framework of the Mohr-Coulomb criterion. In this study, the use of upper-bound solution of limit analysis for bearing capacity of a strip footing resting on the ground surface is proposed for the estimation of the equivalent friction angle and cohesion of rock mass incorporating the generalized Hoek-Brown failure criterion. The upper-bound bearing capacity is expressed in terms of friction angle by use of the relationship between tangential friction angle and tangential cohesion implied in the generalized Hoek-Brown function. The friction angle minimizing the upper-bound bearing capacity is taken as the equivalent friction angle. Through the illustrative implementations of the proposed method, the influences of GSI, $m_i$ and D on the equivalent friction angle and cohesion are investigated.

• The Journal of Engineering Geology
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• v.29 no.4
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• pp.565-578
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• 2019

Basic friction angles of Hwangdeung granite, Berea sandstone, Jeongsun marble, Hongcheon gneiss, Pungam shale and Eumseong sandy shale were measured by direct shear test, tilt test and pull test. Characteristics of basic friction angle and the accuracy of test methods were compared and the optimal method in measuring basic friction angle was suggested. Although basic friction angles might be measured accurately by direct shear test, the test apparatus is expensive and procedures are complicated. Tilt tests which is the suggested method for measuring basic friction angle by International Association for Rock Mechanics also provided similar basic friction angles measured by direct shear test. However, the error measured for the same rock type is higher than 7° and values by repeated measurements in one sample show different trends, such as increasing or decreasing or almost constant as measurements continued. The difference measured in one gneiss sample is higher 12°, indicating that tilt test may be not a reliable method for measuring basic friction angle. Not only pull test provided accurate and consistent results under low normal stresses, but also test apparatus is simple and inexpensive and procedure is not complicated, indicating that pull test may be the optimal method for measuring basic friction angle.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.38 no.1
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• pp.63-71
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• 2018

In this study, the torque and horizontal stress acting on vane were measured and then used to determine a friction angle of sand. A dry Nakdong River sand was prepared for loose and dense conditions in a cell and then pressurized with 25, 50, 75 or 100 kPa from the surface of sand. A vane (5cm in diameter and 10cm in height) was rotated and the torque and horizontal stress were measured at real time. A maximum torque was 3.5-9.5Nm for loose sand and 7.4-17.6Nm for dense sand, respectively. The maximum torque increased as an overburden pressure increased. The maximum torque obtained at 14-20 degrees of vane rotation, which was not influenced by the initial alignment of earth pressure and vane blade. An initial horizontal stress ratio was 0.33-0.35 on the average. The horizontal stress increased initially and then decreased due to particle disturbance. A friction angle was calculated from real time varying horizontal stress and torque, which decreased with increasing overburden pressure. The friction angle of loose sand from vane shear test was similar to that of direct shear test but that of dense sand was overestimated.

• The Journal of Engineering Geology
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• v.19 no.2
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• pp.153-163
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• 2009

In this study, a series of triaxial tests on Jeju basalt were carried out and then rock strength parameters were estimated by the Mohr-Coulomb failure criterion and the Hoek-Brown failure criterion using the test results. The characteristics of both failure criterions were investigated through comparing the estimated rock strength parameters. As the result of the Mohr-Coulomb criterion, the cohesions and the internal friction angles are determined as 5.35 MPa and $50.25^{\circ}$ of Pyoseonri basalt, 16.99 MPa and $60.66^{\circ}$ of Trachy-basalt, and 2.33 MPa and $37.05^{\circ}$ of Scoria, respectively. The cohesions and internal friction angles were estimated by the Hoek-Brown failure criterion in the basis of the results of regression analysis. The cohesions and the internal friction angles are determined as 4.77 MPa and $52.47^{\circ}$ of Pyoseonri basalt, 14.69 MPa and $60.70^{\circ}$ of Trachy-basalt, and 2.22 MPa and $47.60^{\circ}$ of Scoria, respectively. As the result of comparison between the Mohr-Coulomb failure criterion and the failure envelope predicted by the Hoek-Brown criterion, the cohesion estimated by the Hoek-Brown criterion is usually lower than that obtained from the Mohr-Coulomb criterion, whereas the friction angle estimated by the Hoek-Brown criterion is higher than that obtained from the Mohr-Coulomb criterion.