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박성식,황세훈 한국지반공학회 2012 한국지반공학회논문집 Vol.28 No.11
Cemented soils have been used for subbase or base materials of roads, backfill materials of retaining walls and cofferdam. Such cemented soils can be degraded due to repeated wetting and drying or various weathering actions. Unlike rocks, a standard method was not defined for evaluating the durability of cemented soils. In this study, a slaking durability test and an ultrasound cleaner were used for developing a new durability test method for cemented soils. For durability tests, cemented sands with different cement ratios (4, 6, 8, and 12%) with cylindrical specimens were prepared and then air cured or under-water cured for three days. Three-day-cured specimens were dried for one day and then submerged for one day before testing. The weight loss after the slake durability test or ultrasonic cleaner operation for 10 or 20min was measured and used for assessing durability. When a cement ratio was 4%, the weight loss from ultrasonic cleaner test was 7-25% but that from slake durability test was as much as 30-60%. For specimens with cement ratio of more than 8%, the weight loss was less than 10% from both tests. A durability index increased with increasing a cement ratio. The durability index of under-water cured specimen was higher than that of air cured specimen. The ultrasonic cleaner test was found to be an effective tool for durability assessment of cemented sands rather than the slake durability test.
공시체의 직경이 사질토의 비배수 및 배수 전단거동에 미치는 영향
박성식,최선규,김동락 한국지반공학회 2012 한국지반공학회논문집 Vol.28 No.3
An internal friction angle, which is one of strength parameters of granular soils, can be obtained from direct shear tests or triaxial tests. The result of traixial tests can be influenced by various experimental conditions such as confining pressure, shearing rate, specimen diameter and height, and end constraint. In this study, undrained and drained shearing behaviors of Nakdong River sand were investigated for loose (Dr = 40%) and dense (Dr = 80%) specimens with 5, 7, and 10 cm in diameter. Friction angles such as undrained total stress friction angle, undrained effective stress friction angle, and drained friction angle obtained from Mohr’s stress circle slightly increased and then decreased as a diameter of a specimen increased from 5, 7 to 10 cm, regardless of relative densities. The difference between friction angles caused by different specimen size was at maximum 4.5 degrees for undrained total stress friction angle of dense specimen. In most cases, there was little difference between friction angles of large and small specimens, which was less than 2 degrees. The difference between an effective friction angle from undrained tests and a drained friction angle from drained tests was at maximum 7 degrees for loose samples but negligible for dense samples.
박성식,Tan-No Nguyen,문홍득,정승원 한국지질과학협의회 2023 Geosciences Journal Vol.27 No.5
The shear strength and stiffness of granular materials are crucial parameters for evaluating both stability and sustainability in geoenvironmental engineering. Two types of shear loading, static and cyclic, may directly influence the mechanical properties of granular materials; however, the investigation of stiffness improvement of granular materials has not yet been sufficiently investigated using a loading method. The one-way repeated loading method described in this study may overcome the drawbacks of classical testing approaches. One-way repeated direct shear (DS) and direct simple shear (DSS) tests were conducted on a series of poorly graded bead gradations to investigate the shear strength and stiffness of granular materials under a drained and strain-controlled condition with a shear velocity of 0.1 mm/min. All samples in the two tests were prepared using the same method and initial geometry. Three types of samples referring to different curvature coefficient (Cu) values of 1.8, 3.3, and 5.8 and normal stresses (σn) of 50, 100, and 150 kPa were applied. The test results show that an increase in Cu results in an increase in the shear stress of the granular materials at the peak and ultimate states for both the DS and DSS tests. The peak shear stress ratios between the two tests (τDSS/τDS) ranged from 0.75 to 0.95 when Cu and σn were increased. The angles of shearing resistance from the DS test were in a wide range of 25.7°–33.7° at the peak state and 24.9°–31.3° at the ultimate state, whereas those found in the DSS test were in narrow ranges of 26.0°–28.4° and 23.6°–27.9° at the peak and ultimate states, respectively. It should be noted that an increase in Cu led to the increase in the stiffness ratio in both test methods. The difference in stiffness ratios determined from the DS and DSS tests was approximately 14%. Under one-way repeated shear loading, the DS test is recommended to determine the residual shear strength generated along the shear plane, whereas the DSS test is recommended to determine the maximum shear stress at the weakest shear zone developed horizontally owing to repeated shearing.