The frequency of ground motions during earthquakes is typically in the order of a few hertz. As the earthquake-induced liquefaction of soils is widely assessed by performing laboratory tests, it is necessary to consider various loading frequencies generated by real earthquakes. The effect of loading frequency has been studied by cyclic triaxial tests; however, it has rarely been investigated by cyclic direct simple shear tests, which are more similar to the cyclic loading conditions associated with earthquakes. In this study, a series of cyclic direct simple shear tests were performed on clean sand with a relative density (D_r) of 40% (loose sand) and 80% (dense sand), obtained from Nakdong River. The parameters considered are the initial vertical effective stresses (σ_v0′ = 50, 100, and 200 kPa) and the loading frequencies (f = 0.05, 0.1, 0.5, and 1 Hz) to evaluate the effect of the loading frequency on the liquefaction prediction of clean sand. The results showed that the liquefaction resistance of the sand increases with the increase in the loading frequency, regardless of the initial vertical effective stress and relative density. When the loading frequency increased from 0.1 to 0.5 or 1 Hz, the maximum increase in the cyclic resistances were 15%, and 19% for loose and dense sand, respectively. For a given loading frequency, the liquefaction resistance of the sand decreased when the initial vertical effective stress increased.

In this paper, shear rate-dependent rheological properties of mine tailings taken from abandoned mine deposits prone to mass movements are examined using a commercial ball-measuring rheological system. The yield stresses (i.e., dynamic and static yield stresses) and viscosity of sand-rich materials are examined by the shear rate-controlled flow curve and time-dependent stress growth methods. Before yielding, the shear stress reaches a peak value (i.e., yield stress) observed for all flow curves. In the steady-state condition, the materials have a minimum shear stress (i.e., dynamic yield stress). The static yield stress can be determined under a constant applied shear rate with different initial values ranging from 10⁻⁴ to 10⁻¹ s⁻¹. As a result, the Bingham yield stress and viscosity can be used as a first approximation for estimating the debris flow mobility of post-failure materials. However, the Bingham yield stress is competitive with the static yield stress measured from stress growth methods. Upon comparison of the dynamic and static yield stresses, the static yield stress is approximately 35–45 times greater than the dynamic yield stress, and may be strongly related to microstructural changes (i.e., thixotropy). In this context, special attention must be paid to the determination of yield stresses in debris flow mitigation programs.

This study describes the development of a landslide monitoring system for the purpose of reducing damages caused by landslides in natural terrain. The system was developed to analyze the effects of landslide-inducing rainfall and the behavior of slopes through 12 monitoring stations that are distributed across eight national parks in Korea. Several sensors and a data acquisition equipment to monitor landslide were installed in each station. The composition of the system and its operating program were designed to efficiently manage the sizeable amounts of real-time monitoring data that are collected from the various stations. To test the potential of the developed system for reliable landslide hazard evaluations, data measured over a five-year period by the two monitoring stations in Jirisan National Park were analyzed. Subsequently, the suction stress of the soil over the monitoring period was calculated by applying laboratory test result of the geotechnical and unsaturated soil properties in the analysis domain area. The infinite slope stability analysis combined with an effective stress concept based on the suction stress was applied to calculate the factor of safety. This method also enabled the temporal and quantitative evaluation of slope stability in natural terrain. In addition, based on the monitoring and slope stability analysis results, an analysis for the spatial classification of landslide hazards was conducted. The analysis results quantitatively and statistically demonstrated that 98% of historical landslide initiation areas were classified as high hazard levels.

Surface roughness plays an important role in estimating the shear strength of granular materials. A series of ring shear tests with different surface roughnesses (i.e., smooth and rough surfaces) were performed. A large-sized ring shear device, which is applicable for fine- and coarse-grained sediments, was developed to examine the shear strength of large particle sizes (i.e., commercial gravels with a mean grain size of 6 mm). In terms of surface roughness, the drainage- and shear-velocity-dependent shear strengths of the granular materials were examined. In this study, different shear velocities of 0.1, 0.5, and 1 mm/s were applied under drained and undrained conditions. The test results clearly show that shear stress is affected by drainage, shear velocity, and surface roughness. In particular, a typical strain-hardening behavior is exhibited regardless of the drainage and shear velocity condition. The measured shear strength obtained from both drained and undrained conditions increased with increasing shear velocity. All tests showed a large fragmentation using rough surfaces compared to the smooth surfaces of the device. The grain crushing was significant during shearing, even when normal stress was not applied. For a given shear velocity, surface roughness is an important feature in determining the shear strength of granular materials.

Mechanical properties of concrete with bamboo chips as a potential source of aggregates have been investigated in this study.The measurement of this investigation includes slump loss, compressive strength, strain at peak compressive stress, modulus of elasticity, compressive toughness ratio, and splitting tensile strength. A 0.5-cm-thick bamboo chip was cut to a 1 cm (width) × 1 cm (height) piece and then dried, wetted, and coated to minimize water absorption.The coarse aggregates in the concrete specimen were replaced with 10%, 20%, and 30% (by volume) of each bamboo chip. The testing results showed that the compressive strength and splitting tensile strength of concrete with bamboo chips decrease with increasing bamboo chip content (BCC). It is considered that the decrease of strengths is due to the weak bond between the mortar and the bamboo chip.

To build a comprehensive understanding of long-term hydro-mechanical processes that lead to shallow landslide hazards, this study explicitly monitored the volumetric water content (VWC) and rainfall amount for a weathered granite soil slope over a four year period. From the 12 operational landslide monitoring stations installed across South Korea, the Songnisan station was selected as the study site. VWC sensors were placed in the subsurface with a grid-like arrangement at depths of 0.5 and 1.0 m. Shallow landslide hazards were evaluated by applying an infinite slope stability model that adopted a previously proposed unified effective stress concept. By analyzing the variations in the monitored VWC values, the derived matric suctions and suction stresses, and the calculated factor of safety values, we were able to obtain numerous valuable insights. In particular, the seasonal effects of drainage and evapotranspiration on the slope moisture conditions and slope stability were addressed. Preliminary test results indicated that continuous rainfall successfully represented the derived matric suction conditions at a depth of 1.0 m in the lower slope, although this was not the case for the upper and middle slopes. The significance of a future study on cumulative field monitoring data from various sites in different geological conditions is highlighted.