Kuo-Hsin Yang

Mobilization of reinforcement tension within geosynthetic-reinforced soil structures

This paper examines the mobilization of reinforcement tension within geosynthetic-reinforced soil (GRS) structures at working stress and at large soil strains. Fully-mobilized reinforcement tension is assumed in most current design methods for the internal stability of GRS structures. In these methods the mobilized reinforcement tensile load is assumed to be equal to mobilized horizontal soil forces computed using active earth pressure theory. However, comparison with reinforcement tension loads measured in the field has shown that this approach is conservative (excessively safe) by as much as a factor of two. This observation has prompted the current study in which stress data obtained from a numerical study and two instrumented large-scale GRS retaining walls were used to examine the relationship between mobilized reinforcement tensile load and mobilized soil shear strength. The results show that the ratio of reinforcement tensile load and mobilized soil shear strength is not constant Only when the average mobilized soil shear strength exceeds 95%, is reinforcement tensile capacity mobilized significantly. Nevertheless, less than 30% of reinforcement strength is mobilized when the average mobilized soil shear strength reaches peak soil shear capacity. These results help explain why current design methods lead to computed reinforcement loads that are very high compared to measured loads under operational conditions.

Identification and assessment of heavy rainfall–induced disaster potentials in Taipei City

With increasing threat to lives and properties, identifying and assessing disaster potentials has become necessary and prior for effective disaster preparation and rescue planning. This study first introduces practical methods currently used in Taipei City, Taiwan, to identify and assess heavy rainfall–induced potential risks on flood, debris flow, and landslide. The identified disaster potential information is further applied to a series of deterministic and probabilistic risk analyses using Shilin District of Taipei City as a case study. The deterministic risk analyses are conducted to evaluate the impact of various heavy rainfall intensities on the residents. The probabilistic risk analyses are performed to establish risk curves for the population affected by heavy rainfall–induced hazards. The risk curve represents the relationships between the affected population and the annual exceedance probability. This study found the annual exceedance probability is very sensitive to the assumed coefficients of variation of the affected population. It is recommended historical statistical data on the correlation between affected population and rainfall intensity should be recorded and compiled in order to assess the actual probability distribution function of the affected population. Risk analysis results are further applied to assess the community evacuation capacity in this district. Last, short-term and long-term mitigation strategies and recommendations are discussed.

Reliability-Based Design for External Stability of Narrow Mechanically Stabilized Earth Walls: Calibration from Centrifuge Tests

A narrow mechanically stabilized earth (MSE) wall is defined as a MSE wall placed adjacent to an existing stable wall, with a width less than that established in current guidelines. Because of space constraints and interactions with the existing stable wall, various studies have suggested that the mechanics of narrow walls differ from those of conventional walls. This paper presents the reliability-based design (RBD) for external stability (i.e., sliding and overturning) of narrow MSE walls with wall aspects L/H ranging from 0.2 to 0.7. The reduction in earth pressure pertaining to narrow walls is considered by multiplying a reduction factor by the conventional earth pressure. The probability distribution of the reduction factor is calibrated based on Bayesian analysis by using the results of a series of centrifuge tests on narrow walls. The stability against bearing capacity failure and the effect of water pressure within MSE walls are not calibrated in this study because they are not modeled in the cent...

Behavior of Geotextile-Reinforced Clay with a Coarse Material Sandwich Technique under Unconsolidated-Undrained Triaxial Compression

AbstractThis paper presents a series of unconsolidated-undrained (UU) triaxial compression tests for investigating the behavior and failure mechanism of geotextile-reinforced clay and the effects of sandwiching nonwoven geotextile in a thin layer of sand (sandwich technique) on improving the shear strength of reinforced clay. Test variables include confining pressures, the number of geotextile layers, and thicknesses of the sand layers. The mobilized tensile strain of reinforcements, estimated according to the residual tensile strain by using a digital image–processing technique, was used to directly quantify the effects of soil-geotextile interaction on the shear-strength improvement. The test results showed that the shear strength of reinforced clay increased as the number of geotextile layers was increased. Failure patterns were changed from classical Rankine-type failures for unreinforced soil specimens to bulging (barrel-shaped) failures between adjacent geotextile layers. The effectiveness of reinfo...

Effect of Foreshocks of the 2016 Kumamoto Earthquakes on the Aso-Bridge Slope

This paper presents an examination the Aso-bridge slope subjected to the foreshocks of the 2016 Kumamoto earthquakes. While the Aso-bridge slope was marginal stable in the absence of earthquake, it remained stable during the foreshocks that struck on April 14 and 15 2016, with magnitudes of 6.5 and 6.4 Mw, respectively. To validate that the slope did not fail during the foreshocks and the applicability of an enhanced FS method capable of determining the initiation time of co-seismic landslides, relationships between the Aso-bridge slope and the nearest recorded seismic signals, including the main-shock and the foreshocks, were investigated. It was found that the two foreshocks, having PGAs of 43.0 and 129.7 gal, were not strong enough to cause failure of the slope. They were not strong enough mainly because of the smaller magnitude as compared to the main-shock of 7.0 Mw with a PGA of 508.9 gal, leading to a significant difference in the PGAs. The study demonstrates that the enhanced FS method can be one efficient and practical alternative for determining large-scale co-seismic landslides.

Backfill Stress and Strain Information within a Centrifuge Geosynthetic-Reinforced Slope Model under Working Stress and Large Soil Strain Conditions

Numerical methods combined with a centrifuge test are used to investigate the mobilization of backfill stress and strain within a geosynthetic-reinforced soil (GRS) slope under working stress and large soil strain conditions. System stability indicated by the factor of safety (FS) of the GRS slope is calculated using limit equilibrium analysis. The stress and strain information under various soil stress states is evaluated using a finite element model with a soil constitutive model capable of modeling soil softening behavior. The numerical models are verified by data from a centrifuge GRS slope model. Numerical results indicate that soil stress mobilization can be described with soil stress level S, which is defined as the ratio of current stress status to peak failure criteria. As loading increases, backfill stresses develop and propagate along the potential failure surface. Mobilization of soil stress was non-uniform along the failure surface. Immediately after the stress level reaches peak (S=1), strength softening initiates at the top and toe of the slope at approximately FS=1.2. The slope settlement rate and reinforcement tensile load significantly increase when soil softening begins. The softening occurs randomly and irregularly along the failure surface and the formation of soil softening band completes at approximately FS=1.1. The failure surface corresponds to the locus of intense soil strains and the maximum tensile loads at each reinforcement layer.