The Chi-Chi earthquake induced severe landslides in central Taiwan. The Soil and Water Conservation Bureau (2000), identified were more than 20,000 cases of variations based on the SPOT satellite images and aerial photos. The field investigation of geotechnical hazard supported by National Center for Research on Earthquake Engineering also documented 436 landslides. The number of landslides updated from satellite images is more than 30,000 cases. The threshold peak ground horizontal acceleration from the documented cases is approximately 100gal.
The slope behavior subjected to seismic loading is very complicated. The sliding behavior can be defined from sliding displacement and methods for dynamic slope analyses are mainly sliding-block analysis and numerical analysis. The sliding-block analysis using plane failure or infinite slope failure is often adopted in recent years. However, uniform slopes subjected to earthquake are likely to induce non-planar shear surface, in this study log-spiral failure surface is used to derive displacement of slope under seismic loading. Comparing the failure surfaces formed by different methods, it was found that the failure surfaces were very similar when failure surface is limited to toe failure. Accordingly, the proper range for sliding-block analysis is proposed. Within the range, the sliding-block behavior is fairly consistent with results of numerical analysis. Results of this study indicated that sliding-block analysis would provide reasonably good results when slope angle is smaller than 60°.
Landslide cases were selected from satellite images/aerial photos identified variations and ground-based investigation to perform landslide displacement analysis. Results of the analysis suggested that most displacements were larger than 10cm except some cases with 5~10cm. It is proposed that the threshold conditions of yield acceleration ratio of smaller than 0.5 and a toe displacement of 10 cm would be appropriate for defining landslide failure caused by the Chi-Chi earthquake.
The hydraulic facilities in central Taiwan suffered from the severest damages in decades after the strike of the 921 Earthquake. Observably take the remedy measures to vitalize and prevent damages as soon as possible would make the rehabilitation well done. In order to prevent secondary damages and mitigate the possible damages in the future, this project reviewed the water supply facilities and the damages to hydraulic facilities after the 921 earthquake were investigated. The possible secondary damages and their remedies were proposed. As for the completed, a complete review and feasible rehabilitation measures were proposed, especially for those severely damaged rivers. Finally, a hazard mitigation, preparation and rehabilitation plan was recommended to minimize the probable damages to hydraulic facilities after future earthquakes.
On 21 September 1999, the new Sanyi railway tunnel in central Taiwan was seriously damaged by Chi-Chi earthquake. The tunnel was designed and excavated by NATM. It had just been in operation for about one year at that time. For clarifying the damage mechanism, this study proposed a non-linear modified cross section racking deformation method (MCSRD) to analyze the seismic behavior of the NATM-built tunnel. The nonlinear MCSRD method fully considers the nonlinear soil structure interaction between tunnel and surrounding ground. Based on the analysis results, the simulated damage pattern of the tunnel agreed well with the field damage condition. Besides the bad geological factor, the results indicated the bad geometry of the tunnel and the unreinforced second lining were the main causes that may damage the tunnel during earthquake. The results also showed that the second lining substantially sustained seismic loading and the seismic capacity of the lining could be largely increased when the amount of reinforcement of the second lining is over a threshold value. Thus, second lining of the NATM tunnel should be suitably reinforced in a seismically active area.
Fines (particles passing #200 sieve) contents of the soils in Central Western Taiwan often exceed 50%. To avoid confusion in terminology, they will be referred to as the silt/sand soils. The effects of non or plastic fines on the behavior of silt/sand soils especially in terms of cyclic strength have been studied by geotechnical profession for the past half century. The Chi Chi Earthquake of September 21, 2009 triggered extensive liquefaction of silt/sand soils in Central Western Taiwan. The occurrence offered unique opportunities to study the soil behavior related to liquefaction in this region. The author performed laboratory tests, including static/cyclic triaxial tests and cone penetration tests (CPT) in a calibration chamber, as well as field CPT and taking undisturbed silt/sand soil samples, in the past decade. The results show that there are significant differences in soil behavior between the silt/sand soils typically found in Central Western Taiwan and those reported in literatures. The procedures currently used for liquefaction potential assessment and evaluating the effects of fines on soil behavior in this region need to be revised accordingly. The paper summarizes results of studies performed by the author.
During the 1999 Chi-Chi Earthquake, a site named Wu-Feng very close to the fault rupture in central Taiwan has suffered serious damages caused by soil liquefaction induced ground failures. Detailed case study of Wu-Feng is reported in this paper in an effort to investigate both failure causes and damage types. The authors first present soil condition and characterization of the near fault strong ground motion of Wu-Feng. Secondly, both reconnaissance and surveyed results of liquefaction induced ground failures are presented. Finally conclusions on factors that possibly contribute to such extensive ground failures and future research suggestions are provided. Progress of presented study is hopefully to improve our understanding on liquefaction induced ground failures caused by strong ground motions.
Physical model tests using a large biaxial laminar shear box (1.880 m×1.880 m×1.520 m) on the shaking table at the National Center for Research on Earthquake Engineering (NCREE), Taiwan were conducted to study the liquefaction behavior of saturated sand under one- and multi-directional earthquake shakings. Model piles made of steel and aluminum pipes with various masses on their tops were also placed inside the shear box to evaluate the pile performances and soil-pile interaction in liquefiable sand under shakings. Specimens of clean Vietnam silica sand and Mailiao silty sand were prepared using specially designed pluviators. The input shakings included sinusoidal and recorded earthquake accelerations. The test results showed that the induced excess pore water pressures during a multidirectional shaking were substantially higher than those generated under one-dimensional shaking of the same magnitude of acceleration. Significant settlements occurred only when there was liquefaction of the soil. Volumetric strain after liquefaction decreases with relative density of the sand regardless of the shaking amplitude, frequency and direction (one- or multi-directional shaking). The performances of the model piles and soil-pile interactions for liquefied and non-liquefied soil conditions were studied and the effect of the constraint of soil on the pile behavior under shaking was evaluated.
This paper reports some recent local research progress on performance of pile foundation due to ground lateral spreading by using numerical analysis.Interaction between lateral spreading ground and pile foundation is treated by using sophisticated coupling analysis and by using simplified decoupled analysis. For the latter analysis, free field response and pile performance are treated separately in the analysis. Soil parameter reduction , pore water pressure generating model, direct earth pressure and indirect earth pressure approaches are used for lateral ground deformation or earth pressure estimation. For coupling analysis, 3-diemnsional finite element method is adopted in simulating behavior of liquefied soil from initial liquefaction stage to post liquefaction stage. A soil constitutive model considering dynamic stress- strain behavior is used. Ground post liquefaction behavior is numerically simulated by using consolidation concept. Available centrifuge tested examples and real field cases are studied by the aforementioned decoupled and coupled methods. Agreement between the predicted and measured results is compared and discussed in the paper.
Post-earthquake studies on the failure mechanism of geotechnical structures, including soil retaining wall, highway embankments and levees, suggested that a nation-wide strategic program to enhance aseismic designs, construction techniques, regular inspection and retrofit, and risk management for geotechnical structures is necessary. The program is crucial to the minimization of earthquake-induced damage losses; it is also crucial to the rationalization of infrastructure and hazard mitigation budgets.
This article is focusing on the progress and discussion on the disaster and the analysis of the precipitation for further estimation and investigation of Typhoon Morakot, Flood Disaster, Slope-land disaster. Morakot Typhoon attacked Taiwan during the days of August 6 to August 10 in 2009, which brought the worst record to the history of Meteorology in Taiwan. More than 1,500 minimeter of precipitation was collected in one day and more than 3,000 minimeter of the rainfall was accumulated in the mountainous area. The sudden flood, the severe disaster to landslide and debris flow ruined the bridges, roads, broke the transportation to the south east part and the east part of Taiwan of the damage of buildings buried, population with trauma and the casualty of 619 in addition with the loss of cash amount more than 164 billion of NT dollars from different industries of agriculture, timbers, and the farming.
Traditional vegetation engineering methods fail to grow plants on steep concrete slopes and rapidly reach the desired resistance of slope surfaces against erosion. This study tries to tackle these problems by mixing cement, soil, greening additives and grass seeds at particular proportions, and spraying the mixture to steep concrete slopes by hydraulic mulching, so as to evaluate the effectiveness of this technique in the vegetation greening of steep concrete slopes. It has been validated by the 613 flooding in 2005 that the shotcrete vegetation mulching technique (SVMT) used in this study can provide the slopes with sufficient resistance to erosion within a short time and create an environment suitable for plant growth. It is shown that the steeper the slope, the shallower the roots will grow, and the longer it takes for mulch greening to be complete. The growing environment created by the SVMT of this study is suitable for grass species which have a shallow root system, extend by creeping and are highly drought-resistant. This growing environment and the limitation on development of the root system can deter the invasion of alien grass species, and reduce the frequency and cost of slope maintenance.
This paper studies the outward diaphragm wall displacement of an excavation due to column type ground improvement and its implication on the related design problems. Three monitored field cases with different kind of ground improvement method are collected to study the outward displacement behavior. An analytical method using the computer FLAC is proposed in this paper for the quantification of this effect. Analytical results show that the analysis of diaphragm wall displacement due to excavation should take the prior jet grouting effects into account. Otherwise, the prediction may be totally wrong.