The 1999 Chi-Chi, Taiwan earthquake is the largest known onshore thrust-faulting event that has occurred in the 20th century. It provides an excellent opportunity to characterize the surface rupture and ground deformation associated with thrust faulting. The surface faulting along the Chelungpu fault produced by the earthquake extends north-south about 96 km, and has a maximum horizontal slip of about 10 m and a maximum scarp height about 8 m. Ground deformation occurred predominant on the hangingwall, ranged in width from several meters to several tens of meter. In a few cases, the deformation zone was several hundred meters wide, and it was even wider in Tsuolan area. In the deformation zone, there are secondary faults, branch faults, open cracks and/or tilted ground, which damaged existing buildings and structures. Along the length of the Chelungpu fault rupture, hangingwall deformation may be classified into 9 different styles. This includes: (1) Simple thrusting, (2) Monoclinal folding, (3) Simple warping, (4) Thrusting and warping, (5) Thrusting, warping and graben formation, (6) Warping and normal faulting, (7) Thrusting and back-kinking, (8) Thrusting and backthrusting, and (9) Thrusting with multiple zones of open crack. Simple thrusting and monoclinal folding generally represent narrower deformation; whereas more complex zones of deformation tend to be wider. Fault rupture is the direct reason to cause serious damage to engineering structures; the deformation zone away from fault tip is not so critical to common buildings and structures. One method of mitigating hazard from earthquake surface rupture, currently being used in California, is to develop setback zones along fault traces in order to restrict the location of structures for human occupancy across the trace of an active fault. After the Chi-Chi earthquake, the Taiwan Government is also developing a similar rule. However, for complicated faulting like the Chelungpu, it is not easy to draw a simple setback zone to avoid all possible damage from the next earthquake. However, the most important point for a common building or structure is to avoid it from across fault crush zone. But, for a critical facility, like a nuclear power plant or a high dam, sitting and construction on the hangingwall deformation zone should be avoided.
DISASTERS, CHARACTERISTICS, AND CASE ANALYSIS OF SLOPE FAILURES CAUSED BY CHI-CHI EARTHQUAKE
921集集大地震對中部地區造成嚴重的坡地破壞，廖軒吾（2000），根據該地震前後的衛星（Spot）影像、航空照片、以及現場核對，得知該地震一共誘發了9272個面積大於625平方公尺的山崩，總面積約達127.8平方公里。 林美聆、廖洪鈞與翁作新（Lin, Liao and Ueng, 1999）之研究結果顯示：幾乎所有的坡地破壞都發生於車籠埔斷層之右（東）方。國家地震工程研究中心所支持的大地工程震災現地調查，確實掌握了中部山區436處坡地破壞的資料。 本文研究針對這436處坡地破壞的特性作進一步分析，發現本次地震所導致的坡地破壞大都屬於中至小型規模的淺層岩屑滑崩，且其坡度大都較為陡峭，在地震所引致的坡地破壞中，地表運動可歸納為導致坡地破壞的最顯著因素。本文對四大坡地災害（草嶺大崩山、九份二山順向坡滑動、中部橫貫公路谷關德基段之全面崩潰、與九九峰之禿頭）加以討論。 草嶺及九份二山的反算及穩定性分析也於本文之中一併探討
The Chi-Chi earthquake has caused extensive slope failures in central Taiwan. Liao (2000) pointed out that Chi-Chi earthquake caused about ten thousand landslides to the area, each of which is larger than 625 square meters and that the total area of the landslides exceeds a hundred square kilometers. In their paper on the geotechnical hazard caused by Chi-Chi Earthquake, Lin, Liao, and Ueng (1999) showed that almost all of the slope failure sites are located to the right（east）of Che-Lung-Pu fault. Based on 436 landslides investigated by professors and experts in the field of Geotechnical Engineering, the characteristics of slope failure for these landslides are analyzed. The analysis reveals that most of the landslides induced by the earthquake were small to medium size and were typically shallow debris slides of steep slopes. For the earthquake-induced landslides, ground motion is thought to be the most significant factor.Among all the landslides triggered by Chi-Chi earthquake, the most catastrophic and dramatic four will be discussed in greater detail. These are: Tsao-Ling rockslides, Juo-Feng-Err-Shan dip slope failure, stripping of Juo-Juo-Fong (99 peaks), and the Ku-Kuan to Te-Chi section （mileage 34K to 62K）of the Central Cross-Island Highway.Case study on the Tsao-Ling rockslides and the Juo-Feng-Err-Shan dip slope failure was also conducted.
A large-scale landslide happened again at Tsao-Ling Mountain due to the effect of Chi-Chi earthquake on 21st September 1999, a landslide lake formed at upstream of Chi-Shue Creek after the debris blocked the creek. Because the event occurred in flood season, the water level raised up rapidly. In order to understand the stability of residue slope and felled debris and the safety problem of new Tsao-Ling Lake under the effect of flood, also the hydrological condition in this area is severe which made the disposal of natural dam becoming an urgent issue, the Agency of Water Conservancy of Department of Economy proceeded an emergent assessment immediately after the earthquake. The result provided a work guide in short-term and a reference in long-term disposals.
Chi-Chi earthquake is the most damaging earthquake at Taiwan in a century. The shaking intensities at many places in the central part of Taiwan are very high, and extensive soil liquefaction occurred in many areas. Yuen-Lin, Wu-Feng, and Nan-Tou were especially hard hit with severe damages. Soil liquefaction caused house damages, movements and tilting of bridge piers, settlement and sliding of levees and water front structures, tilting and overturning of retaining structures, and cracking, lateral spreading and subsidence of roadways and farm fields. This paper presents the preliminary results of the field investigation, subsurface explorations, in-situ and laboratory tests, and analyses of the soil liquefaction during Chi-Chi earthquake. The information collected for the soil liquefaction during this earthquake will be used in the study of the liquefaction evaluation criteria for the soils in Taiwan.
Chi-chi earthquake, with a magnitude of 7.3 (Ritcher scale), not only caused severe damages to bridges and buildings, but also resulted in soil liquefaction in many areas in central Taiwan. The Yuan-Lin in Chang-Hua County suffered extensive damages, including soil liquefaction, subsidence, settlement and tilt of buildings, lateral movement of ditches, etc.This article presents the process and analysis methods for liquefaction during Chi-Chi earthquake in Yuan-Lin, including site investigation and back analysis result. The liquefaction potential using SPT method and CPT method varies a lot, but the tendency is quite similar. Generally speaking, the results calculated by Seed’s method incorporated with liquefaction potential index well match the field evidence caused by liquefaction. It is considered that Seed’s method is the most suitable one in this study of liquefaction in Yuan-Lin in Chi-Chi earthquake.
The study presented herein focus on the geotechnical disaster occurred in the vicinity of Nantou county from the 921 Chi-Chi Earthquake, including fault movement induced deformation, pile failure nearby the fault zone and soil liquefaction. Based on the investigation results from survey and coring inspection of piles, the connection between damage level and its corresponding distance from the surface fracture of fault was probed. In addition, for well understanding the appropriateness of some familiar approaches applied in analysis of liquefaction potential, the borings data before and after quake were introduced.
A systematic investigation of tunnel structures in central Taiwan after Chi-Chi earthquake has been conducted to evaluate the seismic effect on tunnel engineering. The damage patterns are summarized according to the patterns and the distribution of the cracks on the lining. The results show that the degree of damage is associated to the geological condition and the structural arrangement of the tunnel. Displaced fault zone passing the tunnel is definitely causing demolished damage. The geological weak zone, the distance from the epicenter and the unbalanced force of slope face also play an importance role. The seismic capacity of the tunnel relates to its structural arrangement, lined or unlined section, invert setup and reinforcement in the lining etc. Moreover, several examples are given to illustrate the rehabilitation procedures and technigues.
Due to the extensive slope failure caused by the Chi-Chi earthquake, massive amount of deposit material posed severe threat of potential debris flow hazard. In this study field investigations were conducted to identify and inventory the potential debris flow locations in the central region of Taiwan hit by the earthquake. Models for evaluation of hazard potential and risk were developed and risk assessments of the investigated areas were performed accordingly. The debris flows were rated as with high, medium, and low risk. A total number of 148 locations were investigated, and 49 locations were rated as with high risk. It is suggested that the locations with high risk should be mitigated with highest priority and more researches are need for the newly increased slope-type debris flow.
Lots of buildings settled and tilted in the disaster areas attacked by the Chi-Chi earthquake. For the damaged buildings with stable upper structure, the foundation rehabilitation was performed to recover its original function. The underpinning method and grouting method were used most commonly for the foundation rehabilitation. A total of 3 remedial cases for each remedial method were studied in this paper and the construction processes, rehabilitation effectiveness and the problem encountered during construction were investigated and evaluated. As a result, the comparison of advantage/disadvantage and applicability between two methods are proposed. Results of this study indicate as the detail investigation prior to construction and the proper construction management were conducted during rehabilitation, the damaged foundation can be retrofitted effectively within an appropriate schedule; otherwise, the effective rehabilitation can not be reached and the construction duration will be postponed. Because the local experiences of foundation rehabilitation are limited in Taiwan and some major problems do not resolved yet, it is suggested that the techniques and abilities of foundation rehabilitation should be improved by the engineers with greater efforts.
The Island of Taiwan is at the collision zone of Euroasian and Phillippine Sea Plates, resulting in numerous faults and earthquakes in the process of the Island's formation. Wide range of causes causing diastrous damages during the Chi-Chi Earthquake, including code, regulations, design, construction to disaster prevention system, etc. Regarding geotechnical engineering, in addition to thorough site investigation, the "near fault seismicity study" is imperative for Taiwan's seismologists; the earthquake effect on the earth pressure, water pressure, liquefaction potential analysis and its mitigation measures should be considered seriously; and the design spectrums for both the Maximum Credible Earthquake and the Maximum Probable Earthquake spectrums considering the geologic formation and site soil conditions should be included in geotechnical site investigation reports for structure engineers' use in the design.