Taiwan Report

EEFIT Short Report The Ji-Ji, Taiwan Earthquake of 21 September 1999 Editor: Alan Stewart 1 EEFIT Short Report EEFIT gratefully acknowledges the support of its corporate members: Arup Group British Geological Survey CREA Consultants Giffords Halcrow Risk Management Solutions Sellafield Ltd Sir Robert McAlpine Earthquake Engineering Field Investigation Team Institution of Structural Engineers 11 Upper Belgrave St London SW1X 8BH United Kingdom Tel: +44 (0)207 235 4535 Fax: +44 (0)207 235 4294 www.eefit.org.uk © EEFIT 2007 ISBN 978 1 906335 10 6 2 EEFIT Short Report 1. Introduction A magnitude 7.6 earthquake struck Taiwan at 01.47 local time on 21 September 1999. The epicentre was approximately 7km north west of Ji-Ji, a small town bordering a mountainous area some 155km south of Taipei, the capital. Ground shaking lasted around 40 seconds, and approximately 2,500 people were killed and 11,000 injured. Around 10,000 buildings were destroyed, with the same again being seriously damaged. Damage was greatest in the central regions of Nantou, Taichung and Yunlin, and economic loss has been estimated at US$10 to $12 billion. This was the largest earthquake to hit Taiwan since a magnitude 7.1 event of 1935 which killed around 3,500 people. An EEFIT team visited the area three weeks after the event, and this short report provides a summary of their observations. 2. Tectonic Setting Taiwan lies on the boundary between the Eurasian and Philippine Sea plates and has been created as the Philippine Sea plate moves north-westward at around 7cm a year. The collision area is marked by a north east trending zone of thrust and strike-slip faulting, and the earthquake occurred on the Chelungpu thrust fault near the centre of this zone. The earthquake history of the island has been dominated by large magnitude earthquakes in the eastern part of the country, with less frequent shallow events in the western area where the 1999 event occurred. Only five of 43 recorded 20th century earthquakes were surface fault events like that of 21 September 1999, and much of the west of the island was therefore categorised as only moderately seismic. 2.1 Fault Movements This earthquake was characterised by spectacular fault movements. At places the vertical lift was measured to be close to 10m. Such large fault movements were induced due to the nature of the thrust fault. Due to these large fault movements some of the civil engineering structures suffered quite badly. A few examples are presented below: 3 EEFIT Short Report Figure 1: Fault movement caused this spectacular lift of this building that straddled across the fault Figure 2: The building next to the one seen in the figure before suffered damage but survived without collapse Figure 3: The athletic track at Wu Fang School suffered badly due to fault movements 4 EEFIT Short Report Figure 4: Collapsed Wu Fang school building close to the athletic track Figure 5: Where fault movements were smaller, bridges survived collapse Figure 6: Failure of the dam due to fault movement (right hand side was uplifted by the earthquake) 5 EEFIT Short Report Figure 7: Left hand edge of the dam which indicates the original level of the dam crest Following the above observations there was a move in Taiwan to legislate against construction of buildings close to the known fault lines. Of course, not all fault lines have surface expression and some faults may be below ground and undetectable. The 921 Ji- Ji earthquake itself occurred on a less known Chelungpu fault which did not show much activity in the recent past. 3. Geotechnical Observations 3.1 Liquefaction phenomena Liquefaction phenomena were observed at several locations in the Taichung area. Figures 8 and 9 show a block of single storey houses in the suburbs of Yuan Lin city which suffered extensive liquefaction. The buildings settled and liquefied soil flowed into the rooms through the floor slabs, filling the rooms and lifting the furniture. A nearby well was filled with liquefied soil flowing from the sub-strata (Figure 10), and eye witness accounts suggest that immediately following the earthquake there was high pressure ejection of soil from the sub-strata. 6 EEFIT Short Report Figure 8: Single floor tiled roof house suffers settlement following Figure 9: A view inside the house liquefaction of sub-strata showing the inflow of sub-strata filling the room and lifting the furniture Figure 10: Well filled by liquefied soil Fig.11: Particle size distribution of soil from the well in Fig.10, (courtesy A J Brennan, Research Student, Cambridge University) Fig. 11 shows the particle size distribution of the soil samples collected from the well site, which confirm that the sub-strata consisted of fine grained sandy soils, which are susceptible to liquefaction. Following the EEFIT visit to this site, data was also obtained from bore samples in the area, and the sub-strata interpreted as shown in Fig.12. This indicates that the top soil layer consisted of clayey soil, with liquefiable layers below. It therefore seems likely that the excess pore water pressures generated in the sub-strata were initially retained by the upper clayey soil, leading to a sudden burst into the floor slabs of the houses and the well and an outflow of the material from the sub-strata. 7 EEFIT Short Report A nearby factory compound displayed sand boils and mud volcanoes in the open compound. The factory floor slabs also exhibited classical yield line patterns (Figure 13), indicating that the ground below suffered settlement following liquefaction, offering little support to the floor slabs and causing them to fail. Figure 13: Yield lines in floor slab Fig.12 Soil classification based on bore hole data 3.2 Liquefaction induced settlement In Yuan Lin city, there were many buildings which suffered liquefaction induced settlements. Figure 14 shows rotation of a three storey building towards its neighbour. Figure 15 shows the settlement of this building relative to the road level, while Figure 16 shows a column breaking away from the floor slab at foundation level. Figure 15: Settlement of the ground floor relative to road level Figure 14: Building rotation due to differential settlement following Figure 16: Settlement of a liquefaction of the foundation soil column at the foundation 8 EEFIT Short Report There were many examples of liquefaction induced settlement and the figures above illustrate typical examples. In general the structural damage suffered by these buildings was relatively minor and therefore it is fair to say that these are geotechnical failures. 3.3 Lateral spreading When liquefaction of soil occurs either in the body of, or underneath, a slope, the ground will exhibit lateral spreading. Even gentle slopes of 3o to 6o to the horizontal are known to suffer lateral spreading. There were many examples of lateral spreading in this earthquake, and a typical example is at the site of a new bridge near Nantou city, shown in Figure 17. Liquefaction of the soil was very obvious at this site with sizeable sand boils visible on the ground surface, examples of which are shown in Figure 18. Figure 17: Lateral spreading of the Figure 18: Sand boils at the new slopes parallel to abutments showing Nantou Hsou-Shi bridge site deep tensile cracks 3.4 Landslides Taichung and neighbouring districts witnessed landslides on a massive scale, resulting in major destruction of the overgrowth, as shown in Figure 19. Figure 19: Landslides causing loss Figure 20: Blocking of hill roads of over growth due to landslides 9 EEFIT Short Report By and large, the land slides seem to have occurred at fairly shallow depth, and were not deep seated slope failures. A shallow depth of the top soil has suffered slippage causing the tree and plant debris to slide downhill thereby exposing the soil layers below. Figure 20 shows blockage of the hill roads, which was a common result, leading to a loss of communication between Taichung city and interior villages. The performance of tunnels following the earthquake was mixed. Figure 21 shows one such tunnel which suffered structural damage to its lining. However, the tunnel was located in a hill side which suffered landslides with material from the top of the slope sliding parallel to the tunnel axis. It was therefore not clear whether the damage to the tunnel lining was due to the earthquake loading causing additional stresses in its lining or due to material flow close to the tunnel crest. The tunnel was leaking water and the road was blocked off by the authorities. Figure 22: Landslides caused Figure 21: Tunnel located in the side of failure of the lifelines laid next a hill with landslides at the surface to the roadway The landslides also caused to damage to the lifelines laid next to the road way. In Figure 22, the re-laying of new lifelines can be seen. 3.5 Reinforced slope failure The earthquake provided an example of the performance of a reinforced slope. The slope is located at the campus of the National Chihnan University of Information, and is approximately 120 m high, with failure occurring over a width of about 300 m. Figure 23 shows the failure. Figure 23: Reinforced slope failure 10 EEFIT Short Report A polyethylene cover had been placed on the upper reaches of the slope after the earthquake, to prevent further sliding due to rain water seeping into the slope. The slope angle is very steep and was estimated at approximately 70o. As a result a reinforcement was used which consisted of geotextiles and geonets. The geotextile appeared to have been laid horizontally and was wrapped around a geonet filled with locally available material (rocks plus coarse sand) similar to the rock filled wire mesh gabions used in the UK to stabilise slopes.

Load more