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Liquefaction Potential Studies for
Associate Professor of Amirkabir University of Technology, M.Sc. in Geological Eng.(Tectonics) & Director of Geotechnical Section-Arkan Pooyesh Consulting Engineers, Thran, Iran M.Sc. in Geotechnical Eng.-Managing Director
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ABSTRACT
Vulnerability of Tehran against liquefaction has been the main concern of all inhabitants of the city since long times ago. The fast and great development, and the important political-social position of the city in the past decade have caused the seismic hazards of Tehran to be taken into account more than ever. The liquefaction occurrence and losing the ground strength in susceptible zones of big cities with high seismic potential, is one of the most common hazards caused heavy and considerable damages during strong ground motions. In the city of Tehran, the existing of different faults, having high seismic potential, and location of shallow water table in southern areas, are main factors, which may induce liquefaction in some future earthquakes. In this respect, different studies have been done by some institutes and individual researchers to evaluate the liquefaction potential of the city. In these studies, which have been carried out in a regular, continuous, and supplementary manner, the liquefaction potential of Tehran in Large (macro), and also small (micro) scales have been estimated and specified. In the present paper a great effort has been done to present and describe the background, process, and the outcomes of the studies, together with the results of the latest studies carried out by the author, which has completed the liquefaction potential evaluations of the city of Tehran. The microzonation maps of the city against liquefaction for different future earthquakes have been provided and presented accordingly.
INTRODUCTION
Although liquefaction, which means loosing the shear strength of the soil due to flow failure, may occur at any susceptible points of the ground, however, the most damages due to the phenomenon during past heavy earthquakes have been reported among big cities and urban facilities. The first heavy urban damages belong to Nigata city which happened during the 1964 earthquake and many of recently reconstructed buildings and facilities were severely damaged following the liquefaction of the ground. In spite of the recognition of the phenomenon and its causing factors many other cities around the world repeatedly faced with great damages due to its occurrence afterward. The extensive destruction and damages happened in the city of Kobe during the great Hanshin earthquake in 1995, after numerous researches, studies, and scientific works carried out on the phenomenon for more than a quarter of a century, proved that the continuation of researches and studies in this area to achieve safer design and construction criteria for building and structures are still necessary and vital. In other points of the world and specially in the USA the main part of the damages happened during 1971, 1989, and 1994 earthquakes were due to the liquefaction, which again emphasizes the need for further studies in a universal scale to recognize and control the phenomenon more accurately for finding the more effective methods for mitigation the geotechnical earthquake hazards. In Iran, although damages due to past earthquakes have not been recorded for a long times, but the liquefaction phenomenon in some great earthquakes such as Roodbar- Manjil (1991), was reported to happen in the coastal cities like Astaneh-Ashrafieh and caused many damages to the buildings and facilities of the city. In the great and dense populated city of Tehran (capital of Iran), though there has not been a heavy earthquake during the last century, but taking into account the several active faults
within and around the city, enhances the probability of occurrence a more strong ground motion in the future. Therefore the comprehensive studies of Tehran, in view of assessing the vulnerability points of the city against liquefaction during future earthquakes to mitigate the seismic hazards are of great priority and importance. In the next sections the records and background liquefaction studies of Tehran are presented. and a recent and supplementary study for providing the microzonation maps of the city against liquefaction has been given in details.
THE BACKGROUND OF LIQUEFACTION STUDIES IN TEHRAN
The first liquefaction studies of Tehran was carried out by Berberian et.al (1985). In this work, that can be categorized as grade 1 liquefaction studies, based on the geological characteristics and the alluvial texture of the Tehran ground formation an introductory map of liquefaction potential of the city was provided. According to this map, because of existing shallow water table and layers of sand and silt, the southern part of Tehran have been specified as the susceptible zone to liquefaction. Using other evaluation methods in a more accurate study, Mir Hosseini and Karri (1992) collected all geotechnical data and SPT results already obtained by different bodies in the city, and conducted a comprehensive liquefaction studies for the whole city in grade 2. As a result of this study the southeastern part of the city was found to be the most susceptible zone to liquefaction during the future ground motions. In 1995 Mir Hosseini et.al in a joint project supported by the international institute of earthquake engineering and seismology and the soil mechanics laboratory of road and transportation ministry of Iran, carried out the most accurate studies in some parts of south-east Tehran to evaluate the liquefaction potential of the region. In this project which was done in grade 3, a special sit investigation has been planned and several
boreholes were excavated (Fig. 1) for getting precise geotechnical data of the region. Consequence to this studies different microzonation maps for the selected zones were provided in which the most liquefiable points in future earthquakes have been specified.
Figure 1. The selected regions for liquefaction studies in the past
To complete the liquefaction studies of the city and cover the whole susceptible region, the national research council of Iran urged the university researchers to continue the studies on the remained areas. In a complementary work a new region in southeastern part of the city was selected (Fig. 2), and the liquefaction evaluation by the same scale and precision was continued and carried out. The details of the recent studies, and the specification of the selected area, and also the typical microzonation maps of the region are given in the next sections.
THE SPECIFICATION OF SOUTHERN PART OF TEHRAN
The geological properties of the selected region
Since the selected region is a part of Tehran regional, thus, it has all geological characteristics of southern part of Tehran. The selected region is located in one of the sub series of Tehran mountainous regional, called northern subsidence regional of Figure 2. The selected region for recent liquefaction studies Central Iran (Tehran Flat). The Tehran flat is connected to the high level areas of Tarasht-Abbasabad from the north, and to the south Rey subsidence from the south via the north Rey fault. The type of sedimentations in this region is mostly from the C and D alluvium of Tehran regional.
The seismicity of the selected region used in the studies
The selected region follows the seismicity regime of the south part of central Alborz, since has been located in the Tehran extension. The existing of several long active faults such as Mosha, North of Tehran, Kahrizak, North of Rey, and South of Rey, has increased the seismicity potentials of the region. For specific sites in this region,
the liquefaction studies has to be carried out based on the especial seismic risk analyses to achieve the most probable results. Nevertheless, in the current study to cover the wide ranges of seismic hazards, the magnitude of the anticipated earthquake has been taken 7.5 in Richter scale and the analyses have been done for different peak ground accelerations from 0.3g to 0.4g.
The geotechnical investigations in the selected region
Since the liquefaction studies had to be done in the most accurate scale, according to the manual for zonation on seismic geotechnical hazards (TC4 committee), the region has been divided into grids of 500m.x500m. Then a specific place in the center of each grid has been selected for excavating the geotechnical borehole. The excavating continued up to 20 m. depth provided stiff and hard layers were faced. The excavation machine used in this investigation was a rotary one and the standard penetration test (SPT) at each 2 m. depth and also at changing the kind of layer was carried out regularly. To get the required geotechnical parameters of the layer for liquefaction analyses, some soil samples from different layers were taken and sent to the IIEES soil lab. for doing physical and mechanical tests. According to the past studies of the Tehran Water Supply Company, the suggested underground water levels in the region (Fig. 3), as a basic source for water table contours has been used in the analysis.
Figure 3. The underground water contours in the region
Nevertheless, the direct observations of the water level in the excavated boreholes have been implemented to modify and update the information in this respect. Based on the geotechnical data from excavated boreholes and other field observations the ground profile has been determined and illustrated (Fig. 4).
THE LIQUEFACTION ANALYSES AND MICROZONATION MAPS
To study the liquefaction potential of the region, two analyses methods were used. According to the results of the past liquefaction studies in the area, it was found that the method suggested by Iwasaki proved to be the suitable one for sand to sandy silt grounds, while the Seed and Idriss modified method was the best one for silt and silty sand soils. Thus, upon to the ground texture, one of these methods was used to evaluate the liquefaction potential of the ground alternatively.
Figure 4. The location of the excavated boreholes in the region
Consequent to geotechnical investigations carried out in the region, the physical and mechanical soil parameters (such as SPT numbers, fine content, Atterburg limits, particle size distribution, etc.) at different depths and different points were measured and used in the two mentioned analyses methods for liquefaction evaluation of the region. The liquefaction potential in position of each borehole was initially estimated. Then based on the depth and position of the liquefied layer, its impact on the ground surface in terms of two indices, namely; liquefaction potential (Pl ), and site liquefaction potential (IPL ), were separately calculated and presented.
The analyses process, details of calculations, and safety factor for each layer, as well as, the method of estimating the Pl and IPL, are given in a specific table which has been designed for this study. Fig. 5 shows a typical tables which have been provided for liquefaction analysis in borehole No.(B2). Finally, the microzonation of the whole region, based on the Pl, and IPL, estimations for different peak ground accelerations have been implemented. In figures No.6 and No.7 typical results of microzonations carried out for the peak ground acceleration of 0.35g are presented.
SUMMARY AND CONCLSIONS
The liquefaction potential studies of Tehran (the capital of Iran), have started since 1985. According to these studies there are some liquefiable zones in southern parts of the city. The latest one in this series to cover the whole area was that supported by the national research council of Iran. In this study, as the complementary work, the left region in the southeastern part of the city was selected. Different geological and Figure 5a. A typical log of basic information (borehole No.B2)
Figure 5b. A typical log of final information (borehole No.B2)
Figure 5c. A typical table for liquefaction evaluation procedures (borehole No.B2).
Figure 6a. The microzonation map based on estimated PL and PGA= 0.35g.
Figure 6b. The microzonation map based on estimated IPL and PGA= 0.35g.
Geotechnical investigations were planned and carried out in the region. According to the field and laboratory soil parameters, underground water table, and assumed acceleration data, the liquefaction potential of the region was evaluated. based on the liquefaction analyses, the seismic hazards were specified and the microzonation maps for the region were provided. As can be seen in these maps, for peak ground accelerations above 035g, there are some points will face with the liquefaction in the region. Since these areas are located in the densest populated zones of the city, it is strongly recommended to take into account for special land use in future planning of the city to reduce the human loss during strong ground motions.
REFERENCES
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