Comparison of CPT Based Liquefaction Potential and Shear Wave Velocity Maps by Using 3-Dimensional GIS Muammer Tün, Uğur Avdan, Metin Altan, Can Ayday Anadolu University, Satellite and Space Sciences Research Institute Iki Eylul Campus, 26470 Eskisehir, Turkey {mtun, uavdan, maltan, cayday}@anadolu.edu.tr phone: 222-321-3550; fax: 222-323-9129 SUMMARY Geological and geotechnical researches and studies according to safe construction and proper civilisation must be done before the preparation of construction plans of urban area. This is the first step in the earthquake hazard analysis. The results obtained from these studies have been used as input for earthquake hazard risk evaluation. Utilisation from 3-Dimensional GIS, instead of 2 Dimensional in this stage by using drilling data and analysing them gives more reliable results. Nowadays it is known that, micro-zoning studies and preparation of earthquake risk scenarios depend on these studies are generally used in GIS technology. The overall studied area is nearly 128 hectares and belongs to Seismic Region II. of General Earthquake Risk Map of Turkey. Seismic Cone Penetration Test (SCPT) studies were done at 25 different locations. Cone resistance (q c ), sleeve friction (f s ) and friction ratio (R f ) values were obtained from 279 different layers. CPT based liquefaction map of the interested area were prepared by using these parameters. In addition to that, shear wave velocities (V s ) of different depths were obtained from the same area. Shear wave velocity data of the soil were used for the preparation of shear wave velocity distribution map of the area. All these data were used for making comparison between CPT based liquefaction potential and shear wave velocity distribution model by 3-dimensional GIS techniques. Comparison has indicated that, volumetric distribution of shear wave of soil which has velocity less than 180 m/sec exists between the surface and 5 m depth. On the other hand, liquefiable zones are seen mostly below this depth. Although some overlapped regions were detected with 2-dimensional analysis, it is concluded that, soil which has low V s may amplify ground motions significantly generally exists between 0 m to 4 m depths and liquefiable zone exits between 4 m to 7 m depths. This result have been obtained only by using 3-dimensional GIS. KEYWORDS: CPT, liquefaction potential, shear wave velocity, 3-dimensional GIS INTRODUCTION A methodology for assessing seismic risk study includes regional and local hazard and vulnerability of physical properties. As a first step, earthquake hazard analyses covers the determination of seismic condition of the area and engineering geological parameters of the soil which was obtained from the same area. These values would be used as input for the vulnerability studies sometimes called as loss of estimation. Earthquake hazard studies must be done before the vulnerability investigations. The results of the studies are the main basement for loss estimation studies. Technological development has put forward using computers and related softwares for the study. Geographic information system which is defined as using geographic data for modelling and analysing is the most powerful tool for preparation of these maps. GIS designed specifically for geological work need to be fully three-dimensional, so that each data object is characterised by its location in space with three spatial coordinates (Bonham-Carter, 1994). In 7 th AGILE Conference on Geographic Information Science 29 April-1May 2004, Heraklion, Greece Parallel Session 7.3- Decision Support Systems / Risk Management II 645
these three-dimensional coordinates x, y is used for horizontal surface and z is for vertical drilling. Drilling data obtained from SCPT work were modelled and analysed by using GIS techniques. All these geotechnical parameters, in addition to shear wave velocity data were used for the classification of the soil and determination of the probable liquefiable area in three-dimension space. This study was divided into two stages. SCPT drilling operation was done in the first stage. SCPT was selected for this research, because this test is a very useful method for evaluating geotechnical parameters of soils. SCPT drilling operations were finished in the first stage and then data were prepared for the GIS analysis during the second stage. Totally 25 SCPT drilling were used and data were collected from 279 different layers. Cone resistance (q c ), sleeve friction (f s ) friction ratio (R f ) values and shear wave velocities were obtained from these layers. Suzuki s (1995) CPT based soil liquefaction was used to determine earthquake hazard map of the interested area. Soil liquefaction model of the area was obtained by 3-dimensional GIS technique. The same technique was applied to shear wave velocity (V s ) data and soil condition of the area was obtained according to Borcherdt s (1994) classification methodology. At the end of the comparison between these two different models it was concluded that soil which has high amplifiable property due to earthquake ground motion was seen till 5 m depth. On the other hand, soil which has high liquefiable property was located between 5 m to 7 m depth. This difference was detected by making the comparison between two models with 3-dimensional GIS techniques and availability of application of 3-dimensional GIS for these kind of studies was concluded. THE STUDIED AREA The studied area, Eskisehir is located in the northwest part of Middle Anatolian region (as illustrated by figure 1). It is situated just between Ankara and Istanbul and the population of the city is nearly half million. It is known as one of the biggest and industrial cities of Turkey. Figure 1: Location map of the studied area. The interested area belongs to Seismic Zone II, according to the General Seismic Zone Map of Turkey, which was prepared by Turkish Earthquake Research Institute. Eskisehir has been effected from the 1999 Marmara Earthquake and faced with deaths, injuries and destruction of buildings. Alluvial materials had been deposited around the Porsuk River, which is the major river of the region. The alluvial deposit consists of gravel, sand, silt and clay materials. The sand and silt layers, which are important according to liquefaction analysis is mainly, located 4 to 7 m depths from the surface. Their thickness is approximately 3-4 metres. The water table is 3 to 5 metres from the surface. All these facts effect the liquefaction potential phenomena of the area negatively. 646
LIQUEFACTION POTENTIAL MAP BASED ON CPT DATA Earthquake hazard analysis involves the quantitative estimation of ground shaking hazards at particular site. Even many hazard phenomena are known for earthquake event, liquefaction was selected as the major phenomena for the studied area. For the evaluation of seismic hazard of particular site or region, all possible sources of seismic activity must be identified and their potential for generating future strong motion must be evaluated (Kramer, 1996). Measurement of soil properties is the critical part of the earthquake hazard analysis. Different fieldworks and laboratory techniques are used to detect these hazard problems. CPT has received an increasing interest in Japan in recent years, and empirical correlations using CPT data have been presented (e.g. Suzuki, 1995). Seismic Conic Penetration Test (SCPT) which has additional seismometer measurement facility was selected in this study for the preparation of liquefaction potential map. The CPT can be performed rapidly and inexpensively. Totally 25 SCPT drilling were used and data were collected from 279 different layers (as illustrated by figure 2). Figure 2: SCPT drilling locations in the studied area. CPT can also provide continuous logs of penetration resistance of the soil and it has the capability to detect even very thin layers that the other drilling techniques can not. Standard CPT cone with a 60 degree apex angle and diameter of 35.7 mm (10 cm 2 cross-sectional area) was used in the field. Cone resistance (q c ), is the total force acting on the cone divided by the projected area of the cone. Sleeve friction (f s ) is the total force acting on the friction sleeve divided by the surface area of the friction sleeve and pore pressure (u) is the main parameter obtained from the CPT operation. Friction ratio (R f ) is defined as the ratio of the sleeve friction to cone resistance and expressed as a percentage is the other important parameter obtained from CPT and used for liquefaction analysis. Many empirical parameters were obtained from the test but these were not used in this study. Suzuki s (1995) soil liquefaction methodology based on CPT data were used in this study. Although it is known that grain size and content of fines can not be obtained directly from the CPT results, it is probable to assume fine contents of soil from friction ratio (R f ). Suzuki and his colleagues have suggested a relation between cone penetration resistance and friction ratio. They proposed a boundary line in this relation that separates liquefied and unliquefied areas by using these CPT parameters. In this liquefaction analysis, R f value 0.5 % corresponds to fines content of soil less than 0.5 % and R f value 1 correspond to fines soil content less than 10 %. If cone penetration resistance parameter is taken into consideration, it is seen that, all the liquefied data have less than 15 MPa. If q c - R f relationship is considered together, all the liquefied data fall within a limited range approximately defined by q c < 15 MPa and R f < 1 % (e.g. Suzuki, 1995). Sand, silty sand, sandy silt and silt grained size soil of the studied area have shown high liquefiable property. Most of the q c and R f values are within the liquefiable region of the q c - R f relationship curve (as illustrated by figure 3). 647
100.00 qt1 (MPa) 10.00 line Soil 1 Soil 2 Soil 3 Soil 4 Soil 5 Soil 6 Soil 7 Soil 8 Soil 9 Soil 10 Soil 11 Poly. (line) 1.00 0.10 1.00 10.00 Rf (% ) Figure 3: q c - R f relationship curve and the plots below the boundary line represent liquefiable points. It is more obvious to show these result with the help of 3-dimensional GIS. The distribution and extension of liquefiable soil can be seen not only in two dimension, but also in three-dimensional space. To make interpretation and analysis by using this three-dimensional model is more available than ordinary type two-dimensional analysis. The volumetric distribution of liquefiable soil can be detected and precautions are rapidly ordered. The extension of liquefiable volumes can be seen from the 3-dimensional liquefaction potential map (as illustrated by figure 4). It was concluded that generally most of the liquefiable zone is located below 5 m depths. Figure 4: Liquefiable soil volumes obtained from the 3-dimensional GIS. 648
SHEAR WAVE VELOCITY MAP Shear wave velocity is one of the most known parameter for predicting the ground motion response to earthquake especially in alluvial deposits. The seismic cone of CPT was used for measuring shear wave velocity data and the vehicle that is used for CPT with seismic facility is called SCPT. Using 5 kg hammer from both side of SCPT separately generated the shear wave source. A polarised shear source was located on the surface. The seismic cone was pushed at intervals of 1 m and shear wave velocities were determined from measured differences in arrival times of the shear wave. The seismic data was recorded and saved into the portable type computer like the other CPT parameters. Then special software was used for making interpretation of shear wave velocity of a certain layer. Borcherdt (1994) has proposed a soil classification methodology, which depends on shear wave velocity (V s ) values. He suggested that, the technique provides a general framework for design, as well as sitedependent building-code provisions and predictive maps for earthquake hazards mitigation (e.g. Street, 1997). Then the method was incorporated into the Federal Emergency Management Agency s (FEMA) earthquake loss estimation program. A soil classification that was obtained from Vs was also used to classify the soils according to their potential to amplify earthquake ground motions. The soil classification depends on the average shear wave velocity of the soil. The FEMA methodology defines site classes A, B, C, D, E 1, E 2 and F based on shear wave velocity, thickness and liquefaction potential (Bauer, Kiefer & Hester, 2001). This classification represents the behaviour of a column of soils with no amplification if the soil is classified as A and B type, to more and more amplification for soils representing C through F type. Then National Earthquake Hazards Reduction Program (NEHRP) recommended using average V s in the upper 30 m to assess the susceptibility to amplification (NEHRP, 1997). The same methodology was applied in this study and the interested area was classified according to shear wave velocity (V s ). The thickness of unconsolidated alluvial deposits is taken larger than 20 m and assumed as a constant thickness all over the studied area. Shear wave velocity 180 m/sec was taken as boundary limit for this study. The shear waves velocity value of the soil less than 180 m/sec was defined as high ground motion amplification property. 3-dimensional model was prepared by using these boundary conditions (as illustrated by figure 5). It was seen that the region, which has a high amplification property (mostly E type soil), is located very close to the surface, approximately less than 5 m depth. It was concluded that, there is a high probability of amplification of these regions from the earthquake ground motion. Figure5: Low shear velocity (Vs) value soil zone obtained from 3-dimensional GIS. 649
COMPARISON OF TWO DIFFERENT MODELS Comparison of CPT based liquefaction potential and shear wave velocity maps of the area was prepared by using 3-dimensional GIS (as illustrated by figure 6). It is determined that, making these kinds of study with 3-dimensional GIS would give the results more accurate and more realistic shape comparing with 2- dimensional. Some overlapped areas were seen on the result map (as illustrated by figure 7). It was concluded that, these overlapped regions have the possibility of indicating two different responds to earthquake ground motion. The soil which has low V s may amplify ground motions significantly generally exists between 0 m to 4 m depths and liquefiable zone exits between 4 m to 7 m depths. Generally existence of high % sand and silt layers below 4 m depth creates this phenomena. It was thought that, these overlapped areas are under the influence of dual hazards due to earthquake ground motion. The buildings and utilities above these areas would be faced with liquefaction hazard and significant amplification of earthquake ground motion in this loose soil would increase the intensity of damage. Figure 6: Comparison of liquefaction potential and low shear wave velocity of the area by 3- dimensional GIS. Figure 7: Overlapped areas are seen from the top view 650
CONCLUSION AND RECOMMENDATIONS If drilling data are used in the study, 3-dimensional GIS has indicated more accurate and more realistic results than 2-dimensional GIS. It is not easy to make interpretation without using third dimension especially for geological studies. This kind of GIS has the advantage to see underground into 3- dimensional and to make interpretation about the extension of structures and lithologies. The 3- dimensional distribution of liquefiable soil was detected and interpreted by using GIS techniques. It was understood that for the studied area, the distribution of this zone exits between 4 m and 7 m depths. The distribution of low shear velocity soil was detected by using this techniques and the general low velocity zone was understood that it is close to the surface, above the liquefiable zone. Comparison of the two different distribution models have indicated the relationship of the models. It was thought that these overlapped areas are under the influence of dual hazards due to earthquake ground motion. The buildings and utilities above these areas would be faced with liquefaction hazard and significant amplification of earthquake ground motion in this loose soil would increase the intensity of damage. Responsible people from municipality of this urban area or from governmental organisations must take care of these kind of areas due to earthquake hazard and some precautions must be put into consideration as soon as possible. BIBLIOGRAPHY Bauer, R.A., Kiefer, J. and Hester, N., Soil Amplification Maps For Estimating Earthquake Ground Motions in the Central US. Engineering Geology 62. 7-17, 2001. Bonham-Carter G.F., 1994 Geographic Information Systems for Geoscientists. Pergamon Press. Elsevier Science Inc., 660 White Plains Road, Tarrytown, New York 10591-5153, U.S.A. ISBN 0 08 041867 8, pp 398. Borcherdt, R.D., Estimates of site-dependent response spectra for design (methodology and justification). Earthquake Spectra 10, 617-653, 1994. Kramer S.L., 1996 Geotechnical Earthquake Engineering. Prentice Hall. Upper Saddle River, New Jersey 07458 U.S.A. ISBN 0133749436, pp 653. Street, R., Woolery, E., Wang, Z. and Harik, I.E., Soil Classifications for Estimating Site-Dependent Response Spectra and Seismic Coefficients for Building Code Provisions in Western Kentucky. Engineering Geology 46. 331-347, 1997. Suzuki, Y., Tokimatsu, K., Koyamada, K., Taya, Y. and Kubota, Y., Field correlation of soil liquefaction based on CPT data. Proceedings CPT 95, SGF Report 3:95 Vol. 2. 583-588, 1995. 651