soil liquefaction assessment of earthquake using the shear wave velocity (a case study in ardabil city)

1. introduction as the pressure of porous water increases in sand soils and saturated loose layers during an earthquake, the soil tendency to reduce its own volume leads to a decrease in the soil’s multifaceted tension. this way, the shear strength of the soil strongly decreases and comes close to zero. this phenomenon is called liquefaction. soil liquefaction is the cause of much destruction originated by the earthquake. having observed extensive liquefactions after the destructive earthquakes of niigata (japan) and alaska (america) in 1964, the geotechnical engineers became interested in this phenomenon. liquefaction zoning studies in iran came into being after the occurrence of a destructive earthquake in manjil (2000) by international institute of earthquake seismology and earthquake engineering. the results of macro-zonation map on the scale of 1:1000000 were outlined and provided (pase, 2009:16).over the past 40 years, significant progress has been made in understanding liquefaction mechanisms and the factors influencing such a phenomenon. in the early years, the experts paid much attention to investigate this phenomenon in clean sands as it was thought that liquefaction was just limited to such sands and the fine and coarse sands and soils can’t produce excessive porous water pressure which is the main cause of liquefaction. however, over time and with new types of earthquakes and observing this phenomenon in coarse and fine-grained soils, many researchers have also sought to clarify the factors affecting the liquefaction of the soil. meanwhile, clayey sands did not receive much attention as researchers believed that the adhesion of clay prevented the occurrence of liquefaction. but given the observations of yasuda tohno regarding tokachi-oki earthquake in 1986, soils containing 90% fine grains and 18% clay turned out to be liquefacient. mura et al. have reported the liquefaction of soils with 48% fine grains and 18% clay in the 1993 hokkaido earthquake. according to research findings, the magnitude of earthquake and its duration, the range of shear stress acting on the soil mass during an earthquake and its relative density, the percentage of the fine grains and soil plasticity are the most important factors in the formation of soil liquefaction. the occurrence of this phenomenon in the past earthquakes has caused lots of damages to a large proportion of the life tracks and the foundation of structures. so to counter its destructive effects, it is essential to identify areas prone to liquefaction. it can be done though zoning of areas where the potential risk is deemed to be high. sand and silty sand are the best candidates for liquefaction to occur, especially when they are composed of very tiny grains. on the other hand, those deposits are prone to liquescence that are located in the still water areas and get saturated in this way. ardebil, as the area under the present study, is located on an alluvial plain and surrounded by a chain of faults. of these, we can note some important faults such as neor, astara, and hir. these faults and their seismic records and also the position of ardabil on loose alluvial formations have made this city susceptible to damages coming from earthquakes and soil liquefaction. in recent years, numerous laboratory and field methods have been offered to evaluate the liquefaction resistance of soils. in this study, the measurement method of shear wave velocity has been used to evaluate the soil liquefaction potential of ardabil. location of the study area: ardebil is located in the northwest of iran and the eastern slopes of sabalan, at an altitude of 1340 meters above sea level between 38ْ 11 to 38ْ 18north latitude of the equator and 14ْ 48 to 20ْ 48 east of the meridian greenwich. the city’s population exceeded 480,000 people in 2011. its population and an area of 6300 acres demonstrate a gross density of 76 persons per acre. this area is located on an alluvial plain and is covered by quaternary sediments. generally, the city and its suburbs (north, west, east) spread on the sediments of plains and young alluvial terraces and fans. 2. materials and methods data collection was performed by consulting library references, documentedmaterials, digital resources, websites related to the topic of research, and organizations as well as institutions. in this regard, the data on the depth of still water that was measured monthly by the regional water authority in the city of ardabil were used. this center requires the data through several wells dug around the city. in addition, the related data to the soil aggregation, soil friction angle, density etc. were acquired through ardebil technical soil mechanics laboratory. in order to estimate the soil liquefaction by utilizing shear wave velocity (vsi), the recommended method by the national institutes of america (nist, 1999) has been used. in 1999, the institute presented several curves by reviewing almost all the laboratory studies, analytical studies, the relationship between the results of spt and shear wave velocity data and field measurement of shear wave velocity at that time, and the results of the relationship between the cyclic resistance ratio (crr) and corrected shear wave velocity by robertson et al. (1992), cayenne et al. (1992), lourdes (1994) and andros et al. (1997). 3. discussion the most important parameters affecting the liquefaction process are the following: particle size distribution: everything else being equal, fine-grained and uniform sands compared to coarse sands has a greater capacity to liquefaction. initial relative density of the sand mass: with an increase in the relative density of materials, both settlement amounts and porous water pressure remarkably decreased while exposed to vibratory loadings. so, the probability of liquefaction phenomenon with increasing relative density and density of sand will decrease. the characteristics and nature of the vibratory loads: alluvium in the sand created under horizontal vibration is much greater than the alluvium created through vertical vibration. also the length of the earthquake or the time that the sand remains in the liquid state has a direct relationship with the amount of damage caused by the construction of a liquefaction phenomenon. the nature and amount of loads: homogeneous tension on the mass of sand in the soil will cause initial effective tension in soil and consequently will reduce the probability of liquefaction. period of the formation mass of sand: the untouched sand compared to reconstructed sand of the same type shows up to 75% increase in resistance to liquefaction (hoti, 2005:192). given the important role that the depth of still water and soil characteristics of an area play in the amount of liquefaction, geotechnical data of 9 boreholes drilled by the laboratory of soil mechanics of the province as well as the data of the regional water organization of ardabil regarding the depth of still water have been used to assess the liquefaction hazard. with regard to the effect of physical and mechanical characteristics and type of soil liquefaction on the amount of liquefaction potential, a closer look at these features will be of increasing importance. because clay soils due to the intrinsic viscosity and sandy soils due to faster depreciating of porous water pressure during earthquake are less capable of liquefaction. loose fine sand and silt soils, because of lack of noticeable adhesion, tend to reduce the volume of ground vibrations caused by earthquakes and lack of proper drainage are prone to liquefaction (asgari, 2006:97). assessment of liquefaction-prone soils (sand and silt) using shear wave velocity compared with spt method is a new method. so, studies done in this field are limited. in this method, instead of using the numbers of spt, the shear wave velocity is used to evaluate the liquefaction resistance. 4. conclusions using vs method and its related principles, ardabil fast-making potential was evaluated in two different ways in terms of cement in soil and soil without cement. the results of these studies were appeared in the forms of liquefaction potential maps. (figures 6, 7). as results of the study for cement condition showed, in most parts of central and south west of the area under study, with mostly coarse textured sandy soils, there is no possibility of fast-making potential. that is why the liquefaction potential index (pl) for these regions is zero. also based on the results of the surveys, areas that are located in the north west and south east of ardabil including parts of the third and fourth regions as well as parts of the first and second municipal regions own the liquefaction potential index within 0 to 5. this indicates the high risk of the liquefaction in these areas. therefore, when an important building is built, a detailed research is needed. based on the findings, the northeast parts of the city, that is, some municipal regions of three and four as well as one and two, respectively, have the liquefaction potential from zero to five. therefore, it is necessary to do research and study before building large houses and apartments. moreover, an effort should be made to decrease the high risk of liquefaction. the result of liquefaction potential study for soils without cementation, as was the case before, indicates that liquefaction is less likely to happen in central and southwest parts of the city due to amortizing the pressure of porous water and having the coarse soil texture. the index of liquefaction potential in the northwest part of city, some parts of third municipal region, is between zero and five. based on the output of vs method, the index of liquefaction potential in the north and southeast parts of the studied regions, located in municipal regions of one and four, is more than fifteen. so, it is necessary, on the one hand, to do research and study hard in this case and decrease the high risk of liquefaction, on the other. the findings show that the index of liquefaction potential in the northeast of studied region is between five and fifteen, which demonstrates a high risk of liquefaction formation. due to the limited experience of vs method to determine the liquefaction soil, it is suggested to consider liquefaction potential as an important case to be studied and know how to use the method. its importance comes from the point that the determination of the speed of cutting waves of soil compared to spt is inexpensive and its numerical result is smaller than numerical result of spt and regarding digging and remedy, it has also different conditions.