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Seismic Site Effects of Soil Amplifications in Bangkok

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Seismic Site Effects of Soil Amplifications in Bangkok Research Article Seismic Site Effects of Soil Amplifications in Bangkok Nakhorn Poovarodom* and Amorntep Jirasakjamroonsri Department of Civil Engineering, Faculty of Engineering, Thammasat University Rangsit Campus, Khlong Nueng, Khlong Luang, Pathum Thani 12120, Thailand Abstract In this paper, seismic site effects resulting from amplification of soft soil in Bangkok are presented. Site characteristics were surveyed using the microtremor observation technique. The method of Centerless Circular Array (CCA) was employed to examine the shear wave velocity profile of subsoil from surface to bedrock level of 20 sites. The average shear wave velocity of each site is not significantly different; however, the depth of bedrock varies from 400 to 800 m. The area was subdivided into two zones according to the depth of bedrock. The results of shear wave velocity were then used to construct a model for ground response analysis for prediction of earthquake motion at the surface. The design response spectral accelerations were presented for the two zones. The results show that higher ground accelerations could occur in the area having shallow depth of bedrock, less than 700 m. The design response spectral accelerations are generally high in the periods range from 0.5 to 4 second. Keywords: Site effects; shear wave velocity; microtremor; Bangkok 1. Introduction The first part of the research was an Local soil conditions, especially soft investigation of shear wave velocity (Vs) of soil deposits, always contribute great subsoil using the microtremor observation influence in amplification of seismic waves. technique. Vs profiles of 20 several hundred- To examine these effects, soil characterization meter depth sites were surveyed. Then, the Vs and ground response analysis of the area are profiles were used to construct soil models for required. Understanding of the effects could analysis of seismic ground response of the enable engineers to establish appropriate area. The design spectrum accelerations of measures for earthquake resistant design of Bangkok were proposed for earthquake structures. resistant design of buildings. Problems of soil amplification of The paper begins with an explanation earthquakes in Bangkok have been of the study area and the microtremor occasionally observed. The major sources of technique focusing on the Centerless Circular earthquakes are from active faults in the Array method (CCA). The ground response northern and western part of Thailand, and the analysis using the equivalent linear method subduction zone in the Andaman Sea. Soils was summarized. The results were presented beneath Bangkok are known as soft deposits as Vs profiles and depth of bedrock. Finally, but the effects of seismic amplification have the obtained ground responses were used to not been completely investigated. establish design spectral accelerations for In this study, seismic site effects of Bangkok. soil amplifications in Bangkok were studied. *Correspondence : [email protected] DOI 10.14456/tijsat.2016.22 Thammasat International Journal of Science and Technology Vol.21, No.3, July-September 2016 2. Area of Study 3. Microtremor Observation Bangkok is situated on the central part Techniques of a large lower plain known as the Chao Microtremors represent constant Phraya basin. The plain consists of deep vibration of the Earth’ s surface at seismic Quaternary deposits. Generally, subsoil is frequencies, which are present at anytime. relatively uniform inside the basin. Soil The amplitudes are extremely small, as underlying this area can be described as displacements are in the order of 10-4 to 10-2 alternating layers of clay and sand. The first mm [2]. Recently, techniques of microtremor layer is the uppermost layer of weathered survey have been commonly applied to extract crust of one to five meters. The second layer information for seismic study. In the first part is soft clay with very low shear strength, of the study, key parameters for site known as soft Bangkok clay. The thickness characterization and ground response of this layer is about 15 to 20 meters in the analysis, such as shear wave velocity and central area. The soft clay is underlain by depth of bedrock at a site, were determined by alternate layers of stiff clay and sand. The microtremor survey. Field measurements depth of bedrock is estimated to be 500 meters were conducted to provide site characteristics or deeper, but there is no sufficient data useful for examination of ground motion available at present. Recent research on amplifications. Two techniques of seismic site characteristics by small array determining phase velocities from microtremor observations showed that the microtremors, the Spatial Autocorrelation average shear wave velocity from the surface method (SPAC) and the Centerless Circular to a depth of 30 m (Vs30) was very low over Array Method (CCA), were firstly examined the basin [1]. for further application. The observed phase The area of investigation is located velocities were subsequently inversed to shear within latitudes 13° 30’ N to 13° 57’ N and wave velocity profile of subsoils. longitudes 100° 22’ E to 100° 51’ E. Figure 1 3.1 Spatial Autocorrelation shows the location of the investigated area in method (SPAC) which 20 observation sites are distributed. The basis of this technique, proposed by Aki in 1957 [3], is to simultaneously record the vertical component of microtremors for several positions to measure Rayleigh wave samples propagating from different azimuthal angles. The coherency spectrum is computed for any pair of sensors in the array to evaluate the correlation between them, and to determine dispersive phase velocity characteristics. The coherencies for all measurement pairs having the same spatial distance are then azimuthally averaged to provide the spatial autocorrelation ( SPAC) coefficients of interstation distance r,,r. By assuming that the wave energy Figure 1. Map of observation stations. propagates with only one velocity at each frequency, , it can be shown that the spatial autocorrelation coefficient for a circular array is given by equation (1) [3]. 60 Vol.21, No.3, July-September 2016 Thammasat International Journal of Science and Technology Assuming that the fundamental Rayleigh ReECA,B ,r wave mode dominates the vertical component ECA,A ECB,B of the microtremor field, the ratio of their r power spectra densities, G0 r,r; and J0 kr J0 (1) c G1r,r;, can be written as 2 Where E denotes an ensemble G0 r,r; J 0 rk 2 (4) average over time, CA,B is cross spectra G1r,r; J1 rk of vertical records at two stations, A and B, J 0 is the Bessel function of the first kind with Where J1 is the Bessel function of the the zero-th order, and k and c are the first kind with the first order. The wavenumber and phase velocity are then wavenumber and phase velocity, respectively, estimated by fitting the observed spectral ratio for the Rayleigh waves with the fundamental with a theoretical ratio of mode. The phase velocity can be determined 2 2 by fitting the observed SPAC coefficients at J 0 rk J1 rk. various 푟 for each frequency with 3.3 Inversion of shear wave velocity J 0 r c. profile 3.2 Centerless Circular Array From the dispersion relation of phase method (CCA) velocity and frequency, the results from field observations were compared with those This technique was proposed and derived theoretically from a horizontally developed by Cho et. al. [4] based on spectral layered earth model by iteration procedure. ratio representations which can be considered The results of best-fit shear wave velocity– as a general case of the SPAC method. The depth profile were determined from the spectral ratio which contains information of inversion analysis. phase velocities is an integration of all information on the field of the vertical 3.4 Instrument and field component of microtremors. Field work observation requires deployment of a circular array of The instrument used for microtremor radius r and recording the vertical component observation consists of velocity sensors of the microtremor zt,r, . Define the having frequency range of 0. 1 to 70 Hz average value Z t,r along the connected to the receivers with 24 bit A/ D 0 convertors. There were 4 sets of equipment, circumference and its weighted average each set measured simultaneously by time Z1t,r as synchronization using a GPS clock with a resolution of 1/ 100 second. The array configuration was an equilateral triangular Z t,r z t,r, d array network. Three sensors were placed at 0 (2) the corners of a triangle (or a circle), and one at the center. Figure 2 shows arrangement of Z t,r z t,r, exp(i )d sensors. The radius sizes of the arrays were 5, 1 (3) 30, 100 and 250 meters, respectively. The duration for collecting vibration data was about 40 minutes for each array. 61 Thammasat International Journal of Science and Technology Vol.21, No.3, July-September 2016 Table 1. Dynamic soil properties for equivalent linear analysis. Material Dynamic Soil 120° No. Depth (m) Type Properties 1 0 - 5.5 Clay Ref. [6] 2 5.5 - 10.3 Clay Ref. [6] 3 10.3 -13.5 Clay Ref. [6] 4 13.5 - 20 Clay Ref. [7] 5 20 - 35 Sand Ref. [8] 6 35 - 50 Clay Ref. [7] Figure 2. Array configuration. 7 50 - 75 Sand Ref. [8] 8 75 - 85 Clay Ref. [7] 4. Ground Response Analysis 9 85 - 100 Sand Ref. [8] In order to evaluate seismic site 100 - 10 Sand Ref. [8] effects, ground responses at the site are Bedrock analyzed by one-dimension analysis using the 11 Rock Rock Ref. [9] SHAKE computer program [ 5] . This technique applies an equivalent linear analysis which takes into the account the soil non- linearity by the use of strain compatible shear modulus and damping ratio in a sequence of linear analyses through an iterative process.
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