Open Access proceedings Journal of Physics: Conference series Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 171 Comparison of SPT and Vs-based liquefaction assessment on young volcanic sediment: a case study in Bantul District of Yogyakarta, Indonesia. Anisa Nur Amalina 1,4, Teuku Faisal Fathani 2,4, Wahyu Wilopo 3,4 1 Department of Civil Engineering, Faculty of Civil Engineering and Planning, Islamic University of Indonesia, Indonesia 2 Department of Civil and Environmental Engineering, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta, Indonesia 3 Department of Geological Engineering, Faculty of Engineering, Universitas Gadjah Mada, Yogyakarta, Indonesia 4 Center for Disaster Mitigation and Technological Innovation (GAMA-InaTEK) Universitas Gadjah Mada, Yogyakarta, Indonesia corresponding author's e-mail: amalina.anisa@gmail.com Received 01-07-2022; accepted 27-09-2022 Abstract. On May 26, 2006, an earthquake of moment magnitude (Mw) 6.3 occurred in Yogyakarta. The damages found in Bantul were predicted to be caused by liquefaction. Moreover, liquefaction symptoms were found, such as a sand boil and lateral spreading. It inferred that the damage was controlled by the amplification factors from young redeposited volcanic sediments and altered volcaniclastics from the active Mount Merapi. This study compared subsurface conditions based on two field investigation methods (SPT and Shear Wave Velocity) and determined the liquefaction potential by considering groundwater and the region's seismicity. To obtain the most fitted equation, several equations to represent the N-SPT and Vs data were also analyzed. As a result, several equations used in this study were inadequate to correlate N-SPT and Vs properly. A comparison of safety factor values indicated that the liquefaction potential in the studied area on the Vs-based method is lower than the result from the SPT-based method. Keywords: shear wave velocity, downhole test, N-SPT value, liquefaction potential. 1. Introduction A strike-slip earthquake happened on May 26, 2006, in Yogyakarta. Approximately 5,700 people were killed, and over 156,000 houses and other structures were destroyed. The magnitude was 6.3, and its duration was about 60 seconds, with the hypocenter at the east of the Opak River [1, 2]. Northeast of the Parangtritis, the Bantul and Klaten are the most affected area [3]. Meanwhile, heavy losses were founded near the Opak fault was due to the amplification factors from soft sediments redeposited from the active Mount Merapi [4, 1]. mailto:amalina.anisa@gmail.com Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 172 Besides damaging hundreds of houses, the earthquake destroyed university and school buildings, offices, infrastructures, and the runway at Adi Sutjipto International Airport. Additionally, liquefaction symptoms were detected, such as sand boils and lateral spreading. The area with the highest potential for liquefaction is Patalan, Bantul, part of the Bantul basin, or the Opak River Fault basin [5]. Evaluating soil liquefaction is crucial to minimize future damage, especially in earthquake-prone regions. The method mainly used is the Simplified Procedure [6], originally developed from the standard penetration test (SPT) and correlated with a cyclic stress ratio parameter representing the cyclic soil loading. Meanwhile, the most common approach is in-situ Vs measurements [7]. Vs is a field measurement with less than 10−4% strain [[8], [9]]. The Vs-based liquefaction analysis has obtained considerable relevance compared to SPT-based analysis. Furthermore, Vs and liquefaction resistance are sensitive to relative density, effective stress, and cementation in the same direction [10]. This study aims to compare subsurface conditions based on two field investigation methods (SPT and Shear Wave Velocity) and determine the liquefaction potential by considering groundwater and the region's seismicity. Furthermore, a comprehensive analysis of liquefaction potential on young volcanic sediment was conducted by comparing the N-SPT and Vs values. 2. Methodology The initial stage of this study involves seismic and geotechnical data compilation from the previous research, field test, and desk study. Next, the collected data were analyzed to determine the site classification, soil stratigraphy, and soil parameters. Groundwater and the region's seismicity were considered to calculate the potential of liquefaction. The liquefaction analyses were conducted using the SPT method [14] and Vs measurement [6, 16, 17]. Furthermore, a comprehensive analysis of N-SPT relationships with Vs on young volcanic sediment was explained further. 2.1. Geological Conditions The study was conducted in the Bantul Region of Yogyakarta, Indonesia. Bantul is considered earthquake-prone due to its proximity to the Eurasian Plate's subduction and the Australian plate). Furthermore, based on Rahardjo et al. [11], the Bantul region consists of quaternary young Merapi volcano deposits (Qmi) that have a high potential to liquefy (Figure 1) Deposits in the quaternary period are divided into Holocene and Pleistocene, while deposits older than the Pleistocene are included in the tertiary period. The tertiary period comprised the Kulon Progo mountains and the southern mountains. Meanwhile, most of the quaternary deposits compose Yogyakarta and Bantul. The lithology of the young Merapi Volcano deposits can be classified based on grains size distribution, namely 1) Sand sediment, the most dominant sediment, consists of sand, silt sand, and gravel sand, 2) silt deposits, and 3) clay sediment consists of sandy clay and clay [12]. The microtremor survey was conducted in several severely damaged locations by the 2006 earthquake [13]. The result shows that the depth of bedrock in Bantul area is approximately 30–60 meters. Meanwhile, the deepest bedrock, around 60–100 meters, lay in the east Bantul. In Jetis, Imogiri, and Pundong, a breccia layer reaches 50 meters in thickness. 2.2. Site Classification Besides soil stratigraphy, site classification was also conducted. The classification was based on the average value of N-SPT and Vs until a depth of 30 m. The site classification can be seen in Table 1. Site classification is commonly used to define the Peak Ground Acceleration (PGA) value by determining the seismic zones. Meanwhile, this study applied the PGA value referred to Fathani et al. [17], where the research location was also conducted in Bantul. They calculated the PGA value using an attenuation relationship considering two Scenarios of epicenter coordinate and hypocenter depth based on the Indonesia Meteorological, Climatological, and Geophysical Agency (BMKG) and the United States Geological Survey (USGS). The results are summarized in Table 2. Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 173 Figure 1. The location and geological condition of the study area (modified from [11]). Table 1. Comparison of PGA based on two scenarios [17]. Site class sV (m/s) 30N SE (soft soil) <175 <15 SD (medium soil) 175 to 350 15 to 50 SC (hard/very dense soil and soft rock) 350 to 750 >50 SB (rock) 750 to 1500 N/A SA (hard rock) >1500 N/A SF (special soil) Required specific geotechnical investigation and site response analysis on every site Table 2. Comparison of PGA based on two scenarios [17]. Sample Location PGA (g) BMKG USGS BH-01 BPKP-1 0.24 0.25 BH-02 BPKP-2 0.24 0.25 BH-03 Segoroyoso 0.25 0.30 BH-04 Karangsemut 0.26 0.30 BH-05 Wijirejo 0.28 0.24 BH-06 Bambanglipuro 0.32 0.26 BH-07 Pranti 0.30 0.30 BH-08 Tempuran Kali Opak-Oyo 0.30 0.30 BH-09 Watu 0.32 0.27 2.3. N-SPT and Vs Empirical Correlation for Young Sediment Volcanic The data applied in this study are collected from an extensive geotechnical borehole, downhole and laboratory tests. The data consist of 29 boreholes and nine shear wave velocity data. The data depths vary from 20 m to 50 m (Figure 2). Nine borehole and downhole data were used to calculate the liquefaction potential by comparing those data. Meanwhile, the other available data were used to generate soil stratigraphy. Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 174 Figure 2. The location and geological condition of the study area (modified from [11]). Several equations in Table 3 correlate the N-SPT value with shear wave velocity (Vs) in various types of soils. The selected equation was then used to define Vs value in young sediment volcanic. Table 3. Comparison of PGA based on two scenarios [17]. Author Equation Seed and Idriss [6]: ( ) 0.5 s 61.4V N= (1) Hasancebi and Ulusay [19]: ( ) 0.39 s 90V N= (2) Imai and Yoshimura [20]: ( ) 0.33 s 76V N= (3) Kanai [21]: ( ) 0.6 s 19V N= (4) Akin et al. [22]: ( ) 0.101 0.216 s 121.75 ( )V N z − = (5) Alluvial sands [23]: ( ) 0.292 s 87.8V N= (6) Alluvial soils (Korea) [23]: ( ) 0.319 s 82V N= (7) 2.4. Liquefaction Safety Factor (FS) Parameters that need to be reviewed regarding liquefaction are the earthquake loading and soil strength against earthquake loading. The safety factor is calculated by comparing the cyclic stress ratio (CSR) and the cyclic resistance ratio (CRR). Liquefaction might happen if the CRR is less than CRR. The safety factor of the liquefaction is 1.2 [24]. Referring to Pawirodikromo et al. [25], the de-aggregation results found that the dominant magnitude and the distance were influenced mainly by the shallow crustal instead of the Megathrust earthquak e source. The MD= 6.5 and the RD= 14.5 km. The Opak river fault is located approximately 10 km from Yogyakarta, while the megathrust earthquakes, with a larger magnitude, are located more than 300 km from Yogyakarta. Thus, the moment magnitude of 6.5 is used to calculate MSF (Eq. (8)). MSF 6.9 exp 0.058 4 wM      − = − (8) Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 175 2.4.1. SPT-based Liquefaction Safety Factor (FSL) The safety factor is calculated by the cyclic stress ratio (CSR) and the cyclic resistance ratio (CRR 7.5), as shown in Eq. (9). The CSR value is adjusted to a specific earthquake magnitude (Mw=6.5) by a magnitude scaling factor (MSF). 7.5 L CRR FS MSF CSR = (9) 2.4.2. Vs-based Liquefaction Safety Factors (FSVs) FSVs is calculated using equation given by [6], [15], and [16]. The equation is generally considering both SPT and Vs data. s s s CRR SRR FS CSR SSR V V V = = (10) 2.5. Cyclic Stress Ratio (CSR) The CSR due to earthquake force is usually explained as 0.65 multiplied by the peak value of cyclic shear stress at a particular depth (z). Several parameters, such as surface acceleration and total and effective stresses at different depths, are considered in determining the CSR. 2.5.1. SPT-based Liquefaction Triggering Analysis (CSR) The liquefaction triggering analysis proposed by Idriss and Boulanger [14] is based on trial and error (N1)60cs. The soil is unlikely to liquefy if the clean granular soils or (N1)60cs value is larger than 30 blows/ft. Seed and Idriss [6] calculated the induced stress ratio CSR as shown in Eqs. (11) to (14). σav is the 65% of the peak induced cyclic shear stress triggered by an earthquake, PGA or amax is the peak ground acceleration at the site, g is the acceleration of gravity, rd is a depth factor, σv is the initial total vertical stress, and σ’v0 is the initial vertical effective stress in the ground. max ' ' CSR 0.65 vav d v v a r g       = =       (11) ( ) 1.012 1.126sin 5.133 11.73 zz               = − − + (12) ( ) w 0.106 0.118sin 5.1242 11.28 zz M               = + + (13) ( )wd exp ( ) ( )r z z M = + (14) 2.5.2. Vs-based Liquefaction Triggering Analysis (SSR) The CSR parameter is changed into a shear stress ratio (SSR) in the Vs-based method. However, they have similar physical meanings. The shear stress ratio depends on the soil medium, unit weight, acceleration, and earthquake period [16]. Eqs. (15) to (17) show several parameters. First, t is a predominant period of the earthquake wave. For example, the dominant vibration period suggested for M6.5 is 0.280s [15]. max s ' s SSR V d V a r g       =       (15) ( )s 10.25 n V i s ii T V  = =  (16) ( )s s sa ds i n10.25 ( ) n V i i T V V    =  = − − (17) Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 176 The amax refers to the maximum horizontal ground acceleration (m/s 2), g is the gravitational acceleration (m/s2), σVs is the dynamic vertical stress (kN/m 2), σ′Vs is the effective dynamic vertical stress at the same depths calculated by the same parameters (kN/m2), and rd is the stress reduction coefficient mentioned in Eqs. (12) to (14). 2.6. Cyclic Resistance Ratio (CRR) Soil resistance or CRR is soil's capacity at a particular depth and state to resist liquefaction triggering liquefaction resistance is generally characterized by penetration resistance modified to account for various additional variables that can affect liquefaction resistance. 2.6.1. SPT-based Liquefaction Resistance Analysis (CRR) The CSR parameter is changed into a shear stress ratio (SSR) in the Vs-based method. However, they have similar physical meanings. The shear stress ratio depends on the soil medium, unit weight, acceleration, and earthquake period [16]. The liquefaction safety factor can be calculated with widely used methods such as N-SPT data and corrected with five correction factors as given by [14]. The value of clean sand, (N1)60cs, is then obtained by adjusting the FC (fines content) to the corrected blow count. The empirical procedures to obtain the corrected SPT values based on Idriss and Boulanger [14] are shown in Eqs. (18) to (21). Meanwhile, the SPT-based CRR relationships are presented in Eqs. (22) to (25). ( ) 601 60 N E B R SN N C C C C C= (18) ( )0.784 0.0768 1 60 N ' C 1.7 N a v P  − =        (19) ( ) 2 1 60 9.7 15.7exp 1.63 FC 0.01 FC 0.01 N                = + − + + (20) ( ) ( ) ( )1 1 160 60 60csN N N= +  (21) ( ) ( ) ( ) ( ) 2 3 4 1 60 1 60 1 601 60 ' 1 CRR exp 2.8 14.1 126 23.6 25.4 cs cs cscs atm N N NN                      =                = + − + −       (22) ' ' 1 CRR CRR atm K   = = (23) ' 1 ln min 1.0 vo a C K p    − =              (24) 1 60 1 18.9 2.55 ( ) cs C N  = − (25) 2.6.2. Vs-based Liquefaction Triggering Analysis (SSR) The shear wave velocity was formulated from more than 50 sites measurement as shear resistance ratio (SRR) and determined by corrected Vs and maximum Vs (Vs, max) value, as shown in Eq. (26) [26]. For values of corrected shear waves in the range of 190 to 220 m/s, the curve turns upward sharply where minor changes in Vs1 correspond to significant changes in CRR. The correlation between CRR and Vs of uncemented Holocene-age soils shows in Figure 3 [7]. Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 177 S c S S S max c max 2 1 1 SRR MSF 100 V a b V V V       = + −     −      (26) Figure 3. Correlation between CRR/CSR and Vs [7]. Uyanık and Taktak [15] determined that Vs-max ranges from 220 to 250 m/s based on the fines content. Meanwhile, the a and b values are 0.022 and 2.8. Several researchers [27] suggested the corrected Vs formula as shown in Eqs. (27) to (28). The reference stress or atmospheric pressure (Pa) is 100 kN/m 2. max max max 250 m/s, FC 5% 250 (FC 5) m/s, 5% FC 35% 220 m/s, FC 35% S S S V V V =  = − −   =  (27) 0.25 S S 'c a v P V V  =       (28) 3. Result and Discussion 3.1. Soil Classification Soil classification was conducted by calculating the average value of N-SPT and Vs. Data less than 30 m were approached by the nearest borehole N-SPT values. Table 4 shows a summary of site classification according to [18]. The results show that all soils are considered medium soils. In contrast, several locations (BH-03, BH-04, BH-08, and BH-09) are considered soft soil from the Vs-based calculation. Consequently, this difference will affect the results of the FS calculation. In addition, it might occur due to the uncertainties in downhole field performance. Table 4. Site classification based on SPT and Vs. Sample Location Depth (m) Soil classification SPT Vs BH-01 BPKP-1 30 SD SD BH-02 BPKP-2 20 SD SD BH-03 Segoroyoso 46 SD SE BH-04 Karang-semut 20 SD SE BH-05 Wijirejo 46 SD SD BH-06 Bambang-lipuro 50 SD SD BH-07 Pranti 40 SD SD BH-08 Kali Opak-Oyo 30 SD SE Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 178 BH-09 Watu 34 SD SE Notes: SD = medium soil; SE = soft soil 3.2. Borehole Stratigraphy A total of 23 data were analyzed to interpret the soil stratigraphy. The bedrock depth was estimated from the previous research by Perdhana and Nurcahya [13]. The soil layers are divided into fine sand, medium to coarse sand, silt to clay, breccia, medium to fine sandstone, and bedrock. The A-A' cross- section is made as long sections from north to south while the B-B' is cross-sections from west to east (Figure 2). The borehole data show that fine sand, classified as the lithology of the Young Merapi Volcano Deposits, dominates the upper layer up to 20 m depth. Beneath the 20 m, the soil layer is composed of fine to medium sandstone layers. Figure 4 and Figure 5 present an interpretation of the soil layer. This interpretation is coherent with the research of Buana and Agung [12], where fine sand dominates the area around the east of Bantul. In addition, in the Watu area, Imogiri and Karangsemut consist of a breccia layer. Figure 4. Soil stratigraphy of cross-section A-A′. Figure 5. Soil stratigraphy of cross-section B-B′. 3.3. N-SPT and Vs correlation Table 5 shows that the given equations cannot adequately represent the N-SPT and Vs correlation. Generally, the equation by Akin et al. [22] gives the most insignificant error compared to the other equations. In addition, the error value of BH-04 tends to be small by applying the equation intended for alluvial sediments. Table 5. Summary of relative error for each equation. Location Relative Error, Er (%) Eq. 1 Eq. 2 Eq. 3 Eq. 4 Eq. 5 Eq. 6 Eq. 7 BH-01 120 75 58 58 23 62 65 BH-02 166 108 88 56 21 92 96 BH-03 148 88 71 58 30 73 78 BH-04 78 36 37 47 47 34 34 BH-05 232 149 108 161 34 107 115 BH-06 277 182 156 66 67 159 166 Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 179 Location Relative Error, Er (%) Eq. 1 Eq. 2 Eq. 3 Eq. 4 Eq. 5 Eq. 6 Eq. 7 BH-07 209 188 136 217 50 145 147 BH-08 134 96 62 111 35 65 68 BH-09 175 115 79 127 27 79 85 The previously published research mainly used statistical relation to represent Vs and N60 without considering confining stress. As a result, the graphs (Figure 6) show significant errors in the equations that neglect confining stress (z). Meanwhile, the other equations tend to be overestimated compared to the field test. Hence, the effects of confining stress should be considered to minimize bias and reduce uncertainty. Figure 6. N-SPT and Vs correlation based on given equations. 3.4. Liquefaction analyses The liquefaction analysis was carried out based on two methods (N-SPT and Vs-based) by considering the largest acceleration value taken from Fathani et al. [17]. Figure 7 presents the analysis result from those methods. Vs-based results tend to be much lower than the SPT-based method. The site classification has identified this condition, where some boreholes are classified as soft soil instead of medium soil. The formula given by Idris et al. [14] is susceptible to (N30)cs and FC values, where a value greater than 30 might result in an FS value greater than 2. In contrast, the Vs-based results cannot identify these conditions. Therefore, it aligns with Ghazi et al. [27], where the Vs values decrease mainly caused by the void ratio, while the grain size distribution and relative density do not affect them. Civil and Environmental Science Journal Vol. 05, No. 02, pp. 171-182, 2022 180 Figure 7. FS comparison based on two methods. 4. Conclusions The present study intended to compare the liquefaction potential from SPT and Vs tests. Those methods provide slightly different results. Indonesian code of SNI 8460:2017 [18] was used to determine the soil classification. Several locations showed different results, such as on BH-03, BH-04, BH-08, and BH-09. Based on SPT data, the soil is classified as medium sand, while in Vs -based, it is classified as soft soil. Several equations in this study are inadequate to deliver a good correlation between N-SPT and Vs. The error value varies between 30 - 200%. However, the equation by Akin et al. [22] gave the smallest error number. Therefore, additional borehole and downhole tests must be carried out in the study area to determine the most compatible equation for the Young Volcanic Sediment. The comparison of the safety factor values indicated that the liquefaction potential in the studied area on the Vs-based method is lower than the result from the SPT-based method. 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