Liquefaction potential hazard assessment and its effect on toll road construction in Seyegan Subdistrict, Yogyakarta

Toll road construction is a vital infrastructure connecting the two economic centers on Java Island. However, given the potential disaster risks in the region, it is essential to assess the geological and geotechnical conditions to determine the potential for liquefaction in the area. The special region of Yogyakarta is susceptible to earthquakes and associated disasters. One of the most significant earthquakes occurred on May 27, 2006 with a magnitude of 6.3 Mw resulting from the Opak Fault. Furthermore, the toll road is constructed above the Mataram Irrigation Canal, characterized by a shallow groundwater level and young volcanic deposits that are vulnerable to liquified. The study aimed to determine the potential for liquefaction in the area and predicted the vertical settlement caused by liquified soil. The analysis of liquefaction potential utilized the simplified procedure to obtain factor of safety and Liquefaction Severity Index (LSI) to assess the vulnerability index of liquefaction. Results indicated the research area had a very low to medium liquefaction susceptibility index. In addition, the calculation of vertical settlement using Idriss and Boulanger (2008) equation showed a predicted vertical settlement range of 0.4 cm – 44.64 cm. The predicted vertical settlement range underscores the importance of considering the potential for liquefaction to mitigate any adverse effects on the toll road’s construction and operation.


Introduction
The Trans-Java Toll Road in Indonesia, which extends from Port of Merak in Banten Province to Banyuwangi in East Java Province, is part of the National Strategic Project that connects major cities along the northern region of Java Island [1].It has significantly contributed to enhancing economic growth, expanding employment opportunities, and accelerating socioeconomic progress [2].According to the Indonesia Highway Regulatory Agency (BPJT), the total length of the operated toll road until June 2023 was 1.056,38 km.Nevertheless, the current toll road needs to provide connectivity between the major cities in the northern and southern regions of Java Island.Therefore, constructing the toll road connecting two major cities, Semarang and Yogyakarta, holds significant importance, particularly in strengthening the tourism sector.
IOP Publishing doi:10.1088/1755-1315/1314/1/012120 2 One crucial aspect that needs to be considered when constructing a toll road is the potential impact of natural disasters on the road structure.The research was conducted in the southern area of Java Island, which has a historical record of being prone to earthquakes, with the strongest occurring on May 27th, 2006, registering a magnitude of 6.3 Mw sourced from an active strike-slip fault, Opak Fault.The earthquake resulted in the loss of over 5,500 lives, causing significant damage to buildings and infrastructure.In that seismic event, liquefaction phenomena were observed in the vicinity of the Opak Fault region, especially in Patalan, Bantul [3].
According to Day. [4], several factors contribute to the liquefaction phenomenon, including ground motion characteristics, groundwater level, and soil type.The liquefaction phenomenon occurs during earthquakes on loose, saturated sand, causing a contraction in the material, resulting in the transfer of stress from the solid matrix to the pore water.This transfer of stress reduces the effective stress, strength, and stiffness of the sand, ultimately leading to soil deformation.Here are several consequences of liquefaction: reduction in shear strength that leads to instability of slope or embankments, lateral spreading of gently sloping ground, and settlement caused by reconsolidation of the liquefied soils [5].Hence, it is essential to assess the potential of liquefaction and its consequences on toll road construction.
Numerous studies on determining potential liquefaction using empirical methods in Yogyakarta Province have been presented by many researchers [6][7] [8].Previous studies have indicated that in the area of Yogyakarta province, peak ground acceleration of about 0.3 to 0.4 g can trigger liquefaction.
This study provides an empirical analysis of soil liquefaction in the Seyegan subdistrict focusing on geological, hydrological, and geotechnical conditions.The simplified procedure proposed by Boulanger and Idriss (2014) is utilized to calculate the potential for liquefaction triggering using Standard Penetration Test (SPT), while the Liquefaction Severity Index method developed by Sonmez and Geockglu (2005) is employed in order to generate susceptibility maps.Additionally, the settlement postliquefaction is evaluated using the equation Idriss and Boulanger (2008) proposed to examine the vertical displacement reconsolidation of the liquefaction phenomenon.

Geological Condition of Study Area
The research area is situated in Seyegan Subdistrict, Sleman Regency of the Yogyakarta Special Region, which is close to the boundary of Central Java Province.A toll road spanning a length of 4.2 kilometers, mainly located over the Mataram Irrigation Canal.Notably, the area is characterized by a relatively low groundwater level.Nevertheless, a recent analysis has indicated that the Canal is experiencing sedimentation, which has the potential to cause fluctuations in the groundwater level of the surrounding area [9].
The geological structure that influences the area about 18 km east-south east (ESE) is the Opak Fault.This active fault has the potential to generate seismic events with a maximum magnitude of 6.6 Mw [10].A seismic event that occurred on May 27 th , 2006, it was observed that Sleman Regency experienced the second most significant number of casualties among the district within Yogyakarta Province [11].
Based on the geological map in Figure 1, the study area includes the Quaternary Merapi deposits (Qmi) consisting of undifferentiated tuff, ash, breccia, agglomerate, and lava flow.To specify the type of soil in the study area as shown in Figure 2, a comprehensive geological survey was conducted along the constructed toll.The upper strata comprise tuffaceous sandstone, with a maximum thickness of 20 meters; the middle and lowest unit consist of breccia, reaching a maximum thickness of 40 meters.In locations near Mataram Irrigation Canal and River, the upper unit comprises a mixture of clay or silt soil, with thickness varying from 2 to 6 meters.This type of soil, rock formation, structural geology condition, and groundwater level are prone to liquefied [4][5] [12].

Data Collection
The data employed in this study was acquired from soil investigation: Standard Penetration Test (SPT), boring, laboratory, and groundwater level data obtained by PT Jasamarga Jogja -Bawen.Additionally, the geophysical investigation was conducted through the acquisition of microtremors.A total of 137 boreholes and 13 data microtremors were carried out along the construction of the toll road, as seen in Figure 3.
Standard Penetration Test (SPT) data and boring data collected until a depth of 30-40 meters, with N-SPT values recorded at intervals of 2 meters.Based on the microtremors survey, the study area is predominantly within site class D with a value of shear wave velocity (Vs) until the depth of 30 meters ranging from 195 m/s to 311 m/s [14].

Seismic Hazard Analysis
According to the Deagregation Map [10], the study area's magnitude varies from 6.2 Mw to 6.4 Mw.The history of earthquake events in Yogyakarta for the past 30 years showed that the 2006 Yogyakarta earthquake with a magnitude of 6.3 Mw became the region's largest shallow crustal earthquake, proven to induce liquefaction and the closest distance to the study area.Therefore, it becomes the scenario ground motion.To estimate Peak Ground Acceleration (PGA) value, the attenuation formulated by Boore -Atkinson NGA 2014, Campbel -Bozorgnia NGA 2014, and Chi -Youngs NGA 2014 are utilized.However, the result of seismic hazard analysis indicates the presence of uncertainty in each function attenuation.A weighting logic scheme is applied to accommodate the uncertainty in the analysis [10].
Based on SNI 2833:2016, the value of PGA and amplification factor (  ) are required to calculate the value PGA in the ground surface (As) as shown in equation (1).

Analysis of Microtremor Data
Microtremor data plays a pivotal role in discerning site classification.An effective technique for appraising ambient excitation site attributes is the Horizontal to Vertical Ratio (HVSR) method.This study uses the Geopsy software to procure the H/V curve.After this, the Geopsy Dinver program is harnessed to execute an inversion process on the H/V curve, yielding a value of shear wave velocity and bedrock depth.
The derived shear wave velocity (  ) outcomes in m/s can be harnessed to compute the site classification using an equation by incorporating parameters related to the soil layer's thickness (  ) up to a depth of 30 meters [14]. (2)

Liquefaction Susceptibility Criteria
Identification of soil type based on field survey and laboratory data indicates that the area is composed of volcanic rocks with various grain-size distributions, from clayey silt to breccia.As stated previously, loose saturated sand indicates an area susceptible to liquefaction.Thus, to understand whether the soil type of study area is susceptible to liquefaction.New liquefaction susceptibility criteria for saturated fine-grained soil based on the value of plasticity index (PI), water content (  ), liquid limit (LL), and clay content of soil (particle size < 0.005 mm) is carried out.
The following are the criteria proposed by Boulanger and Idriss [15], fine-grained with PI ≥ 7 exhibits "clay-like" behavior.Bray and Sancio propose another criterion to observe the liquefaction susceptibility of fine-grained soil [16] for loose soil with PI < 12,   / > 0.85 were susceptible to liquefaction, and loose sand with 12 < PI < 18,   / > 0.8 were significantly more resistant to liquefaction.However, in certain conditions, moderate plasticity of fine-grained soil is possible to experience liquefaction when subjected to shaken intensely over a substantial number of loading cycles.

Liquefaction Potential Analysis
This study utilized a standard penetration Test (SPT) to examine soil liquefaction characteristics.Additionally, the liquefaction potential analysis is assessed using a simplified procedure developed by Boulanger and Idriss [17], which takes into account the influence of soil type, soil density, and fine contents.
The simplified procedure employs a stress-based methodology initially proposed by Seed and Idriss in 1967.This approach compares the earthquake-induced cyclic stress ratio (CSR) and the cyclic resistance ratios (CRR) of the soil to estimate the value of the factor of safety as expressed in equation (3).If the value of factor of safety < 1 indicates the possibility of liquefaction of the soil layer during the seismic earthquake and the factor of safety > 1 is classified as non-liquefiable soil.
The cyclic stress ratio (CSR), as shown in equation ( 4), is a quantitative indicator of the seismic load exerted on a soil stratum in an earthquake.The term "shear stress ratio" refers to the proportion between the cyclic shear stress generated by an earthquake (commonly expressed as a representative value that is equivalent to 65% of the maximum cyclic shear stress ratio) and the initial vertical effective stress (  ′ ) of the soil layer [17] [18].The other parameters to calculate CSR are vertical total stress (  ), PGA in the ground surface (  ), factor of shear stress reduction (   ), factor correction of effective overburden stress (  ), and magnitude scaling factor (MSF).CRR can be defined as soil's capacity to resist liquefaction during an earthquake.The CRR value can be determined by the corrected SPT blow count ( 1 ) 60 and the value of corrected fines contents ( 1 ) 60 as presented in equation ( 5) [5] .

Liquefaction Severity Index
The liquefaction severity index, as developed by Sonmez and Geockglu [19] is designed for creating liquefaction susceptibility maps by inputting the depth of the mid-point of the soil layer in meters (()) using equation ( 6) and factor of safety (FS) in equation and equation (8) to obtain the value of   ().
The upper threshold for soil that will not undergo liquefaction in this method is FS > 1.411.However, these criteria are not applicable if the soil layer considered a non-liquefiable layer due to its clay content and liquid limit.Table 1 presents the classification of the liquefaction severity index and its corresponding elucidation.

Settlement Post Liquefaction
Settlement post-liquefaction in this study primarily calculates the vertical displacement resulting from the reconsolidation of liquefied soil in the context of liquefaction occurrences.This settlement analysis considers the vertical strains equated to volumetric strain (  ) in one dimensional reconsolidation and integrates them across the relevant depth interval as expressed in equation ( 9) [5].
To categorize the damage and estimate approximate settlements resulting from settlement postliquefaction reconsolidation using the Ishihara and Yoshimine [20] methods, the following classification is shown in Table 2.

Liquefaction Susceptibility Criteria Analysis
The liquefaction susceptibility criteria analysis in the study area was conducted based on Standard Penetration Test (SPT) data on 137 boreholes along the constructed toll road.Investigation of geological conditions shows that the area consists of volcanic soil deposits from Merapi Volcano.Observing 137 boreholes with 250 soil samples indicates the presence of a cohesive layer on top of the soil stratum, especially near the Mataram Irrigation Canal and River, followed by a cohesionless soil layer of silty sand with various grain-sized distribution.Specifically, to the south, the soil layer is dominated by coarse-grained soil.The identification based on geological survey and SPT data indicates that the soil's particle size is heterogeny.It is proved by the density of soils consisting of very loose sand and loose sand at various depths but predominantly at a depth of 2 up to 12 meters; medium dense also observed at various depths from 2 up to 30, and underlain by dense sand until the depth of 40 meters.
The groundwater level is one-factor influencing liquefaction susceptibility; the deeper groundwater level increases normal effective stress on saturated soil, reducing liquefaction risk; this condition also correlates with higher sediment age, cementation, and compaction [21].Figure 4 shows that the study area is dominated by shallow groundwater less than 3 m, mainly in STA 72+000 till STA 74+000, and deep groundwater level encounters in the northern area of constructed toll road, indicate artesian water pressure.The area with a shallow groundwater level of less than 3 is a high liquefaction susceptibility.Liquefaction susceptibility is low for depths less than 15.2 m [21].In terms of soil condition, the study area is dominated by loose to a very loose saturated fine-grained soil, mainly at a depth of up to 12 meters.Hence, the criteria to assess the susceptibility of liquefaction using the criterion of the value of plasticity index (PI), liquid limit (LL), and water content (  ).In a total of 137 boreholes, 37 boreholes found to be lacking sufficient data, and 100 boreholes is analyzed according to its adequacy.A comprehensive analysis was conducted on 250 soil samples from 100 boreholes, employing a liquefaction susceptibility criterion specifically designed for saturated finegrained soil.
According to Boulanger and Idriss [15], fine-grained soil with PI ≥ 7 exhibits "clay-like" behavior, and PI < 7 demonstrates "sand-like" behavior.From 250 soil sample, 100 soil sample indicates "sandlike" behavior with predominantly SM classification.However, Figure 5 shows soil with CL-ML classification would behave fundamentally like sands.The susceptibility of fine-grained soils to liquefaction, as per the criterion proposed by Bray and Sancio [16], is determined based on the Plasticity Index (PI) and Water Content ratio to Liquid Limit (Wc/LL) derived from 250 soil samples.Based on the analysis suggests that out of these samples, 154 are vulnerable to liquefaction under seismic conditions.In Figure 6, soil samples from the study area are graphed against the liquefaction susceptibility criterion.Most of these samples fall under the 'susceptible to liquefaction' category.

Liquefaction Potential Analysis
An exhaustive evaluation of liquefaction potential was carried out using a dataset of 100 SPT after eliminating factors associated with susceptibility criteria of fine-grained soil to liquefaction.Employing microtremor data as a critical diagnostic tool to evaluate the potential of liquefaction.The outcomes of this meticulous analysis unveil a predominant site class corresponding to the medium site class (SD) according to SNI 2833:2016 [14], as visually depicted in Figure 7. Once the site classification has been established, the next crucial step involves calculating the Peak Ground Acceleration (PGA).To achieve this, we utilize specific attenuation models, namely the Boore -Atkinson NGA 2014, Campbel -Bozorgnia NGA 2014, and Chi -Youngs NGA 2014.These models are accompanied by a weighting logic scheme designed to encompass and address the inherent uncertainty within the analysis.
The results of the seismic hazard analysis are succinctly portrayed in Figure 8.This figure visually represents the outcomes derived from assessing seismic risks across a dataset of 100 boreholes.The values obtained from this analysis indicate a range of PGA, spanning from 0.27 extending up to 0.31 g to the southern area.
A comprehensive analysis was undertaken using the available dataset to evaluate the susceptibility of liquefaction phenomena.The outcomes of this discerning examination are succinctly elucidated through Figure 9, which effectively portrays the calculated factor of safety values.Notably, this quantitative metric is a pivotal indicator of the inherent stability within the soil strata.
Ranging from 2 to 20 meters, where the soil composition transitions from a loose to a mediumdensity soil, several samples indicate that the factor of safety falls below 1, indicating a liquefaction potential.

Liquefaction Severity Index
The method developed by Sonmez and Gokceoglu [19] to determine the liquefaction severity index as a base for conducting liquefaction susceptibility maps.Based on the calculation, there are 29 boreholes exhibit no sign of potential liquefaction, 55 boreholes have very low potential liquefaction, 15 boreholes categorized as having low potential liquefaction, and 2 boreholes are classified as having medium potential liquefaction.The result of the LSI method was then used to create a liquefaction susceptibility map using Inverse Distance Weight (IDW) methods in ArcGIS software.10, a positive connection is shown between the LSI value, the PGA value, and the groundwater level.This suggests that an increase in the PGA value and groundwater level is associated with an increase in the LSI value.Hence, it can be deduced that the construction of toll roads in the southern region is more prone to encountering liquefaction in comparison to the northern region.

Settlement Post-Liquefaction
The study reveals the likelihood of liquefaction in the area, emphasizing the need to assess its effects, such as post-liquefaction settlement, particularly vertical displacement due to reconsolidation.Based on the calculation using Idriss and Boulanger (2008), the area possibly undergoes a vertical settlement ranging from 0 -44.64 cm.The use of Ishihara and Yoshimine [20] classification is employed to assess the damage level during settlement post-liquefaction.The study area is primarily characterized by low to no damage extent from liquefaction.However, specific locations such as STA 71, STA 73, and STA 74 show indications of significant settlement exceeding 30 cm, classified as extensive damage.A comparison between LSI and settlement post-liquefaction results reveals that specific boreholes exhibit suitability.For instance, STA 73 indicates moderate liquefaction potential with extensive damage.The boreholes on settlement post-liquefaction analysis show that out of 100 borehole points, 63 experienced low to no damage, 33 boreholes exhibited medium damage, and 4 boreholes indicated extensive damage.
The estimation of surface manifestations is facilitated by comprehending the extent of settlement post-liquefaction.In the study area, if liquefaction occurs, surface cracks or oozing may occur Figure 11 shows that post-liquefaction settlement distribution in the southern region tends to be larger, potentially due to the higher PGA value.

Conclusions
The liquefaction potential analysis conducted in the study is based on Standard Penetration Test (SPT) data from 137 boreholes, providing valuable insights into the geological complexity of the study area, primarily consisting of volcanic soil with heterogeneous distribution of grained-size.A cohesive layer atop soil strata near Mataram Irrigation Canal, followed by cohesionless soil layers with loose to medium density and shallow groundwater level, contributes to liquefaction potential.
The assessment, incorporating PI, LL, and   indicates a significant portion of the soil sample as 'susceptible to liquefaction'.From a total of 137 boreholes, only 100 boreholes are suitable for liquefaction potential analysis.
The potential liquefaction analysis incorporates microtremor data and an attenuation model to establish the site classification.PGA resulted in specific areas showing a factor of safety < 1 showing liquefaction potential.The liquefaction severity index (LSI) method is carried out to generate liquefaction susceptibility maps.Based on the analysis, the study area is dominated by low potential analysis that spreads in the region and highlights a positive correlation between LSI value, PGA, and groundwater level.
Considering the effect of the potential of liquefaction, the study delves into settlement postliquefaction focusing on vertical displacement due to reconsolidation.From settlement post liquefaction calculation, the southern area of constructed toll road shows an indication of experiencing a greater vertical settlement than in the northern area; it can be related to the higher value of PGA due to the proximity to the active strike-slip fault of the Opak fault that became the ground motion scenario of this study.
This research underscores the intricate interplay between geological conditions, soil properties, groundwater level, and seismic hazard analysis in influencing liquefaction susceptibility.The findings emphasize the need for comprehensive analysis and site-specific considerations in assessing and mitigating the risks associated with liquefaction-induced hazards, particularly in infrastructure development like toll road construction.

Figure 2 .
Figure 2. The detailed geological survey in the study area shows the domination of tuffaceous sandstone on the surface.

Figure 3 .
Figure 3.The location of soil investigation and geophysical investigation.A total of 137 boreholes and 13 microtremor data are acquired

Figure 4 .
Figure 4. Groundwater level along with the susceptibility of liquefaction in the study area (green color indicates the threshold of high susceptibility of liquefaction, and the yellow color indicates the range of low susceptibility of liquefaction)

Figure 5 .
Figure 5.The Atterberg limit chart displays representative data for each soil type, demonstrating their respective behaviours categorized as clay-like, sandlike, or intermediate (modified from [15])

Figure 6 .
Figure 6.Plot illustrating soil samples from the study area against proposed liquefaction susceptibility criteria (modified from [16])

Figure 7 .Figure 8 .
Figure 7. Site class of study area based on SNI 2833:2016 and calculation of microtremor data

Figure 9 .
Figure 9.The value safety of factor results less than 1 signifies the vulnerability of liquefaction, shown by a red box.

Figure 10 . 12 Figure 11 .
Figure 10.Liquefaction severity index result indicates that very low vulnerability of liquefaction dominates the research area.