Analysis of liquefaction potential in Opak Fault nearby area (case study: Solo-Yogyakarta-NYIA Kulon Progo Toll Road construction section I.2

. Construction of the Solo – Yogyakarta – NYIA Kulon Progo Toll Road Section I.2 is a part of Java's toll road network system that connects Solo and Yogyakarta. The presence of this opak fault allegedly cut the toll road alignment. Based on Indonesia's Liquefaction Vulnerability Zone Map, the alignment location is in a medium liquefaction vulnerability zone. This study aims to determine the potential of liquefaction in the construction of Solo – Yogyakarta – NYIA Kulon Progo Toll Road Section I.2 (stationing 29+000 - 36+000). The maximum peak ground acceleration on the surface was calculated using the Deterministic Seismic Hazard Analysis calculation. Calculating liquefaction potential analysis uses the simplified procedure method developed by Idriss and Boulanger. The research shows that ten borehole points with sandy soil types 2 m – 18 m below the surface have liquefaction potential. The depth of the groundwater table is between 0.2 m – 10 m below the ground surface. Based on the Liquefaction Potential Index analysis, the research location is in a low - high level of liquefaction vulnerability.


Introduction
Java Island is an island in Indonesia with the most significant population; as the population grows, so does economic activity.One of the factors supporting economic growth is the development of the infrastructure.The Solo-Yogyakarta-NYIA Kulon Progo Toll Road is the government's chosen approach to fostering and boosting economic activity on Java Island.This toll road is a part of the network of toll roads on Java Island.The Solo-Yogyakarta-NYIA Kulon Progo Toll Road Construction section 1.2 will be built by passing through Klaten-Purwomartani (stationing 22+300-42+375), which is also a part of Section I through the length of ±42,375 km.
As it is well known, earthquakes often happen in and around the Yogyakarta region.According to earthquake history compiled by the USGS (United States Geological Survey), over the past 50 years, at least the Yogyakarta region has had earthquakes more significant than 5 Mw more than 50 times [1], as shown in Fig. 1.
The geological characteristics on Java's island, particularly in Yogyakarta and the adjacent areas, strongly correlate with the occurrence of the earthquake.On May 27, 2006, an earthquake with 6.3 MW struck Yogyakarta, inflicting fatalities and causing damage to the city's infrastructure.The Opak fault allegedly caused that year's earthquake.Liquefaction phenomena indicated by fine sand in soil fissures and lateral spreading were also discovered.According to the Atlas of Indonesian Liquefaction Risk Zones published by the *Corresponding author: hary.christady@ugm.ac.idGeological Agency of the Ministry of Energy and Mineral Resources, the Solo-Yogyakarta-NYIA Kulon Progo Toll Road is in a moderate liquefaction risk zone [2], as shown in Fig. 2. Liquefaction is the loss of soil strength brought on by increased pore water pressure, typically in sandy soil layers and shallow water.Due to earthquake-induced ground movement, the pore water pressure rises.One of many issues with infrastructure development is liquefaction, which can damage buildings, roads, and bridges, as well as the failure of dams.This study is necessary for liquefaction potential analysis calculations, specifically on the Toll Road Construction of Solo-Yogyakarta-NYIA Kulon Progo Section I.2 (stationing 29+000-36+000) to determine the depth and thickness of layers that have the liquefaction potential, so it can provide suitable mitigation solutions to reduce the risks involved.

Fig. 2. Map of the liquefaction vulnerability zone of Central
Java Province [2].

Research methods
This study uses data from a soil investigation conducted by PT Jogjasolo Marga Makmur.The study results are presented as a Standard Penetration Test (SPT), carried out to 30 m depths under the surface with 2-meter intervals in each drill point.Groundwater investigation data were collected between April 2021 and September 2021, with 3-4 days of measurement after the completion of drilling activities.Laboratory testing is carried out after samples from the drill results are obtained.

Geological conditions of research locations
The research location is on the Solo-Yogyakarta-NYIA Kulon Progo Toll Road Section 1.2, explicitly stationing at 29+000 and 36+000.According to the geological map, the research location is above the Quaternary Merapi lithology, consisting of volcanic breccia, lava, and tuff [3], as shown in Fig. 3. Fig. 3. Regional geological map at the research location [3].These Quaternary deposits are generally susceptible to liquefaction events due to recent deposition.According to Walter et al. [4], this volcanic deposit provides a high amplification effect of seismic waves along the Opak River basin, which caused more damage from the earthquake in 2006 than the lithology of the constituent rocks that were not too affected by the earthquake [4].In addition, the condition of the geological structure at the study site is near an Opak fault.The movement of the Opak fault was the cause of the Yogyakarta earthquake, which occurred on May 27, 2006.History records that apart from the Yogyakarta earthquake in 2006.There had been an earthquake of up to 6.9 MW in 1987.The earthquake source extended in a southwestnortheast direction close to the Opak River flow [5].
This situation raises the possibility that an earthquake will be caused by the active Fault moving at any time shortly.The Center for Geological Research and Development's 1:100.000scale regional geological map shows that the Opak Fault is mapped from Parangtritis to Prambanan in a direction that corresponds to the flow of the Opak River.As shown in Fig. 4, an active fault known as the Mataram fault, which is a continuation of the Dengkeng fault, has been discovered based on the findings of mapping work that PT Jogjasolo Marga Makmur has done.This Fault extends westward toward Yogyakarta along the toll road alignment in the Purwomartani region.

Calculation of maximum peak ground acceleration (PGAM.)
One of the critical parameters in the analysis of liquefaction potential is seismicity.Based on the National Center for Earthquake Studies [7] for locations close to the Fault, a review is needed regarding the Deterministic Seismic Hazard Analysis [7].The determination of the maximum ground acceleration value (M) using a deterministic analysis approach while also applying the Next Generation Attenuation (NGA)-WEST 2 equation of The Boore -Atkinson model [8].The attenuation function is customized according to the geological conditions of the research location, which is near the Opak fault with an earthquake source mechanism using the history of the 2006 Yogyakarta earthquake.This earthquake is included in the shallow earthquake source caused by the strike-slip movement from the Opak fault.The calculation of the value of  using an online application via https://ngawest2.berkeley.edu/with input data in the form of moment magnitude, the shortest distance from the rupture plane (RRUP), the shortest distance to the surface projection of the rupture plane (RJB), down-dip width of the rupture plane (Width), the dip angle of rupture plane (DIP) and depth of the peak of the rupture plane (ZTOR) as shown in Fig. 5.

Determination of soil grain size distribution
The soil's Grain size distribution was measured using sieve analysis/sieve test.In this case, the grain size distribution of the soil is the percentage of the weight value of the soil in a sieve with a specific diameter.The soil grain size analysis results can be used for a simple initial analysis of potential liquefaction soils.Tsuchida [10] has made a boundary curve for most of the liquefiable and potential liquefiable soils, as shown in Fig. 6.Fig. 6.Grain size distribution curve of soils vulnerable to liquefaction [10].

Liquefaction potential analysis
Earthquakes, certain soil types, local geology, and shallow groundwater depths are several factors that can cause liquefaction.High groundwater levels can lead to water-saturated soil conditions, which makes the soil susceptible to liquefaction during an earthquake.By calculating the value of soil to liquefaction due to cyclic loading, cyclic resistance ratio (CRR), and soil shear stress due to cyclic loading, cyclic stress ratio (CSR), analysis of liquefaction potential was carried out in this study using a simplified procedure method developed by Idriss & Boulanger (2008) [11].The CSR value is the cyclic stress that triggers liquefaction.The CSR value can be calculated using Equation 1.
where  is the maximum acceleration of the earthquake on the ground surface (m/s 2 ),  is the acceleration of gravity (9.81 m/s 2 ),  is the total vertical stress of the soil (kN/m 2 ).At the same time, ′ is the effective vertical stress of the soil (kN/m 2 ), and  is the stress reduction coefficient (dimensionally).Figure 0.65 represents the cyclic stress due to the earthquake, representing 65% of the maximum cyclic stress.The parameter  is expressed as a function of the depth and magnitude of the earthquake (M), which can be calculated using Equations 2-4.
where  is the depth (m),  is the moment magnitude, and the sin angle is in radians.The CRR value is the ratio of soil resistance to cyclic loads that cause liquefaction.The factors that affect the CRR value are the SPT value corrected for 60% efficiency, overburden pressure, and fine grain content.The SPT value is corrected for efficiency (R) 60% (()60) and can be calculated using Equation 5.
() 60 =           (5) where Nm is the N-SPT value of the field test results, CE is the correction factor for the striking energy ratio, CB is the correction for the borehole diameter, CR is the correction for the length of the borehole, and Cs is the correction for the SPT sampler tube type.The corrected SPT value for 60% efficiency and 1 atm effective stress ((1)60) is calculated using Equation 6, where  is an effective stress correction factor of 1 atm and can be calculated by Equation 7.
The SPT value for correcting fine grain content {( 1 ) 60− } can be calculated using Equation 8.The fine grain content value can be calculated using Equation 9. ( where  is the percentage of fine content obtained from the results of the grain analysis test on grains that pass sieve No. 200. To get the value of , an iterative method is needed between ((1)60−) and , namely by defining the function f (x) by entering the values of N60, σ'vc, and Δ(1)60 in Equation 10.
The safety factors against liquefaction triggers can be calculated as the ratio of the CRR of sand to an earthquake that induces CSR (Equation 12).

Liquefaction potential index (LPI)
Sonmez [12] has modified the liquefaction potential category that previously had been produced using Equation 13 after knowing the value of the factor of safety to determine the liquefaction potential based on the severity of the liquefaction and the depth of the liquefaction zone.
where FL is the damage level of the soil layer and w(z) is the depth factor.Table 1 shows the liquefaction potential category based on LPI with very low to very high potential levels.

Groundwater level conditions
The results of measuring the groundwater level at the study site, which is located at 0-2 m -10.08 m below the ground surface.Referring to the study of Youd et al. 1979, it is known that based on the identification of liquefaction based on the depth of the groundwater table, the research location is at a low level of liquefaction vulnerability -high liquefaction [13], as shown in Fig. 7.

Fig. 7.
Graph of variation of groundwater level.

Soil grain size distribution
Soil conditions at the review site were dominated by sandy soil with loose to dense density levels; besides that, there were also layers of silt and clay at several points.Based on the results of Grain Size Analysis data carried out by PT Jogjasolo Marga Makmur, the authors chose four boreholes at BH 01, BH 02, BH 03, and BH 04, which will be used as examples of plotting on the curve proposed by Tsuchida [10].Based on the plotting results on the Tsuchida curve, it is known that most of the grain sizes in the four boreholes are susceptible to liquefaction potential, as shown in Fig. 8-11.

Liquefaction potential analysis
The liquefaction potential was analyzed using the simplified procedure by Idriss & Boulanger (2008) at forty-three (43) borehole points along stationing 29+000 -36+000.The magnitude of the earthquake strength used was the magnitude of 6.3 Mw, and the PGAM value used refers to Table 2. Fine-grained soils with a fines content of > 50% were not analyzed and considered will not experience liquefaction because the fines content values were included in the silt soil and clay category.Based on the calculation results, there are ten (10) Boreholes with liquefaction potential, as shown in Table 3.     3 shows that liquefaction occurs at the depth of the soil layer between 4 m -18 m below the soil surface with water-saturated conditions.Based on the evaluation of liquefaction at the research location, it is known that the characteristics of the soil that experienced liquefaction are sandy soil types with N-SPT between 5; this is to previous studies, under the sand with N less than 20 more prone to liquefaction while sanding with N more than 30 not easily liquefied.The lowest FS value is at BH 04 at a depth of 2 m, with an FS value of 0.44.

Liquefaction potential index
The calculation of the liquefaction potential index refers to the classification according to Sonmez (2003) with the results of the category that in 10 boreholes with liquefaction potential, there is a liquefaction potential index classification with low -high class shown in Table 4.

Conclusion
Solo -Yogyakarta -NYIA Kulon Progo Toll Road Section I.2 (Stationing 29+000-36+000) Based on the results of this study, there has been found three liquefaction trigger factors, including an earthquake with a magnitude above 5 Mw, sandy soil conditions with an N value of less than 30 and low groundwater table (0.2 m -10 m).By taking an empirical approach to an earthquake with a magnitude of 6.3 Mw and a maximum peak ground acceleration (PGAM) value between 0.38 g -0.50 g, it is also discovered that out of 43 boreholes, ten boreholes have the potential of liquefaction.The liquefaction potential occurs at the depth of the soil layer between 2 m -18 m below the soil surface with water saturation conditions.After acknowledging the potential for liquefaction at the research location with a reasonably high level, it is necessary to calculate land subsidence that may occur after liquefaction and its mitigation plan to reduce the structure damage of Solo -Yogyakarta -NYIA Kulon Progo Toll Road Section I.2 (stationing 29+ 000 -36+000).
The author would like to thank for the support given to the Directorate of Freeways, the Directorate General of Highways, the Ministry of Public Works and Public Housing, and PT.Jogjasolo Marga Makmur.

Fig. 4 .
Fig. 4. Map of the active Fault in the Yogyakarta area and the alignment of the Solo -Yogyakarta -NYIA Kulon Progo toll road [6].

Table 1 .
Liquefaction potential category based on LPI.

Table 2 .
Calculation of PGAM at each borehole point.

Table 3 .
Results of the potential liquefaction analysis of 10 boreholes with liquefaction potential.

Table 4 .
Liquefaction potential index at research locations.The level of potential liquefaction is expressed in the LPI index, with high categories shown at BH 02, BH 04, BH 05, BH 56, and BH 79.Moderate liquefaction categories at BH 01, BH 08, BH 10, and low liquefaction categories at BH 03 and BH 84.The highest LPI value is shown at BH 04, with a value of 12,98.