Assessment of liquefaction potential and damage on the toll road construction in earthquake-prone area, Sleman Regency, Yogyakarta

The construction of toll roads in earthquake-prone areas presents significant challenges due to the potential occurrence of liquefaction, a seismic phenomenon in fine-grained soils losing strength during earthquakes. The research was conducted at a toll road construction site in Sleman Regency, Yogyakarta, an area near the active Opak fault which increases its vulnerability to liquefaction. The objective of this study is to analyse the liquefaction potential and damage on the surface in research area. The method used is simplified procedure by Idriss and Boulanger which assessing the cyclic stress ratio and cyclic resistance ratio based on standard penetration test and laboratory data. Then, Liquefaction Potential Index (LPI) and Liquefaction Severity Index (LSI) are calculated. Finally, vertical displacement caused by liquefied soil is estimated using the post-cyclic reconsolidation model to understand its impact on the surface. The study shows low to very high liquefaction potential at the toll road construction site in Sleman Regency. With a vertical displacement of 4.69 cm to 38.83 cm, surface damage can range from low to extensive, emphasizing the need for liquefaction mitigation in earthquake-prone areas like Yogyakarta during toll road infrastructure planning.


Introduction
Indonesia is frequently impacted by earthquakes since it is surrounded by the Eurasian Plate, the Indo-Australian Plate, the Pacific Plate, and the Philippine Sea Plate [1].The Bantul earthquake, which occurred in Yogyakarta province, is one of the earthquakes that have struck Indonesia.This earthquake killed thousands of people and destroyed much of the infrastructure in Yogyakarta and its surroundings [2].The earthquake happened within the Opak Fault, which extends from south coast of Java Island to Prambanan, Klaten Regency [3].The Opak fault is an active strike-slip fault that moves 5.0 mm per year [4].Places that have had or are prone to earthquakes need to be taken into account when building infrastructure, especially in places near faults where there are risks of earthquakes and disasters caused by earthquakes, such as liquefaction.
Seismic hazard assessment, including liquefaction, are required in earthquake-prone areas, particularly in areas less than 10 kilometers from known active faults with a potential magnitude of at least 6 [5].One of the significant infrastructural developments in Sleman Regency involves to the 2 construction of toll roads.This toll road serves to connects two cultural cities, Surakarta and Yogyakarta.The region crossed by this toll road is lies within area of moderate susceptibility to liquefaction [6].
Recent earthquakes have shown that liquefaction of sandy soils due to earthquake ground motion affects civil engineering structures [7].Sand becomes liquid when the saturated soil experiences seismic shocks, resulting in a reduction in effective stress.In the worst case, the soil's effective stress is zero [8].Several conditions can influence the occurrence of liquefaction, including the type of soil, groundwater level, and cyclic loads.During seismic occurrences, several ground surface impacts might manifest, such as slope instability and lateral spreading in regions characterized by high slopes, and ground settlement caused by liquefaction in predominantly flat locations [9].In a previous study, Rahman (2020) analyzed the potential for liquefaction in the Yogyakarta area in the Yogyakarta underpass area, Kulon Progo Regency, using the Seed and Idris (1970) methods and the Liquefaction Potential Index (LPI) method, with low and high potential outputs [10].
The objective of this study is to analyse the liquefaction potential and damage on the surface in research area, Sleman, Yogyakarta.Liquefaction potential was assessed through utilization of the simplified procedure developed by Idriss and Boulanger (2008).The Liquefaction Potential Index (LPI) and Liquefaction Severity Index (LSI) are also used in the assessment of liquefaction potential.The vertical displacement was calculated as part of the analysis to assess the potential settlement resulting from liquefaction.The result of this study is expected to provide a comprehensive evaluation of the liquefaction risk and its impact on the surface settlement.

Site Condition
Research area is located close to Mataram Canal in Purwomartani, Kalasan District, Sleman Regency, Special Region of Yogyakarta Province (Fig 1).Certain regions of Sleman Regency have susceptibility to natural disasters, especially seismic events, which have the potential to induce liquefaction phenomena [12].Geological condition is one of the ways to figure out how vulnerable soil is to liquefaction [13].On a geological map, it can be seen about rock formations, types of materials, and the locations of active faults that could cause earthquakes [11].
According to the geological map, the construction of the toll road lies within the Quaternary volcanic deposit.Previous research shows that layers of soil with a susceptibility to liquefaction usually develop within quaternary volcanic deposit [14].Geological composition of the research area in Sleman Regency consists of young volcanic deposits originating from Gunung Merapi (Qmi).These deposits primarily consist of lava, tuff, and volcanic breccia which are the products of Mount Merapi's volcanic activity [11].

Methods
The analysis conducted utilizing empirical method developed by Idriss and Boulanger (2008), based from the analysis of soil investigation data.The utilization of empirical approaches for liquefaction analysis resulted in more conservative outcomes in comparison to numerical methods [15].The data utilized in this research is derived from secondary sources [16].Fig 1 shows the distribution of boreholes location in research area.In this method, the cyclic resistance ratio (CRR) is compared to the cyclic stress ratio (CSR).CRR is a way to evaluate the soil's resistance to cyclic loading, and the CSR represents the cyclic stresses induced by earthquakes.The CSR is influenced by factors such as the peak ground acceleration (PGA) that occurred at a ground level and the CRR is influenced by N-SPT value after several corrections have been made.The assessment also conducted through the utilization of LPI developed by Iwasaki et al. (1981) [17] and LSI developed by Sonmez and Gokceoglu (2005).After that, the calculation of vertical displacement resulting from the liquefaction of soil was performed.

Simplified Procedure
This study uses empirical method developed by Seed and Idriss known as simplified procedure to figure out safety factors through the comparison of the earthquake-induced cyclic stresses (CSR) and the cyclic resistance of the soil (CRR).To estimate CSR, the semi-empirical method according to Seed where the coefficient of 0.65 is equal to 65% of the peak cycle stress, amax is the peak ground acceleration, v is the total stress, ′v is the effective stress, and d is the stress reduction coefficient.
With the help from the PEER NGA-West2 Spectrum website, the PGA is taken from the calculations of Next Generation Attenuation (NGA)-West2 Boore et al. (2014).The approach was formulated using seismic occurrences of shallow crustal earthquakes, specifically those associated with active strike-slip faults ranging from magnitude 3 to 8.5 Mw [19].These earthquake events share similarities with the Opak Fault.Furthermore, this approach offers a low root mean square (RMS) error value when estimating the maximum horizontal acceleration, as indicated by reference [20].
The cyclic resistance ratio (CRR) represents the soil's capacity to withstand cyclic pressures triggered by seismic activity.The variables influencing CRR include the N-SPT value obtained after correcting for soil investigation drilling equipment, the percentage of fine soil present, and the correction made for effective overburden pressure ((1)60).Idriss stands for the overburden correction factor,  for the fines content,  for the energy ratio correction factor,  for the width of the borehole,  for the length of the rod, and  for the liner.

LPI
This method uses an integral approach to the level of damage and depth factor in order to determine the quantified liquefaction potential level of an area.Iwasaki et al. (1981) [17], as shown in equations ( 14) -(17), developed the LPI equation.

𝐿𝑃𝐼 = ∫ 𝐹(𝑧). 𝑤(𝑧). 𝑑𝑧
is volumetric strain due to the post-cyclic reconsolidation,  is the highest shear strain, and  is the shear strain limit.Table 3 shows relationship between the level of damage and the approximate settlement.

Simplified Procedure
This approach conducted using soil investigation data from 11 boreholes located in Sleman Regency, which were crossed by the site construction.Coordinates and locations from soil investigation data can be seen in Fig 1 .Based on soil investigation data, the N-SPT curve and the groundwater table (GWL) ranges from 1.5 and 3.5 meters deep can be seen in Fig 2.
Boore et al. ( 2014) method for figuring out PGA in bedrock requires a number of factors, such as distance from site to fault.In this study, the earthquake was based on the 6.3 Mw Yogyakarta Earthquake.The PGA value on bedrock is then multiplied by a factor (F PGA ) based on the site class according to requirements of the Indonesian National Standard determined by the average SPT value [5].The result obtained from the calculation is the PGAM value varies between 0.36 to 0.47 g showed by Table 4.
Figure 3 show outcomes of the liquefaction potential study based on safety factor at BH 01 to BH 11 where liquefaction happens under the GWL.Most of potential occurred in loose to medium-density sand.The results then utilized for examining at the liquefaction potential for each index category.

Vertical Displacement
As shown in equation ( 22      Figure 4 shows that the LPI ranged from low to very high.Analysis shows that BH 01 and BH 09 have very high level of liquefaction potential with a significant damage to the surface of the ground.This area is dominated by liquefaction with a moderate to high level of severity, as shown in Fig 5.This area also has a significant potential of ground surface damage, illustrated in Fig 6, which is classified by Ishihara's (1996) study of the relationship between the amount of damage and the amount of settlement.Meanwhile, in BH 02 and 08, although they also have a very high level of liquefaction potential, they have a medium level of damage potential.This is because in BH 02 and 08, the thickness of the liquefied layer is not as thick as in BH 01 and 09, which can be seen in Fig 3.

Conclusions
Research area located in earthquake-prone area caused by its proximity to active fault.Soil conditions in research area is mostly have low N-SPT value, and shallow GWL with depth between 1.5-3.5 m.The result shows there is a low to very high liquefaction potential with PGA of at least 0.36g.The highest liquefaction severity levels occurred at BH 09.From the results of the vertical displacement evaluation, it was found that the potential for settlement due to liquefaction was found with dominantly medium to extensive damage level.Where the highest settlement potential is found at BH 01 with an approximate settlement of 38.83 cm.It was also found that the thickness of layers that have the potential for liquefaction, influence the settlement potential.
The results of this study are expected to be input for further studies in the assessment of the toll road construction in this area, especially for the soil bearing capacity and foundation requirements.And as suggestions for future research, soil investigation data greatly determines the results of the evaluation that has been carried out, the feasibility and validity of the data is needed to generate a well evaluation.

Figure 1 .
Figure 1.Site conditions around research area in Sleman Regency and the location of boreholes (modified from [11]).

Table 3 .
Relationship between the level of damage and the approximate settlement[24]

Table 4 .
Data used for PGA calculations FS and the GWL of 11 boreholes.4.2.Liquefaction Potential Index Evaluation11 boreholes, as illustrated in Fig 4, have the potential for liquefaction based on the LPI category.Very high LPI with values of 20,42, 15.43, 30.67, 30,34 occurred at BH 01, 02, 08, and 09.High LPI with values of 9.05, 11.82, 10.71, 6.05, and 5.61, occurred at BH 03, 04, 06, 07, and 11.Moderate and low LPI occurred at BH 10 and 05. Figure 6 shows that the liquefied soil caused low to extensive damage with 4.69 -38.83 cm of vertical ground settlement after the earthquake.The results show that the ground surface is damaged significantly with approximate settlement of 38.83 and 37.19 cm at BH 01 and 09.Then, 8 boreholes, BH 02 to 04, 06 to 08, 10, and 11, had moderate damage.1314 (2024) 012064