Liquefaction potential analysis based on standard penetration test in coastal area (Case study: Loh Buaya, Rinca Island, Indonesia)

An area has liquefaction potential when it has a shallow groundwater level, loose sandy soil, and is prone to earthquakes. There are several areas with such criteria that have not been analysed for liquefaction potential. This study aims to analyse and plot the liquefaction potential in the coastal area of Loh Buaya, Rinca Island, East Nusa Tenggara Province. Soil investigation data, such as SPT, sieve analysis, and groundwater level, as well as earthquake history that occurred from 1922-2022, served as the main data for liquefaction potential analyses. The methods used were Ground Motion Equation Prediction (GMPE) to calculate Peak Ground Acceleration (PGA), Simplified Procedure, and Liquefaction Severity Index (LSI) to make a liquefaction hazard assessment. LSI scores were used to provide micro-zonation of liquefaction potential with Inverse Distance Weighted (IDW) interpolation in QGIS. The result obtained is very dense gravel has no liquefaction potential whereas loose sandy soil has very high in LSI classification because loose sandy soil has liquefaction potential up to 20 meters of depth. The applying of micro-zonation LSI by IDW interpolation method can estimate the potential level of liquefaction hazard on Loh Buaya, Rinca Island with limited soil investigation data.


Introduction
Liquefaction research has become a popular topic in Indonesia, especially since the liquefaction in Palu in September 2018.An earthquake with 7.5 mw triggered the liquefaction that caused large numbers of mortalities [1].Besides causing deaths, liquefaction in Palu in 2018 caused massive infrastructure damage and huge economic losses [2].In other places, liquefaction also occurred in Wenchuan China in 2008 [3,4], Kanto Region Japan in 2011 [5], Christchurch New Zealand in 2011 [6], Kumamoto Japan in 2016 [7], Tibetan Plateau China in 2021 [8], and Central Thessaly Greece in 2021 [9].
Liquefaction occurs in loose sandy soils, and shallow groundwater levels and is initiated by large earthquakes.Liquefaction is a phenomenon where saturated cohesionless soil loses its shear strength and behaves like a liquid because the effective stress of the soil drops to zero [10].During an earthquake, the pore water pressure will increase so that the sandy soil, which is a non-cohesive soil, 1314 (2024) 012123 IOP Publishing doi:10.1088/1755-1315/1314/1/012123 2 will be saturated and lose its strength.Liquefaction causes ground instability such as foundation settlement, lateral spreading, and soil damage.[11].
Coastal areas with loose sandy soil types and close to active faults have liquefaction potential.Palu Bay, located near the Palu-koro Fault, has a liquefaction potential at 16 boreholes out of 22 borehole SPT samples [12].In the Long Beach area on the east coast of Cyprus, the predominantly loose sandy soils have liquefaction potential at depths of 2-18 meters.[13].An alluvial flood plain in a coastal area of Catania has liquefaction potential at depths of 3 and 7 m [14].In Jizan coastal area of Saudi Arabia has liquefaction potential in fine sand to silty clay soil with a groundwater level of less than 2 m depth [15].The coastal regions of Albania and Montenegro are formed of sediments prone to liquefaction [16].Also in Eco-Delta City Busan has the potential for liquefaction in coastal areas and alluvium soils [17].
Various soil investigation data can be used as a basis for analyzing liquefaction potential.The soil investigation data are able to use standard penetration test (SPT), cone penetration test (CPT), and microtremor measurement.Among these soil investigation data, SPT is the most used data.SPT data was also used in the study of liquefaction potential in the Jammu Region, India [18], Roorkee Region, India [19], Kathmandu Valley in Nepal [20], Imperial Valley, California [21], Surat City, India [22].Currently, there are limited number of studies on liquefaction potential analysis in coastal areas in Indonesia.

Figure 1. Study site in Loh Buaya, Rinca Island, Indonesia
There are three objectives in this study.The first is the Study of liquefaction potential in the coastal area.The case study took place in the coastal area of Loh Buaya, Rinca Island (Figure 1).This liquefaction potential analysis used the simplified procedure by Idriss-Boulanger [23] according to 8 SPT boreholes.It procedure has also been applied in Jono Oge-Paneki River [24], Gumbasa Irrigation Area [25] dan Bangga River [26] for liquefaction potential.The second is to determine the Liquefaction Severity Index (LSI).Factor of Safety (FS) from simplified procedure is used to make a liquefaction hazard assessment using Liquefaction Severity Index (LSI) [27,28].The third is to determine micro-zonation maps of LSI.micro-zonation map was created to determine the extent of liquefaction-prone areas [29][30][31][32].Micro-zonation map created using Inverse Distance Weighted interpolation method based on LSI number in each borehole [33,34].

Tectonics and Geology of Loh Buaya, Pulau Rinca
The vulnerable location for earthquakes is characterized by its location near active faults or subduction zones.Rinca Island is located between several active faults, which are Flores back arc thrust, Sape strike-slip, and Bondowatu fault.Rinca Island is also located near the subduction zone between the Indo-Australian and Eurasian plates [35].As a result of being near 3 active faults and subduction zone, Rinca Island is categorized as a high-risk earthquake area [36].According to the Modified Mercalli Intensity (MMI) scale, Rinca Island has the potential to be impacted by earthquakes more than VIII MMI [37].Soil conditions in Loh Buaya Rinca Island are composed of quaternary period of alluvial sediments and young volcanic rock [37] The quaternary period is the youngest age compared to other geological ages so it will be easily found loose and poorly compaction soil layers.

Subsurface Geotechnical Investigation
The geotechnical investigation included standard penetration test and sieve analysis.Soil investigation was obtained from Standard Penetration Test (SPT) results on 8 boreholes in Loh Buaya, Rinca Island.B-1 to B-5 are on land while B-6 to B-8 are underwater.Based on land cover, B-1, B-3, B-4, B-5 are swamp areas; B-2 is shrubs and B-6, B-7, B-8 are mangroves (Figure 2).
The SPT value obtained various results (Table 1).B-1, B-3, B-4, B-5 obtained values below 20 blows.B-2 has 60 blows at 2 to 6 m and 12 to 16 m depth.However, at 8-10 m depth, the SPT blows decreased to 30 and 32.At B-6, B-7, and B-8, the SPT value obtained 60 blows from 2 to 14 m depth, indicating that the soil is classified as stiff soil.
The groundwater depth value found that B-1, B-2, B-3, B-4, and B-5 have a shallow groundwater level that is below 2m of depth.B-6, B-7, and B-8 are located underwater so that the groundwater level is above the ground.The results of the sieve analysis test found that the type of soil at points B-1, B-3, B-4, and B-5 is relatively similar, which is dominated by fine sand (Figure 3).While B-2, B-6, B-7, B-8 are gravel.

Peak Ground Acceleration (PGA)
PGA calculations are performed to calculate the level of ground acceleration that occurs due to earthquakes based on hypocenter distance, epicenter distance, magnitude, and soil conditions.The greater the PGA value, the more the impact of earthquake shaking will be felt.Liquefaction may occur if the PGA value is more than 0.09 g [38].In predicting PGA, a deterministic approach using Ground Motion Equation Prediction (GMPE) is used.This study chose GMPE Kanno [39] according to the equation ( 1) dan (2): For focal earthquake depth D ≤ 30 km For focal earthquake depth, D > 30 km: with y is PGA (cm/s 2 ); a 1 = 0.56; a 2 = 0.4; b 1 = -0.0031;b 2 = -0.0039;c 1 = 0.26; c 2 = 1.56; d 1 = 0.0055.The PGA obtained needs to be corrected according to the V S30 .The equations are as follow equations ( 3) and (4): with G is correction to VS30 (g); p = -0.55;q = 1.35 and log   is the corrected log PGA (g).To derive VS30, the correlation of SPT blows to Vs by Iyisan (1996) was used.Iyisan correlation has a degree of correlation efficiency of 81% [40].The correlation equation is as follows equation ( 5): = 51.5 0.516 (5) with N is SPT value.N is the SPT value.The earthquake data used in the calculation is based on earthquake records from the United States Geological Survey (USGS) from 1922 until 2022.To analyse liquefaction potential, the largest PGA value would be used.

Liquefaction Potential Analysis
Analysis of liquefaction potential begins with a preliminary study based on grain size distribution.Grain size distribution by Tsuchida chart [41].This chart describes the grain boundaries in areas that have the potential and most potential for liquefaction.Fine sand is the highest liquefaction vulnerability and silty sand is the soil with the high liquefaction vulnerability (Figure 4).Further analysis of liquefaction potential using the simplified procedure by Idriss and Boulanger [23].The data required are SPT value, Ground Water Level (GWL), Moment Magnitude (Mw), Peak Ground Acceleration (PGA), and Fines Content (FC).When the soil is not saturated, it has no liquefaction potential.This potential analysis is used to determine the Factor of Safety (FS) value in each soil layer.The FS value is obtained from the comparison between Cyclic Resistance Ratio (CRR) and Cyclic Stress Ratio (CSR).When FS < 1, it has the liquefaction potential.The FS equation is as follows equation ( 6): Figure 4. Grain size distribution by Tsuchida [41] The calculation of the FS value is limited to 20m of depth because liquefaction incidents only occurred at a maximum of 20m of depth [23].The CSR equation is as follows equation ( 7): with r d is shear stress reduction coefficient, σ v is vertical total stress (KPa), σ′ v is vertical effective stress (KPa), a max is maximum peak ground acceleration (g), MSF is magnitude scaling factor and Kσ is overburden correction factor.The CRR equation is as follows equation ( 8): with ( 1 ) 60 is equivalent clean sand SPT value count.

Liquefaction Severity Index
The level of liquefaction vulnerability was calculated using Liquefaction Severity Index (LSI) by Sonmez [42].This method evaluates the vulnerability of liquefaction potential according to the value of the Factor of Safety (FS).The LSI equation is as follows equation (9)(10)(11)(12): () =  Grain Size (mm) Boundaries for potential liquefable soil Boundaries for most potential liquefable soil with LS is liquefaction severity, PL is liquefaction probability, dan z is depth of soil layer (m).The LSI has 6 classifications of liquefaction vulnerability (Table 2).The higher the LSI value, the higher the level of potential liquefaction vulnerability.To predict the surrounding liquefaction-prone area, microzonation was made.The micro-zonation method uses Inverse Distance Weighting (IDW).IDW will determine the value of an unknown point using a combination of linear weights from sample points.The IDW equation is as follows equation ( 13): with Z is predicted point value, Zi is value at sample points, n is value of total sample data, di is the distance between the sample point and the predicted point, and p is weighting power.IDW method is an interpolation method that has been widely used in spatial data, images, and optimization algorithms [43].IDW interpolation is performed based on the results of the LSI calculation in each borehole with QGIS version 3.22.

Peak Ground Acceleration Calculation
The maximum corrected PGA at Loh Buaya on Rinca Island was obtained as 0.57 g (table 4).The PGA correction value is derived from the SPT value at B-3 which is correlated to VS30.This PGA is derived from the 1968 earthquake with 6.8 Mw, 30.6 km of depth, and 64.91 km of epicenter distance from Loh Buaya [44].In terms of depth, the earthquake was a shallow earthquake (depth < 70 km).

Liquefaction Potential Analysis
Figure .5 is the sample at 3m below ground surface at B-1 until B-5.In B-6, B-7, and B-8, the grain size distribution graph was not conducted because the soil is gravel.The results obtained that B-1, B-3, B-4, and B-5 are included in the range of grains that have the liquefaction potential.While B-2 is not included in the range of potential liquefaction because more than 40% of the soil consists of coarse sand with grain size > 2mm.The results of the FS calculation obtained that B-1, B-3, B-4, and B-5 have liquefaction potential while B-2, B-6, B-7, and B-8 do not have liquefaction potential (Table 4-5).B-6, B-7, and B-8 have similar results because the SPT results obtained are the same at each depth.The FS value obtained is very low due to 4 factors, which are: SPT blows < 20%, and the soil is dominated by fine sand and cohesionless; FC<35% up to 20m depth; shallow groundwater level, GWL<2m; and large of PGA value, PGA>0.09g.The low SPT value and saturated condition resulted in a low CRR value and thus a low FS.Meanwhile, B-2, B-6, B-7, and B-8 do not have the potential for liquefaction even in saturated conditions because the soil conditions are dominated by gravel with SPT blows > 60, so the CRR and FS values obtained will be high.
This proves that the coastal area in Loh Buaya, Rinca Island has a potential liquefaction hazard.Engineering is needed to mitigate the occurrence of liquefaction.Liquefaction mitigation methods need to consider the location and construction costs.Utilizing local resources can reduce the cost of mitigation.Locations close to rock deposits can be utilized to create stone columns or soil reinforcement to increase SPT.

Micro-zonation of Liquefaction Severity Index
The LSI level varies from no liquefaction to very high level (Table 4).Very high LSI located in swamps area (B-1, B-3, B-4, and B-5), while B-2 and B-6 to B-8 are areas that are secure from liquefaction because the FS<1 with land cover types such as mangroves and sea.The very high LSI in B-1, B-3, B-4, and B-5 are due to up to 20 meters of depth has the potential for liquefaction (Table 5).
The FS value obtained is not more than 0.3 in each soil layer.If there are layers that do not have the potential liquefaction (FS > 1), the resulting LSI can be decreased to moderate or low.

Figure 6. LSI mapping using IDW interpolation in QGIS
Micro-zonation mapping is the result of interpolation using IDW method to predict the potential for liquefaction in locations around the SPT boreholes.IDW interpolation is limited to a maximum distance of 50 m from the borehole.The soil condition in the swamp area is dominated by loose sandy soil with GWL < 2 m.By using the PGA value of 0.57 g, the swamp area has high until very high LSI level (Figure 6).No Liquefaction until low of LSI classification outside swamp area.
In this research, there may be errors in predicting liquefaction potential because of limited data and SPT boreholes that spread far from each other.To improve the accuracy of mapping, more soil investigation sites and other interpolation methods can be used.Soil investigations can include SPT, CPT, and microtremor tests.

Conclusions
Based on the analysis to calculate the safety factor of liquefaction potential, some parts of the coastal area in Loh Buaya, Rinca Island, were concluded to have liquefaction potential.Four boreholes have liquefaction potential until 20m depth and four borehole points do not have liquefaction potential.This dissimilarity is obtained due to differences in soil types in each borehole.The result of the Liquefaction Severity Index (LSI) analysis using 8 boreholes showed that the coastal area in Loh Buaya, Rinca Island was classified as very high severity due to liquefaction by 4 boreholes and no liquefaction by the rest.Saturated sandy soil resulted in very high level of LSI.Meanwhile, gravel soils with a very dense consistency remain secure from potential liquefaction.Micro-zonation map based on the LSI classification showed that swamp area with sandy soil has moderate-Very High LSI classification, while outside swamp area with gravel soil has Low-No Liquefaction of LSI classification.Micro-zonation Mapping accuracy can be improved by increasing the number of soil investigation sites.This needs to be a concern to increase awareness when constructing around the coastal area.

Figure 2 .
Figure 2. Map of SPT boreholes location and land cover at the study site

Figure 5 .
Figure 5. Grain size distribution of 5 boreholes at 3.0 m depth from the ground surface.

Table 4 .
Calculation of FS at each borehole at 2, 6, and 10 m depth and LSI

Table 5 .
Recapitulation of factor of safety at 20 meters depth