Liquefaction potential analysis based on standard penetration test data at irrigation canals in Sibowi Area-Central Sulawesi Province

A phenomenon in which non-cohesive soil in saturated conditions loses its carrying capacity owing to the reduced stiffness and strength of the soil during earthquake shaking or rapid loading is called liquefaction. This occurs under the structure and can cause damage during earthquakes. In this study, we estimated the possibility of liquefaction in the main irrigation canals in the Sibowi area of Central Sulawesi Province, considering that the soil type in this area is sandy. Liquefaction analysis uses a simplified procedure using Standard Penetration Test data to obtain the Safety Factor and estimate the potential level of liquefaction using the Liquid Potential Index (LPI) method. According to the analysis, with the 7.5 Mw earthquake and groundwater levels observed, most of the soil layer below the irrigation canals still has liquefaction potential at 15 m to 19.5 m. The LPI result that 5 (five) out of 10 (ten) boreholes have the potential for liquefaction in low to moderate categories, and the boreholes that have liquefaction potential have groundwater depths between 14 and 16 m.


Introduction
Many countries worldwide, including Indonesia, have a high risk of earthquakes because Indonesia's geographical position is on an active tectonic plate, which is a meeting between the Sunda and Indo-Australian plates.Apart from volcanic activity, the source of earthquakes in Indonesia also originates from subduction sona and active faults on land.An earthquake with a destructive impact occurred in the city of Palu and its surroundings in September 2018.The 7.5 Mw magnitude earthquake triggered as a result of the activity of the Palu-Koro fault is located on land, precisely in Lompio village, Donggala Regency, at a depth of 10 km [1].This shallow tectonic earthquake also triggered three secondary disasters: tsunamis on the coast of Palu Bay, liquefaction, and massive mudflows [2,3].In the Sigi Regency area, the earthquake was followed by large-scale soil flows caused by liquefaction in several locations, such as densely populated areas in Petobo Village, Jono Oge Agricultural Area, and Sibalaya Area, which also damaged the irrigation canal infrastructure and regional roads [4].
The Indonesian government conducted post-disaster rehabilitation and reconstruction in Central Sulawesi following the damage caused by the 2018 earthquake.The research site was in the Gumbasa irrigation main canal section in Sibowi villages, as shown in Figure 1.The condition of the irrigation canal infrastructure at Sibowi after the 2018 earthquake is presented in Figure 2. Liquefaction is the 1314 (2024) 012035 IOP Publishing doi:10.1088/1755-1315/1314/1/012035 2 condition of changing the condition of non-cohesive soil in a loosely packed soil to become more liquid due to an increase in pore water pressure, whose value becomes equal to the total pressure due to the occurrence of rapid or cyclic loads, so that the soil loses the ability to support the load [5].Many factors affect the liquefaction in the subsoil caused by earthquakes.Based on many studies, Seismic parameters, soil characteristics, and site conditions are key factors affecting liquefaction due to earthquakes [6].
Previous studies on several segments of irrigation canals affected by liquefaction concluded that factors such as geological site conditions, amplitude, moment magnitude, groundwater table, and soil properties significantly affect liquefaction susceptibility.Petobo and Jono Oge at the downstream of the Sibowi segment irrigation canals tend to have shallow groundwater levels between 1.0 meter to 3.0 meters, with the dominant soil condition being sand from alluvial deposits, and this area has the potential to experience liquefaction at depths between 6-10 m with peak soil acceleration (PGA) occurring at 0.25 g in the Petobo area and 0.4 g in the Jono Oge area [7,8].
This study aims to determine whether the main irrigation canal in the Sibowi village segment still has liquefaction vulnerability.To determine this, safety factors were calculated based on the soil resistance ratio required to withstand liquefaction to loads caused by earthquakes.This calculation used seismic data and N-SPT test results.The results of the safety factor calculation were then used to estimate the LPI scale and were presented in the form of a microzonation map.The results of this study are expected to contribute to anticipating potential disasters and as a consideration in determining effective mitigation for the ongoing reconstruction process.

Material and Methods
Soil investigations and laboratory analysis, soil sample drilling, and N-SPT tests were conducted by ministry of public works and housing after the earthquake [9], and were used as data in this study.There were 10 (ten) drilling points at Sibowi Village.The layout of boreholes data distribution in Sibowi presented in Figure 3, where the drilling point distribution is lengthwise in the direction of the main canals.Based on monitoring, the groundwater level elevation at the drilling point ranged from 14 m to

Geological Condition
Information about geological formations is essential to determine the nature and geological characteristics of possible liquefaction risks in an area.The regional geological conditions in the Sigi Regency area are dominated by Quaternary deposits, namely the Pakuli Formation (Qp), which consists of unconsolidated sedimentary rocks and alluvium formations (Qa), which range from sand-shaped river deposits to clay [10,11].
Deposits consisting of clay, silt, and sand in the area were formed by alluvial deposits at lower elevations in the central area of the valley closer to the Palu River.Surface sediments are deposits of young and old alluvium fans on hills extending west and west-east of Palu Valley [9].
Youd and Perkins (1978) classified the liquefaction vulnerability indices into continental zones [12].Based on this classification, the geological characteristics in the Sigi Regency area, especially in the Palu Valley area, are types of deposits originating from alluvial fans with deposit ages with moderate to high liquefaction vulnerability categories.This empirical approach based on geological aspects is still general, and the determination of liquefaction potential is more accurately carried out empirically using a safety factor calculation analysis.

Maximum Horizontal Acceleration at Ground Surface (PGAM) and Moment Magnitude (MW)
The Peak Ground Acceleration (PGA) value is determined using help from a web-based application, Indonesia Design Response Spectrum 2021[13], accessed at the https://rsa.ciptakarya.pu.go.id/2021 site address.This application requires the input of the coordinates of each borehole and site-class information.Referring to the applicable standards in Indonesia regulated at SNI 1726:2019, the PGA value in the bedrock must be corrected using the appropriate site class coefficient [14].
The analysis was limited to data from soil investigations of 10 (ten) boreholes scattered along the main canals in the Sibowi area.The results of groundwater level monitoring in each borehole in November 2021 after the 2018 earthquake showed that the groundwater table elevation varied from 14 m to 16 m.The 2018 Palu-Donggala earthquake, with a magnitude of 7.5 Mw was used as the earthquake parameter in the analysis [15].Idriss and Boulanger (2008) made adjustments by correcting the N-SPT value based on the parameters of the test equipment [17].CRR is a parameter that represents the liquefaction resistance of soil deposits.The sensitivity of soil liquefaction was demonstrated by the fact that the CSR value of each sediment deposit was higher than its CRR value.The CRR value can be calculated using Equation (1) CSR is the ratio between the maximum shear stress due to cyclic load and the vertical effective stress of the ground.CSR values can be calculated using equation ( 5) amax is the maximum horizontal acceleration at ground level due to earthquakes, σ v are total stress, σ′ vc are effective stress, and rd are stress reduction coefficient factors.The Equation is then updated by adding an overburden correction factor (K σ ) and adjustments to the MSF scale [5,17].Such additional parameters are calculated using Equation (6) After each CRR and CSR value at each depth is calculated, the safety factor (SF) of each layer is determined by comparing the CRR value to the CSR value.If the CSR value has a higher tendency than the CSR value, the soil shear stress that occurs is greater than the vertical effective stress of the soil that resists liquefaction.The safety factor (SF) value was calculated based on Equation (15).

Liquefaction Susceptibility Microzonation using Liquefaction Potential Index (LPI)
Microzonation is a technique for breaking up a large zone into smaller zones, with criteria determined based on the purpose of zoning itself.The liquefaction zone in this study was spatially mapped based on the LPI values.The LPI method is used to evaluate the level of liquefaction potential in each soil layer based on the safety number, which is directly proportional to the thickness of the layer undergoing liquefaction [18].This Equation gives a depth limit of up to 20 m because the given function gives an index value that tends to shrink close to zero at depths up to 20 m and a higher value closer to the surface.Calculation of the LPI index using Equation ( 16) with conditions and criteria in Equation ( 17) up to Equation (20).
W(z) = 10-0.5zfor z ≤ 20 m The value of (z) is the depth of the soil layer, and the value (FS) is the safety number based on CRR and CSR calculations.The division of vulnerability categories proposed by Iwasaki (1981) states that the probability of liquefaction is very high at index values above 15, and index values below 5 are improbable for liquefaction.Based on these conditions, Sonmez (2003)  when analyzing the soil liquefaction potential [19].Equation ( 16) was then tested and verified for the liquefaction cases in Turkey.The criteria in Table 2 proposed by Sonmez are more acceptable, based on statistics and validation with field data from several earthquakes in Turkey.In this study, we adapted these criteria to map the liquefaction vulnerability index in the Sibowi area.

Result and Discussions
The results of SPT testing on each borehole (SIB.01 to SIB.10) are shown in Figures 4 and 5.There are 4 (four) drill holes with drilling depths of up to 15 meters and 6 (six) boreholes with drilling depths of up to 19.50 meters.Boreholes SIB.02, SIB.03, and SIB.04 in Figure 4 show a tendency for N-SPT values of less than 30 blows to a depth of 20 m.These results show a tendency of similarity in soil characteristics with previous studies in the downstream location at Petobo segment, where layers that experience liquefaction show N-SPT values ranging from to 7-25 blows on non-cohesive soils [8].
Groundwater level elevation on the Sibowi section tends to be deeper, between 14 m to 16 m after the earthquake.The calculation results using the simplified procedure method to determine the safety number (SF) at the SIB.02 point with a groundwater level elevation of 14 m and an earthquake moment magnitude Mw = 7.5 are described in Table 3.The soil layer in SIB.02, which underwent liquefaction, was at a depth of 15 m and between 18 m and 19.50 m.This layer is below the groundwater table and is classified as non-cohesive soil with an FC content below 10%.The graph presents the SF value at each borehole depth by comparing the CRR and CSR values described in Figure 6 using Eq.(15).The liquefaction layer with thickness varies between 3.0 m to 4.50 m at 5 (five) boreholes, namely SIB.02, SIB.03, SIB.04, SIB.08, and SIB.09.The SIB.01, SIB.05, SIB.06, and SIB.07 borehole points do not experience liquefaction because the depth of the groundwater table is greater than the test depth, so the soil layer is assumed to be dry (unsaturated), although the characteristics of sandy soil in the layer show indications of liquefaction when it is in a saturated condition, assuming an increase in groundwater levels.SIB.02 is the borehole point with the most significant layer thickness that experiences liquefaction, which is 4.50 m.Point SIB.03 has a critical SF value of 1.10 points at a depth below the surface of 16.00 m, and SIB.04 has a critical SF value of 1.09 points at a depth below the surface of 18.00 m.The next step in this analysis after obtaining the FS value is to calculate the level of liquefaction potential using the LPI scale and make a micro zoning mapping of the study area by interpolating the LPI value at each borehole point and interpreting the category of potential liquefaction levels on the map using mapping software Table 4.As a recap of the calculation of the LPI value at each depth that undergoes liquefaction and a description of the vulnerability level category, the SIB.02 point has an LPI index of 4.10 on a moderate scale because the thicker the layer that undergoes liquefaction, the higher the LPI index.The LPI scale is representative and is used to assess the level of potential liquefaction in an area because it can group the level of vulnerability into each category, facilitate data analysis, and clarify the information contained in the data [20].Micro-zonation map of liquefaction vulnerability levels based on the calculation of the LPI values in the main irrigation canalss in the Sibowi area described in Figure 7. Based on the analysis of safety factors on liquefaction and the results of potential level analysis using the LPI method, the irrigation infrastructure in the Sibowi section is still potentially liquefied at several review points in the area around SIB.02, SIB.03, and SIB.04, shown in green and yellow, have liquefaction potential with a low to moderate vulnerability level category at points around SIB.08, and SIB.09 also shows the same analysis results.This area has a low-to-moderate vulnerability level, as shown on the map in green and yellow.

Conclusion
Based on this study, sandy soils from sediment formations dominate soil characteristics at the Sibowi irrigation project site.Groundwater level elevation based on post-earthquake observations at depths of 14-16 m indicates a layer of sandy soil that is approximately 4-6 m thick under saturated conditions.Sandy soils with low resistance, as indicated by N-SPT values below 30 strokes, have higher liquefaction susceptibility.This area is at SIB.02, SIB.04, and SIB.09, with moderate levels in the LPI category.
In this study, the safety factor (FS) values at five drill points (SIB.02,SIB03, SIB.04, SIB.08, and SIB.09) under the requirements of FS<1.0 at depths varying between 15 meters to 20 meters indicate that the area along this main canals is categorized as an area still potentially liquefaction in the 7.5 Mw earthquake and PGA value of 0.7g.However, the earthquake intensity and maximum soil acceleration are not the main factors that cause liquefaction, but the groundwater level and soil characteristics below it are essential factors that determine the liquefaction potential.Therefore, to prevent the possibility of future disasters due to liquefaction, the effect of liquefaction needs to be considered in mitigating the resilience of infrastructure built in this area to potential liquefaction that may occur in the future.

Figure 3 .
Figure 3.The layout of boreholes data distribution in Sibowi.
modified the liquefaction vulnerability index criteria because the range of safety factor values from 1.0 to 1.2 should be considered IOP Publishing doi:10.1088/1755-1315/1314/1/0120356

Figure 4 . 5 .Figure 5 .
Figure 4. SPT test results and FC content (%) in each soil layer in borehole SIB.01 (a), SIB.02 (b), SIB.03 (c), SIB.04 (d), and SIB.05 (e).The N-SPT value at the SIB.09 point ranges from 11-16 to blows to a depth of 20 m, as shown in Figure 5.With different results in the SIB.06,SIB.07, SIB.10 boreholes; at these three points, the N-SPT value at depths above 10 m tends to increase to a depth of 15 m and 20 m with an N-SPT value of more than 60 blows, indicating that the soil layer at this depth is more rigid

Figure 7 .
Figure 7. Liquefaction potential microzonation map in Sibowi according to LPI categories

Table 1
presents the PGAM values, groundwater table elevations, and site classes used in the analysis.
[16]Empirical Analysis of Liquefaction Based on SPT DataThe Simplified Procedure method is one of the methods used to determine the value of soil shear strength based on site soil investigation data, such as SPT data.This method considers the soil depth, N value (number of blows SPT), and correction factor.SPT is a soil investigation method used to determine soil strength and density.The Simplified Procedure method is based on a comparison of the ground Cyclic Resistance Ratio (CRR) and Cyclic Stress Ratio (CSR) owing to seismic shock.The CRR value was directly obtained from the soil strength test data at the test point.The Equation for calculating the CRR value was proposed by Seed andIdriss (1970)based on the N-SPT value obtained from direct testing[16].

Table 3 .
Safety Factor of Liquefaction Calculation based on SPT at Borehole SIB.02 at GWL=14 m and Earthquake Magnitude 7.5 Mw.

Table 4 .
LPI category for 7.5 Mw Earthquake at the Sibowi.