Liquefaction Potential Analysis in Terboyo Industrial Park

Terboyo industrial park is one of the most important industrial areas in Semarang City. It is located on the north sea coast of Java, where in fact the land is alluvial soil. The characteristic of alluvial soil which is loose and tends to store much water can cause liquefaction phenomena when it is hit by an earthquake. Moreover, Semarang city is passed by an active fault along 34 km which has the potential to cause an earthquake measuring 6.5 magnitudes. Liquefaction is one geotechnical phenomena that occur due to cyclic loads. It causes the soil to lose its carrying capacity and function like a liquid, so it is very dangerous for the infrastructure above it. The research objective to analyze the possibility of liquefaction in Terboyo industrial park. This research method was begun with collect secondary data in the form of soil data and earthquake data. After collecting data, then the data was processed and analyzed using semi-empirical methods. The data processing produces cyclic stress ratio (CSR) and cyclic resistance ratio (CRR) values which were analyzed for safety factor (SF) as a parameter for the level of liquefaction potential. Then, know the level of liquefaction potential from the Liquefaction Potential Index (LPI) method is the following procedure. Based on the results of the liquefaction potential analysis, the soil in Terboyo Industrial Park is very vulnerable to liquefaction hazards because the LPI value is 23.57.


Introduction
The Terboyo industrial location is one of the busiest regions in Semarang city.It is positioned at the coast of the North Sea.The soil in the Terboyo industrial place is dominated by means of alluvial soil.The characteristics of alluvial soil are that it is unfastened and numerous water in order that it can cause liquefaction phenomena whilst hit with the aid of an earthquake.Moreover, consistent with professionals, the town of Semarang is traversed by means of an energetic fault 34 km long.This fault can reason an earthquake measuring 6.5 SR [1].Liquefaction is a geotechnical phenomenon that happens due to huge earthquake hundreds and lengthy duration.Normally this phenomenon occurs in lost sandy soil and saturated.This liquefaction event reasons the soil to lose its bearing potential and behave like a liquid, making it very dangerous for the infrastructure above it.Therefore, it is necessary to perform an analysis of the potential for liquefaction inside the Terboyo commercial place to determine the vulnerability of the soil to the dangers of liquefaction.

Method
This research was started with the collection data.There are soil data and earthquake data.After accumulating records, they became processed and analyzed the usage of semi-empirical method.From processing the data, Cyclic stress Ratio (CSR) value and Cyclic Resistance Ratio (CRR) value whose safety values had been analyzed or factor of safety (FS) as an indicator parameter of liquefaction had happened.To decide the level of capability liquefaction, then it was analyzed using Liquefaction Potential Index (LPI) method.The description of the research levels above can be summarized in Figure 1.To determine the level of liquefaction potential, the result of the LPI analysis is matched with Table 1.

Result And Disscusion
Based on the geological map, Terboyo Industrial Park was built on quaternary alluvial sedimentary soils.Alluvial sedimentary soil is a sediment that has loose characteristics so that it can cause liquefaction phenomena when an earthquake occurs.Moreover, it is located near the coast, which incidentally has a shallow groundwater table.The Geological Map of Semarang City can be seen in Figure 3.The potential for liquefaction can also be known from soil investigation data, one of which is the NSPT and borlog values.Before analyzing the potential for liquefaction, it is necessary to identify the type of research soil by correlating the NSPT data with the empirical data of soil parameters shown in Table 2, Table 3, and Table 4 Based on Table 2 and Table 3, the type of soils in Terboyo Industrial Park at a depth of 0 to 5 meters is sand with a relatively loose density while Table 2 and Table 4 show that at a depth of more than 5 meters, the soil is clay with a soft consistency.As stated by the method of Seed et al, the liquefaction potential can be found by comparing Liquid Limit (LL) with Plasticity Index (PI).If the LL is less than 37% and the PI is less than 12%, the liquefaction potential is high.If the LL is between 37%-47% and the PI is between 12%-20% then it has the potential to experience liquefaction.If the LL exceeds 47% and the PI exceeds 20%, there is no great potential to experience liquefaction [5].According to the result of the Atterberg limit test, it is known that at a depth of -7.5 m to a depth of -15 m.It has an LL value of less than 47% and a PI value of less than 20% so that there is a risk of liquefaction.This soil identification can show that the soil in Terboyo Industrial Park is less dense and vulnerable to liquefaction hazards if the groundwater is shallow water table and the magnitude of the earthquake at the location is more than 5 Mw.

Calculating CSR
The following is an example of calculating the liquefaction potential analysis at a depth of 2.5 m below the soil surface.Calculate the total stress and effective stress at a depth of -2.5 m, it is known Jsat = 19,9 kN/m 3 and at a depth of -0.8 m there is a groundwater table.Therefore, J' comes from Jsat subtracted by Jw the result is 9,9 kN/m 3 .From the calculation above, the total stress at a depth of -2.5 m is 49.75 kN/m 3 while the effective soil stress is smaller, namely 32.75 kN/m 3 .The effective stress is smaller than the total stress because at a depth of -0.8 m.The presence of water causes pore water pressure in the soil.
The calculation result of the reduced stress coefficient at a depth of 2.5 m from the ground is 0.93 and the deeper the soil layer, the smaller the reduction coefficient (rd) of the subsoil under review.The reduced stress coefficient is a function of depth so that the value of the reduced stress coefficient on the surface is 1 and will decrease with soil depth.
After calculating the value of the reduced stress coefficient to calculate the CSR value, it takes the soil surface acceleration value.In this research, it is used with the help of the PUSKIM website.From the PUSKIM website search results, Terboyo Industrial Park has a PGA value of 0.435 as shown in Figure 4. From some of the data above that is needed in calculating the CSR value, the CSR calculation can be completed as follows.The result of calculating CSR at a depth of 2.5 m below ground is 0.40

Calculating CRR
Based on the N-SPT data, at a depth of 2.5 m underground it has an NSPT value of 5 beats.
= 0,07 From the calculations, the CRR value obtained at a depth of 2.5 m below the soil surface is 0.07.The CRR value is very small because the value of the N-SPT is also small and the position is still close to the soil surface.

Calculating FS
In reference, Semarang city has experienced the largest earthquake of 6.5 Mw, so this largest earthquake data is used for the M value in MSF calculations.
Based on the results of analysis of liquefaction potential, the FS value of the study area at a depth of 2.5 meters below ground is 0.26, which is less than 1, so that depth has a vulnerability to liquefaction.For a summary of the calculation results, see Table 5 and Figure 5. Based on Figure 5, every depth at the research location has the potential for liquefaction.This can be seen from the FS value for each depth which is to the left of the value 1, which means the value is less than 1.According to the provisions, if the FS value is less than 1, it indicates the potential of liquefaction.
The FS value in the research is very small due to the effect of soil conditions.The low N-SPT value of the soil and the relatively shallow groundwater table make the stress or strength value of the soil decrease when it is hit by an earthquake.This is because the soil stress value decreases due to the increase in pore water.The FS value was also influenced by the magnitude value of the earthquake that occurred in Semarang city, which is quite large with value of 6.5 Mw.The bigger the earthquake, the more vulnerable it is to liquefaction.4.4.Calculating LPI LPI is a method that was first delivered by means of Iwasaki (1978) to evaluate the degree of liquefaction potential.LPI used to improve of FS.FS is the very last end result of the liquefaction capability evaluation which handiest indicates the soil layer is liquefied or not.The LPI value is proportional to the thickness of the liquefied soil, the strength of the liquefied soil layer and the safety factor.LPI analysis is calculated to 20 meters from the soil surface.
The following is an example for calculating LPI at a depth of 2.5 m below the soil surface: The LPI result at a depth of 2.5 m below the soil surface is the combined value of the LPI above it.The LPI value depends on the FS value and depth.From the results of LPI calculations for all the depths of the soil layers, it is known that the potential for liquefaction in the research location is very high.This is because the value of the LPI for all depths is 23.57.The result of the LPI calculation can be seen in Table 5.

Conclusion
The soil classification in Terboyo Industrial Park based on the geological map is included to alluvium deposits which have loose characteristics.Meanwhile, based on NSPT data, the soils at the research location are loose sand and soft clay.From the calculation results of the liquefaction potential analysis, the soil in Terboyo Industrial Park has a very high vulnerability to liquefaction if it experiences an earthquake with a magnitude of 6.5.

Figure 4 .
Figure 4. Earthquake Acceleration Spectral Values in Terboyo Industrial Park

Table 1 .
Categories of liquefaction potential based on LPI (Iwasaki, 1984)The research location is in Terboyo industrial park, Terboyo Wetan Village, Genuk District, Semarang City.It is 300 hectares.The Figure of research locations can be seen in Figure2.

Table 2 .
. NSPT data and test results of Atterberg limit of one point in Terboyo Industrial

Table 4 .
Correlation data for clay/loam soils(Terzaghi and Peck, 1967) International Conference on Coastal and Delta Areas

Table 5
Summary of liquefaction potential analysis results