The decrease of the bearing capacity of single pile foundation during earthquake on liquefiable soil (Study Case: Birobuli Area, South Palu)

Birobuli is located South Palu, Indonesia. In Indonesia, earthquakes are frequent events. Consequently, it is crucial to consider the safety of building structures against earthquakes when calculating the factor of safety of the building. It is important to not only analyse the impact of earthquakes on the superstructure but also take into account their effect on the substructure. One potential substructure failure caused by earthquakes is soil liquefaction, which refers to a decrease in soil bearing capacity resulting from seismic activity. To assess this phenomenon, a quantitative approach was employed in this study, involving soil investigations and sampling from the field. The findings from the soil investigation were used to calculate the susceptibility of soil liquefaction in different soil layers and the axial bearing capacity of single pile foundations both before and during earthquake and liquefaction. The study site predominantly consisted of silty sand with ground water table (GWT) level at -4m with low CPT value, making it susceptible to liquefaction from depths ranging between -4 meters and -12 meters. Two single pile models were simulated in the study, with respective reductions in their axial pile bearing capacity of 71,6% and 38,9%


Introduction
Indonesia experiences a high frequency of earthquake activity due to its location at the convergence of four major tectonic plates: the Eurasian Plate, Indo-Australian Plate, Pacific Plate, and Philippine Plate [1].This geographical situation necessitates the inclusion of seismic effects in structural safety analyses.Apart from causing damage to upper structures, earthquakes can also lead to failures in lower structures or building foundations.Structural and extensive damage to both soil and buildings can result in soil liquefaction [2].Soil liquefaction refers to the loss of strength or load-bearing capacity in soil when subjected to earthquake-induced loads.This phenomenon typically occurs in saturated loose sand during seismic events.Soil experiencing liquefaction undergoes a transformation from a solid to a liquid-like state Instances of liquefaction have occurred in various countries, resulting in significant destruction and loss of life.An example of liquefaction is the event that took place in Palu, North Sulawesi, Indonesia, in 2018 [3].The Indonesian National Board for Disaster Management (BNPB) reported that the earthquake and liquefaction event in Palu led to 2,034 casualties, 671 people missing, and 152 people being buried alive [4].Given the considerable danger posed by soil liquefaction, preventive measures against this disaster must be implemented.Mitigation of severe damage from liquefaction can involve designing earthquake-resistant foundations, taking into account the potential for liquefaction.The design of liquefaction-resistant foundation systems can be achieved by calculating the safety factors for foundation systems in the event of liquefaction occurrences.

Method
The method used in this study is a qualitative descriptive method, where the research was conducted using data collection and analysis methods.The data used were CPT and SPT field soil tests, soil sampling, earthquake data collection, building loads and foundation models.The primary data in this study is soil data obtained by field testing.Secondary data in this study is historical data of earthquakes that occurred.

Research Location
This research conducted in Birobuli area, South Palu, Indonesia.

Research Stages
The research stage of this study is as follows: 1. Conduct CPT soil test, SPT and soil sampling to obtain soil stratigraphy and physical and mechanical parameters of the soil 2. Collect earthquake data history in the nearby area 3. Do the soil susceptibility analysis 4. Calculate single pile bearing capacity before liquefaction and during liquefaction  The analysis of the soil susceptibility to liquefaction is done by the magnitude of earthquake of Mw = 7.5.The result is display as the following drawing:

Soil Test and Liquefaction Susceptibility Analysis
CPT 1 CPT 2

Figure 2. Soil Liquefaction Susceptibility Result From CPT Data
From the analysis, shows that the soil layer is tend to liquefied from depth -4m to -12m.The safety factor on that layer is SF < 1. Soil from -12m and below has high CPT value, thus the factor of safety against liquefaction is high or the soil layer not tend to liquefied.The drawing of the soil layer from the analysis is showed in picture below: From the field data analysis and soil liquefaction calculation, the soil layer will be liquefied from depth -4m to -12m due to low CPT value.Soil layer from 0m to -4m meter is not liquefable because of the ground water table is found on -4m.All layer above -4m is not likely to liquified.Soil layer from -12m and below is not liquefiable due to its high value of CPT and likely the bedrock is already found at those depth.

Single Pile Bearing Capacity Before Liquefaction
For calculating the single pile bearing capacity, 2 type of pile foundation is modelled.First type is floating pile type and second type is end bearing type.The first type, the end of the pile is not reaching the bedrock and the second type pile, the end of the pile is sat on the bedrock.The properties and drawing of the pile are as drawing below:

Single Pile Bearing Capacity During Liquefaction
In this case, to calculate the pile bearing capacity during liquefaction, undrained strength calculation is conducted.The internal angle of friction of the liquefiable soil is 15 [6] , and the effective vertical stress is 0 on those liquefiable soil.From the calculation above, during liquefaction, skin resistance of the pile is mainly decrease because of the soil layer liquified and cannot give the resistance it used to have before liquefaction.Table 3 shows that the decrease of the ultimate pile bearing capacity for type 1 is greater than type 2 pile foundation.This is because on the type 2 pile foundation, the end of the pile is sat on the nonliquefiable soil, so there still resistance from the bottom of the pile for type 2 foundation.On the type IOP Publishing doi:10.1088/1755-1315/1314/1/0121137 1 foundation, the end of the pile is sat on the liquefiable soil, so the end bearing capacity on the type 1 foundation is decrease a lot during liquefaction.

Conclusion
Series of steps is conducted in this study.From this study, the soil in the study area is tend to liquefied from depth of -4m to -12m at Earthquake Magnitude Mw = 7.

Figure 3 .
Figure 3. Soil Liquefaction Susceptibility Result From CPT Data

Table 1 .
Soil field test conducted by CPT and SPT test.In this research, test is conducted in 2 point to obtain soil stratigraphy and the condition of the ground water table (GWT).The soil condition on the research area is sandy soil for 12 meters and ground water table at depth of -4 meter.The soil properties are as follows: Soil

Table 2 .
Foundation properties for input model For type 1 foundation, soil internal angle of friction is assumed 28 and for type 2 is assumed 36.

Table 3 .
Ultimate Pile Bearing Capacity 5. Single pile bearing capacity before liquefaction for type 1 and type 2 respectively are 248 KPa and 1393 KPa.Single pile bearing capacity during liquefaction for type 1 and type 2 respectively are 70,28 KPa and 850,06 KPa.Decrease of Single pile bearing capacity during liquefaction for type 1 and type 2 respectively are 71,6% and 38,9%.The larger of decreased in bearing capacity found in type 1 pile foundation is caused by the end of the pile is sat on the liquefiable soil.