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CASE STUDY AND NUMERICAL MODELING FOR LIQUIFACTION OF SOIL WITH THREE POSSIBLE PHENOMENONS CAUSE OF LIQUIFACTION

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CASE STUDY AND NUMERICAL MODELING FOR LIQUIFACTION OF SOIL WITH THREE POSSIBLE PHENOMENONS CAUSE OF LIQUIFACTION
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  Proceeding of SLOPE 2019   September 26 –  27, 2019   1   C SE STUDY ND NUMERIC L MODELING FOR LIQUIF CTION OF SOIL WITH THREE POSSIBLE PHENOMENONS C USE OF LIQUIF CTION ABSTRACT :   An earthquake measuring 7.4 on the Richter Scale has hit Palu in Central Sulawesi. This earthquake was not the first, but until now it was the strongest earthquake. The Central Analysis of Volcanology and Geological Disaster Mitigation (PVMBG) states that a large 7.4 SR earthquake was triggered by the Palu-Koro slip-strike fault activity. Earthquakes that occured cause liquefaction. Liquifaction basically occurs in the loose sand material. The impact of earthquake shocks that occur causes high porewater pressure, high porewater pressure causes a reduction in the effective pressure of the soil so that the soil becomes liquid. One area at the town of Palu which liquifaction occurred massively is Balaroa. There are three initial conditions that may occur at Balaroa liquefaction, the first is the soil having a high water level due to rain, the second is the soil having high porewater pressure due to seepage and the third the soil has a high porewater pressure due to the rupture of aquifer layers. Numerical modeling was tried to model the effect of those initial conditions to see how the phenomenon of the liquefaction occurs. Keywords: fault, earthquake slip-strike, liquefaction, high water pressure, rainfall, seepage, aquifer INTRODUCTION There are a number of initial conditions of soil which liquefied during earthquakes in Palu, among others the first is the initial condition due to the effect of rising water level because of rainfall, the second is the present of seepage from the adjacent hill of the zone of liquefaction and the third is the existence of aquifer which indicate by geoelectric test which then rupture during the earthquake. Base on the layer of soil in Palu where it was dominated by younger sediments of sand and easy to liquify, this numerical study is intended to mimic those initial condition and its contribution to soil liquefaction. GEOLOGICAL CONDITION OF PALU From data submitted by the National Agency for Disaster Management (BNPB), earthquakes have occurred in the Gulf of Palu as long as recorded on December 1st, 1927. The location of Donggala and Palu is at the Palu-Koro Fault and has become a region prone to earthquakes and M.F. Syofyan Erka Konsultan Enjiniringerka.enj@gmail.com P.P. Rahardjo Professor of Engineering FacultyParahyangan Chatolic University Bandungpprahardjo@gmail.com Y.P. Arudia Islamic University of Sultan Agungyosidapermata@gmail.com R. Karlinasari Doctor of Engineering Faculty Sultan Agung Islamic University Semarangrkarlinasari@gmail.com  2   tsunamis. This fault is a fault with the second largest movement in Indonesia after Yapen Fault in West Papua. Palu-Koro Fault movement is up to 46 mm per year. From the google earth topografic map, we can learn that there are a number of streams from hills that is suddenly lost (missing) and dissapeared from the surface. This condition was caused by the previous earthquakes. These conditions means that the flow of water were exsist inside the soil layer below the surface. Figure 1. Slip- strike fault zone under the hills of Palu (Watkinson, I.M and Hall, R.,2016) Figure 2. The flow of the stream were cut off in the Balaroa area. SOIL STRATIFICATION OF PALU Soil investigation post earthquake has been done by Geotechnical Engineering Center (GEC) in several locations at Palu which are affected by the earthquake. The results of soil investigation show the dominance of the sandy soil layer of very soft consistency to medium consistency, with varying thickness (2- 5 meters) below the surface of clay. Soft soil layers - accordance with the results of soil investigation are defined as follows: - The first layer is clay soil with Medium Clay consistency (N SPT 5-8) - The second layer is sand with Loose Sand consistency (N SPT 4-10) - The third layer is sand with Dense Medium consistency (N SPT 25-35) - GWL is -2.7 meters below the ground surface. MODELING Numerical models are made with Geostudio Program SEEP / W, SIGMA /W and QUAKE / W. Parameters - parameters defined by consistency and soil types in Table 1, Table 2 and Table 3 as follows:  3 Table 1. SEEP/W parameters Consistency of soil Saturaturated Permeability (Ksat) [m/day] Volume Water Content at Saturation   (   %    Coef.of Volume Compressibility mv [m 2  /kN]  Medium Clay 2.16E-03 38 1.00E-07 Loose Sand 4.66E+00 30 1.00E-06 Dense Sand 4.66E+00 30 1.00E-06 Tabel 2. SIGMA/W parameters Consistency of soil Modulus of Elasticity E [kN/m 2 ] Cohesi c [ kN/m 2 ] Shear angle [deg] Medium Clay 8.00E+03 18 16 Loose Sand 1.00E+04 4 24 Dense Sand 3.50E+04 4 30 Table 3. QUAKE/W parameters Consistency of soil Modulus of Elasticity E [kN/m 2 ] G Modulus [ kN/m 2 ] Poisson Ratio Damping Ratio Medium Clay 8.00E+03 3.00E+03 0.35 0.1 Loose Sand 1.00E+04 3.75E+03 0.25 0.1 Dense Sand 3.50E+04 1.31E+04 0.35 0.1 I.   Modeling of Saturated Initial Condition due to Rainfall. The impact of rainfall can affect the increase in groundwater level which makes the soil conditions in that location become saturated. In this case numerical modeling is intended to model phenomenon that might occur because of the high intensity of rainfall. The rainfall data that is used are the highest rainfall data in 4 years from 2010 - 2014. Rainfall intensity data was applied to the model by putting hydraulic boundary, using flux units (q). Hydraulic boundary was placed at the top of the soil surface. Rainfall data were from Palu Statistic data from the years of 2010 to 2014.  4   Tabel 3. The Rainfall BPS of Palu 2010 –  2014 Month Curah Hujan [mm] Month Curah Hujan [mm] Januari 137 Juli 41.9 Febuari 34.8 Agustus 119 Maret 33.4 September 30.8 April 42.2 Oktober 29.5 Mei 68.8 November 37.1 Juni 25.6 Desember 105 Rain is simulated to occur within 1 month (744 hour). Porewater pressure during the rain is as follows, Figure 3. Pressure Head at SEEP/W Figure 4. Pore-Water Pressure   SIGMA/W   The Palu earthquake data input is obtained from Time History data with the highest acceleration of 0.75 g (www.iris.edu). The provisions of horizontal seismic analysis of the magnitude of half of the value of the vertical earthquake since the earthquake has characteristics slip -    earthquake strike. Figure 5. Time vs Acceleration for Horizontal   Earthquake wave with peak acceleration at 0.37 g   Figure 6. Time vs Acceleration for Vertical Earthquake wave with peak acceleration of 0.75 g    5 The number of earthquake wave cycles that cause a reduced shear stress ratio and the number of wave cycle ratios that cause the rise in the pressure pressure ratio used to model the loose sand material are as follows: Figure 7 . Cyclic Number vs Shear Stress Function  Figure 8. Cyclic Number vs   Pore Pressure Function From the QUAKE / W analysis using earthquake data in the case of rainfall, it was found that liquefaction were occured in the consistency of loose sand soil. The occurrence of liquefaction in the loose sand layer is due to the increase in pore water pressure and a drastic decrease in effective pressure after the earthquake. The increase of pore water pressure after the earthquake in the simulated layer was 63 kPa. Figure 9. Deformation   from QUAKE/W modelling   Figure 10. Liquefaction zone at the end of earthquake Figure 11. Pore Water Pressure Contour at the end of the earthquake Figure 12. Graph of Increasing Pore Water Pressure in simulated soil

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