ANALISIS POTENSI LIKUIFAKSI DAN GERAKAN TANAH AKIBAT GEMPABUMI

ABDUL JALIL, Prof. Ir. T. Faisal Fathani, S.T., M.T., Ph.D., IPU., ASEAN.Eng

2022 | Disertasi | DOKTOR TEKNIK SIPIL

Likuifaksi disebabkan oleh guncangan gempabumi yang energinya merambat ke lapisan tanah sehingga memicu eskalasi tekanan air pori berlebih dan massa tanah mencair. Kondisi ini telah mereduksi tegangan efektif, menghasilkan penyebaran lateral dan penurunan tanah. Gempabumi tektonik 9,2 Mw terjadi di Kota Banda Aceh pada tanggal 26 Desember 2004, dan 7,5 Mw terjadi di Kota Palu pada tanggal 28 September 2018. Kedua peristiwa tersebut menghasilkan urutan bencana, seperti kerusakan masif infrastruktur, gerakan tanah, likuifaksi, dan tsunami. Kerusakan yang terjadi di Kota Banda Aceh signifikan karena gempabumi dan tsunami, sedangkan kerusakan di Kota Palu dominan pengaruh gempabumi, likuifaksi, dan tsunami. Lokasi yang sangat terdampak likuifaksi di Kota Palu adalah daerah Balaroa, Petobo, dan Jono Oge. Tujuan penelitian ini adalah untuk memetakan potensi likuifaksi dan gerakan tanah yang terjadi di Kota Banda Aceh dan Kota Palu. Kajian potensi likuifaksi dan gerakan tanah dilakukan dengan pendekatan empiris dan numeris. Pendekatan empiris menggunakan metode Idriss dan Boulanger (2008), sedangkan pendekatan numerik menggunakan program DEEPSOIL dan QUAKE/W. Kedua pendekatan ini menggunakan data N-SPT dan properties tanah, seperti berat jenis, sudut geser dalam tanah, shear wave velocity, level muka air tanah, dan kohesi. Analisis likuifaksi Kota Banda Aceh menggunakan data uji 16 uji penetrasi standar (N-SPT), Sedangkan Kota Palu dengan data 7 situs N-SPT dan 150 stasiun mikrotremor. Dalam pendekatan empiris, indeks kerentanan likuifaksi (LPI) menggunakan pendekatan Iwasaki (1981). Data peak ground acceleration Kota Banda Aceh dan Kota Palu mengacu pada SNI 03-1726-2012 dan SNI 03-1726-2019. Penyusunan peta menggunakan program ArcGIS dengan mengacu pada peta dasar dari DEMNAS dan peta rupa bumi Indonesia (RBI) untuk Kota Banda Aceh dan Palu. Pendekatan numerik menggunakan model analisis nonlinear dan equivalent linear dalam program DEEPSOIL dan tanah dimodelkan dengan generalized/Hyperbolic constitutive (GQ/H+PWP) serta membangkitkan pore water pressure (PWP). Program SEEP/W menggunakan model analisis equivalent linear dan model tanah. Program SEEP/W digunakan untuk menganalisis matrix suction dalam lapisan permukaan tanah. Hasil penelitian di Kota Banda Aceh menunjukkan bahwa zona potensi likuifaksi kerentanan tinggi terjadi di Kecamatan Baiturrahman, Kuta Alam, dan Syiah Kuala. Sementara itu, potensi likuifaksi kerentanan rendah dan sangat rendah tersebar di bagian timur laut, barat, dan selatan Banda Aceh. Hasil penelitian di Kota Palu menunjukkan sebaran nilai regangan geser tanah di Kota Palu berkisar 0,75���10-4 hingga 2,56���10-4. Distribusi lokasi peristiwa likuifaksi berhubungan dengan distribusi nilai regangan geser tanah. Nilai regangan geser tanah yang tinggi ditemukan di lembah Palu. Pada situs dengan regangan geser tanah tinggi dan muka air tanah dangkal mengindikasikan potensi likuifaksi sangat tinggi. Metode empiris menunjukkan likuifaksi dalam (deep liquefaction) sangat potensial terjadi pada kedalaman maksimum 16 m di bawah permukaan tanah secara luas di Kota Palu. Pasir dengan kadar air tinggi dan rentang kepadatan relatif (Dr) antara 0-15% memiliki risiko tinggi likuifaksi. Lereng landai alam Balaroa, Petobo, dan Jono Oge berpotensi likuifaksi menghasilkan aliran lumpur besar seperti banjir bandang akibat gempabumi. Potensi likuifaksi di Balaroa dan Petobo masing-masing memiliki kedalaman 9 m dan 10 m. Sedangkan di Jono Oge, potensi likuifaksi terjadi pada kedalaman 16-17 m, dengan ketinggian muka air tanah 14 m di bawah permukaan tanah. Hasil ini menunjukkan bahwa potensi likuifaksi di Jono Oge dipengaruhi oleh infiltrasi air irigasi. Respons situs Balaroa, Petobo, dan Jono Oge dengan masing-masing faktor amplifikasi sekitar 1,25, 1,71, dan 1,13 kali. Respons tanah Kota Banda Aceh untuk kelas situs SD dan SE masing-masing adalah 1,10 dan 1,06. Faktor amplifikasi untuk kelas situs SD dan SE Kota Banda Aceh sangat sesuai dengan ketentuan yang tercantum dalam SNI-03-1726-2019.

Liquefaction is caused by earthquake shocks containing energy that propagates to the soil layer, triggering an escalation of excess pore water pressure and soil mass to liquefy. This condition reduces the effective stress and generates lateral spread and soil subsidence. In December 26, 2004 and September 28, 2018, tectonic earthquakes of Mw = 9.2 and Mw = 7.5 occurred in the cities of Banda Aceh and Palu, respectively. The two events resulted in a sequence of disasters such as massive infrastructure damage, ground movement, liquefaction, and tsunami. The damage in Banda Aceh City was significant due to earthquakes and tsunami, while Palu City was dominantly affected by earthquakes, liquefaction, and tsunami. Due to liquefaction in Palu City, the most affected locations covered Balaroa, Petobo, and Jono Oge areas. The objective of this study was to map the liquefaction and soil movement potentials in Banda Aceh City and Palu City. This study observed the liquefaction and soil movement potential that occurred in the cities of Banda Aceh and Palu by using the empirical approach of Idriss and Boulanger 2008 methods and numerical approach of DEEPSOIL and QUAKE/W programs. Both approaches used the N-SPT data with soil properties such as density, soil friction angle, shear wave velocity, groundwater level, and cohesion. Sixteen boreholes were used to obtain Standard Penetration Test (SPT) data in Banda Aceh City. Soil data in the Palu City area consisted of 7 boreholes and 150 microtremor stations. Under the empirical approach, Iwasaki�¢ï¿½ï¿½s (1981) was used to obtain the liquefaction potential index (LPI). The seismic hazard maps published by the Indonesian National Standard (SNI) were used to obtain the peak ground acceleration data of the two cities. Mapping was carried out by applying for the ArcGIS program in referring to DEMNAS and administrative maps of respective cities. Nonlinear and linear equivalent analysis models in DEEPSOIL program were used for the numerical approach, while the soil was modeled by using generalized/Hyperbolic constitutive (GQ/H+PWP) and by generating the pore water pressure (PWP). The SEEP/W program used the linear and soil equivalent analysis models. This program analyzed the suction matrix in the subsoil. The research results indicated that zones with a high vulnerability of liquefaction potential in Banda Aceh City included the districts of Baiturrahman, Kuta Alam, and Syiah Kuala. Meanwhile, zones with low and very low vulnerability covered the Northeast, West, and South parts of Banda Aceh. The location distribution of liquefaction events was related to soil shear strain value distribution. Research results in Palu City showed the distribution of soil shear strain values ranged from 0.75���10-4 to 2.56���10-4. High values of soil shear strain were indicated in Palu valley. Such high value and water table exhibited liquefaction in the city of Palu were absolute to occur. Empirical methods ensured deep liquefaction potential to occur widely in Balaroa, Petobo, and Jono Oge at a maximum depth of 16 meters below ground level. Due to loose sand with high water content, the soil became a massive quantity of mud when liquefaction occurred. Then, the slope and shaking of the ground due to the earthquake created a massive mud flow similar to a flash flood. At Balaroa and Petobo sites, the depths of liquefaction potential were 9 m and 10 meters, respectively. With a groundwater level of 14 meters below, the depth of liquefaction potential at Jono Oge was 16-17 meters. These results indicated that liquefaction potential in Jono Oge was influenced by infiltration of irrigation water from the surface, which caused liquefaction on the soil surface. The estimation showed that peak ground acceleration amplification factors for Balaroa, Petobo, and Jono Oge were 1.25, 1.71, and 1.13 times, respectively. In Banda Aceh City, amplification factors predicted for SNI class site SD and SE by equivalent linear model were obtain 1,10 and 1,06 times, respectively. These results have a good agreement with SNI-03-1726-2019.

Kata Kunci : Liquefaction, soil movement, mudflows, ground shear strain, amplification