Analisis Potensi Likuefaksi Dan Upaya Mitigasi Pada Pulau Reklamasi Di Utara Jakarta

Muharram Nur Fikri, Artifical Land, Earthquake, Response Spectrum Analysis, Liquefaction, Liquefaction Mitigation, Ground Settlement

2024 | Tesis | S2 Teknik Sipil

Pertambahan jumlah penduduk di Jakarta yang semakin meningkat seiring bertambahnya waktu menjadi alasan pemerintah setempat mengambil kebijakan untuk melakukan pembangunan lahan buatan di pantai utara Jakarta yang dikenal dengan istilah pulau reklamasi. Selama proses perencanaan pembangunan pulau, diketahui lapisan tanah hingga kedalaman tertentu memiliki lapisan pasir lepas yang tebal. Hal ini menyebabkan adanya kerentanan pada wilayah pembangunan, khususnya berkaitan dengan fenomena likuefaksi. Sehingga diperlukan suatu usaha dalam mengidentifikasi, memitigasi dan memetakan sebaran likuefaksi di area penelitian.

Penelitian dimulai dengan interpretasi kondisi lapisan tanah hingga kedalaman 20 m menggunakan data SPT dan borehole yang tersebar di seluruh area pulau. Dalam tahapan identifikasi likuefaksi digunakan data gempa 7,5 Mw Rengasdengklok tahun 2007 serta metode yang disederhanakan sehingga diperoleh nilai faktor aman dan indeks potensi likuefaksi. Selanjutnya, pengelompokkan kelas situs dilakukan dengan membandingkan nilai percepatan puncak tanah dari Peta Sumber dan Bahaya Gempa tahun 2017 dengan perangkat lunak DEEPSOIL v7. Alternatif desain seperti stone column (SC), sand compaction pile (SCP) dan vibroflotation (VF) dipilih sebagai teknik perbaikan yang biasa digunakan dalam perbaikan tanah pada pembangunan pulau reklamasi. Tahapan desain tiap metode perbaikan diberikan batasan hingga nilai faktor aman mencapai 1,2 dengan tujuan untuk menghindari biaya berlebih dalam pelaksanaan proyek. Signifikansi peningkatan nilai faktor aman tanah dievaluasi dengan menghitung tingkat penurunan tanah sebelum dan sesudah diberikan perbaikan. Seluruh hasil analisis kemudian dipetakan dengan menggunakan perangkat QGIS.

Hasil penelitian menunjukkan adanya lapisan tanah kohesif pada kedalaman dangkal tiap lapisan sedangkan lapisan pasir akan ditemukan mulai dari kedalaman 14 m hingga 20 m. Hasil perhitungan dengan SPT dan data PGA menunjukkan wilayah penelitian didominasi oleh tanah lunak dan sebagian lainnya digolongkan sebagai tanah sedang. Hasil simulasi metode satu dimensi non linear memberikan hasil PGA dengan rentang 0,21 g hingga 0,37 g, sedangkan Peta Sumber dan Bahaya Gempa Indonesia tahun 2017 memberikan nilai PGA 0,36 g hingga 0,46 g. Berdasarkan hasil analisis diperoleh rentang nilai faktor aman dan indeks potensi likuefaksi beruturut-turut sebesar 0,57-0,98 dan 0,00-9,86. Perbaikan metode SC memiliki desain diameter seragam 1 m dan jarak antar kolom 1,5-3,5 m. Metode perbaikan SCP didesain memiliki interval 2 m dengan diameter tiang 0,25-0,5 m. Terakhir, teknik perbaikan VF dirancang dengan spesifikasi alat berkekuatan 30 HP (22,4 kW) dengan spasi pemasangan 2,5-3,0 m. Seluruh perhitungan metode perbaikan tanah dilakukan dengan pola segitiga. Akibat peningkatan nilai FS, terjadi pengurangan nilai penurunan tanah signifikan yang sebelumnya 4,67-20,08 cm menjadi 0,81-3,44 cm di seluruh area pembangunan pulau reklamasi. Terakhir, pemetaan lokal digunakan untuk memudahkan dalam memahami seluruh hasil analisis penelitian.

The increasing population in Jakarta over time has prompted the local government to implement policies for the development of artificial land on the northern coast of Jakarta, known as the reclamation islands. During the planning process of the island construction, it was discovered that the soil layers to a certain depth have a thick layer of loose sand. This condition poses vulnerability to the construction area, particularly concerning to liquefaction phenomena. Therefore, efforts are needed to identify, mitigate, and map the distribution of liquefaction in the research area.

The research commenced with the interpretation of soil conditions to a depth of 20 m using Standard Penetration Test (SPT) data and scattered boreholes across the entire island area. In the identification stage of liquefaction, data from the 7.5 Mw Rengasdengklok earthquake in 2007 were utilized along with a simplified method to determine the safety factor and liquefaction potential index. Subsequently, site class categorization was performed by comparing peak ground acceleration values from the 2017 Seismic Hazard and Source Map with DEEPSOIL v7 software. Design alternatives such as stone columns (SC), sand compaction piles (SCP), and vibroflotation (VF) were selected as common ground improvement techniques for reclamation island development. The design stages for each improvement method were constrained until the safety factor reached 1.2 to avoid excessive costs in project implementation. The significance of increasing the soil's safety factor was evaluated by calculating the level of ground settlement before and after improvement. All analysis results were then mapped using QGIS software.

The research results indicate the presence of cohesive soil layers at shallow depths for each layer, while sand layers are found starting from a depth of 14 m to 20 m. Calculations using SPT and Peak Ground Acceleration (PGA) data show that the research area is dominated by soft soil, with some portions classified as medium soil. One-dimensional nonlinear simulation results provide PGA values ranging from 0.21 g to 0.37 g, while the Indonesian Seismic Hazard and Source Map gives a PGA value of 0.36 g to 0.46 g. Based on the analysis, the range of safety factor values and liquefaction potential indices are 0.57-0.98 and 0.00-9.86, respectively. The stone column (SC) improvement method is designed with a uniform diameter of 1 m and column spacing of 1.5-3.5 m. The sand compaction pile (SCP) improvement method is designed with a 2 m interval and pile diameter of 0.25-0.5 m. Finally, the vibroflotation (VF) improvement technique is designed with a 30 HP (22.4 kW) equipment specification and installation spacing of 2.5-3.0 m. All soil improvement method calculations are performed with a triangular pattern. Due to the increase in the safety factor (FS), there is a significant reduction in ground settlement values, previously ranging from 4.67-20.08 cm, reduced to 0.81 and 3.44 cm across the entire reclamation island development area. A local mapping is used to facilitate a comprehensive understanding of all research analysis results.The increasing population in Jakarta over time has prompted the local government to implement policies for the development of artificial land on the northern coast of Jakarta, known as the reclamation islands. During the planning process of the island construction, it was discovered that the soil layers to a certain depth have a thick layer of loose sand. This condition poses vulnerability to the construction area, particularly concerning to liquefaction phenomena. Therefore, efforts are needed to identify, mitigate, and map the distribution of liquefaction in the research area.

The research commenced with the interpretation of soil conditions to a depth of 20 m using Standard Penetration Test (SPT) data and scattered boreholes across the entire island area. In the identification stage of liquefaction, data from the 7.5 Mw Rengasdengklok earthquake in 2007 were utilized along with a simplified method to determine the safety factor and liquefaction potential index. Subsequently, site class categorization was performed by comparing peak ground acceleration values from the 2017 Seismic Hazard and Source Map with DEEPSOIL v7 software. Design alternatives such as stone columns (SC), sand compaction piles (SCP), and vibroflotation (VF) were selected as common ground improvement techniques for reclamation island development. The design stages for each improvement method were constrained until the safety factor reached 1.2 to avoid excessive costs in project implementation. The significance of increasing the soil's safety factor was evaluated by calculating the level of ground settlement before and after improvement. All analysis results were then mapped using QGIS software.

The research results indicate the presence of cohesive soil layers at shallow depths for each layer, while sand layers are found starting from a depth of 14 m to 20 m. Calculations using SPT and Peak Ground Acceleration (PGA) data show that the research area is dominated by soft soil, with some portions classified as medium soil. One-dimensional nonlinear simulation results provide PGA values ranging from 0.21 g to 0.37 g, while the Indonesian Seismic Hazard and Source Map gives a PGA value of 0.36 g to 0.46 g. Based on the analysis, the range of safety factor values and liquefaction potential indices are 0.57-0.98 and 0.00-9.86, respectively. The stone column (SC) improvement method is designed with a uniform diameter of 1 m and column spacing of 1.5-3.5 m. The sand compaction pile (SCP) improvement method is designed with a 2 m interval and pile diameter of 0.25-0.5 m. Finally, the vibroflotation (VF) improvement technique is designed with a 30 HP (22.4 kW) equipment specification and installation spacing of 2.5-3.0 m. All soil improvement method calculations are performed with a triangular pattern. Due to the increase in the safety factor (FS), there is a significant reduction in ground settlement values, previously ranging from 4.67-20.08 cm, reduced to 0.81 and 3.44 cm across the entire reclamation island development area. A local mapping is used to facilitate a comprehensive understanding of all research analysis results.

Kata Kunci : Lahan Buatan, Analisis Respons Spektrum, Likuefaksi, Mitigasi Mikuefaksi, Mikrozonasi