Evaluasi Kinerja Seismik Pada Bangunan Gedung Dengan Fondasi Spun Pile Terhadap Potensi Likuefaksi Secara Non-Linear Time History Dan Direct Method

Suraida Nur Kumala, Dr. Eng. Ir. Sito Ismanti, S.T., M.Eng., IPM ; Prof. Ir. Iman Satyarno, M.E., Ph.D ; Dr. Eng. Ir. Fikri Faris, S.T., M.Eng., IPM

2026 | Tesis | S2 Teknik Sipil

Bangunan eksisting dengan fungsional sebagai bangunan berkategori risiko tinggi berdiri di atas tanah pasiran NSPT sedang pada kawasan rawan likuefaksi akibat aktivitas Sesar Opak dan zona subduksi selatan Jawa. Evaluasi seismik bangunan eksisting umumnya mengasumsikan tumpuan jepit tanpa memperhitungkan direct method maupun likuefaksi. Penelitian ini bertujuan mengevaluasi potensi likuefaksi berdasarkan Liquefaction Severity Index, menganalisis karakteristik gerakan tanah, membandingkan periode natural struktur fixed base dan direct method, menganalisis respons dinamik tanah dan fondasi spun pile sebelum dan sesudah likuefaksi, serta menilai kelayakan fondasi terhadap SNI 9273:2025 pada level BSE-2E kala ulang 1000 tahun.
Penelitian menggunakan data sekunder SPT tiga titik bor dan shop drawing. Potensi likuefaksi dianalisis dengan prosedur Idriss–Boulanger (2014) untuk memperoleh LPI dan LSI. Struktur atas dimodelkan pada MIDAS Gen, diintegrasikan dengan tanah setebal 50 m dan fondasi spun pile pada MIDAS GTS NX melalui direct method, dengan lapisan rentan likuefaksi dimodelkan UBCSAND. Beban gempa diterapkan sebagai riwayat waktu non-linear dua arah menggunakan tiga rekaman PEER yang diskalakan terhadap spektrum target situs. Hasil dievaluasi terhadap SNI 8460:2017 dan SNI 9273:2025.
Likuefaksi terkonsentrasi pada pasir dangkal 0 – 4 m dengan LPI sangat tinggi namun LSI rendah. Ketiga mekanisme memiliki durasi dan frekuensi berbeda yang menentukan pola pemicuan likuefaksi. Interaksi  memperpanjang periode natural hingga 6,67 kali dibanding fixed base. Rasio tekanan air pori berlebih mencapai hampir 1,0 pada seluruh mekanisme disertai perpindahan lateral transient hingga 19,2 cm dan permanen 5 cm. Setelah likuefaksi, momen dan geser tiang meningkat drastis akibat hilangnya kekangan lateral tanah, sedangkan gaya tarik membesar akibat ayunan tiang. Berdasarkan SNI 9273:2025, fondasi spun pile belum memenuhi kriteria kinerja saat likuefaksi terutama pada megathrust, sehingga diperlukan perbaikan tanah dan/atau perkuatan struktural.

An existing building functioning as a high-risk category facility stands on medium-NSPT sandy soil in an area prone to liquefaction due to the activity of the Opak Fault and the southern Java subduction zone. Seismic evaluation of existing buildings commonly assumes a fixed-base condition without accounting for the direct method or liquefaction. This study aims to evaluate the liquefaction potential based on the Liquefaction Severity Index, analyze the characteristics of the ground motion, compare the structural natural period between the fixed-base and direct-method models, analyze the dynamic soil response and the spun pile foundation before and after liquefaction, and assess the foundation's adequacy against SNI 9273:2025 at the BSE-2E level with a 1000-year return period.
The study used secondary data consisting of SPT results from three borehole points and shop drawings. Liquefaction potential was analyzed using the Idriss–Boulanger (2014) procedure to obtain the LPI and LSI values. The superstructure was modeled in MIDAS Gen and integrated with a 50-m-thick soil model and the spun pile foundation in MIDAS GTS NX through the direct method, with the liquefiable layer modeled using UBCSAND. Seismic loading was applied as a bidirectional non-linear time history analysis using three representative PEER records scaled to the site's target spectrum. The results were evaluated against SNI 8460:2017 and SNI 9273:2025.
Liquefaction was concentrated in the shallow sand layer at 0–4 m, with a very high LPI but a low LSI. The three mechanisms had different strong-motion durations and frequency content, which governed the pattern of liquefaction triggering. The interaction lengthened the natural period by up to 6.67 times compared to the fixed-base model. The excess pore water pressure ratio reached nearly 1.0 across all mechanisms, accompanied by a transient lateral displacement of up to 19.2 cm and a permanent displacement of 5 cm. After liquefaction, the pile moment and shear increased drastically due to the loss of lateral soil confinement, while the tensile force increased due to pile rocking. Based on SNI 9273:2025, the existing spun pile foundation did not meet the performance criteria under liquefaction, particularly for the megathrust mechanism, so soil improvement and/or structural strengthening are required.

Kata Kunci : liquefaction, spun pile, time history, direct method, Midas GTX