Pengaruh Likuefaksi terhadap Stabilitas Underpass

Anky Rakhmat Prasetya, Dr.Eng. Ir. Fikri Faris, S.T., M.Eng.; Prof. Ir. Adam Pamudji Rahardjo, M.Sc., Ph.D., IPM.

2024 | Tesis | S2 TEKNIK PENGELOLAAN BENCANA ALAM

Kulon Progo beralih fungsi menjadi kawasan aerotropolis Bandar Udara Internasional Yogyakarta (YIA). Underpass YIA merupakan infrastruktur jalan berupa struktur terowongan di bawah tanah yang melintas area bandara. Yogyakarta memiliki aktivitas seismik tinggi sehingga rawan gempa bumi, termasuk likuefaksi, akan tetapi data atau informasi kegempaan di wilayah tersebut sangat terbatas. Underpass YIA didesain tahan terhadap likuefaksi dengan skenario periode ulang gempa 1.000 tahun. Namun, hasil observasi lapangan menunjukkan bahwa ketinggian muka air tanah lebih dangkal dibandingkan pada saat perencanaan. Tujuan penelitian ini adalah untuk mengetahui karakteristik dinamis tanah akibat naiknya muka air tanah, evaluasi potensi likuefaksi menggunakan kecepatan gelombang geser hasil pengamatan mikrotremor, dan mengetahui potensi uplift serta mitigasi bencana yang diperlukan.

Metode yang digunakan dalam penelitian adalah melakukan pengukuran mikrotremor dan muka air tanah pada sumur observasi. Data pengukuran mikrotremor diproses menggunakan metode HVSR (Horizontal-to-Vertical Spectral Ratio) dengan bantuan perangkat lunak geopsy dan pedoman yang diberikan oleh Site Effects Assessment Using Ambient Excitations (SESAME).

Hasil penelitian menunjukkan nilai indeks kerentanan seismik (Kg) lebih dari 20 yang mengindikasikan bahwa area penelitian memiliki potensi bahaya seismik yang tinggi. Berdasarkan interpretasi kurva HVSR, area penelitian memiliki frekuensi dominan rendah antara 0.214 hingga 0.289 Hz dan kecepatan gelombang geser yang bervariasi. Titik U01, U04, dan U09 paling rentan terhadap bahaya likuefaksi karena nilai kecepatan gelombang geser ternormalisasi di beberapa lapisan tanah kurang dari 215 m/s. Analisis ini menunjukkan bahwa area penelitian rentan terhadap bahaya gempa karena memiliki lapisan deposit tanah yang tebal dan muka air tanah yang dangkal. Berdasarkan evaluasi faktor keamanan terhadap gaya angkat air, nilai faktor keamanan (FS) saat terjadi gempa adalah 1,43 dan dinilai masih aman. Monitoring fluktuasi muka air tanah merupakan aspek kritis dalam perancangan dan pemeliharaan struktural bagi stabilitas underpass. Untuk memverifikasi hasil pengukuran mikrotremor diperlukan pengeboran dan uji laboratorium karena pengamatan mikrotremor tidak mendapatkan jenis dan sifat fisik tanah.

Kulon Progo has transitioned into an aerotropolis region centered around the Yogyakarta International Airport (YIA). The Underpass of YIA is a subterranean tunnel infrastructure traversing the airport area. The Yogyakarta region is known for its high seismic activity, making it susceptible to earthquakes, including the risks associated with liquefaction; however, seismic data or information for this area is notably scarce. The Underpass of YIA was designed to be resistant to liquefaction based on a 1000-year earthquake return period scenario. Field observations have indicated that the groundwater table level is shallower than anticipated during the planning stages. The objective of this study is to discern the dynamic characteristics of soil attributable to rising groundwater levels, evaluate the potential for liquefaction using shear wave velocity obtained from microtremor observations, and understand the potential for uplift as well as the requisite disaster mitigation strategies.

The methodology employed in this research involved conducting microtremor measurements and groundwater level assessments at observation wells. Microtremor measurement data were processed using the Horizontal-to-Vertical Spectral Ratio (HVSR) method with the assistance of geopsy software, guided by the recommendations of the Site Effects Assessment Using Ambient Excitations (SESAME) protocol.

The results of the study reveal that the seismic vulnerability index values exceeding 20 indicate that the research area possesses a high potential for seismic hazards. Interpretations of the HVSR curve show that the area features low dominant frequencies between 0.214 and 0.289 Hz with variable shear wave velocities. Points U01, U04, and U09 are most susceptible to liquefaction risks, as normalized shear wave velocity values in certain soil layers are less than 215 m/s. This analysis suggests that the research area is vulnerable to earthquake hazards due to the presence of thick soil deposits and a shallow groundwater table. Based on the safety factor evaluation against uplift forces, the safety factor during seismic events is calculated to be 1.44, which is deemed to be within safe limits. Monitoring the fluctuations of the groundwater table is a critical aspect in the design and maintenance of structural stability for underpasses. To verify the results of microtremor measurements, drilling, and laboratory testing are necessary, as microtremor observations do not provide information on the type and physical properties of the soil.

Kata Kunci : Kulon Progo, Indeks kerentanan seismik, Mikrotremor, Likuefaksi, Uplift