KARAKTERISTIK MASSA BATUAN FORMASI KEBO-BUTAK DI KECAMATAN GEDANGSARI DAN NGAWEN, KABUPATEN GUNUNGKIDUL, YOGYAKARTA
FATHAN HANIFI MADA M, Ir. I Gde Budi Indrawan, S.T., M.Eng., Ph.D., IPM; Dr. Ir. Sugeng Sapto Surjono, S.T., M.T., IPU., ASEAN Eng
2020 | Tesis | MAGISTER TEKNIK GEOLOGIDaerah penelitian secara administratif berada di Kecamatan Gedangsari dan Ngawen, Kabupaten Gunungkidul, Provinsi Yogyakarta yang tersusun oleh batuan sedimen klastik, volkaniklastik, dan volkanik primer dari Formasi Kebo-Butak bagian bawah dan merupakan formasi tertua di Pegunungan Selatan yang mengalami fase tektonik terbanyak. Lebih lanjut, tektonisme akibat pengangkatan Pegunungan Selatan pada Pleistosen Tengah disusul pelapukan intensif telah memberikan dampak terhadap kualitas massa batuan di daerah penelitian. Tujuan dari penelitian ini adalah untuk melakukan karakterisasi massa batuan pada Formasi Kebo-Butak. Beberapa metode diaplikasikan untuk mengetahui karakteristik massa batuan yang dimulai dengan pembagian litofasies batuan dari stratigrafi terukur dilanjutkan analisis menggunakan metode geological strength index (GSI) dan rock mass rating (RMR) yang didukung analisis petrologi, diskontinuitas, dan laboratorium (sifat indeks dan kuat tekan batuan).Hasil penelitian pada 5 lokasi dengan 28 panel yang tersebar di dusun Bantengwareng, Tegalrejo, dan Tumpang Pondok menunjukkan 9 fasies utama terdiri atas lapillistone, batupasir tufan, perselingan batupasir-tuf dengan sisipan lanau, perselingan batupasir gradasi normal-tuf, batulempung, perselingan batupasir-batulanau dengan sisipan tuf, perselingan batupasir gradasi normal-batupasir tufan, batupasir, dan tuf. Hasil uji kuat tekan menunjukkan kisaran 1,68 MPa hingga 72,33 MPa, sedangkan nilai densitas menunjukkan rentang 2,15-2,66 g/cm3. Hasil analisis GSI menunjukkan bahwa daerah penelitian secara dominan tersusun atas massa batuan kelas II (good) dengan persentase 71,43%. Sementara itu, analisis RMR juga didominasi massa batuan kelas II (good) dengan persentase 67,56%. Korelasi antara 2 metode ini menunjukkan nilai 0,76 (76%) yang tergolong baik. Daerah penelitian mengalami derajat pelapukan dari fresh rock sampai dengan residual soil dan didapatkan empat profil pelapukan dominan dengan interval zona pelapukan yang bervariasi.
The research area is administratively located in Gedangsari and Ngawen Districts, Gunungkidul Regency, Yogyakarta Province predominantly composed of clastic, volcaniclastics, and primary volcanic rocks from the Lower Kebo-Butak Formation which is the oldest formation in Southern Mountain that underwent many tectonic phases compared to other formations in the same physiography. Tectonism particularly due to the Middle-Pleistocene uplift of the Southern Mountain followed by intensive weathering had a significant impact on the rock mass quality of this formation. Therefore, the aim of this research is to characterize the rock mass quality in the Kebo-Butak area. Several methods were applied to determine the rock mass quality starting with the determination of rock facies from measured section method followed by geological strength index (GSI) and rock mass rating (RMR) methods which were supported by petrological analysis, discontinuity analysis, and laboratory analysis (index properties and uniaxial compressive strength).The results of the study at 5 locations with 28 panels in Bantengwareng, Tegalrejo, and Tumpang Pondok exhibited 9 main sedimentary facies consisted of lapillistone, tuffaceous sandstones, interbedding of sandstones-tuff with siltstone intercalation, interbedding of normal-graded sandstones-tuff, claystone, interbedding of sandstones-siltstone with tuff intercalation, interbedding of normal-graded sandstones - tuffaceous sandstones, sandstones, and tuff. The compressive strength test results are approximately between 1.68 MPa to 72.33 MPa (very low-high stregth), while the density value ranged between 2.15-2.66 g / cm3. The results of the GSI analysis indicated predominant class II (good) rock masses with a percentage of 71.43%. Meanwhile, the RMR analysis was also dominated by class II (good) rock mass with a percentage of 67.56%. The correlation between these 2 methods shows a value of 0.76 (76%) which is classified as strongly correlated. Furthermore, the study area experienced the degree of weathering from fresh rock to residual soil that can be summarized by four dominant weathering profiles with varying intervals. In general, Rock facies associated with fine grain size such as tuff, claystone, and siltstone tend to have a lower rock mass quality, which is characterized by short space, long persistance, and a high degree of weathering in the Kebo-Butak Formation.
Kata Kunci : karakterisasi massa batuan, Formasi Kebo-Butak, GSI, RMR, klasifikasi massa batuan, derajat pelapukan
