STABILITY OF ALTERED INTRUSIVE ROCK SLOPES IN KULON PROGO MOUNTAINS, YOGYAKARTA SPECIAL REGION, INDONESIA

Hnin Thandar Phyu, Dr. Heru Hendrayana; I Gde Budi Indrawan, Ph.D.

2020 | Disertasi | DOKTOR TEKNIK GEOLOGI

This study was performed to investigate the characteristics of Mt Ijo slopes which consist of altered intrusive andesite rock in Kulon Progo Mountains. The rock slopes were identified in the field, petrographic, XRD, XRF and engineering properties analyses of altered intrusive andesite rock samples in the laboratory, upon which rock genetic classification was determined. The key minerals assemblages determined types of rock hydrothermal alterations identified in XRD analysis. The percentage of primary minerals to secondary minerals determined hydrothermal alteration intensities and rock weathering degrees observed in the point counting method and CIA from XRF analysis results. Rock weathering degrees were determined in the field based on the visual characteristics, the rock uniaxial compressive strength (UCS) from point load index and compression tests in the laboratory. The results showed that the parentrock consists of 6.37 to 74.30% plagioclase, 1.52 to 6.82% quartz, 7.81 to 17.20% hornblende, and 4.81% pyroxene minerals, therefore, was classified as intrusive hornblende andesite. The presence of secondary minerals assemblage (chlorite, calcite and epidote) and (halloysite, montmorillonite and mixed chlorite/smectite) in the andesite rock samples indicated that the parent andesite had undergone propylitic alteration and argillic alteration. Meanwhile, the percentages of primary minerals to secondary minerals assemblages and CIA results indicated that the altered andesite rock had undergone weak to high alteration intensities. The CIA values of the altered andesite rock samples increased 47.41 to 97.01% with the increasing of rock weathering. Based on the field observation,mineralogical, geochemical and engineering characteristics from laboratory results are good agreement, and then the altered andesite rock slopes were divided into the strong andesite rock zone consists of fresh, slightly, moderate weathered andesite and weak zone consists of highly, completely weathered andesite and residual soil by rising rock weathering. The UCS values of fresh andesite has 75.09 to 222.02 MPa (high strength) and moderate weathered andesite has 95.09 MPa that classified very strong rock occurred at depth (~17m). The mean value of mi of the FR andesite rock was 19.30 to 22.81 and the decline in the SW andesite rock which changed 17.14 to 17.66. The UCS value of highly and completely weathered residual soil had 4.78 to 0.41 MPa (very weak). The SF value of cohesion loss CH was 0.93 and 1.07 in the altered andesite rock slope.

This study was performed to investigate the characteristics of Mt Ijo slopes which consist of altered intrusive andesite rock in Kulon Progo Mountains. The rock slopes were identified in the field, petrographic, XRD, XRF and engineering properties analyses of altered intrusive andesite rock samples in the laboratory, upon which rock genetic classification was determined. The key minerals assemblages determined types of rock hydrothermal alterations identified in XRD analysis. The percentage of primary minerals to secondary minerals determined hydrothermal alteration intensities and rock weathering degrees observed in the point counting method and CIA from XRF analysis results. Rock weathering degrees were determined in the field based on the visual characteristics, the rock uniaxial compressive strength (UCS) from point load index and compression tests in the laboratory. The results showed that the parentrock consists of 6.37 to 74.30% plagioclase, 1.52 to 6.82% quartz, 7.81 to 17.20% hornblende, and 4.81% pyroxene minerals, therefore, was classified as intrusive hornblende andesite. The presence of secondary minerals assemblage (chlorite, calcite and epidote) and (halloysite, montmorillonite and mixed chlorite/smectite) in the andesite rock samples indicated that the parent andesite had undergone propylitic alteration and argillic alteration. Meanwhile, the percentages of primary minerals to secondary minerals assemblages and CIA results indicated that the altered andesite rock had undergone weak to high alteration intensities. The CIA values of the altered andesite rock samples increased 47.41 to 97.01% with the increasing of rock weathering. Based on the field observation,mineralogical, geochemical and engineering characteristics from laboratory results are good agreement, and then the altered andesite rock slopes were divided into the strong andesite rock zone consists of fresh, slightly, moderate weathered andesite and weak zone consists of highly, completely weathered andesite and residual soil by rising rock weathering. The UCS values of fresh andesite has 75.09 to 222.02 MPa (high strength) and moderate weathered andesite has 95.09 MPa that classified very strong rock occurred at depth (~17m). The mean value of mi of the FR andesite rock was 19.30 to 22.81 and the decline in the SW andesite rock which changed 17.14 to 17.66. The UCS value of highly and completely weathered residual soil had 4.78 to 0.41 MPa (very weak). The SF value of cohesion loss CH was 0.93 and 1.07 in the altered andesite rock slope.

Kata Kunci : STABILITY OF ALTERED, INTRUSIVE ROCK SLOPES