Analisis Pengaruh Derajat Pelapukan Terhadap Sifat Fisik dan Sifat Mekanik Batuan Andesit Porfiri di Daerah Kokap, Kompleks Pegunungan Kulon Progo, Yogyakarta

Abstrak

Porphyritic andesite in Kokap District, Kulon Progo Mountains, Yogyakarta is a geologically weak material and subject to weathering-related breakdown processes which may affect the stability of slope and strength of geotechnical structure throughout the region. Quantitative studies that established correlations between the weathering degree, microstructural changes and the degradation of mechanical properties of porphyritic andesite from Kokap District are lacking. In this study, quantitative classifications of weathering degree were carried out based on digital petrographic observations in order to measure the abundances of secondary minerals, porosity (void), and Microcracks. Three rocks samples with different weathering degree (ADS-01, ADS-02, and ADS-03) were taken from outcrops along the Kokap District. Petrographic analysis showed increasing amounts of secondary minerals with weathering class from 12.4% (Class I) to 48.2% (Class IV), increase porosity content from 0% to 2.9%, and Microcrack density from 0.2% to 1.2%. Changes in rock microstructure were concluded to play an important role in degrading mechanical properties of porphyritic andesite. Test results of Uniaxial Compressive Strength (UCS) showed decreasing trend of compressive strength with advancing weathering degree. Declines in cohesion and friction angle were also consistently observed with reduction of Geological Strength Index (GSI) and Rock Mass Rating (RMR) value. Natural, saturated and dry unit weight showed a consistent decrease.

Kata kunci

• Pelapukan
• Sifat Fisik
• Sifat mekanik
• andesit porfiri