Recommendations of excavation and support systems of Pamukkulu Dam diversion tunnel based on correlation of rock mass classification between RMR and GSI

¹ Department of Geological Engineering, Engineering Faculty, Gadjah Mada University, Yogyakarta, Indonesia.
² Ministry of Public Works and Housing, Jakarta, Indonesia.

^* Corresponding Author: agung_setianto@ugm.ac.id

Abstract

This paper presents the results of geological engineering research conducted to determine the character of rock masses, recommendations of tunnel excavation method and support system based on standup time estimates in unsuported conditions. The investigation was conducted by observing rock mass quality based on the newest bore log sample test results in 2019 using Rock Mass Rating (RMR) and Geological Strength Index (GSI) rock mass classification. The results showed that area consist of lithology in the form of porphyryc lava basalt and pyroclastic volcanic breccia. Rock mass has a slightly weathering alteration rates. Intact rocks have Uniaxial Compressive Strength (UCS) values ranging from 100-250 Mpa to >250 Mpa and are a category of strong rocks. Rock mass has fair to good rock quality class III-II based on RMR values between 53-69, GSI values between 48-64. The roof span required is obtained from the tunnel planning roof span of 10 meter, with a stand-up time of 70 hours without support system and immediate collapse for 5 days. The recommended excavation methods are excavation by drill and blast on top heading and bench: 1,5-3 meter advance in top heading tunnel face, and then can be recommended support system using rock bolts (20 mm diameter, fullly bonded): systematic bolts 4 meter long, spaced 1,5-2 meter in crown and bench with wiremesh in crown then shotcrete: 50-100 mm in crown, and 30 mm in sides, without steel ribs support.