Breccia for Diversion Tunnel Construction at Bener DAM in Purworejo, Central Java, Indonesia: Fragments, Matrix, RMR

This study aimed to explain the detailed engineering geology investigation which has breccia geological units on Diversion Tunnel at Bener Dam, Purworejo Regency, Central Java Province. The investigation includes geological conditions both breccia fragment and matrix, to Rock Mass Rating (RMR) values each unit. The data were taken from face mapping tunnel with interval 10 meters at inlet 1-390 and 841-865 meters. The findings showed that geological engineering units consisted of Monomict Breccia. The geological units can be divided into geological engineering; Monomict Breccia Matrix (RMR 72-92), Polymict Breccia Matrix (RMR 82), Andesite Fragmen (RMR 52-62), and Tuff Fragmen (RMR 79-84). The discovery of Andesite and Tuff Fragments needs to be handled differently than Monomict Breccia or Polymict Breccia. Andesite Fragment unit with discontinuity of sheeting joint requires rock bolt reinforcement to be installed perpendicular to the discontinuity plane. While the dominant unit is the Tuff Fragment which has a UCS value of 5-7 MPa, it is not recommended to use the blasting excavation method even though it has an RMR value of 79-84.