Designing excavation method and support system of Bintang Bano irrigation tunnel, Sumbawa Barat, Nusa Tenggara Barat based on Rock Mass Rating (RMR)

. This paper presents the characterization of igneous rock masses for the design of tunnel excavation methods and support systems. The subject of this research is the Bintang Bano Irrigation Tunnel located in Sumbawa Barat, Nusa Tenggara Barat. This tunnel was built as a container to channel water potential from the Bintang Bano Dam to the surrounding area. The purpose of this study was to determine the tunnel excavation method and support system during tunnel design and construction and estimate standing time based on rock mass classification. This research was conducted using primary data and secondary data collection was carried out to achieve the research objectives. The results showed that there was only one type of rock unit along the tunnel, namely andesite rock unit. The andesite rock along the tunnel has a uniaxial compressive strength (UCS) with various values, which is between 4.24 – 93.23 MPa with very weak to strong rock classification. Based on the RMR value, the andesite rock in the tunnel appears to be divided into two categories, namely poor rock and fair rock. Suggested excavation techniques and tunnel support systems along the tunnels vary based on rock mass categories calculated using the RMR method.


Introduction
Excavability is the simplicity with which soil and rock can be removed using the best methods.Whether executed on stone or soil media, excavation is a crucial task in construction activities [1].To reduce the potential risks of tunnel construction, it is crucial to use the right excavation techniques and support systems.Tunneling media can be stable when used for construction, one of which depends on the stability of the rock mass [2].
RMR, one of the techniques for classifying rocks by rock mass, was created in 1973 and was initially applied to sedimentary rocks [3].The RMR method was created as a planning method for determining the tunnel support system.RMR was first used to determine the stable condition of the tunnel without support after the tunnel was excavated [4][5].The purpose of this study was to determine the value of rock mass along the Bintang Bano Tunnel based on the RMR method.Need to know that the diameter of the Bintang Bano Tunnel will be 3 meters, and its length will be 1,224 meters.It is situated at Tepas Village in the Nusa Tenggara Barat, Sumbawa Barat (Figure 1).
Several geological surveys have been carried out on the tunnel plan, but the geological mapping has not been done; only rock drilling has been carried out with points along the tunnel [6].Determining rock mass conditions * Corresponding author: dindawahyusetiarini@mail.ugm.ac.id around the tunnel site is essential for tunnel design.Engineering geological investigations of rock masses must be carried out on and below the surface [7].Henceforth, the rock mass classification along the tunnel can be carried out to determine the tunnel excavation support system and strategy.
Engineering rock mass classification has improved their performance in later a long time.This rock mass classification is broadly utilized within the plan and development of rock tunnels.In underground structures that experience complex conditions, much of this is done using the RMR classification approach [1 and 8].At the project's initial design stage, the rock mass classification system assists in identifying rock mass.The composition and important parts of the rock mass are described by one or more rock mass classification methods, however, in order to offer information about early estimations of the rock mass's strength, deformation, and support estimates [9].

Research methodology
Geological mapping is carried out to get data about rock and soil masses, geological structures, and groundwater influencing tunnel construction.The scale used for surface engineering geological mapping is 1:10.000as development activities, including excavation and development materials, ranges from 1:5,000 to 1:25,000 based on accuracy and purpose [11].
The use of RMR in this study was carried out for convenience and adaptability for different practical purposes within the field of building with rock mass classification.This strategy utilizes 6 (six) parameters where estimations can be made within the region and interpretation of drill log data [3], as seen in the example core drill zone with borehole TR1-3 at 120-125 m depth (Figure 3).The variables are 1) UCS of in tact rock material, 2) RQD or Rock Quality Designation, 3) Discontinuity Spacing, 4) Discontinuity Conditions, 5) Groundwater Conditions, and 6) Discontinuity Orientation.[3].Of the six values, then, a determination is made based on the Bieniawski table [3] (Table 1), which is an appropriate design tool for determining the actual behavior of soil and rock [12].The research flow chart is shown in Figure 4.

Result and discussion
Based on the slope map of the research area, the tunnel's location is in a hilly area with the highest peak reaching 205 meters above sea level with a slope around the tunnel development location, which is very steep, extending from 45o to 60o (Figure 5).Field mapping around the research site using a scale of 1:10.000consists of two rock units from oldest to youngest, namely the andesite breccia unit, with sandy tuff, pumiceous tuff, and tuffaceous sandstone, and the andesite rock unit, with volcanic breccia, lahar, tuff, ash, and lava [Figure 6]; with many geological structures found in the form of joints (Figures 7 and 8).However, there is only one rock unit along with the tunnel: the andesite rock unit.Based on drilling hole data, andesite rock units have very weak to strong rock weathering levels.Eight points were drilled throughout the tunnel, resulting in compressive strength (UCS) values (Table 2).
The andesite rock unit along the tunnel has been categorized agreeing to the RMR strategy [3], with rock classification results along the tunnel ranging from poor to fair rock masses (Figure 9).The suggested excavation method for the rock mass classification along the tunnel based on RMR [3] may be shown in table 1 based on the evaluation of each parameter in Table 2 and Bieniawski [3].Lithology and weathering level are two categories that can be determined by the quality of the rock mass.Figure 9 shows the two categories of rock mass quality: Fair Quality Andesite (RMR value 41-60), and Poor Quality Andesite (RMR value 21-40).Based on geological criteria, Table 3 explains the characteristics of each rock unit.The excavation method and the support of the rock tunnels that will be connected are different since it is known in Figure 9 that the RMR values along the tunnel are varied.Based on stand-up time of excavated tunnel [8] for each RMR value under the rock mass characteristics, the stand-up time of the tunnel that was not supported during excavation is for poor rock classification, it is 70 hours and for the fair rock classification, it is 3000 hours (125 days) or no support required.

Conclusion and recommendation
Geological mapping, determining how surface and subsurface rocks have weathered, classifying drilling core rock mass, and laboratory tests are all part of this study.From the discussion, it can be concluded that along with the tunnel construction site, there is only one type of andesite rock unit with a uniaxial compressive strength (UCS) between 4,24 to 93,23 MPa with a very weak to strong rock classification.According to the RMR value, the andesite in the tunnel appears to be divided into two categories: poor rock and fair rock.The excavation technique for the fair rock mass class is heading, benching, and advancing 1.5-3 m.Support work begins after each blast, and it is finished 10 m from the face.With wire mesh and a shotcrete tunnel support system, the poor rock mass class is excavated 10 meters from the front while also making 1 to 1.5 meters of progress in the top heading.The initial design of the tunnel benefits from the use of empirical techniques.Numerical techniques can also be used in study to find the tunnel support system's most effective design.
The Authors would like to thank The Ministry of Public Works and Housing of Indonesia which is gratefully acknowledged by the authors for providing financial support, and so The River Basin Organization Nusa Tenggara I Mataram, PT Nindya-Lestari, KSO, and PT Indra Karya Ciriajasa Parama, KSO, were kind enough to provide the authors permission to perform this study and to share information about the location examination.

Fig 9 .
Fig 9. RMR Value along the Tunnel

Table 1 .
[9]es for the RMR Framework's Excavation Method and Support of Rock Tunnels[9] Fig 7. Andesite and Andesite Breccia Lithology Contact Fig 8. Joints in Surface Andesite Rock Units

Table 3 .
Engineering geological characteristic of the rock mass along the tunnel