Empirical design for the excavation method and support system at tunnel no 2, Bintang Bano irrigation area, West Nusa Tenggara

Bintang Bano Irrigation Area aims to optimize water consumption downstream of the Bintang Bano Dam and distribute it to irrigate agricultural land in Rempe, Seteluk, Seloto, and Senayan villages. Bintang Bano Irrigation Area uses tunnel construction as its network. This research aims to provide recommendations for the tunnel excavation method and support system and predict the tunnel stand-up time based on an empirical method from Rock Mass Rating (RMR89). The research method includes analysis of surface geological mapping and analysis of the core drill along the tunnel to determine the classification of rock masses. Surface engineering geological mapping was conducted to validate the Rock Mass Rating parameters determined by drill core evaluation. The results show the tunnel consists of andesite having poor to good rock masses. The excavation methods for poor to fair rock masses are heading and bench; good rock mass is full face and uses complete support. The tunnel requires bolts, shotcrete, and steel ribs for the support systems. Stand-up time predictions for poor rock mass are less than one day, and fair to good rock is over a month. Although the empirical approach is functional, numerical analysis is needed to verify the empirical design and effect of geological structures for tunnel stability, including tunnel deformation during construction.


Introduction
Engineering design can be obtained using a variety of approaches, including analytical, observational, empirical, and numerical methods.The empirical methods based on rock mass classifications are the most commonly used in geology and geotechnics due to their ability to handle uncertainty [1] [2].Between 1972 and 1973, Z.T. Bieniawski developed Rock Mass Rating (RMR), which underwent numerous modifications until 1989, when it became known as RMR89 [3].RMR89 gives guidelines for characterizing rock masses and recommendations for excavation methods and support system tunnels with parameter ratings [4].
Directorate General of Water Resources plans to construct a tunnel for irrigation networks in Bintang Bano Irrigation Area.Bintang Bano Irrigation Area aims to optimize water consumption downstream of the Bintang Bano Dam and distribute it to irrigate agricultural land in Rempe, Seteluk, Seloto, and Senayan villages.This research will focus on the inlet portal in Bintang Bano Irrigation Tunnel No. 2, 1314 (2024) 012004 IOP Publishing doi:10.1088/1755-1315/1314/1/012004 2 located in Seloto Village, Taliwang District, West Nusa Tenggara Province, Sumbawa Island.The tunnel is designed with a total length of 352.46 m [5].
The stability of the surrounding area will be disturbed during tunnel excavation, potentially leading to landslides or collapse [6].The cause is stress changes, as the stress condition of the rock mass was in equilibrium before excavation.The transformation of the situation eventually results in tunnel deformation.The geological conditions in the area impact the area's stability level.To solve this problem, an appropriate excavation method and support system is necessary.The stability of the rock mass in tunnel design is influenced by various factors, including intact rock strength, Rock Quality Designation (RQD) rating, discontinuity spacing, and discontinuity conditions [7].This research aims to provide recommendations for the tunnel excavation method and support system and predict the tunnel stand-up time based on an empirical method from Rock Mass Rating (RMR89).This research will highlight the importance of surface and subsurface engineering geological mapping and empirical approaches for tunnel design.

Regional geological condition
According to the Sumbawa Sheet regional geological map, the tunnel location is formed by Breccia-Tuff Unit (Tmv), as shown in Figure 2. Andesitic volcanic breccia with tuffaceous sandstone, sandy tuff, and pumiceous tuff form this formation (with local basaltic lavas, andesitic, and lahar).The rock unit is propylitised, mineralized, and silicified.The formation age ranges from Early to Middle Miocene [6].Based on Garwin [8], there are many ancient volcanic domes on Sumbawa Island [9] and the current volcanic on Mount Tambora, Mount Sangeang, and Mount Sangenges.In addition, there are also many faults and lineaments.Around the research location, some faults have a relative North-South trend.Figure 3 shows the lineament has relative North-South and Northeast-Southwest trends.

Surface geological condition analysis
Surface geological data can be collected by observing rocks exposed to the surface at outcrops.Figure 4 displays the boundary area of the surface geological condition analysis.This observation was done to recognize lithology (rock unit), geological structure, geomorphology, weathering degree, and sample collection for Uniaxial Compressive Strength (UCS) [11] and Point Load Testing [12].Surface geological data was collected through detailed geological mapping, scanlines to determine the distribution of discontinuity areas in the field, and data collection on the distribution of weathering degrees on the surface by ISRM [13], taking samples for analysis of the engineering properties of soil and rock which include the index and mechanical properties.

Subsurface geological condition analysis
Data from the drill core along the tunnel was used to observe subsurface geological conditions.The objective is to acquire lithology and identify subsurface rocks using the rock mass rating parameters (RMR 89).

Rock mass rating (RMR89)
RMR89 is a rock mass classification that uses six parameters [3].RMR89 comprises five main parameters: intact rock strength, Rock Quality Designation (RQD) [14], discontinuities spacing, discontinuities condition, and groundwater conditions.In contrast, one parameter is adjustment rating for effect discontinuities strike and dip orientation [3].All the rating parameters are based on the table of the RMR89 rating system proposed by Bieniawski [3].Equation 1 illustrates the Final RMR, resulting from the sum of RMR89 parameter ratings.Based on the geological surface and subsurface observations, this equation describes the rock mass along the tunnel.Figure 6 shows the RMR89 evaluation scheme used in this research.

Figure 6. Scheme for RMR89 evaluation
The intact rock strength is obtained from Point Load [12] and UCS testing [11].Drill core samples are taken based on the rock weathering degree at each drill core depth.Table 1 shows the result of intact rock strength from samples on the drill core locations BP-01 (80 m), BP-02 (35 m), and BP-03 (70 m) at each depth.RQD values are calculated by dividing the total number of pieces over 100 mm (4 inches) in length by the length of the core run [14].Pieces with a dimension of 100 mm or more that are not hard should not be counted.Furthermore, if the rock mass is highly weathered, the RQD is zero [14].The spacing distance between two discontinuities is used to calculate discontinuity spacing.Meanwhile, discontinuity conditions include the length of the discontinuity, the aperture, the roughness, the infilling condition, and the weathering.In this study, the general condition of groundwater is determined based on groundwater elevation on bor log data and the actual site condition.The strike and dip of joints on surface geological mapping around the tunnel area determined the discontinuity orientation.Because determining joint orientation from the drill core is difficult while tunnel excavation is ongoing, plot the strike data in the rosette diagram based on the joint orientation measurements.The radial histogram in the rosette diagram describes the direction of a strike from which the orientation of the main joint can be obtained.Plot the primary joint orientation to the tunnel axis, then make a discontinuity orientation adjustment using the RMR 89 table proposed by Bieniawski [3].
After determining the quality of the rock mass, recommendations for excavation methods and tunnel support systems can be made.Furthermore, using a correlation graph between the maximum unsupported roof span or tunnel width and RMR89 rating [15], the RMR89 classification can predict tunnel stand-up time [3].

Geological condition
The research location on the surface geological mapping shown in Figure 7 consists of alluvial deposits and andesite.Around 79% of the research area comprises andesite; the remaining was an alluvial deposit formed by sedimentation around the Brang Bulu River.Andesite has a greenish-grey color in fresh condition and reddish brown in weathered condition, porphyry aphanitic, holocrystalline, and mineral composition is plagioclase, biotite, quartz, pyrite, and chlorite.Figure 8 describes the geological section along the tunnel result of drill core observation, which is dominated by andesite with some inserted tuff in only a few locations.According to the geological map shown in Figure 7, the geological structures found at the research location are fractures or joints.The joints found were shear and tension, as shown in Figure 9. Shear joints are often found in several outcrops, while tension joints are only found in a few locations.Figure 10 shows the distribution of joint orientation in this research location based on joint strike data in a rosette diagram.According to the measurements of joint orientation, it was found that the strike of the joint was mainly found in Northeast -Southwest (NE-SW) relative direction.

Rock mass characterization of drill core
The quality of the rock mass is assessed in four drill cores along the tunnel using RMR89 parameters.Table 2 shows the evaluation of RMR89 parameters from drill core B. PND-03 (depth 15 m -19 m) demonstrates good rock mass quality (Final RMR89 rating = 68).According to the plotted results, the primary joint orientation to the tunnel axis is perpendicular to the tunnel axis and drives against dip with dip 45-90.With a rating of -5, the qualitative rating adjustment value is fair.Then, the RMR89 parameters are assessed using Table 3 for each drill core along tunnel depth.As shown in Figure 11, every depth with the same rock mass quality is correlated with other drill cores as the interpretation of rock mass quality between drill core locations areas.The rock mass quality in the tunnel opening area must be emphasized to determine the most suitable excavation method and tunnel support system.The tunnel is divided into sections based on the rock mass quality that was previously determined.Furthermore, each drill core's representation determines each section's rock mass quality assessment.Section 1 is represented by drill core B.PND-01, Section 2 by drill core B.PND-03, and Section 3 by drill core B.PND-04 for this research.Table 3 captures the quality of the rock mass along each tunnel section.
Table 3. Rock mass quality along the tunnel Figure 11 shows the rock mass quality cross-section along the tunnel.The rock mass along the tunnel consists of Section 1 having a domination of poor rock mass, Section 2 having a good rock mass, and Section 3 having a fair rock mass.

Excavation method, support system, and stand-up time
The RMR89 empirical method could be used to estimate the excavation method, support system, and stand-up time depending on the rock mass quality along the tunnel, as shown in Table 3. Table 4 explains the tunnel excavation method recommendation, which can be used in any tunnel section.Poor rock mass excavation is top heading and bench with 1,0 to 1,5 m advance in the top heading; good rock mass excavation is full face with 1,0 to 1,5 m advance; and fair rock mass excavation is heading and bench with 1,5 to 3 m advance in heading.Table 5 describes the recommended support system for each tunnel section based on rock mass quality.Rock bolts, shotcrete, and steel sets are 3 (three) principal components used to support the tunnel.The tunnel support system for the poor rock mass consists of systematic bolts 4 to 5 m long, 100 -150 mm shotcrete in the crown, and 100 mm insides.The good rock mass consists of 3 m long systematic bolts with 50 mm thick shotcrete in the crown as needed, whereas the fair rock mass is 4 m long systematic bolts with 50 -100 mm shotcrete in the crown and 30 mm insides.The unsupported roof span or tunnel width determines the stand-up time prediction parameter.The tunnel has an average width of 4 m.As a result, the stand-up time is predicted using the graph from the relationship between unsupported roof span and RMR89 rating, as shown in Figure 12.Section 1 has a poor-quality rock mass and a stand-up time of 2x10 1 hours.Section 2 has a stand-up time of 3x10 3 hours, and Section 3 has a stand-up time of 4x10 3 hours.Based on these results, it can be summarized that stand-up time predictions for poor rock mass are less than one day, and fair to good rock is over a month.

Conclusion
The lithology of the research location consists of andesite and alluvial deposit.Meanwhile, andesite is dominated by some inserted alluvial deposits in a few locations.The geological structures found at the research location were tension and shear joints.According to drill core evaluation, the tunnel consists of andesite rocks and some tuff with poor to good rock masses.Poor rock mass excavation is top heading and bench with 1,0 to 1,5 m advance in the top heading; good rock mass excavation is full face with 1,0 to 1,5 m advance; and fair rock mass excavation is heading and bench with 1,5 to 3 m advance in heading.The tunnel support system for the poor rock mass consists of systematic bolts 4 to 5 m long, 100 -150 mm shotcrete in the crown, and 100 mm insides.The good rock mass consists of 3 m long systematic bolts with 50 mm thick shotcrete in the crown as needed, whereas the fair rock mass is 4 m long systematic bolts with 50 -100 mm shotcrete in the crown and 30 mm insides.Stand-up time predictions for poor rock mass are less than one day, and fair to good rock is over a month.Although the empirical approach is functional, numerical analysis is needed to verify the empirical design and effect of geological structures for tunnel stability, including tunnel deformation during construction.

Figure 7 .
Figure 7. Geological map of tunnel

Figure 8 .
Figure 8. Geological section along the tunnel

Figure 12 .
Figure 12.Stand-up time prediction of tunnel

Table 1 .
Result of intact rock strength testing from borehole location along tunnel

Table 2 .
Assessment of drill core B.PND-03 based on RMR89

Table 4 .
The recommendation for the excavation method based on RMR89

Table 5 .
The recommendation of a support system tunnel based on RMR89