Recommendation of excavation method and support system based on rock mass classification of the Matenggeng Dam Tunnel, Cilacap, Central Java

The Matenggeng Dam is situated in Matenggeng village, Dayeuhluhur, Cilacap district, Central Java. It features a diversion channel equipped with a tunnel that has a diameter of five meters and is formed like a horseshoe. Therefore required engineering geological analysis for tunnel planning. The purpose of this study is to ascertain the tunneling technique and the corresponding support systems. This study will provide suggestions and input before and during the construction. This research uses rock mass classification and the Point Load Test for several samples. Tunneling method and support systems using empirical method Rock Mass Rating (RMR) and Geological Strength Index (GSI). The study location is composed of sandstone, conglomerate, andesite, and siltstone with alluvium deposits in Cijolang River area. Classification of rock masses is fair to good rock. The GSI and RMR analyses suggest that the excavation technique uses hammer and blasting methodologies and a max stand-up time without support of 20 to 144 hours. The tunneling method uses top heading and bench and support systems using rock bolts, wire mesh, shotcrete, and steel sets.


Introduction
Dams are constructed structures comprised of landfills, rock piles, and concrete.Their primary purpose is to store and regulate water, but they can also be utilized for the containment of mining waste or clay, resulting in the formation of reservoirs [1].Within the context of dam construction, there exist various ancillary structures, among which is the river diversion facility.The primary purpose of this structure is to divert the channeling the river's current to aid in the construction of the dam.There are other classifications of elusive structures, including tunnel-based elusive constructions, particularly diversion tunnels.The Matenggeng Dam is situated in Matenggeng Village, in the Dayeuhluhur District of the Cilacap Regency, which is part of the Central Java Province.
According to a detailed study of the design of the Matenggeng Dam phase II [2], it is evident that the Matenggeng Dam serves multiple purposes in facilitating the advancement of irrigation (both intensification and extensification), hydroelectric power generation (PLTA), flood mitigation, provision of raw water for potable purposes, support for fisheries, promotion of tourism, and various other functions.The Matenggeng Dam is proposed to be constructed as a closed channel structure, specifically utilizing a horseshoe-shaped design with a length of 705 meters and a width of 5 meters.The 1311 (2024) 012072 IOP Publishing doi:10.1088/1755-1315/1311/1/012072 2 Matenggeng Dam is proposed to be constructed over the Cijolang River, spanning around 807.65 meters in length, with a dam height of approximately 117.00 meters above the lowest point of excavation.
The presence of a structure at the designated location of the Matenggeng Dam has the potential to significantly impact its construction and subsequent operational processes.According to research [3], the geological features observed at the location consist of two dextral shear faults oriented in a northeastsouthwest direction, as well as two sinistral shear faults oriented in a northwest-southeast direction.A different kind of rock structure encountered in geological formations is shear joints, which have the potential for increased strength.The presence of a geological structure in the form of a fault or fault is a movement zone that is frequently employed as an erosional weakening line that gives rise to linear topographical features [4].The study location is at the border of West Java and Central Java, specifically at Kaso Village, Ciamis Regency, and Matenggeng Village, Cilacap Regency.The area scope is 2.25 km 2 (1.5 km x 1.5 km) as shown in Figure 1.

Geological condition
Taken on the geological map of the Majenang sheet in Java [5], the study area is situated within the Tapak Formation (Tpt).This formation mostly consists of coarse-grained sandstone with a greenishgrey coloration.The presence of molluscs inhabiting saline environments provides evidence that the calcareous sandstone including marl deposits can be attributed to the early to middle Pliocene period.The width of the unit measures 500 meters.The results obtained from the study of surface rocks contain sandstone, conglomerate, andesite, and siltstone, with alluvium deposits found in the floodplain of the Cijolang River as shown in Figure 2. The geological structures identified throughout the mapping process consist of normal faults and dextral shear faults.

Methodology
A. Rock mass rating (RMR) Rock Mass Rating (RMR) classification method was established in 1989 [6].This method uses six parameters.The parameters of interest in this method are the Uniaxial Compressive Strength (UCS) of intact rock material, the Rock Quality Designation (RQD), the spacing of cracks/discontinuities, the condition of cracks/discontinuities, and the groundwater condition (flow rate per 10m, ratio, and general conditions).There is also a weighted adjustment parameter from the adjustment to the orientation discontinuity.The parameters in the study are determined through the analysis of field data, which includes the use of borehole data.Each parameter is associated with a specific computation and rating [6].Table 1 displays the state of discontinuities and ratings.The assessment of underlying rocks through core drilling results is conducted using a quantitative approach.GSI measurements necessitate the utilization of a standardized measurement of rock strength in the field, which is assessed by the Rock Quality Designation (RQD).This assessment serves to represent the strength of the rock mass and the current conditions of its joints, the surface conditions of the rock are indicated by these factors [7].This relationship can be mathematically expressed using the following equation: (1) Figure 3. GSI estimation value [7] Figure 3 represents an estimated GSI value, which pertains to the continuity requirements as defined before [8], as illustrated in Table 1.Table 2 presents the rock mass classifications according to the Geological Strength Index (GSI).The Rock Quality Designation (RQD) is a technique employed to forecast the quality of a rock mass based on data obtained from drilling results.RQD is calculated by dividing the length of the core piece that is longer than 10 centimeters by the total length of the core collected during drilling.Below is the equation for the RQD:

C. Point load test
The Point Load Test is a laboratory test conducted on intact rock samples, where pressure is applied at one point until the specimen breaks failure.The ease and efficiency of the testing equipment make it highly useful for field applications, enabling accurate predictions of rock strength.The equation for determining the Index Point Load (Is) of an identified sample is as follows [10]: ‫ܫ‬ ௦ = Point load strength index (Index Franklin) P = The maximum load at which the sample fractures D = The spatial separation between the two compression cones The point load test can be utilized as an estimation for the Uniaxial Compression Strength (UCS) [11].This can be achieved by employing the following equation:

E. Excavation and support systems
The determination of excavation and support systems in rock tunnels, specifically in the diversion tunnel, is based on the lowest value of the RMR at each borehole site.In this case, the tunnel is assumed to have a horseshoe-shaped cross-section with a roof span width of 10 meters.The vertical stress is expected to be below 25 Mpa.Table 3 presents the guidelines for excavation and support systems.

Result and discussions
This study involved the drilling of three boreholes within the surrounding area of the diversion tunnel.The depths of the boreholes are as follows: BMT-1 has a depth of 55 meters, BMT-3 has a depth of 110 meters, and BMT-4 has a depth of 75 meters.The geographical locations of each borehole are displayed in Table 4.The strength of the rock mass in the trace diversion tunnel was evaluated by considering the results of the point load test and analyzing the RMR and GSI classification criteria.The findings are displayed in Table 5  The research area passed through by the tunnel trace is in the form of breccia lithology with different strong pressures.The occurrence of this happening can be referred to various weathering conditions and the infiling at several points.The presence of infilling produces a drop in the pressure strength of the rock.Relating to the classification of RMR, the BMT-1 point is assigned a value of 55, indicating its categorization within the fair rock mass class (41-60).According to the stand-up time diagram depicted in Figure 5, it is evident that the RMR value for BMT-1 is 55.This particular configuration exhibits a stand-up time of approximately 120 hours, which is approximately less than one week.Moreover, the roof span of BMT-1 measures around 13 meters without support.In this particular segment, it is advisable to immediately implement a support system at a distance not exceeding 13 meters and within a timeframe of five days, to ensure the preservation of tunnel stability.The roof span of BMT-3 is approximately 13.5 meters in the absence of any form of support.In this specific section, it is recommended to promptly establish a support system within a maximum distance of 13.5 meters and a timeframe of six days, to guarantee the maintenance of tunnel stability.The diagram presented in Figure 8 is utilized based on the point load test data acquired for each sample at the borehole, where the Is50 value is less than 3 MPa.The GSI assessments indicate that the excavation method employed for boreholes BMT-1, BMT-3, and BMT-4 is classified as very good to good, using hammer and blasting techniques.According to the RMR assessment, the observed classification is in the fair category.This classification fits the recommendations provided in Table 3, which suggest particular steps for top heading and bench excavation.Specifically, it is advised to advance the top heading by 1.5-3 meters before using support measures.Additionally, it is recommended to commence support activities following each explosion, support measures should be promptly implemented at a separation of 10 meters from the excavation face.The recommended support system comprises of systematic bolts measuring 4 meters in length, with a distance of 1.5-2 meters in both the crown and walls.In addition, wire mesh is used in the crown.The shotcrete applied at the crown of the structure has varying thicknesses ranging from 50 to 100 mm, but the sides have a uniform thickness of 30 mm.

Conclusions
The rock mass classification based on the obtained RMR value at each drilling location indicates a fair of rock quality.The recommended duration for rock mass courses, based on the Rock Mass Rating (RMR) value at each drilling point, varies between 20 and 144 hours.Additionally, the maximum roof span for these classes ranges from 8.5 meters to 13.5 meters.Concerning the evaluation of RMR, the lithology of the calcarenite unit with sandstone interlayer in the Sigli-Aceh toll road area exhibits parallels to earlier research [14].More precisely, it belongs to the fair rock mass categorization.In the fair class environment, it is recommended to use consistent excavation methods, such as top heading and bench excavation methods.Before applying support measures, it is recommended to move the top forward by 1.5 to 3 meters.In addition, it is recommended to start support operations immediately after each blast and start support procedures 10 meters from the exposed excavation surface.The recommended support system consists of systematically placed bolts 4 meters long, spaced 1.5 to 2 meters apart on the wall and crown.Additionally, the crown contains wire.On the sides, there is a layer of shotcrete that is 30 mm thick, while on the crown, the shotcrete thickness ranges from 50 to 100 mm.Each point's GSI value falls within the range of good to very good, indicating that the preferred excavation technique is the utilization of hammers and blasting.

Figure 2 .
Figure 2. Geological map of Matenggeng DAM Site Plan (Source: Basemap ESRI.LiDAR data Citanduy River Basin Organizations) UCS = Compressive Strength (Mpa)D.Stand-uptimeBieniawski established the notion of stand-up time in 1989.The duration of stand-up time depends on how effective it is at maintaining an unsupported span within the portal.The span is determined by the lesser of two measurements: either the length of the entrance or the distance between the tunnel face and the final support[12].After gathering data on the classification of the rock mass along the tunnel path, an assessment was conducted to establish the roof span's capacity to remain intact without any support system.The most critical rock mass class values along the tunnel are selected.The roof span and standup time, as per the RMR category, are of significant importance are shown in Figure4.

Figure 5 .
Figure 5. Stand-up time and roof span according to the RMR value at BMT-1

8 Figure 6 . 3 Figure 6
Figure 6.Stand-up time and roof span according to the RMR value at BMT-3 Figure 6 is an illustration of stand-up time and roof span based on the RMR value at BMT-3.The RMR value for BMT-3 is 56.The given configuration demonstrates a stand-up time of around 144 hours.The roof span of BMT-3 is approximately 13.5 meters in the absence of any form of support.In this specific section, it is recommended to promptly establish a support system within a maximum distance of 13.5 meters and a timeframe of six days, to guarantee the maintenance of tunnel stability.

Figure 7 . 4 Figure 7
Figure 7. Stand-up time and roof span according to the RMR value at BMT-4 Figure 7 displays the stand-up time and roof span of BMT-4.The figure clearly shows that BMT-4 has been assigned an RMR value of 46.The given configuration demonstrates a stand-up duration of around 20 hours.The unsupported roof span of BMT-4 measures roughly 8.5 meters.In this particular

Table 4 .
Coordinates and depths of each borehole

Table 5 .
: RMR and GSI parameters for each borehole.