Geological Mapping and Stand-up Time Estimation based on Core Drilling Evaluation using Rock Mass Rating Method on The Main Spillway and Diversion Tunnel at Manikin Dam, Kupang Regency, Nusa Tenggara Timur

The Main Spillway and Diversion Tunnel at Manikin Dam is one of the dams under construction in the Kupang City area, where several soil or rock investigations have been carried out around it. However, the field investigations are not accompanied by a surface geological investigation or evaluation of subsurface geological conditions based on core drill results. Therefore, the purpose of this paper is to evaluate the engineering geological condition in tunnel construction by displaying surface and subsurface geological mapping with a scale of 1:10,000 and evaluate samples of core drilling using the Rock Mass Rating method in order to have more accurate stand-up time assumptions. The result of engineering geological mapping shows that the research area has three lithologies, namely a basalt unit, a carbonaceous sandstone unit, and a scaly claystone with a chaotic rock unit. The core-drill evaluation indicates that the research area is classified into three quality rock masses, namely Good Rock with an RMR Rating of 64, Fair Rock with an RMR Rating of 46, and Poor Rock with an RMR Rating of 36. The stand-up time value for each rock mass quality category is obtained from the quality rock mass results. The stand-up time value for the Good Rock category is 2200 hours, the Fair Rock category is 30 hours and the Poor Rock category is 1.2 - 1.7 hours. Based on the low stand-up time values in the Fair Rock and Poor Rock categories, it is expected that a support system will be installed immediately after tunnel excavation.


Introduction
The Main Spillway and Diversion Tunnel at the Manikin Dam construction project in Kupang (Figure 1) is one of the tunnels that serves to divert river water during the construction process and also controls the volume of inundation water storage at the dam.The tunnel is being constructed by the main contractor, PT Pembangunan Perumahan (PT PP) and supervised by the River Basin Authority NTII, Ministry of Housing and Public Works.The tunnel length reaches 830.88 m with tunnel excavation diameter dimensions of 8.25 m and 10.25m [1].As the supervising consultant, PT Innako (2021) has conducted a field geological investigation at the tunnel site.The results are in the form of core drill 10 1307 (2024) 012007 IOP Publishing doi:10.1088/1755-1315/1307/1/012007 2 observation points with a depth variation of 36m -85m.The drill points are evenly located along the main and diversion tunnel.From the drilling results, there are 4 types of lithology, namely siltstone, claystone, andesite and mudstone [1].However, the Supervisory Consultant did not conduct further studies on the quality value of the rock mass in determining the stand-up time for tunnel excavation.Determination of stand-up time in tunnel excavation work is very necessary in setting the work cycle time and excavation method.So this research is aimed at evaluating the value of stand-up time through determining the value of rock mass quality using the rock mass rating (RMR) method [2].The rock mass rating value is not only limited to underground geological investigations but also surface geological investigations.The results will show a more precise correlation of observations around the tunnel location.

Geological Condition
The Regional Geological Map of Kupang -Atambua Sheet of Timor, Southwest Section [3], shows that the study area and its surroundings consist of the Bobonaro Complex (Tmb) and Noele Formation (QTn).The Bobonaro Complex consists of two main parts: (a) scaly clay, (b) foreign boulders of varying sizes.Scaly clay has uniform properties that show fault mirrors, soft, and variably colored (dark red, greenish, brownish red, bluish grey, and pink) [4].The technical properties of the clay include moderate to high plasticity.The Noele Formation is a member of the Viqueque sequence.The formation is of Late Miocene to Early Plistocene age and consists of silty sandstone, silty conglomerate sandstone and conglomerate [4].(Figure 2)

Geological Mapping
Geological and engineering geological mapping of rocks and soils at the surface and excavation face of the tunnel is a stage in the feasibility study specified by the Ministry of Housing and Public Works [5].Geological and engineering geological mapping is carried out on the surface and subsurface.Subsurface mapping is carried out by two methods, namely face-mapping and core drill analysis.Geological mapping was conducted to determine the lithology of rocks at the research site.Engineering geological mapping aims to quantify data and information on rocks and soils (or deposits) obtained from geological mapping and mapping the character, classification or engineering behavior of rocks and soils on the surface and in the subsurface.In this study, geological and engineering geological mapping was carried out with a research area of 2 x 2 km with a 40-day implementation time.The contours applied are the results of field measurements by the Contractor and Consultant in the field with a scale of 1:10,000.The number of observation tracks reached by researchers is 76 observation points.

Rock Mass Rating (RMR)
Rock quality classification based on Rock Mass Rating (RMR) (Bieniawski, 1989) [2] is one of the easiest classification methods and can be applied for various testing purposes.The RMR classification uses five parameters to determine the rock mass rating.The fifth parameters are uniaxial compressive strength (UCS), rock quality designation (RQD), discontinuity spacing, discontinuity conditions and groundwater conditions.The estimated UCS value in this study was obtained from field observations based on the ISRM (1981) guidelines [6].UCS parameter values can be seen in Table 1.The RQD value is obtained from the percentage of the total core length of more than 4 inches (10 cm) to the full length of the core.The parameter values of the RQD values can be seen in Table 2.The discontinuity distance parameter rating is to find the perpendicular distance between discontinuity planes (joints, faults, folds, bedding planes, etc.) along the core drill.The discontinuity distance parameter rating value can be seen in the Table 3.The greater the discontinuity distance, the smaller the RMR rating value.This is due to the distance between discontinuity planes getting smaller, the strength of the rock mass will decrease and vice versa.The discontinuity condition parameters used in this study are divided into two, namely conditions with filler and no filler.Parameter ratings with conditions with filler are shown in Table 4.If there are fillers, then there are sub-parameters that are considered, namely persistence (length of the discontinuity field), roughness (level of roughness on the surface of the discontinuity field), separation (distance between the surfaces of the discontinuity field), infilling (filler material in the discontinuity field) and weathering (level of weathering in the discontinuity field).The values of these sub-parameters can be seen in Table 5. Slightly weathered 5

Moderately weathered 3
Highly weathered 1 Decomposed 0 The Groundwater Condition parameter is assessed by feeling the surface of the core drill results by hand.
The right condition for this assessment is after the core drill is completed, or the core drill results are still kept moist by being covered with aluminum foil.Groundwater Condition parameter values can be seen in Table 6.

Stand Up Time
The stand-up time is determined by the opening's effective (unsupported) span, defined as the space's width or the distance between the tunnel face and the final support (whichever is less).The stand-up time for arched openings would be substantially longer than for a flat roof.Controlled blasting extends the stand-up duration by reducing damage to the rock mass.According to the table of relationship between the stand-up time and span for various rock mass classes in Bieniawski (1989) [2], the standup time for tunnels with arched roofs is related to the rock mass rating (RMR).Do not postpone supporting the roof in a rock mass with a long stand-up time because this may cause the rock mass to deteriorate, reducing the stand-up time.Lauffer (1988) observed that the stand-up time improves by one class of RMR value in excavations.

Results and Discussions
The results of surface geological mapping at the research location show four lithological units, namely the Alluvial Deposits Unit, Limestone Unit, Carbonaceous Sandstone Unit and Scaly Claystone with a Chaotic Rock Unit (Figure 4).The chaotic rock consist of breccia and basalt (Figure 5).The mapping results can be seen in Figure 3.The geological process that occurred at the research location was caused by the collision activity of the northwestern Australian Plate with the Banda Islands Arc so that the Australian Continental crust subducted under the archipelago arc with a northward tilt direction that IOP Publishing doi:10.1088/1755-1315/1307/1/0120076 occurred in the Late Miocene age.After the collision process, obduction of the Banda Arc plate occurred above the passive boundary of the Australian Continental plate which caused Banda Allochthon deposits to appear in the crust of the arc face, covering the Perm-Triassic age Australian Continental deposits.

Figure 3. Geological Map of Research Location
Figure 5 shows observation points with data on strike/dip layers, bridles and faults mainly in carbonate sandstone units, and also on basalt exactly 130m from the tunnel outlet towards the tunnel inlet.At the observation point of the basalt, shear and thrust faults were found but categorised as minor because the continuity of the structure was not found.Based on the regional geological stratigraphy [3], the age of chaotic rock older than the bobonaro clay (scaly caly) ranges from perem to early Miocene.The results of subsurface mapping observations based on the core drill field 10 points along the tunnel (results can be seen in Figure 6) found four lithologies namely scaly claystone unit, siltstone unit, basalt and slate.Followed by subsurface mapping with face-mapping in the tunnel inlet section.There is the same lithology as the core drill, namely the scaly claystone unit.

Figure 6 .
Figure 6.Geological Cross Section of TunnelThe results of the 10-point core drill observation/analysis are shown in Table7.There are three zonations of rock mass quality along the tunnel elevation, namely Good Rock (II), Fair Rock (III) and Poor (IV).With an excavation diameter of 8.25m, the stand-up time can be determined based on the rock mass quality zonation.

Figure 7 .
Figure 7. Rock Mass Quality of The Cross Section Tunnel