GEOTECHNICAL INVESTIGATION ON HEADRACE TUNNEL CONSTRUCTION AT HYDROPOWER FIELD OF PAPUA PROVINCE

Mohammad Realdy HASTARIANZA1*, Sofyan RACHMAN1, Harry PRAMUDITO1

1Geological Engineering Study Program, Faculty of Earth Technology and Energy, Trisakti University, Jakarta, INDONESIA

*Corresponding Author: muhammadrealdy@gmail.com

Abstract

The tunnel stability is a very important thing for the construction of headrace tunnel on hydropower. In order to become a safe mining method, it is necessary to analyze the Rock Mass Rating (RMR) and the selection of temporary support using empirical analysis, i.e. input parameter of structure mapping, physical and mechanical properties of materials, and characteristic of joint. Empirical analysis uses the RMR classification. This research aims to know the RMR value in the research area and to determine the temporary support in accordance with the RMR. Based on the analysis, there are five types of rocks passed by the headrace tunnel, i.e. calcareous sandstone, limestone, claystone, silty claystone and sandy claystone with RMR value ranged from 60 to 68, fair rock class to good rock class, 5 meters span, stand-up time ranged from 1 week to 6 months, full face and top heading and bench excavation method, rockbolt and shotcrete support system with thickness between 47 to 60 mm.

Keywords: Engineering geology, tunnel stability, RMR, temporary support

1. INTRODUCTIONThe study of geotechnical investigation is the first step to understand the state of geological conditions,soil conditions, and rocks mechanic. This study will produce the information as the basis for planningthe development of hydropower headrace tunnel. In some cases, the stress generated by theexcavation of the tunnel increases beyond the strength of the soil and / or rock around the tunnel. Inthis condition there will be collapse on the surface of the tunnel excavation. The above must berealized by the engineering geologist, because it plays an important role in the making of the tunnelfrom planning, determining the method of work and determining the temporary support in accordancewith conditions in the field. So it can be planned the structure of the tunnel that is strong enough towithstand the load around it. The purpose of this research is to know the Rock Mass Rating (RMR) asthe basis of excavation planning and the temporary support of the headrace tunnel at hydropower fieldof papua province.

2. LITERATURE REVIEWTunnelAccording to Soetrisno Arifin (2009), the tunnel is a closed corridor that connects two open sidesor one side open with the aim of a particular object. What is meant by connecting two open sidesfor example are tunnel waterways, road tunnels, railroad tunnels and others that essentially pierceboth sides open. One open-ended example is a tunnel for mine work.

Rock Excavation MethodsThere are several methods of rock excavation. The methods are selected based on several things,including the size of the drill, the available equipment and the formation conditions of the existingsoil / rock. In general, this method is divided into three, i.e.:

1. Full face method

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2. Top heading and bench method

3. Drift method

3. METHODOLOGYThe research methods used in this research are:

1. Literature ReviewLiterature review is done by looking for supporting materials. Determining the boundaries ofthe problem so as not to expand, not out of the existing problems, and the data taken can beutilized effectively, ie research conducted along the tunnel path.

2. Data CollectionSecondary Data:1) Uniaxial Compressive Strength (UCS)2) Rock Quality Designation (RQD)3) Spacing of Discontinuities4) Condition of Discontinuities5) Groundwater Conditions6) Orientation of Discontinuities7) Geological DataThe support standard used are based on recommendations by Bieniawski (1984).

Figure 1 Full Face Method (Made Astawa Rai, 1988)

Figure 2 Top Heading and Bench Method (Made Astawa Rai, 1988)

Figure 3 Drift Method (Made Astawa Rai, 1988)

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4. RESULT AND DISCUSSIONBased on the RMR value can be made a support design that will be used in this location so it is knownthe condition of the tunnel stability based on the calculation of support load, shotcrete thickness andshotcrete maximum pressure.

4.1. Rock Mass Rating (RMR) Rock Mass Rating was developed to solve problems in the field quickly and not be used to replace analytical studies, field observation, measurement, and engineering judgment. Bieniawski's Geomechanic Classification provides RMR value based on its quality. The geomechanical classification is based on six parameters, i.e. Uniaxial Compressive Strength (UCS), Rock Quality Designation (RQD), spacing of discontinuities, condition of discontinuities, groundwater conditions and discontinuity orientation (Bieniawski, 1989).

Table 1 RMR value

Lithology RMR Rock Quality Calcareous sandstone 68 Good (II)

Limestone 63 Good (II) Claystone 60 Fair (III)

Silty claystone 60 Fair (III) Sandy claystone 60 Fair (III)

Temporary support recommendations based on RMR value a. Calcareous sandstoneFrom the results of the data analysis, on calcareous sandstone with rock quality II (Good Rock),the general recommendation for the excavation method is Full Face Method. This method iscarried out with advances of about 1 - 1.5 meters and supporting begins after blasting as far as 20meters from the face. Installation of rockbolt with a length of 3 meters, spacing between rockboltalong 2.5 meters and with 0.047 meter shotcrete thickness can be a temporary support with amaximum shotcrete pressure of 7.104 ton/m2.

b. LimestoneFrom the results of the data analysis, on calcareous sandstone with rock quality II (Good Rock),the general recommendation for the excavation method is Full Face Method. This method iscarried out with advances of about 1 - 1.5 meters and supporting begins after blasting as far as 20meters from the face. Installation of rockbolt with a length of 3 meters, spacing between rockboltalong 2.5 meters and with 0.056 meter shotcrete thickness can be a temporary support with amaximum shotcrete pressure of 8.446 ton/m2.

c. ClaystoneFrom the results of the data analysis, on calcareous sandstone with rock quality III (Fair Rock),the general recommendation for the excavation method is Top Heading and Bench Method.Excavation starts from the top of the tunnel sections excavated before the bottom of the crosssection. After excavated the top of the tunnel section to reach the length of 1.5 to 3 meters(heading), the excavation of the bottom of the cross section is done (bench cut) to form the desiredcross section of the tunnel and the support begins after the 10 meter detonation of the face. Thesupport recommendation uses a 4 meter long rockbolt on the roof and on the tunnel wall, a 1.5 to 2meter spacing of rockbolts, with wire mesh on the roof of the tunnel, 0.060 meter shotcretethickness, and a shotcrete maximum pressure of 9.040 ton/m2.

d. Silty claystoneFrom the results of the data analysis, on calcareous sandstone with rock quality III (Fair Rock),the general recommendation for the excavation method is Top Heading and Bench Method.Excavation starts from the top of the tunnel sections excavated before the bottom of the crosssection. After excavated the top of the tunnel section to reach the length of 1.5 to 3 meters(heading), the excavation of the bottom of the cross section is done (bench cut) to form the desiredcross section of the tunnel and the support begins after the 10 meter detonation of the face. Thesupport recommendation uses a 4 meter long rockbolt on the roof and on the tunnel wall, a 1.5 to 2

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meter spacing of rockbolts, with wire mesh on the roof of the tunnel, 0.052 meter shotcrete thickness, and a shotcrete maximum pressure of 7.850 ton/m2.

e. Sandy claystoneFrom the results of the data analysis, on calcareous sandstone with rock quality III (Fair Rock),the general recommendation for the excavation method is Top Heading and Bench Method.Excavation starts from the top of the tunnel sections excavated before the bottom of the crosssection. After excavated the top of the tunnel section to reach the length of 1.5 to 3 meters(heading), the excavation of the bottom of the cross section is done (bench cut) to form the desiredcross section of the tunnel and the support begins after the 10 meter detonation of the face. Thesupport recommendation uses a 4 meter long rockbolt on the roof and on the tunnel wall, a 1.5 to 2meter spacing of rockbolts, with wire mesh on the roof of the tunnel, 0.058 meter shotcretethickness, and a shotcrete maximum pressure of 8.743 ton/m2.

5. CONCLUSIONFrom the research that has been done on the headrace tunnel construction at hydropower field ofpapua province, it can be concluded that:

1. Geotechnical investigation is a very important thing in the planning and implementation ofheadrace tunnel construction for a hydropower plant. Given the existing geological data, it can bedetermined rock quality. From the rock quality can be determined the initial design of the tunnel inaccordance with the conditions in the field and the most enabling method of implementation andpreparation of all aspects as well as possible, especially on the security aspects of the constructionof the tunnel construction.2. Based on the calculation, the research area has two rock quality, namely class II (good rock) andclass III (fair rock) with RMR value ranges from 60 to 68.3. Excavation method of this area with Full Face method and Top Heading & Bench method, andtemporary support using rockbolt and shotcrete, without using steel support according toBieniawski (1989).4. Calculations of support load, shotcrete thickness and shotcrete maximum pressure are veryimportant, because each rock type has different density.

ACKNOWLEDGEMENT The writer would like to thank PT. PLN (PERSERO) which has given opportunity to writer to do this research and collect the data around the tunnel location, so that writer can finish the research.

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