Engineering geology for site characterization in Tunnel 2 Bintang Bano irrigation area, West Sumbawa, West Nusa Tenggara Province

Site characteristics develop a better understanding of geological conditions to prevent tunnel failure. Tunnel 2 Bintang Bano Irrigation Area in Seloto, West Sumbawa, West Nusa Tenggara Province, was designed with different interpretations of geological conditions that could lead to inaccuracy of the tunnel design. This paper aims to identify the characteristics of the tunnel site area through engineering geology, including morphological and lithological unit, geological structure, and geomechanics, which are rock mass characterization using Rock Mass Rating (RMR) and Tunnelling Quality Index (Q-system), for the excavation planning area. This area is identified as volcanic landforms categorized as lava ridges with steep slopes and alluvial plains with very sloping slopes. The lithologies are altered andesite and gravelly sand unit. Altered andesite with tuff intercalation was found in the subsurface. The tunnel is projected to be excavated in rock mass with dominantly fair rock with ratings (±40≤RMR≤65/±4.33≤Q-System≤8.67) approximately 60% of the excavation area, ±30% area of good rock (63≤RMR≤69/10.67≤Q-system≤11.40) and 10% poor rock (26≤RMR≤40/1≤Q-system≤3.57).


Introduction
The site characterization could be defined as a three-dimensional geological characteristics process of the site, including surface and subsurface, to develop a better understanding of the materials, engineering properties, groundwater, or even man-modified conditions such as utilities that influenced site conditions [1] [2].This process needs to gather necessary information from the site for construction or engineering works.Engineering geology is a fundamental part of geotechnical site characterization that is used in civil engineering projects.Incompletion or inaccurate of this characterization will lead to incorrect models, geotechnical properties, or design values [3].According to Waltham (2002) in his book Foundations of Engineering Geology, six component areas of engineering geology are ground materials and structures, regional characteristics, surface processes and materials, ground investigations, material properties, and difficult ground conditions [4].Some elements could be referred to as characteristics of the site, such as stratigraphy, groundwater regime, top of rock, rock mass characterization, presence of weak rock, potential problems related to sedimentological, structural, or 1249 (2023) 012017 IOP Publishing doi:10.1088/1755-1315/1249/1/012017 2 geomorphological conditions, potential instability, likely excavation characteristics, and geological constraints [1].
Tunnel 2 Irrigation Area of Bintang Bano is planned to be built in Seloto, Taliwang Sub-district, West Sumbawa, West Nusa Tenggara Province, UTM 488109.4866,9039050.1327(inlet) -UTM 487850.3299,9038929.7395(outlet) based on datum 50 S. Geologically, West Nusa Tenggara is located on Sumbawa Island, which is part of Lesser Sunda Island.It was formed from the subduction zone of the Indo-Australia plate in the Tertiary [6] [7].Based on the regional geology map of the Sumbawa section, the research area consists of two formations.The oldest formation is Tuff Breccia (Tmv), formed in the early Miocene.The formation consists of andesite with sandy tuff intercalation, tuff pumice, tuff sandstone, lava with pillow structural, and chert intercalation, with some areas containing volcanic mudflow.This formation is proliferated, mineralized, and silicified, quartz and calcite veins are shown.This formation has approximately 150-200 m in depth for the west area and ±400 m in depth for the east area of Sumbawa.The Tuff Breccia formation is unconformably covered by Alluvial (Qal), formed in the Holocene, consisting of mixed pebble to gravel, sand, clay, and mud.This formation has approximately 30-50 m in depth [8].
A tunnel is high-risk underground construction with a complex design following the geological conditions along the tunnel [5].Therefore, understanding the geological condition of the areas of tunneling is important to prevent design failure.Based on the previous studies, because of the wide definition of the two formations in regional geology map, different interpretations were executed in the design process of the tunnel which enlarge the uncertainties of geological informations.As the tunnel design is defined based on empirical analysis, mineral composition, strength, and rock mass quality would directly influence the design.This paper aims to re-evaluate lithology, geological structures, and rock mass characterization, that should be considered before determining the tunnel design to prevent some pitfalls in site characterization that usually occurred because of misinterpreted in geology [1] [2].

Methods
The research area was investigated in both surface and subsurface.Generally, the research method is determined into three phases: (1) Lithological (surface and subsurface), geomorphological, slope, and structural mapping (lineaments and fractures), (2) Field test and laboratory analysis, geomechanics mapping (rating of rock mass) (3) Integrated analysis for site characteristics for tunneling.
In the first phase, data were collected by surface investigation and drilling result analysis.Samples were taken during this phase and tested in the laboratory to calculate the rock mass quality.In the second phase, laboratory tests performed mechanical tests, including uniaxial compressive strength (UCS) and point load.All data were used to classify the rock masses for subsurface calculations based on rock mass classifications, RMR and Q-system.The RMR has six parameters: (1) Strength of intact rock, (2) Rock Quality Designation (RQD), (3) Joint discontinuity spacing, (4) Joint Condition, (5) Groundwater condition, and (6) Joint Orientation [9].The Q-system also has six parameters:

Result and Discussion
The results were divided into 4 (four) subsections that explained geomorphology, lithology, structural geology, and geomechanics (rock mass classification).

Geomorphology
The research area generally defines hills with an elevation of 200-300 mean sea level (msl), formed by endogenic processes from volcanic activities.The slope gradients range from 21-55% and are categorized as steep slopes.As details, the other morphology was formed by the exogenic process through fluvial transportation between the hills with 30-50 msl.and slope gradients of 3-7% and categorized as a very sloping slope.According to BMB Classification proposed by Brahmantyo and Bandono [11], the first geomorphology unit could be categorized as a lava ridge with a steep slope and alluvial plains with a very sloping slope.Based on Figure 5 Lava ridge with a steep slope dominated the site by approximately 54% with an elevation of 50-278 msl.Aside from the endogenic process, the exogenic process also occurred in this unit in the form of weathering.On the top of the hills in the research area, the rocks have already turned into residual soil.Outcrops are found in the middle to foothills with dominantly highly weathered rocks.The rest site was identified as alluvial plains with a very sloping slope with approximately 46% of the site.The elevation of this unit is in the range of 30-50 msl.

Lithology
Lithology aspects of this research area are defined as two lithological units, (1) The altered andesite unit.The rock exhibits dark grey (fresh) to greenish grey (altered) and, in a certain area, shows a milky-white color (Figure 6b).The altered andesite was identified as lava, shown by the trachytic texture of plagioclase based on the petrographic analysis.The texture of the rock is porphyro-aphanitic, massive, with a euhedral-subhedral crystal shape.Based on the drilling core in the subsurface (Figure 7a), tuff is found as intercalation in the altered andesite (Figure 7b).

Structural Geology
Structural geology assessment is based on two kinds of analysis: desk study and field investigation, to compare general and specific perspective evaluation results.Desk study used a digital elevation model to analyze the lineaments predicted in the research area.No fault or fold was identified in the research area based on regional geological maps [8].Furthermore, the field investigation is used to confirm structural geology in detail that could influence the tunnel design.

Geomechanics (Rock mass characterization)
Rock mass for the tunneling area is evaluated by 4 (four) drilling cores and uses two different rock mass classifications, which are RMR and Q-system.Rock mass classification for subsurface calculated on dry conditions which the water table placed under the tunnel excavation area.The stress condition, defined as Stress Reduction Factor (SRF) in Q-system, is calculated based on depth tunneling of the tunnel area (for competent rocks) and categorized as low stress due to open joint and near-surface excavation.A specific condition also appeared in the 2 nd borehole area (PND-2A), which was identified as a single weakness zone containing clay or chemically disintegrated rock because of alteration that changed the hardness of the rock, as shown in Figure 13.Rock mass discontinuity conditions were mostly undulating, smooth, and slightly rough, with only a staining surface and no infilling materials except for completely weathered rock and a single weakness zone that contains soft material fillings.
Strength intact rock of the altered andesite, parallel to weathering scale of the rocks.The altered andesite is divided into completely weathered, highly weathered, moderately weathered, and slightly weathered based on typical rock characteristics in the research area [12].The fresh rock is unidentified due to alteration.Tuff intercalation is measured as the specific entity.Completely weathered and highly weathered altered andesite has similar ranges in strength intact rock, approximately 1-3 MPa.Moderately weathered rocks valued from 5-50 MPa (±30-40 MPa dominated).Slightly weathered of altered andesite valued more than 50 MPa (±60-90 MPa dominated).Tuff intercalation has a similar value to highly weathered altered andesite (±2-3 MPa).The subsurface rock mass conditions are classified as extremely poor rock (0.01≤Q-system≤0.04),poor rock (26≤RMR≤40; 1≤Q-system≤3.57),fair rock (41≤RMR≤59/4.17≤Q-system≤8.94),and good rock (63≤RMR≤69; 10.67≤Q-system≤11.40)classes.Extremely poor rock (Q-system) and poor rock (RMR) is a crushed rock mass with earth-like conditions.The aperture is 1-5 mm with infilling soft materials that prevent rock wall contact.The intact rock has strength in the range of completely and highly weathered (1-3 MPa).Meanwhile, the poor rocks category portrayed by Q-system has disintegrated rock with more than two joint sets, smooth undulating, and slightly altered joint walls.
Poor rocks have strong, intact rock similar to moderately weathered rocks (5-50 MPa) and less rating of RQD (<20%).Fair rocks have strong intact rock mostly in the range of moderately weathered (5-50 MPa), with aperture (1-5 mm) and without infilling, spacing discontinuities >10 cm, and RQD 29%-65%.Good rocks have a strong intact rock in the range of slightly weathered rocks (>50 MPa) with RQD >65% and joint spacing 10-88 cm.Furthermore, staining is found on the joint's surface.The configuration of subsurface quality rocks is illustrated in Figure 15.  Figure 15 shows that the tunnel is designed in elevation 37.5-41.5 msl with 338.63 m length.The tunnel is projected to be excavated in fair and good rocks area, with some parts identified as poor rock due to tuff intercalation nearby the outlet section.Fair rock area approximately covered 60% or ±200-245 m of the tunnel length.While good rocks covered ±30% tunnel excavation area, the rest of the excavation area was identified as poor rocks (RMR: ±35; Q-system: ±3.33).

Conclusion
Tunnel 2 Bintang Bano irrigation area is designed under the hills of lava ridge with steep slope (21-55%) landforms formed by altered andesite.Tuff intercalation is found estimated in the outlet tunnel area.Weathering occurred strongly in this area where the top of the hills became residual soil, and the bedrock became slightly weathered rocks because of the alteration.The dominant force in this area was suggested as north-south (N-S) or reversible due to northwest-southwest (NW-SW) and northeastsouthwest (NE-SW) based on the dominant joint orientation in the area.Based on two different rock mass classifications, the tunnel is estimated to build dominantly in fair rock areas, which is approximately 60% of the tunnel length, ±30% is estimated to be good rocks, and the rest percentage is defined as poor rock classes for tuff intercalation near the outlet of the tunnel.
Based on this site characteristic, tunnel excavations, especially in the portals, need to consider the steep slope of geometry with residual soil and weak rock conditions due to intense weathering at the top of the excavated section (extremely poor to poor rock mass classes).The excavation method and support system in the tunnel depth section must consider RMR and Q-system recommendations.From the illustrations, as shown in Figure 15, the Q-system portrayed the weak zone as specifically more sensitive than RMR, which includes an extremely poor rock class.For this reason, to determine the tunnel's excavation, portal, and support system, consider the lowest rock class quality based on the rock mass classification.The strength of the tuff intercalation intact rock is similar to highly weathered altered andesite, which potentially becomes a critical part of the tunnel.

Figure 5 .Figure 4 .
Figure 5. (a) Slope (%) of the research area (b) Geomorphological map of the research area.

Figure 9 .
Figure 9. Lithological map of the research area.

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Desk study showed lineaments are from northwest (NW) -southwest (SW) orientations based on 21 data lineaments orientation.The major force orientation of the sites is predicted from north (N) to south (S) or reversible.This structural geology confirmed by field investigation that the dominant joint orientations have NW-SW and northeast-southwest (NE-SW) trending orientations based on 63 data, shown in Figure10 (c).

Figure 12 .
Figure 12.Measurement of joint orientation (a) inlet tunnel area (b) outlet tunnel area (c) rose diagram of joint orientation.

Figure 15 .Figure 14 .
Figure 15.Borehole correlation of rock mass classification (a) based on RMR (b) based on Q-system.