Application of Soil Nailing for Landslide Mitigation in Bhutan: A Case Study at Sorchen Bypass Dr. Raju Sarkar* Professor Department of Civil Engineering and Architecture, College of Science and Technology Royal University of Bhutan, Bhutan. *Corresponding Author e-mail ID: [email protected] Ritesh Kurar Undergraduate Student Department of Civil Engineering, Delhi Technological University, Delhi, India. Sangay Zangmo, Ugyen Dema, Sujan Subba, Daman Kumar Sharma Undergraduate Students Department of Civil Engineering and Architecture, College of Science and Technology, Royal University of Bhutan, Bhutan ABSTRACT Soil nail walls constructed by reinforcing the underlying soil with driving reinforcement are one of the numerous emerging slope stabilization approaches. The provided reinforcement tends to increase the shear strength of the soil as the nail affixes to stable base beneath, thus, making the slope more stable. The chief intent of this study is to establish feasibility of constructing soil nail walls at Sorchen, 17.5 km from Phuentsholing, along Phuentsholing-Thimphu highway in Bhutan; by evaluating the geological reviews and surveying the site, additionally, considering physical and chemical specifications of soil and geotechnical landslide characterization. The analysis, design and construction of permanent soil nail wall in this study has been influenced by soil nailing construction reference manuals published by Federal Highway Administration of United States Department of Transportation. This paper proposes an applicable design of soil nail wall at Sorchen bypass; verified by SNAP_2 software developed by Federal Highway Administration of United States Department of Transportation; considering the existing site soil parameters and drainage conditions. This study has been undertaken particularly at Sorchen bypass landslide, such that, the same approach/method may be utilised to arrest other similar landslide prone areas of Bhutan. KEYWORDS: Slope Stability, Soil nailing, SNAP_2, Landslide INTRODUCTION Soil nailing technique to stabilize slopes and excavations in soils has been drawn-out from New Austrian Tunnelling method (NATM), which is a system for underground excavations in rock supports. In NATM, passive metallic reinforcement, known as rock bolts, are inserted and grouted - 4963 - Vol. 22 [2017], Bund. 13 4964 into the ground, in conjunction with shotcrete facing [1-3]. Subsequently, this concept of combining passive steel reinforcement and shotcrete facing extended significantly into rock-slope and soil-slope stabilization projects [4]. Soil nails are basically rigid bars which driven into soil or pushed into boreholes which can be subsequently filled completely with grout [5]. Together with the in situ soil, they chart a coherent structural body supporting an excavation or holding the movement of an unstable slope. Soil nail walls are a widely used technology for retaining vertical cuts, nearly vertical cuts in soil and any slope which is at an angle steeper than the soil parameters would normally permit [5]. The methodology has been proven to be cost-effective and the construction faster than any other conventional support methods. On increasing account of soil–nailing technology application to reinforce soil slopes, a throng of studies on the design of nail-reinforced slopes has also been proposed. The effective designing method banks on the vigorous evaluation for stability level of reinforced slopes, which solely depend upon judicious understanding of failure behaviour and reinforcement mechanisms [6]. The interaction between soil nails, the soil behind the wall, and the facing is complex and causes redistributions of tensile forces in the nails. The mobilized shear stress along the grout-soil interface is in general not uniform and changes in direction along the nail length [7]. The tensile force that can develop in a tendon depends on the location where the nail crosses the slip surface. The location of maximum nail tensile forces is close to, but generally does not coincide with, the critical slip surface established in stability analyses. The location of the maximum load also changes from nail to nail [8] (Fig. 1). The intersection of a soil nail with the slip surface determines the length of that soil nail that can develop pull-out resistance. A multitude of methodologies have been recommended for investigating the stability of nail-reinforced slopes, such as, the finite-element method, the limit equilibrium method, and the kinematics method. By establishing varied rationales for slope failure surface and the soil– nail interaction model, numerous researchers have analysed soil nail behaviour by optimizing the design with respect to various parameters, including site conditions, geometric arrangements, length, inclination, diameter and spacing [9-14]. For practicable study, analysis and design of soil nail wall at an instable site, which has been affected by landslides for more than 20 years now [15], has been chosen (Fig. 2a, 2b, 2c). The site is near Sorchen bypass, 17.5 km away from Phuentsholing-Thimphu Highway, Bhutan. The study follows the recommendations and guidelines presented in [8] and [16]. Vol. 22 [2017], Bund. 13 4965 Figure 1: Location of maximum tensile forces in soil nails [8]. 2(a) Figure 2: Continues on the next page. Vol. 22 [2017], Bund. 13 4966 2(b) 2(c) Figure 2: Studied site of Sorchen bypass, Bhutan. (a) Site location from College of Science and Technology, Royal University of Bhutan (CST-RUB) (b) Render image of site at Sorchen bypass (c) Landslide effected zone of Sorchen bypass The manual presents the information on the design, analysis and construction of soil nail walls. It also provides LRFD (Load and Resistance Factor Design) procedures for site and lab investigation which are carried out for the suitability of construction of soil nail walls. The reference manual contains empirical formulae and range of design parameter charts to be used in the design. The Vol. 22 [2017], Bund. 13 4967 minimum factor of safety as recommended by the manual for various modes of failure of soil nail wall are mentioned in Table 1. Table 1: Minimum Factor of safety Failure modes of soil nail wall Minimum factor of safety Global stability 1.35 Sliding stability 1.3 Nail pull-out failure 2 Nail tensile failure 1.8 Facing flexure failure 1.1 Facing punching failure 1.1 METHODOLOGY AND MATERIALS The main research strategy and measures for our research processes are shown in Fig. 3. Vol. 22 [2017], Bund. 13 4968 Study on application of soil nailing at Sorchen landslide site Selection of study area Laboratory tests Chemical tests Preliminary survey Field Test Site Sieve analysis Reconnaissance Resistivity investigatio Moisture content survey Proctor test Atterberg limit test Triaxial test Result Analysis and design parameters Design and Analysis of soil nail wall Verification using SNAP -2 Soil nail wall Drawing Conclusion and documentation Figure 3: Process of adopted research methodology Vol. 22 [2017], Bund. 13 4969 FIELD SURVEY AND SUBSURFACE EXPLORATIONS 1. Total Station survey The profile of Sorchen bypass landslide site has been developed by using Total Station (Fig. 4). • Surface area of affected area or study area = 1272.431 m2 • Height of slope = 30m • Angle of slope = 600 Figure 4: Profile of Sorchen landslide site. 2. Soil Resistivity Test The Wenner 4-Point Method by far is the most used test to measure the resistivity of soil. The test provides information about the level of ground water table at certain depth and soil type of the tested area. The test has been performed in accordance to the set International standards [17]. Fig. 5 depicts the typical arrangement of Wenner 4-Point method at Sorchen bypass site. Fig. 6 depicts position of data collected from effected slope at Sorchen bypass site. Vol. 22 [2017], Bund. 13 4970 Figure 5: Typical arrangement of Wenner 4-Point method as per [18]. Figure 6: Typical arrangement of Wenner 4-Point method at Sorchen bypass site. The electrode depth (B) is kept small compared to the distance between the electrodes (A) and R is the Megger earth tester reading in ohms. The following formula gives the soil resistivity of depth A in ohm-m. ρ = 2π AR, where, A = 20B (1) Vol. 22 [2017], Bund. 13 4971 Top Middle Toe Figure 7: Position of data collected from effected slope at Sorchen bypass site. Table 2: Resistivity at various depth Type of Soil Depth(m) Resistivity(ohm-m) Top 1 1067.6 Sand mixed with various sizes of gravel 2 1632.8 Gravely silt 3 2505.72 Gravely silt Middle 1 910.6 Sand mixed with various sizes of gravel 2 653.12 3 866.64 4 678.24 5 659.4 6 640.56 7 615.44 Toe Vol. 22 [2017], Bund. 13 4972 1 558.92 2 628 3 546.36 Sand mixed with various sizes of gravel 4 628 5 590.32 6 547.8672 In Table 2, relatively higher values of resistivity test show that the ground water table is not near the ground surface and soil is basically sandy gravel. LABORATORY TESTS 1. Chemical Analysis Chemical test results of soil are found to be non-aggressive for soil nailing and as per the Soil nail manual recommendations [8, 16]. Table 3: Chemical properties of collected soil sample S. No. Properties Value Recommendations 1 Chloride content (mg/l) 1.47 < 100 2 Conc. of Sulphate (mg/l) 30.4 < 200 3 pH of sample 6.9 5-10 2. Geophysical Tests For the present study, soil sample has been amassed from Sorchen bypass, Phuentsholing- Thimphu Highway, Bhutan. The top layer of the soil has been withdrawn with the help of a shovel up to a depth of 0.5m before gathering the soil sample. The geotechnical properties of soil sample used in this study are given in Table 4. Table 4: Physical properties of soil sample Properties Value Unified soil classification system SW Maximum mass dry density (g/cc) 1.899 Optimum moisture content (%) 12.5 Internal angle of friction of soil (φ) 32.9º Cohesive strength (c) Negligible Liquid limit (%) 25.11 Plastic limit (%) 13.39 Plasticity index (%) 11.72 Vol.