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Home , Landslide mitigation, Soil nailing

Application of Nailing for 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 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 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 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 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 which can be subsequently filled completely with [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 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].

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Figure 1: Location of maximum tensile forces in soil nails [8].

2(a)

Figure 2: Continues on the next page.

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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.

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Study on application of soil nailing at Sorchen landslide site

Selection of study area Laboratory tests  Chemical tests Preliminary survey Field Test Site   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

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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.

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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)

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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 mixed with various sizes of 2 1632.8 Gravely 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 system SW Maximum mass dry density (g/cc) 1.899 Optimum moisture content (%) 12.5 Internal angle of of soil (φ) 32.9º Cohesive strength (c) Negligible Liquid limit (%) 25.11 Plastic limit (%) 13.39 Plasticity index (%) 11.72

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• Sieve analysis The classification of the soil follows Unified soil classification system and has been graded according to International standards [19]. The soil contains sand and gravel with 10.38% of fine grained soil. • Compaction test The test has been performed as per International standard [20]. Two modified proctor tests have been conducted for a sample where both tests gave similar Maximum dry density and Optimum moisture content. • Undrained Unconsolidated Triaxial test Triaxial testing has been performed, as per International standards [21], such that the shear strength parameters of the soil can be computed. In this test, all round pressure is applied to the soil specimen after being set in the triaxial cell assembly and the shearing process is started immediately without any consolidation and without allowing any drainage of water. • Atterberg limit tests The tests have been performed as per International standards [22].

TEST RESULTS INTERPRETATION

Table 5: Comparison of obtained test results against laid guidelines in reference manual for favourable condition for soil nailing [8, 16]. As par Circular No. 7 – Soil Nail Walls Selected site data Remark reference manual (GEC 7) Dense to very dense granular soils Sieve analysis: soil contents Favourable for soil with apparent 43.28% gravel, 10.38% slit nailing Weathered rock with adverse and , 46.34% sand. The Ground weakness planes soil is classified to be dense condition Stiff to hard fine-grained soils granular with apparent Residual soils cohesion. Glacial till LL ≥ 50% LL = 25.21% Soil does not meet the PI ≥ 20% PL = 13.39% criteria for creep Soil creep Liquidity index (LI) ≥ 0.2 LI = -0.225 potential. Favourable potential Soil is organic Soil is not organic for soil nailing

pH : 5

Sulfates: Less than 200ppm 30.4ppm Chloride: Less than 100ppm 1.47ppm Ground water Presence of higher ground water Resistivity test does not Favourable for soil table is unfavourable for soil nailing show presence of ground nailing water table till 6m depth from toe Vol. 22 [2017], Bund. 13 4974

DESIGN OF SOIL NAIL WALL The Design provided in this paper is in accordance with the Geotechnical Engineering Circular No. 7 – Soil Nail Walls reference manual (GEC 7) published by U.S Department of Transportation, Federal Highway Administration [8].

DESIGN PARAMETERS

Table 6: Adopted design parameters conforming to reference manual guidelines [8, 16] Sl.no Parameters Recommendation as par Soil Nail Walls Parameters adopted in reference manual (GEC 7) the design 1 Nail length 0.7H 17m 2 Nail inclination 10o to 20o 15o 3 Drill hole diameter 100mm to 200mm 150mm 4 Nail spacing 1.22mm to 1.83m 1m 5 Cantilever distance 0.61m to 1.1m 1.1m 6 Spacing of last nail 0.61m to 0.92m 0.5m from the base 7 Thickness of initial 100mm to 150mm 100mm facing 8 Thickness of final 150mm, 200mm and 250mm 150mm facing 9 Bearing plate 200mm to 250mm side length, thickness of 250, 250mm x 19mm to 25mm 250mm x 25mm 10 Shotcrete strength Minimum 20.7 N/mm2 to 27.6N/mm2 30N/mm2 11 Bond strength Bond strength for sandy and gravel soil is 15- Bond strength, 26psi qu=20psi or 137.95KN/m2

12 Cohesion Field data 0 13 Angle of friction - Ф = 32.9o 14 Interface frictional s = (2/3)Ф s =21.93degree angle 15 Batter angle 𝛿𝛿 - 𝛿𝛿 0o 16 Slope angle Field data 60o 17 Back angle slope - 0o 18 Surcharge load Minimum surcharge load= 250psf 12kN/m2

ADOPTED DESIGN SPECIFICATIONS Following parameters have been adopted for the constructed the designed soil nail wall after several trials and error calculations. • Stepped vertical wall of 10m height each • Adopt top to down construction process • Adopt rotatory drill method to drill hole • Provide grouting of nail without corrosion protection • Provide initial facing (hi) of 100mm thick Vol. 22 [2017], Bund. 13 4975

• Provide final facing (hf) of 150mm thick • Provide nail inclination (i) of 15° to the horizontal • Provide surcharge load for 70R loading • Provide bench width of 0.92m • Area of steel rod

ANALYTICAL SOLUTIONS • Global Stability Safety Factor Global Stability Safety Factor method is used to determine nailed soil wall stability which is inclusive of geometry problem, soil properties, and the nail tension. The analysis proceeds as per the guidelines given by FHWA.

Teqcos( i) + [(W + QT)cos + Teqsin( i)]tanФ FSG = = ψ − ψ ψ − ∑ R (W + QT)sin ∑ D FSG 1.35 𝜓𝜓 where,≥ Teq = Equivalent nail force Teq = (Tall)j Sh 1 n Tall = Allowablej=1 axial force carrying capacity of the soil nail (RP or RT whichever is smaller) ∑ o o 32.9 o = 45 + 2 = 45 + 2 = 61.45 qs = SurchargeФ ψ 2 qs = 12 kN/m W = Weight of the failure wedge W = 0.5γH2cot( ) BL = Length of horizontal slip surface RP = Nail pull outψ capacity RP = πdLPqu qu = Estimated Bond Strength of soil nails in coarse-grained soils as per Table 4.4a in [8] LP = Pull out length or soil nail length behind slip surface ( ) LP = L – ( ) H−z cosψ z = Moment arm in effective width of facing section sin ψ+i i = Inclination of soil nail wall from horizontal RT = Nail tensile capacity 2 RT = πd fy d = Distance from the outer edge of a facing section in compression to the centroid of the reinforcement. fy = Yield strength of steel

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Table 7: Analytical solutions for FSG conforming to reference manual guidelines [8, 16] Facing section wall FSG First Wall 3.56 Second Wall 2.71 Third Wall 2.2

• Sliding Stability b L ( T A ) FSSL = A c B + W+Q +P sinß tanФ FSSL 1.5 P cosß where,≥ PA = Active earth pressure (kN/m) 2 PA = 0.5KaγH

cb = Cohesion of soil along the base of soil block QT = qsBL

Table 8: Analytical solutions for FSSL conforming to reference manual guidelines [8, 16] Facing section wall FSSL First Wall 7.87 Second Wall 8.3 Third Wall 8.74

DETAILING OF SOIL NAIL WALL Figures 8-12 depict detailing of various Soil Nail wall elements.

Vol. 22 [2017], Bund. 13 4977 Section B

View A

View B

Figure 8: Front View of wall (30m X 22m). Figure 9: Section XX of Soil nail wall.

Figure 10: Enlarged view Section B showing element specifications. Vol. 22 [2017], Bund. 13 4978

Figure 11: Enlarged viewpoint of View A depicting element specifications.

Figure 12: Enlarged viewpoint of View B depicting element specifications.

SNAP_2 VERIFICATION SNAP_2 (Soil Nail Analysis Program) is a computer program developed for designing all components of soil nail retaining structures, including nail and facing elements by Federal Highway Administration of United States Department of Transportation [23]. SNAP_2 can evaluate the internal (facing and nail) components of a soil nail wall, external stability, and global stability based on the current standards in the Allowable Stress Design method. All designing aspects are heeding to the FHWA guidelines presented in [8] and [16]. Giving the input parameters of the design, the software evaluates the components of soil nail wall and performs stability check. The following table shows the check for external stability calculate by the software and through manual calculation.

Table 9: Factor of Safety comparison Factor of safety As per Soil Nail reference manual Manual calculation SNAP_2 (GEC 7) Goble stability FSGS ≥ 1.35 2.82 1.52 Sliding stability FSSL ≥ 1.5 8.3 3.9 Vol. 22 [2017], Bund. 13 4979

VIEWS OF SOIL NAIL WALL AS GIVEN BY SNAP_2

Figure 13: Initial facing without and with shotcrete

Figure 14: Final facing without and with shotcrete Vol. 22 [2017], Bund. 13 4980

Figure 15: Slope before soil nailing Figure 16: Slope after soil nailing

CONCLUSIONS This paper presents the details of feasibility of designing a soil nail wall at Sorchen bypass landslide effected site in Bhutan. The soil wall has been designed in accordance with LRFD (Load and Resistance Factor Design) procedures contained in [8]. Also, the laboratory and field tests results have then been compared with parametric study of [8] and [16]. Based on the study presented in previous sections, following site-specific conclusions can be drawn. 1. The collected soil sample has favorable properties, in accordance with permissible values given by [8] and [16], for soil nailing. Considering chemical and resistivity tests results of the soil sample, the corrosiveness of the ground/soil can be established as less aggressive. Therefore, no additional corrosion protection of Soil-Nail wall at Sorchen bypass is required. Thus, making the project even more cost-effective. 2. Moreover, the creep potential of the tested soil sample can be categorized as poor. Furthermore, there is no presence of ground water till 6m from toe of the slope. Hence, the site conditions make this slope stability process even more effective. Also, any drainage and problem that might arise at the site can be encountered by providing shotcrete and geo-textile mats. 3. It has been noted that as the angle of the nail increases, the stability of the slope increases up to a certain limit. Thereafter, further increase in the angle decreases the slope stability. Additionally, the stability decreases with increase in the spacing between the nails. 4. For legitimate feasibility of designed soil nail wall, the design has been verified, both by manual calculation and SNAP_2, where the determined Factor of Safety is under permissible limit contained in [8]. The soil nail wall so designed can stabilize the slope of the chosen site at Sorchen bypass, such that, the transportation system of Phuentsholing- Thimphu Highway can be enhanced by reducing the existing slope-failure caused inconvenience.

REFERENCES [1] Rabcewicz, L.V. (1964a). “The New Austrian Tunnelling Method,” Part 1, Water Power, Vol. 16, November, London, England, 453-457. [2] Rabcewicz, L.V. (1964b). “The New Austrian Tunnelling Method,” Part 2, Water Vol. 22 [2017], Bund. 13 4981

Power, Vol. 16, December, London, England, 511-515. [3] Rabcewicz, L.V. (1965). “The New Austrian Tunnelling Method,” Part 3, Water Power, Vol. 17, January, London, England, 19-24. [4] Long, J.H., Chow, E., Cording, E.T., and Sieczkowski, W.J. (1990). “Stability Analysis for Soil Nailed Walls,” Geotechnical Special Publication No. 25, American Society of Civil Engineers, Reston, VA, 676-691. [5] Giacon, L. (2010). “Flexible Facing for Soil Nailing Retaining Systems”, Dissertation, University of Bologna. [6] Zhang, G., Cao, J., and Wang, L. (2014). “Failure behavior and mechanism of slopes reinforced using soil nail wall under various loading conditions”, Soils and Foundations, 54(6):1175–1187. [7] Babu G.L.S., Murthy B.R.S. and Srinivas A. (2002). “Analysis of Construction Factors Influencing the Behaviour of Soil Nailed Earth Retaining Walls”, Ground Improvement, 2002, 6(3):137–143. [8] Carlos, A.L., Helen, R., Jesús, E.G., Andrew, B., Allen, C., and Ryan, B. (2015). “Geotechnical Engineering Circular No. 7 Soil Nail Walls - Reference Manual”, Report FHWA-NHI-14-007, Federal Highway Administration, Washington, D.C. [9] Shen, C.K., Bang, S., and Hermann, L.R. (1981). “Ground movement and analysis of earth support system”, Journal of Geotechnical Engineering, 107(12):1609–1624. [10] Cheuk, C.Y., Ng, C.W.W., and Sun, H.W. (2005). “Numerical experiments of soil nails in loose fill slopes subjected to rainfall effects”, Computers and Geotechnics, 32(4):290–303. [11] Kim, J.S., Kim, J.Y., and Lee, S.R. (1997). “Analysis of soil nailed earth slope by discrete element method”. Computers and Geotechnics, 20(1):1–14. [12] Guler, E., and Bozkurt, C.F. (2004). “The effect of upward nail inclination to the stability of soil nailed structures”, Geotechnical Engineering for Transportation Projects, 2213–2220. [13] Gui, M.W., and Ng, C.W.W. (2006). “Numerical study of a nailed slope excavation”. Geotechnical Engineering, 37(1):1–12. [14] Patra, C.R., and Basudhar, P.K. (2005). “Optimum design of nailed soil slopes”, Geotechnical and Geological Engineering, 23(3):273–296. [15] Keunza K, Dorji Y, Wangda D (2004) Landslides in Bhutan. Country Report, Department of and Mines, Royal Government of Bhutan, Thimpu, 8 p [16] Byrne, R.J., Cotton, D., Porterfield, J., Wolschlag, C. and Ueblacker, G. (1998). “Manual for Design and Construction Monitoring of Soil Nail Walls”, Report FHWA- SA-96-69R, Federal Highway Administration, Washington, D.C. [17] ASTM G57-06 (2012). “Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Method”, ASTM International, West Conshohocken, PA. [18] Getting down to Earth – A practical guide to Earth resistance testing, Megger, www.megger.com Vol. 22 [2017], Bund. 13 4982

[19] Indian Standard: 1498, “Classification and Identification of soils for general engineering purposes”, Bureau of Indian Standards, New Delhi, India, 1970 (Reaffirmed 2007). [20] Indian Standard: 2720 - (Part VIII), “Methods of Test for Soils: Determination of -Dry density relation using Heavy compaction”, Bureau of Indian Standards, New Delhi, India, 1983 (Reaffirmed 2008). [21] Indian Standard: 2720 - (Part XI), “Methods of Test for Soils: Determination of the shear strength parameters of a specimen tested in unconsolidated undrained triaxial compression without the measurement of ”, Bureau of Indian Standards, New Delhi, India, 1993 (Reaffirmed 2002). [22] Indian Standard: 2720 - (Part V), “Methods of Test for Soils: Determination of Liquid and Plastic Limit”, Bureau of Indian Standards, New Delhi, India, 1985. [23] Barry, D.S. (2014). “SNAP_2 (Soil Nail Analysis Program) User’s Manual”, Report FHWA-IF-14-016, Federal Highway Administration, Washington, D.C.

Other EJGE papers on Soil Nailing for Slope Stability [24] S. Rawat and A. K. Gupta: “An Experimental and Analytical Study of Slope Stability by Soil Nailing” Electronic Journal of Geotechnical Engineering, 2016 (21.17), pp 5577- 5597. Available at ejge.com. [25] Surender Singh: “Soil Nailing for Stability of Slopes” Electronic Journal of Geotechnical Engineering, 2014 (19.Z2) pp 9889-9895. Available at ejge.com. [26] Jefferson Lins da Silva, Francisco Shigueo Urakawa, Lucas Deroide do Nascimento, and Clever A. Valentin: “Global Stability of Reinforced Soil with Nail and ” Electronic Journal of Geotechnical Engineering, 2014 (19.E), pp 1123-1133. Available at ejge.com.

© 2017 ejge

Editor’s note. This paper may be referred to, in other articles, as: Dr. Raju Sarkar*, Ritesh Kurar, Sangay Zangmo, Ugyen Dema, Sujan Subba and Daman Kumar Sharma: “Application of Soil Nailing for in Bhutan: A Case Study at Sorchen Bypass” Electronic Journal of Geotechnical Engineering, 2017 (22.13), pp 4963- 4982. Available at ejge.com.