Assessment of Rock Slope Stability Along Minas Road, Tons Valley, Uttarakhand Himalaya

INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 9, ISSUE 03, MARCH 2020 ISSN 2277-8616

Assessment Of Rock Slope Stability Along Minas Road, Tons Valley, Uttarakhand Himalaya

Gambhir singh chauhan, h.c. Nainwal

Abstract: Slope stability analysis along the Minas road was carried out at 7 locations. Limestone, sandstone and slates are the main rock types. Geotechnical Data for Rock Mass Rating (RMR) were taken from each location for the rock mass classification. The rock samples were also collected for the lab analysis. The stereographic projections of rock joints were plotted for Kinematic analysis and different slope failures (planar and wedge) were identified. Geological cross section of each rock slope was prepared to show the orientation of different sets of joints with respect to the slope face. Finally the Factor of safety (FOS) of each rock slopes were determined.It was found that among the 7 rock slopes 3 falls in good and 4 in fair RMR class. The Kinematic analysis shows that 2 rock slope form planar failure and 5 wedge failures. The Factor of safety analysis depicts that 3 rock slopes have a FOS value above 1 and form stable slopes while 4 rock slopes shows a FOS value near and below 1 which represent partially stable and unstable slopes.

Keywords : Slope stability, Tons valley, Kinematic analysis, Factor of safety, RMR, UCS, Planar failure ——————————  ——————————

1 INTRODUCTION Stability analysis of slope failures along NH-305, India [11], Slope instability problems are common in hilly terrain Stability analysis of landslides along Balia Nala, Nainital mainly along the roads and highways. Rocks in the Uttarakhand [12], Stability analysis of the Pawari landslide, Himalayan terrain are highly deformed with a number of Himachal Pradesh [13], Landslide stability analysis in structural discontinuities. Slopes of an area become Rudraprayag and Agastyamuni, Uttarakhand [14], Stability unstable either due to natural process or anthropogenic analysis of rock slopes for planar mode of failure [15].Study activities. Road cutting causes the steepening of slopes area lies along Minas road which passes along the Tons and exposer of more discontinuities along which a slope valley and connect the pilgrimage, tourist places such as may fail. Slope stability of an area depends on different Hanol, Tuni, Harkidoon and various villages of Uttarakhand properties of rock mass of that area. The Rock mass along with the main city. Minas road gets blocked in raining a road section can be classified into different RMR classes season due to the slope instability problem which cause the to analyze the slope stability condition. The FOS analysis of interruption in transportation. Slope stability study along the an area is carried out to estimate the stability condition of road section is helpful to know the causes of slope failures the slope. For this purpose various numerical methods are and their mitigation measures. In present work stability used to calculate the FOS of a slope. In this analysis a analysis of unstable Rock slopes located along the minas slope is categorised into stable, partially stable and road was carried out (Fig 1). unstable classes. Slope stability and landslide study along the highways and roads were carried out in past by various 2. STUDY AREA researchers like, [1] carried out the Study of landslides in The area of study is located between latitude 30°36'30"N- Mandakini valley, Stability analysis of Kaliyasaur landslide 30°46'0''N and longitude 77°40'0''E-77°49'0''E and lies in along NH-58 [2], Landslide study on the Berinag Munsiyari the survey of India toposheet no 53F/14 and 53 F/10 along road, Pithoragarh [3], Cut slope stability analysis in the minas road in the Tons valley, Dehradun district of Rudraprayag district of Uttarakhand using CSMR and Uttarakhand (Fig.1). The study area falls in the lesser kinematic analysis [4], Numerical slope stability analysis [5], Himalaya zone and is characterized by the tropical The study of rockslides /cut slopes and their mitigation monsoon climate. The major rainfall in the study area measures [6], Rock mass Assessment along the Sutlej occurs from mid-June till early October. river, [7], Assessment of Rock fall hazard along a NH-58 in the Alaknanda Valley using Kinematic analysis [8], Study of 3. GEOLOGICAL SETUP slopes along the NH-58 using SMR and kinematic analysis The present study area is located in the lesser Himalaya approach [9], Stability analysis of Lakhwar dam reservoir covering sedimentary and metamorphic rocks of the [10], Chakrata, Deoban, Mandhali, Chandpur Formations and Bansa limestone. Slate, sandstone and quartzite are the rock types of the Chakrata Formation and limestone is the ______main rock type of Deoban Formation. The Mandhali and Chandpur Formations are dominantly consists of slate.  Gambhir Singh Chauhan Research Scholar Department of Geology, HNB Garhwal University, Srinagar Garhwal, Uttarakhand, 246174 E- mail: [email protected]  Proff. H.C. Nainwal, Department of Geology, HNB Garhwal University, Srinagar Garhwal, Uttarakhand, 246174 E-mail: [email protected]

At the first step stereographic projections were prepared and joint set responsible for the slope failure were identified using kinematic analysis [25]. This method is highly applied in rock slope stability analysis. Orientation of discontinuities, slope angle and angle internal friction (Փ) are used in kinematic analysis. FOS is calculated by dividing the resisting forces from driving forces of a rock slope. The FOS analysis of plane and wedge failures of different rock slope was performed according to Hoeks and Bray [26]. The value of FOS above 1 indicates stable slopes and value below 1 depicts unstable slope condition [27]. The value of cohesion (C) and internal friction (Փ) for each rock type were taken according to Hoek and Brey [26]. Internal friction angle of 25￮ was taken for slate and 35￮ for sandstone and limestone while Cohesion of 25 MPa was taken for slate and 30 MPa for sandstone and limestone. The equation 1 and 2 were used for FOS analysis of planar and wedge failures. FOS analysis of planar failure is determined for dry condition.

F = C. A + W. cosѱp. tan ɸ/ W. sinѱp (1)

Where cohesion, area and weight of sliding block is represented by C, A and W respectively. ѱ and ѱ Fig.1 Geological map of the study area with line of p f represents the dip amount of joint and angle of slope face. geological cross section

1 Geology of study area has been contributed by different 퐹 = sin β tanɸ sin⁡(휉/2)푡푎푛 휑 authors as, [16], [17], [18] etc. The structural features of (2) study area are Tons thrust, Amtiyar gad thrust and Deoban fault. Different types of fold have developed in the rock Where angle of intersection between discontinuities and mass due to multiple phases of deformation [19]. There are the bisector is denoted by β, angle of internal friction, 3 sets of joints with random sets present in the rock mass wedge angle and plunge of wedge are represented by ɸ, ξ of the study area. and φ respectively. Stereographic projection was used to determine the different factors used in the equation. 4. METHODOLOGY Using the geological map of Valdiya [20] as the base map, 5. RESULT AND DISCUSSIONS the detail geological mapping of the study area was carried Results of the preset study are presented in tables and out. Data of each slope were taken in field using GPS and figures. Rock mass of different slopes falls in good and fair cross section profiles were prepared using Alos Palsar Dem RMR category (Table.1). RMR rock mass of an area are with 12.5 m spatial resolution. Field data was imported in used in civil engineering construction and slope stability Arc-GIS 10.5 and final map was prepared. analysis. Kinematic analysis of rock mass depicts the type and mode of slope failures. Wedge failures were found 4.1 Rock Mass Rating more common in the rock mass (Table. 2). RMR of the rock slopes is based on the 5 parameters such as, Uniaxial compressive strength (UCS), Joint spacing, Table.1 Results of RMR of each location Joint condition, Rock quality designation (RQD), Water Joint Joint Water condition [21]. The condition of discontinuities is depends Locati RL UCS Rating & RQD & spacing & condition Cod & RMR RMR -on rating rating Rating & Rating Rating Class on the aperture, persistence, filling material and weathering L1 39 103 100-250 33.5% 6-10 cm 19 Dry 62 Good Rock (12) (8) (8) (15) condition of joints. The UCS data was collected from the L2 42.2 85.83 50-100 42.4% 6-12 cm 15 Dry 53 Fair Rock field using L-type Schmidt hammer rebound according to (7) (8) (8) (15) L3 42.5 60 50-100 82.6% 20-50cm 18 Dry 67 Good Rock ISRM [22] and converted into UCS using equation given by (7) (17) (10) (15) L4 35 60 50-100 55.4% 30-60 cm 22 Dry 67 Good Rock Derre and Miller [23]. RQD was determined based on (7) (13) (10) (15) number of joints per unit volume (J ) according to L5 25 45.07 25-50 45.7% < 6 cm 15 Dry 47 Fair Rock V (4) (8) (5) (15) Palmstorm [24]. L6 25 45.07 25-50 32.5% 8-14 cm 17 Dry 52 Fair Rock (4) (8) (8) (15) L7 37.5 77.25 50-100 72% 6-10 cm 22 Dry 65 Good Rock RQD = 115-3.3 *J (7) (13) (8) (15) V

4.2 Kinematic and Factor of safety analysis The NX size core samples of different rock were prepared

for UCS analysis according to ISRM [22]. UCS of 2 core samples of Bansa limestone is identified 54 and 123 MPa and 2 core samples of purple sandstone shows an UCS value of 28 and 98 MPa (Fig.5). Gray sandstone and slate shows a low UCS value. UCS value of rocks identified in lab and from Schmidt hammer have a minor difference. Description of each rock slope (L1 to L7) with cross section is given below.

Table .2 Results of Kinematic analysis and Factor of safety

Location Slope face Failure Responsible Rock type Joint trend Angle (￮) type joint FOS J1 210°/25 L-1 Limestone J2 300°/70 N 175￮/55 Planar J1 J 240°/85 1.20 3 J1 N70/40 L-2 Sandstone J2160°/75 N225￮/70 Wedge J1 &J2 J 29°/70 0.91 3 J1 50°/65 L-3 Slate J2 305°/78 N225￮/65 Wedge J1 &J2 1.02 J3 200°/55 J1 50/60 L-4 Sandstone J2 280°/55 N305￮/70 Wedge J1 &J2 0.73 J3 155°/55 J1 55°/60 L-5 Slate J2 335°/75 N/70 Wedge J1 &J2 1.31 J3 55°/80 J1 220°/55 L-6 Slate J2 330°/75 N300/70 Wedge J1& J2 1.05 J3 65°/65 J1 200°/50 L-7 Slate J2 50°/25 N200/65 Planar J1 1.43 J3 320°/70

Fig.2 Geological cross Section of different locations (A-A', L-1 – Siliceous Bansa limestone is the rock type at L-1 B-B', C-C', D-D', E-E', F-F', G-G') along the section line which falls in fair RMR class (Table.1). The cross section shown in Fig.1 along A-A' was prepared which shows relationship between the different sets of joint and slope face (Fig. 2). The height L-3 - Slate of the Chakrata Formation is the main rock type of slope is 220 m and road is located on the slope at an at L-3. Rack mass falls in good RMR category. The cross- elevation 120 m. The slope is unstable and form planner section of slope along C-C' shows a height of 120 m and is failures along J-1 joint sets (Fig.3). The direction of dip of ￮ covered by debris (Fig. 4a). The dip of natural slope angle slope is N175 with a dip amount of 55 . The trend of joint is about 45￮ but due to road cut the slope angle has ￮ set J-1 is N 195/30 . The Slope wash material is present on increased about 65 to 70￮. A wedge failure is found between the slope. FOS determined using equation 1 is 1.20 which J1 and J2 with a plunge of N13/60 (Fig 4b). FOS of 1.02 shows that slope is partially stable (Table.2). was determined which shows that slope is partially stable.

L-2- The purple color sandstone of the Chakrata Formation L-4 – Rock slopes at L-4 is consisting of gray color is the main rock type at Location 2 which falls in fair RMR sandstone of the Chakrata Formation. Rock mass falls in category. The cross-section of slope along B-B' and fair RMR class and slope is covered by overburden stereographic projection are given in Fig.3a & 3b. The ￮ material. The cross section profile along D-D' with height of road cut slope is 150 m and average dip is 55 . stereographic projection are presented in Fig 5a and 5b. Rock slope is unstable and form wedge failures between The height of slope is 260 m with a dip direction of N 305 J1-J2 and J2 - J3 joint sets. The plunge of wedge failure and dip amount of 40￮. Kinematic analysis shows wedge ￮￮ between J1- J2 is N 82/40 and J2-J3 is N85/45 . The rock failure between J-1 and J-2 with a plunge of N342/30￮. The mass is highly deformed and moderately weathered and FOS analysis depicts that slope is unstable as value of covered by overburden material. Factor of safety value FOS is 0.73. 0.91 was determined for wedge failures between J1 and J2 joint sets.

Fig.4.Geological cross section and stareographic projection of location-7

L-7- Slate is the rock type at L-7 which forms planar failures along J-1 joint sets which has a trend of N220/60￮. The orientation of discontinuities is shown in cross-section G-G' and stereographic projection. The height of slope is 180 m and the road is located on the slope at an elevation of 70 m. Rock mass falls in good RMR class and is covered by overburden. The angle of internal friction is kept 25￮ and cohesion 25 MPa to determine the FOS of 1.43 for this slope.

Fig.3 Stereographic projection of different locations (L1, L2, L3, L4, L5, L6, L7) showing different types of failures with respect to the slope

L-5 – Rock mass at L-5 falls in good RMR category and slate of The Mandhali Formation is the main rock type. The cross section along E-E' and stereographic projection shows wedge failures between J1 - J2 and J2-J3 joint sets. The dip of wedge between J1- J2 and J2-J3 is N39/58￮ and N340/60. The height of slope is 140 m along the cross- section and road is located at an elevation of 30 m on the slope. Factor of safety value of 1.31 is determined for this slope.

Fig.5 Strain versus stress curve of UCS. Sample 1&2 L-6- Rock mass falls in fair RMR category and slate is the Bansa limestone, Sample 3&4 purple sandstone. main rock type at L-6. Geological cross section along F-F' was drawn and kinematic analysis shows wedge failures between J1 and J2 with a plunge of 260/48￮ (Fig 2&3). The slope height is 130 m and road is located on the slope at an elevation of 60 m. The FOS value of 1.05 is identified for this slope which depicts that slope is partially stable.

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