APPLICATION OF COMPOSITE CLAY AS CORE MATERIAL IN EARTHFILL EMBANKMENT DAMS

International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 8, August 2018, pp. 790 797, Article ID: IJCIET_09_08_080 Available online at http://www.iaeme.com/ijciet/issues.asp?jtype=ijciet&vtype=9&itype=8 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 IAEME Publication Scopus Indexed APPLICATION OF COMPOSITE CLAY AS CORE MATERIAL IN EARTHFILL EMBANKMENT DAMS Rupali Sarmah Assistant Professor, Department of Civil Engineering, Assam Engineering College, Guwahati - 781013, Assam, India Anshul Kumar, Arpit Chaudhary, Sourav Bhardwaj, Pankaj Thakur Undergraduate Student, Department of Civil Engineering, National Institute of Technology Hamirpur, Himachal Pradesh-177005, India ABSTRACT Earthfill and rockfill embankment dams are man-made finite slopes, used for retaining water within river banks. Such dams are consists of two main parts- a pervious zone to give structural stability to the slope and an impervious zone to prevent seepage of water through the dam, absence of which may result in seepage related slope instability. The mechanical properties of core material are different from the same of embankment dam shell material. This affects the stability of the embankment dam slope. In this study, the possibility of application of composite clay, which is a mixture of clay and gravel, as a core material is studied using Slope/W tool by GeoStudio, which is a limit equilibrium software package. Morgenstern-Price method is used for the stability analysis of the upstream slope. Stability analysis of the embankment dam with the varying percentage of gravel is conducted, for high flood level (HFL) and low flood level (LFL) in upstream of the embankment dam. The effect of gravel content on interslice normal force, interslice shear force and the factor of safety of the upstream slope of an embankment dam is observed. Key words: Composite clay, Earthfill embankment dams, Limit equilibrium analysis, Morgenstern-Price method. Cite this Article: Rupali Sarmah, Anshul Kumar, Arpit Chaudhary, Sourav Bhardwaj, Pankaj Thakur, Application of Composite Clay as Core Material in Earthfill Embankment Dams. International Journal of Civil Engineering and Technology, 9(8), 2018, pp. 790-797. http://www.iaeme.com/ijciet/issues.asp?jtype=ijciet&vtype=9&itype=8 1. INTRODUCTION Earthfill and rockfill embankment dams are man-made finite slopes used for retaining water within river banks or riverbank stabilization. Such dams are consists of two main parts- a pervious zone to give structural stability to the slope and an impervious zone to prevent http://www.iaeme.com/ijciet/index.asp 790 editor@iaeme.com

Application of Composite Clay as Core Material in Earthfill Embankment Dams seepage of water through the dam, absence of which may result in seepage related slope instability. Clay is the preferred impervious core material due to this reason. Composite clay is a mixture of clay, as the main body, and aggregates, which are floating within the clayey matrix [4]. Composite clay can also be used as the embankment dams core material when aggregates are mixed with highly plastic clay. The core is provided in earthfill embankment dams to restrict the seepage flow through the dam, making the later impervious. The mechanical properties of core material are different from the same of embankment dam shell material. This affects the stability of the embankment dam slope. Moreover, very steep slope may not be stable. Rapid drawdown, a hypothetical condition which assumes that the water level goes down rapidly and suddenly, can result in failure of the slope. The failure of the slope may lead to loss of life and structure. Therefore, it is advisable to check the slope stability of embankment dams during rapid drawdown condition. 1.1. Literature Review Examples of stability analysis of embankment dams using analytical and numerical methods are found in literatures [2] and [11]. Gottardi et al., 2016 [1] evaluated the riverbank stability under transient seepage condition to estimate the riverbank probability of failure. Khanna et al., 2015 [6] undertook an analytical study to identify the zone of thickness of vertical core, which has the influence on the stability of upstream slope of earth and rockfill dams under rapid draw-down condition. The study suggested that the magnitude of reduction of factor of safety beyond critical thickness is not significantly influenced by height of dam and depth of drawdown. Sarkar et al., 2009 [9] conducted stability analysis of soil slope in Luhri area, Himachal Pradesh using GALENA software. The study revealed that the slope was critically stable at dry condition whereas unstable in wet condition especially during monsoon time. Tafti et al., 2008 [10] carried out a dynamic analysis on Karkheh Dam in Iran to observe the effect of core materials on permanent displacement. The study reported that there will be an increase in displacement due to seismic pore pressure build up in composite clays. Holtz and Ellis, 1961 [3] conducted the experimental investigation on the effect of gravel content on the shear strength of clayey soils. The authors stated that, the variation in shear strength will vary be dependent upon the clay matrix and the gravel type. The authors also revealed that the soil had shear properties similar to the clay matrix for an addition of gravel upto 35%. Beyond 50% gravel content, the effect of gravel was apparent because of large particle interference. 1.2. Objective The objective of this study is identified as to carry out numerical analysis to study the effect of variation in gravel content on upstream slope stability of an earthfill embankment dam in Swan River, Himachal Pradesh, India during high flood level and low flood level conditions using Slope/W. 2. STABILITY ANALYSIS The stability analysis was carried out with the help of Slope/W module of GeoStudio to determine Factor of Safety of the finite slope. GeoStudio is a software suite which uses limit equilibrium method for analysis of slopes. Morgenstern Price Method was adopted for the analysis. Half-sine function was taken as the side function. Slip surface direction was considered from left to right or vice versa as according to the upstream or downstream condition. After considering all the basic conditions, cross-section of given embankment was modelled by joining specific coordinates. http://www.iaeme.com/ijciet/index.asp 791 editor@iaeme.com

Rupali Sarmah, Anshul Kumar, Arpit Chaudhary, Sourav Bhardwaj, Pankaj Thakur 2.1. Geometric Properties A simplified diaphragm embankment dam similar to an embankment in Swan River, Una district of Himachal Pradesh, India was considered for this study. The embankment had base width 11m and core height 3 m. The thickness of impervious core was considered up to 7 m. The side slope of local earth was considered to be 2.5:1 for upstream side and downstream slope ratio to be 1.5:1. A granular sub grade course 25 cm thick was provided at the crown of the embankment. Also, a slope protection in the form of stone pitching of unit weight 26.5 kn/m 3 on the downstream (d/s) side (0.50 m height and 0.15 m thick) was provided. The overall embankment was laid over the impervious bedrock like sandstone. For before drawdown condition or high flood level (HFL) condition, the water surface level was considered up to 2.6 m from the river base. For rapid drawdown or low flood level (LFL) condition, the water surface level was considered 0.5 m above the upstream river base. Simplified cross sections during HFL and LFL used for the analyses are shown in Figure 1 and Figure 2 respectively. Figure 1 Cross-section of embankment dam during high flood level Figure 2 Cross-section of embankment dam during low flood level 2.2. Material Properties The upstream casing was considered to be made up of local saturated sand and the downstream casing was considered to be made up of local dry sand. The granular subgrade course was considered as concrete. The material properties of local saturated sand in upstream, local dry sand in downstream, stone protection works and cement overburden for given embankment are given in Table 1. http://www.iaeme.com/ijciet/index.asp 792 editor@iaeme.com

Application of Composite Clay as Core Material in Earthfill Embankment Dams For embankment dam core, the material properties with the variation in the percentage of gravel are mentioned in Table 2. Table 1 Material properties of different components of the dam [5], [7] and [9] Material Type Saturated sand Dry sand Cement overburden Model Mohr-Coulomb Mohr-Coulomb Mohr-Coulomb Unit Weight (kn/m 3 ) 22.77 17.65 23 Cohesion, c (kn/m 2 ) 0 0 10.335 Friction angle, φ 30 30 37 P.W.P (Piezometric line) 1 1 1 Table 2 Material properties considered for the dam core [3] Gravel Content (%) For 0% gravel For 20% gravel For 50% gravel Model Mohr-Coulomb Mohr-Coulomb Mohr-Coulomb Unit Weight (kn/m 3 ) 16.17 17.72 19.28 Cohesion, c (kn/m 2 ) 59.923 48.23 31 Friction angle, φ 24 26 32 P.W.P (Piezometric line) 1 1 1 Beyond 50% gravel content, the properties of the composite would be governed by the gravel matrix. That would not fulfil the purpose of providing core in an embankment. Hence, gravel content was limited to 50% for this study. 2.3. Morgenstern Price Method In this research, Morgenstern-Price method is used for the stability analysis of upstream slope. Morgenstern and Price, 1965 [8] developed a method for analysis of finite slope, where interslice normal force and interslice shear forces both are considered during the analysis. This method satisfies both force and moment equilibrium conditions in each slice. The interslice shear force is computed as a percentage of the interslice normal force as Where X = interslice shear force E = interslice normal force λ = the percentage of function used f(x) = interslice force function representing the relative direction of the resultant interslice force 3. RESULTS AND DISCUSSION 3.1. Effect on Interslice Normal Force Figure 3 depicts the influence of gravel content in the core on the interslice normal force for HFL and LFL both. For HFL, the maximum interslice normal forces are found to be 6.81 kn, 6.35 kn and 6.02 kn for 0%, 20% and 50% gravel content respectively. For LFL, the maximum interslice forces are found to be 2.49 kn, 1.61 kn and 1.61 kn for 0%, 20% and 50% gravel content respectively. It is observed that, for the same gravel content, the maximum interslice normal force during LFL condition is decreasing significantly when compared to maximum interslice normal force during HFL. It is also observed that the size range of slice subjected to interslice normal force is decreasing with increase in gravel content for both before and after rapid drawdown conditions. As it is well established fact that, normal forces contribute to the stability of slope; therefore, it can be concluded from the above http://www.iaeme.com/ijciet/index.asp 793 editor@iaeme.com

Rupali Sarmah, Anshul Kumar, Arpit Chaudhary, Sourav Bhardwaj, Pankaj Thakur observation that factor of safety will decrease with increase in gravel content for this particular case study. HFL with 0% gravel content LFL with 0% gravel content HFL with 20% gravel content LFL with 20% gravel content HFL with 50% gravel content LFL with 50% gravel content Figure 3 Interslice normal force for HFL and LFL with variation in gravel content 3.2. Effect on Interslice Shear Force Figure 4 depicts the influence of gravel content in core on interslice shear force for HFL and LFL both. For HFL, the maximum interslice shear forces are found to be 0.60 kn, 0.50 kn and 0.50 kn for 0%, 20% and 50% gravel content respectively. For LFL, the maximum interslice shear forces are found to be 0.97 kn, 0.64 kn and 0.33 kn for 0%, 20% and 50% gravel content respectively. It is observed that for the same gravel content, the maximum interslice shear force during LFL is increasing when compared to the maximum interslice force during HFL upto 20% gravel content and then decreasing at 50% gravel content. Moreover, for the same gravel content, the position of maximum interslice shear force is http://www.iaeme.com/ijciet/index.asp 794 editor@iaeme.com

Application of Composite Clay as Core Material in Earthfill Embankment Dams shifting to right with increase in gravel content for low flood level. Furthermore, it is also observed that the range of slices subjected to interslice shear force is decreasing marginally, before rapid drawdown and the same is decreasing significantly, after rapid drawdown with increase in gravel content. Again, it is established that shear forces contributes to the instability of slope; therefore it can be concluded from the above observation that factor of safety will decrease with increase in gravel content for this particular case study. HFL with 0% gravel content LFL with 0% gravel content HFL with 20% gravel content LFL with 20% gravel content HFL with 50% gravel content LFL with 50% gravel content Figure 4 Interslice shear force for HFL and LFL with variation in gravel content 3.3. Effect on Factor of Safety The effect of gravel percentage in composite clay on the factor of safety for the Swan river embankment is obtained as given in Table 3. http://www.iaeme.com/ijciet/index.asp 795 editor@iaeme.com

Rupali Sarmah, Anshul Kumar, Arpit Chaudhary, Sourav Bhardwaj, Pankaj Thakur Table 3 Factor of safety (F. O. S.) obtained with variation in gravel percentage Gravel Percentage in Core F.O.S. during H.F.L F.O.S. during L.F.L 0% 1.720 0.935 20% 1.716 0.909 50% 1.710 0.885 Figure 5 Critical failure surface for 20% gravel during Low Flood Level For all the variation in percentage of gravel, the factors of safety are found to be more than 1.2 for HFL which indicates that the slope is safe during HFL. However, the factors of safety are found to be less than 1.2 for LFL which is not acceptable indicating that the slope is not safe during rapid drawdown for this particular case study of embankment in Swan River with composite clay. 4. CONCLUSIONS After studying the effect of variation in gravel content on upstream slope stability of an earthfill embankment dam in Swan River, Himachal Pradesh, India during HFL and LFL conditions using Slope/W, the following conclusions are obtained- For the same gravel content, the maximum interslice normal force during LFL condition is decreasing significantly compared to the maximum interslice normal force during HFL. For the same gravel content, the maximum interslice shear force during LFL is increasing compared to the maximum interslice shear force during HFL upto 20% gravel content. Whereas for 50% gravel content opposite trend can be seen. The factor of safety is found to follow a decreasing trend with increase in gravel content for HFL and LFL both. The upstream slope is not safe during LFL or rapid drawdown for this particular case study of embankment in Swan River with composite clay. REFERENCES [1] Gottardi, G., Carmine, G. G., Rocchi, I. and Bittelli, M. Assessing river embankment stability under transient seepage conditions. Procedia Engineering, 158, 2016, pp. 350-355. http://www.iaeme.com/ijciet/index.asp 796 editor@iaeme.com

Application of Composite Clay as Core Material in Earthfill Embankment Dams [2] Hasani, H., Mamizadeh, J. and Karimi, H. Stability of slope and seepage analysis in earth fills dams using numerical models (Case study: Illum dam-iran). World Applied Sciences Journal, 21(9), 2013, pp. 1398-1402. [3] Holtz, W.G. and Ellis W. Triaxial shear characteristics of clayey gravel soils. 5 th International Conference on Soil Mechanics and Foundation Engineering, Paris, vol. 1, 1961, pp. 143-149. [4] Jafari, M.K., and Shafiee, Mechanical behavior of compacted composite clays. Canadian Geotechnical Journal, 41(6), 2004, pp. 1152-1167. [5] Joyee, M. J., Geotechnical Evaluation Report: Embankment and Foundation Stability and Settlement, Housatonic river and Naugatuk river flood protection projects, section I, Ansonia and Derby, Connecticut. PS&S Integrating design and engineering, Warren, New Zealand, 2010. [6] Khanna, R., Datta, M. and Ramanna, G. V. Influence of core thickness on stability of upstream slope of earth and rockfill dams under rapid-draw-down. Proceedings of 50 th Indian Geotechnical Conference, Pune, Maharastra, India, 2015. [7] Maddukuri, N. N., Ravali, N. V. N. and Vasudeo, A. D. Design of embankments and bank protection works for hilly rivers. Journal of Civil Engineering and Environmental Technology, 2(9), 2015, pp. 58-62. [8] Morgenstern, N. R. and Price, V. E. The analysis of the stability of general slip surfaces. Geotechnique, 15, 1965, pp. 79-93. [9] Sarkar, K., Sazid, M., Khandelwal, M. and Singh, T. N. Stability analysis of soil slope in luhri area, Himachal Pradesh. Mining Engineers, Journal, 10(6), 2009, pp. 21-27. [10] Tafti, S. R., Shafiee, A. and Rajabi, M. M. The influence of clay core composition on the permanent displacement of embankment dams. Proceedings of the 14 th World Conference on Earthquake Engineering, Beijing, China, 2008. [11] Zhang, L. L., Zhang L. M. and Tang, W. H. Technical note on Rainfall-induced slope failure considering variability of soil properties. Geotechnique, 55(2), 2005, pp. 183-188. http://www.iaeme.com/ijciet/index.asp 797 editor@iaeme.com