IGC. 50 th. 50 th INDIAN GEOTECHNICAL CONFERENCE

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1 INFLUENCE OF CORE THICKNESS ON STABILITY OF UPSTREAM SLOPE OF EARTH AND ROCKFILL DAMS UNDER RAPID-DRAW-DOWN Rajesh Khanna 1, Manoj Datta 2, G. V. Ramana 3 ABSTRACT Earth and rockfill dams are characterized by two zones - an inner zone of earth or soil (core) and an outer zone of rockfill (shell). The core material has lower shear strength parameters than the shell material. A designer has to arrive at a dam section which has steep upstream and downstream slopes so that quantity of earth work is minimum. The steepness of the slopes is governed by stability considerations which in turn are influenced by the strength of the core and shell materials as well as their geometry. In earth and rockfill dams, the dimensions of the core and shell are usually determined by the types of soils and their quantities available near the dam site. When rockfill material is available in abundance and core material is scarce, a dam with thin core is adopted and vice versa. If both fine grained soil for the core, as well as coarse grained soil/rock fill for the shell are readily available in abundance, one can design a large variety of dam sections with different thicknesses of core. To arrive an economical dam section, it is necessary to choose a core thickness which results in steepest side slopes and minimum earth work. An analytical study has been undertaken to identify the zone, within which the thickness of a vertical core has no influence on the stability of upstream slope of earth and rockfill dams under rapid-draw-down condition. A 180 m high earth and rockfill dam section, founded on strong base, having section and slopes similar to a high dam in the northern India was used as a base section for analysis. Various sections of the dam, with different thicknesses of core were analyzed for slope stability using standard available software to arrive at the factors of safety of the upstream slope of the dam for each case. The influence of parameters such as drawdown level, relative strength of core to shell and height of dam was studied. The critical core thickness was identified, beyond which the factor of safety of upstream slope of the dam was observed to undergo reduction. The study shows that for the base case, the factor of safety of upstream slope under rapid-draw-down remains high when the thickness of vertical core is less than of height of dam. Beyond this thickness there is a drop in factor of safety. The magnitude of critical thickness varies from 100 to 200% of height of dam depending on the relative strength of core to shell of the dam. The magnitude of drawdown (between ¼ to ¾ dam height) does not affect the slope stability results. The reduction of factor of safety, beyond critical thickness is higher for lower relative strength of core to shell, however the reduction is not significantly dependent on height of dam. It is concluded from the study that vertical core causes a reduction in the stability of upstream slope of earth and rockfill dams under rapiddraw-down if it is positioned such that it lies in the zone beyond that defined by the critical thickness. 1 Dr. Rajesh Khanna, CSMRS, New Delhi, India, rajesh12khanna@yahoo.com 2 Prof. Manoj Datta, Civil Engineering, IIT Delhi, New Delhi, India, mdatta@civil.iitd.ac.in 3 Prof. G. V. Ramana, Civil Engineering, IIT Delhi, New Delhi, India, ramana@civil.iitd.ac.in

2 Rajesh Khanna, Manoj Datta, G. V. Ramana Thus, this study is useful for demarcating the safe zone within an earth and rockfill dam for positioning of core considering the strength properties of available construction materials and other relevant parameters to provide maximum stability and minimum earth work to minimize the cost of the project. Keywords: Earth and rockfill dam, core, rapid-draw-down Simplified dam section with vertical core having variable thickness

3 INFLUENCE OF CORE THICKNESS ON STABILITY OF UPSTREAM SLOPE OF EARTH AND ROCKFILL DAMS UNDER RAPID-DRAW-DOWN Rajesh Khanna, Scientist, CSMRS, New Delhi, India; Manoj Datta, Professor, IIT Delhi, New Delhi, India; G V Ramana, Professor, IIT Delhi, New Delhi, India; ramana@civil.iitd.ac.in ABSTRACT: In earth and rockfill dams, it is necessary to choose a core thickness which results in steepest side slopes and minimum earth work. An analytical study has been undertaken to identify the zone, within which the thickness of a vertical core has no influence on the stability of upstream slope of earth and rockfill dams under rapid-draw-down condition. A 180 m high earth and rockfill dam section, founded on strong base, having section and slopes similar to a high dam in the northern India was used as a base section for analysis. Various sections of the dam, with different thicknesses of core were analyzed for slope stability using standard available software to arrive at the factors of safety of the upstream slope of the dam for each case. The influence of parameter such as drawdown levels, relative strength of core to shell and height of dam was studied. The critical core thickness was identified, beyond which the factor of safety of upstream slope of the dam was observed to undergo reduction. The study shows that for the base case, the factor of safety of upstream slope under rapid-draw-down remains high when the thickness of vertical core is less than of height of dam. Beyond this thickness there is a drop in factor of safety. The magnitude of critical thickness varies from 100 to 200% of height of dam depending on the relative strength of core to shell of the dam. The variation of drawdown levels between H/4 to 3H/4 and the variation of height of dam do not influence the slope stability. The reduction of factor of safety, beyond critical thickness is higher for larger relative strength of core to shell, however the reduction is not significantly dependent on height of dam. It is concluded from the study that vertical core causes a reduction in the stability of upstream slope of earth and rockfill dams under rapid-draw-down if it is positioned such that it lies in the zone beyond that defined by the critical thickness. Thus, this study is useful for demarcating the safe zone within an earth and rockfill dam for positioning of core considering the strength properties of available construction materials and other relevant parameters to provide maximum stability and minimum earth work to minimize the cost of the project. INTRODUCTION To arrive at an economical dam section in earth and rockfill dams with vertical core, it is necessary to choose a core thickness which results in steepest side slopes and minimum earth work. The dimensions of the core and shell are usually determined by the types of soils and their quantities available at the dam site. If fine grained soils (for the core) as well as coarse grained soil/rock fill (for the shell) are readily available in abundance, one can design a large variety of dam sections with different thicknesses of core. The dimensions of core and its thickness is dependent not only on consideration of stability but also on the other factors such as seepage loss, proper sealing at the foundation, resistance to piping, resistance to cracking during seismic loading etc. There are very few studies reported in literature which delineate the zone in which the influence of a vertical core is minimum on stability of upstream slope of dams under rapid-draw-down condition. The main objective of the present investigation is to arrive at the safe zone for the location of vertical core in an earth and rockfill dam within which the stability of upstream slope of dams is not affected under rapiddraw-down condition. OBJECTIVE The present study was undertaken with objective of identifying how the incremental increase in thickness of vertical core in an earth and rockfill

4 Rajesh Khanna, Manoj Datta, G. V. Ramana dam influences the factor of safety of upstream slope of the dam under rapid-draw-down. From such a study, it is possible to identify the critical thickness beyond which core causes a reduction in factor of safety of the dam slopes under rapiddraw-down. Further, it was studied how the critical thickness is influenced by a variety of parameters such as drawdown levels, relative strength of shell and core materials, height of dam. On the basis of the study, a zone has been identified, within which the location of vertical core does not influence the stability of the upstream slope of a dam for the entire range of parameters studied under rapiddraw-down condition. LITERATURE REVIEW A review of literature of earth and rockfill dams reveals the following ((Sherard, 1967; Singh and Sharma, 1976; Datta, 1979; Champa and Mahatharad, 1982; Kutzner, 1997; Singh and Varshney, 2004; World Register of Dams, 2011; Honkanadavar, 2010; CSMRS, 2009, 2011, 2012); Khanna et. al., 2014: a) In earth and rockfill dams, the upstream slope is usually in the range of 1 V: 2.50 H to 1 V: 1.70 H. However in some cases the upstream slope is as flat as 1 V: 3.00 H or as steep as 1 V: 1.40 H. The downstream slope is usually in the range of 1 V: 2.25 H to 1 V: 1.50 H. However in some cases the downstream slope is as flat as 1 V: 2.50 H or as steep as 1 V: 1.25 H. b) The minimum thickness reported for core is 17 % of height of dam c) The maximum thickness reported for the core is 200 % of height of dam. d) In most cases, the value of effective angle of shearing resistance for shell material lies in the range of 35 to 45 and for the core material the values of effective cohesion intercept and effective angle of shearing resistance lie between 0 to 50 kpa and 14 to 25 respectively. e) The value of pore water pressure ratio in core material for end-of-construction condition ranges from 0.35 to f) The influence of core thickness on stability of earth and rockfill dams has not been a subject of detailed studies. Some studies by Singh and Sharma, 1976; Datta, 1979; Datta and Gulhati, 1991and Datta et. al 1994, indicate the influence of core thickness on stability of dam slopes but do not delineate the safe zone for locating the core. The present study attempts to fulfill the gap by identifying the zone within which the location of a core does not influence the stability. It thus enables economic design of earth and rockfill dams through proper location of a core in a dam body resulting in steep slopes and minimum earthwork. ANALYTICAL STUDY A 180 m high earth and rockfill dam, founded on strong base having section and slopes similar to a high dam in Northern India was selected as a base section for the study. A simplified cross-section having flat base and no berms on the upstream and downstream slope of dam, with variable thickness of core has been adopted for the present study as shown in Fig. 1. The core thickness has been increased, in increments symmetrically (Fig.1) till it merges with the upstream and downstream slopes. The factor of safety of the upstream slope was analysed for each thickness of the core, to identify the critical thickness beyond which the factor of safety begins to reduce due to the influence of the core. The parameters of shell and core used in the study for the base case are as follows: Shell Material: c = 0 kn/m², ϕ = 42 (for both end-of construction and steady-state-seepage) Core Material: c = 50 kn/m², ϕ = 24 (for unsaturated soil, endof-construction condition)

c = 0 kn/m², ϕ = 24 (for saturated soil, steadystate-seepage condition and rapid-draw-down condition) When the thickness of core is beyond 150 % of dam height, the factor of safety begins to decrease

The minimum value of factor of safety is obtained when the upstream slope of core merges with the upstream slope of dam. Fig.

5 c = 0 kn/m², ϕ = 24 (for saturated soil, steadystate-seepage condition and rapid-draw-down condition) When the thickness of core is beyond 150 % of dam height, the factor of safety begins to decrease as the failure surface passes through the core (Figure 3 (b)). This is the critical thickness of the core. The minimum value of factor of safety is obtained when the upstream slope of core merges with the upstream slope of dam. Fig. 1 Simplified dam section with vertical core The influence of the following parameters was studied: a) Thickness of the core: From 25 % of dam height to full base width in increments of 25 %. b) Magnitude of drawdown levels: H dd = H/4 to 3H/4 c) Relative strength of shell to core: ϕ shell = 30, 42, 54 with ϕ core = 24 (constant). d) Height of dam: 180 m and 45 m In the present study, the stability analysis has been performed using Bishop s Simplified Method for upstream slope of dam under rapid-draw-down condition. STABILITY OF UPSTREAM SLOPE Influence of Core Thickness Figure 2 depicts the influence of core thicknesses on factor of safety of upstream slope of the dam with H dd = H/2, where H dd is drawdown level and H is height of dam. It is noted form Figure 2 that for variation of vertical core thickness from 25 % to 150 % of dam height (by varying upstream and downstream slopes of vertical core from 1 V: H to 1 V: 0.75 H) there is no change in factor of safety which has a value of 2.45 (this corresponds to tanϕ shell / tanβ for infinite slope in cohesionless soil), because the critical failure surface continue to pass through the shell material (Figure 3 (a)). Thickness of core (as % height of dam) Fig. 2 Influence of vertical core thickness under rapid-draw-down on stability of upstream slope for 180 m high vertical core earth and rockfill (H dd = H/2) Fig. 3 (a) Critical failure surface for 180 m high earth and rockfill dam under rapid-draw-down with vertical core of thickness 100 % of dam height

6 Rajesh Khanna, Manoj Datta, G. V. Ramana Fig. 3 (b) Critical failure surface for 180 m high earth and rockfill dam under rapid-draw-down with vertical core of thickness 200 % of dam height Influence of Depth of Drawdown Levels Figure 3 depicts the influence of depth of drawdown on the stability of upstream slope for different thicknesses of the core. Once again it is noted that the factor of safety decreases when the thickness of vertical core exceeds 150 % of dam height. It is observed that magnitude of drawdown in dam has no influence on factor of safety of upstream slope and the manner in which core thickness of vertical core influences the factor of safety of upstream slope under rapid-draw-down condition because the factors of safety obtained from stability analysis overlap for H dd = H/4, H/2 and 3H/4 (Fig. 3). Influence of Relative Strength of core to shell Figure 4 depicts the influence of relative strength of core to shell on the stability of upstream slope for different thicknesses of the core. It is evident from Fig. 4 that vertical core begins to influence the stability of the dam when its thickness exceeds 100 % for high difference in relative strength of shell to core (i.e. ϕ shell = 54 and ϕ core = 24 ). In contrast when the difference in relative strength is low (i.e. ϕ shell = 30 and ϕ core = 24 ), the factor of safety begins to decrease only after thickness of core exceeds 200 % of dam height. Thickness of core (as % height of dam) Fig. 4 Influence of relative strength under rapiddraw-down (u/s) on stability of upstream slope of 180 m high dam (H dd = H/2) Influence of Height of Dam Figure 5 depicts the influence of height of dam on the stability of upstream slope for different thicknesses of the core. Once again it is noted that the factor of safety decreases when the thickness of vertical core exceeds 150 % of dam height. It is observed that results for height of dam of 180 m and 45 m overlap indicating that height of dam has no influence on factor of safety of upstream slope under H dd = 0.5 H and the manner in which core thickness of vertical core influences the factor of safety. Thickness of core (as % height of dam) Fig. 3 Influence of depth of drawdown on stability of upstream slope of 180 m high dam

7 Thickness of core (as % height of dam) Fig. 5 Influence of height of dam under rapiddraw-down (u/s) on stability of upstream slope of 180 m high dam DISCUSSION This study shows that a vertical core begins to reduce the factor of safety of the upstream slope under rapid-draw-down when thickness of vertical core is beyond a critical value of around 150 % height of dam (for base case). As long as the thickness of core is below the critical value, there is no influence of core on stability of upstream slope of dam. The critical core thickness varies with relative strength of shell to core and reduces to 100 % height of dam when the difference in strength of shell and core is large. In contrast, the value of critical core thickness may be as high as 200 % of dam height when relative strength difference is low. The magnitude of reduction of factor of safety beyond critical thickness is not significantly influenced by height of dam and depth of drawdown. CONCLUSIONS The results of present study are tabulated in Table 1. One notes from this table that the critical core thickness values range from as low as 100% to as high as 200 % of dam height. The following can be concluded: A core does not influence the stability of outer slopes of earth and rockfill dams, if it is positioned such that its thickness is less than 100 % of dam height (for vertical core). A core causes a reduction in the stability of outer slopes of earth and rockfill dams if it is positioned such that its thickness is more than 200 % of the dam height (for vertical core). For the intermediate cases of cores positions, the influence of core depends on factor such as relative strength of shell Thus, the present study is useful for demarcating the safe zone within an earth and rockfill dam for positioning a vertical core under rapid-draw-down. The range of parameters used in the study covering strength properties, drawdown levels and height of dam make the results applicable for a large variety of earth and rockfill dams on strong foundations. REFERENCES 1. Champa and Mahatharadol. (1982). Construction of Sinagarind dam. 14 th ICOLD Congress, Rio de Janero. 2. Central Soil and Materials Research Station (2009). Report on large size triaxial shear test of rockfill material for Salma dam project, Afghanistan. 3. Central Soil and Materials Research Station (2011). Report on large size triaxial shear test on of rockfill material for Suntale dam project, West Bengal, India. 4. Central Soil and Materials Research Station (2011). Report on large size triaxial shear teston of rockfill material for Punatasangchu dam project, Bhutan. 5. Central Soil and Materials Research Station (2012). Report on large size triaxial shear test on of rockfill material for Polvavaram dam project, Andhra Pradesh, India. 6. Datta, M. (1979). Design of Beas dam embankment. International Water Power and Dam Construction J., London, 31 (6), Datta, M., and Gulhati, S.K. (1991). The influence of core thickness and inclination on stability of dams. Proc.9th Asian Regional Conference.

8 Rajesh Khanna, Manoj Datta, G. V. Ramana 8. Datta, M., Kumar, Manjay., and Tankha, Anurag. (1994). The effect of core size and position on the slope stability of zoned embankment dams. Ground Engrg. J., 27 (9), Honkandavar, N.P., (2010). Testing and modeling the behavior of model and prototype rockfill materials. Ph. D. thesis, Indian Institute of Technology Delhi, India. 10. Kutzner, C. (1997). Earth and rockfill dams. Oxford and IBH Publishing, New Delhi. 11. Khanna,R., Datta. M., Ramana. G. V. (2014). End of construction stability of earth and rockfill dam having vertical core. 2014, Kakinada, India. 12. Khanna,R., Datta. M., Ramana. G. V. (2014). Influence of inclination of thin core on stability of upstream slope of earth and rockfill dam, Electronic Journal of Geotechnical Engineering, Boundle (U), pp Rocscience (2006). SLIDE 5.0. Manual of SLIDE 5, 2D limit equilibrium slope stability analysis 14. Sherard, J.L., Woodward, R.J., Gizienski, S.F., and Clevenger, W.A. (1963). Earth and rockfill dams. John Willy & Sons, New York. 15. Singh, B., and Varshney, S.R. (1995). Engineering for embankment dams. Oxford and IBH publishing company New Delhi. 16. Singh, B., and Sharma, H.D. (2004). Earth and rockfill dams. Sarita Parkashan, Meerut. 17 World register of dams (2011), Published by International Commission on Large Dams, Paris

9 Table 1 Critical thickness of vertical core beyond which factor of safety decreases for upstream slope of the dam Upstream slope Critical Condition Influence of Variation of Parameters Critical Core thickness as % of height of dam Drawdown levels H dd = H/4 H dd = H/2 H dd = 3H/4 Rapid-Draw-Down Relative strength of shell and core ϕ shell = 30 ϕ shell = 42 ϕ shell = % 100 % Height of dam 45 m 180 m

APPLICATION OF COMPOSITE CLAY AS CORE MATERIAL IN EARTHFILL EMBANKMENT DAMS

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