Stability Analysis of Kelau Earth-Fill Dam Design under Main Critical Conditions

Transcription

1 Stability Analysis of Kelau Earth-Fill Dam Design under Main Critical Conditions Mohd Ashraf Mohd Ismail School of Civil Engineering, USM, Engineering Campus, NibongTebal, Seberang Perai Selatan, P. Pinang, Malaysia. Soon Min Ng School of Civil Engineering, USM, Engineering Campus, NibongTebal, Seberang Perai Selatan, P. Pinang, Malaysia. Eng Keat Gey School of Civil Engineering, USM, Engineering Campus, NibongTebal, Seberang Perai Selatan, P. Pinang, Malaysia. ABSTRACT The increased development and expansion of population in urbanize area in Malaysia have created a definite need to construct an inter-basin water transfer tunnel to fulfil the increase demand of water in the water scarcity region. As one of the main component in the inter basin raw water transfer tunnel project, the Kelau Dam had been designed to serve as a regulating dam in order to maintain a sufficient water level in the Kelau River for the water intake. The successful construction and operation of the dam over the full range of loading require a comprehensive evaluation of the dam designed before the construction proceeds. Thus, this study serves to investigate the factors that can cause instability of the earth-fill dam slope and to evaluate the earth-fill dam performance under main critical conditions by determining the factor of safety (FOS) of the dam conceptual design. Four main critical conditions considered in the analysis are end of construction, steady state, steady state with seismic loading and rapid drawdown conditions. This study began with detailed site characterization and investigation for the engineering information needed in the slope stability analysis. The dam slope stability analysis considering of the critical conditions were carried out using limit equilibrium method (LEM), shear strength reduction method together with seepage analysis using finite element method (FEM). It is found that FOS of rapid drawdown and steady state with seismic loading conditions are close to each other and have obvious lower value compare to steady state and end of construction conditions. The 2-D conceptual model of the earth-fill dam can be classified as stable under all critical loadings as the FOS are more than the minimum requirement specified by the design. Factors of safety computed by both LEM and FEM are close to each other where differences in percentage are less than 6 %. Based on the seepage qualitative and quantitative results analysis, piping failure at the toe of downstream slope is unlikely to occur due to systematic arrangement of filters and drains at internal surface of slope. KEYWORDS: Earth-fill dam, slope stability, limit equilibrium method, finite element method, factor of safety, end of construction, steady state, steady state with seismic loading, rapid drawdown

2 INTRODUCTION 3210 A dam is a hydraulic structure constructed across a stream, river, or waterway for the purpose of confining and controlling the flow of water. Dams are built for specific functions such as for water supply, irrigation, flood control and also to generate hydroelectric power [1]. There are two types of modern dam namely embankment dam and concrete dam. Embankment dam consist of homogeneous earth-fill dam, zone type rock-fill dam, impervious core type rock-fill dam, facing type rock-fill dam, roller compacted concrete (RCC) embankment dam, and also blasted rock-fill dam. Besides that, types of concrete dam consist of concrete gravity type dam, masonry dam, arch type dam, buttress type dam, and trapezoidal-shaped cemented sand and gravel (CSG) dam [2]. Generally several factors are considered in selecting the type of dam such as site conditions, topography, geology and foundation conditions, material available, environmental and economic [3]. In Malaysia, embankment dam is more commonly constructed compare to concrete dam due to economic reasons and the availability of construction materials at the site. All dams are designed and constructed to meet specific criteria. First, a dam should be built from locally available materials wherever possible. Second, the dam must remain stable under all conditions, during construction, and ultimately in operation, both at the normal reservoir operating level and under all flood and drought conditions. Third, the dam and foundation must be sufficiently watertight to control seepage and maintain the desired reservoir level. Finally, it must have sufficient spillway, outlet works capacity and freeboard to prevent floodwater from overtopping the dam crest [4]. Thus, evaluation on the dam design must be performed to ensure these criteria are satisfied. Stability of the dam against slope failure is an essential component for the design. Besides than stability, the dam also must be able to exhibit satisfactory serviceability where the dam should be able to perform the intended function throughout the service life [3]. Hence, in this study, limit equilibrium method (LEM) and finite element methods (FEM) were utilized to evaluate the performance of an earth-fill dam under main critical stability conditions. CASE STUDY: KELAU DAM SITE This study was carried out based on the case study of Kelau Dam located in Pahang, Malaysia as shown Figure 1. Kelau Dam is an earth-fill dam designed to serve as a regulating dam in order to maintain a sufficient water level in the Kelau River for water intake work in the inter basin raw water transfer tunnel project. Detailed site investigation comprised of boreholes, drill holes, test pits, field and laboratory test were carried out in the study area. It was found that the bedrock around the dam site comprises of alternating beds of carbonaceous shale and siltstone which are represented by phyllites. The physical properties of the soil change with the degree of weathering and the soil material is basically ranges from stiff clay to silt with varying amounts of sand and gravel. The soil zone has low permeability and the rock zone is quite impervious due to the joints being tight and infilled with clayey matrix.

3 3211 Figure 1: Location of study area The dam is designed as a rolled earth-fill embankment using residual soils of sandy silt or sandy clay with total height of 30 m. Earth-fill materials can be sourced from the areas nearby the site and also from the necessary excavation. The embankment consists of a central clay core with upstream and downstream shoulders constructed with residual soil of sandy silt and sandy clay. The clay core have a minimum width of 6.25 m at the top and side slopes of 3.2 to 1 on upstream face with slope angle of approximately 18 and 2.8 to 1 on the downstream face with slope angle of approximately 20.The downstream slope of the dam has a 4 m wide berm. Drainage channels on the berm will intercept surface water run-off thereby protecting the slopes below from erosion. The developed 2D conceptual model for Kelau Dam is shown in Figure 2.

4 3212 Figure 2: 2D conceptual modell for Kelau Dam The design parameters for the dam were obtained from the results of laboratory soil testing carried out during the site investigation stage. Soil strength tests using UU and CU triaxial test were carried out on saturated specimens at a stress range comparable to that expected in the dam. Meanwhile, stability analysis were carried out under four main critical conditions namely at the end of construction, steady state, steady state with seismic loading and rapid drawdown using LEM and FEM analyses. LEM is a conventional and well established methodd that is widely used by engineers and researchers to perform slope stability analysis. This method provides an estimation of the factor of safety (FOS) even without the knowledge of initial conditions [5].. One of the weaknesses of LEM is the failure of this method to consider stress-strainn relationship of the soil. This will result in unrealistic stresses and variations of locall factor of safety along the slip surface are disallowed [6]. Due to technology advancement such as personal computers and development of computer software, an alternative to the conventional LEM was developed whichh is known as finite element method (FEM). FEM is applicable to complex conditions and is able to produce information regarding the stresses, strains, soil movements and even pore pressures which cannot be done by LEM. Thus, the performance of the slope can be investigated throughout its service life by using FEM. In this study, both LEM and FEM approaches were adopted by utilizing Rocscience Slide 6.0 and Rocscience Phase program to analyze the dam stability. Both results will then be compared and verified with the minimum required FOS emphasized by USACE (2003) [7]. Slope stability analyses were performed in terms of effective stress parameters and at the anticipated pore pressure ratios or piezometric heights within the dam. High construction pore pressure is likely to build up in the clay core and pore pressure dissipation is expected to occur. The end of construction pore pressure ratio, r u is estimated to be 0.5 in the most critical areas of the core. The pore pressure response to rapid shoulder fill may be quite high but it is anticipated that pore pressure dissipation will occur during construction with provisions of sand blanket at each berm level. The end of construction pore pressuree ratio, r u is estimated to be 0.3. Sand blankets were used in the upstream and downstream shoulder fill at each berm level to relieve high pore water pressure in the internal surface of slope during construction and rapid drawdown of reservoir. Table 1 shows the characteristics of the materials used forr the stability analysis.

5 3213 Table 1: Characteristics for dam slope stability analysis Material Effective Stress Parameters Unit Weight ɸ' Pore pressure ratio at end of ɣ' (kn/m3) C' (kpa) ( ) construction (r u ) Shoulder Core Foundation Filter STABILITY ANALYSIS USING LIMIT EQUILIBRIUM METHOD The FOS under four main critical conditions were determined using five different limit equilibrium methods namely Ordinary/Fellenius, Bishop s Simplified, Janbu s Simplified, Spencer s and GLE/Morgenstern-Price method. All the FOS at the end of construction obtained for both upstream and downstream slope has a maximum value of and a minimum value of These results met the minimum required FOS stated in USACE (2003) which is 1.3 [7]. Excess pore water pressure is expected to increase due to shoulder filling during construction stage. However, with proper installation of sand blanket at each berm, the pore pressure dissipation can occur and thus will improve the slope stability at the end of construction. For steady state condition with full supply of water level at 85 m LSD (Land-surface datum), the FOS obtained for both upstream and downstream ranges from to The results showed that the FOS satisfied the minimum required FOS which is 1.5. During steady state condition, the ponded water will exert external force on the upstream surface to counter balance the internal force exerted by the pore water pressure. Hence, the FOS computed under this condition is higher indicating a stable slope. Seismic hazard for Kelau Dam is also taken into consideration in the dam stability analysis. The value of 0.1g was adopted as the critical acceleration under steady state condition. The maximum and minimum FOS for the upstream and downstream under this condition is and respectively. The results satisfy the minimum required FOS of 1.1 according to [7] but has a lower value of FOS compared to the steady state condition without seismic loading. This is mainly due to the effect of ground shaking that will weaken the soil strength and thus will decrease the FOS. For rapid drawdown, the condition was analysed with maximum flood level of m LSD lowered to a minimum operating level of 73 m LSD. The analysis results showed that the FOS for the upstream slope ranges from to The FOS is higher than 1.2 which is the minimum required FOS stated by [7] and thus the dam is categorized as stable during rapid drawdown condition. From the results, it can be observed that rapid drawdown is more critical compared to the steady state and end of construction. This is mainly due to the internal pore water pressure in the dam that is unable to dissipate out as quickly as the lowering of water level in the reservoir resulting in unbalance force equilibrium. The upstream slope will tends to be pushed outward by the force of the internal pore water pressure in the dam and thus reducing the stability. However, this problem can be overcome by proper geometry design of the dam and also integration of

6 3214 drainage system to the dam such as sand blanket or internal drain. The results of the dam stability analyses for 4 critical conditions using LEM are summarized in Table 2. Table 2: Results of dam stability analysis by limit equilibrium method (LEM) Critical stability conditions Minimum FOS (USACE, 2003) End of construction Steady state Steady state with seismic loading of 0.1g Rapid drawdown Upstream Downstream Upstream Downstream Upstream Downstream Upstream Downstream Ordinary/Fellenius Bishop s Simplified Janbu s Simplified Spencer s GLE/Morgenstern- Price Remark Stable Stable Stable Stable Stable Stable Stable Stable Among the 5 LEMs, Janbu s Simplified method shows the lowest FOS. This is mainly due to the moment equilibrium equations that are not satisfied by Janbu s Simplified method compared to other methods [8]. Generally, the FOS analysed by Bishop s Simplified, Spencer s and GLE/Morgenstern-Price method produced approximately similar results and all the FOS obtained under 4 main critical conditions indicated that the dam has satisfactory stability. STABILITY ANALYSIS USING FINITE ELEMENT METHOD Other than LEM, the stability of the dam was also analysed by finite element method (FEM) with shear strength reduction technique which was proposed by Zienkiewicz et al. (1975) [9]. This method is effective for assessing the performance together with the safety factor of slope and locating the failure surface that were proven by past studies [10]-[12]. The essence of FEM with shear strength reduction technique is the reduction of the soil strength parameters until the soil fails [13]. The FOS produced by this method is known as the strength reduction factor (SRF). The dam was modelled with 3 noded triangular meshes with a total of numbers of elements as shown in Figure 3. The material type model utilized in this study is the elastic plastic soil model with Mohr Coulomb failure criterion. The results of SRF determined under the 4 main critical conditions for upstream and downstream slope are summarized in Table 3.

7 3215 Figure 3: Dam meshing using finite element method with shear strength reduction technique Table 3: Results of dam stability analysis using FEM with shear strength reduction technique (FEM) Minimum required Critical stability SRF factor of safety (FOS) condition (USACE, 2003) Upstream Downstream End of Construction Steady state Steady state with seismic loading of 0.1g Rapid drawdown The results of SRF showed that all the analyzed conditions have met the minimum required FOS established by USACE (2003) [7] and the dam is considered stable for its design. Among the 4 conditions analyzed, steady state with seismic loading of 0.1g exhibits the criteria of the most critical condition based on the SRF obtained. STABILITY ANALYSIS USING LIMIT EQULIBRIUM METHOD FOS from GLE/Morgenstern-Price of the LEM is chosen to be compared with SRF of the FEM with shear strength reduction technique. This is because the FOS computed by GLE/Morgenstern-Price satisfies all the static equilibrium conditions which consist of moment and force equations and thus will produce a more realistic FOS [8]. Comparison of the results as shown in Table 4 indicates that the percentage of difference between FOS and SRF is less than

8 3216 6%. This indicates that the results obtained agree very well with each other and is acceptable. The small difference between the FOS and SRF is mainly due to the incorporation of stress strain relationship in FEM while the slice forces in LEM are only determined by static equilibrium. From the comparison between LEM and FEM, each method presents different critical condition. The critical condition for LEM is the rapid drawdown condition while the FEM is the steady state condition with seismic loading of 0.1g. This is also due to the stress strain relationship that is incorporated into FEM for the analysis. Since the seismic loading of 0.1g involved ground motion, therefore the results produced by FEM is more sensitive compared to LEM as stress strain distributions are taking place. Hence, with cross referencing between the results of LEM and FEM, both conditions of rapid drawdown and seismic loading are considered critical for the design of an earth-fill dam. Loading condition Table 4: Comparison of stability analysis results between LEM and FEM LEM Percentage of difference FEM GLE/Morgenstern-Price (%) Upstream Downstream Upstream Downstream Upstream Downstream End of construction Steady state Steady state with seismic loading of 0.1g Rapid drawdown SEEPAGE ANALYSIS The earth-fill dam is subjected to seepage through embankment and foundation. Therefore seepage control is essential to prevent dam failure due to excessive uplift pressure, piping through the dam body and also foundation, erosion etc. Rocscience Phase program was utilized to analyze the seepage problem using finite element method (FEM). The hydraulic conductivity parameters of the dam materials used in the seepage analysis are listed in Table 5. These parameters are obtained based on the typical hydraulic conductivity parameters, k given in Look (2007) [14].

9 Section Table 5: Hydraulic properties for seepage analysis [14] Material used Hydraulic Conductivity, k ( m/s ) Shoulder Sandy silt and sand clay Core Wet compacted clay Foundation Sandstone and phylite Filters and drains Sands and gravels Curtain grouting Cement grout and bentonite (5 Lugeon) 3217 Based on the seepage analysis result determined by FEM as shown in Figure 4, the phreatic line has been lowered down effectively through the wet compacted clay core and thus the pore water pressure in the internal surface of downstream are expected to be under control. The results of total head also show that the seepage in the dam is within the acceptable limit. Drainage facilities such as filters and drains are also incorporated into the embankment design to control seepage. Drains are position mainly in the downstream shoulder of the dam to intercept seepage flow through core, foundation and abutments and to convey water out of the dam. Filters are incorporated in the drain design to prevent erosion of core and foundation soils into the drain. Figure 4 also shows the seepage water indicated by the flow lines are conveyed to the downstream toe of the dam and the drainage facilities have helped to minimize the development of pore water pressure in the downstream. Therefore, the stability of the downstream slope can be enhanced. Figure 4: Seepage analysis during steady state condition without seismic loading CONCLUSION Based on the results in this study analyzed by both limit equilibrium methods (LEM) and finite element method (FEM), the conclusions that can be drawn are:

10 3218 The conceptual model of the earth-fill embankment dam can be classified as stable under all critical loadings because the FOS and SRF obtained are more than the minimum requirement. The FOS and SRF computed by LEM and FEM with shear strength reduction technique for the four critical conditions are close to each other where the percentages of difference are all less than 6%. Hence, both methods are satisfactory for engineering usage in the stability analysis of earth-fill dam. Based on the seepage qualitative and quantitative results, the pore water pressure is unlikely to build up at the downstream slope due to the systematic arrangement of drainage facilities such as filters and drains of the dam. ACKNOWLEDGEMENT The authors gratefully acknowledge the assistance and cooperation given by KeTHHA (Ministry of Energy, Green Technology and Water Malaysia) and Tokyo Electric Power Services Co., Ltd. (TEPSCO) to carry out this study successfully. REFERENCES [1] Novak, P., Moffat, A.I.B., Nalluri, C. and Narayanan R. (2001) Hydraulic Structures Fourth Edition, Taylor & Francis Group, London. [2] Kutzner, C. (1997) Earth and Rockfill Dams: Principles of Design and Construction, A. A. Balkema, Netherlands. [3] USACE (2004) General Design and Construction Considerations for Earth and Rock-Fill Dams, Department of The Army, U.S. Army Corps of Engineers, Washington DC, United States of America. [4] Ratnayaka, D.D., Brandt, M.J. and Johnson, M. (2009) Twort's Water Supply 6 th Edition, Butterworth-Heinemann: Elsevier. [5] Cheng, Y. M., Lansivaara, T. and Wei, W. B. (2007) Two-dimensional slope stability analysis by limit equilibrium and strength reduction methods. Journal of Computers and Geotechnics, Vol. 34, pp [6] Duncan, J.M. (1996) State of art: limit equilibrium and finite element analysis of slopes, ASCE, Journal of Geotechnical Engineering, 122(7); [7] USACE (2003) Engineering and Design: Slope Stability,Department of The Army, U.S. Army Corps of Engineers, Washington DC, United States of America. [8] Krahn, J. (2004) Stability modeling with SLOPE/W: An engineering methodology. Alberta: Geo-Slope/W International Ltd. [9] Zienkiewicz, O. C., Humpheson, C. and Lewis, R. W. (1975) Associated and nonassociated visco-plasticity and plasticity in soil mechanics. Geotechnique 1975; 25(4): [10] Griffiths, D. V. and Lane, P. A. (1999) Slope stability analysis by finite elements. Journal of Geotechnique, 49(3):

11 3219 [11] Lane, P.A. and Griffiths, D.V. (2000) Assessment of stability of slopes under drawdown conditions. J Geotech Geoenviron Eng, ASCE 2000; 126(5): [12] Cai, F. and Ugai, K. (2004) Numerical analysis of rainfall effects on slope stability. Int J Geomech, ASCE 2004; 4(2): [13] Huang, M. and Jia, C. Q. (2008) Strength reduction FEM in stability analysis of soil slopes subjected to transient unsaturated seepage. Computers and Geotechnics, 36 (2009): [14] Look, B. G. (2007) Handbook of Geotechnical Investigation and Design Tables,Taylor & Francis /Balkema, London, UK, pp ejge