Modelling the Construction of a High Embankment Dam

Transcription

1 KSCE Journal of Civil Engineering (2014) 18(1): Copyright c2014 Korean Society of Civil Engineers DOI /s TECHNICAL NOTE Geotechnical Engineering pissn , eissn Modelling the Construction of a High Embankment Dam Qun Chen*, Yu Hua Zou**, Min Tang***, and Chang Rong He**** Received July 1, 2011/Revised 1st: July 22, 2012, 2nd: December 27, 2012/Accepted April 17, 2013 Abstract Appropriate layer quantity has been considered to model the sequential construction of a high embankment dam over 300 m and its effect on the predicted settlement of the 314 m high Shuangjiangkou dam has been discussed. The simulation results have demonstrated that at least 25 layers are required to accurately model the stage construction of a high embankment dam over 300 m. The stress and deformation within the dam and the foundation during the dam construction and reservoir filling have been simulated using finite element analysis. Saturated-unsaturated seepage theory is used to analyse the transient seepage field in the dam and in the foundation. Two different cases about the construction and the operation are modelled. One involves gradual reservoir filling after the completion of sequential construction, whereas the other involves sequential construction and reservoir impounding by several interleaved stages. The simulation results for both cases have been compared and discussed. Keywords: embankment dams, reservoirs, coupled analysis, numerical modelling 1. Introduction Many high dams had been constructed in the last decade. At present, there are many ones in the design or under the construction in China. Embankment dams are one of the most common types of them. Their height has increased from just 100 m around to more than 300 m. During the construction of the high embankment dam, both the stresses and deformations developed in the dam and the foundation are key factors to the safety of the dam, so engineers and researchers engaged in the design of such high embankment dams are very interested in these issues. The finite element method is a powerful tool to analyze and solve problems in constructions of the embankment dam as it can calculate the internal deformation of the core and shell so that the stress distribution and load transfer within a dam section can be obtained. Many researchers have used this method to study the deformations and stresses in embankment dams. A twodimensional plane strain finite element method was used to study the stresses and deformations of an embankment and was proposed to solve the problem of nonlinear material properties (Clough and Woodward, 1967). Based on different testing results, Boughton (1970) gave formulas for the nonlinear elastic modulus and Possion s ratio and computed the deformations of a dumped rockfill dam. Duncan and Chang (1970) proposed a hyperbolic constitutive model for the nonlinear stress-strain relationship of soil, which has often been used for stress and deformation analysis in embankment dams (Kulhawy and Duncan, 1972; Sharma et al., 1979; Adikari and Parkin, 1982; Shen, 1987; Khalid et al., 1990; Yang, 1995; Sun et al., 2006). Naylor et al. (1981) proposed a nonlinear K-G model for finite element analysis in geotechnical engineering and used it to predict the construction performance of the Beliche dam (Naylor et al., 1986). Martin (1978) used an isoparametric element with a quadratic displacement function and the substructure method to perform a three-dimensional finite element analysis for an 80 m high rockfill dam. Shen (1990) proposed a double yield surface hardening model for stress-strain analyses of soils, and it has been widely used in stress and deformation analyses of embankment dams in China (Shen and Wang, 1990; Shen and Zhang, 1991; Liu et al., 1999). Alonso et al. (1990) proposed a constitutive model for partially saturated soils that permits stress-strain analyses of earth and rockfill dams (Alonso et al., 2005; Costa and Alonso, 2009). With the development of computational techniques, the stressstrain relationship of soil and rockfill has developed from simple nonlinear elastic models to complex elasto-plastic ones. The 314 m high Shuangjiangkou core wall rockfill dam, located on the Dadu River in the southwest of China, will be the second highest embankment dam in the world. The main zones *Professor, State Key Laboratory of Hydraulics and Mountain River Engineering, College of Hydraulic and Hydropower Engineering, Sichuan University, Chengdu , China (Corresponding Author, chenqun@scu.edu.cn) **Assistant Researcher, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu , China ( nrdb_003@ 163.com) ***Engineer, Guodian Dadu River Zhentouba Hydropower Construction Co. Ltd., Leshan , China ( tangmin009@163.com) ****Professor, State Key Laboratory of Hydraulics and Mountain River Engineering, College of Hydraulic and Hydropower Engineering, Sichuan University, Chengdu , China ( hechr@163.com) 93

2 Qun Chen, Yu Hua Zou, Min Tang, and Chang Rong He Fig. 1. The Riverbed Cross Section Profile of the Shuangjiangkou Dam in the embankment, shown in Fig. 1, are composed of a gravelclay core, a double-layer filter, a transition, a rockfill shell and a kentledge. The core is placed on hard granite through a concrete base slab having a grouting gallery. There is a 96 m deep grout curtain in the bedrock below the concrete base slab. The rockfill shell is built on three overburden layers above the granite. The overburden layers consist of boulder cobble gravel on the top (overburden 3), sandy cobble gravel (overburden 2) in the middle and cobble gravel with boulder (overburden 1) at the bottom. The rockfill section has an upstream slope of 2.0H:1.0V with a berm at an elevation of m and a downstream slope of 1.9H:1.0V. The elevation of the crest and the base of the dam are m and m, respectively. The crest is 8 m wide. The top and bottom width of the core are 4 m and 128 m, respectively. In order to study the stress and deformation in the dam, appropriate layer quantity for modelling the sequential construction of a high embankment dam over 300 m has been proposed based on the impact of the layer quantity on the predicted settlement of the dam at the end of construction. Coupled stress-pore pressure analyses have been carried out on a two dimensional plane strain condition using the finite element program Geostudio2007 (Geoslope International Ltd, 2007) for two possible cases, one is gradual reservoir filling after the sequential construction of the dam, and the other is sequential construction and reservoir filling by several interleaved stages. 2. Coupled Analysis Method 2.1 Constitutive Equation for Soil Structure Considering soils as ideal elasto-plastic materials, the incremental strain-stress relationship for an unsaturated soil medium can be written as follows (Fredlund and Rahardjo, 1993): { σ} { m} u a = ([ D e ] [ D p ]){ ε} [ D e ]{ m H } ( u a u w ) where { σ} and { ε} are stress and strain incremental matrices, respectively; u a and u w are the pore-air pressure and pore-water pressure, respectively; {m} = { } T, is the unit isotropic tensor; {m H } = , where, H is the H H T unsaturated soil modulus for a soil structure with respect to matric suction (u a u w ). [D e ] and [D p ] are drained elastic and plastic constitutive matrices, respectively. [D e ] is the relation matrix between the stress and elastic strain. [D p ] reflects the (1) impact of plastic strain on the relationship between the stress and the total strain and it can be obtained by the following formula: [ D p ] where, F is the yield function and G is the plastic potential function; σ is the effective stress. It can be assumed that the air pressure remains at atmospheric pressure at all times, thus Eq. (1) becomes: When the soil is fully saturated, Eq. (3) should be: Comparing Eq. (3) and (4), it can be seen that, when the soil is fully saturated, the following condition should be satisfied: Therefore, the modulus H can be obtained by solving Eq. (5). 2.2 Flow Equation for the Water Phase The equation governing the water flow through unsaturatedsaturated soils can be obtained by introducing Darcy s law into the continuity equation as follows (Fredlund and Rahardjo, 1993): where k x and k y are the hydraulic conductivity in the x and y directions, respectively; γ w is the unit weight of water; θ w is the volumetric water content; and t is time. The increment of the volumetric water content, θ w, can be given by the following expressions (Dakshanamurthy et al., 1984): where, [ D e ] G F T [ D e ] σ σ = F T [ D e ] G σ σ { σ} = ([ D e ] [ D p ]){ ε} + ([ D e ] [ D p ]){ m H } u w { σ} = ([ D e ] [ D p ]){ ε} + { m} u w ([ D e ] [ D p ]) m H { } = { m} k ---- x 2 u w k y 2 u w θ w = 0 γ w x 2 γ w y 2 t θ w = β ε v + ω u w E β = H( 1 2µ ) (2) (3) (4) (5) (6) (7) (8) 94 KSCE Journal of Civil Engineering

3 Modelling the Construction of a High Embankment Dam 1 3β ω = R H where ε v is the increment of the volumetric strain; E is the elastic modulus; µ is the Poisson s ratio; R is a modulus relating the change in volumetric water content to the change in matric suction, which can be obtained by inversing the slope of the volumetric water content curve. Strain can be expressed by a displacement based on the deformation compatibility conditions. Considering increment of displacement and pore-water pressure as field variables, Eq. (1) and Eq. (6) can be solved simultaneously using finite element method, therefore, the stress, displacement and pore-water pressure can be obtained synchronously. In addition to the constitutive model of the material, the volumetric water content function and the hydraulic conductivity function are also required to conduct a coupled analysis. 3. An Elasto-Plastic Model for Soils In this analysis, a nonlinear elasto-plastic constitutive model is used to simulate the stress-strain relationship of the soil. The Mohr-Coulomb yield criterion (Chen and Zhang, 1990) is used as the yield function, F, of the elasto-plastic model: F J 2 sin θ + π -- (10) J 2 3 cos θ + π -- I = sinϕ sin ϕ ccosϕ 3 where J 2 is the second deviatoric stress invariant; I 1 is the first stress invariant; and θ is the Lode angle. The soil properties are defined as effective parameters. The plastic potential function, G, used in the analysis has the same form as the yield function except that the effective friction angle, ϕ, is replaced by the dilation angle, ψ. If the soil is unsaturated, the soil cohesion can be computed as: c c ( u a u w )tanϕ θ w θ = r (11) θ s θ r where c is the effective soil cohesion; θ s and θ r are the saturated and residual volumetric water contents, respectively. A water content equal to 5% of θ s is taken as the residual water content θ r. The elastic modulus of the soil is usually not a constant and depends on the stress state. The nonlinear elastic modulus of the soil is calculated using the following formula (Byrne et al., 1987): σ E= Kp v a p a n (9) (12) where σ v is the effective vertical stress; K and n are the coefficient and exponent, respectively, defining the influence of the vertical stress on the elastic modulus; and p a is the atmospheric pressure. 4. Analysis Cases and Boundary Conditions The incremental construction analysis results obtained by Clough et al. (1967) showed that the construction process has a strong effect on the displacement of the embankment. Many researchers have used about 10 layers to simulate the sequential construction process of dams (Clough and Woodward, 1967; Kulhawy and Duncan, 1972; Adikari and Parkin, 1982; Shen and Wang, 1990; Shen and Zhang, 1991; Yang, 1995; Liu et al., 1999; Sun et al., 2006). However, there are few reports studying on the sequential construction of a high dam with its height over 300 m. In order to determine appropriate simulation layers for the stage construction of 314 m high Shuangjiangkou dam, five different simulation layers (10, 15, 20, 25 and 30) were chosen to simulate the construction process. In this analysis, total stress analysis was used and the pore pressure was not considered. Two analysis cases (Case I and Case II) were carried out to study the effects of construction and reservoir filling processes on the stress and deformation of the dam using coupled analysis. The total stress analysis was used for the foundation. Case I involves a gradual filling of the reservoir after the sequential construction of the embankment, whereas Case II involves reservoir filling by interleaved stages during the sequential construction of the embankment. In both cases, the construction of the dam was simulated using 36 lifts. The first 6 lifts were used to simulate the construction of the cofferdam and the other 30 lifts were used to simulate the construction of the main embankment. In Case I, it was simulated to place one more layer of the embankment every 30 days, and then to fill the reservoir from an elevation of m (dam heel) to the normal water level of m after 1530 days. In Case II, the dam construction and reservoir filling were simulated by 14 stages (as listed in Table 1). Fig. 2 shows the change in dam lifting elevation and the water level versus time in both analysis cases. Analyses were conducted for both the embankment and the foundation. The upstream boundary of the foundation is 330 m beyond the dam heel, and the downstream boundary is 333 m beyond the dam toe. The underside boundary of the foundation is 396 m below the concrete base slab and 300 m below the bottom Table 1. Simulation Stages of Dam Construction and Reservoir Filling in Case II Stages Time (d) Duration (d) Elevation of Dam Lifting (m) Water Level (m) Vol. 18, No. 1 / January

4 Qun Chen, Yu Hua Zou, Min Tang, and Chang Rong He 5. Material Parameters Fig. 2. Change in Dam Lifting Elevation and the Water Level versus Time of the grout curtain. The initial ground water table was assumed to lie at the base of the dam and the foundation was submerged. The initial vertical stresses in the foundation were computed from the gravity of the overburden soils and foundation rock. The initial horizontal stresses were calculated using Poisson s ratio of the soil and the rock. The same initial stress and hydraulic conditions were applied to both analysis cases. The upstream and downstream boundaries of the foundation were fixed in x direction. The base of the foundation was fixed in both x and y directions. In addition to the aforementioned displacement boundaries, the hydraulic boundaries need to be defined in the coupled analysis. The upstream boundary of the foundation and the upstream slope of the dam below the normal water level of m were defined as the total head condition. The head is a function of time and rises from an elevation of m to an elevation of m by stages with the time (as shown in Fig. 2). A total head with a constant elevation of m was applied to the downstream ground surface and to the downstream boundary of the foundation. The downstream slope was defined as a zero flux boundary condition if the total head was less than the elevation head; otherwise it was considered as a free outflow boundary. The other boundaries of the simulation domain were defined as zero flux boundaries. The foundation rock was modelled using a linear elastic model. All of the soils in the embankment and the overburden layers were modelled using a nonlinear elasto-plastic model, which can account for dilation of the soil. Considering the foundation rock, concrete cutoff wall and grout curtain are much more rigid than the surrounding soils, they were assumed to be a linear elastic material. The property parameters of the concrete for the finite element analysis were adopted according to code GB (The Construction Ministry of People s Republic of China, 2002). The material parameters of the soils and rocks were synthetically determined based on many test results obtained by several research institutes (Hydro-China Chengdu Engineering Corporation, 2008). The elastic modulus and Poisson s ratio for the foundation rock were obtained using a uniaxial compression test. The strength parameters, the Poisson s ratio and the dilation angle of the soil were obtained using a drained triaxial test. The parameters K and n used to fit the nonlinear elastic modulus of the soil were obtained from a confined compression test and from the relationship between the modulus of compressibility, the Poisson s ratio and the elastic modulus. The saturated hydraulic conductivity k sat was obtained by large scale permeability test. All the parameters for various materials used in the finite element analyses are listed in Table 2 and Table 3. Two soil property functions, which are the volumetric water content function and the hydraulic conductivity function, are required to solve the seepage equation in the coupled analysis. In this paper, they were estimated following the procedures proposed Table 2. Linear Elastic Material Parameters used in Analyses Material Concrete Rock Unit Weight γ (kn/m 3 ) Elastic Modulus E (GPa) Poisson s Ratio µ Saturated Conductivity k sat (m/d) Table 3. Nonlinear Elasto-Plastic Material Parameters used in Analyses Material Effective Effective Dilation Saturated Unit Weight Coefficient Exponent Poisson s γ (kn/m 3 Cohesion c Friction Angle Angle Conductivity ) K n Ratio µ (kpa) ϕ ( ) ψ ( ) k sat (m/d) Overburden Overburden Overburden Cofferdam Core Filter Filter Transition Upstream Rockfill Downstream Main Rockfill Downstream Second Rockfill KSCE Journal of Civil Engineering

5 Modelling the Construction of a High Embankment Dam least 25 to obtain accurate simulation results. In this work, 30 layers were used to simulate the main embankment dam and to study the stresses and deformations in the dam during construction and reservoir filling. Fig. 3. Hydraulic Conductivity Functions of the Soils by Fredlund et al. (1994, 2002). Fig. 3 shows estimated hydraulic conductivity functions of the gravel-clay core, the filter, the transition and the rockfill. 6. Results and Discussion 6.1 Effect of Simulation Layers of Construction on the Deformation The change in the maximum settlement and upstream horizontal displacement at the end of dam construction with the quantity of simulation layers is shown in Fig. 4. Both the settlement and the displacement nonlinearly decrease with the increasing quantity of simulation layers. There is only a slight change in the deformation when simulation layers are more than 25. The maximum settlements are m and m for the simulation layers of 25 and 30 respectively, and their difference is only m or 0.90%. Accordingly, the maximum upstream displacements are m and m respectively and their difference is only m or 0.38%. However, the differences of the maximum settlements and the maximum upstream displacements are m (3.74%) and m (1.03%) respectively between simulation layers of 20 and 25. Therefore, it is suggested that the quantity of simulation layers for the sequential placement of a high dam about 300 m should be at 6.2 Pore-Water Pressures in the Dam In addition to the stresses and deformations in the dam, a coupled analysis also considers the hydraulic conditions, such as pore-water pressures and total water heads. Figs. 5 and 6 show the contours of pore-water pressures in the dam in Case I and Case II, respectively. At the end of the sequential placement of the dam, the phreatic line where the pore-water pressure in the dam is zero, is a little higher than the initial water level at the ground surface, especially in the gravel-clay core. The reason for this result is that the excess pore-water pressure in the core cannot dissipate immediately during construction due to lower conductivity of the core than that of the shell. During reservoir filling, the pore-water pressures in the upstream shell and the core increase with the increasing water level, but there is almost no change in the downstream shell. Because the conductivities of the core is about 4 orders of magnitude lower than that of the shell, the phreatic line exits the back of the core at a very low level and shows little change with the increasing water level. The distributions of the pore-water pressure in the dam in both Fig. 5. Contours of Pore-Water Pressures in the Dam in Case I (Units: kpa): (a) At the End of Construction, (b) At Normal Water Level Fig. 4. Change in the Maximum Displacement and Settlement with the Quantity of Simulation layers Fig. 6. Contours of Pore-Water Pressures in the Dam in Case II (Units: kpa): (a) At Water Level m (Dam Crest Elevation m), (b) At Normal Water Level Vol. 18, No. 1 / January

6 Qun Chen, Yu Hua Zou, Min Tang, and Chang Rong He cases I and II are almost identical when the reservoir water level rises to the normal water level. It can be concluded that the construction and filling processes have little effect on the final distribution of pore-water pressure in the dam. 6.3 Stresses in the Dam The contours of the horizontal and vertical effective stresses in the dam are shown in Figs. 7 and 8. At the end of dam construction in Case I, both the horizontal and vertical effective stresses gradually increase with the dam construction. At corresponding level of equal depth below the crest of the dam, the stresses in the core are less than those in the filter, transition and shell due to the lower elastic modulus of the core. In both Case I and Case II, the horizontal and vertical stresses reduce considerably in the upstream half of the dam when the reservoir is filled because the application of the reservoir water load results in excess porewater pressure and uplift pressures in the upstream half of the dam. The horizontal stresses in the downstream shell increase when the reservoir is full because the seepage force in the core is transferred to the downstream shell. The vertical stresses in the downstream shell do not have much change when the water has filled the reservoir because the phreatic line and the pore-water pressure almost do not change during the filling process (Fig. 6). The distributions of the stresses in the dam are almost the same in both analysis cases when the water has fully filled the reservoir. This indicates that the construction and reservoir filling processes have little effect on the final distribution of effective stresses in the dam. Figures 9 and 10 show the distributions of the horizontal and vertical effective stresses during different stages at an elevation of m. The elevation of the dam crest and the water levels corresponding to a given time in the two figures can be found in Fig. 2 and Table 1. In Case I before the reservoir is filled (before 1080 days), the distributions of the horizontal and vertical stresses along width direction of the dam at a given elevation are nearly symmetrical Fig. 7. Contours of Horizontal Effective Stresses in the Dam (Isoline Units: kpa): (a) At the End of Construction in Case I, (b) At Normal Water Level in Case I, (c) At Normal Water Level in Case II Fig. 8. Contours of Vertical Effective Stresses in the Dam (Isoline Units: kpa): (a) At the End of Construction in Case I, (b) At Normal Water Level in Case I, (c) At Normal Water Level in Case II Fig. 9. Distributions of Horizontal Effective Stresses at Different Stages at an Elevation of m: (a) Case I, (b) Case II 98 KSCE Journal of Civil Engineering

7 Modelling the Construction of a High Embankment Dam days in Case I and from 1920 days to 2010 days in Case II, the water level rises in elevation from 2460 m to 2500 m, but the decreases of the stresses in both the horizontal and vertical directions are less than that during previous reservoir filling steps in both Case I and Case II. Fig. 10. Distributions of Vertical Effective Stresses at Different Stages at an Elevation of m: (a) Case I, (b) Case II to the dam axis (x = 0) (Fig. 9a and Fig. 10a). As the reservoir is filled, the horizontal stress decreases gradually in the upstream half of the dam and increases gradually in the downstream half of the dam. In Case II, both the construction and the reservoir filling result in horizontal stresses increasing in the downstream half of the dam (Fig. 9b). In comparison with Case I, the decrease in the horizontal stress in the upstream half of the dam over time is less than that in Case II. The difference is because the construction of the dam introduces an increase in the horizontal stress while filling the reservoir does a decrease in the horizontal stress. The magnitude of the decrease is larger than that of the increase, so the final change in the horizontal stress is decrease. In Case II, there is no dam construction, only water level increases between 930 days and 990 days, 1110 days and 1200 days, 1920 days and 2010 days. It can be seen in Fig. 10 that the vertical stress in the downstream half of the dam has little change during the reservoir filling, while it increases during the dam construction in both cases I and II. The vertical stress in the upstream half of the dam increases during the dam construction but decreases when the reservoir is filled. When the water level is close to the capacity of the dam, the effect of the water level on the stresses in the upstream half of the dam is minor. For example, from the time of 2490 days to Deformations in the Dam The contours of horizontal displacement in the dam are shown in Fig. 11. At the end of the dam construction in Case I, the maximum upstream and downstream horizontal displacements occur at about one third of the dam s height in the upstream and downstream shell, and they are m and m, respectively. When the water fully fills the reservoir, the upstream displacements decrease and the downstream displacements increase due to the effect of the water load. The upstream displacement distributions in the dam are similar in both cases I and II, and the maximum upstream displacements are m and m, respectively. They occur near the heel of the upstream cofferdam. However, the distributions of the downstream displacements are clearly different in cases I and II. The maximum downstream displacement occurs at the crest of the dam in Case I while it appears at about three fifths of dam s height in the core in Case II. They are m and m, respectively. The discrepancies of the distributions and values of the downstream displacements between the two cases are significant. This is because the stress paths that the soils of the dam went though are different between the two cases. Figure 12 shows the distributions of the horizontal displacements along width direction of the dam at different stages at an elevation of m. In both cases, the downstream horizontal displacements increase during the dam construction and reservoir filling. The upstream horizontal displacements increase with the dam construction (before 1080 days in Case I as shown in Fig. 12a, and from 680 days to 930 days, from 990 days to 1110 days and from 1200 days to 1920 days in Case II as shown in Fig. Fig. 11. Contours of Horizontal Displacements in the Dam (Units: m): (a) At the End of Construction in Case I, (b) At Normal Water Level in Case I, (c) At Normal Water Level in Case II Vol. 18, No. 1 / January

8 Qun Chen, Yu Hua Zou, Min Tang, and Chang Rong He Fig. 12. Distributions of Horizontal Displacements at Different Stages at an Elevation of m: (a) Case I, (b) Case II Fig. 14. Distributions of Settlements at Different Stages at an Elevation of m: (a) Case I, (b) Case II Fig. 13. Contours of Vertical Settlements in the Dam (Units: m): (a) At the End of Construction in Case I, (b) At Normal Water Level in Case I, (c) At Normal Water Level in Case II 12b). However, they decrease with the increasing reservoir water level (after 1080 days in Case I, and from 930 days to 990 days, from 1110 days to 1200 days and from 1920 days to 2610 days in Case II). The contours of settlements in the dam are shown in Fig. 13. At the end of the dam construction in Case I, the maximum settlement occurs at about half of the height of the dam in the core and amounts to m. When the reservoir is fully filled, the locations of the maximum settlements move downwards and downstream a little. The maximum settlements are m and m in cases I and II, respectively. The difference between the maximum settlements in cases I and II is m. The distributions of the settlements in the dam are generally similar in both cases. In Case II, the contours have a zigzag pattern in the upstream half of the dam, which results from the construction load of the dam being applied by stages. Figure 14 shows the distributions of the settlements along the width direction of the dam at different stages at an elevation of m in both cases. The settlements in both the upstream shell and the core increase during the dam construction (before 1080 days in Case I as shown in Fig. 14a, and from 680 days to 930 days, from 990 days to 1110 days and from 1200 days to 1920 days in Case II as shown in Fig. 14b) but decrease during reservoir filling (after 1080 days in Case I, and from 930 days to 990 days, from 1110 days to 1200 days and from 1920 days to 2610 days in Case II) in both cases. The settlements in the downstream shell of the dam increase during the dam construction 100 KSCE Journal of Civil Engineering

9 Modelling the Construction of a High Embankment Dam and change little during reservoir filling due to the small changes in the pore-water pressures behind the core of the dam. Except for the placement of the kentledge (corresponding to 570 days in Case I and to 680 days in Case II) and it just being inundated by the water, the effect of other dam construction and reservoir filling stages on the settlement of the kentledge ( m < x < m) is small because the load of the embankment above is not applied over the kentledge, whereas it is applied to the main dam directly. The historical maximum settlement is m at the end of the dam construction (1080 days in Fig. 14a) in Case I and is m at the time that the dam construction is completed (1920 days in Fig. 14b) in Case II. The former is m more than the latter. This result indicates that the processes of the dam construction and reservoir filling have a significant influence on the horizontal displacement and settlement of the dam. 7. Conclusions In this work, an appropriate quantity of simulation layers for construction has been chosen and studied to simulate the sequential construction of an embankment dam over 300 m. Stresses and deformations in the high Shuangjiangkou embankment dam are investigated using coupled stress-pore pressure analyses for two possible construction and operation cases, gradual reservoir filling after the sequential construction of the dam (i.e., Case I) and sequential construction and reservoir filling by several interleaved stages (i.e., Case II). The results provide a reference for designers and researchers concerned about the behaviour of the dam. The simulation layers for construction have little effect on the stresses in the dam but have a significant effect on the deformations of the dam. Both settlements and displacements of the dam decrease nonlinearly with the increasing quantity of simulation layers. The change in the deformation with the simulation layers is small if the quantity of simulation layers is more than 25. In Case I, pore-water pressure is produced in the lower part of the core at the end of the construction of the dam. As the reservoir is filled, the pore-water pressures in the upstream shell and the core increase with the increasing water level, but in both cases, there is almost no change in the downstream shell. The construction and reservoir filling processes have little effect on the final distribution of pore-water pressures in the dam at a given reservoir water level. At the same elevation, the stress in the core is less than that in the other parts of the dam. Both the horizontal and vertical effective stresses gradually increase during the dam construction. The horizontal and vertical stresses in the upstream half of the dam are reduced considerably when the dam is submerged. However, the horizontal stress in the downstream half of the dam increases when the reservoir is filled. The vertical stress in the downstream half of the dam changes little with the increasing water level. The construction and reservoir filling processes have little effect on the final distribution of effective stresses in the dam. Deformations of the dam increase with the construction of the dam. The upstream horizontal displacements decrease and the downstream horizontal displacements increase as the reservoir is filled. However, the settlements in the dam decrease as the water level rises. The distributions and the quantities of the deformation in the dam are different for different construction and reservoir filling processes. The maximum historical displacement and settlement from the analysis of Case I are more than those anticipated in Case II. Because the collapse deformation cannot be simulated using current model, the water impoundment of the upstream shoulder causes effective stress reduction and some rebound. This produces some uplift of the soil compensating part of the compression calculated during construction (Case I). In order to include collapse, an elasto-plastic model for unsaturated soils which can simulate collapse behaviour due to wetting, such as BBM (Alonso et al., 1990) will be required. In addition, the initial conditions of suction, preconsolidation stress and water content according to compaction should be included, and the incorporation of rain events and evaporation on the exposed surfaces both during construction and operation should be considered. Acknowledgements This research was substantially supported by the Ph.D. Programs Foundation of Ministry of Education of China (Approval No ) and the New-Century Training Program Foundation for Talents from the Ministry of Education of China (Approval No. NCET ). References Adikari, G. S. N. and Parkin, A. K. (1982). Deformation behaviour of Taibingo Dam. International Journal for Numerical and Analytical Methods in Geomechanics, Vol. 6, No. 3, pp Alonso, E. E., Gens, A., and Josa, A. (1990). A constitutive model for partially saturated soils. Geotechnique, Vol. 40, No. 3, pp Alonso, E. E., Olivella, S., and Pinyol, N. M. (2005). A review of Beliche Dam. Geotechnique, Vol. 55, No. 4, pp Boughton, N. O. (1970). Elastic analysis for behavior of rockfill. Journal of Soil Mechanics and Foundation Division, ASCE, Vol. 96, No. SM 5, pp Byrne, P. M., Cheung, H., and Yan, L. (1987). Soil parameters for deformation analysis of sand masses. 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