Mechanical and Hydraulic Behavior of Cut off-core Connecting Systems in Earth Dams

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1 Mechanical and Hydraulic Behavior of Cut off-core Connecting Systems in Earth Dams Zakaria Zoorasna Graduate Student Faculty of Engineering, Tarbiat Moallem University, Tehran, Iran Amir Hamidi Assistant Professor Faculty of Engineering, Tarbiat Moallem University, Tehran, Iran Ali Ghanbari Assistant Professor Faculty of Engineering, Tarbiat Moallem University, Tehran, Iran ABSTRACT Seepage through foundation of earth dams can be controlled using concrete cut off walls. The increase in hydraulic gradients in connecting zone of cut off wall and core usually results in erosion and water leakage. Also the difference between stiffness of clayey core and concrete cut off wall results in stress concentration and increase in deformations in connecting zone. As a result, connecting systems are usually used between cut off wall and core to reduce the hydraulic gradients and stress concentration. In the present research, Karkheh storage dam in Iran is considered as the case study. Seepage and stress-strain analysis are conducted to investigate the effect of different connection systems on the maximum gradient and stress concentration in connecting zone. In this regard, the most appropriate systems with the most effective characteristics and suitability in construction are recommended. KEYWORDS: Cut off wall; connecting system, hydraulic gradient, stress-strain distribution, Karkheh storage dam. INTRODUCTION Seepage can be controlled in foundation of earth dams using different methods. To select an under seepage control method for a particular dam and foundation, the relative merits and efficiency of different methods should be evaluated (U.S. Army Corps of Engineers, 24). Concrete cut off walls are one of main methods of seepage control and are divided to the following categories according to the material type used in construction: Slurry trench cut off wall

2 Vol. 13, Bund. K 2 Bentonite-cement cut off wall Concrete cut off wall Plastic concrete cut off wall The plastic concrete is an appropriate kind of material due to its high deformability (ICOLD, 1985). The cut off wall construction causes an increase in hydraulic head at the upstream and a reduction in downstream part of foundation. As a result, the maximum gradient happens in connection zone of the cut off wall and core (Shahbazian Ahari et al., 2). The maximum gradient should be less than an allowable limit. Also the difference between stiffness of cut off wall and core results in some stress concentration in connection zone. Connecting systems should be designed to reduce stress concentration and hydraulic gradients in connection zone. This may be achieved by different details for connecting system. Six common details for this goal are as follows and their characteristics are shown in Fig. 1: - Penetration of the cut off into the core - Thick concrete slab at the base level of the core - Combination of cut off penetration into the core and the concrete slab - Compaction grouting around the connection zone in foundation - Clayey soil and a concrete cap over it - Clayey trench In the present study, seepage and stress-strain analysis are used to investigate the effect of different connecting systems on maximum hydraulic gradient and stress-strain distribution in cut off-core connection zone of Karkheh storage dam. System 1: Penetration of the cut off wall into the core System 2: Thick concrete slab at the base level of the core System 3: Combination of cut off penetration into the core and the concrete slab System 4: Compaction grouting around the connection zone in foundation System 5: Clayey soil and a concrete cap over it System 6: Clayey trench Figure 1: Details of different connecting systems

3 Vol. 13, Bund. K 3 CASE STUDY Karkheh storage dam is among the largest dams, in terms of reservoir and volume of fill placed, constructed in Iran. It is a central core, zoned embankment dam, 127 meters high, 33 meters long, with an embankment volume of 32 million cubic meters. The dam crest is located in +234 MSL and the minimum level of the foundation is +16 MSL. Normal water level is in +22 MSL and reservoir has 5572 million m 3 volume at the maximum level (Karkheh dam section engineers, 1998). Foundation of this large earth dam consists of alternative layers of conglomerate and mudstone, in which the conglomerated layers have much more impermeability, resistance and elastic modulus than the mudstone layers. Figure 2 depicts the cross section of the dam, its foundation and cut off wall. Specification of materials used in dam construction are also shown in Table 1. The figure indicates that cut off wall is used in this dam for the control of seepage. Depth of wall is determined based on seepage analysis done in different stages and economical factors. Also the thickness of wall is determined based on allowable hydraulic gradient, hydraulic fracturing pressure, and the drilling facilities. The depth of wall in deepest section is about 8 meters while the average of depth is about 5 meters. With a length of 33 m, it was vertically built in dam foundation along the dam axis. Thickness of the wall is 1 meter at the valley and in the right abutment. At some location of the left abutment, the thickness of the wall is chosen to be.8 meter (Shadravan et al., 24 and Karkheh dam section engineers, 1998). 1. Impervious core (mudstone mixed with sandy gravel) 8. U/S slope protection using soil cement 1A. Impervious core (mudstone) 9. Plastic concrete cut off wall 2. Sandy gravel 1. Pre-coffer dam 3. Conglomerate or sandy gravel 11. Main cofferdam 4. Sand filter 12. Mudstone No. (-1) 5. Gravel filter and drain 13. Mudstone No. (-2) 6. Sand-gravel filter 14. Conglomerate 7. U/S slope protection using limestone riprap 15. Inspection gallery Figure 2: Cross section of Karkheh storage dam (Karkheh dam section engineers, 1995)

4 Vol. 13, Bund. K 4 SEEPAGE ANALYSIS Seepage analysis of dams has been done by different numerical methods in literature (Li and Ming, 24; Ghobadi et al., 25 and Lee et al., 25). In this paper, dam, foundation and different seepage control systems of Karkheh dam were modeled using GMS software in the largest section for seepage analysis (Zoorasna et al., 28). In this section, the cut off wall continues 25.5 meters below the core and fixed in a mudstone layer. Figure 3 shows the finite element mesh used in seepage analysis. The soil anisotropy is modeled using different permeability coefficients in horizontal and vertical directions. The total flow and the maximum hydraulic gradient were determined in connection zone of cut off wall-core connecting systems. Table 1: Specifications of different parts of the dam (Karkheh dam section engineers, 1998) Parameters Shell Core Filter Cut off wall Mudstone layers Conglomerate layer (1) Conglomerate layer (2) Conglomerate layer (3) Dry unit weight (kn/m 3 ) Saturated unit weight (kn/m 3 ) Permeability coefficient (cm/s) Elastic modulus (kn/m 2 ) Poisson's ratio Undrained cohesion (kn/m 2 ) Drained cohesion (kn/m 2 ) Undrained friction angle (degree) Drained friction angle (degree) Dilation angle (degree) Figure 3: Finite element mesh generated for seepage analysis

5 Vol. 13, Bund. K 5 The effect of main characteristics of each connecting system which was previously shown in Fig. 1 is investigated on total flow or maximum hydraulic gradients. A summary of the results is presented in the following sections. Connection system 1 In this case, the cut off wall simply penetrates into the core without any interface material. The length of penetration which was previously defined as "h" in Fig. 1 is the main variable for this connecting system. The effect of penetrating length on total flow is shown in Fig. 4 for different cut off wall permeability values. In this figure "H" is the total length of the cut off wall. For cut off wall permeability values less than that of the core, the total flow decreases when the penetrating length of cut off wall increases. While, there is an increasing trend in total flow for permeability values more than that of the core. 2 Penetration of the cut off wall into the dam core, b=1. (m) Q[(m3/day)/m] K cut off wall=1^-6 (cm/s) K cut off wall=1^-7 (cm/s) K cut off wall=1^-8 (cm/s) (1/3) (1/15) (1/1) (1/9) (1/8) (1/7) (1/6) (1/5) h/h Figure 4: Effect of cut off wall permeability and penetration length on total flow (Detail No. 1) Connection system 2 For this detail, a concrete slab is placed at the base level of the core over the foundation. The cut off wall moves through this slab but does not penetrate into the core. This was previously indicated in Fig. 1. The effect of slab length (B) and slab thickness (t) on total flow is shown in Fig. 5. As the figure illustrates, total flow increases with the increase in slab length or thickness. 1 Thick concrete slab at the base level of the core, k=1 1-7(cm/s).8 Q[(m3/day)/m] t=1.m t=1.5m t=2.m B(m) Figure 5: Effect of slab length and thickness on total flow (Detail No. 2)

6 Vol. 13, Bund. K 6 Connection system 3 The only difference between this case and the previous one is that the cut off wall penetrates into the core after moving through the concrete slab. In this case, the effect of penetrating length, which is previously defined by "h" in Fig. 1 on total flow, is indicated in Fig. 6. As the figure shows, total flow slightly decreases with the increase of penetrating length. Q[(m3/day)/m] Combination of cut off wall penetration into the core and the concrete slab, B=6. (m) K cut off wall=1^-6 (cm/s) K cut off wall=1^-7 (cm/s) K cut off wall=1^-8 (cm/s) (1/3) (1/15) (1/1) (1/9) (1/8) h/h Figure 6: Effect of cut off wall permeability and penetration length on total flow (Detail No. 3) Connection system 4 Grouting process is used in this connecting system to decrease the hydraulic gradients at the point intersection. The reduction in permeability of grouted zone is an important factor affects the amount of maximum hydraulic gradients. The amount of permeability can be controlled using the amount of cement in grout and the pressure used in grouting process. Figure 7 shows the effect of permeability in grouted zone on the maximum hydraulic gradients near the zone of intersection. As the figure shows, the hydraulic gradients sharply reduce with the reduction in permeability of grouted zone. Connection system 5 Clayey soil besides a concrete cap is used in this method as the connecting system as was previously shown in Fig. 1. As an example, the effect of the concrete cap angle (θ) on the total flow is depicted in Fig. 8. As the figure shows, total flow increases with the increase of cap inclination. It can be concluded that the use of a flatter cap can result in reduction of the flow from cut off wall connecting system. However, suitability in construction and also the executive remarks should be considered an important factor in this case.

7 Vol. 13, Bund. K Compaction grouting around the connection zone in foundation, s=3.(m),g=3.(m) 5 i K cut off wall=1^-6 (cm/s) K cut off wall=1^-7 (cm/s) K cut off wall=1^-8 (cm/s) 1*1^-5 5*1^-6 1*1^-6 5*1^-7 1*1^-7 5*1^-8 K Grouted Zone (cm/s) Figure 7: Effect of soil permeability in grouted zone on the maximum hydraulic gradient 2 Clayey soil besides a concrete cap, K Cut off Wall=1 1-7 (cm/s) Q[(m3/day)/m] 1 K clay=5*1^-6 (cm/s) K clay=5*1^-7 (cm/s) K clay=5*1^-8 (cm/s) θ Figure 8: Effect of concrete cap angle on total flow (Detail No. 5) Connection system 6 A small trench filled by clayey soil at the intersection zone can extremely affect the amount of hydraulic gradients. Figure 9 indicates the reduction of the maximum hydraulic gradients in intersection zone with decrease in fill material permeability. The permeability of fill material is dependents on the type of filling soil, degree of compaction and executive considerations which can be controlled to achieve the desired values.

8 Vol. 13, Bund. K Clayey trench, l/l=3/9 i 4 3 K cut off wall=1^-6 (cm/s) K cut off wall=1^-7 (cm/s) K cut off wall=1^-8 (cm/s) 2 1 5*1^-6 5*1^-7 5*1^-8 5*1^-9 K Fill material Figure 9: Effect of permeability of fill material on hydraulic gradients (Detail No. 6) Comparison of the results of seepage analysis In order to compare different connecting systems, total flow was considered to be identical for all six connection systems, and then the amounts of maximum hydraulic gradients were compared. The results are shown in Table 2 for constant flow discharge of.25 m 3 /m/day and cut off wall permeability of 1-8 cm/s. The values of different variables for connecting systems used in analysis are indicated in this table. The case with no connecting system is considered as the base for comparison and reduction percent of hydraulic gradient with respect to this case is determined for different connecting systems. The table shows that all connecting systems except system 5 in which the hydraulic gradient is nearly the same as case without connecting system are effective in reducing of the hydraulic gradient. The maximum reduction percent belongs to connection systems 2 and 3 which show the most desirable performance in leakage control. The results of comparison for other cut off wall permeability values confirm this result (Zoorasna et al., 28). Table 2: Comparison of maximum hydraulic gradients in constant flow discharge of.25m 3 /m/day, K cut off = cm/s No system System (1) System (2) System (3) System (4) System (5) System (6) Variable values - h/h=1/3 s=2m g=2m θ=3 B=4m B=4m t=1m l=1m K t=1m h/h=1/3 grouted zone = K clay = K fill material = cm/s cm/s cm/s Hydraulic gradient Reduction percent (%) STRESS-STRAIN ANALYSIS Stress-strain analysis was performed using PLAXIS software for two different stages of dam construction, i.e. end of construction and steady state seepage. It was assumed that the dam is constructed in 15 layers. Moreover, elastic-plastic Mohr-Coulomb criterion was considered as

9 Vol. 13, Bund. K 9 an appropriate model for the soil. Material parameters for stress-strain analysis are shown in Table 1. Also the geometry of connection systems was modeled using the values of variables which were previously mentioned in Table 2. Figure 1 shows the finite element mesh generated in stress-strain analysis. In case of stress-strain analysis, the maximum shear stress, shear strain, failure criterion and the number of plastic points were investigated around each connecting system to investigate the stress-strain behavior of different connecting systems. Results of comparison are summarized in the following sections. Figure 1: Finite element mesh generated for stress-strain analysis Shear stresses Figure 11 shows the maximum shear stress values in the vicinity of different connecting systems at the end of construction and steady state seepage stages. The case with no connecting system is shown with dashed line in this figure. This shows that although the hydraulic gradients decrease when a connecting system is used, the stress concentration increases in connection zone. Comparison of different connecting systems shows that the least stress concentration occurs in system 2 while it is maximum for system 3 at the end of construction stage. This can be considered as an advantage for system 2 because both systems 2 and 3 have a similar effect in reducing the hydraulic gradients. Both systems nearly indicate similar condition at the stage of steady state seepage. Shear strains The shear strain values were determined at the connection zone for all connecting systems. Figure 12 shows the shear strains for two different stages of analysis. As the figure illustrates, the minimum shear strain occurs in system 2 for both stages. The maximum shear strains belong to system 5 which was previously determined as a non appropriate system in reducing the hydraulic gradients in seepage analysis. The calculated values of shear strains again show the advantage of system 2 among other connecting systems due to the lower stress or strain concentration in connection zone.

10 Vol. 13, Bund. K 1 25 End of construction 25 Steady state seepage 2 No Connecting System 2 No Connecting System Max τxy (kpa) 15 1 Max τxy (kpa) Number of connecting system Number of connecting system Figure 11: Comparison of shear stresses in different connecting systems 1 9 End of construction 1 9 Steady state seepage 8 No Connecting System 8 No Connecting System 7 7 Max γxy (%) Max γxy (%) Number of connecting system Number of connecting system Figure 12: Comparison of shear strains in different connecting systems Failure criterion and plastic stress points Figure 13 shows the status of principal stresses for all stress points considered in connection zone for steady state seepage stage. The Mohr-Coulomb failure criterion is also drawn for the clayey core material. This figure can be used to determine the percent of plastic points in each connecting system. The ratio of the number of plastic points to the total stress points considered in each connecting system is determined and the results are shown in Fig. 14. As indicated in this figure, the minimum percent of the plastic points occurs for System 2. This figure also shows that the maximum percent of plastic points belongs to connection System 5. This demonstrates the advantage of System 2 among other connecting systems.

11 Vol. 13, Bund. K Mohr-Coulomb failure envelope σ' 1 (kpa) σ' 3 (kpa) No connection system Connection No. 1 Connection No.2 Connection No. 3 Connection No. 4 Connection No. 5 Connection No. 6 Figure 13: Failure criterion for different connection systems at the steady state seepage stage Percent of plastic points (%) Steady state seepage No Connecting System Number of Connecting system Figure 14: Percent of plastic points in each system at the steady state seepage stage SUMMARY AND CONCLUSION Seepage and stress-strain analysis used to investigate the mechanical performance of cut off wall-core connecting systems in earth dams. Karkheh storage dam in Iran was used as the case study and six different connecting systems were modelled. Total flow, maximum hydraulic gradient, shear stress, shear strains and percent of plastic points were determined in connection zone. The following results obtained according to the analysis. 1. The characteristics of cut off-core connecting system affects total flow discharge and maximum hydraulic gradient in connection zone. 2. Using of a concrete slab at the base level of core with or without penetrating cut off into the core results in an extreme reduction of the hydraulic gradients at the vicinity of the intersection zone. This can help in reducing erosion and leakage from connection zone.

12 Vol. 13, Bund. K Using of a concrete slab at the base level of core without penetrating cut off into the core also results in less stress and strain concentration in connection zone. The analysis performed for the end of dam construction and steady state seepage stages approve this result. However, static analysis in other stages of dam construction besides dynamic analysis should be performed to confirm this conclusion. 4. Using clayey soil above cut off wall besides concrete cap, is not recommended due to the difficulties in construction, high hydraulic gradients and stress-strain concentration which results in a high percent of plastic points near the connection zone. 5. For future investigation in this topic, evaluation of uplift pressure in cut off-core connection zone and seismic performance of different connection systems are recommended. REFERENCES 1. Ghobadi, M.H., Khanlari, G.R. and H. Djalaly (25) "Seepage problems in the Right Abutment of the Shahid Abbaspour Dam, Southern Iran," Engrg. Geol., 82, GMS, SEEP2D (1998) "User's Guide," Brigham Young Univ., Engrg. Comp. Lab. 3. ICOLD, (1985) "Filling Materials For Watertight Cut Off Walls," Bull. No Karkheh Dam Section Engineers (1995) "Karkheh Dam Cutoff Wall Technical Specification," Mahab Ghodss Co. Eng., Tehran. 5. Karkheh Dam Section Engineers (1998) "Karkheh Project Technical report of Dam-Body and Foundation," Mahab Ghodss Co. Eng., Tehran. 6. Lee, J.Y., Choi, Y.K., Kim, H.S. and S.T. Yun (25) "Hydraulic Characteristics of a Large Rock fill Dam: Implications for Water Leakage," Engrg. Geol., 8, Li, X.S. and H. Ming (24) "Seepage Driving Effect on Deformations of San Fernando Dams," Soil Dyn. Earthquake Engrg., 24, Plaxis 8.2 "Plaxis V.8 Reference Manual," Web: Shadravan, B., Mirghassemi, A.A. and M. Pakzad (24) "Karkheh Storage Dam Cutoff Wall Analysis and Design," Proc. of 5th Int. Conf. on Case Histories in Geotech. Engrg., NY., Shahbazian Ahari, R., Mirghassemi, A.A. and M. Pakzad (2) "Investigation of the Interaction between Dam, Foundation and the Concrete Cut off Wall," Proc. of 4th Conf. on Dam Engrg., Iran, U.S. Army Corps of Engineers (24) "General Design and Construction Considerations for Earth and Rock-fill Dams," EM , Washington DC. 12. Zoorasna, Z., Hamidi, A. and A. Ghanbari (28) "Seepage through Different Concrete Cut off Wall Connection Systems, Case Study: Karkheh Storage Dam," Proc. of 6 th Int. Conf. on Case Histories in Geotech. Engrg., VA., No ejge