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EVALUATION AND COMPERING OF FACED SYMETRICAL HARD-FILL DAMS (FSHD) AND CONCRETE FACED ROCK-FILL DAMS (CFRD)1

Mohammad Esmaeilnia Omran

University of Kurdistan / Mahab Ghodss Consulting Engineering Co., Tehran, Iran

Hamed Mahdiloo Torkamani University of Kurdistan, Sanandaj, Iran

1. INTRODUCTION

In the studies Stage of dam projects, determination of appropriate dam site and dam type is the first and important step of dam projects design; because determination of appropriate location of dam site and dam type is very important technically and economically. For choosing optimal option of dam, the dam types are evaluated. Finally, the optimal option of dam is determined with technical, economic, environmental and social consideration.

Trapezoid-shaped dams are good options for the sites with poor foundation.

Faced symmetrical hardfill dams (FSHD) and concrete faced rockfill dams (CFRD) have symmetrical trapezoid-shaped cross section with concrete face slab in upstream which prevent water penetration into dam body. Usually trapezoid-shaped dams have much bigger weight than conventional gravity dams. Therefore, they do

                                                            1  Evaluation et comparaison de Des barrages avec du béton rouleuse et surface symétrique en béton (FSHD) et Des barrages rocailleux avec symétrique en béton (CFRD)  

not require the high shear strength of bedrock in order to satisfy the safety against sliding. As results, trapezoid-shaped dams can be constructed even on the poor foundation.

In this paper, properties of FSHD and CFRD are evaluated. Static and dynamic

analysis for both of dams are performed using of finite element software’s (ANSYS for FSHD and PLAXIS for CFRD) and safety of them are evaluated against static and dynamic loads. At end, both of dams are evaluated and compared technically and economically.

2. FACED SYMETRICAL HARD-FILL DAMS (FSHD) FSHD is a new type of dam, which is called CSG (cemented sand and gravel)

dam in Japan. It is a symmetrical trapezoid-shaped dam with an impervious face in the upstream, using of a low cost cemented sand and gravel material known as hardfill. The FSHD has some advantages such as high safety, strong earthquake- resistance, low demands for foundation, simple and quick construction, low cast, small negative effects on the environment and so forth. The first proposal for hard-fill dam was made by J. M. Raphael [1] and P. Londe [2]. Since the beginning 1990 s, this dam type had already been caused and paid close attention to and been used in the actual project in Japan, and where is called CSG dam. Figure 1 shows Cross section of hardfill dam.

Fig.1

Cross section of hardfill dam [3]

A trapezoid-shaped hardfill dam has much bigger weight and longer length for shear resistance than a conventional gravity dam. It is said the high shear strength of

dam foundation is not required in order to satisfy the safety against sliding. As a result, a trapezoid-shaped hardfill dam can be constructed even on the poor foundation. It is another advantage of the dam. Furthermore, a trapezoid-shaped hardfill dam can install the outlet works in its body, since the dam designed to as elastic materials [3].

3. CONCRETE FACED ROCK-FILL DAMS (CFRD)

Concrete Faced Rockfill Dams (CFRD) are very popular all over the world, especially in regions, which receive heavy rain and where impervious clay are insufficient. The CFRD has become popular in the last 40 years because of its good performance and low cost compared with rockfill dams with an inner earth core. In these dams, Concrete face prevents water penetration into dam body.

A chronicle of modern rockfill dam design, including a description of current

practice in CFRD design, is presented by Cooke [4]. Hunter [5] and Hunter and Fell [9] explained the characteristics of rockfill behavior using actual CFRD cases. It is often necessary to rely on historic performance data from other dams to estimate dam properties. In recent years, many researches were performed about the properties, design, construction and behavior of the CFRDs. Figure 2 shows a typical Cross section of CFRD.

In the years 1967 and 1993, New Exchanger CFRD with 155 m height and

Aguamilpa CFRD with 187 m height were built in USA and Mexico, respectively. And in China, Shuibuya CFRD with dam height of 233 meters was completed. Therefore, it is necessary to attend in design and construction aspects of this dam type more than before.

Fig. 2

Cross section of CFRD [6]

4. FINITE ELEMENT MODELS OF FSHD AND CFRD In this section, finite element models of FSHD and CFRD dams are presented.

For General properties of dam such as geometric properties, material properties of foundation, dam height and level of reservoir has been used by profile of Kahir dam site and same for both of FEM models.

The Kahir dam will be the first Faced Symmetrical Hardfill dam to be

constructed in Iran. Kahir dam is located in Oman Sea Region, northwest of Kanarak town of Sistan province, on Kahir River in Southeast of Iran. River basin Area in Kahir dam site is equal to 4596 . The average height of annual rainfall in the basin is 150 mm [7]. In tables 1 and 2, geometric and material properties of both dams used in the finite element models are presented respectively:

Table 1

Geometric properties of FSHD and CFRD models (meter)

FSHD CFRD ropertiesP

48.5 48.5 Dam height

4.0 10.0 Width of dam crest

71.9 165 Width at base of dam

378 378 Length of dam crest

42.5 42.5 level of reservoir

0.7H:1V 1.6H:1V Slope in upstream and downstream

0.45 0.45 Concrete face

Table 2

Material properties of finite element models

foundation Concrete face

Rockfill (CFRD)

Hardfill (FSHD)

material properties

- 2400 2100 2400 Unit weight 0.3 0.18 0.25 0.2 Poison’ ratio

1000 28000 210 12000 Modulus of elasticity in static case

1250 28000 250 15000 Modulus of elasticity in dynamic case

- - 1.0 - cohesion - - 45 - internal friction (degree)

Modeling of FSHD is performed by using of ANSYS finite element software [8].

Two-dimensional FEM model was conducted for static and dynamic analysis. Solid structural elements PLANE 82 is used for the dam body (Hardfill), concrete face and the foundation. This is an 8 node element. Contact element is adopted to simulate between dam body and concrete face. This FEM model has plane-strain behavior. Also, dam body, concrete face and foundation are assumed as elastic material. The 2D FEM model of FSHD is shown Figure 3.

Fig.3

FEM model of FSHD

Modeling of CFRD is performed by using of PLAXIS finite element software [9]. Two-dimensional FEM model was conducted for static and dynamic analysis. Six-node element is used for the dam body (Rockfill), concrete face and foundation. The element between dam body and concrete face is Contact element. This FEM model has plane-strain behavior. Also, concrete face and foundation are assumed as elastic material and model of dam body (rockfill) is Mohr-Coulomb model. This is plastic model. Figure 4 shows 2D FEM model of CFRD.

Fig.3

FEM model of CFRD

Loads applied in the static and dynamic analysis of FEM models for both of dams are as follows:

1- Weight of dam body 2- Hydrostatic pressure of reservoir water is applied on the upstream dam

surface. 3- Uplift pressure is applied at the base of dam

4- Inertia force of dam body is applied from upstream to downstream (dynamic load).

5- Hydrodynamic pressure is applied on the upstream of dam. Earthquake coefficient for both dams is considered 0.15.

5. TECHNICAL EVALUATION OF FSHD AND CFRD

Table 3 shows the maximum deformation in dam body and concrete face under dynamic loads. Comparing with the CFRD, deformations at dam body and concrete face of FSHD is much smaller: the maximum horizontal and vertical deformations at FSHD dam body are 1:11 and 1:14 comparing to CFRD. In this favorable condition, the facing Safety for FSHD has great improved.

Table 3

The maximum deformation in dam body and concrete face (dynamic loads)

Ratio CFRDFSHD Item

1:11 180 17 Horizontal deformation of dam body(mm)

1:14 170 20 Vertical deformation of dam body(mm) 1:6 94 16 Horizontal deformation of face(mm)

1:9 157 18 Vertical deformation of face(mm)

Table 4 shows the principle stress and safety factor for FSHD. According to

USBR [10], safety factors for stress distribution in the dam body for Static and Dynamic load case are considered 3 and 1.5, respectively. From table 3 can be seen that tensile and compressive stresses in the dam body are much less than allowable stresses. As a result, dam is in the safe condition. Also, for construction of FSHD hardfill materials with low cement lower strength can be used.

Table 4

The principle stress and safety factor for FSHD

Safety factor Allowable stress

Dam body stress

Item

2.16 1.5 0.694 Tensile stress (Mpa)

4.36 -15 -3.44 Compressive stress (Mpa)

In the FEM model of CFRD, tensile and compressive stresses distributed in the

dam body are very low. Maximum tensile and compressive stresses created in dam body are -0.5Mpa and 0.3Mpa, respectively. As a result, dam is in the safe conditions, completely.

Local safety factor against sliding at baseof dam formula [3]:

Where σ is normal stress in vertical direction at basement of dam; τ is shear stress at basement of dam; f is friction against shearing; c is cohesion against shearing, and A is width of the dam base [3].

Safety factor against sliding at base of FSHD and CFRD are equal 2.1 and

3.25, respectively. Figures 5 and 6 show the distribution on local safety factor against sliding at base of FSHD and CFRD, respectively. From the viewpoint of structural stability, it can be seen that CFRD has greater safety factor than the FSHD Because a CFRD has much bigger weight and longer length for shear resistance than FSHD. According to the safety factor result, the safety against sliding of CFRD is about 50% more than FSHD, but both dams are safe against sliding.

Fig. 5

Distribution on local safety factor against sliding at base of FSHD

Fig. 6

Distribution on local safety factor against sliding at base of CFRD

6. ECONOMICAL EVALUATION OF FSHD AND CFRD

In this section, FSHD and CFRD are evaluated economically and construction costs. For this purpose, both of FEM model in the previous section has been considered as a sample case for economic evaluation.

In the executive process of the dams, the most important items of dam

construction are: 1- water diversion system 2- Excavation of foundation and abutment of the dam and modification of them. 3- Construction of cutoff wall 4- Construction of dam body 5- Construction of spillway 6- Instrument installation [10].

It is considered that the dam site for both FSHD and CFRD are same, so some

of the executive items and costs for these items are equal. Items 2 and 3 items are same for both dams.

Design of water diversion system for rockfill and earth dams are based on the

7 to 10 years flood return periods where as for Roller compacted concrete (RCC) dams are based on 3 to 5 years flood periods. This is because; the construction

period for rockfill dams are more than RCC dams. Thus, the cost of water diversion system for CFRD is more than FSHD.

Cost of instrument installing can be considered same for both dam. The

number and type of instruments depend on the type and design of dam. It is reminded that the settlement and deflection in CFRD is more than FSHD, so from the past experience it shows that, the durability and life of instrument in FSHD is more than CFRD.

Construction of dam body and spillway of FSHD and CFRD are different from

each other. According to Table 1 (geometric properties), the amount of materials Used in the dam body and concrete face for both dams is presented in table 5.

Table 5

The amount of materials consumed in the dam body and concrete face

CFRD FSHD Materials ()

- 696654 Hard-fill 1604138 - Rock-fill

15564 10070 Concrete face

Based on table 5 can be seen that the volume of materials consumed for

CFRD are more than FSHD. But unit price of rockfill materials are cheaper than hardfill. Based on existing prices at 2009, table 6 shows unit prices of above materials [11].

Using tables 5 and 6 for these dams, the total cost of dam body and facing

construction for FSHD and CFRD has been calculated and presented in table 7. According to the obtained results, the construction costs of dam body and

concrete face for CFRD is less than FSHD and this cost for CFRD is about 78% FSHD.

Table 6 Unit prices of materials

Materials ( ) unit prices ( US Dollars)

Hardfill 20

Rockfill 6.5

Concrete face 60

Table 7

The total cost of dam body and facing construction

Total cost Concrete face costDam body costDam/ Cost (Dollars)

14,537,280 604,200 13,933,080 FSHD

11,360,737 933,840 10,426,897 CFRD

In the above cost, the cost of spillway is not included. In CFRD, spillway is constructed separately from the dam body its costs are about 30% to 35% of the total cost of dam, While in FSHD, spillway is constructed on dam body and the spillway costs are very low and about 5% total cost of dam construction [11]. So in General, CFRD costs are more than FSHD.

7. SUMMARY AND CONCLUSIONS

In this paper, FSHD and CFRD dams were evaluated technically and economically and compared with each other. There are some conclusions in the following:

1- Faced symmetrical hard-fill dam is a new type of RCC dam with the shape intervenient between gravity dam and CFRD. It has good quality and some unique advantages such as high safety, strong earthquake resistance, low demands for foundation, simple and quick construction, low cast, small negative effects on the environment and etc.

2- concrete faced rock-fill dam has symmetrical trapezoid-shaped cross section with concrete face slab in upstream which prevent water penetration into dam body. This type of dam is suitable for sites with alluvial foundation and gravel materials and especially in regions, which receive heavy rain and where impervious clay is insufficient.

3- Results show that, deformations at dam body and concrete face of FSHD is smaller than the deformations at the CFRD.

4- The maximum horizontal and vertical deformations at FSHD dam body are 1:11 and 1:14 comparing to CFRD, respectively.

5- Results obtained from the analysis shows that, tensile and compressive stresses in the both dam body are much less than allowable stresses and both of the dams are in the safe condition

6- Hard-fill materials with low cement and lower strength can be used for the construction of FSHD.

7- The safety factor against sliding of CFRD is about 50% more than FSHD, but both dams are safe against sliding.

8- Evaluating the total costs of dam construction including the cost of diversion system, dam body, facing and spillway construction, one can see the total construction costs of FSHD is less than CFRD.

9- Construction of FSHD on sites with seasonal rivers with high floods is in better condition than CFRD; because, during unexpected seasonal floods, FSHD has greater than CFRD.

8. REFERENCES

[1] J. M. RAPHAEL. The Optimum Gravity Dam. Proceedings Roller Compacted

Concrete III. ASCE, San Diego, California, 2-5 February 1992. [2] P. LONDE AND M. LINO. The Faced Symmetrical Hardfill Dam: a New

Concept for RCC. International Water Power & Dam Construction, 1992. [3] YUNFENG P., YUNLONG H., KUN X., Study on the Structural Safety of CSG

Dam. New progress on Roller Compacted Concrete Dams, China Water Power Press, 2007.

[4] J.B. COOKE, Progress in Rockfill Dams. Journal of Geotechnical Engineering, 1984.

[5] G. HUNTER. The Pre- and Post-failure Deformation Behavior of Soil Slopes. PhD Thesis, University of New South Wales; p. E1–10, 2003.

[6] Siah Bisheh CFRD technical reports. Ministry of Energy, Iran, 2006. [7] Kahir dam, Reports on Second Phase study of KAHIR DAM, Ministry of

Energy, Iran, 2004. [8] ANSYS User Manual, ver.11, 2007. [9] PLAXIS User Manual, ver.7.2, 2005. [10] J. ABRISHAMI, N.V. RAJAII, Concrete Dams; Design and Construction.

Astan Ghods Publications, Iran, 2005. [11] Price List of Dams, MAHAB GHODS Consulting Engineering, Iran, 2008.

SUMMARY

Faced symmetrical hard-fill dams (FSHD) and concrete faced rock-fill dams (CFRD) have symmetrical trapezoid-shaped cross section with concrete face slab in

upstream which prevent water penetration into dam body. Usually trapezoid-shaped dams have much bigger weight and longer length for shear resistance than conventional gravity dams. Therefore, they do not require the high shear strength of bedrock in order to satisfy the safety against sliding. As results, trapezoid-shaped dams can be constructed even on the poor foundation.

In this paper, properties and respected cost of the materials for FSHD and

CFRD dams were compared, also both of dams have been evaluated technically and economically. Static and dynamic analysis for both of dams were performed by use of finite element software’s (ANSYS for FSHD dam and PLAXIS for CFRD dam) and safety of them were evaluated against static and dynamic loads keeping height ,level of reservoir and dynamic loads same for both of the dams. The results show that FSHD and CFRD dams are safe against applied loads, but the deformation obtained in FSHD dam is smaller than CFRD dam. From construction point of view FSHD dam is more economical than CFRD dam for dam site with high flood rivers.

SOMMAIRE

Des barrages avec du béton rouleuse et surface symétrique en béton (FSHD) et Des barrages rocailleux avec symétrique en béton (CFRD) ont coupe symétrique trapézoïdal avec une couche imperméable en béton dans supérieur flot que cette couche empêche de pénétrer d’eau dedans de cote de barrage. Ordinairement, des barrages trapézoïdaux ont beaucoup poids et plus de longueur pour résistance tondre que des barrages pesanteur conventionnel. Donc, ils ne demandent pas grande force tondre dans la fondation pour assurer le coefficient de sécurité contre glissement. En conséquence, des barrages trapézoïdaux peuvent construire sur la fondation faible.

Dans cette recherche, les propriétés et le cout des matériaux pour les

barrages de FSHD et de CFRD sont comparé. Aussi, les deux barrages sont évalués techniquement et économiquement. L’analyse statique et dynamique est accomplie pour les deux barrages en emploi du logiciel d’élément fini (ANSYS pour barrage de FSHD et PLAXIS pour barrage de CFRD) et la sureté des barrages est évaluée contre les charges statique et dynamique. L’hauteur de barrage, l’hauteur de l’eau de réservoir la quantité de la force de séisme influence est égal pour les deux. Au point de vue de construction, dans les sites avec les hautes inondations, le barrage de FSHD est plus économique que le barrage de CFRD.

KEY WORDS

FSHD Dam, CFRD Dam, dynamic analysis, safety