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Comparing Faced Symmetrical Hardfill and Concrete-Faced Rockfill Dams

With both faced symmetrical hardfill and concrete-faced rockfill dams being suitable for sites with poor foundations, the authors set out to determine unique characteristics of the two dams types. Their research revealed a smaller amount of deformation and lower costs for FSHDs.

By Mohammad Esmaeilnia Omran and Hamed Mahdiloo Torkamani

The first and most important step in designing a new dam is determining an appropriate type of dam to build, based on the unique site conditions. The optimal design of the dam is determined after consideration of technical, economic, environmental and social factors.

For sites with poor foundations, trapezoid-shaped dams are good options because they typically have a much greater weight than conventional gravity dams and therefore do not require the high shear strength of bedrock to satisfy requirements for safety against sliding. Both faced symmetrical hardfill dams (FSHD) and concrete-faced rockfill dams (CFRD) have symmetrical trapezoid-shaped cross sections with an upstream concrete face slab that prevents water from penetrating into the dam body.

In this article, the authors evaluate the properties of FSHD and CFRD. Static and dynamic analyses for both dam types were performed using finite element analysis software, and safety was evaluated for static and dynamic loads. Finally, both dam types are evaluated and compared technically and economically.

The results show that FSHD and CFRD are safe against applied loads, but the deformation obtained in FSHDs is smaller than that for CFRDs. Also, FSHDs are more economical to build than CFRDs for sites with high flooding in the river.

Understanding these dam types

FSHD is a fairly new type of dam, first proposed in 1992,1,2 which is called CSG (cemented sand and gravel) in Japan. This type of dam is built using a low-cost cemented sand and gravel material known as hardfill. A FSHD has several advantages, including a high degree of safety, strong earthquake resistance, low demands for the foundation, simple and quick construction and minimal negative effects on the environment.

Because of its trapezoid shape, a hardfill dam has much greater weight and a longer length for shear resistance than does a conventional gravity dam (see Figure 1). It is said the high shear strength of the dam foundation is not required to satisfy safety requirements against sliding, meaning this type of dam can be constructed even on a poor foundation.3

CFRDs are popular all over the world, especially in regions that receive heavy rain and where impervious clay is insufficient. CFRD has become popular over the past 40 years because of its good performance and low cost compared with rockfill dams with an inner earth core.

A chronicle of modern rockfill dam design, including a description of current practice in CFRD design, is available,4 as are explanations of the characteristics of rockfill behavior using specific CFRD cases.5,6 It is often necessary to rely on historic performance data from other dams to estimate dam properties. In recent years, a significant amount of research has been performed with regard to the properties, design, construction and behavior of CFRDs.

There are several examples of CFRDs built worldwide, including New Exchanger, a 155 meter-high dam built in the USA in 1967; Aguamilpa, a 187 meter-high dam built in Mexico in 1993; and Shuibuya, a 233 meter-high dam built in China in 2008.

Finite element models of FSHD and CFRD

The authors performed finite element modeling of FSHDs and CFRDs to evaluate safety of both dam types against applied loads. To supply the general properties of the dam — geometry, material properties of the foundation, dam height and reservoir level — the authors chose a profile of the Kahir Dam site for both models (see Tables 1 and 2 for more information on the two dam types studied).

Kahir Dam, located on the Kahir River in southeastern Iran, will be the first FSHD constructed in the country, and this work is scheduled to begin in 2012. The dam is 68 meters high, with a width at the dam crest of 4 meters and width at the base of 71.9 meters. The upstream and downstream slope of this dam is 0.7H:1V. The river basin area at the Kahir Dam site is equal to 4,596 km2. And average annual rainfall in the basin is 150 mm.7

Modeling of the FSHD was performed using ANSYS finite element analysis software.8 A two-dimensional FEM model was built for static and dynamic analysis. The solid structural element PLANE 82 is used for the dam body (hardfill), concrete face and foundation. This is an eight-node element. A contact element is adopted to simulate the element between the dam body and concrete face. This FEM model has plane-strain behavior. Also, the dam body, concrete face and foundation were assumed as elastic material.

Modeling of the CFRD was performed using PLAXIS finite element software.9 A 2D FEM model was built for static and dynamic analysis. A six-node element was used for the dam body (rockfill), concrete face and foundation. The element between the dam body and concrete face is a contact element. This FEM model has plane-strain behavior. Also, the concrete face and foundation were assumed to be elastic material, and the model of the dam body (rockfill) is a Mohr-Coulomb (plastic) model.

Loads applied in the static and dynamic analysis of the FEM models for both dam types are:

— Weight of the dam body;

— Hydrostatic pressure of the reservoir water on the upstream dam surface;

— Uplift pressure at the base of the dam;

— Inertia force of the dam body from upstream to downstream (dynamic load); and

— Hydrodynamic pressure on the upstream side of the dam.

The earthquake coefficient for both dams was considered to be 0.15.

Technical evaluation of FSHD and CFRD

The maximum deformations in the dam body and concrete face under dynamic loads for a FSHD are much smaller compared with the CFRD: the maximum horizontal and vertical deformations are 1:11 and 1:14. In this condition, the facing safety for a FSHD is better than that for a CFRD.

Table 3 shows the principle stress and safety factor for a FSHD. Safety factors for stress distribution in the body of a FSHD for the static and dynamic load cases are considered to be 3 and 1.5, respectively.10 Tensile and compressive stresses in the dam body are much less than allowable stresses. As a result, the dam is in the safe condition. Also, for construction of a FSHD, hardfill materials with low cement strength can be used.

In the FEM model of a CFRD, tensile and compressive stresses distributed in the dam body are very low. Maximum tensile and compressive stresses created in the dam body are -0.5 Mpa and 0.3 Mpa, respectively. As a result, the dam is in the safe condition.

The formula for the local safety factor against sliding at the base of the dam is:3

Equation 1

K = (∑σ x f + c x A) / τ

where:

— σ is normal stress in the vertical direction at the base of the dam;

— f is friction against shearing;

— c is cohesion against shearing;

— A is the width of the dam base; and

— τ is shear stress at the base of the dam.

Safety factors against sliding at the base of a FSHD and a CFRD are 2.1 and 3.25, respectively. Figure 2 shows the distribution of local safety factors against sliding at the base of a CFRD and FSHD, respectively. From the viewpoint of structural stability, it can be seen that a CFRD has greater safety factor than the FSHD because a CFRD has much bigger weight and longer length for shear resistance than a FSHD. According to the safety factor result, the safety against sliding of a CFRD is about 50% more than a FSHD, but both dams are safe against sliding.

Economical evaluation of FSHD and CFRD

In this section, FSHDs and CFRDs are evaluated economically. For this purpose, both of the FEM models in the previous section were considered as sample cases for economic evaluation.

From an executive point of view, the six most important aspects of dam construction are:

— Water diversion system;

— Excavation of foundation and abutment of the dam and modification as needed;

— Construction of cutoff wall;

— Construction of dam body;

— Construction of spillway; and

— Instrument installation.10

With the dam site for both FSHD and CFRD being essentially the same, the executive items and associated costs are equal. The second and third items are the same for both dam types.

Design of a water diversion system for rockfill and earth dams is based on the seven- to 10-year flood return periods, whereas roller-compacted-concrete (RCC) dams are based on three- to five-year flood periods. This is because the construction period for rockfill dams is longer than for RCC dams. Thus, the cost of a water diversion system for CFRD is more than for FSHD.

Costs for excavation of the foundation and abutment and construction of a cutoff wall are considered identical.

Construction of the dam body and spillway for FSHDs and CFRDs are different. The amount of materials used in the dam body and concrete face was 696,654 m3 of hardfill for the FSHD and 1,604,138 m3 of rockfill for the CFRD, along with 15,564 m3 and 10,070 m3 for the concrete face of the CFRDs and FSHDs, respectively.11

Using the above numbers, the total cost of dam body and facing construction for FSHDs and CFRDs has been calculated. The total cost of a FSHD is $14,537,820, with $13,933,080 for the dam body and $604,200 for the concrete face. The total cost of a CFRD is $11,360,737, with $10,426,897 for the dam body and $933,840 for the concrete face (see Table 4).

In the above costs, the cost of the spillway is not included. For CFRDs, the spillway is constructed separately from the dam body, and its costs are about 30% to 35% of the total cost of the dam. For FSHDs, the spillway is constructed on the dam body and the spillway costs are very low, only about 5% of the total cost of dam construction.11 Thus, in general, CFRD costs are greater than FSHD costs.

Summary and conclusions

FSHDs and CFRDs were evaluated technically and economically and compared with each other. Based on the above technical and economical evaluation, the authors have drawn the following conclusions:

— FSHD, a new type of RCC dam with a shape between a gravity dam and CFRD, has good quality and some unique advantages such as high safety, strong earthquake resistance, low demands for foundation, simple and quick construction, low cast and small negative effects on the environment.

— CFRD has a symmetrical trapezoid-shaped cross section with a concrete face slab on the upstream side that prevents water penetration into the dam body. This type of dam is suitable for sites with alluvial foundation and gravel materials and especially in regions that receive heavy rain and where impervious clay is insufficient.

— Results show that deformations at the dam body and concrete face of FSHDs are smaller than deformations for CFRDs.

— The maximum horizontal and vertical deformations at FSHD dam body are 1:11 and 1:14, respectively.

— Results obtained from the analysis show that tensile and compressive stresses in the bodies of both dam types are much less than allowable stresses, and both of the dam types are in the safe condition.

— Hardfill materials with low cement and lower strength can be used for the construction of FSHD.

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

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

— Construction of FSHD on sites located on seasonal rivers with high floods is a better option than CFRD because during unexpected seasonal floods, FSHD has a greater safety factor than CFRD.

Notes

1. Raphael, J.M., "The Optimum Gravity Dam," Proceedings of Roller Compacted Concrete III, American Society of Civil Engineers, Reston, Va., USA, 1992.

2. Londe, P. and M. Lino, "The Faced Symmetrical Hardfill Dam: a New Concept for RCC," International Water Power & Dam Construction, February 1992.

3. Yunfeng, P., H. Yunlong, and X. Kun, "Study on the Structural Safety of CSG Dam," New Progress on Roller Compacted Concrete Dams, China Water Power Press, Beijing, China, 2007.

4. Cooke, J.B., "Progress in Rockfill Dams," Journal of Geotechnical Engineering, Volume 113, No. 10, October 1984.

5. Hunter, G., The Pre- and Post-failure Deformation Behavior of Soil Slopes, PhD Thesis, University of New South Wales, 2003.

6. Siah Bisheh CFRD technical reports, Ministry of Energy, Tehran, Iran, 2006.

7. Reports on Second Phase Study of Kahir Dam, Ministry of Energy, Tehran, Iran, 2004.

8. ANSYS User Manual, version 11, 2007.

9. PLAXIS User Manual, version 7.2, 2005.

10. Abrishami, J. and N.V. Rajaii, Concrete Dams: Design and Construction, Astan Ghods Publications, Iran, 2005.

11. Price List of Dams, MAHAB GHODS Consulting Engineering, Iran, 2008.

Mohammad Esmaeilnia Omran is assistant professor with the University of Kurdistan and project manager with Mahab Ghodss Consulting Engineering Co. in Tehran, Iran. Hamed Mahdiloo Torkamani is a masters student with the University of Kurdistan in Iran.

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