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Modern structural and technological solutions for new large dams

 

Such "newer" dam types as facing symmetrical hardfill and rockfill with an asphalt concrete core provide solutions to site-specific challenges for dams in Russia and can be used worldwide.

By Yury Lyapichev and Yury Landau

Dr. Yury Lyapichev is an international consultant on dams in Russia and a member of ICOLD's Committee on Dam Analysis and Design. Dr. Yury Landau is deputy director of URK-Hydroproject in Ukraine and worked on the Kankunskaya project in Russia with Dr. Lyapichev.

There are several "newer" types of dams being built in such countries as Russia, Turkey, Colombia, China, Japan, Norway, Iran and Canada. These include facing symmetrical hardfill (FSH) and rockfill with an asphalt concrete core (ACC) dams. These dams are well-equipped to deal with difficult site conditions, such as poor foundation and high seismicity.

In Russia, new large hydro projects with high (greater than 100 meters) dams are concentrated mainly in the remote regions of Siberia, Yakutia and the Far East, with more difficult natural conditions than in any other country. Therefore, Russian engineers are developing new structural and technological solutions for FSH and rockfill dams with ACC.

FSH dams

Gravity dams at least 100 meters high that are made using roller-compacted concrete and feature the traditional vertical upstream facing and sloping downstream facing (0.8h/1v) on a rigid foundation frequently are unsafe in an earthquake with horizontal acceleration of 0.2 g or more. Another serious restriction of traditional RCC dams is that they are not feasible on a soil or weak rock foundation.1

These restrictions can be overcome by changing the profile to a symmetrical triangular cross-section with low cementitious content RCC, without horizontal joint treatment and with a watertight upstream concrete facing. This new type of lean RCC dam — called FSH, with both slopes of about 0.7h/1v — was introduced in 1992 and constructed in 1996 in the Dominican Republic (25 meter-high Moncion afterbay dam).2 Because of the symmetrical shape, the RCC does not require high shear or compressive strengths, and there is no tensile stress at least for an earthquake with a pseudo-static acceleration of 0.2 g.1

Further optimization led to a new type of FSH dam with outer zones of lean RCC and an inner wide zone of rockfill enriched with cement-flyash mortar (REC), proposed in 1998.3 A 100 meter-high FSH-REC dam4 (see Figure 1) was developed for a high seismic region and later used in some dam projects in Russia.

The outer zones of this dam, with slopes of 0.5-0.7 and width of 3+0.1 H meters (where H is head), can be made with low cement content (<70 kg/m3). By placing a watertight membrane on the upstream slope (instead of a reinforced concrete facing), the uplift in RCC joints or cracks is eliminated with no consequence on watertightness or safety.4 The membrane is placed after completion of the dam to overcome any difficulties with thermal cracking in RCC zones.5

Material consisting of rockfill with a diameter of 5-300 mm can be placed in the central zone of the dam in a 60 cm-thick layer. Then 10-15 cm-thick cement-flyash mortar is spread and penetrates into the coarse pores. This penetration can be facilitated by two passages of a sheep roller, and compaction can be achieved by two to three passages of a vibrating roller. This roller also can used to compact the 30 cm-thick layers of RCC placed in the outer zones of the dam.6,7

Because the rockfill layers are 60 cm thick compared with the 30 cm typically used for RCC dams, and a membrane is placed on the upstream slope instead of a reinforced concrete facing, the speed of construction for an FSH-REC dam will be faster than for a homogeneous FSH dam. One caveat: The structural (seismic) analysis of a new design of FSH-REC or FSH dam should be performed in advance of the final design because there are limited seismic analyses available for these dam types.4,6,7,8

Seismic analysis of a 100 meter-high FSH-REC dam (see Figure 1) shows a minimum cohesion between the RCC in the outer zones and the REC of 0.5 MPa. This cohesion value corresponds to the minimum cohesion of RCC joints without treatment during dam construction. For RCC and REC material, a minimum inner friction angle of 45 degrees is assumed, which corresponds to the preliminary design of RCC dams.7,9

The comparative analysis was made in terms of factors of safety against sliding at the foundation, of a 100 meter-high RCC dam with vertical upstream and sloping downstream faces and an FSH-REC dam of the same height and sloping upstream and downstream faces. Three foundation types were considered: rock (with the angle of inner friction 45 degrees), alluvial (35 degrees) and moraine (30 degrees and cohesion 0.1 MPa).

Two operating cases were considered: static case with a maximum reservoir level and seismic (pseudo static) case with ground acceleration of 0.2 g. In the seismic case, the shear wedge method was used to calculate the acceleration distribution because this method corresponds to the shear movements of RCC dams during earthquakes. For both dams, uplift was taken at 40% of the force developed by a straight percolation line from full head upstream to no head at the dam.

According to Russian design codes for gravity dams,10 the minimum allowable factors of safety against sliding on the contact dam-rock foundation for static and seismic cases are 1.32 and 1.18, respectively. Analysis results showed that a 100 meter-high RCC or conventional concrete gravity dam are not feasible on a soft foundation (such as alluvium or moraine). On the contrary, a 100 meter-high FSH-REC dam with both slopes of 0.6h/1v is quite feasible on a soft foundation and of 0.5h/1v on a rock foundation.

The Russian anti-seismic design codes for dams released in 2003 indicate seismic (dynamic) analyses are to be performed for high dams (100 meters and higher) in moderate or high seismic regions.11

Dynamic analysis of a 100 meter-high FSH-REC dam with slopes of 0.5v/1h has been performed using a method employed at the Geodynamic Center of the Hydroproject Institute.8 Synthetic horizontal and vertical accelerations with peak values of 0.8 g were normalized as the maximum design earthquake (MDE) with peak ground horizontal and vertical accelerations of 0.2 and 0.14 g, respectively, and as the maximum credible earthquake (MCE) with peak ground horizontal and vertical accelerations of 0.4 and 0.28 g, respectively. The same shear strength values of RCC and REC joints were adopted in the dynamic analysis as in the previous pseudo-static analysis.

Results of the dynamic analysis indicated the FSH-REC dam is safe for the MDE case and there is no development of tensile stresses or opening of RCC joints.

Figure 2 shows results of the dynamic analysis for action of the MCE with ground peak horizontal and vertical accelerations of 0.4 and 0.28 g, respectively. The cracking pattern in the dam body for the MCE case is deteriorated compared to the MDE. In the lower part of the dam, the cracks (joint opening) propagated from the upstream slope toward the dam axis. However, owing to the upstream impervious membrane, uplift propagation through the RCC and REC joints is impossible and seismic safety of the dam is provided.

Cracking in the RCC outer zones during the MCE can be excluded, or at least decreased, by joint treatment in these zones (bedding mix), which can increase RCC joint cohesion twice or more. And there is another solution: to decrease the steepness of both slopes from 0.5 to 0.6, excluding any treatment of the RCC joints.

Thus, the 100 meter-high FSH-REC dam with both slopes of 0.5h/1v has, at least, double the seismic (dynamic) safety against action of the MCE compared with a traditional RCC dam.

The new type of FSH-REC dam on rock or soil foundation is an attractive alternative to traditional RCC or conventional gravity dams and is recommended when developing new projects in seismic regions of Russia and other countries.

Examples of FSH dams that could be built using very lean RCC include:

— Cindere, a 107 meter-high FSH dam constructed in Turkey in 2005 on a soft rock foundation in a seismic region;

— Yumagazinskaya, a 65 meter-high FSH-REC dam on a soil foundation in a seismic region in Russia, which was developed as an alternative to a rockfill dam with a clay core (the rockfill dam was built because of lack of experience in construction of FSH dams in Russia); and

— Ituango in Colombia, a 180 meter-high FSH dam on a rock foundation in a seismic region, which was developed as an alternative to a concrete-faced rockfill dam (the CFRD was built because of the same lack of experience).

Rockfill dams with compound ACC

By 2010, about 120 ACC rockfill dams were operating worldwide, including about 30 dams more than 100 meters tall. Some of these dams are in China (170 meter Quxue, 198 meter Houziyan and125 meter Yele), Norway (128 meter Stor-glomvatn and 100 meter Storvatn), and Canada (109 meter Romaine).

These dams provide four advantages:

— Greater operational safety compared with high rockfill dams with clay cores and concrete facings, especially in difficult climatic, geological and seismic conditions.

— Benefits over clay cores, concrete facings and geomembranes, including: water tightness, allowing construction of a thin core; stability against erosion and aging; high resistance to seismic loads; significant tensile and shear deformations without cracking, even at negative temperatures; and several grades of bitumen and admixtures may be used to improve the mechanical properties of the asphalt concrete to satisfy design requirements for a severe climate.

— ACC exhibits viscoelastic-plastic, ductile behavior and thus can relieve any stress concentrations and self-heal any tendencies to fissure or crack formation. These types of dams also can tolerate foundation settlements and embankment deformations due to static and earthquake loading better than clay cores and concrete facings, allowing the builder to accept the use of lower-quality rockfill. ACC is protected from impact loads and damage by reservoir debris, deterioration due to weathering and ice loadings.

— ACC easily adapts to displacements of the adjacent transition zones. Contrary to clay cores, practical application of ACC is not hampered by extreme weather, which allows extension of the construction season in Siberia by nearly two months.

Alternatives for Kankunskaya rockfill dam

The 1,200 MW Kankunskaya plant is to be built in Southern Yakutia from 2013 to 2025. Under the contract between FNK Engineering and the St. Petersburg branch of the Hydro-project Institute, FNK Engineering developed four alternatives of rockfill dam with ACC for the design documents of Kankunskaya.

The basic requirements of developing the 232 meter-high Kankunskaya dam are dam safety, technological adaptability and economic efficiency of construction. Four alternatives were considered:

1. Compacted asphalt concrete core: Hot (160-170 C) asphalt concrete is placed and compacted with upstream and downstream transition zones of 0.2 meter-thick and 1.5 meter-wide filters. Placing and compacting ACC at negative temperatures can result in a low core quality.

2. Liquid ACC: Use of liquid (flowable) ACC has some disadvantages, the main one being danger of squeezing of bitumen in adjacent transition zones.12

For both of the above alternatives, arching of cores on more rigid adjacent transition zones results in a decrease in vertical normal stresses in cores during construction, which can lead to inadmissible tensile deformations and cracking in the base of the core during reservoir filling.

3. Compound ACC formed by upstream and downstream facings from precast concrete plates with a waterproof geomembrane on their external sides and subsequent filling of the cavity between the plates with liquid asphalt concrete.

4. Compound ACC formed by upstream and downstream facings from steel sheets with a waterproof geomembrane on their external sides and subsequent filling of the cavity between the sheets with liquid asphalt concrete.

Compound ACC, with a flexibility practically the same as liquid ACC during construction and reservoir filling, follows displacements of the adjacent transition zones and prevents squeezing of bitumen in these zones. Facings from precast concrete plates or steel sheets, covered by geomembrane, carry out the function of sliding joints: decrease friction factor between the compound ACC and transition zones. These facings also lower arching of the compound ACC, which can lead to formation of vertical tensile deformations and horizontal cracks in the base of core.

The safety of Kankunskaya rockfill dam is defined by the stress-strain state of the ACC. Estimation of durability of the ACC is carried out on the basis of the stress-strain state, using the absence of tensile deformations as a safety criterion.

Analysis of the seepage regime in the ACC rockfill and its foundation was carried out by solution of stationary seepage flow problems in the dam foundation. The required depth of the grout curtain in the river channel and its bank slopes, including section of right bank abutment in zone of seepage in river banks. Unloading of 80% of the seepage flow through the dam foundation will happen in the downstream part of the dam behind the concrete gallery.

Analyses of the thermal regime in alternatives to the ACC rockfill dam were performed using a program called Abaques.13 When construction is complete, the zone of negative temperatures covers almost all the dam body including the upstream part (see Figure 3). In the foundation, there is a small zone of positive temperature connected with the influence of these temperatures on the rock base. Near the bottom of the ACC, there are positive temperatures. Over time, as the temperature field stabilizes, there is a zone of positive temperatures in nearly all the upstream part of the dam. Near the foundation of the downstream part, there is a narrow zone of positive temperatures (see Figure 3). These results show that, over time, the thermal deformations of the dam body, including ACC, are stabilizing excluding a narrow zone of downstream slope with seasonal changes in negative and positive temperatures, which has no influence on this slope stability.

There are some basic results from analyses of the stress-strain state of alternatives to an ACC rockfill dam. By the end of construction of the liquid and compacted alternatives of ACC rockfill dam, there is unloading of vertical stresses or non-uniform arching of cores on transition zones, lesser in the upstream part of the dam and greater in the downstream part.

In alternatives 1 and 2, tensile stresses can arise in the base of both cores that can lead to cracking of the core base and loss of its water tightness.

Analyses of an ACC rockfill dam with a compound core have shown that the increase of deformation modulus of rockfill in the downstream part of the dam from 60 to 160 MPa results in a much more favorable stress-strain state of the compound core — in 1.2 times decrease of core settlement and 2.6 times decrease of its deflections.

Results of coupled analyses of thermal regime and stress-strain state of the ACC rockfill dam, taking into account the sequence of dam construction and reservoir filling in alternatives 3 and 4, have shown the following:

— Installation of a liquid concrete core with an external geomembrane considerably improves the stress-strain state of the liquid core and increases its cracking resistance and water tightness; and

— In an ACC rockfill dam with a compound liquid concrete core, there are three waterproof contours that in such difficult operating conditions at low temperatures and high water pressures greatly increase the dam safety.

During analyses of static strength of a rockfill dam with a concrete core, values of safety factor of strength and stability of the dam are 1.71 by the end of dam construction and reservoir filling and 1.65 after 30 years of operation. These values are much more than the admissible value of 1.25.

Analyses also were performed regarding seismic resistance of ACC dams, using spectral and dynamic theories. The normative value of safety factor of Kankunskaya dam under action of the MPE is 1.06. Strength and stability of the ACC rockfill dam is provided in all four design cases with normative safety factors. In analyses of stability of dam slopes by the circular sliding surfaces method, values of safety factor are 1.25 for the basic combination of loadings (static case) and 1.063 for special combination of loadings. Analyses of seismic resistance by the linear spectral and wave (dynamic) theories have shown that seismic resistance of the dam with compound liquid ACC, located between concrete facings, is provided.

Comparison of alternatives 3 and 4 has shown that they are characterized by: dam safety under difficult operating conditions; technological features of full mechanization of ACC construction and quality and maximum lengthening of ACC construction time in the winter; and close cost indexes with a RUB1.46 billion (US$44.7 million) excess cost of alternative 4 compared with 3, which is 4% of the total cost of the ACC rockfill dam.

It is recommended to develop in detail alternatives 3 and 4 for a choice of the most effective ACC rockfill dam design.

Notes

1. Londe, P., and M. Lino, "The Faced Symmetrical Hardfill Dam: A New Concept for RCC," International Water Power and Dam Construction, February 1992, pages 19-24.

2. "The Gravity Dam: A Dam for the Future – Review and Recommendations," Bulletin 117, International Commission on Large Dams, Paris, 2000.

3. Lyapichev, Yury, "Presa de Concrete Compactado con Rodillo (CCR) y Presas Mixtas de CCR y Escollera (Aspectos de Diseno y Construccion)," Seminar Sobre Presas de CCR, Isagen, Medellin, Colombia, 1998.

4. Lyapichev, Yury, "Seismic Stability and Strength of New Combined Symmetrical RCC Dam with Rockfill Enriched with Grout," Proceedings of 4th International Conference on RCC Dams, Taylor & Francis, 2003.

5. "Water Proofing and Protection with Flexible Synthetic Geomembrane," Carpi Tech S.A., www.carpitech.com.

6. Lyapichev, Yury, "Design and Construction of Modern High Dams (RCC-FSH Dams; Rockfill Dams with Asphaltic Concrete Cores and Concrete Facings," (in Russian), RUDN Publishing, Moscow, 2009.

7. Lyapichev, Yury, et al, "Structural and Technological Solutions to Accelerate Construction of Dams and Reduce their Costs under Different Natural and Climatic Conditions," Proceedings of 22nd ICOLD Congress on Large Dams, International Commission on Large Dams, Paris, 2006.

8. Lyapichev, Yury, and M. Groshev, "Stability and Strength of New RCC Dam under Maximum Seismic Action," (in Russian), Structural Mechanics of Engineering Constructions and Buildings, Volume 3, March 2008, pages 48-60.

9. State-of-the-Art of RCC Dams, Bulletin 125, International Commission on Large Dams, Paris, France, 2003.

10. Codes of Design of Concrete Dams, SNiP 2.06.06-85, GosStroi, Moscow, Russia, 1986.

11. Codes of Design of Hydraulic Structures in Seismic Regions, SNiP-33-03, GosStroi, Moscow, Russia, 2003.

12. Lyapichev, Yury, "Safety Problem of the Boguchansk Rockfill Dam with Asphaltic Concrete Core," Proceedings of 22nd ICOLD Congress on Large Dams, International Commission on Large Dams, Paris, France, 2006.

13. www.simulia.com

Reference

Lyapichev, Yury, "Modern Structural and Technological Solutions for New Projects of Large Dams in Russia and Some Other Countries," Proceedings of HydroVision Russia 2011, PennWell, Tulsa, Oklahoma, USA, 2011.

Acknowledgment

The authors thank the general directors of the Moscow and St. Petersburg branches of Hydroproject Institute, FNK Engineering, UKR-Hydroproject Institute, and the professionals from these institutes who contributed to the study and design of some dams presented in this article. Mr. Lyapichev also appreciates his cooperation with Isagen SA in Colombia on hydro projects and dams in 1998.

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