SECOND INTERNATIONAL SYMPOSIUM ON ROCKFILL DAMS RIO DE JANEIRO ,RJ – BRAZIL, OCTOBER 27 – 28, 2011

DESIGN CRITERIA AND CONSTRUCTION FOR VERY HIGH CFRDs – QUESTIONS AND CONCEPTUAL TRENDS

Abstract

CFRDs as an inherent safety structure has allowed that high (> 200 m) rockfill dams have been built with different granular materials on sound or weathered rock or alluvium foundation in the last 10 years. Progress in CFRDs construction has embracing to speed required rock excavation production, hauling placement and compaction with heavy vibratory rollers, concrete and slipping form methods improvement and have reduced slabs concreting time and implement high CFRDs, as a feasible and profitable alternative. Unpredicted face slabs cracking and concrete spalling phenomenon during first reservoir impounding associated with high leakage values (> 1,000 liter/s), might cause additional costs for owners in view remedial treatments and further repairing works. Current design criteria and specifications for rockfill compaction, slab design reinforcement and vertical joints materials and anti-spalling reinforcement have been implemented to aiming more protection against these undesirable issues. In view CFRDs ranged 300 m high being designed currently, this article comments design criteria and construction procedures to face this challenge target and aiming progress in these future extra high Dams.

Key words: cracks, waterstops, pre-settlements, leakage , spalling, valley effect.

1. Introduction

CFRDs around 200m high were built in the last decade such as, Campos Novos (Brazil, 202m) , Shuibuya (China, 233m), Bakun (Malaysia, 205), Kárahnjúkar (Iceland, 196 m), El Cajón ( Mexico, 188m), La Yesca (Mexico, 208, under construction) and Jiangpinghe (China, 221m). All listed CFRDs are excellent examples of safety dams in operation and construction method progressing. Since 2006, based on lessons from cracks and spalling slabs collapse occurred during first impounding recorded on Campos Novos, Barra Grande (2005) and Mohale (2006), such current design criteria were adopted as : i) vertical soft joint filling (wood, or EPDM or GB material) at slab compression areas ; ii) slabs 7.5 m wide in compression areas; iii) anti spalling reinforcement; iv) 0.50 to 0.80 m rockfill thinner layers and extending Zone B to Zone C downstream concept; v) upstream fill up topping. In addition, rockfill pre-settlement time before slab concrete works starting (delaying slabs facilities) were implemented in Shuibuya and Hongjiadu CFRDs, seeking to minimize further rockfill deformations.

2. Design and Construction new trends - Comments and Concerns

2.1 Slab Cracks issue Cooke, B. stated that face slab CFRDs leakage can be accepted and does not involve safety [Ref.1]. Meanwhile, fissures and slab cracks during construction and impounding can be classified according to its opening and causes [Ref. 2]. Cracks treatment must always be implemented by contractors

Xu Zeping , Professor, PhD China Institute of Water Resources and Hydropower Research (IWHR) No.20, West Chegongzhuang Road Beijing, 100044, P.R. China xuzp@iwhr.com

Manoel S. Freitas Jr., Senior Engineer Hydrogeo Engenharia Ltda Independent Consultant Av. Brig. Faria Lima, 2355 Room 707 01452 000 São Paulo - SP – Brazil hydrogeoeng@uol.com.br

SECOND INTERNATIONAL SYMPOSIUM ON ROCKFILL DAMS RIO DE JANEIRO ,RJ – BRAZIL, OCTOBER 27 – 28, 2011

(construction period) or owners (operation phase) due spoiling potential future consequences, such as, leakage increasing and/or reinforcement corrosion. Further reinforcement deterioration may affects concrete slabs durability, and must be concern and controlled. Brazilian Concrete Code tolerance states 0.3 mm cracks opening for structures, as general, and 0.2mm in case water contact. In Chinese Code, tolerance limit for face slab cracks is 0.2mm. For cracks opening larger than 0.2mm, obligatory seal treatment is required. For very cold weather conditions or frequent water level change, more rigorous cracks tolerance will be implemented. Specifications and bidding design criteria might indicate a “cracks criteria acceptance “ for the end of construction phase and mainly after impounding, in authors’s opinion. In Tianshengqiao I (TSQ.I), a maximum -912μ horizontal strain was recorded ( strain gage SGH20) in June 2003 [ Ref. 3], at center spalling area, corresponding to a compressive rupture ≈ 22 MPa. In Mohale, horizontal strains ranged -600 to -620 μ have been recorded, along vertical joint L 17/ L18 , which means a failure compressive rupture ≈ 20 M Pa [ Ref 4 ]. Therefore, Mohale and TSQ.1 slabs should not have collapsed by horizontal compressive strain only, but due to the flexion stresses, too. Horizontal reinforcement bending recorded during site inspections confirmed this hypothesis. Barra Grande and Campos with a similar spalling phenomenon might be followed the same behavior. So,

monitored strain gauge values ≤ - 1000 µ in slabs collapsed showed that flexion/tensile stresses should play a major role than compressive stresses in view concrete resistance ranged 25 to 30 MPa for current slab design. Face slab deformations due to rockfill settlements and hydraulic head, induce compressive and tensile stresses along the slab area and preferentially along slab slope. Rockfill arching effect due abutment geometry can induce flexion effects in center parts. At perimeter joint it seems to be less affected since plinth structure is founded on competent rock foundation. Pre-settlement rockfill period around 3 – 7 months before slab construction have been implemented in some high CFRDs in China such as, Hongjiadu [ Ref. 4] and Shuibuya, seeking to reduce further tensile stresses in slabs. 2.2 Leakage control Leakage records around 1,300 liters/s have been recorded at Barra Grande and Campos Novos CFRDs during operation phase. Mohale CFRD leakage ranged currently 500 to 600 liters/s. These CFRDs are operating quite well and in safety conditions and no slab treatments were implemented, so far. In authors’ view, a dam leakage value higher 500 liters/s, should concern owners and designers regard face slab quality control or grouting plinth performance and must deserves attention and monitoring and urgent treatments based on in situ inspections.

2.3 Slabs Thickness ( T) design criteria The empiric relationships : 0.30 + K H has been applied in the last 30 years for slab thickness dams up to 100 m high and hydraulic gradients around 200 ≈ 220. Empiric K parameter ranged 0.002 ≤ K ≤ 0.0035, is been used in several high CFRDs, as presented in Table 1. However, T = 0.30 + 0.002H current empiric formula has been satisfactory applied for several CFRDs higher than 200m currently. Meanwhile, K conservative criteria values ≥ 0.003 has still been used in few current CFRDs (Table 1) with costs increasing. 2.4 Reinforcement ratio

SECOND INTERNATIONAL SYMPOSIUM ON ROCKFILL DAMS RIO DE JANEIRO ,RJ – BRAZIL, OCTOBER 27 – 28, 2011

For practical purpose, 0.4 % to 0.5% (vertical) and 0.30 % to 0.35% (horizontal) steel bars ratio in each direction and double mash criteria, has became an international practice and should still been applied for extra high CFRDs. Anti spalling double reinforcement is being commonly used for current design, since the accidents occurred between 2003 and 2006 years. Double reinforcement design should avoid cracks due to compressive and flexion tensions, in addition. Mohale and Kárahnjúkar have applied reinforcement ratio ranged 0.39 to 0.52 % and 0.66 to 0.96, respectively [Ref.5]. Bakun CFRD reinforcement is arranged in one layer, two directions. For the central part of the second stage slab, reinforcement is arranged in two layers, two directions. Reinforcement ratio is 0.4% in both directions. Shuibuya’s reinforcement is arranged in one layer, two directions, reinforcement ration 0.4 % in slope direction, dam axis direction is 0.35%. High reinforcement ratios ( above > 0.50% ) are quite conservatives concept application, in authors’ opinion, and bring construction difficulties for concrete placement for instance, close to waterstops, as expensive practices, unnecessary. Table 1: CFRDs features in the last decades

Dam Name Country Year Height (m) (m)

Slab thickness (T) A/H2

Alto Colombia 1974 140 0.30 +0.003H 1.14 Golillas Colombia 1978 130 o.30 +0.0037H 0.92

Foz do Areia Brazil 1980 160 0.30 +0.0034H 5.43 Salvajina Colombia 1983 148 0.30 +0.0031H 2.62 Segredo Brazil 1992 145 0.30 +0.0035H 4.14

Aguamilpa Mexico 1993 187 0.30 +0.003 H 3.92 Xingó Brazil 1993 145 0.30 +0.0034H 6.0

Messochora Greece 1995 150 0.30 +0.003 H 2.27 Tianshengqia China 2000 178 0.30 +0.0035H 5.68 Machadinho Brazil 2002 125 0.30 +0.0033H 4.93

Mohale Lesotho 2002 145 0.30 +0.0035H 4.14 Barra Grande Brazil 2005 185 0.30 + 0.002H (<100m)

0.0050 H (>100m) 3.16

Campos Novos

Brazil 2005 202 2.60

EL Cajón Mexico 2006 188 0.30 +0.003 H 3.21 Hongjiadu China 2006 180 0.30 +0.003 H * 2.20

Kárahnjúkar Iceland 2007 196 0.30 +0.002 H 2.42 Sanbanxi China 2007 186 0.30 + 0.0034 H 2.40 Shuibuya China 2008 233 0.30 +0.003 H 2.21

Bakún Malaysia 2009 205 0.30 +0.003 H 3.02 Sogamoso Colombia UC 191 0.30 +0.003H 2.06 Porce III Colombia 2010 155 0.30 + 0.0024H 2.40

Mazar Ecuador 2009 166 0.30 +0.003 H 1.70 La Yesca Mexico UC 208 0.0045H * 2.55

* Water Power & Dam Construction Year Book 2010 UC under construction

Slipping forms: From international construction practice, slabs 15 m or 16 m wide have been used commonly. Currently, FEM - Finite Element Method design analysis have recommended to split slabs in 7.5 m or 8.0 m wide at compressive parts (Shuibuya, Bakun and Mazar), in view high horizontal compression and flexion stresses predictions in these areas. Complementary, a soft material ( wood, EPDM) has been inserted in some vertical joint at slab center part to face compressive stresses from

SECOND INTERNATIONAL SYMPOSIUM ON ROCKFILL DAMS RIO DE JANEIRO ,RJ – BRAZIL, OCTOBER 27 – 28, 2011

abutments to center part. In the authors’ view, 7.5 m (or 8.0 m) wide slabs criteria, brings facilities costs increase in vertical joints construction, more waterstops, more consuming materials in pre casting preparation facilities, significantly, and has no deep scientific support. Rockfill parameters during impounding and stresses interaction slab – rockfill are not clearly know, so far. Current FEM analysis are important design tools but cannot estimate confident stresses interaction between the rockfill deformations and concrete face, so far. For extra high CFRDs this issue concerns dam behavior predictions and further behavior must be better focus on by designers and constructors.

2.5 Waterstops

Reservoir water head pressure induces rockfill stresses / deformations from abutments to center areas and toward dam top and consequently slab high deflections along all these areas. Since CFRDs spalling issues ( 2003 – 2006), vertical center joints have been designed by inserting a flexible filler material between slabs (soft filling concept) , such as wood or PVC or similar materials, to mitigate compression stresses [Ref. 7]. This “compressive fill concept” was also applied during Barra Grande and Campos slabs remedial repairing works and were adopted consequently in several high dams construction, such as, Shibuya, El Cajón, Kárahnjúkar , Bakun and Mazar.

Several materials are been used for joints sealing purpose such as : i).mastic, PVC band placement; (Campos Novos, Barra Grande) ; ii) self-healing sealing-clog - fly-ash or coal-ash (Aguamilpa, TSQ.I, El Cajón, La Yesca - UC); iii) GB material combined with a corrugated GB rubber ( Figure 1), designed and manufactured by China Institute of Water Resources and Hydropower Research (IWHR), and used in Shuibuya, Bakun and Mazar. Besides, bottom copper waterstops over PVC or bituminous strip placed on curb surface are still been used to complete the joint design. GB flexible filling has been placed to fill up joints also without the corrugate rubber. This joint concept brought interesting contribution for sealing and seeking movements from compressive and flexion stresses at abutments and perimetric joint .

Figure 1 : Corrugate waterstop – IWHR [Ref. 6] ( Courtesy of OFICINA DE TEXTOS, Editor)

SECOND INTERNATIONAL SYMPOSIUM ON ROCKFILL DAMS RIO DE JANEIRO ,RJ – BRAZIL, OCTOBER 27 – 28, 2011

Fly ash has been used successfully in Mexican CFRDs , along perimeter joint and vertical joints such as such as Aguamilpa, El Cajon and La Yesca ( 208 m, under construction). However, this joint concept is complemented with a copper waterstop at joint top and bottom, both. Copper waterstops have been applied in CFRDs since 1970’s. PVC or GB synthetic materials are excellent alternatives instead of copper waterstops, in vertical joints mainly. Synthetic materials present several benefits and reasons for use, such as: i) quality control in factories; ii) easily installed and shipping to site; iii) competitive costs; iv) more easily connected facilities by welding, etc. Owners, constructors and designers must implement efforts to incentive synthetic materials use in high and extra high future CFRDs as a copper’s alternative.

2.6 Horizontal joints

Horizontal slab construction joints in two to three stages are being built, as an international practice. Joint treatment consists in removing few centimeters of the pre-existent concrete, surface cleaning with air or green cutting facilities to remove exposed aggregates. Reinforcement steel bars being continue overlapping the next concrete pouring stage. Horizontal construction joint should not be considered as contraction joints, so, waterstops are not required according to a good engineering practices. For practical and costs purpose, constructor is preferable to define how many slab horizontal joints must be built ( two, three or only one slab stage), considering the construction schedule and civil works Contract bench marks, in authors’ view.

2.7 Upstream fill

An upstream blanket impervious soil is adopted on the lower bottom part of the concrete face since Alto Anchicaya Dam (1974) and this concept was followed in Foz do Areia, Khao Laem, and Golillas Dams and in several others CFRDs dams since 70s’. The purpose is to cover the perimeter joint and slab lower elevations with impervious soil, for leakage control in case cracks or joint openings. Barry Cooke ‘s recommendation of an upstream fill about half of the total dam height for high or extra high CFRDs ( ≥ 240 m) must be currently re analyzed, in the authors’ opinion. CFRDs site areas are geologicaly and commonly shortage of available fine or silt /clay materials. In addition, the recent cases of Campos Novos Barra Grande, and Mohale upstream fills have apparently few effect for leakage control after cracking phenomenon issues. In addition, upstream fill might be kept as wasting areas also, for required excavation fine materials from dam foundations, as an construction alternative purpose.

3. Rockfill

Rockfill construction proceedings have adopted currently a thinner layers thickness (max. 50 cm at Zone 3B and center parts and 80 cm at Zone 3C), and high waterfill (> 250 ≈ 300 liter/m3) supply, and a high energy compaction 6 to 8 passes of a heavy 13.6 – 25 ton vibratory compactor. Medium to high rockfill modulus of deformability ranged 90 to 150 MPa, must be seek by designers and owners. In addition, to extend upstream Zone 3B zone to at least two-thirds of the cross section has been adopted as a design criteria. Improvement in rockfill compaction facilities aiming to achieve high modulus of deformability ( > 90 MPa). For gravel natural material highest modulus (> 250 MPa) are achievable. After impounding, modulus can increase substantially, achieving around 3 three times construction modulus [Ref. 8]. However, this issue must deserves quite more design and site investigations based on instrumentation monitoring analysis for future extra high CFRDs.

SECOND INTERNATIONAL SYMPOSIUM ON ROCKFILL DAMS RIO DE JANEIRO ,RJ – BRAZIL, OCTOBER 27 – 28, 2011

79-75 Kg Cement/m360-75 KgCement/m3

4. Curb concrete

An extruded concrete curb , 40 cm trapezoidal blocks over the cushion upstream [Ref. 9], The Itá Method , 1999) has been adopted in all CFRDs already constructed in the world. Figure 2, shows curb details and curb extruding machine.

Curb Detail Extruding machine Figure 2: Upstream curb protection

Application of a bituminous emulsion bond breaker between curb surface and face slab has been [ Ref. 10] to minimize inter action slab –curb surface. In authors’ view, bond breaker use is expensive, and there is no current trust monitoring data and design analysis related stresses- strain inter relation between slab and rockfill/curb surface contact, so far.

5. Parapet wall

Double parapets is currently used in high dams, seeking to speed up time construction, improving quality control. In situ or precast parapet walls are being used for modern CFRDs as a practice construction alternative. Post settlements ( creep ) predictions for operation period must be taken in consideration by the designer, to determine the final upstream top parapet elevation. Downstream rockfill zones after construction ( creep) deformation depends on the dam zoning, compaction specifications, rainy season effect ( in tropical countries), valley effect and in same cases seismic events. Monitoring settlements from cells and bench marks installed along crest and downstream parts must go on be implemented as an important role for collecting dam behavior information for future extra high CFRDs movements predictions.

6. Valley geometry effect ( A/ H2)

Valley geometry effect in rockfill dam behavior has been pointed out by several authors [Refs. 11, 12, 13). This point still remains a very controversial matter among senior engineers. Narrow Valley Factor ( ≤ 3.0) deep influence in rockfill performance and slab deformations are not deep clearly explained, so far. Intense instrumentation programs along abutment areas must still be implemented for next high CFRDs to investigate this question. 7. Rockfill Construction facilities Access roads, internal ramps combining with tunneling temporary facilities allow shortest routes between different parts of the rockfill dam. In addition, during construction stage, grid steel bars and steel mesh reinforced cofferdams and rockfill zones can permit river water flows (overtopping) during

SECOND INTERNATIONAL SYMPOSIUM ON ROCKFILL DAMS RIO DE JANEIRO ,RJ – BRAZIL, OCTOBER 27 – 28, 2011

floods period in first year of construction, as built in TSQ 1 Dam ( 1996), to minimize diversion structures facilities and optimizing construction schedule.

8. Earthquake

A recent high earthquake (2008) that occurred in Sichuan province, China, of 8,0 magnitude on the Richter scale, with the epicenter at 17 km and horizontal acceleration of 2g at dam crest level reached de Zipingpu CFRD, 156 m high. Settlements of upper rockfill parts, spalling parapet wall, cracking and rupture of the 2nd and 3rd stages of slabs, gaps between slabs and rockfill , and however, dam leakage was not increased, recording a flow up to 191 l/s, were the only consequences [ Ref. 14]. The dam resisted quite well the earth quake, no further leakage increasing, confirming COOKE and SHERARD’s predictions. 9. Final Considerations

CFRDs as safety structure are feasible and profitable alternative in time and costs comparatively with RCC - Roller Compacted Concrete and Arch gravity dams alternatives.

Monthly rockfill pick production around 1, 200 x 103 m3 were achieved in TSQ.I and Hogjiadu CFRDs and others dams. Rockfill zones could be built up to different levels using internal temporary ramps and high management and flexibility facilities in rock materials placing from required excavations directly in dam zones and minimizing stock pilings. Rockfill layers thickness, ranged 50cm to maximum 80 cm, and water facilities fill (> 250 ≈ 300 liter/m3) supply and high energy compaction (13.6 – 25 ton heavy vibratory rollers) have allowed to achieve modulus of deformability ranged from 90 to 150 MPa or higher. This proceeding seeks to minimize large potential rockfill arching effects or differential settlements and potential further cracking issues along face slabs.

Additional settlements after construction at Zone 3C due to the “ watering breakdown phenomenon” after heavy rain seasons ( at tropical climate areas) has probably affected Zone 3B deformations and caused face slab and crest areas further cracking effects, after two or three years of dam operation. Face slab further movements in TSQ.1 would explain cracks and concrete spalling in May – June 2003 and 2004, on rainy periods.

From international construction practice, slabs 15 m or 16 m wide must still be used commonly, for slipping form works. Current design recommendations to split slabs in 7.5 m or 8.0 m wide , based on FEM analysis has increasing works facilities ( more waterstops, joint filling materials, etc) and costs.

PVC, EPDM or GB are excellent synthetic materials and good alternatives instead of copper waterstop, in vertical joints, mainly. Synthetic materials present practical good reasons to be used, related to easy quality control, speed shipping facilities and placement at site and must be deeply considered to increase application for future extra high CFRDs.

REFERENCES [1] The High CFRD Dam”- J. Barry Cooke- Invited Lecture- International Symposium on Concrete

Faced Rockfill Dams – Beijing- China- September, 18th- 2000.

SECOND INTERNATIONAL SYMPOSIUM ON ROCKFILL DAMS RIO DE JANEIRO ,RJ – BRAZIL, OCTOBER 27 – 28, 2011

[2] Cracks and Flows in Concrete Face Rock fill Dams (CFRD), Cruz. P.T.; Freitas, M.S. F. Jr. – Dam

Engineering , 14 – 16 Feb. 2007, Lisbon, Portugal.

[3] Design Research for very High CFRD, Keming, C, Zhongping and Caichang Guang (2007) , Workshop on High Dam Know-How, May 22 -24, Yichang, China.

[4] Deformation Control of high the 200 m High Hongjiadu Concrete Faced Rockfill Dam, Yang Zeyan, Jiang Guocheng, 1st International Symposium on Rockfill Dams, Ocot. 2009, Chengdu, China).

[5] Updated Assessment of Mohale Dam Behavior, including of slab cracking and seepage Evolution, Johannesson, P., Tohlang, S., ( 2007), Third Symposium on CFRD –Dams, Oc. 2007, SC, Brazil.

[6] Concrete Face Rockfill Dams, Cruz, Bayardo, Freitas 2009 , Oficina do Texto, Editor.

[7] Design Criteria for CFRD An actual review of Barry Cooke and James Sherard papers of 1987, Cruz, Bayardo, Freitas, Hong Kong Dec. 2008.

[8] The design and construction of extra high CFRDs , N. L. de S. Pinto, Hydropower & Dams Issue Three 2009.

[9] Itá Method – New Construction Technology for the Transition Zone of CFRDs, Resende, F.; Materon, B., (2000) ICOLD, Beijing, China 2000.

[10] Construction Kárahnjúkar Face Slab, Gianni Porta, Richard Graham, Modern Rockfill Dams, I Simposium on Rockfill Dams, Chengdu, China, 2009.

[11] Colombian Experience in the design and construction of concrete face rockfill dams”, Amaya F.; Marulanda, A- J. Barry Cooke Volume Concrete Face Rockfill Dams, International Symposium on Concrete Faced Rockfill Dams, 18 September, 2000, Beijing, P.R. China. [12] Discussion on behavior of Brazilian Rockfill dams , Viotti, C.B., Second Symposium on CFRD, Florianopolis, Brazil, October 1999 . [13] Very High CFRDs: Behavior and design features , Pinto N.L. de S., The International Journal on

Hydropower & Dams, Issue Four , 2008.

[14] Performance of the Zipingpu CFRD during Wenchuan earthquake, Xu Zeping, Hydropower & Dams Magazine, Issue Three, 2009.`