ANALYSIS OF THE CRFD BEHAVIOR RELATED TO THE ROCKFILL DEFORMABILITY AND ITS LEAKAGE CONSEQUENCES AFTER RESERVOIR FILLING

SILVEIRA, J.F.A. e BANDEIRA, O.M.

SUMMARY

This paper presents an analyses of the rockfill deformability in a total of 18 (eighteen) CFRD – Concrete Face Rockfill Dams, in: Australia, Nigeria, Thailand, Brazil, Colombia, Malaysia and Mexico, trying to correlate it with the leakage measured through the dam after the reservoir filling.

TYPICAL SECTION OF XINGÓ CFRD

Central section of the Xingó CFRD, instrumented with hydraulic settlement

cells (CR) and magnetic meters (MM).

Hydraulic Settlement Cell

Plate extensometer KM for horizontal displacements

Settlement Cell and KM extensometer.

ITAPEBI CRFD WEIR GAUGE

General view of the Itapebi CFRD.

Weir gauge measuring device.

TYPICAL SECTION OF CRFD UNTIL 1994

Until mid-90s the CRFD were divided into two "zones" with the amount formed by tightly compacted rockfill and good wetting, to ensure high modulus deformability and a good support for the concrete slab. The downstream zone has been constructed with thicker layers and accepts larger size of rocks.

Typical section of CRFD until 1994 (Ref. Marengo, 2015).

TYPICAL SECTION OF CRFD FROM 2002

From 2002 generally it went up to employ a new concept, dividing the cross section of CFRD into three "zones“ : 3B Zone: Rockfill with blocks and layers with thickness between 0,6 and 0,8 m; T Zone: similar Rockfill with blocks and layers with thickness between 0.8 and 1.0 m; 3C Zone: Rockfill with blocks and layers with thickness between 1,2 and 1,4 m.

Typical section CRFD from 2002 (Ref. Marengo, 2015).

3B ZONE T ZONE 3C ZONE

MAXIMUM SETTLEMENTS AT THE END OF THE CONTRUCTION PERIOD

DAM Finished Year Height (m) Rock Type Vertical Modulus

(MPa)

Cethana 1971 110 Quartzite 145

Mackintosh 1981 75 Greywacke 40

Bastyan 1983 75 Rhyolite 160

Winneke 1980 85 Siltstone sound 55

Alto Anchicaya 1974 140 Hornfels 145

Shiroro 1984 125 Granite 76

Khao Laem 1984 130 Gravel 40/50

Golillas 1984 125 Gravel 160

Salvagina 1984 148 Gravel / Sandstone e

Siltstone

390 54

The results of the rockfill deformability modulus observed in sixteen (16) different CRFD dams with heights ranging from 75 to 160 m, and using different types of rock types are presented below.

MAXIMUM SETTLEMENTS AT THE END OF THE CONTRUCTION PERIOD

DAM Finished Year Height (m) Rock Type

Vertical Modulus (MPa)

Foz do Areia 1980 160 Basalt sound/ Basalt+breccia

37/56 26/30

Segredo 1993 145 Basalt (Mont)

Basalt (Jus) 60 38

Xingó 1994 150 Granite (Mont)

Granite (Jus) 32 20

Itá 1999 125 Sound Basalt

Basalt+breccia

30/60

20/30

Itapebi 2002 121 Gneiss (Mont)

Gneiss (Jus)

55

28

Campos Novos 2006 202 Basalt (Mont)

Basalt (Jus)

60

30

Bakun 2009 205 Greywacke 94

Modulus of deformability above 100 MPa have been observed at Cethana, Bastyan, Alto Anchicaya and Salvagina CFRD, employing up rocks like quartzite, rhyolite, diorite and gravel.

DEFORMABILITY MODULES

Deformability Modules obtained from the instrumentation during the construction period

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70

FOZ DO AREIA SEGREDO XINGÓ ITÁ ITAPEBI

Defo

rmab

ilit

y M

od

ule

s (

MP

a)

CFRD DEFORMABILITY MODULES

UPSTREAM ZONE DOWNSTREAM ZONE

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50

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150

200

250

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350

CAMPOS NOVOS AGUAMILPA EL CAJÓN LA YESCA

Defo

rmab

ilit

y M

od

ule

s (

MP

a)

CFRD DEFORMABILITY MODULES

UPSTREAM ZONE CORE DOWNSTREAM ZONE

COMPARSIONS OF THE DEFORMABILITY MODULUS

Comparison of the deformability modulus of the rockfill versus leakage.

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300

600

900

1200

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2100

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XINGÓ ITÁ ITAPEBI CAMPOSNOVOS

AGUAMILPA EL CAJÓN LA YESCA BAKUN

Leakag

e (

L/s

)

Defo

rmab

ilit

y M

od

ule

(M

Pa)

DEFORMABILITY MODULE x LEAKAGE - CFRD

UPSTREAM ZONE CORE DOWNSTREAM ZONE LeakageRelation between "Form Factor" and the rockfill deformability modulus for several CFRD.

Three CFRD’s in red on the chart presented open cracks and concrete rupture in the central slabs, due to high longitudinal compression.

BAKUN DAM IN MALASYA

Some CFRD dams have been built in stages, as can be seen in Bakun CFRD, in Malaysia. The first stage, the dam was raised to El.125 (50% of final height), to be able to act as a cofferdam for the 500 years flood.

Main Cross section of the Bakun Dam, Malaysia

CUTTING JOINTS OPERATION

Cutting operation in the central joints of the concrete face slab of the Bakun CFRD.

The Bakun owner, the design company and the Board of Consultantes decided cutting six joints between the central slabs, with a gap of 50 mm/joint.

ANALYSIS OF LONG-TERM SETTLEMENTES

While the CFRD built about 30 to 50 years ago indicated settlements occurring mainly during periods of construction, reservoir filling and start operation, the settlements observed in CFRD are usually in the long-term, occurring settlements over several decades.

Illustration of the downstream zone affected by slow wettability during the rainy season.

ANALYSIS OF LONG-TERM SETTLEMENTES

Evolution of settlements of the crest of Foz do Areia and Segredo CFRD.

-100

0

100

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700

0 20 40 60 80 100 120 140

Defl

exão

(m

m)

Distância ao Plinto (m)

UHE ITAPEBIBaragem de EFC - Rio Jequitinhonha

Eletroníveis EN-01 a EN-06 - Laje 9 ( Seção A - Estaca 21+ 00m )

Deflection of the Slab 9 measured by the eletrolevels in Itapebi CFRD.

COMPARSION OF THE CREST SETTLEMENTES

PERCENTAGE OF SETTLEMENTS

Measured and Theoretical curves of the maximum percentage settlements observed in several CFRDs.

AVEREGE RATE CREST SETTLEMENT Crest Settlement average rate in some CFRDs (~ 100 <H <~ 200 m)

CFRD Average Rate Crest settlement (cm/year)

After 5 years After 10 years After 30 years Segredo, Foz Areia,

Alto Anchicaya, Cethana e Murchison

2,8 1,6 0,6

Mackintosh e Xingó (OD e center)

5,2 2,9 1,1

Xingo (OE), Itapebi e Campos Novos

8,2 4,6 1,8

It can be observed that for Xingó (left abutment), Itapebi and Campos Novos CRFDs, in average terms would be occurring variations of settlements of 1.8 cm / year, with direct implications for the heightening of the crest, given that every 10 years would still be experiencing a settlement of around 18.0 cm in the center section of the dam crest. It should be kept in mind, therefore, that the settlements of CFRD would occur over several decades, with significance even in dams built with the best quality of rockfill, with no use of water at the downstream zone.

LEAKAGES AFTER RESERVOIR FILLING

The leakage through the CFRD generally occur through cracks in the concrete face slabs, as well in the joints between slabs. In the Xingó CFRD leakage increased suddenly after one year of reservoir filling, as a consequence of open crack slabs at the left embankment.

Settlements cells installed at Est 62 + 00 (left abutment) Xingó. CFRD

Xingó CRFD – settlements increase observed at Est. 62+00

ITAPEBI CRFD LEAKAGES

Leakages at the Itapebi CFRD left embankment, as a consequence of layers of weathered rock at the downstream zone.

COMPARSION OF LEAKAGE X FORM FACTOR

Figure bellow shows a comparison between the maximum leakage observed in some CFRDs in Mexico, Malaysia and Brazil, versus the form factor given by the ratio between the slab area by dam height square. With the exception of the Xingó CRFD, is generally observed that the lower form factor correlated with the lower measured leakages.

MAIN CONCLUSIONS

• The Chinese with their experience in building a great number of CFRDs have recommended, for dams H≥200 m, the use of rockfill with at least an average deformability (30 <E <80 MPa). The compaction density must be increased to achieve high deformability modules, with low porosity less than 20%;

• The occurrence of extensive cracking in the concrete face slab in the Aguamilpa CFRD, in Mexico, has shown that the good behavior of the face slab depends not only on the rigidity of the upstream rockfill, but also of the deformability in the central and downstream zones. The occurrence a long horizontal crack in the Aguamilpa face slab, 165 m long, resulting in high leakage, having not been prevented by the high deformability modulus of the upstream zone (E = 320MPa). It clearly showed that it was also influenced by the modules of the Central ( E = 140 MPa) and downstream Zones (E = 45 MPa);

In the actual paper the authors tried to correlate the rockfill deformability in a total 18 different CFRD’s, in Australia, Colombia, Nigeria, Thayland, Brazil, Malaysia and Mexico, with the leakages measured after reservoir filling. The more relevant points are:

• The Chinese experience also recommends strengthening the slab reinforcement in the region of the central joints, for the highest CFRDs, as well as providing the installation of reinforcement steel bars (anti-spalling) among the most central slabs.

MAIN CONCLUSIONS

• While Aguamilpa, El Cajon and La Yesca CFRDs, in Mexico, showed percentage of settlement in the range of 0.40% to 0.90% of the dam height, while the Brazilian CFRD have presented much greater modules of deformability, with percentages of settlements much higher, varying from 1.20% to 2.24% of the height;

• While rockfill dams built about 30 to 50 years ago indicated settlements primarily during construction periods and first years of operation, the Brazilian CFRD’s are presenting settlements along several decades, due to slow wetting of downstream rockfill, during the rainy season, every year.

• To avoid rupture or cracking of the central concrete slab in CFRD, it is of great importance the installation of flexible elements in the central concrete face joints. In Bakun CRFD in Malaysia, after the rupture of concrete face observed in the most central region of Mohale, Barra Grande and Campos Novos CFRD’s, it was decided to proceed the cut of the six central joints between slabs, to ensure an average opening of 5 cm/joint. Then, the joints were filled with a pine soft wood covered with a special protective blanket, which ensured a good performance for the dam.

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