SEMINAR ON RCC DAMSOrganized by VNCOLD

Hanoi, xx September 2011

RCC DAMS WORLDWIDE AND IN VIETNAM M. Ho Ta Khanh (VNCOLD)

<mhotakhanh@yahoo.com>

RCC Dams Worldwide

Design Criteria and Analysis for Gravity RCC Dams

The same as for CVC gravity dams but with different values for the parameters.

Mechanical conditions

- Strength and Sliding stability

- Deformation, settlement : FEM analysisHydraulic conditions

- Permeability- Hydraulic gradient

Adopt Vietnamese code if any, but it is possible to use all available codes or

guidelines (USA, Canada, GB, France, Germany, Russia, China, Japan, India, etc).

No code is better or worse than the others! All these codes are coherent and adopt the

same general principles (which are a simplification of the reality). They differ mainly by

the presentation and lead to very comparable results! The most important is to adopt

homogeneous parameters and criteria (global safety factors and partial safety factors

for friction, cohesion and max tensile strength). Don’t mix these codes and don’t retain,

for safety reason, the most unfavourable results!

Application with good judgment by experimented engineers is more important than the

origine of the code !

The conception of a RCC dam must be adapted to the

particularities of the RCC technique (Basic principles)

• Use as much as possible the local materials (aggregates, cementitious

materials).

• Reduce as much as possible the quantities of cementitious materials, in

particular the fly ash if it is not available near the site (< 200 km).

• Adapt the cross-section of the dam to the characteristics of the RCC, …and

not the opposite! (See examples of the Moroccan and French RCC dams).

• Each RCC dam must be optimized according to the conditions of the site

(flexibility of the design) : avoid to normalize the conception of the dam and the

composition of the RCC materials !

• Facilitate as much as possible the placement of the RCC:

Avoid if possible the structures including large openings.

Separate, if possible the location of the dam and the the powerhouse (example of Long Tan

and Salto Caxias HPP).

Select the construction equipment adapted to the rate of placement.

• If useful, the design can separate the mechanical and the watertight functions.

Design and Construction Trends in the U.S RCC Dams by F.Y.Abdo (2010)

“Perhaps the most notable development in recent RCC gravity dams in U.S.A in

the design is:

• Increasing the dam size in order to reduce the required RCC strength

provided an opportunity to use marginal on-site aggregates.

• Designing the dam to resist full hydrostatic uplift pressure eliminated the

need for foundation drains and drainage gallery (for low and medium-sized

dam).

• Eliminating the construction of a stilling basin”.

The purpose of the next slides is to illustrate these recommendations by

some examples of recent RCC dams worldwide.

Flexibility of RCC Dam Designs Very different cross-sections according to the quality of the foundation, the aggregates,

the type and content of the cementitious materials

Longtan Dam (China) 2007

Very good aspect of the downstream face

with the GEV-RCC method and an intact

core 15 m long in the dam.

No seepage in the gallery →

LONGTAN DAM

A good example of separation

between the dam and the

powerhouse.

This implementation allows a

separation between the CVC

and the RCC placements, a

continuous regular placement

of the RCC and a

commissionnig of the 3 first

units before the end of

construction of the dam

(shorter delay than for the

powerhouse).

SALTO CAXIAS (Brazil)1998

A good example of separation between the dam and the powerhouse and a

RCC without fly ash.

Same advantage as for the Long Tan Dam concerning the powerhouse

implementation.

Miel I RCC Dam (Colombia) H = 192 m V = 1 400 000 m3

A good example of a very high dam with cement contents adjusted to the

stresses and without fly ash.

MIEL I RCC Dam in a seismic area

• RCC cement contents in the different parts of the dam = 85 to150 kg/m3.

• No fly ash.

Breña II RCC dam in Spain, H= 119 m, V= 1.6 hm3

The largest RCC dam in Europe

• Limestone filler used

as cementitious

material in the mix.

• Fixed crest spillway.

• Stepped spillway on

the downstream face.

• Large crest width to

increase the dam

volume and lower the

max stress.

Rapid Development of Dams in Morocco since 1985 due to RCC advent (low cost and rapid construction)

913

2027

67

98

132

0

20

40

60

80

100

120

140

1940 1950 1960 1970 1980 1990 2000 2010 2020

Nu

mb

er o

f dam

s

Year

RCC

advent

The Aoulouz RCC dam in Morocco H= 79 m , V= 900 000 m3

First large RCC dam in Morocco (designed in 1987)

← 1992. End of construction

2011. Flow over the spillway →

Construction of the RCC Aoulouz dam in 1990(Note the aspect of the RCC with low cement content and no flyash)

AOULOUZ DAM

RCC with 100 kg cement/m3

and clayey fines, no flyash.

R365 =10 MPa

Progress in RCC Dams in Morocco since 1987Various cross-sections of RCC dams

No flyash in all the RCC Moroccan dams

Low RCC DamsSince 1987, the Moroccan experience proved that RCC technique, in place of

masonry or CVC, is an economical solution even for low dam (< 30m).

Examples of unconfined compressive strengths

for 3 Moroccan RCC dams with additional fines

With100 kg cement/m3 and additional fines (clayey for Aoulouz, limestone for Sidi Saïd

and high quality limestone for Rmel), no flyash.

0

5

10

15

20

25

0 50 100 150 200 250 300 350 400

Days

UC

S (

MP

a)

Aoulouz Sidi Said Rmel

0

5

10

15

20

25

0 50 100 150 200 250 300 350 400

Days

UC

S (

MP

a)

Aoulouz Sidi Said Rmel

Hassan II RCC dam in Morroco (2005):120 m high, 660 m long

Granit + limestone filler

Dmax: 63mm

Cement content : 80 to 100 kg/m3

R365 =16 MPa

Hassan II RCC dam: Cross-sections and details

Wirgane RCC dam with gated spillwayH=70 m (2008)

• Cement content = 100

kg/m3 with filler.

• First placement of RCC

by the sloped layer

method. The advantages

of this method were so

evident that it was

adopted for all the next

Moroccan RCC dams,

even with medium sizes.

Note the 3m high steps

on the downstream face

corresponding to the

height of the10

continuous layers of 0.3m

high each.

The Taskourt RCC dam in Morocco (2011)

Low cement content (100 kg/m3), no flyash

… and no leakage on the downstream face !

Taskourt dam, H= 75 m, L= 416 m

Cross sections through spillway and bottom outlet

Taskourt dam : placement of the RCC by the sloped layer method

Visit of the Taskourt dam (06/06/2011)

The Tiouine RCC dam (H= 84m) in Morocco 2011

Cross sections through the spillway and the bottom outlet

Production of the inert filler and grading curves of filler and sands

The RCC (100 kg cement/m3, no fly

ash, 7% of inert filler) is placed by the

sloped layer method

Tiouine RCC material

Local fine sand and ground inert filler, without fly ash, provide sufficient density, strength

and watertightness for the dam, with the minimal cost !

Tiouine dam: Placement of the RCC

(Note the dry aspect of the RCC very easy to compact)

Visit of the Tiouine RCC dam (07/06/2011)

Rizzanese dam (France), H= 40.5 mAn example of RCC dam on weak foundation, low quality aggregates and low

percentage of cement, without fly ash.

Rizzanèse dam

Spreading the RCC (100 kg of cement/m3 without F.A) on the bedding mix (mortar)

Cementitious content

Comments

• Increase of «High paste RCC» is due

mainly Chinese RCC dams (China

has a lot of coal fired thermoplants

with low cost of fly ash).

• Increase of Lean RCC is due mainly

to Brazilian RCC dams (The Brazilian

RCC dams are far from thermoplants).

• High increase of the proportion of

Hardfill dams (they are not numerous,

although very interesting on

weathered foundation).

• Relative decrease of RCD (higher

cost, only adopted in Japan).

These values reflect the particularities

of the site and the conception of the

dam but not the proof of the

superiority of one technique on the

others !

1996 2006

High paste

(> 150 kg/m3

cementitious material)

43.3 % 53.4 %

Medium paste

(100 < CM < 145)

21.7 % 16.9 %

Lean RCC

(CM< 99 kg/m3)

12.7 % 13.3 %

Hardfill 0.6 % 2.9 %

RCD

(Japan)

18.5 % 12.8 %

Unknown 3.2 % 0.8 %

Cementitious materials

Comments

• Decrease of the use of (cement + low-

lime FA), which remains however the

large majority of cases.

• Increase of the use of (cement +

natural pozzolans), due to the

expansion of RCC dams to regions

where fly ash (and slag) are not

available.

• Increase of the use of (Portland

cement alone), due to the expansion

of RCC dams to regions where fly ash

(and slag) are not available.

1996 2006

Cement + low-lime

FA

66.2 % 60.8 %

Cement + high-

lime FA

1.3 % 0.9 %

Cement + ground -

granulated slag

4.5 % 5.1 %

Combination of

pozzolans (no

cement)

4.5 % 2.1 %

Cement + natural

pozzolans

7.6 % 15.3 %

Cement +

manufactured

pozzolans

2.5 % 1.2 %

Portland cement

alone

10.2 % 14.7 %

Unknown 3.2 %

Cementitious content of Brazilian Dams

Cementitious content of Brazilian Dams

RECENT TRENDS IN RCC MATERIALS

• High or low paste content ?

All recent RCC materials are in reality «High paste content», it is more exact

to replace in this classification : «paste» by «cementitious».

• The cementitious content

The «cementitious materials» include cement and slag but also all the materials that present a «pozzolanic reactivity» (fly ash, natural or artificialpozzolan, some natural fines and rock powder, etc).

• The use of powdered aggregates

More and more used everywhere fly ash or pozzolan are too costly.

• The use of admixtures in RCC

More and more used as they can lengthen the setting time of the RCC (to

improve the bonding between the layers) and reduce the water content and

consequently the cementitious content.

Use of admixtures

Comments

- Use of a plasticizer–retarder

admixture (0.8 to 1.12 kg/m3).

- There is a reduction of VB time up to

40% for the same water content, or a

reduction of circa 10% of water

content for the same VB time.

- There is an increase of VB density.

- There is an increase of the mix

efficiency.

- For the same consistency and

compressive strength, the

cementitious content can be reduced

(15 to 30% ).

Without

admixture

With

admixture

VB (s) 67 23

Density VB

(kg/m3)

2 540 2 565

Mix efficiency

at 180 days

(MPa)/(kg/m3)

0.10 0.13

Cementitious

content in

(kg/m3)

100

120

85

80

Retarding

admixtures

Construction

under high

temperature

To avoid

cracks

(China)

Cost savings

(cement)

Brazil

Morocco

10 kg/m3

40 kg/m3

Use of powdered aggregate in Elk Creek Dam (USA)

The use of fines (in particular limestone powder) is generally very beneficial

in the RCC and allows to lower the amount of cementitious materials.

RECENT TRENDS IN RCC CONSTRUCTION (1)

• The use of conveyors: the main advantages are the possible high rate of construction

and the non pollution of the RCC layers. This use is now almost generalised for the very

large dams.

• The Sloped Layer Method (SLM): this method is at present more and more applied

when the volume of RCC to be placed on each layer is large compared with the capacity

of the batching plant.

• The bedding-mix: used generally in particular cases (cold joints between the RCC

layers, medium and low paste RCC, etc).

• The Grout Enriched RCC (GEV-RCC): more and more used for the upstream and

downstream faces of the dam and between the RCC and the CVC structures or between

the RCC and the foundation. Give very good results, if correctly applied. To obtain a

good result, it is necessary that the grout (cement+water or mortar) is poured at the base

and/or in the middle of the new layer (or in a small trench dug in this layer), before its

vibration by the needles. This technique is valid even with low cementitious RCC

(Chraibi 2010).

RECENT TRENDS IN RCC CONSTRUCTION (2)

• The cooling of RCC: for low and medium high dams (<100 m) : use of low heat

cement and fly ash if not too expensive, pre-cooling of the aggregates by air, water

spraying of the layers, induced intermediate vertical joints (see photo of the upstream

face of Nam Theun 2 dam) to prevent crack extension,For high dams (>100 m) : same

precautions, plus an ice cooling plant and an internal cooling of the dam, if necessary.

• The use of geomembrane: can be an interesting solution when the function of

watertightness is separated from the mechanics and the stability functions. For example

for the low paste RCC (without fly ash) gravity dams, or for FSHD and CSG dams with

very low cement contents. Some designers prefer to adopt a gemembrane protected by

precast concrete panels for the upstream face of the dam.

Rialp RCC Dam (Spain) Transportation of RCC by conveyor and swinger : quick placement and clean layer

surface !

Some recent Chinese construction techniques for RCC dams

Sommaire

SummaryUse of Geomembrane

Balambano Dam (Indonesia)

Sommaire

Summary

For the Balambano dam the total

leakage through the dam is virtually

zero (some seepage appeared

through the foundation and the

abutments) : the geomembrane was

thus very efficient for the dam

watertightness.

Use of Geomembrane

Sommaire

SummaryOvertopping protection of

embankment dam by RCC : Brownwood Country Club Dam

(USA)

• Initially 6 m high earth dam

• First earth dam in USA to

receive RCC overtopping

protection (1984)

• Initial Flood = 74 m3/s

• Revised Flood (PMF)= 330

m3/s

• Overtopped 6 times since its

construction with no damage

• Volume of RCC = 1 070 m3

placed in 2 days

• 1/3 of the cost for increasing

spillway capacity by traditional

method

Two Vietnamese RCC Dams

Dinh Binh Dam

Son LA HPP

Dinh Binh Dam

Cementitious content of RCC (per m3)

Cat

Cement

(kg)

Fly Ash

(kg)

Sand

(kg)

A

.0.5x2

(kg)

A.2x4

(kg)

A.4x6

(kg)

Water

(l)

TM-20

(l)

P-96

(l)

RCC

15070 175 772 531 219 605 110 1.47 0.42

RCC

200126 141 746 852 468 0 132 1.6

Materials UnitsCement Flyash Total

kg/m3 VND kg/m3 VND VND

CONCRETE MIX

1x2, OK6-8M150, coarse

aggregate m3 296 251 600 488 095

M150, coarse aggregate

2x4, OK6-8m3 281 238 850 457 698

M150, coarse aggregate

4x6, OK6-8m3 266 226 100 426 709

RCC MIX

RCC MIX , M200 m3 126 110 754 141 97 572 440 953

RCC MIX, M150 m3 70 61 530 175 121 100 408 342

The RCC material costs (2007) are almost the same than the conventional concrete

material costs due to :

• the high percentage of cementitious materials,

• the similar treatment of aggregates.

Comments about the Dinh Binh RCC

The RCC cost of Dinh Binh dam (as other RCC dams in Vietnam) is

high compared with CVC. Why and how to lower it?

• Not optimal design: the design must optimize the cross-section of the dam, and

avoid as much as possible openings in the RCC. It is unecessary to design several

costly watertight barriers in the dam body. The most important is to select an adapted

RCC material, to optimize consequently the design and to have a good control during

the RCC placement.

• High fly ash cost: use fly ash only if there is a thermal powerplant near the site.

• Too high content of cementitious materials : avoid to normalize a minimum RCC

strength (for example RCC150 or RCC200), as for the CVC! Adjust this minimum value

according to the results of each optimization (materials/analysis) of the design.

The strengths of the RCC are too large compared to the required strengths. The

watertightness of the dam and its durability can be obtained by other cheaper

alternatives. The cost of the cementitious material must be lower than 30% of the total

cost of the RCC material, it is here almost equal to 50%!

• Low rate of construction: improve the organization of the works, adopt as much as

possible a continuous placement.

• Poor construction equipment: for dams with large volume (> 1 to 2 millions of m3),

select the RCC transportation by conveyor belt.

Is the RCC always the most economical alternative?

The advantages of the RCC technique are not conclusive for low and medium dams,

with large openings, built for flood control.

Son La Dam

Mix Proportions per m3 : Cement PCB 40 = 60 kg/ m3, Pulverized Fly Ash = 160 kg/ m3

Comment:

The required high tensile strength to resist to the design earthquake loadings is linked to the shape of the cross section of the dam.

Son La : The penstocks and the

powerhouse

• In this part of the dam, the RCC is

used only in the bottom and, not

easiliy, in the upper part, downstream

the intake.

• The placement of the RCC cannot

be continuous on the dam.

• The commissioning of the power

house cannot be done before the end

of construction of the dam.

Son La RCC sequence and rate of placementThe placement of the RCC is very discontinuous with peak near 10 000 m3/day (costly

construction equipment) and many weeks without placement .

Son La Hydropower Project

Daily RCC Production from 11 January 08 - 30 April 10

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

6500

7000

7500

8000

8500

9000

9500

10000

10500

11-J

an-0

8

25-J

an-0

8

8-Fe

b-08

22-F

eb-0

8

7-M

ar-0

8

21-M

ar-0

8

4-A

pr-0

8

18-A

pr-0

8

2-M

ay-0

8

16-M

ay-0

8

30-M

ay-0

8

13-J

un-0

8

27-J

un-0

8

11-J

ul-0

8

25-J

ul-0

8

8-A

ug-0

8

22-A

ug-0

8

5-S

ep-0

8

19-S

ep-0

8

3-O

ct-0

8

17-O

ct-0

8

31-O

ct-0

8

14-N

ov-0

8

28-N

ov-0

8

12-D

ec-0

8

26-D

ec-0

8

9-Ja

n-09

23-J

an-0

9

6-Fe

b-09

20-F

eb-0

9

6-M

ar-0

9

20-M

ar-0

9

3-A

pr-0

9

17-A

pr-0

9

1-M

ay-0

9

15-M

ay-0

9

29-M

ay-0

9

12-J

un-0

9

26-J

un-0

9

10-J

ul-0

9

24-J

ul-0

9

7-A

ug-0

9

21-A

ug-0

9

4-S

ep-0

9

18-S

ep-0

9

2-O

ct-0

9

16-O

ct-0

9

30-O

ct-0

9

13-N

ov-0

9

27-N

ov-0

9

10-D

ec-0

9

24-D

ec-0

9

7-Ja

n-10

21-J

an-1

0

4-Fe

b-10

18-F

eb-1

0

4-M

ar-1

0

18-M

ar-1

0

1-A

pr-1

0

15-A

pr-1

0

29-A

pr-1

0

Date

Volu

me,

m3

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000

16000

17000

18000

19000

20000

21000

22000

23000

24000

25000

in H

undr

eds

Acc

u. V

olum

e, m

3

Total Volume of RCC Produced

Accumulated Volume

Maximum Daily

Production to Date

9918.75 m3

Sinking of the truck in the RCC

•Too much paste and water in the RCC.

Sufficient water content is required for

good bond between the layers but too

much water (bleeding and laitance) is

detrimental.

• Avoid as much as possible the use of

trucks on the RCC layers. Use as

possible conveyor belt and swinger.

Comparison Long Tan/Son La RCC Dams

Long Tan

• Volume of the dam : 6.6 hm3

• First concrete placement : November 2003

• End of concrete placement : November

2007

• Duration of dam construction : 48 months

(137 500 m3/month)

• Commission of the 3 first units : May 2007

• Delay between the concrete dam

placement and commission of the 3 first

units : 3.5 years

Son La

• Volume of the dam : 4.6 hm3

• First concrete placement : April 2007

• End of concrete placement : August

2010

• Duration of construction : 40 months

(115 000 m3/month)

• Commission of the first unit :

December 2010

• Delay between the concrete dam

dam placement and commission of

the first unit : 3 years

The Son La Design

1. The fly ash contents a high percentage of L.o.I and it is far from the site. It has to be

transported by trucks on roads frequently cut by landslides during the rainy season. It is

expensive and depends on an unique Vietnamese provider (Pha Lai powerplant).

2. The most economical alternative is to reduce as much as possible the quantity of fly ash. Is it

possible and how to do ?

– By giving a batter of 0.15 to 0.20 to the upstream face, instead of vertical, it will be

possible to reduce the maximal tensile strength (at maybe 0.3 to 0.5 MPa).

– With this last value of the tensile strength, the max required compressive strength of the

RCC can be reduced to 15 MPa.

– This value of RCC compressive strength (at 365 days) can probably be obtained with only

130 kg of cement* per m3 (and some 7% milled fines) without fly ash, or with 100 kg/ m3 of

cement and 50 kg/ m3 of flyash (total=150 kg/ m3 compared with 220 kg/ m3 used).

* This relative high percentage is due to a rather low grade cement (PCB 30 or 40), with then a relative

high cost for the transportation.

– The volume of the dam will be a little higher but, as the unit cost of the RCC is lower, the

total cost of the dam will certainly decrease.

- To improve the watertightness, the upstream face of the dam could be enriched in cement

and fly ash by the GEV-RCC method or by CVC. A light reinforcement mesh can be put, if

necessary.

- Even with 130 kg of cement per m3, the cooling of the RCC will not pose more problems

than the present situation, provided the vertical joints are correctly implemented.

Conclusion about some Vietnamese RCC dams

• An optimal RCC dam should not be a traditional gravity dam in which the

conventional concrete is simply replaced by RCC.

• The studies of the RCC materials should be carried out before the design

and the analysis of the structure (and not the opposite as in many

Vietnamese projects!), as they depend on the most available and

economical materials which can be obtained on the site.

• The most economical solution is not always the minimum dam volume

with a large amount of fly ash whenever this material is not available near

the site.

•The conception of a RCC dam must be flexible and must be optimized

among all the possible RCC alternatives (different cross section, RCC

composition, RCC zoning, separation of mechanical and watertight

functions, etc). Don’t adopt the same cross section and the same RCC for

all the sites!

A particular type of RCC dam:

The Face Symmetrical Hardfill Dam (FSHD) and Cofferdam

• A new shape : fit with

incompetent or low

resistance foundation

• A cheap material : hardfill

– low cost aggregates

• natural alluviums

• mug from excavation

• soft rock

– low cement content Untreated natural alluviumsRio Grande dam in Peru

AVANTAGES OF

SYMMETRICAL

PROFILE

- low and uniform vertical

stress repartition

- little change in the vertical

stress with reservoir filling,

- no tension at the dam heel,

- uniform and reduced shear

stress at the base with the

seismic load

- small influence of uplift forces

improved stability

conditions in case of

earthquake and

large overtopping

10

0 m

C r itical r esultant

For ear thquake 0.2 g

A

1 2 C

D

2B

= 0.63

Empty

Full

0.8

= 24 kN / m 3

0 0.40 1.0Uplift

42°32°

10°

20°

30°

40°

27°

u d

(MPa)(MPa)

A = 0.84B = 1.56C = 2.40D = 0.00

PG

FSHD = 23 kN / m 3

0.7

0.7Critical resultantFor earthquake 0.2 g

u

(MPa) d

(MPa)

= 0.36

1

00

m

2

AC

2

DB

0 0.40 1.0

U plift

10°

20°

14°

22°

18°

Full

E m pt y A = 1.39B = 1.41

C = 1.15D = 1.15

1

1

11

11

Some examples of FSHD and FSH cofferdams:

- Cidere and Oyuk FSHD in Turkey

- Koudiat Acerdoune FSHD in Algeria

- Saf Saf FSHD and FSH cofferdam in Algeria

Sommaire

SummaryCINDERE DAM

(Turkey)2002

H = 107 m L = 280.60 m

V = 1 680 000 m3 (RCC = 1 500 000

m3 , CVC = 180 000 m3 )

Q (Peak Flood) = 3 600 m3 /s

Foundation : Micaschist

Es = 2.75 to 3.70 GPa

Rcs = 3.3 to 15.3 MPa

Seismicity

OBE = 0.20g MCE = 0.40g

RCC cementitious materials :

50 kg/ m3 P.C + 20 kg/ m3 F.A

Rc = 6 MPa (180 days)

Covered geomembrane upstream

OYUK Dam (Turkey)2007

H = 100 m L = 212 m

Q (Peak Flood 1/10 000) = 530 m3/s

Foundation : Gneiss and micaschist

Seismicity : OBE = 0.24g MCE = 0.40g

Cementitious materials : 50 kg/m3 P.C + 100 kg/m3 F.A

Rc = 6 MPa (90 days)

Koudiat Acerdoune (Algeria), H = 121 m, Crest Length = 493 m

A high RCC dam located in a seismic

area with low grade aggregates and with

very bad foundation (schist and marl)

with important rock slides during the

construction.

This dam was designed and constructed

by French consultants and contractors.

Choice of a FSHD cross-section to

adapt the design to the very low

quality of the foundation and of the

aggregates, with a reduction of Rc.

Replacement of the costly fly ash by

a limestone filler ground in situ.

Koudiat Acerdoun : Composition and characteristics of RCC

Initial design Final design

Quantity of RCC 1 070 000 m3 1 515 000 m3

Cement content 77 kg/m3 140 kg/m3

Fly Ash content 87 kg/m3 0

Limestone filler content 0 150 kg/m3

Required compressive strength 19 MPa at 90 days 11 MPa at 90 days

Max temperature 25°C 25°C

Diversion works for the Saf Saf FSHD (Algeria)

Low protection against flood during the construction

The Q10-yearflood = 890 m3/s, but the capacity of the diversion canal is only 150 m3/s (annual flood).

- In October 2008, a peak discharge flood of 500 m3/s overloaded

the canal capacity and the dam was overtopped with a 1.5 m

overflow depth, the base of the dam (3 m) was under construction.

- No damage resulted from this flood (no erosion of the crest, the

U/S and D/S faces of the dam). The works could start again after

a 2 weeks cleaning period.

Failure of the Cua Dat CFRD (Vietnam) during the construction

To minimize the cost of the diversion structures, it was admitted to divert the flow during the wet

season of 2007 by only one tunnel (D=9 m) in place of 2 tunnels (D= 11 m) of the initial design, with

a possible overtopping of the main dam 25 m higher than the river bed. Unfortunately an extreme

flood (8000 m3/s), much higher than expected (5300 m3/s), destroyed the gabion protection, the

cofferdam and a part of the dam during the construction (but without serious damage downstream).

Observation about FSHD for dam and cofferdam

1. A FSHD is particularly interesting for the sites with weak foundation, high floods

(often difficult to estimate precisely) and in seismic area. A FSHD can be

overflowed without serious damage during the construction permitting significant

savings in diversion works. FSHD may be consequently an interesting

alternative to CFRD for the sites with high floods and highly weathered rocks in

foundation. For this reason several FSHD are presently under construction in

Morocco in place of the traditional CFRD alternatives (Mr. Chraibi).

2. FSHD (or CSG) cofferdams - as demonstrated by their very good resistance to

large overflows - seem to be the best solution in case of overtopped structures,

even they may be a little more expensive than an embankment.

3. The failure of the Cua Dat CFRD must not lead to rule out the method of

diversion with overtopped structures - which allows generally important cost and

delay savings - but to adopt an adequate mode of protection of the downstream

slope of the embankment and, if necessary, of its toe and abutments. It is

probable that, if the downstream slope of this dam were protected by a

downstream FSHD in place of the gabions, the main dam and the RCC would

have resisted to the flood or be only superficially damaged.