INNOVATIVE DESIGN AND CONSTRUCTION METHODS
FOR CFRDs
Bayardo Materón
Consulting Engineer
Bayardo Materon & Associates
Brazil
INTRODUCTION.
The recent development of innovative design and construction methods, for concrete
faced rockfill dams (CFRDs), are generating competitive layouts for structures in
Hydro Power, Irrigation and Mining.
The demand for rapid construction imposed by the EPC (Engineering, Procurement
and Construction) type of contract has motivated Designers and Contractors to
develop these techniques.
This article presents improvements in design and construction recently adopted in
South America, which are being applied in other countries where CFRD’s are
selected.
Improvements in Design
New techniques in design can be summarized as follows:
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Plinth foundation design.
Internal slab plinth.
New slab design formulas.
Required reinforcement criteria.
External water stops.
Design of parapets walls.
Dam priority section design.
Improvements in construction
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Slip forming for plinth
Extruded curb
Transition material construction
Use of Heavier Vibratory Rollers
Plinth construction priority.
Dam priority section construction
Underground fuse
Underground access
Precast parapet walls
Construction of Face slab - rockfill simultaneously.
Following, a description of the above items will be commented:
IMPROVEMENTS IN DESIGN
Plinth foundation design.
A good correlation has been obtained between the Bieniawski RMR rock
classification and the required gradients for a conservative design of the plinth
dimension as follows:
RMR
80 -100
60 – 80
40 – 60
20 – 40
< 20
GRADIENT
18 – 20
13 – 18
10 – 13
04 – 10
Deeper foundation or cut – off
The table above illustrates the selection of the gradient for a defined RMR value. It is
interesting to indicate that the first use of calculated gradients was established during
the construction of Salvajina CFRD (148m), Colombia, where a good correlation
between the foundation rock classification and the dimension of the plinth was
reached. Later using very well known methods of rock classification ( Bieniawski,
Hoek) has been possible to design the plinth dimension with conservative
gradients based on experience.
Internal slab plinth
Recently, some CFRD’s have adopted an external constant plinth which is the
grouting cap for the foundation treatment. Generally is 4 - 6m. An internal slab plinth
will be required to guarantee the hydraulic gradient of design. Example for a
foundation rock with RMR= 55 and a reservoir hydrostatic pressure of 100m. The
internal slab is computed using the interpolated values as follows:
RMR= 55
G= 12
Plinth width = 100 / 12 = 9m.
External slab = 4m.
Internal slab = 9 – 4 = 5m
Design for new Brazilian Dams (Itá, Machadinho, Monjolinho, Barra Grande,
Campos Novos) adopted this new method of design. Photo 1.
Also international dams as Caracoles, Argentina; Bakun, Malaysia; Merowe, Sudan,
and Berg River, South Africa are adopting this design to optimize excavation and
scheduling.
New slab design formulas
The traditional formula for designing the slab thickness has a limit in its use.
T= 0.30+0,002 H m
Where:
T= slab thickness in meters
H= Hydraulic head in meters
Since, this formula follows a straight line and the gradient (G) over the slab increases
in a curve equation, it is important to increase the thickness after a critical head,
where tensile strains are becoming high.
Successful experience with Australian dams indicates good slab behavior with
gradients around 220. (Reece, 122m)
Tensile strains higher than 0, 0001 produce cracks in the face slab.
A good control can be obtained by increasing the slab thickness or increasing the
concrete strength (Fck) which is proportional to the modulus of elasticity (E).
A practical rule is:
When the Hydrostatic pressure reaches 120m increase the thickness of the slab
using the formula:
T = 0, 0045 H m
Photo 1
Plinth with internal slab. – Slipforming
Use pozzolanic cement to increase the 28 day strength in more than 20% increasing
the concrete modulus of compressibility..
Example:
Hydrostatic head = 180m
Fck = 20 MPa at 28 days or 25MPa at 60 days
T =0,0045 x 180m = 0.81m
G = H/T or 180m/0.81 = 222 >220
Applying Hook laws: σ = E x ε
E= 25.000 MPa
and ε = 1.8MPa/ 25000 MPa = 0,00007 < 0,0001
Required Reinforcement
The reinforcement in the slab generally is calculated as a percentage of the slab
cross section in both directions:
Horizontal bars = 0.3 - 0.35%
Vertical bars = 0.4 - 0.5%
However,some cracks have been observed close to the abutments when
percentages used are 0.3%for horizontal and 0.35% for vertical. Recently double
mats have been introduced close to the perimetric joint, when the dam is higher than
120m, using values 0.5 % in both directions with good results.
External Waterstops
Traditionally two kinds of water stops are being used:
A copper water stop located at the slab bottom and a central PVC or rubber water
stop in the middle of the slab.
Recently, some CFRD’s have eliminated the central PVC water stop, pursuing
external water stops of Neoprene or EPDM, Jeene type. These external water stops
have been tested to water pressures of 20MPa.
The most used is the omega type (Ω) waterstop with the concave shape in the same
direction of the pressure of the reservoir. Photo 2
Design of Parapets Walls
The use of parapets in CFRD’s is a general practice, due to the savings in rockfill
and quarry exploitation.
When the parapet cost is lower than the saving in rockfill the design of these
structures is justified. Double parapets have been selected for high dams with cost
effective solutions.
Priority Section Design
Upstream priority sections have been designed as incorporated cofferdams into the
dam, to make the handling of the river economical.
The main cofferdam is designed for a relative low period of recurrence building the
upstream portion of the dam for the second period of rains with a safe period of
recurrence.( 1: 500 years)
The main cofferdam controls the overtopping risk during the first rainy period. A fuse
dike is built external or underground to prevent losing the main cofferdam in case of
overtopping. The priority section will avoid any overtopping over the main rockfill.
Usually main cofferdams are designed for 1:20;1:30 years and priority sections for
1:500 years.
Photo 2
External Waterstop Omega Type
IMPROVEMENTS IN CONSTRUCTION
Slip forming for the plinth
The internal slab design of the plinth is generally built by slip forming when long
stretches are required. The slip form is a combined solution of a metallic structure
with timber. This form is moved by hydraulic jacks or winches with productions
ranging between 1.5 -2m/hour.
Photo 1 illustrates plinth slip forming at Itapebi dam, Brazil.
Extruded Curb
The extruded curb was first fabricated in Ita Brazil as a construction method to
protect the upstream face of the dam, to eliminate upstream slope compaction and to
reduce segregation of the transition material located under the face slab.
The method has being used in South America, and recently in Africa, Europe and
Asia. Photos 3 - 4 illustrate the use of extruded curb at Barra Grande, Brazil
Photo 3
Extruded curb construction
Photo 4
Extruded face
Transition material construction
The placement of this material is carried out by using an open bottom metallic
dispenser where the hauling trucks unload the transition 2B material. The dispenser
has a hook for connecting it to the truck. The truck pushes ahead the metallic
dispenser spreading the material for later compaction. Use of graders and leveling
dozers are eliminated reducing costs. Segregation of the material is reduced. Photo
5.
Heavier Rollers Vs Layer Thickness
Traditionally 10 ton vibratory rollers were produced for compacting rockfill. The 10
ton is defined as the weight acting on the compacting drum. Recently, the rollers
industry has produced drums weighing 12 ton, with lengths of the drum distributing
the load to values close to 5 ton/m of drum.
These new vibratory rollers are excellent for compacting rockfill in layers up to 1.50
m thick.
The modern tendency in high CFRD’s is to compact better the fill in layers ranging
between 0.60 to 1.50m.
Photo 5
Transition 2B construction with metallic dispenser
Priority for Plinth Construction
Experience in CFRD Construction has proven that giving priority to the plinth
construction helps to the quality and productive placing of materials into the dam.
The plinth should be built always simultaneously with the diversion tunnel
construction leaving only the plinth at the riverbed to the built after diversion.
This strategy applied in very high dams (180m-200m) has permitted placing rockfill
with productions exceeding 1 million cubic meters by month. (Barra Grande, Campos
Novos, Brazil) Photo 6.
Priority Section
Construction of the priority section of the dam shall be always concentrated at the
upstream portion of the dam. This sequence is the best for protection of the dam
since the transition zone 2B will be acting as restriction of the flow in case of
overtopping the main cofferdam, due to the semi pervious permeability of this
material.
Construction of the priority section of the dam requires construction of the plinth with
high priority as indicated above.
Underground Fuse Dykes
The main cofferdam of CFRDs has to be protected in case of an eventual
overtopping. Generally, the protection is provided by a fuse dyke built in a channel
Photo 6
Priority in plinth construction - Barra Grande Brazil
besides the cofferdam. In some narrow places the same protection can be carried
out with an underground by pass with an internal fuse dyke built inside the by pass
tunnel.
Underground access
Underground access for placing rockfill into the dam has been pursued in projects
where the abutments are very steep and construction of hauling roads are extremely
difficult and expensive.
There are many examples of rockfill dams where shafts and tunnels have been built
to speed up construction with excellent results of productivity and costs.
Guavio dam, 250m, Colombia; Barra Grande 185m and Campos Novos,202m, Brazil
are examples where underground accesses were pursued for the construction of
the dam with high productivity.
Precast Parapets
A general solution applied in modern dams is the adoption of precast parapet walls
built during the construction of the dam.
Precast parapets are fast for installation reducing the time consuming operation of
building parapets cast in place.
Usually the parapet walls are built when the dams are complete and the contractor is
in the process of demobilization of major installations as the batching plants. The
precast parapets will permit the removal of industrial installations since the elements
of the wall are already built and ready for installation.
Face Slab and rockfill built simultaneously
There are some projects where the schedule requires to start filling the reservoir
before completion of the CFRD due to economical and technical reasons.
In these dams it is possible to build a portion of the slab simultaneously with the
upstream placing of rockfill.
Construction of a temporary platform over the upstream face of the dam has
permitted to continue rockfill placing simultaneously with the construction of the face
slab located under this platform.
The platform is built with anchorages into the fill providing reaction for climbing the
slipform using hydraulic jacks. Photos 7 - 8
Photo 7. Platform for simultaneous construction of face slab and upstream rockfill
Photo 8 – face slab built simultaneously with upstream rockfill.
Conclusions
The innovative design and construction methods referred here, applied mainly in
rockfill dams, have produced in South American Projects excellent results from the
viewpoint of productivity and economic. These techniques are being applied in other
countries as well.
REFERENCES
1. Materón, B, ; Resende, F.; Construction Innovations for the Itapebi CFRD,
Hydropower & DAMS, Volume eight, Issue Five, 2001
2. Pinto, N.L. de S.; Questions to Ponder on Designing Very High CFRDs,
Hydropower & DAMS, Volume eight, Issue Five, 2001.
3. Marques, F.P.L.; et als; Pichi Picun Leufu. A CFRD of Compacted Gravel.
Second Symposium on CFRD, Florianopolis, Brazil, October, 1999.
4. Resende, F.; Materon, B.; Ita Method. New Construction Technology for the
Transition Zone of CFRDs, Proceedings of International Symposium on
Concrete Faced Rockfill Dams. Beijing, China, September 2000.
5. Materon, B.; Construction Evolution for the Highest CFRD’s. Symposium on
CFRDS and the 20th Anniversary of China’s CFRD Construction. Yichang,
China, September 2005.
SUMMARY
New and innovative methods for design and construction of Rockfill dams are
being applied in some South American and international projects to cope with
the requirements of productivity and quality imposed by the EPC (Engineering,
Procurement and Construction) type of contracts. This article discusses these
methods especially for Concrete Face Rockfill Dams. (CFRD’s)
.