Using examples from South America, Bayardo Materon looks at recent developments in the design and construction of modern CFRDs
THE demand for rapid construction, as imposed by an engineering, procurement and construction (EPC) contract where the contractor is fully responsible for design, schedule and quality, has motivated the development of new techniques in the design and construction of concrete face rockfill dams (CFRDs) in South America.
Plinth conception and construction
Traditionally, the width of the plinth in a CFRD has been designed taking into account empirical relations between the height of the reservoir and the existing geology.
B = K H
Where: B: Width of plinth in meters
K: Empirical value ranging between 0.05 and 0.125 for fresh and weathered groutable rock respectively.
H: Hydraulic head in meters
Recently, Brazilian and international dams have adopted the design of an external constant plinth of 4-5m, with an internal slab the required length to guarantee an adequate hydraulic gradient. Examples of this design are Babagón, 63m in Malaysia; and Ita, 125m; Machadinho 125m; Itapebi, 120m; Barra Grande, 185m; and Campos Novos, 200m all in Brazil. Two gravel dams under construction, Caracoles, 130m and Punta Negra, 85m, in Argentina, are adopting this design as a means to optimise excavation costs, stability and scheduling.
This internal slab design allows slipforming in long stretches of the plinth in a rapid and efficient manner. To facilitate construction, the plane of the internal slab is excavated in the same plane of the external plinth as shown schematically in Fig 1 (figures currently not available). At Itapebi dam, the slip form is a combined solution of a metallic structure with heavy timber, weighing 1.5t. Some ballast is placed avoiding floating of the light form during the concrete operation. Hydraulic jacks, 12t each, were used for the slip forming of stretches up to 70m long. Average production is 1.5m/hr. The grouting operation is easier over a constant external plinth, since the drilling equipment can be mobilised using cables operated by hydraulic winches.
Face slab criteria
The design of the face slab thickness has followed empirical formulae of the type:
t = 0.30 + KH m.
Where: t: variable thickness in meters
K: Empirical value = 0.002
H: Hydraulic head in meters
As commented by Pinto, this straight-line formula implies a physical limit, since the hydraulic gradient acting on the face slab increases at a more rapid rate than the slab thickness. The hydraulic gradient follows a curve equation becoming critical when the hydrostatic pressure exceeds a certain dam height.
Fig 2 shows the variation of slab thickness (t), following the classical lineal equation with K= 0.002, and the hydraulic gradient curve (G) which increases with dam height.
Presence of cracks in some new high dams has indicated that the concrete tensile strain has been exceeded.
Practical experience reported by Australian articles indicate good experiences with CFRDs with hydraulic gradients up to 289 (Reece, 122m, Australia).
Control of the tensile strain can be obtained by increasing either the slab thickness or the specified strength of the concrete, which is related with the modulus of elasticity.
The design of the concrete strength for the face slab (fck) should be zoned, for higher dams, varying the strength at the bottom to increase the modulus of elasticity. The tensile strain may be reduced to adequate values (< 0.0001) increasing the strength in the face slab in areas with high pressures. The use of pozzolan or pozzolanic cement will increase the strength increasing the modulus of elasticity (E) in an economical way. Experience shows that pozzolanic cement increases at 60 days, the strength at 28 days in more than 20%.
Consequently, it appears practical and conservative to apply the concept proposed by Pinto using a formula of t = 0.0045H after a dam height over 125m, as shown in Fig.2 limiting the hydraulic gradient (G) to close to 225.
Fig 3 illustrates this concept for a hypothetical dam of H= 200m; face slab = 150.000m_ and upstream slope of 1.3H: 1V. The lower portion of the dam (H > 125m) is designed for fck = 25Mpa or E = 25.000MPa. At 60 days.
Example: For 200m head the pressure at the bottom of the slab is:
s = p/t if t= 0.0045×200 =0.90m ; p= pressure 2MPa
s = 2/0.90 = 2.22MPa
Applying Hook Law: s =E. e
Where E: 25.000MPa and e tensile strain
e =2.22/25000 = 0.00009 < 0,0001
Since reservoir filling always happens after some months of construction of the lower portion of the slab, the pozzolanic action will give a safer modulus of elasticity for developing tensile strain.
Using slip forms operated by hoists or hydraulic jacks is the practical way for construction of the face slab. Normally, the slab is constructed in phases in such a manner that the first phase of the slab is built simultaneously with the downstream placement of rockfill avoiding interference.
However, there are some projects where the schedule imposes to start filling the reservoir ahead of time due to technical or economical reasons. In those projects it is possible to accelerate construction, building the lower phase of the slab simultaneously with the upstream rockfill placement.
The face slab at Itapebi was divided in three phases. Fig. 4 shows the face slab and Fig. 5 the construction method adopted by building an external platform, which was used for providing services such as water, air and energy and logistical accesses for personnel and concrete.
This temporary platform, supported by anchorages, was structurally calculated to provide reaction for climbing the slip form using hydraulic jacks and cables. The Phase l was built at the same time as the upstream rockfill was placed permitting the lower fill over the slab, suspending pumping and improving the construction schedule.
The general practice for reinforcement design is to provide an empirical percentage of the slab cross section, in both ways, which ranges between 0.4-0.5% for vertical bars and 0.30-0.35% for horizontal.
Reduction of percentage steel to values of 0.35% vertical and 0.3% horizontal in the middle of slab is probably the cause of horizontal or sub parallel cracks to the perimetric joint observed in some dams as described by others. Double mats close to the perimetric joint in higher dams than 125m have been used as a prudent manner to avoid fissuring (Machadinho).
Placement of the reinforcement bars during the face slab construction controls production. A semi-mechanised system of placing reinforcement has been carried out successfully in countries where the hand labour cost is low. Pre fabricated mats has proven an effective method where scheduling is critical. Generally, it is initially placed some re-bars as a guide for sliding the prefabricated mat, which is assembled in the crest of the dam.
The method has been used at Ita, Mohale, Machadinho and Itapebi with good results of productivity.
Rebar placement outputs ranges between 300-50t/month but with pre-fabricated mats productions close to 1000t/month have been reached .
The technique of using an extruded curb was initiated at Ita and followed in subsequent dams in Brazil as a rapid method for protecting the upstream face and minimising segregation of the transition material. The method was used in Africa (Mohale), Peru (Antamina) and it has been proposed for Canada (Toulnustouc and Digue Sud), Panama (Fortuna), Argentina (Caracoles – Punta Negra), Colombia (Cercado), Ecuador (San Marcos) and some Korean dams (Chong Son). Application of the method has been thoroughly discussed in the technical literature and it is a practical construction innovation favouring cost and scheduling.
The use of the extruded curb has permitted reduction in the width of the transition material to values of 3m for materials 2B and 3A as depicted in Fig. 6 (above). Placement of the material is carried out using an open bottomed steel dispenser, where the hauling truck unloads the material. The same truck pushes ahead the dispenser eliminating use of conventional graders and dozer and reducing cost.
The method was used for the first time in Machadinho dam and it has been adopted in recent Brazilian dams.
The flexibility of the dam zoning is one of the best advantages for adopting this type of dam in high structures. Fig. 7 (below) shows the typical conception of material distribution and the conventional layers and compaction method. The presence of a dead zone in the middle of the dam has permitted using materials of lower quality without affecting the performance of the whole structure.
It is common during the exploitation of a quarry to find weathered or fragmented material at the first blasting operations. The dead zone may be extended at lower levels providing a logistic place for depositing these materials, thus reducing stockpiling of waste material and cost.
Fig 7 (below) shows also the conventional method of compaction and layer thickness normally used. However, the presence of cracks observed in high dams (TSQ1 and Aguamilpa) favours the proposal of better compaction downstream. It appears reasonable for higher dams than 150m to increase the compaction downstream using heavier rollers (15-18t) and reducing thickness of layers. The 187m high El Cajón, Mexico 0.6m, 0.8m and 1.2m for layers of 3A, T and 3B, respectively is proposed.
The most recently built dams have eliminated central PVC waterstops, keeping the copper waterstop in the perimetric joint and in the tension joints.
Over the compression and tension joints have been proposed and used external waterstops of neoprene or EPDM (ethylene-propylene-dyene), JEENE type, after exhaustive testing in laboratories simulating opening, settlement and upstream shear with water pressures up to 20MPa.
The Ita dam installed for the first time an external mushroom type of waterstop. Machadinho dam installed the Omega W JEENE type, which has the property of using the convexity shape in the same direction of the pressure of the reservoir. External waterstops are simpler to install and supervision and quality control are accessible to inspectors. All construction joints at Itapebi, tension and compression, are W Type. Joints are cleaned and prepared carefully to fit the W shape into the joint and then the upper portion is glued to the slab using a special epoxy.
The use of parapet walls upstream and downstream is a general practice nowadays. The reason for this is that the incorporation of a parapet wall saves a volume of rockfill giving additional space at the crest for the face slab construction. When the saving in rockfill exceeds the parapet wall, it is economical to incorporate parapet walls at the crest. In some places such as Yacambu in Venezuela, 160m; Salvajina in Colombia, 148m; or Itapebi in Brazil, 120m, it is justified to install an additional parapet downstream. Parapet walls ranging between 3-5m are economical solutions.
A negative construction aspect is that the execution of parapets is effected at the end of the dam construction when the contractor wants demobilisation of major installations such as the batching plant. Since the parapet construction is a time consuming operation, efforts have been devised to accelerate construction. A recent solution adopted in modern dams is the adoption of precast parapet walls built during the construction of the fill.
A precast parapet wall, 2m high, was built in Pichi Picún Leufu, a CFRD dam 45m high built with gravel, in Argentina. Precast parapets are being installed in Itapebi in a fast an economical way.
Conclusions and recommendations
The construction of the plinth with internal slab is an effective solution when designed in long stretches. The plinth may be built by slipforming in a fast and economical manner meeting the specified quality.
The use of pozzolanic cement or standard Portland cement plus pozzolan permits optimising the face slab thickness as follows:
For dams up to 125m:
Use t = 0.30 + 0.002H m.
fck = 20 MPa at 60 days
For dams > 125m:
t = 0.0045 H m
fck = 25 MPa at 60 days.
This design is economical for high dams and meets the requirements of tensile strain for avoiding undesirable cracks.
The first phase of the face slab may be built simultaneously with the upstream rockfill placement, when technical or scheduling requirements are necessary. Use of temporary external platforms and slip forms applying hydraulic jacks on cables is feasible and cost effective.
Placement of reinforcement in pre-fabricated mats may be simplified by using rebars as a guide for sliding the mat as described in this article. Productions higher than 500t/month are easily obtained.
The use of extruded curb for transition 2B protection is a practical construction technique generally adopted in recent dams. It helps in reducing transition width, avoids segregation and eliminates compaction on the upstream face.
The flexibility of the CFRD zoning is one of the major advantages for selection this type of dam. The dead zone located
in the middle of the dam providing a logistic place for materials of lower quality, reducing stockpiling, waste material and cost.
External waterstops with durable materials such as Neoprene or EPDM are simpler in installation and easier for controlling quality.
Installing pre-cast parapet walls has solved the time consuming operation of building parapets at the end of dam construction.
For further information, please contact:
Bayardo Materon Associados,
Avenida Giovanni Gronchi 5445. sala 172,