Construction analysis of Grand Poubara RCC Dam27 September 2011
Cesar Alvarado-Ancieta and Patrick Yalis Ongalla present important technical aspects regarding the construction analysis of the Grand Poubara Dam on the Ogooue River in Gabon
Located at Poubara Falls on the Ogooue River in Gabon, the 160MW Grand Poubara project features a 37m high rockfill-roller compacted concrete dam. The crest of the dam is 300m long and it has been designed with a stepped spillway with discharge capacity of 1100m3/sec. A bottom outlet is featured as an environmental measure to provide permanent downstream water releases. The following article will analyse the construction measures employed at this important hydropower dam project as a follow on to an article which detailed tunnelling works at the project published in IWP&DC in April 2010.
Dimensioning and dam geometry
RCC has been applied on the section of the dam arranged along the river valley width as shown in Figure 1. The RCC application took into account the defined geometry and cross section based on the following issues:
• The only section of the dam constructed using the RCC technique includes the stepped spillway.
• A river width of 120m – considering that the width of the spillway is 120m too, this means that there is a constant width of 120m from the top to the bottom of the stepped spillway right up to the river bed.
• The height of the dam along the stepped spillway section of 120m with BCR is 29m.
• The abutments or shoulders of this section are vertical.
• The downstream slope of the RCC dam section is 1:0.8 (V:H).
• The width of the crest of the RCC dam section is 4.6m and the width of the base is 24.5m at elevations 407m asl and 378m asl respectively.
Based on these geometrical characteristics a dimensioning of dam geometry at the stepped spillway section is shown in Figures 2 and 3.
Design of the dam body and appurtenance structures
Grand Poubara Dam is a medium size dam and the construction practice over the last 20 years has been to simplify the dam body as much as possible. The advantage of this is ease and speed of construction.
The simplification of the design of Grand Poubara Dam in the RCC section was chosen for the following reasons:
• Use of a lower number of concrete mixtures in the dam body.
• To diminish as much as possible the interference between the RCC and the appurtenance structures of reinforced concrete or the hydro-mechanical equipment. In this sense a common practice is to design the intake and bottom outlet in reinforced concrete blocks, which are then executed during independently from the RCC dam body, next to the shoulders of the valley.
• The design of the access and inspection galleries are compatible with the principle of non-interference during the placement of RCC (Figures 4 and 5). In general, it is recommended that the inclined galleries inside the dam body or parallel reaches in the direction of the river should be avoided. Instead the net of galleries and access should be built as horizontal galleries, which are joined by access vertical shafts.
Design of the stepped spillway
As part of the design, the release channel for Grand Poubara is a stepped concrete structure placed on the downstream slope of the dam. The walls, release channel, stilling basin and the river realignment were adjusted to the geometrical requirements of the close valley in the dam area. In the case of Grand Poubara Dam, the design unit discharge for the spillway is 9.16m3/sec, taking into account that the width of the spillway is 120m. This magnitude allows for the inclusion of a a stepped spillway because the maximum unit discharge is less than 40 m3/sec.
Monolithic blocks, layers and of the transversal joints
Due to its total height of 37m, Grand Poubara is a gravity dam of right alignment. The design of an RCC dam like this one needs to include independent monolithic blocks, similar to the design of a dam of conventional concrete. However, as is common knowledge, the construction of RCC dams is carried out in continuous process from one abutment to the other, similar to earthfill dams, employing similar machinery during the earthmoving works.
In order to incorporate monolithic blocks, it was necessary to divide the process of continuous construction by means of transversal joints.
There are a number of procedures that can be followed to arrange the transversal joints. The most common method used nowadays is to insert in the fresh RCC mass a separating element in the alignment of the joint plane, which work as joint inductors. These sheet metal joints are placed in each RCC layer in the same transversal position where the joint is located. If the RCC has a good design, it is relatively easy to insert these elements immediately after finishing the compaction of the RCC. As a result this operation of settling transversal joints will not interfere with the placement of concrete.
The selection of the position and distance among the transversal joints is determined by:
• Geometry of excavation.
• Location of the spillway, intake and bottom outlet.
• Thermal stresses calculated by a sophisticated thermal-structural computation.
Based on the conditions in-situ, the distance among transversal joints is between 20 and 30m for the Grand Poubara Dam.
The transversal joints located in the upstream slope are impermeabilized by waterstops. Downstream from the waterstops is a drain, which is connected with the drain system of the dam.
Design of the RCC mixture
The RCC mixture at Grand Poubara needs to fulfil the following basic requirements:
• RCC mixture designed using local materials.
• A cohesive, dense and uniform mixture.
• Avoid segregation during his transport and placement.
• Adequate consistency and workability.
• Minimize the generation of heat.
• Reach the required resistance to the design age.
• Fulfil the required properties in situ to the design age without an extensive treatment of the joints on each layer.
In order to meet these requirements, the mixture design takes into consideration the following parameters:
• VeBe times (measure of RCC consistency) of 10 to 15 seconds.
• High content of pozzolan material as cement element.
• Limitation of the maximum size of the fine aggregate (lower than 40-50mm).
• Mixture design with high content of paste.
• Use of additives in the retard of curing and water content reductor.
During the final design phase, investigations were carried out on the available materials, and laboratory tests were conducted to find appropriate concrete mixtures. This test campaign allowed some recommended limits to be altered.
The main parameters to be used in the RCC design were strength values which should be derived from the structural computation. The usual critical parameter in the design of RCC mixture in a dam with the characteristics of Grand Poubara is the vertical tensile strength in the joints among each layer. This was determined by the behaviour of seismic type. In some designs, it is advantageous to include zones of different RCC mixtures in the dam body, each one with different resistance characteristics depending on the magnitude and localization of tension states in the structure. However, no more than two or three different RCC mixtures should be included, and no more than two at the same level inside the dam body.
Upstream and downstream slopes
The downstream and upstream slopes of the RCC dam have a similar design to dams built with conventional concrete and vibrated by immersion. The construction process here is related to assure a good quality of materials included in the technical specifications, in order to avoid honeycomb during the set out of the workforms.
This problem is also resolved specifying the use of RCC with GEVR (grout-enriched vibratable RCC) in order to obtain a good finishing and durability surface in the exposed faces of the RCC dam. In such a case, the properties of resistance and impermeability of the structure are in the RCC mixture.
Design of the joint of RCC material with rock basement
The contact between the RCC of the dam body and the rock of the dam slopes and lower zone of the dam foundation is carried out in a similar way as structures of conventional concrete. Therefore, for the RCC placement and consolidation, a transition concrete is used, which will consolidate by internal vibration the RCC and the rock.
Usually a concrete CVC type is placed in the overburden area of the RCC dam foundation. Over this area, the treatments of consolidation are carried out prior to RCC placement.
In the abutments, the contact RCC-rock was designed in similar conditions.
Planning and construction
The dam of Grand Poubara is considered to be of medium size among the existing BCR dams.
An analysis of the dam geometry was carried out considering a layer volume of 300mm of BCR. As it can be observed in Figure 6 the RCC volumes per layer are always decreasing from around 900m3 to 170m3 per layer. This is considered a positive aspect taking into account that only the spillway section will be made of RCC and therefore the shoulders or abutments are vertical.
The total volume of RCC has been estimated for Grand Poubara Dam to be less than 52,000m3. Therefore, taking into account the optimization of the structure layout and the application of some modern concepts, dam construction was expected to take less than four months.
In order to reach this objective, it was necessary during the design phase to prepare the general guidelines in the construction process, which could then be adjusted to the project objectives. In this sense, it is a usual practice to perform the technical specifications defining the minimum outputs and characteristics of the machinery, which will be used in the construction process, transport, and placement of RCC.
The RCC mixture was designed in order to reach in situ, not in the laboratory, the properties determined and derived from structural computations, carried out in the detailed design phase. During this phase the vertical stresses in the joints between the upper and the lower layers will be of special interest, as they are the critical parameters of RCC design in dam located in a region where there is seismic activity. Also, the thermal-structural analysis is another aspect to be considered in this type of dam, which is rapidly constructed and with a few of possibilities for heat dissipation generated by the hydration of the conglomerates.
Several issues specifically related to RCC construction methodology should be taken into account. The construction process has been focused on quality and speed of construction by an accurate control of:
• Supply of aggregates.
• Location of RCC production plant.
• Testing programmes.
• Facing systems and techniques.
• Lift surface.
• Placement procedures.
• Control of cracking.
• Installing joints, waterstops and drains.
• Galleries for grouting and drainage.
• Obstruction of outlet works and spillway.
• RCC production controls.
• RCC production plant.
• RCC transportation systems.
• Quality control and quality assurance in RCC construction.
Supply of aggregates
Aggregate usage during RCC placement is generally very high because of the continuous placement of RCC at maximum practical production rates. This usually requires large aggregate stockpiles to be used during RCC placement since aggregate production occurs at a slower rate. Large areas for aggregate stockpiles in the vicinity of the Grand Poubara dam must be provided. Access to these areas are necessary for time periods in advance of RCC placement or during off hours. The alternative to constructing large onsite stockpiles is to utilize extensive truck hauling or extensive conveying at a rate to match the RCC placement rate.
Location of RCC production plant
The RCC production plant location is in the upstream reservoir, near the right abutment (Figure 7). Obviously, a location near the aggregate stockpiles was advantageous to minimize the transportation of aggregates from stockpiles to the plant. The plant is accessible and provides the required staging area for trucks hauling cementitious materials. Such material handling can be an extensive and continuous operation during production of RCC at moderate to high production rates. Access for the resupply of other materials, service vehicles, and auxiliary hauling, such as loaders or dump trucks, was considered.
A critical part of the construction of the project with the RCC technique is the testing and evaluation of materials and construction techniques. The timing and extent of such testing depends on several factors. As with conventional concrete, projects utilizing materials not previously used require a responsible level of quality evaluation.
Aggregates, cementitious materials, admixtures, and other constituent materials are evaluated to ensure basic quality performance. Some of these physical properties are specific to RCC and need not be evaluated the same as for conventional concrete. Optimization of material properties by material selection, mixture proportioning, or structural design changes can result in significant cost savings which will benefit from more intensive testing. Less testing may be acceptable where testing yields no such benefits.
It is important to control the following tasks during testing in construction:
• Evaluate mixture performance.
• Fabrication of a density block for calibration of density gauges.
• RCC transport and movement activities.
• RCC placement activities.
• Avoid segregation.
• RCC compaction.
• RCC curing.
• Evaluate equipment performance.
• Evaluate plant production and operation.
• Personnel training.
• Installation techniques for panels or other structures.
• Hand work and compaction of RCC.
• Use of bedding mortar.
• Lift joint preparation.
• Evaluate fresh and cold joints.
• Determine a target density.
• Performance density testing.
• Other sampling and testing.
Facing systems and techniques
Most RCC structures use some form of facing system to construct one or more of the RCC faces. Natural RCC slopes, that is RCC placed at a slope equal to or less than the natural angle of repose of the material, have been used satisfactorily on many dams with RCC technique. Facing systems are used with RCC structures for several reasons:
• Form for RCC face. RCC placed as a granular material cannot stand vertically. Facing systems provide a vertical or sloped form against which RCC is placed.
• Provide a durable surface.
• Control seepage. Some facing systems provide a means to control seepage. Panel systems with embedded or attached membranes provide a barrier to seepage.
• Hydraulic performance. Spillway or outlet surfaces constructed of RCC may not provide the erosion resistance or the dimensional control to serve as high-velocity surfaces. Facing systems are used in this case to provide a cast-in-place concrete surface on the designated slope. Slip-formed elements have been used to provide a stepped spillway surface.
It may be necessary to clad vertical and near-vertical exposed surfaces of RCC with precast or cast-in-place conventional concrete to provide a more durable exposed surface and to provide a restraint against which the outside edge of each lift of RCC is placed. Cast-in-place conventional concrete may also provide increased water tightness for the upstream face and will provide increased resistance to erosion and damage by freezing and thawing.
When cast-in place conventional concrete is placed on the upstream face of a dam constructed of RCC, or when conventional concrete is placed against rock abutments, care must be taken that the interface between the conventional concrete and the RCC is thoroughly consolidated and intermixed. Consolidation should take place in a sequence so that the entire interface area is intermixed and becomes monolithic without segregation or voids in the material or at the interface itself.
The design and constructed quality of lift surfaces during the construction phase are critical to the stability of a structure and to the seepage performance of a structure. The design of a structure will dictate the shear and tensile strength required at the lift joints. The formulation of the mixture proportions and subsequent testing programs are the first steps in ensuring that required performance is attained. Proper specification of construction procedures and field control of construction operations are just as vital to ensuring that required performance is attained. The design team must balance the structural requirements, the material performance, and the required and allowable construction activities in preparation of a viable project design.
RCC has been successfully placed in lift thicknesses ranging from a minimum of 150mm (compacted thickness) to well over 1m. Lift thickness can vary depending on mixture proportions, plant and transport capability, placement rates, spreading and compacting procedures, whether or not a bedding layer is used, and size of placement area. For most applications, an initial lift thickness of 300mm was suggested, with subsequent adjustments based on results of specified preconstruction investigations.
Control of cracking
As is the case with most concrete structures, cracks do occur in RCC structures, and, if the structure involved is a dam or other water-retention structure, the results can range from simple leakage to instability of the structure. Cracking is often the result of mass volume changes resulting from long-term cooling of the structure or from short-term cooling of the RCC surfaces. Other cracking may result from abrupt changes in foundation grade and from high stresses generated by re-entrant corners of structures embedded in the RCC. Cracking may occur in spite of preventative measures. The possibility of thermal and restraint-based cracking should be anticipated in the design by incorporating appropriate jointing, as well as secondary features such as drainage conduits and sumps, where necessary, to remove water from the structure. The consequences of such cracking may range from destabilization of the structure to operational and maintenance problems. Remedial measures can be extensive and costly.
In order to avoid cracking the contractor will carry out an accurate control of temperature, precooling techniques, transverse contraction joints, foundation-induced cracking, re-entrant corner cracking, and waterstops and membranes.
Installing joints, waterstops and drains
Placing vertical transverse contraction joints in dams constructed with RCC and installing waterstops in these joints near the upstream face were considered for crack control.
Galleries for grouting and drainage
In dams that are greater than 30m in height, galleries are included in the design as in Grand Poubara Dam. The gallery is necessary to provide a location from which to drill drain or grout holes, provide drainage for leakage, and provide access for inspection. Several different gallery designs are used in RCC construction. They include construction of a gallery with gravel or sand fill followed by excavation of the fill after the surrounding RCC has hardened, construction using a slip form curbing system for walls with precast reinforced ceiling elements, and construction using conventional forming systems for walls with precast reinforced ceiling units.
Obstruction of outlet works and spillway
Outlet structures and conduits can provide obstacles to RCC placement. The preferred practice in placement of outlet works in RCC dam is to attach an intake structure to the RCC structure and locate the conduits in or along the rock foundation to minimize delays in RCC placement.
RCC production controls
The concerns regarding production of RCC can be divided into two main issues, those affecting the quality of RCC and those affecting RCC production rates. However, the primary advantage of RCC over other materials is the relative economy of the final product. This economy is a direct result of the high production rates that are possible with RCC.
One of the cost-saving features of RCC is the rapid rate at which it can be placed and consolidated by earthmoving and compaction equipment. Generally, as with most other construction processes, the faster the placement is made, the less expensive the RCC becomes. In the case of a dam, the faster placement will mean less time between placement of lifts, resulting in lift joints with improved strength and seepage performance. Typical production rates may range from 280 to 1200m3/d for a small RCC project, 1200 to 2800m3/d for a moderate-size RCC project, and 2800 to 6000m3/d for a large RCC structure. In the case of Grand Poubara Dam from our computations the production rates could be around 1000m3/d and therefore an estimated construction time of less than four months as it is shown in Figure 8.
The estimated production rate of RCC in Grand Poubara Dam shows that during the first month this rate is in the range of 1000 to 1500m3/d, in the first 10m of dam height (i.e. up to reach the elevation 387m asl) during the elevation process. During the following two months this production rate decays to an average output of 640m3/d for the next 18m of dam height (i.e. up to reach the elevation of 405m asl). Finally, the next half month this production rate is 290m3/d for the final 2m (i.e. up to reach the elevation of 407m asl, which is the crest of the spillway made of RCC), see Figure 9. Above this level shall be massive concrete and conventional concrete (CC) up to the elevation 411m asl implemented.
The production rate for RCC is the result of the concurrent, coordinated operation of several systems such as aggregate production; material batching and mixing; RCC transportation, placing, spreading, and compacting; quality control testing; and other related operations.
These related operations include bedding placement, facing system placement, gallery construction, and intake works and spillway construction. It is generally necessary to accumulate large aggregate stockpiles before starting RCC placement so that adequate stockpile reserves are available at all times during production. Adequate stockpiles are especially important if the aggregate requires additional processing or transportation from offsite sources. The potential for rapid RCC placement also provides the contractor the option of limiting placement to specific time periods to take advantage of cool or warm weather to aid in controlling the temperature of the RCC. It also provides the opportunity to reduce the extent of cofferdam and diversion requirements. The contractor must consider the relationship of each of these systems and balance specifications during construction in such a way that the individual system requirements are compatible with the overall production requirements. Whenever possible, the contractor should be given the flexibility to manage the RCC production rates as long as overall schedules are met. This will allow the most economical match of material, equipment, and labour resources. However, required schedule dates must be clearly defined in the specifications, with workable controls to enforce them.
Segregation is one of the most detrimental conditions that can occur in the production and placing of RCC. Handling of materials must be controlled during each phase of the operation to minimize or prevent segregation of the aggregate. Many of the preferred procedures and equipment used for RCC construction are based, in part, on favourable performance with regard to segregation.
RCC production plant
The RCC plant includes the aggregate stockpiles; the materials feed system, the mixer, and the discharge system. Many of the practices recommended for conventional concrete production apply to the production of RCC as explained below:
• Aggregate stockpiles. Segregation is the primary condition to avoid when handling aggregates.
• Aggregates are supplied to the proportioning and mixing plant by one of three methods. The simplest method, usually employed for low-production projects, is the use of a front-end loader to charge aggregate feed bins at the plant. The loader removes aggregate directly from the stockpile and deposits the aggregate in feed bins. Standard implementation ranges from one bin for feeding a single aggregate group or two bins for feeding a fine and coarse aggregate to three bins for feeding one fine and two coarse aggregate groups. A variation of this process is to use remote feeders and conveyors to charge the plant feed bins. Again, front-end loaders haul the aggregate from the stockpile to the bin that feeds the batch plant. This is more typical of projects where the loader haul distances must be minimized. A tunnel is advantageous for large projects requiring higher volumes of aggregate. This option eliminates the use of front-end loaders by directly feeding the stockpiled aggregate into a tunnel under the stockpile and then conveying the aggregate to the batch bin.
• Mass batching of aggregates involves transferring aggregates from the feed bin to the mass hopper.
• Continuous feed systems are used to provide a continuous, uninterrupted flow of material and RCC.
• The twin-horizontal shaft mixer is the predominant continuous mixer used for production of RCC.
• Uniformity of the mixing operation is critical to good-quality RCC. Mixer uniformity testing is the primary means to establish whether consistent mixing of materials is occurring.
RCC transportation systems
The selection of a transportation system for RCC is an important item during construction. The quality of the lift surface is affected by the process used to transport material to the placement area. In general, high-quality lift surfaces, particularly those requiring high lift strength, are better constructed using a transportation system that uses conveyors for transportation on the dam. Vehicle placement systems are more appropriate for placements where lift surface quality and consequent lift strength are not as critical. The apparent high relative cost of the conveyor system compared with vehicle haul systems may be tempered when consideration is given to haul road logistics, placement areas, and damage control measures. Transportation systems that combine conveyor and vehicle methods have been effective on many projects.
Extension of the RCC technique to 90% of the structure
For the case of extension of the RCC technique to almost 90% of the dam structure, the following analysis was performed taking into account the dam profile (Figure 10).
Dam conception profile
The extension of the RCC technique will vary the dam conception profile as show in Figure 10.
Dam body execution
A brief analysis of the dam geometry in the close of the valley has been carried out considering a layer volume of 300mm of RCC, which is presented in Figure 11.
As it can be observed in Figure 11 the RCC volumes per layer are always decreasing from around 900m3 to 350m3 per layer. This is considered a positive aspect taking into account that only the spillway section will be made of RCC and therefore the shoulders or abutments are vertical.
Also shown in Figure 11 is a wide zone in the lower-middle part of the dam height, around 15m high, in which the RCC volumes per layer are almost similar, around 930 m3 per layer. This is considered a positive aspect, which indicates that the close valley is optimum for a good design of a RCC dam.
Therefore, taking into account the optimisation of the structure layout and the application of some modern concepts on the design of RCC, it is possible that the designed structure will be constructed in four to fivr months (Figure 12). This is considered an advantage for the project.
RCC production control
The estimated production rate of RCC in Grand Poubara Dam under this conception of dam profile shown that during the first month this rate is in the range of 900 to 930m3/d, in the first 15m of dam height during the elevation process. During the following two months this production rate decay to an average output of 780m3/d for the next 18m of dam height. Finally, the next month this production rate is 415m3/d for the final 10m.
Cesar Adolfo Alvarado-Ancieta (Peru), Civil Engineer - Director Hydropower & Dams, Dipl.- Ing., M. Sc., Hydropower & Dams Engineer, Nuremberg, Germany. E-Mail: [email protected]
Patrick Yalis Ongalla (Gabon), Ing. Electrical - Deputy, Coordinator / Resident Engineer, Direction Générale de l’Energie et des Ressources Hydrauliques, Republic of Gabon, Franceville. E-Mail: [email protected]