2. DESIGN OF RCC DAMS
2.4. Design considerations
2.4.1. General
The key to ensuring the realisation of the full advantages of RCC construction is simplicity (Dunstan, 2012) and a significant contribution to construction simplicity is achieved through appropriate dam design. In this regard, the RCC dam designer will benefit significantly from a greater understanding of construction requirements than is typically the case for a CVC dam. As a consequence, an inexperienced RCC dam designer would be wise to seek assistance, or to commission a review of constructability. In reality, it is evident that the potential efficiencies of RCC dam construction have often been frustrated as a consequence of poor, or inappropriate dam design.
A specific criterion of the dam design should be to allow the maximum rate of RCC production to be maintained, with as few interruptions as possible until completion. Similarly, the RCC construction configuration should be designed for the same objective, minimising as far as possible the interruptions on the critical path, such as warm and cold layer joint preparations, bedding layers, facing concrete, complex formwork (setting and lifting), internal formed drains, formed galleries, embedded instrumentation, conduits and inserts and other items that disrupt RCC placement.
The design of a dam constructed in RCC is usually finalised with a Full-Scale Trial (FST) early during the construction phase, using the equipment, materials and labour to be applied for the main dam construction. This exercise and subsequent testing serve to confirm the achievement of the various important design parameters under real construction conditions. On the basis of the findings of the FST, the final construction methodologies and processes will be defined (and approved) to assure the achievement of the target RCC design parameters on the main dam construction. Full-scale trials are essential for all RCC dams where the structural design parameters are important, where the RCC mix requires optimisation, where bond between layers is important, where set retardation is to be used, where new construction systems are to be applied, or where an inexperienced contractor needs to demonstrate his capabilities, or to train key personnel. The same opportunity should be used to train supervision and inspection personnel. In view of differing in-situ and laboratory test RCC setting times and the fact that testing is the only way to establish the variation of layer bond with joint maturity and treatment method, a full-scale trial remains an essential requirement for almost all RCC dams.
The first RCC placed in the dam will typically be at the lowest point of the foundation and consequently subject to the highest hydrostatic pressures and maximum stress levels. Accordingly, the full-scale trial should be undertaken outside the dam body or in a less-critical section of the works, such as high on one abutment, or as part of a stilling basin foundations. Furthermore, the full-scale trial should be continued until all construction procedures are perfected, to ensure fully effective construction during first RCC placement in the base of the dam structure.
Impermeability and durability in RCDs are ensured through a wide section of cementitious rich CVC on the upstream and downstream faces, with only the lower cementitious content core zone compacted by roller. Adjacent monolith blocks are separated from each other by cutting joints through the full section at 15 m centres. Consequently, RCD gravity dams are designed as 2-dimensional structures according to the same design approach applied for CVC dams and correspondingly RCDs demonstrate the same performance characteristics as CVC dams.
2.4.2. Typical RCC strength characteristics
Table 2.1 provides a broad indication of the typical strength parameters that can be anticipated for different types of RCC:
Table 2.1
Indicative RCC Strength Parameters
Strength Characteristic at 365 days (MPa)
RCC Type
LCRCC MCRCC HCRCC RCD
Compressive Strength
Typical 12.5 17 23.5 17.3
Range 7.5 - 16 7.5 - 30 11 - 40 12 - 25
Parent Direct Tensile Strength
Typical 0.6 0.9 1.5 -
Range 0.3 – 1.2 0.5 – 2.0 0.7 – 2.9 0.8 – 1.8
Joint Direct Tensile Strength
Typical 0.4 0.65 1.1 -
Range 0.2 – 0.7 0.3 - 1.1 0.6 – 1.9 -
Joint Cohesion
Typical 1.1 1.0 1.6 2.4
Range 0.7 - 1.4 0.6 – 1.6 0.8 – 4.0 1.5 – 4.0
In preliminary RCC dam design, a value of 45˚ is typically assumed for the angle of friction in lift/layer joint shear calculations. Testing has demonstrated that the friction angle on any type of lift joint is usually within 1˚ of that of the parent RCC (Schrader, 2012) and while a value of 45˚ is generally a reasonable initial assumption for typical concrete aggregates, the actual friction angle can vary between 30 and 60˚, depending on the type and nature of the aggregates used. It should accordingly be acknowledged that the modern practice of applying lower factors of safety for sliding stability analysis (USACE, 1997) is predicated on the availability of actual, tested shear strength parameters.
As is common in all dam engineering, the RCC dam designer must be confident that his design assumptions are achievable with the materials available and under the construction conditions anticipated at the project site.
2.4.3. Bond between layers
RCC Layer Bond
As a result of placement in layers, the performance of an RCC dam will be largely determined by the respective performance of the bond between the placement layers.
While the important characteristics of bonding between successive RCC layers are horizontal shear strength, permeability and vertical tensile strength, the significance of each will vary for different RCC types, design approaches and construction methodologies. For large dams subject to significant seismic loading, for example, tensile strength will often be the critical design parameter, whereas for a smaller gravity dam, subject to only reasonable hydrostatic loadings, the primary bond requirement will often be low permeability.
In principle, the shear and tensile strengths and the permeability of the parent RCC can only be reproduced with confidence at the joint interface between placement layers when successive layers are placed rapidly and typically before the initial set of the receiving layer. Thereafter, as the surface of the exposed receiving layer matures, tensile and shear strengths reduce and permeability progressively increases. It should be noted that it is the set condition of the surface of the layer that determines subsequent bond with the new layer above and this may differ from the set condition in the majority of the layer beneath. The lost shear strength, impermeability (particularly in the case of LCRCC) and tensile strength can subsequently be restored through the application of a bedding layer (grout, mortar, or concrete), with the measure of benefit regained again reducing with increasing surface maturity. Beyond the age of the final set of the receiving layer and usually after a period of approximately 2 days, significant tensile strength can only be recovered by exposing well-embedded aggregate (and the
application of a bedding layer where necessary), as typically applicable for conventional mass concrete.
It should be noted that it has also been established that the application of a bedding mix will not necessarily increase joint performance in high-workability HCRCC mixes (Dunstan & Ibañez-de- Aldecoa, 2003).
In order to distinguish between the necessary treatments to be applied to achieve the required inter layer bond characteristics, layer joints are typically differentiated into “Hot”, “Warm” and “Cold”
conditions as follows:
• A joint is typically defined as Hot when the receiving RCC of the layer below is still workable (initial set has yet to occur) at the time the subsequent layer is spread.
• A joint is defined as Cold when the surface of the receiving layer is judged to be such that little, or no penetration of aggregate will occur during the compaction of the subsequent RCC layer. Typically, this condition will develop after final set of the receiving RCC layer surface.
• A joint is generally defined as Warm when its condition lies between Hot and Cold.
It should be noted that deviations from the above experience have occurred, with cases of bond starting to reduce at some time both before and after the initial setting times (Dunstan & Ibañez-de- Aldecoa, 2015).
In terms of bond and required layer treatments, whatever might be applicable for a particular set of cement chemistry, pozzolan type and fineness, admixture type and dosage, wind, sun and solar conditions may change when any of these change, which will occur regularly. Consequently, the maturity definitions developed to distinguish between joint conditions must be adjusted as necessary when conditions change. Furthermore, penetration methods used to measure setting times in screened mortar (ASTM C403) can give substantially different results in the laboratory, compared to field conditions.
Typically, actual RCC setting times in the field are substantially less than the times measured in laboratory testing, although in different conditions, longer setting times have also been experienced.
Construction specifications must correspondingly always be developed to recognise actual performance under the range of field conditions anticipated. Realistically, indicators developed and specified for the determination of layer joint treatment should be viewed as no more than guidelines, which anticipate appropriate adjustments as and when necessary.
When an earlier set of the RCC in the top surface of the layer is apparent, caution must be considered in the application of aggressive cleaning procedures, which may cut through a set surface and damage the less mature RCC beneath.
While the final bond properties between layers will depend on the characteristics of the RCC materials, the applicable curing, the receiving-layer preparation and the climatic conditions during exposure, the dam design and construction must adequately take cognisance of the specific associated requirements and the realistically achievable parameters for RCC layer bond. For example, a super- retarded RCC, or the application of sloped layers, can ensure a level of bond between layers that approaches the parent properties only if the applied construction methodology can consistently ensure successive layer placement within the initial concrete setting time. On the same premise, a dam section design requiring no tensile strength, with only friction for shear resistance and allowing permeability implies a significantly more flexible construction methodology that is insensitive to receiving layer joint maturity, as generally applicable for a Hardfill section design. Between these two extremes in RCC methodologies, any design variation combining different areas of bedding layer coverage and levels of joint maturity is possible.
Site-specific tests, including slant/shear and large-scale shear and tensile tests, are the only way to confidently establish the actual cohesion, friction, and tensile strength properties that can be achieved for various lift joint conditions, maturities and treatments. Testing must be undertaken on samples created under full-scale, representative construction conditions, using the actual materials and
possible to extrapolate test data from other projects constructed using similar aggregates, aggregate gradings, cementitious materials types/sources, mix designs and construction methods.
The in-situ tensile and shear strength parameters achievable for layer joints can be tested by the following means:
• Direct shear tests, under various confining loads can be undertaken on blocks cut into full- scale trial placements;
• Drilled cores (minimum 150mm (6-inches)) can be recovered from full-scale trials for laboratory testing (including slant/shear); and
• Samples can be sawn out from large-scale shear test pads and tested in a laboratory.
Testing of laboratory-manufactured RCC samples should be used to compliment, but never in place of, the testing of in-situ placed samples.
For a given set of aggregates, testing has demonstrated that the angle of friction on a joint surface is largely independent of the RCC mix, the layer maturity, or the surface condition. Conversely, the residual friction angle can decrease more in HCRCC than LCRCC, while higher joint friction angles can be anticipated for RCCs and bedding layers manufactured with crushed aggregates rather than alluvial materials.
The primary factors affecting in-situ tensile strength between RCC layers can be defined as (Schrader, 2012):
• the ultimate strength of the RCC mix and the rate of strength development;
• the properties of the fresh RCC (consistency, setting time, placing temperature, etc.);
• the degree of segregation at the point of placement;
• the maturity and treatment of the layer surface and the curing applied;
• the compacted density (should exceed 96% and ideally 99% of the theoretical-air-free density); and
• the use of bedding mixes (although not necessarily in HCRCC mixes (Dunstan & Ibanēz- de-Aldecoa, 2015).
2.4.4. Design for horizontal construction
In traditional CVC dam construction, placing in discrete monoliths ensures that gravity loads are carried directly downwards into the foundation and designers are warned against the use of shear keys when the lateral transfer of load is specifically undesirable (Indian Standard, 1998 & Shaw, 2012).
Applying the typical system of joint inducing in RCC dams, achievable horizontal and vertical joint alignment tolerances are of the order of ± 50 mm and, with induced joints opening typically no more than a few millimetres, 2-dimensional shear keys between adjacent blocks are effectively created. This situation is even more significant during construction, before induced joints open to accommodate the contraction of adjacent blocks. At this time, bridging over foundation depressions and the partial transfer of gravity load from taller cantilevers to adjacent shorter cantilevers can influence the short-term and long-term behaviour of the dam structure. While such stress transfer does not occur in a CVC structure constructed in separate vertical monoliths, deleterious consequences can include a reduction in the normal forces, and consequently shear resistance, on the critical lift surface joints and at the foundation contact.
While the consequences of horizontal construction must be acknowledged and recognised as a difference between RCC and CVC dams, the most significant effects will be found in large dams, where temperatures can remain elevated for an extended period, and in dams constructed using a low stress-relaxation creep RCC, where induced joints may not open to any significant extent. The designer should be aware of these effects and apply appropriate measures, such as a jointing system that assures no shear transfer, or formed block joints, in instances where no lateral bridging or stress transfer between blocks is tolerable. Particularly critical conditions include narrow, steep-sided dam sites and foundations with significantly variable rock mass deformation moduli. Although the specifics of each
situation should always be considered, the transfer of stress between adjacent blocks should generally be taken into account when the crest length/height ratio of a dam is less than 6 and/or the foundation rock mass deformation modulus varies by a factor of more than 50% between adjacent blocks.
2.4.5. Gravity dams
In respect of loadings, stability and allowable stresses, RCC gravity dams are designed in accordance with the same criteria and principles applicable for CVC gravity dams; with exceptions in some cases related to thermal loads.
A concrete gravity dam is a structure that is proportioned such that it transfers all static and dynamic loads into the foundation through the action of its own mass. Consequently, conventional concrete gravity dams are usually designed as 2-dimensional structures (plane stress). In modern practice, finite element analyses are typically used to support simple equilibrium stability calculations, particularly for a more accurate evaluation of structural response under earthquake loading and to enable a more realistic analysis of non-linear behaviour. In the case of an RCC gravity dam, similar principles apply, but additional consideration should be given to reviewing potential lateral load transfer between adjacent blocks and the consequential impact on internal and overall dam stability.
2.4.6. Arch dams (see Chapter 9)
In the economic feasibility evaluation of a dam, the volume of construction materials is a particularly important consideration. A gravity dam is an inefficient structure with respect to concrete volume, with much of the mass experiencing levels of stress at only a fraction of the concrete strength.
Accordingly, where the site topography and geology allow, benefit in dam design can be realised in taking advantage of 3-dimensional load transfer to reduce the required volume of concrete. Unlike the gravity dam, for which RCC has now effectively globally replaced CVC as the optimal solution in all but exceptional circumstances, not all arch dam sites will be more suited to an RCC arch than a CVC arch.
The topographical factors that increase the efficiency of an arch structure, typically a low crest length/height ratio and a narrow valley bottom, will tend to compromise the full achievement of the efficiencies associated with RCC, favouring vertical, rather than horizontal construction. Additional requirements for an arch, such as consolidation grouting on steep abutments, post-cooling and joint grouting, further compromise the time advantages of RCC construction. Accordingly, an optimum RCC arch can often involve a simpler, heavier section, which does not require post-cooling, or joint grouting before impoundment. As a consequence, RCC arch dams are typically more efficient solutions at sites best suited to an arch-gravity configuration (or a “thick arch”), in temperate climates and when constructed using low stress-relaxation creep RCC. All RCC arch dams to date have been constructed using HCRCC.
Three particular benefits of RCC arch dams are found in a mitigation of the inherent sensitivity of RCC dams to shear strength on the layer joints, a reduced structural sensitivity to vertical heel tensions under high seismic loading and a more efficient utilisation of the inherent strengths of HCRCC.
RCC arch dams have been demonstrated to offer time and cost savings over RCC gravity dams on numerous prospective sites and all of those in operation to date have performed above and beyond expectations.
A full 3-dimensional structural analysis is essential for an RCC arch dam, using a finite-element analysis system, with non-linear, dynamic and thermal analysis capabilities.
The various technologies applied to date for RCC arch dams, the associated benefits and drawbacks and the important related design considerations are addressed in greater detail in Chapter 9.