While the broad behaviour of RMC in dams can most closely be likened to that of conventional mass concrete, there are specific differences that must be addressed in the process of design. To understand the important factors of influence on design and the approach applied to the design and con- struction of RMC structures, it is first necessary to understand the nature of the material itself. While the general composition, mixes and behaviour vari- ations of the material are discussed in subsection 2.2 of this guideline (Materials requirements), the structural characteristics and performance of RMC are addressed in this sub-section.
RMC – the nature of material
Wedging Diagonal shear ‘Scissors jack’
understanding of all related important factors of influence on design and construction.
While the history in Zimbabwe of more than 15 years of ‘new generation’ RMC arch dam construction provides very useful reference, the prevalent level of knowledge of the material characteristics and the lack of published data and information leave many questions unanswered. Construction in Zimbabwe has seen the almost exclusive use of granitic aggregates, on granitic foundations, while in South Africa. There is a significantly broader range of geologies and construction material characteristics. Furthermore, climatic variations across Zimbabwe are less significant than across the breadth of South Africa and accordingly, while temperature related effects have been rather inadequately addressed previously in RMC arch dam design, information concerning successful precedents exposed to environments as extreme climatically as certain parts of South Africa is sparse.
4.2.2 Construction without contraction joints
RMC dams, particularly arch dams, are usually constructed without contraction joints, to allow for the effects of thermal shrinkage. While the consequences of thermal effects on arch action within a dam wall are addressed later in this section, it is important that the designer consider very carefully the necessity to include contraction joints and the structural implications, should they be omitted.
Multiple arch buttress dams of well over 200 m in length and arch dams of up to 150 m in length have been constructed in RMC without suffering any cracks. On the basis of past experience it is considered that joints within arch sections can and should be avoided, if possible, as a joint in an arch would require grouting to re-establish structural continuity. A single joint, or widely spaced joints in an arch, if left un-grouted, may well result in greater stress concentrations than would occur in a structure with no joints.
4.2.3 Construction without shutters
RMC dams will generally be constructed without shutters, as the drier external zones are placed first to contain the more flowable mortar matrix into which the rock particles are embedded. Construction in such a manner, however, implies that different structural characteristics can be anticipated for the internal and external zones of the dam wall. Whilst the mass of both zones will contribute to the overall structural stability, only the internal zone will be capable of carrying the full design compression, shear and tension stresses and only this zone will contribute to the water-tightness of the structure.
If the technique of placing a flowable RMC within drier external walls is used, careful attention should be given to the possibility of rapid drawdown of the reservoir, when considering rendering (plastering) of the upstream face. If the dam design allows the impounded reservoir to be drawn down rapidly and a
There are certain circumstances for which the inclusion of contraction joints is considered advisable, including: • when differential foundation settle-
ment, or movement is possible, • when the length to height ratio of a
straight section of wall becomes excessive,
• in extreme climatic conditions.
4.2.4 Practical construction constraints
A large part of the success of RMC dam construction to date has been found in its simplicity and an ability to utilise low skill labour. The need for simplicity and the nature of this shutterless construction, however, limits the practicality of more complex dam geometries and arch shapes. Above a certain height, significant benefit is derived in creating additional sliding resistance for a concrete multiple arch dam by sloping the arch forward, to mobilise the weight of the water and develop additional vertical load on the foundation. To date, no economical solution has been found to allow inclination of the arches of an RMC multiple arch dam.
4.2.5 Ineffective self-sealing
When particularly pure, aggressive water is impounded, self-sealing may not occur as effectively and careful consideration should be given to appropriate section dimensions and indeed even the appropriateness of RMC in such environments.
4.3 Dam design considerations
4.3.1 RMC elastic modulus and thermal expansivity
It is likely that a relatively low elastic modulus and the use of material with a low thermal expansivity in relatively temperate climates are important factors in the apparently successful behaviour of RMC to date. When investigating higher RMC arch dams, for which higher strength RMC is necessary, it is very possible that increased cement contents and/or the use of very hard rock aggregate could compromise RMC behaviour, producing a material more closely emulating concrete.
The relative sensitivity of RMC arch dam performance to thermal expansivity and elastic modulus can be illustrated through the typical effects indicated in a study of stresses for a typical RMC arch dam when subjected to a uniform structural temperature drop of 6°C. Table 3 below presents an indicative summary of the impact of typical materials variations on stress and displacement for this relatively mild drop in body temperature. Obviously, neither thermal expansivity, nor elastic modulus variations are of any influence on structural stress levels if no temperature variation load is applied; only displacements being affected.
4.3.2 Climatic- and environment-specific conditions
In observing an empirical approach to RMC dam design, on the basis of Zimbabwe experience, we must bear in mind that elsewhere in southern Africa, we can experience a significantly wider range of construction material types, foundation geologies, climatic conditions and site specific circumstances. For instance, it is significantly possible that surface freeze-thaw conditions within RMC, in through seepage water, could occur in the colder areas of South Africa and Lesotho. Furthermore, aggressive, ion hungry water may not allow the In considering the construction of a high
(above perhaps 16 m) RMC arch dam in South Africa, it is suggested that the establishment of actual RMC mech- anical properties through realistic testing is essential for any constitutive materials save for granite applied in a mild climate.
Testing essential