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Concrete In Australia : June 2014
Concrete in Australia Vol 40 No 2 29 and nitrogen (for conventional concrete) cooling, with insulation of concrete pours critical in the harsh Canberra winters where temperatures were often below freezing. Thermal and stability analysis One of the most severe loads to be considered in mass concrete dams is the thermal loading. The sheer volume of concrete and the speed at which it is placed means that near full adiabatic conditions are present in the centre of the dam. This means that the full amount of heat generated from hydration of the cementitious materials is accumulated in the structure with negligible losses. Thermal loading is particularly severe in highly variable climates such as Cotter Dam where the dam is exposed to a large range of ambient temperatures. A key design objective was therefore to keep the cementitious content of the mix (in particular the cement content), to a minimum, while still obtaining the required strength characteristics. This was achieved through careful RCC mix design, as described below. This limits the thermal potential and thereby limits the thermal stresses in the dam. A thermomechanical finite element model was developed using Strand 7 (along with models in several other packages) to analyse the stresses in the dam for the thermal loading combined with a range of other loads for different scenarios, including flood loading and earthquake. A full time history seismic analysis was undertaken to investigate how the dam would respond to seismic loading. Highly sophisticated cracked analysis of the dam was undertaken to investigate the likelihood of the cracks developing as well as the risk of them propagating through the structure for the range of loads considered, including seismic loading. The model enabled the stability of the dam to be confirmed for these complex, extreme load conditions. The modelling pushed the bounds of the software available and together with the initial analysis, is among the most extensive analysis known to have been undertaken for a gravity dam. Details of the analysis was presented to ANCOLD in 2011. Foundation design A comprehensive geological model was developed for the dam foundation based on the core hole information, seismic tomography and field mapping. This was used to define the minimum geological excavation profile, which set the baseline for the foundation excavation. Potential global and local instability mechanisms, defined by the interaction of the major joint sets in the foundation, were considered in the development of the dam foundation design. As the dam was excavated, the model was updated as more surface exposures were revealed. While construction requirements meant that the foundation was typically deeper than the minimum geological excavation line, the final excavated profile was characterised by the major defect sets identified during the investigations. The geological model was ultimately used to assist in defining the scope of dam foundation grouting works. This was used to define the depth and orientation of the grout holes, in interpreting the grout results and making decisions for higher order hole requirements. The model also enabled likely areas of high permeability to be identified in advance to assist with construction planning and forecasting. Key facets of the geological investigations were presented to ANCOLD 2012 as well as the Australia and New Zealand Geomechanics Conference 2012. Hydraulic design The dam was designed to safely pass the PMF, or an inflow of approximately 6,000m3/s. Hydraulic design of a spillway to safely pass this flow presented several challenges. The dam has a tapered central primary spillway section that is 70m wide with secondary spillways on either side. The total crest length of the secondary spillways is 220m. The downstream face of the dam was stepped which serves to dissipate much of the energy, particularly for smaller floods. To address the risk of cavitation damage to the spillway from high velocity discharge during extreme floods, air is introduced into the flow via an air intake system situated part way down the downstream face. Flow from the secondary spillway is returned to the main river via large spillway chutes down the abutments. Water from the main and secondary spillway converges at the toe of the dam into a custom designed stilling basin. The stilling basin design details, including the arrangement of the baffle blocks and deflector blocks, were trialled with a physical model to provide optimal energy dissipation. The spillway design was analysed though both numerical and physical hydraulic modelling. The physical model was a 1:45 scale and stood approximately 2.5m high. A good correlation was established between the numerical and physical model, which enabled potential cost saving options to be evaluated in an economical manner without sole reliance on the physical model. One key outcome from the hydraulic modelling was narrowing of the secondary spillway on the right abutment, which generated significant cost savings for the project. The outcomes of the hydraulic modelling were published by Hydropower & Dams International Journal, presented at ICOLD in 2012 and a paper was also presented in Spain. Conclusion The project was delivered under the Alliance contract model. The collaboration this model brought between the design and construction teams, combined with the pressures of the construction budget and program, has facilitated a drive for innovations, particularly during the construction phase of the project. In many respects the delivery of the Cotter Dam project has exceeded world’s best practice, with multiple design and construction innovations made on the project. This not only benefits future RCC dam projects, it has significance to other concrete applications. 22-29 - cover story.indd 29 22-29 - cover story.indd 29 22/05/14 11:49 AM 22/05/14 11:49 AM