J.H. Green; C. Beesley; C. The and S. Podger
Rare design rainfalls for probabilities less frequent than 1% Annual Exceedance Probability (AEP) are an essential part of spillway adequacy assessment as they enable more accurate definition of the design rainfall and flood frequency curves between the 1% AEP and Probable Maximum events.
Estimates for rare design rainfalls were previously derived using the CRC-FORGE method which was developed in the 1990s. However, as the method was applied on a state-by-state basis, there are variations in the approach adopted for each region. Differences in the cut-off period for data, the amount of quality controlling of the data undertaken, the base used for the 2% AEP estimates, gridding settings and smoothing processes have created inconsistencies which are particularly apparent in overlapping state border areas.
The Bureau of Meteorology has derived new rare design rainfalls for the whole of Australia using the extensive, quality-controlled rainfall database established for the new Intensity-Frequency-Duration (IFD) design rainfalls. These data have been analysed using a regional LH-moments approach which is more consistent with the method used to derive the new IFDs and which overcomes the limitations of the spatial dependence model in the CRC-FORGE method. In particular, the selection and verification of homogenous regions and the identification of the most appropriate regional probability distribution to adopt relied heavily on the outcomes of the testing of methods undertaken for the new IFDs. However, to focus the analysis on the rarer rainfall events, only the largest events have been used to define the LH-moments.
Keywords: Rare design rainfalls; Intensity-Frequency-Duration (IFD); Annual Exceedance Probability
Michael McKay and Francisco Lopez
Mt Bold Dam impounds the largest reservoir in South Australia. The dam wall comprises 19 concrete monoliths, 11 forming a central arch section and 8 forming gravity sections on the left and right abutments. The upstream face of the arch section is vertical, but the top portion overhangs on the reservoir side. The dam was originally constructed in the 1930s, and was raised by 4.3 m in the 1960s. In this upgrade the gravity abutments were raised using mass concrete blocks and the arch non-overflow crest was raised with hollow, reinforced concrete portals. On the spillway section a pier and gate system was installed on top of a hollow ogee section. The maximum height of the dam in its current configuration is 58 m.
GHD has been conducting a staged safety review of Mt Bold Dam since 2011. This included a detailed finite element nonlinear, time-history seismic analysis of the dam-foundation-reservoir system. The analysis was carried out using finite element techniques and included a detailed 3D model of all major components of the dam and different domains of the foundation rock. The nonlinearity of the model was included by explicitly incorporating contact elements at the dam-foundation interface, at the monolith contraction joints, and at some identified unbonded horizontal concrete lift joints within the dam wall. The seismic analysis was conducted for three different accelerograms corresponding to Maximum Design Earthquakes (MDEs) with 1 in 10,000 Annual Exceedance Probability (AEP).
This paper explains the purpose of the study, the adopted methodology and material properties, the results of the modelling phases, and the anticipated seismic behaviour and damage on the main components of the dam resulting from the MDEs. Finally, a conclusion is made in regards to whether or not Mt Bold Dam passes the adopted performance criteria for seismic loading.
Keywords: Arch, gravity, seismic, nonlinear, damage prediction.
Paul Southcott, Tim Griggs & Jamie Campbell
Suma Park Dam is the principal water supply dam for the City of Orange in central NSW. The 30m high single curvature concrete arch dam has a High A consequence category and required an upgrade due to an inadequate spillway capacity. To maximise the benefits of this major capital works, the Council also sought to increase the storage capacity and modernise the outlet works to help supply the rapidly growing population of the city.
Challenges that needed to be overcome to develop an affordable and safe solution included: very high flood inflows; limited freeboard; a highly stressed arch with a narrow crest width; poor access to the toe and right abutment; and a saddle dam located on a deeply weathered foundation.
Innovations incorporated into the design of the works included: Monte-Carlo based modelling of the flood hydrology that better estimated the design inflows resulting in a significant reduction in flood upgrade requirements; precast parapet crest units that incorporated crest widening to improve constructability; an anchored toe block to ensure the toe of the arch is stable; an upgrade to the stilling basin; and an auxiliary spillway incorporating Fusegates at the saddle location designed only to operate in floods in excess of the 1;1,000 AEP event with minimal loss of storage.
Construction of the works is now well underway. A number of challenges have been overcome during the construction stage including a re-design of the auxiliary spillway to use Fusegates and discovery of Naturally Occurring Asbestos (NOA) on site. Construction of the upgrade works is expected to be completed by the end of 2015.
Keywords: Concrete arch dam, flood upgrade, pre-cast, fuse gates, anchoring.
Richard R. Davidson, P.E., CPEng Kenneth B. Hansen, P.E.
Early in the twentieth century, placing concrete core walls within embankment dams was a popular construction technique for small to medium height dams. It became in vogue as a replacement for the popular British dam construction technology of puddle clay core dams which were used between the 1860’s and 1920’s. It avoided the many problems with semi-hydraulic / manned placement methods of the puddle clay cores within narrow trenches. However, after the mid 1930’s this concrete core wall construction fell out of favour because of the improvements made in embankment compaction methods and the difficulties in building reinforced concrete core walls to more significant heights.
Today concrete core wall embankment dams are now reaching an age where their continued performance is being questioned. This dam building technology has become extinct and is unknown to the last few generations of dam engineers. Therefore, it is relevant to re-examine this dam building technology in a modern context and work on answering the following questions. How have these dams performed after almost a century of service? Are there unanticipated performance features that have produced positive results when subjected to extreme flood and seismic events? Does the concrete provide enhanced performance over time? What role does steel reinforcement play in the performance of the core wall? Are there lessons here that can be applied to the more common concrete cutoff wall solutions being applied to embankment dams with seepage problems? This paper examines these questions with a number of illustrative case histories to provide a retrospective illumination of this forgotten dam building technology.
Keywords: Embankment dams, Concrete core walls, Dam construction history.
T. I. Mote, M.L. So, N. Vitharana, and M. Taylor
This paper explores the sensitivity of selection of earthquake design magnitude to liquefaction triggering in Australia for ground motions typically used for dams. The low seismicity of Australia creates a situation where liquefaction triggering is marginal at design hazard levels and this low level of seismic hazard makes the liquefaction trigger analysis very sensitive to the derivation of the seismic inputs. A methodology is presented that couples the probability of liquefaction triggering with the distribution of earthquake contribution to the hazard from the magnitude-distance deaggregation. The results show that for the “typical” soil profile and input ground motions approximately equivalent to the maximum design earthquake for Australia, the probability of liquefaction triggering varies significantly with the design magnitude selected. Using the maximum credible earthquake or mean magnitude may provide significantly different liquefaction triggering implications. Combining the probability of liquefaction triggering with the contribution of varying magnitudes to calculate liquefaction probability is a useful method to understanding the sensitivity of liquefaction to design magnitude.
Keywords: Liquefaction Assessment, Design Magnitude, Probability of Liquefaction, Magnitude-distance deaggregation, Australia
Nihal Vitharana, Nuno Ferreira
The raising and/or stabilising of existing concrete gravity dams by continuous concrete buttressing is a viable solution and, in some cases, it is the only solution available. There are few medium-large dams in Australia currently under consideration for raising with continuous buttressing.
Two of the major issues to be surmounted are: (a) the existing dam should not be subjected to cracking (particularly on the upstream face) due to heat-hydration effects, and (b) the requirement for the two dam bodies to resist the hydrostatic and other loadings as a monolith (unified dam).
However, there is great need for understanding the mechanisms involved in selecting an appropriate heat-of-hydration model and in calculating thermal stresses rationally. Due to such lack of understanding, expensive precautions, mostly with compounding conservatisms, would be adopted in concept and detailed designs eg. shear-keys on the interface, artificial cooling, post-grouted interface, anchor bars at the interface, concrete with high cement contents. On the other hand, unsafe designs could be the result.
The paper discusses these issues highlighting that a rational approach can be adopted to economise the design and construction processes. An example is also presented to demonstrate how the potential for temperature-induced cracking in new and old dam bodies can be evaluated with reduced uncertainty by considering all the mechanisms involved in a holistic way.
Keywords: Heat-of-Hydration modelling, raising concrete dams, thermal stresses, concrete buttressing