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.
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
Maz Mahzari and Chi-Fai Wan
Upgrading of an existing dam often faces challenges in both static and seismic safety assessment. The use of new hydrological and seismological data and improved design methods often mean more severe loading which outdates the original design and demands expensive upgrade works. Establishing the design criteria for checking the structural adequacy of an existing dam for multiple unusual load events occurring within a relatively short time frame presents another challenge.
A probabilistic approach is presented to rigorously address the effects of multiple load events while maintaining a consistent risk of failure for the structure. This is based on a probabilistic conditional combination where probability of each event is defined and used to develop a joint probability distribution. For instance if an earthquake occurs following a severe flood, the seismic hazard curve of the site can be used to adjust the seismic loading with shorter average recurrence interval to be used in conjunction with the pre-earthquake flood when assessing the structural adequacy of the dam. With this method of adjustment, the design can benefit from the choice of a reduced seismic design loading and hence a more cost effective design solution.
The proposed method is straightforward and can be effectively used in most engineering practices, including the design of hydraulic structures such as dams.
Keywords: Dams, Seismic Hazard, Post-earthquake, Risk analysis
Gavan Hunter, Andrew Pattle and Mark Foster
A piping incident occurred during first filling of Rowallan Dam, Tasmania in 1968. The incident occurred at the interface of the embankment with the spillway wall, a 15 m high near vertical wall, where the contact earthfill eroded into the single stage downstream filter. Repairs were undertaken in 1968/1969 and the reservoir has operated largely without incident since.
A risk assessment in 2009 identified that piping through the embankment at the spillway wall interface remained a significant dam safety risk. Investigations in 2010 encountered cracking within the earthfill core at the spillway wall interface.
Dam safety upgrade works were undertaken in 2014/15 to address the piping failure mode at the spillway walls and also within the upper portion of the embankment. The works required excavation down to a rock foundation at depths up to 18 m adjacent to the spillway walls and this excavation provided an unusual opportunity to closely examine active piping features that had been preserved when interim repairs in 1968/69 had arrested the progression of piping. The repair comprised reconstruction of a significant portion of the embankment at the spillway and the reconstruction of the upper 7 m of the crest, which included dual filters downstream of the earthfill core.
The findings from the forensic investigations of the deep excavations adjacent to the right spillway wall are described in this paper along with a summary of finding from the 1968/69 repair works and a discussion of the piping mechanism at the spillway wall. The paper also covers the design and construction of the repair work. The focus of this paper is on advancements in our understanding of piping risk arising from the Rowallan Dam work.
In conclusion, (i) the upgrade works successfully reduced the dam safety risk of Rowallan Dam; (ii) the findings support the methodologies of the piping toolbox; (iii) the case study provides insight into filtering and crack filling mechanisms that have a broader implication for estimating the risks of internal erosion within existing dams; and (iv) the findings support the assessment of the low residual risks for piping through the embankment away from the upgrade work areas (crest reconstruction and spillway walls).
Keywords: Earth and rockfill embankment, piping incident, piping mechanism, dam safety upgrade.
Monique Eggenhuizen, Eric Lesleighter, Gamini Adikari
St Georges Dam is located on Creswick Creek approximately 2km southeast of the township of Creswick and 135km northwest of Melbourne. The reservoir, located within the Creswick Regional Park and originally constructed to supply water for the Creswick quartz crushing plant in the 1890s, has since been established as a popular recreational storage and is the responsibility of Parks Victoria. The dam is approximately 16m high and located across a relatively steep gully. The embankment consists of earthfill with an upstream face of rock beaching and a grass covered downstream face. The primary and secondary spillways are cut into the right and left abutments respectively.
At the completion of a detailed design review, St Georges Dam was assessed to be within the top three of Parks Victoria’s dams portfolio in regards to Public Safety Risks. The detailed design review assessed that the risk position for the dam plotted within the unacceptable region of the ANCOLD Guidelines for the static, earthquake and flood failure modes. As such, upgrade measures were considered to be required. In 2010 and 2011, a number of significant flood events emphasised the importance of upgrade works at this dam, particularly in regards to upgrading the spillway capacity, and consequently Parks Victoria assigned these works a high priority.
SMEC was engaged to design the upgrade works for the dam. A number of arrangements to increase the spillway capacity of the dam were considered, with the most cost effective option being assessed to be a secondary spillway over the dam embankment in the form of a rock chute.
This paper describes the decision making process associated with the option selection and the methodology for designing the overbank spillway which utilised the findings in ‘Riprap Design for Overtopping Flows (Abt & Johnson, 1991), and US Army Corps of Engineers, Waterways Experiment Station, publications of standard riprap gradations and computer program CHANLPRO.
Keywords: Embankment Dams, Spillway, Rock Chute, Erosion Protection
Chahnimeh reservoirs with 1.4 billion cubic metres storage capacity have a critical role in water supply for both drinking water and agricultural purposes for the whole Sistan region in eastern Iran. Sistan river used to be the only source for agricultural purposes, so that several gated diversion weirs were constructed on the river in the past 50 years. Because of climate change and upstream development causing flow fluctuations, the river alone is no longer a reliable source for irrigation purposes. So the idea of storing water in Chahnimeh reservoirs and optimised operation of reservoirs have become a necessity. In order to achieve this, development of structures to have efficient operational plan of the river and reservoirs system is underway.
Several projects have been built for more efficient use of the reservoirs, some projects still being designed. One of the latest is the project of “Development of Operational Infrastructures for Chahnimeh Reservoirs” designing a structure to regulate flow between Chahnimeh I and III reservoirs. This kind of structure operating between two connecting reservoirs is so rare, so that innovation is needed to design a cost effective structure covering different operational conditions. Different structures were investigated and the summary of selection of structure types are presented. The paper illustrates challenging design of the project, useful for engineers who might be or will be dealing with such a project. By designing gates with pre-compressed rubber sealing, huge amount of costs associated with having two different gates for different directions of flow are avoided. Because of saturated foundation, by designing a diversion system between two reservoirs, it is possible to undertake pre-consolidation of foundation soil and to drain saturated foundation water. This would reduce settlement of the foundation of the structure after construction to the extent that by construction of a pile group, the gated structure will perform with high reliability for gates function. This type of structure is so rare and the methods and experiences of the presented design can be used by other engineers and consultants in similar projects. The estimated cost of the project is 15 million dollars and with construction under way, completion is expected in 2017.
Keywords: regulating structure, gates, reservoirs, reservoir operation