Zivko R. Terzic, Mark C. Quigley, Francisco Lopez
The Mt Bold Dam, located in the Mt Lofty Ranges in South Australia, is a 54m high concrete arch-gravity dam that impounds Adelaide’s largest reservoir. The dam site is located less than 500m from a suspected surface rupture trace of the Willunga fault.Preliminary assessments indicate that Mt Bold Dam is likely to be the dam with the highest seismic hazard in Australia, with the Flinders Ranges-Mt Lofty region experiencing earthquakes of sufficient magnitude to generate shaking damage every 8-10 years on average. Prior evidence suggests that the Willunga Fault is likely capable of generating M 7-7.2 earthquakes.As part of the South Australia Water Corporation (SA Water) portfolio of dams, Mt Bold Dam is regularly reviewed against the up-to-date dam safety guidelines and standards. SA Water commissioned GHD to undertake detailed site-specific geophysics, geotechnical and geomorphological investigations, and a detailed site-specific Seismic Hazard Assessment (SHA) of the Mt Bold Dam area. The results of this investigation will be used to inform decisions related to planned upgrade works of the dam.Geomorphological mapping of Willunga Fault, detailed geological mapping, analysis of airborne lidar data, geophysical seismic refraction tomography and seismic reflection surveys,and paleoseismic trenching and luminescence dating of faulted sediments was conducted to obtain input parameters for the site-specific SHA.Discrete single-event surface rupture displacements were estimated at ~60 cm at dam-proximal sites. The mean long-term recurrence interval (~37,000 yrs) is exceeded by the quiescent period since the most recent earthquake (~71,000 yrs ago) suggesting long-term variations in rupture frequency and slip rates and/or that the fault is in the late stage of a seismic cycle. The length-averaged slip rate for the entire Willunga Fault is estimated at 38 ± 13 m / Myr. Shear wave velocity (Vs30) of the dam foundations was estimated based on geotechnical data and geological models developed from geophysical surveys and boreholes drilled through the dam and into the foundation rock. The nearest seismic refraction tomography (SRT) lines were correlated with the boreholes and those velocity values used in the Vs30 parameter determination. All relevant input parameters were included into seismic hazard analysis with comprehensive treatment of epistemic uncertainties using logic trees for all inputs.Deterministic Seismic Hazard Analysis (DSHA) confirmed that the controlling fault source for the Mt Bold Dam site is Willunga Fault, which is located very close to main dam site (420m to the West).For more frequent seismic events (1 in 150, 1 in 500 and 1 in 1,000 AEP), the probabilistic analysis indicates that the main seismic hazard on the dam originates from the area seismic sources (background seismicity).Based on deaggregation analysis from the site specific Probabilistic Seismic Hazard (PSHA), the earthquakes capable of generating level of ground motion for the 1 in 10,000 AEP event can be expected to occur at mean distances of approximately 22km from the dam site(with the mean expected magnitude atMt Bold Damsite estimated at Mw >6).For less frequent (larger) seismic events, the contribution of the Willunga Fault to the seismic hazard of Mt Bold Dam can be clearly noted with Mode distance in the 0-5 km range, which indicates that most of the seismic hazard events larger than the 1 in 10,000 AEP comes from the Willunga Fault. The Mode magnitudes of the events are expected to be Mode Magnitude at Mw= 6.6 for a segmented Willunga Fault scenario, and Mw= 7.2 for a non-segmented fault scenario.Consideration was also given to the upcoming update of the ANCOLD Guidelines for Earthquake, which calls for the determination of the Maximum Credible Earthquake (MCE) on known faults for the Safety Evaluation Earthquake (SEE) of “Extreme” consequence category dams. The MCE for Mt Bold Dam was estimated from the DSHA; in terms of acceleration amplitude, the MCE event approximately equals the 1 in 50,000AEP seismic events.
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James Toose, Lelio Mejia, Jorge Fernandez
The recently completed Panama Canal Expansion project required construction of a new, 6.7-km-long channel at the Pacific entrance to the Panama Canal, to provide navigation access from the new Post-Panamax locks to the existing Gaillard Cut section of the Canal. The new channel required construction of four new dams adjacent to the existing canal, referred to as Borinquen Dams 1E, 2E, 1W, and 2W. The dams retain Gatun Lake and the Canal waterway approximately 11 m above the level of Miraflores Lake and 27m above the Pacific Ocean.The largest of the dams, Dam 1E, is 2.4km long and up to 30 m high. The dam abuts against Fabiana Hill at the southern end, and against the original Pedro Miguel Locks at the northern end. This paper provides an overview of the key challenges in construction of Dam 1E including the foundation, seepage cut-offs and embankment.
A common concern for large spillways is erosion of the receiving plunge pool and potential impacts on the stability of the dam.Devils Gate Dam is an 84m high, double curvature arch concrete dam, located in northern Tasmania and constructed between 1968 and 1970.The full 134m long crest is designed as a free-overflow spillway and spill flows impact the downstream valley sides and plunge pool below, where energy is dissipated to reduce riverbank erosion downstream.To protect foundation rock,the plunge pool and large portions of the valley sides were concrete lined with 450mm thick reinforced and anchored concrete. During spill events the area is inundated by up to 12m of tail-water.In 2016 damage to the plunge pool concrete was discovered by divers during a special inspection of the impact areas, but poor visibility limited the understanding of the extent and severity. Subsequent investigations, including detailed sonar scanning, improved the understanding but it was not until the plunge pool was fully dewatered that the full extent of the damage was quantified.The damage commenced around 35m downstream of the dam arch and consisted of approximately 330 square metres of moderately to severely eroded concrete and exposed, deformed, and in some areas completely removed reinforcing bars. The most significant feature was a penetration through the concrete up to 2.5m into the foundation rock.A number of stressed anchor heads were also damaged or destroyed.A full appreciation of the damage necessitated the decision for immediate repairs given the impending power station refurbishment (commencing January 2018) which will subject the plunge pool to nine months of constant spill.This paper outlines the diving and sonar investigations undertaken in 2016, discusses the challenging tasks of dewatering the plunge pool and gaining access through the narrow canyon, and presents the physical works to strengthen the damaged areas.It discusses the difficulty of identifying and treating such damage, and serves as a cautionary tale for other owners who have fully submerged plunge pools downstream of spillways.
Alberto Scuero, Gabriella Vaschetti, John Cowland
Waterproofing geomembranes have been used for new construction and rehabilitation of dams since 1959. Research for underwater rehabilitation with geomembranes started at the beginning of the 1990s. The first installation was made in 1997 at Lost Creek arch dam in USA, where a SIBELON PVC geomembrane system was installed partly underwater, to restore watertightness to the upstream face. Techniques for underwater cracks/joints repair, and for staged repair, were developed and first adopted in 2002 and 2010 respectively. The paper presents through some significant case histories the range of underwater applications available today. The paper also presents a new underwater technology, the Sibelonmat®mattress, that allows water-tightening canals without reducing water flow.The Sibelonmat®can be used in embankment dams, to waterproof the upstream. face or as upstream blanket
Tom Ridgway, Chris Topham, Aaron Brimfield
A significant number of dams across Australia are of earthen construction and may be susceptible to internal erosion of their earth core, also known as piping. In January of 2016, during an annual inspection of the Tarraleah No 1 Pond Levee it was found that the embankment was experiencing significant seepage at the toe. Further investigations found actively developing piping holes through the embankment. To better understand the condition of the dam, HydroTasmania’s remote monitoring trailer was deployed to provide telemetered seepage data to further understand the developing issue. It was found that the leakage was increasing dramatically, and carrying suspended core material, resulting in the need for prompt resolution to protect the embankment from further loss of material. A sheet piling wall was installed in the centre of the embankment to cut off the flow of water through the embankment. After the installation of the sheet piling wall, post works monitoring showed the seepage through the embankment reduced to virtually zero, only peaking in rainfall events. This paper outlines the investigation and management of the incident, and the mitigation measures put in place from the time of identification including the use of a sheet piling wall to mitigate a developing piping failure. The paper will conclude with the outcomes of the work and how a similar solution could be utilised for other dam owners in a piping event.
Elaine Pang, Robert Fowden
There are numerous established methods available for assessing the consequences of failure for earthen water dams.The estimation of breach dimensions and failure times remains the greatest common area of uncertainty, particularly for dams under 10m in height, where the number of historic records behind the established methods reduces considerably.Also, various factors can have a significant impact on the strength of small dam embankments, potentially contributing to the likelihood of failure.Consequently, failure impact assessments for smaller dams may rely more heavily on the engineering judgement of the responsible engineer. Although the consequences of failure may indeed be lower for smaller dams, the large number of unknown or unregulated dams in some locations means that it can be difficult to quantify their overall contribution in terms of dam safety risk. This paper presents an on-going project to compile and analyse observed small earthen dam failures with the intent of refining existing statistical breach relationships for smaller dams.Context is provided through an overview of DEWS’ investigative program, including the presentation of several case studies which highlight field data collected throughout the program.