Andrew Barclay, Greg Kotze
The Enlarged Cotter Dam (ECD) is under construction on the Cotter River, 18km west of Canberra. The new dam comprises an 85m high roller compacted concrete gravity dam, located 120m downstream of an existing 31m high concrete dam. This paper describes the geological structures that prevail at the site and their significance with respect to design and construction considerations.
Geological mapping has confirmed that the abutment slopes are characterised by zones of prominent rock outcrop and thin mantles of colluvial soil that form overall slope angles of 45 degrees. The Cotter River valley in the ECD area has been eroded through a geological sequence of Early to Late Silurian age, comprised predominantly of porphyritic rhyolite and lapilli tuffs of the Walker Volcanics.
Geotechnical investigations for the ECD were extensive and comprehensive. The results obtained have enabled the compilation of a detailed geological model of the dam site. Particular attention was paid to defining, characterising and kinematically analysing prominent geological structures, including intersecting sheared or crushed seams and zones that traverse the dam footprint.
Prominent geological structures that were encountered during the abutment excavation had significant design and construction implications for:
Abutment stripping and foundation preparations;
Rock slope stabilisation;
The foundation of the intake tower that comprises a 66m high concrete structure; and
The foundations for 1 x 56m high and 2 x 78m high tower cranes that required positioning on the steep abutment slopes during construction.
This paper highlights the importance of understanding the geological origin, nature and distribution of rockmass defects within a complex rock foundation. Site specific construction requirements and engineering design solutions used to successfully negotiate adverse geological structures are described.
Keywords: Dam, Roller Compacted Concrete, Geological Structures, Abutment, Foundation.
Barton Maher, Michel Raymond, Mike Philips
The Queensland Bulk Water Supply Authority (trading as Seqwater) owns and operates North Pine Dam, situated on the North Pine River in the Northern Suburbs of Brisbane. North Pine Dam is an Extreme Hazard Dam consisting of a concrete gravity dam with earthfill embankments at both abutments and three earthfill saddle dams. The spillway consists of five radial gates which are manually operated. Flood operations at the dam are controlled in real time by the Seqwater Flood Operations Centre.
In January 2011, North Pine Dam experienced the flood of record at the dam site with a peak inflow of approximately 3,500 m3/s and a corresponding outflow of approximately 2,850 m3/s. This inflow was more than double the previously recorded flood of record. The inflow was generated by high intensity rainfall both at the dam and in the upper catchment resulting in a rapid rise of the storage. The system which caused this rainfall was also contributing to the major flooding occurring in the adjacent Wivenhoe – Somerset catchment, also being managed by the Seqwater Flood Operations Centre. The rapid rise and fall of the storage presented difficulties for both the Seqwater Flood Operations Centre and the operators at the dam site.
Following the flood event, an analysis of the rainfall and the resulting inflows indicated a significant difference between the Annual Exceedance Probability (AEP) of the rainfall in the catchment and the estimated AEP of the inflow and peak water levels from previous hydrology studies. A detailed review of the flood event was commissioned by Seqwater and undertaken by URS Australia Pty Ltd.
This paper presents details of the flood event, lessons learned for the operation of the dam, upgrade works undertaken to date, results of the hydrology review and the conclusions of the Acceptable Flood Capacity (AFC) study. A key implication for dam owners was the increase in the estimate of the Probable Maximum Flood (PMF) by over 30% due to changes in calibration of the hydrologic model for the catchment.
Keywords: Probable Maximum Flood, Flood Operations, North Pine Dam, Flood Estimation
Dr. Mark Locke, Jiri Herza
Gördes Dam is a nickel and cobalt mine tailings dam situated in a seismically active zone in Manisa Province, Western Turkey. The dam is a conventional cross valley earthfill structure with a fully lined storage basin. The starter embankment with a maximum height of 50 m will be raised in downstream lifts to an ultimate height of 90 m. The total storage capacity is 19 million m3. Construction of the starter embankment is planned to commence in late 2012 and the dam will be commissioned in June 2013.
The tailings will be discharged from the dam crest and return water will be collected by a floating decant pump at the opposite site of the storage. Decant water has high calcium sulphate levels and will require treatment before re-use in the plant or release. The tailings contain about 33 % of solids and are classified as high plasticity silts and clays with more than 90 % of particles passing the 0.075 mm sieve.
The dam is founded on a complex formation of altered sedimentary and metamorphic rocks including mudstones, siltstones, limestones and serpentines. The mudstone blocks, the predominant foundation materials, are juxtaposed with siltstones and serpentines via a complex arrangement of faults. Where exposed, the mudstones are highly to completely weathered with a well-developed structure of smooth bedding surfaces leading to anisotropic strength characteristics. Several landslides, likely associated with the anisotropic character of the mudstones, were identified within the area including a significant landslide under the upstream shoulder of the dam.
Mining development in Turkey has a complex legislative environment. There is also standard practice which is not legislated but expected, this can be considerably different to normal design practice in Australia. The Turkish legislation is based on waste management guidelines and may be more appropriate to landfills than large tailings storages. The legislation is very prescriptive in some aspects and silent in others, with little consideration of risk or consequence based design.
This paper discusses the design difficulties associated with the challenging foundation conditions, which have been magnified by the requirements and limitations embedded in the approval documentation and the legislative environment in Turkey. It will also address some of the key differences between the design philosophy in Australia and in Turkey with a focus on the major risk elements of the design.
Keywords: Tailings, Turkey, Liner, HDPE, Nickel laterite
Karen Riddette, Chee Wei Tan, Alan Collins, David Ho
Due to a number of historical stilling basin slab failures around the world, modern basin slab stability assessment approaches now require allowance for hydrodynamic pressure fluctuations. Extreme fluctuations in uplift pressures have been found to occur in hydraulic jumps and plunge pools resulting in high-pressure pulses being transmitted via joints and drainage openings to the underside of the slab. If, peak uplift forces beneath the slab coincide with minimum pressure fluctuations on the top of the slab, the resulting pressure differential can be sufficient to lift a slab. As a result, simple static design based on tailwater depth and mean floor pressures is now considered highly non-conservative.
Through a case study on the Waipapa Dam spillway stilling basin, this paper examines the use of CFD modelling to compute mean hydrodynamic slab pressures taking into account the location of the hydraulic jump and the effect of the impact blocks on the pressure distribution over the slab. By combining the CFD results with empirically-derived pressure fluctuations, uplift scenarios are applied in a FEA model to compute the maximum load in the slab anchors and examine the sensitivity of the stilling basin slabs to uplift failure.
Keywords: Stilling basin, hydrodynamic modelling, CFD, pressure fluctuation, slab stability.
Eric Lesleighter, Peyman Andaroodi, Colleen Stratford
In January 2011 major flooding was experienced across a large part of Southern Queensland. The flood discharges through the Wivenhoe Dam spillway caused extensive erosion of the rock in the plunge pool. While not an issue in relation to the spillway structure’s security, the rock erosion experience was dramatic for a number of reasons. The paper presents details of the extent of erosion under head conditions that can be classed as moderate only when compared with many taller dams. The discharges over several days resulted in a pile of huge rock blocks downstream of the plunge pool.
The paper describes the plunge pool design dimensions, the geology, the hydrology of the releases, the hydraulics of the plunge pool, the surveys of the pool and rock mound, and moves on to discuss the mechanism of the fracturing and transport of the rock. Similar relevant experiences will be cross referenced, especially from details of recent experiences at the Kariba Dam and the study of remedies in the context of the dam’s actual safety.
From an actual major experience of erosion, and the sheer volume of rock that was lifted up and out of the plunge pool, the occurrence stands as a timely demonstration of what can happen in similar spillway situations, and suggests the type of awareness that spillway design needs to accommodate for energy dissipation facilities in unlined spillways plunge pool.
Keywords: Spillways, plunge pools, rock erosion, scour, plunging jets, pressure transients.
Mike Phillips, Kelly Maslin
A spillway upgrade conceptual design and selection process was undertaken to identify options for upgrading the Dartmouth Dam to pass the Probable Maximum Flood (PMF). A number of upgrade options were investigated, including variations of dam raise heights and spillway modifications. One of the options, the piano key weir, was initially developed from the limited available publications on the weir design, and further developed with the use of a 1:60 scale model. The piano key weir, a variation of the labyrinth weir, is a passive spillway that utilises a total weir length several times that of the effective spillway width. For the Dartmouth Dam study, the piano key weir design that was developed consisted of a 7-cycle, 9 m high structure, with a total weir length of nearly 600 m, or more than 6 times the existing effective spillway width of 91 m. The spillway was designed to pass the routed PMF outflow of approximately11,500 m3/s with a head of approximately 11 m.
The piano key weir design was developed using the following analyses:
Initial 1:60 scale physical model of the piano key weir based on published papers on piano key weirs and design manuals for labyrinth weirs;
Structural analysis and weir member sizing using initial physical model results;
Computational Fluid Dynamics (CFD) modelling to improve the hydraulic efficiency of the weir for the range of flows;
Revised 1:60 scale physical model of the piano key weir; and
Confirmation of conceptual structure design.
This paper describes the process of developing the piano key weir option for the Dartmouth Dam spillway and lessons learned.
Keywords: Piano key weir, CFD, spillway, physical model