M. B. Barker, R.M. Holroyde, J Williams and T. Qiu
Grahamstown Dam is a major water supply source for the Newcastle area and it is proposed to raise the full supply level by 2.4m from RL 10.4m to RL 12.8m. The present spillway is inadequate to pass the PMF without overtopping of the existing embankments at the new FSL and part of the raising comprises construction of a new embankment of about 10m high with a right bank spillway upstream of the existing spillway capable of passing the PMF. The Pacific Highway is located some 600m downstream of the new spillway and a 60m wide culvert below the Pacific Highway is being constructed with capacity sufficient to pass the PMF. Significant changes were made to the feasibility design for the spillway and the Pacific Highway culvert using a labyrinth spillway and a baffle chute energy dissipator respectively. Both of these designs are uncommon and the process of finalising the designs as well as some of the problems in the use of a labyrinth spillway and the cost savings realised in the use of these designs are presented.
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Mark Locke, Buddhima Indraratna, Phillip Cummins and Gamini Adikari
ABSTRACT: Australia has a large number of older embankment dams, which have been in service and performed adequately for over 50 years. However, current industry practice in embankment dam design predicts that the granular filters within these dams may not be adequate. This may require refurbishment of the dam by retro-fitting a new filter to ensure the continued safety of the structure. This paper outlines the potential problems with older embankment dam designs, and the reasons for constructing a new filter. Potential problems may include inadequate or non-existent filters, risk of failure due to earthquake, piping, or excessive foundation seepage. Design methods for granular filters are described briefly, concentrating on whether an existing filter is adequate, and the potential improvement by constructing a new filter. Construction issues for placing filters on existing dams are also discussed.
A new analytical method, developed to describe the time dependent erosion and filtration within embankment dams, is described briefly. The model predicts particle erosion, transport and retention based on fundamental fluid mechanics and geotechnical concepts. The application of this model to the design of filters for new and existing dams will be described. The predictions of such analytical modelling can give a designer a significantly clearer picture of the purpose of a granular filter, the extent of core erosion that can be expected, and the effect of retrofitting a new filter to an existing dam.
P.W. Heinrichs and R. Fell
Ben Boyd Dam, a 29 m high earthfill embankment built in 1978, has had an unusual history. In 1979, a number of seeps developed during first filling with water 5 m below FSL indicating unexpectedly high pressures. Investigations concluded the coarse filter permeability was very low due to excess fines. Remedial works in 1982 included a drainage filter beyond the toe and a new stability berm above. New piezometers were installed, including several in the blanket filters in the existing dam. These later indicated up to 10.5 m head in isolated areas within the filter. Pump out tests partially lowered the water level in the standpipes but in 1995 the water level rose by 4 m back to its previous high level. All this during a period of relatively low rainfall. Stability analyses were carried out and further investigations in 1999 concluded that apart from general leakage from the foundation abutment into the filters, the rise in pressures was due to leakage from a riser hole from one of the nearby foundation piezometers. A potential for piping along the piezometer tubes within the dam was also identified.
This situation was managed without resort to costly capital works, because it was concluded that the pressures from the vertical riser were not a potential failure mode, and potential piping failure would be adequately handled by the existing chimney drain, intersecting the piezometer tubes trench. Any potential piping failure would also give warning signs which increased frequency of monitoring (now in place) would pick up in time to allow lowering of the storage.
Failure modes and effects analysis (FMEA) has been increasingly utilised to prioritise and investigate dam safety deficiencies. It can be used to enhance dam safety programmes. Dam Surveillance, O & M procedures, and emergency plans can all be evaluated for their effectiveness in detecting and mitigating the applicable failure modes for a dam. Experience with a workshop process to carry out this evaluation and some of the improvements that have been identified are described.
A. Ash, D. S. Bowles, S. Abbey and R. Herweynen
A preliminary risk assessment was undertaken of its three dams by the South East Queensland Water Board (SEQWB) in 1999. The risk assessment process used included a series of workshops that proved to be an important part of ensuring a worthwhile result. The combined expertise of the consultants together with that of staff from the Board and the Queensland’s Department of Natural Resources were used to improve the outcome. The results of the assessment showed that the process had both advantages as well as difficulties in comparison to a standards based approach for making dam safety decisions. Risk Assessment was seen to be a useful management tool for managing dam safety. It gave the owner the ability to quickly rank upgrade requirements or maintenance options on the basis of probability of failure, life safety risks and financial risks to the owner or economic risks to all stakeholders.
This paper describes the use of a high strength woven geotextile and preloading to stabilise the surface of a very low strength tailings pond, and the incorporation of a geosynthetic clay liner (GCL) within the final capping design to complete closure. The pond, which contains tin and copper tailings, formed the lower tailings containment area of a three-tiered tailings storage, located directly above the Wild River in North Queensland. Stabilising the lower pond (area 2,500 m2), which contained tailings of “zero strength” in the central area involved the placement of a woven geotextile over the surface, which was anchored around the perimeter. The placement of finger berms (preloading fill) on the geotextile was successful without exceeding the bearing capacity of the tailings overall. Settlements of the berms were closely monitored to allow the system to support construction plant. After the finger berms were joined, they were widened until the area was covered. A sand layer was then placed over the area followed by a GCL to form an impermeable barrier prior to the placement of clay and topsoil.