M. B. Barker and D. Holroyde
A detailed study was completed for the Stage 2 works of the Grahamstown Dam augmentation to investigate various alternatives for the slope protection of the Saddle Dam and Subsidiary Dam embankments, including a standards based and a risk management approach. The standards based approach required an evaluation of the slope protection level and least cost option based on the hazard rating of the dam. Due to the sand construction of the embankments, it was possible to apply a wave erosion model SBEACH to develop an economic risk model for optimising the slope protection alternatives. The erosion model included the effects of the wind direction, reservoir level and wind speed variation during flood events, embankment profile and material parameters. The risk management approach clearly showed that significant cost savings could be achieved by using the risk management approach. Furthermore, the cost curves indicated the sections of the embankments for which present capital works would not be economically justified and for which ongoing maintenance works would be economically advantageous.
Now showing 1-12 of 27 2964:
P.J. Ritchie and N.A. Currey
Kidston Gold Mines commenced operations in 1984 and built a dam to safely store the tailing waste from the ore processing. The dam was progressively raised 5 times (3 downstream and 2 centreline lifts) and has an active surface area of 310 hectares; stores 66 Mt of tailing and is 32 metres high at its maximum height. The dam was decommissioned in September 1997.
Rehabilitation planning for the tailing dam commenced in 1994 with an 11 hectare direct revegetation trial established in March of that year. A 40 ha trial was established in 1998. Both sites have been the subject of intensive scientific research by the (University of Queensland) Centre for Mined Lands Research group. This research assisted in understanding the issues of revegetation stability and sustainability, biological cycling, soil chemistry and surface erosion.
The aims of rehabilitation is to meet the Queensland Department of Mines and Energy (DME) key closure criteria. These include; creating a stable landform, not only for the dam wall structure but also of low surface erodibility, maintenance of acceptable downstream water quality by controlling poor quality seepage and runoff and by meeting an acceptable final end land use criteria for the structure.
Ongoing research is addressing the long term hydrology of the tailing dam with an aim towards understanding the overall water balance. Three consulting groups are involved in what is considered to be a novel approach. Evapotranspiration rates from pasture and tree species have been measured during the 1999 wet and dry season. This information, along with climatic and soil suction data is then used as one of the key parameters for the unsaturated zone modeling. One output from the “Soilcover” model is seepage into the saturated zone in the tailing dam. Water movements in the saturated zone are being modelled using Modflow. The acid oxidation potential for the dam is also being evaluated in light of the long term water movements in the saturated and unsaturated zones of the dam. This process will allow short and long term prediction of dam seepage quality and quantities.
The geotechnical stability of the final dam wall structure as defined by the Factor of Safety, ranged from 2.0 to 2.3, which meets the long term DME recommended stability target FOS of 1.5 for slopes.
In order to evaluate the impact of metal toxicities in grazing cattle, a grazing trial has been established on the pasture covering the surface sediments of the tailing dam. This work is being supported by the Qld EPA, Qld DME, Qld Health and the NRCET, and will assist in understanding metal uptake in grazing animals on rehabilitated mined lands.
Richard R. Davidson, Roger Vreugdenhil and Mark Foster
Significant cracking was observed on the crest of the main embankment at Lake Eppalock for many years, but in recent years increasing movement upstream during low reservoir levels indicated a progressively deteriorating stability situation. Investigations also revealed cohesive filter material that would allow a crack to propagate. A fast-tracked remedial works program was completed in 1999 to rebuild the highly vulnerable upper rockfill shells and filters, both upstream and downstream. To manage construction risk, the works were carried out directly by G- MW with innovations in removal, protection and replacement of the downstream shoulders, and placement of a new multi-zone filter.
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.
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.
Glen Hobbs and Danny Azavedo
Recent years have seen a growing awareness and understanding of the factors that contribute to the reliability of spillway gates and the incorporation of reliability data into overall dam risk studies.
The study of a number of spillway gate failures shows that no one component or incident leads to gate failure, but rather a combination of factors have resulted in gate failure. A rigorous reliability assessment should consider all factors, not only the equipment condition and performance but the complete system, from the receipt of data through to the actuation of the gates. It should take into account issues such as human factors, poor design, maintenance history and policy. Unfortunately one of the main hindrances to quantifying gate reliability is the lack of information on spillway gate equipment and system performance and failures.
This paper considers a number of gate failures, then looks at some of the tools of reliability assessment and the role of human factors in gate reliability.
The paper then discusses a recent study of four gated dams. For this study a systems approach was adopted and human factors were considered. The results compare favourably with other similar critical structures, and show that for these well designed and maintained structures human factors are the limiting criteria in multiple gate operations. The study also shows that the probability of opening all the spillway gates at a dam improves with time (2-4 hours) during the flood operation, and it is considered that time based reliability provides a more meaningful and useful assessment of overall spillway gate reliability.