Bruce Walpole and Craig Scott
Monitoring and surveillance is crucial to managing the ongoing performance of dam structures.
The true value of appropriate monitoring, surveillance and review processes is only realised when
potential dam safety issues arise. TrustPower’s civil safety monitoring and surveillance program
includes nineteen hydro schemes throughout New Zealand and incorporates structures with
Potential Impact Classifications (PIC) ranging from Low to High.
TrustPower promotes a continual improvement policy on its management of safety issues and
conducts inspections on a regular basis. Routine and periodic independent inspections of the key
components within a scheme are paramount to the viability of the safety management system. The
importance and purpose of these inspections has recently been highlighted by the discovery of two
sinkholes on the face of the earth dam associated with the Cobb hydro electric power scheme.
This paper provides an example of the need for continual monitoring and surveillance, vigilance
of observations, good archiving systems and documentation. It discusses the broader issues
surrounding the subsequent response processes to potential dam safety deficiencies, and the
success (or otherwise) of investigative methods. It also highlights that an adequate dam safety
compliance system has commercial value as there is a measurable reduction in dam performance
uncertainty and hence greater efficiency in the speed at which accurate resolutions can be drawn.
Keywords: Dam safety, embankment, sinkholes, foundations, dam drainage, geophysical
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Now showing 1-12 of 17 2971:
Stuart Read and Laurie Richards
Many dams in New Zealand are founded on greywacke, a typically hard, closely-jointed rock mass. This paper describes the characteristics of greywacke rocks based on field mapping, laboratory testing and rock mass classification, and gives examples of design inputs for dams, in particular concrete structures. Unweathered, intact rock materials have unconfined compressive strengths generally above 100 MPa and moderate to high modulus ratios. The rock masses, which comprise sandstones and mudstones, are commonly tectonically disturbed and have an unusual combination of very high intact strength and joints with low persistence. The effect of these properties on rock mass deformability and strength is illustrated by estimation of dam foundation deformability from tiltmeter measurements and assessment of critical foundation failure mechanisms from estimates of defect and global rock mass strengths.
Keywords: foundations, dam design, rock mass strength, rock mass deformability, greywacke
Paul Hurst, Tom Ewing, Steven Fox and Bob Wark
For an ogee-shaped spillway crest, it is well recognised that sub-atmospheric pressures will develop on the lower-nappe profile for operating heads greater than design head. This effect is useful in providing an increase in efficiency of the spillway discharge for small increases in operating head. However, there is limited data on the formation of sub-atmospheric crest pressures for high-head operation above 1.3 times greater than the design head
This paper reports on modelling work done by GHD and the Water Corporation for the Wellington Dam Remedial Works Project in Western Australia where the current design flood has increased to more than twice the original design head. Two-dimensional physical scale modelling and 3-D Computational Fluid Dynamics (CFD) modelling of the existing Wellington Dam spillway profile was carried out to determine the discharge coefficient and uplift force generated by the formation of sub-atmospheric crest pressures under high-head operation.
The paper compares the results of the physical scale model and the CFD model and earlier published data by Cassidy (1970) and concludes that there exists a good correlation between the three data sets.
Keywords: Ogee, sub-atmospheric, crest pressures, Wellington Dam
Nigel Connell, Tim Logan and Tim Mills
Leakage from Tekapo Canal, between km 11 and 12, is investigated. A groundwater model is formulated based on construction records, detailed monitored seepage flow and groundwater levels in the canal embankment over the 30 year life of the canal, chemical analysis and flow history. Sieve analysis of embankment materials confirmed embankment fill was sourced from glacial outwash graves excavated from canal cut upstream. Anisotropic permeability of the fill embankment, inferred from the construction method using motor scrapers and vibratory rollers, contributed to explaining inflow to the model primarily from a source up to 500 m from the leakage outflows. Stability of the canal embankment is reassessed considering length of the seepage paths, which are long, hydraulic gradients, which are relatively flat, and resistance of the glacial outwash gravels to piping. The groundwater model that is developed indicated that stability of the canal embankments is not reduced significantly due to the seepage.
Keywords: Tekapo canal, groundwater model, canal leakage.
David Ho, Chee Wei Tan and Glen Dominish
Upper Cordeaux Number 2 Dam is founded on an igneous intrusion rock mass which overlays sedimentary rock layers above the Wongawilli Coal seam. The coal mining company, BHP Billiton Illawarra Coal, planned to extract coal close to the dam. Although the dam is classified as a low hazard dam, its importance, both as part of the Sydney Catchment Authority’s water supply system and for its significant heritage value, mean that the proposed mining should not have undesirable impact on the structure. This paper describes how the mining impact on the dam was assessed using a nonlinear 3D finite element model. The model considered the pre-existing cracks in the dam wall, uplift water pressure along the dam/foundation interface and the hydrostatic pressure at full supply level. Mining-induced movement such as valley opening, closure and upsidence were applied to the model. Stability and strength assessments were made against a set of acceptance criteria developed for mining impact. The development of different stabilising mechanisms was examined. From the numerical investigation, WorleyParsons was able to provide technical advice to the mining company, the dam owner and the NSW Dam Safety Committee to facilitate the mining application and to satisfy dam safety requirements.
Keywords: Mining subsidence, Arch/gravity dam, Nonlinear numerical analysis, Safety assessment
Appurtenant structures associated with a dam play and important part to the dam’s operation. For these structures it may be important that their functional and structural integrity is retained in the event of a notable earthquake, particularly when they are required to release water from the reservoir in a controlled manner to lower the storage following an earthquake. Research has been conducted into the current state of practice for the seismic design and analysis of these structures, including review of the main issues for seismic effects, documentation of case histories and review of current research, international guidelines and standards. The general assessment philosophy was found to be relatively consistent internationally, however, the adopted assessment procedures were found to vary. The status of the current ANCOLD earthquake guidelines has been provided in relation to the current international state of practice for various types of appurtenant structures.
Keywords: Appurtenant structures, performance criteria, seismic performance, seismic analysis.