Mohammad Okhovat, Viculp Lal, Neil Sutherland
The precast, prestressed concrete penstocks at Meridian Energy’s Benmore power station in New Zealand have attracted attention since construction about 50 years ago because of their unusual design. They are listed as the world’s first prestressed penstocks. However, their seismic capacity has been determined to be insufficient when measured against Meridian’s current asset management objectives aimed at avoiding significant damage to generating assets in a 1:2,500 year AEP earthquake. The deficiency is mainly due to the relatively narrow base width of the penstocks.
In this study, a series of linear analyses was performed to obtain an improved understanding of seismic behaviour of the penstocks. Various strengthening solutions are under consideration for the penstocks to meet the acceptance criteria. Additionally, nonlinear analysis of the penstocks was carried out to investigate the use of seismic damping devices fitted to the penstocks, similar to damping applications in seismic response control of buildings and bridges.
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Alan P. Jeary, James O’Grady, Thomas Winant
Mainmark are introducing the STRAAM system of full scale non-destructive testing for dams into Australia and New Zealand. Advances in measuring extremely low amplitude vibrations combined with methods for extracting the unique dynamic signature have now enabled the rapid measurement of the response of earthen and concrete dams. This ability allows the quick calibration of Finite Element Models that can be used to accurately assess the strength of a dam. Furthermore, this information allows dam owners to efficiently track changes in the capacity of their dams due to aging, earthquake or flood activity through changes in the dynamic.
The STRAAM system measures the vibration of the dam structure to establish the natural frequencies, mode shapes and associated damping ratios of the dam. The field measurements are correlated with a three-dimensional finite element model to fine tune the effects of abutments and foundations on the three dimensional model. Because of the sensitivity of the instrumentation and the novelty of the analysis techniques, the information available to dam managers allows information-based decisions to be made in a way that optimizes the financial implications. In addition, the techniques are non-invasive and non- destructive and they give additional information about the connectivity of the dam with the surrounding terrain, and whether that connectivity is compromised by water seepage.
This paper discusses the results obtained from field measurements from four dams located in Switzerland, USA and Scotland.
Mark Arnold, Gavan Hunter and Mark Foster
Following the dam safety risk assessment for Greenvale Dam in 2008, Melbourne Water implemented a 3.0 m reservoir level restriction on the operation of the storage as an interim risk reduction measure. The 3.0 m restriction coincided with the ‘as constructed’ top of the chimney filter in the main embankment. This interim action reduced the dam safety risk to below the ANCOLD limit of tolerability.
Dam safety upgrade works were undertaken in 2014/15 to bring the dam in-line with current risk based guidelines and to enable the removal of the interim reservoir restriction, bringing the storage back to full operating capacity. Greenvale Dam was required to remain operational throughout the works and this required careful consideration of the dam safety risk during construction.
Deep excavations were required within the crest and downstream shoulder of the embankments, that,, without adequate management, had the potential to increase risk to the downstream population. Excavations up to 18 m depth were required into the wing embankments for construction of full height filters from foundation to crest, excavations up to 7 m deep were required in the main embankment to expose and connect into the existing filters and secant filter piles up to 13 m deep were used to connect the new chimney filter of the wing embankments with the original chimney filter of the main embankment.
A key element of the design and construction of the upgrade works was managing dam safety during construction. Dam safety considerations included (i) design based decisions to manage the level of exposure; (ii) implementation of further restrictions on reservoir level by the owner Melbourne Water; (iii) construction methods to manage exposure; (iv) an elevated surveillance regime during the works and (v) emergency preparation measures including emergency stockpiles and 24 hour emergency standby crew. The construction based dam safety requirements were focused on early detection and early intervention, and were managed via the project specific Dam Safety Management Plan.
This paper focuses on dam safety management including the decisions made, actions taken and construction requirements and touches on how these relate to the key project features.
David Scriven, Lawrence Fahey
Paradise Dam is located approximately 20 km north-west of Biggenden and 80 km south-west of Bundaberg on the Burnett River in Queensland. The dam was designed and constructed under an alliance agreement with construction completed in mid 2005. It is a concrete gravity structure up to 52 m high, the primary construction material being roller compacted concrete (RCC).
In January 2013 the flood of record was experienced at the dam with a depth of overflow on the primary spillway reaching 8.65 m following heavy rainfall in the catchment from ex-tropical cyclone Oswald. The peak outflow was approximately 17,000 m3/s. This equated to a 1 in 170 AEP flood event. When the flood receded it was discovered that the dam and surrounds had suffered severe damage in a number of locations including: extensive rock scour downstream of the primary dissipator and the left abutment, damage to portions of the primary dissipator apron, and the loss of most of the primary dissipator end sill.
SunWater initiated a staged remediation program to manage the dam safety risks and by November 2013 had completed the initial Phase 1 Emergency and Phase 2 Interim repairs. Phase 3 of the program was to implement a comprehensive Dam Safety Review (DSR) and a Comprehensive Risk Assessment (CRA). The DSR became arguably the largest ever undertaken by SunWater and included: extensive geotechnical investigations, large scale physical modelling, numerical scour analysis, stability analysis, and an extensive design assessment. This paper describes some of the key aspects of the DSR undertaken related to the flood damage.
Kristen Sih, Richard Rodd
Melbourne Water currently manages over 235 stormwater retarding basins. The process of assessing the risk posed by these assets began in 2006, and at the end of 2015 full risk assessments were completed for around 30 of the basins that were estimated to pose the highest societal risk. However, when analysing the results of these risk assessments, there was some concern that the results were inconsistent and often too conservative, given the few incipient or actual failures that had been experienced.
It was found that one of the key areas causing the conservatism was poor documentation of design and construction details, and the fact that the tools used for assessing the Potential Loss of Life (PLL) were aimed at larger storages that cause much higher depths and velocities in dambreak events than these (generally) small storages. To remedy this situation, advice was sought from specialist practitioners to develop guidance notes on the assessment of PLL and failure likelihoods for retarding basins.
On the back of these guidance notes, Melbourne Water initiated an accelerated program of assessing the risk associated with 78 retarding basins over a 6 month period. This paper describes the key recommendations from the guidance notes, compares the results of the risk assessments performed pre- and post-guidance notes and provides a summary of the portfolio risk assessment outcomes, what they mean for Melbourne Water and what the organisation intends to do to manage this risk into the future.
Woodrow Lee Fields
Although flooding can lead to many types of severe consequences, the primary objective of the US Army Corps of Engineers (USACE) dam and levee safety programs are to manage risk to the public who rely on those structures to keep them reasonably safe from flooding. Thus, reducing the risk associated with loss of life is paramount. This paper discusses new methods that have been developed for estimating life loss with uncertainty from flood events.
HEC-LifeSim is a dynamic simulation system for estimating life loss with the fundamental intent to simulate population redistribution during an evacuation in conjunction with flood wave propagation. The population redistribution process has been revised from the ground up as an agent based model. In addition to the agent based model, uncertainty analysis has been enhanced. Through Monte Carlo sampling, the natural variability of warning and mobilization timing and likelihood of fatality varies delivering a range of potential life loss from a hazard. Knowledge uncertainty about parameters, such as warning issuance time, can also be defined. To accommodate the new HEC-LifeSim computation engine, an innovative GIS interface has been developed to quickly summarize and animate results. The methods that are discussed in the following provide new tools to estimate life loss and educate local authorities.