N. Vitharana, G. McNally, C. Johnson, A. Thomas, K. Dart and P. Russell
Millbrook Reservoir is an offline storage with an earthen embankment dam containing a puddle clay core and a moderately sized upstream catchment. The dam is 31m high and has a capacity of 16.5 GL when the storage water level is at the Full SupplyLevel (FSL). The reservoir is 25km NE of Adelaide on Chain of Ponds Creek, a tributary of the River Torrens. The dam was constructed during the years 1914-1918. Earthworks were carried out only during summer as the five winters during the construction period were very wet.
Dam safety reviews and geotechnical investigations, undertaken between 2001 and 2004 by SKM, showed that these winter recesses would have created weak layers, increasing the potential for piping due to the lack of a filter. This was highlighted by the large deformations which occurred at the end of construction in 1918. The spillway was assessed as able to pass a flood event with AEP of 1:1,300,000. Given the location of the dam, ANCOLD(2000b) Guidelines suggest the dam should be able to safely pass the PMF flood event. Accordingly, the dam required upgrading to modern guidelines.
The 2005 detailed design of the upgrade included the construction of a 70m wide unlined spillway, construction of filters on the downstream face of the dam with a stabilisation (weighting) fill, installation of instrumentation and seismic protection of the outlet tower. The construction of these works is currently underway.
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B Simmons, N Mudge
In 2004 the NSW Government released its Metropolitan Water Plan (MWP). This plan detailed the government’s initiatives to secure Sydney’s water needs during the current drought and into the future. The MWP outlined a range of both demand and supply side measures. These included modification to Warragamba and Nepean dams so that the water at the bottom of the dams that is currently unavailable for water supply can be accessed.
Accessing this deep water will increase the available water supply by an additional six months in the immediate drought and will provide, on average, an additional 40GL/annum to our long term available water supply.
The Warragamba Dam Deep Water Access Project involves accessing and transferring water from deep in Warragamba Dam to the existing water supply system.
Phase One of the project saw an abandoned underground pumping station 1.5km downstream of the dam wall, being enlarged and upgraded to pump water from the low level pipeline into the existing water transfer pipelines.
Phase Two of the project involved making a penetration low on the dam wall, some ninety metres below full storage level to access the deep water. This enabled the water to flow into the new pumping station, through an existing underground pipeline.
This project and in particular Phase Two was extremely unique due to the saturation diving systems and specialist tooling systems needed to create the penetration in the dam wall. The project provides a reference point for the water industry for future similar works.
This paper describes the project that was initiated at Warragamba Dam to access the deep water and is focused on the extremely difficult and unique works associated with creating the low level penetration in the dam wall.
Joseph Matthews, Dr Mark Foster, Michael Phillips
Pykes Creek Dam is a 39m high earthfill dam with a central clay puddle core, first completed in 1911 and raised in 1930. A detailed risk assessment of the dam indicated that the risk did not satisfy ANCOLD societal risk criteria and that remedial works were necessary to address piping deficiencies and inadequate flood capacity. The risk assessment identified that piping at the embankment/spillway interface accounted for over 80% of the total risk. Therefore, interim risk reduction works were implemented in 2005 to address this risk issue while investigations and design studies were progressed for the second stage of works. Following the Stage 1 works, Pykes Creek Dam remains the highest risk in Southern Rural Water’s portfolio of dams and Stage 2 works are planned to commence in 2007 to reduce piping risks and increase flood capacity. The aim of the Stage 2 works is to reduce the risk below the Limit of Tolerability for Existing Dams (ANCOLD 2003) and to increase the flood capacity to a level more appropriate for an Extreme consequence category dam based on ALARP principles. The upgrade will stop short of meeting the PMF as there are other dams in Southern Rural Water’s portfolio requiring attention before an upgrade to this standard would be considered. The design of the works was complicated by the fact that the dam is bisected by a major freeway and has a complex spillway layout. This paper discusses the decision-making process and the methods used to analyse the dam from the initial risk assessment studies through to the design of the remedial works.
Internal erosion and piping within embankment dams may initiate in cracks caused by differential settlement or desiccation, in cracks caused by hydraulic fracture and in very poorly compacted layers of soil. It generally cannot occur unless one of these defects is present because backwards erosion, the other mechanism for internal erosion, will not occur in embankments under normal gradients and will not occur in cohesive soils unless gradients are exceptionally high.
As a result it is very unlikely that it will be possible to detect initiation of erosion with piezometers, and the most likely successful method is seepage observation and monitoring. However the time from the first detection of increased seepage to breach of the dam may be very short-a matter of hours in some situations.
Thoughtfully positioned and read piezometers are more likely to be successful in identifying the critical gradients which may lead to the onset of backwards erosion in cohesionless soils in the foundation of dams.
Piezometers are more useful in establishing the pore pressures for use in analysis of stability, but in most cases where stability is marginal undrained strength analysis is required and the pore pressures and effective strengths alone are not sufficient to assess stability. In a number of cases differential settlements, and acceleration of settlements have proven valuable in detecting the on-set of instability and the conditions in which internal erosion and piping to initiate. Once these conditions are recognised more detailed survey monitoring and borehole inclinometers can be valuable in better defining the geometry of instability.
Dr. J. M. Rüeger
After a brief review of the origin and early days of the technique, the present role of geodetic deformation measurements is discussed. The design of geodetic measurement schemes is then considered, followed by a review of geodetic measurement, analysis and reporting techniques. An overview of the important discussions, that need to take place between engineers and surveyors in the design phase, follows. This covers the definition of the engineering needs and the resolution of surveying issues.
David M. Schaaf, P.E., Jeff Schaefer, Ph.D., P.E., P.G
The United States Army Corps of Engineers (USACE) has an inventory of over 600 dams. The main purpose of many of these dams is for flood control, but there are a significant number of dams primarily used for navigation. Additional benefits at many of these projects are provided through hydropower generation, recreation, and irrigation for farmers. Many of the dams are quite old and represent an aging infrastructure across the inventory. In addition, leaner budgets relative to the need for repairs across the aging system require that USACE invest wisely in order to efficiently use available funds to reduce the greatest risks across the inventory. Previously, individual projects with perceived deficiencies were evaluated separately by the responsible district. This evaluation was not compared in any programmatic way to other USACE dams being evaluated for deficiencies.
In order to improve the process of making risk-based decisions across the entire spectrum of USACE dams, the Screening for Portfolio Risk Assessment (SPRA) for the USACE Dam Safety Program was initiated during the summer of 2005. This effort represents the first level of a multiple phased effort to bring full scale risk assessment to the decision-making regarding making investment decisions associated with dam safety by linking engineering reliability with economic and life loss impacts on a relative scale. The SPRA effort involved the development of a tool for evaluating the relative life and economic risk of dam failures for a variety of deficiencies across the inventory of USACE dams. This paper will focus on the basic aspects of the evaluation tool as well as the process by which the screening was completed.