Upstream construction methodology has been used to raise tailings dams in Western Australia (WA) for more than three decades, and the tailings storage facilities (TSFs) built in this manner have performed satisfactorily so far. The maximum design earthquake (MDE) for most of the existing, upstream-raised TSFs in WA was that corresponding to a 1-in-1,000 year annual exceedance probability (1:1,000 AEP). However, the recommended MDE loading for the High/Extreme Failure Consequence Category in the 2012 ANCOLD Guidelines on Tailings Dams is that of a 1:10,000 AEP. This more stringent seismic design criterion may restrict the use of upstream TSF construction in some areas of WA and Australia in general.
To evaluate the viability of upstream construction for a new or existing TSF, the effects of the earthquake design ground motion (EDGM) on the liquefaction and deformation response of the structure must be understood. The results of such analyses are an essential component in determining whether upstream raising will be feasible, or whether more robust but much more costly centreline or downstream construction methods are required.
A parametric study was completed to investigate the liquefaction and deformation behaviour of a typical, upstream-raised tailings dam under different earthquake design ground motions with different response spectra. The study utilized two-dimensional finite difference code FLAC2D effective stress dynamic analysis, in which the UBCSAND constitutive soil model was incorporated. Twenty-eight earthquake ground motions (matched and unmatched to the target response spectrum) were used in the study and the liquefaction response of the tailings dam model under those ground motions was analysed.
The results of the study demonstrate the importance of appropriate ground motion and response spectrum selection in assessing the seismic performance of an upstream-raised TSF. Liquefaction response was shown to vary with different response spectra, even though the corresponding EDGMs had similar peak ground acceleration (PGA) values. The importance of earthquake frequency content and duration, which in turn are affected by earthquake magnitude, distance and ground motion response, is emphasized. Scaling and matching the earthquake input motion to the uniform hazard response spectrum (UHRS) may result in overly-conservative design. Thus, selection of the most representative EDGM is essential to evaluating expected seismic performance for an upstream-raised TSF, and scaling or matching the earthquake input motions must be done cautiously.
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Nicole Anderson and Nihal Vitharana
A large number of aging concrete dams in Australia may not meet the requirements of modern dam safety practices. In addition, there is an ever-increasing demand for the supply of water. Continuous concrete buttressing is a method of strengthening existing dams which allows the dam to be raised to augment the storage capacity at an incremental cost.
This paper explores the key design considerations involved in concrete-buttressing existing concrete gravity dams. Critical aspects considered include storage level during construction, interface drainage, interface shear transfer, the relative strength of existing and new concrete and the behaviour during the heating and cooling phases of the heat-of-hydration. The discussions will be of relevance to asset owners and water authorities faced with upgrading existing dams in a time where there is an increasing demand for security of supply of water resources.
Nigel Connell, Karina Dahl, Steve Agnew and Brent Walton
The Waimakariri Irrigation scheme was initially built from 1997 to 2001 and irrigates approximately 18,000 hectares in North Canterbury with canals between the Waimakariri and Ashely Rivers. This was an enlargement from an existing stockwater scheme originally constructed in 1890. The owner and operator of the scheme, Waimakariri Irrigation Ltd, propose to construct a storage pond to supplement irrigation supply when take is restricted due to low flow in the Waimakariri River.
The footprint of the proposed pond is approximately 1 km x 1 km, with maximum dam height of 12 m and an 8.2 Mm3 maximum storage capacity. Accommodation for hydro-power development has been incorporated into the design of the irrigation storage ponds to provide multiple use of the reservoir contents.
The embankments are to be constructed from on-site granular material that forms the Canterbury Plains and lined with geomembrane. Careful consideration has been given in the seismic design for this High Potential Impact Classification (PIC) structure, which takes into account lessons from recent major earthquakes in the Canterbury Region. In addition, an understanding of the rapidly growing community downstream of the proposed dam has been crucial to ensuring that the potential risk of the dam is managed appropriately.
John Duder, David Bouma and Paul McCallum
The authors have been involved in the safety inspection and remediation of many older (pre-dating the 2004 Building Act) farm dams over the past decade coupled with considerable corporate knowledge from dams inspected by Tonkin & Taylor Ltd in its 50+ year history. This paper presents a summary of the varied benefits and risks of these older dams and the difficulties encountered in bringing them into alignment with current practice.
The many farm dams around New Zealand provide considerable benefit to the owners and often to the environment and wider community including the obvious stock water and irrigation, but also micro hydro, recreation, flood detention, release of environmental flows and flows for downstream users, and wetland habitat.
However, when applying current dam safety practice, and looking forward to the implementation of the Dam Safety Regulations, some of the older farm dams have significant dam safety issues that are often challenging to address. Although there is a high degree of variability, typical issues include:
Little or no documentation of geotechnical investigations, design or construction,
Design standards, particularly for spillway capacity have generally increased,
Little or no formal surveillance or maintenance carried out or recorded since commissioning,
Many farm dam owners have a poor understanding of their obligations under the Building Act and the Conditions of their Resource consents,
Consent conditions may not require dam safety related monitoring and maintenance, and/or the conditions may not have been historically enforced.
Many of these farm dams have been constructed by small contractors at the request of the farmers, often with only “standardised” engineering design and little specific geotechnical investigation. Typically there are no as-built records and the dam owners have been left with a general lack of understanding of owner’s responsibilities to monitor and maintain the dam.
Given that there are often very limited funds available for upgrade work, it has proved important to apply sound engineering judgement and a high degree of pragmatism to realise the greatest possible reduction in dam safety related risk for the available funds. Good cooperation between the Regional Authority, the Building Consent Authority for dams (often they are different organisations), the dam owner, and the dam engineer, together with a pragmatic approach is vital in moving toward current best practice for management of these dams.
Case studies are presented for the Northland Region, where the farm dams are typically homogenous earth fill dams in the order of 8 to 12 m high, fulfilling functions as irrigation, stock water supply, recreation and flood detention structures. The findings are considered relevant to earth fill farm dams across the country.
Peter Hill and Rory Nathan
The ANCOLD Acceptable Flood Capacity (AFC) guidelines were published in 2000 and provide guidance on the selection of design flood capacities for dams and specifically a deterministic fallback provision for spillway capacities. Since the guideline was published, there has been a continual evolution in dam safety management practices and related guidelines, including the 2003 ANCOLD guidelines on risk assessment and the current revision of Australian Rainfall and Runoff by Engineers Australia. This paper describes the scope of the current AFC guidelines and perceived opportunities for refinement. A survey of users was used to test and identify issues and gauge the need for the guideline to be updated. A number of topics were identified that would benefit from clarification or further guidance. These topics include consistency with other ANCOLD guidelines, clarity on the selection of the AFC, definition of the dam crest flood, freeboard and application to gated structures.
J.P. Giroud, Neil Jacka, Christopher Dann and Jeremy Eldridge
The remediation of a large hydropower canal included the lining of selected reaches of the canal with a geomembrane to extend the life of the canal and enhance seismic resilience. This paper presents a summary of innovative analyses performed to select and design the geomembrane liner system. Two mechanisms that induce tensile stress and strain in the geomembrane following the development of cracks in the supporting subgrade resulting in the deflection of the geomembrane over the cracks under the applied water pressure were analysed. The analysis uses the concept of ‘co-energy’, a geomembrane property that evaluates its ability to withstand stresses and strains together. A range of ballast configurations undertaken to assess the tension, strain and deflection of the geomembrane while evaluating the resistance to hydrodynamic forces and other loads were analysed. Stability analyses showed that geosynthetic reinforcement of the ballast over the upper canal slopes was required.
Keywords: Canal, Lining, Geomembrane, Design, Seismic resilience.