Susantha Mediwaka, Nihal Vitharana, Badra Kamaladasa
Nalanda dam is the oldest concrete gravity dam on the Island built in the 1950s by the Ceylon Department of Irrigation. The dam was built in 9 monoliths having a dam crest length of approximately 125m and a maximum height of about 36m. The spillway consists of: (1) a low-level uncontrolled ogee-crested horse-shoe section with a crest length of 46m, and (b) a high-level broad crested weir with a crest length of 43m.
It was designed and constructed according to the then standard practices adopted throughout the world. Over the years, Nalanda dam has been showing signs of deterioration which is suspected to be Alkali-Aggregate Reaction (AAR). The dam was also shown to be deficient with respect to the stability levels required by modern standards. Under a program of dam safety improvement of the dams throughout Sri Lanka, it was decided to stabilise Nalanda dam as the first step in addressing a series of issues affecting the dam.
This paper presents the construction history, current issues, design assumptions and salient construction features in the upgrading of the dam to modern dam safety requirements.
Keywords: Concrete dams, dams Sri Lanka, concrete buttressing, upgrade, horse-shoe spillway
Mark R. Sinclair & Richard J. Rodd
Over the last six years there have been ongoing significant developments in the design, fabrication and particularly of the corrosion protection details for high capacity ( >13,500kN MBL ) re-stressable ground anchors used to improve stability of gravity dams. These Australian based developments and the resultant specifications and details have now become the de-facto standards adopted.
The ANCOLD Register dams to have had this generation of cables installed have included; Ross River Dam, Lake Manchester Dam, Catagunya Dam, Tinaroo Falls Dam and Wellington Dam. These projects include the highest capacity permanent ground anchors installed to date worldwide. Some smaller capacity anchors installed into dams have also benefited from this technology.
The Recent Developments and Application of Large Ground Anchors for
Craig Johnson, Mark Arnold
Toorourrong Reservoir is a small storage reservoir which was constructed in 1885 and forms an important part of Melbourne’s water supply network. As part of Melbourne Water’s dam safety upgrade program, remedial works at Toorourrong Reservoir were identified to address deficiencies in flood capacity, embankment stability and to provide protection against piping. These works included an engineered filter system, downstream stabilising berm and raising of the dam crest level by 2.3m through a combination of earthfill and a concrete parapet wall. The existing spillway also required substantial enlargement and the existing scour and outlet structures were to be reconfigured. These works were designed and undertaken by the Water Resources Alliance (WRA).
Preliminary geotechnical investigations indicated the dam was founded on soft alluvial deposits, with the potential for foundation liquefaction under earthquake loading. During the course of subsequent investigations, the full complexity of the dam foundation was realised using numerous techniques including geophysics, CPT
u probes and seismic dilatometer testing. The results of these investigations were used to develop a detailed geotechnical model and embankment design sections. A range of analytical methods were utilised to characterise the liquefaction potential of the foundation, with these making reference to recent developments in this area of practice. Through an extensive assessment and review process, the design soil properties for the foundation were established and the liquefaction potential determined.
Based on these assessments, it was found that the potential for liquefaction existed across the majority of the dam foundation, with discrete soil layers liquefying depending on the intensity of the design seismic event. Strain-weakening (sensitive) soils were also identified in the foundation. A quasi risk-based stability assessment was undertaken for a range of post-liquefaction strength parameters and FoS to determine the sensitivity of the foundation response. Stability analyses were performed which indicated that additional stabilising berms were required at several locations. However, even with these berms, the extremely low post-liquefaction strengths meant that further ground improvement was required. This was assessed further and Grouted Stone Columns (GSC) were ultimately selected as the preferred foundation improvement method for the critical design sections with GSC to be installed both upstream and downstream to reinforce the dam foundation. This is the first time GSC have been used in Australia and some key “lessons learned” will be discussed.
2011 – Toorourrong Reservoir – Small Dam, Big Problems
Rob Campbell, Tom Kolbe, Ron Fleming, Christopher Dann
Hinze Dam is an Extreme hazard category water supply dam situated in the Queensland Gold Coast hinterland, owned and operated by Seqwater (formerly owned by Gold Coast City Council). The Hinze Dam Stage 3 works involved raising the previously 65m high central core earth and rockfill embankment approximately 15m to a maximum height of approximately 80m.
The Stage 3 works included a program of foundation curtain grouting, consisting of six discrete grout panels, five of those beneath areas where the embankment was extended and one beneath part of the spillway enhancement works. Five of the six grout panels were essentially single row panels, with one or more partial rows added in specific areas of high grout take. The remaining grout panel (Panel 4) was constructed as a triple row panel.
A number of challenges were encountered and overcome during the Stage 3 foundation grouting works due to highly variable foundation conditions, ranging from extremely low strength residual soil to highly fractured and permeable high strength rock.
The grouting works were undertaken using downstage grouting techniques, with manual recording of data, manual control of grout pressures and injection rates and use of predominantly neat cement grout mixes.
A key issue in the execution of the foundation grouting works was the maximum grout pressures applied to the foundation and this was discussed in detail between the project design team and external review panel. This paper presents the results from project specific grout trials and production grouting to demonstrate that closure of the foundation was consistently achieved (with one exception discussed herein), which supports the grouting approach employed and the adopted grout pressures.
This paper presents a case study description of the Stage 3 foundation curtain grouting works, including a summary of key learnings which may be of benefit to future dam foundation curtain grouting projects.
The enlargement of the Cotter Dam is being undertaken by ACTEW to provide a greater security of water supply to Canberra. The project involves constructing a larger, higher new dam wall immediately downstream of the existing Cotter Dam, to allow the present dam to continue functioning and supplying water while construction is underway. The project raised a number of environmental issues partly because the Cotter Dam currently supports a self-sustaining population of (endangered) Macquarie Perch, and because the Bendora Dam, upstream of Cotter Dam, contains a breeding population of (endangered) Trout Cod. Bendora Dam will not be physically affected by the works on Cotter Dam, but its operations may be altered. An ecological risk analysis was conducted to identify critical environmental risks that would need to be investigated and managed or ameliorated and management strategies were put in place to reduce risks. ACTEW have adopted an adaptive management approach to the project, but in order to implement that approach it is necessary to conduct effective monitoring of the fish populations of concern. These potentially include the two endangered species, as well as potential predators (such as cormorants) and competitors (such as trout). Power analysis has been used as a tool to evaluate whether it is feasible to monitor key populations sufficiently rigorously to be able to confidently detect a change (either an increase or decrease in a population). For Macquarie Perch and trout it should be possible to detect population changes statistically with a logistically feasible monitoring program.
2011 – Using risk analysis, power analysis and adaptive management to minimise ecological impacts of the Cotter Dam enlargement