Paul Maisano, Water Storage Team Leader for GHD in Victoria, Australia; and Angus Paton, Manager of Asset Management at the South Australian Water Corporation, give details about the upgrading of Kangaroo Creek Dam.
Kangaroo Creek Dam is located on the Torrens River approximately 22km north east of Adelaide, South Australia. It was originally constructed between 1966-69 to provide additional storage for Adelaide's metropolitan water supply, and was later raised and the spillway upgraded in 1983 for the purpose of providing additional flood storage and to reduce flood outflows. The dam is owned and operated by the South Australian Water Corporation (SA Water).
Kangaroo Creek Dam is a concrete face rockfill dam with a side channel spillway as shown in Figure 1. Also included in the design were a grout curtain in the foundation, an inclined reinforced concrete tower with outlet pipework through a tunnel on the right abutment (originally part of the diversion works) and a seepage measuring weir. The dam is currently 63.5m high with a crest length of 138m.
In 1983 the dam was modified to provide a flood mitigation function by reducing the full supply level with the inclusion of two low flow ducts in the original spillway structure and a raising of the ogee crest level on the spillway (see Figure 2). The embankment crest level was also raised through the installation of a reinforced concrete U-shaped structure (see Figure 3).
Repairs were undertaken in 2001 on some of the upstream concrete face slab joints.
Previous safety reviews and a risk assessments identified that the dam does not comply with modern safety standards with respect to flood capacity and structural strength. The dam may be vulnerable to rare strong seismic loading that may cause the perimetric joint and some vertical joints in the upstream face concrete slab to open up below the reservoir level. This would result in leakage into the dam rockfill. Considering the zoning of the dam and the materials used for construction, these leaks might result in saturation of the rockfill and instability and/or unravelling of the downstream slope and failure of the dam. Current Australian National Committee on Large Dams (ANCOLD) guidelines require the dam to be able to safely pass the Probable Maximum Flood.
GHD was engaged by SA Water to design upgrade works to ensure the dam meets modern standards and complies with relevant ANCOLD Guidelines by providing increased spillway capacity, improved resistance to extreme seismic loading and increased drawdown capacity through the outlet works. The scope of GHD's engagement includes all required investigations and a spillway physical model study.
The main components of the upgrade works include:
- Raising the embankment height from 63.5m to 69.4m.
- Extending the concrete face slab.
- Providing a full height downstream rockfill berm with suitable transition zones.
- Upgrading the vertical and perimetric joint waterstops.
- Widening the spillway chute.
- Extending the eastern ogee crest structure by 40.2m.
- Raising the spillway walls.
- Anchoring the existing spillway ogee crest structure.
- Modifying the outlet works to accommodate the embankment raise and construction of a new seepage measuring weir.
The key challenges faced by the designers have been:
- Competing requirements to provide a stabilising fill for stability in the event the downstream rockfill becomes fully saturated, and to provide an internal drainage system to reduce potentially high seepage gradients to prevent unravelling of the downstream rockfill;
- Unfavourable conditions in the spillway training wall foundations for post tensioned anchors, requiring the design to proceed on the basis of raising the walls using gravity sections, with resulting impacts on flow within the spillway chute; and
- The requirement to maintain the existing Dam Crest Flood capacity at all times during the works which has required the design of three separate cofferdam arrangements.
Geotechnical Investigations
Kangaroo Creek Dam is situated on the western side of the Mount Lofty Ranges which form part of a major mountain chain, including the Flinders Ranges, which extends some 800km inland from the coast near Adelaide. The Kangaroo Creek Dam and reservoir area are underlain by two principal lithologies:
- Schist, quartz-sericite-feldspar-chlorite, mainly fine grained with a prominent flaky foliation; and
- Gneiss, quartz-feldspar-sericite, medium to coarse grained, with a prominent flaky foliation, including massive and minor masses of granitic gneiss.
Sandiford (2003) describes the Mount Lofty and Flinders Ranges in South Australia as one of the most seismically active parts of the Australian continent. The Mount Lofty Ranges are bounded by a set of discrete, curvilinear scarps, with four faults on the western range front and three faults on the eastern side of the ranges. It is suggested that these faults have experienced significant Quaternary displacement.
As part of the design work, GHD undertook geological mapping and drilling of eight inclined diamond cored boreholes (total drilled length 255.4m) which included in-situ permeability testing. The results of the borehole drilling indicated unfavourable conditions in the spillway training wall foundations for post tensioned anchors, requiring the design to proceed on the basis of raising the walls using gravity sections. The boreholes also informed the design of the batter slopes for the spillway widening.
Embankment raising
The existing embankment consists of five zones (Zones 1 to 4 are rockfill and Zone 5 is a small amount of alluvium left in place in the central valley section). It has a downstream face batter slope of 1.4H:1V. The embankment raising will require adding a less steep stabilizing berm on the downstream face, removing the existing U-shaped concrete section from the crest and raising the crest level by increasing the embankment height and extending the upstream concrete face slab to the design level, thus providing a freeboard of approximately 1m for the PMF.
The embankment design requirements are based on a postulated post-earthquake scenario assuming the joints and waterstops in the concrete face slab have breached and the design leakage is flowing into the dam body. It is further assumed that the reported "skins" (i.e. broken down rock on the top of the compaction layer) in the downstream rockfill have reduced the permeability of the rockfill so that it is no longer free draining, to the extent that the design leakage has saturated the existing embankment body. This may result in the downstream slope becoming unstable. A stabilising berm is thus required to provide adequate post-earthquake stability under these severe loading conditions.
Conversely, a potential high horizontal permeability (assuming a reduced vertical permeability due to the presence of the reported "skins") may result in erosion of the downstream face if the exiting hydraulic gradient of the leakage flow exceeds the critical gradient. An internal drainage system is required to control the exit flow rate, or the downstream zone modified to prevent unravelling on the downstream face. The design incorporates internal transition zones for drainage with a drainage zone incorporating an outer larger rockfill designed to prevent unravelling. A typical cross section is shown in Figure 4.
The design of the embankment zoning in the raised portion of the embankment was based on the recommendations to control seismic-induced settlement in the upper portion of the embankment contained within Materón and Fernandez (2011). Materón and Fernandez also recommend a flatter downstream slope in the upper portion of the embankment. The 1.7H:1V effective downstream slope adopted for the upper portion of the embankment is significantly flatter than for most CFRDs. Materón and Fernandez also recommend a well graded rockfill material that is well compacted to provide better earthquake resistance in the upper portion of an embankment where ground motions are amplified. The design incorporates a selected rockfill grading for the zone above the top of the existing rockfill to provide the recommended properties for improved seismic resistance.
The internal transition zone consists of a three-layer arrangement, with all the layers being no erosion filters for the adjacent materials. The weighting berm will consist of three different gradings. In the upper portion of the stabilising berm acts solely as weighting fill to the internal transition (filter/drainage) zone. In the lower portion, an extension of the existing drainage zone contains larger rock for improved flow capacity. A 6m wide zone of selected rock of adequate size to prevent unravelling protects this extended drainage zone from both internal leakage exit gradients and from backwater effects when the spillway is operating. The downstream slope of the lower portion of the embankment is significantly flatter at 1:2.25 to meet stability requirements.
The extension of the concrete face slab will be supported on an extruded concrete kerb in line with current practice for CFRDs. A layer of 100mm minus well graded rockfill (similar to Zone 2B in ICOLD Bulletin 141) will be placed adjacent to the kerbs. The design utilises the concrete kerbs as a temporary cofferdam until the face slab extension is completed.
Spillway widening
The required additional spillway capacity will be provided by extending the existing ogee crested structure by 40.2m and excavating a widened spillway chute. Five different computational fluid dynamics (CFD) models of the proposed spillway arrangement were analysed. For all arrangements the intent was to retain control on the ogee crest and not to allow control to move into the chute. The end result was an optimised arrangement of a tapered chute that provided the required flow capacity but reduced the volume of rock excavation.
The existing spillway terminal structure is a flip bucket with a plunge pool in the river course. During the course of the design it became apparent that modifying this arrangement to have a plunge pool at the end of the spillway chute could provide cost savings. The design proceeded with the removal of the flip bucket and addition of a plunge pool, with this arrangement being modelled in a physical hydraulic model constructed at the Manly Hydraulics Laboratory. Initial model runs showed unfavourable conditions in the plunge pool at large flows and the decision was made to revert the design back to the original flip bucket arrangement. At the time of writing, the physical modelling of the flip bucket is in the process of being finalised. Figure 5 shows the physical spillway model in operation.
A statistical fracture survey was undertaken on the existing spillway excavation using a scanline mapping methodology. The purpose of this mapping was to identify, assess and document the nature and distribution of rockmass fractures on the spillway excavation slopes. The information collected was used to enable statistical analysis of both fracture characteristics and kinematic behaviour. The end result was the adoption of a 50º batter slope (similar to the existing batter slope) with benches at approximately 15m vertical spacing.
The existing embankment fill is retained on the left abutment by a large gravity concrete wall - referred to as the Left Abutment Block - which also acts as a spillway training wall. Due to the unfavourable conditions for post tensioned anchors, this wall has been designed as a gravity structure. This has necessitated the use of 3-D finite element analysis - both linear and non-linear - to check the stability of the wall under seismic loading. In order to improve the wall's stability against sliding, the design incorporates an anchored thrust beam to transfer loads into the rock batter on the far side of the spillway cutting.
Waterstops
The existing perimetric and vertical face slab joints contain a single PVC waterstop. In line with current practice, the existing joints and the new joints associated with the face slab extension will be protected with external EPDM Omega type waterstops. The extended perimetric joint will have a second line of defence in the form of a copper waterstop, also in line with current practice. Two external waterstop profiles were chosen based on the expected joint openings as a result of earthquake-induced settlement.
Construction
SA Water used a tender process to identify two contractors who were contracted during concept design works for the purpose of:
- Providing constructability advice to ensure that the final design minimised construction costs.
- Ensure competition in tendering for the construction.
Constructability was a major concern as excavated material needed to be transported across the spillway and placed on the embankment. The haul road up the downstream face of the embankment will be incorporated into the embankment design and left in place to provide permanent access and a cost saving.
Nearing completion
Whilst the Kangaroo Creek Dam has performed satisfactorily since the completion of construction, changes to dam safety standards have meant that upgrade works are required to meet current criteria for resistance to seismic loading and flood capacity.
The design of upgrade works is nearing completion with construction work underway in the spillway cutting and also at the downstream toe with the extension of the outlets. At present, the design is focussing on post-tensioned anchors to stabilise the existing ogee crest spillway structure for extreme loads.
References
- Sandiford, M. (2003). Neotectonics of southeastern Australia: linking the Quaternary faulting record with seismicity and in situ stress. Chapter 8, Geological Society of Australia Special Publication 22, pp 101 - 113.
- Trudinger, J. P. (1973). Engineering Geology of the Kangaroo Creek Dam. Department of Mines, Geological Survey of South Australia.
- ICOLD (2010) Bulletin 141: Concrete face rockfill dams - Concepts for design and construction.
- Considerations on the Seismic Design of High Concrete Face Rockfill Dams (CFRDs), Materón and Fernandez (2011).