Elmgrove and Kinlyside Reservoirs

Project Location: Gungahlin, ACT
Client: Actew Corporation Ltd
Client’s Representative:
Superintendent: ActewAGL
Superintendent’s Representative: Kula Thenuwara
Project Start Date: December 2008

Description:

To design and construct two post tensioned concrete tanks, connecting pipework, access roads, drainage and instrumentation.

Elmgrove Reservoir- scope of works:

  • capacity of 20 mega litres
  • diameter 52m
  • excavation 22,000m3 including 6600m3 of rock
  • post tensioned cast is situ base 620m3 concrete
  • 54 precast post tensioned panels 2.745m wide, 10.66m high, .350m thick, approximately 24 tonne each, connected by 300mm wide stitch joints
  • Structural steel roof structure and cladding including access and ventilation
  • Over 550 lineal metres of pipework, including inlet/outlet pipework, scours and storm water, ranging in size from 100mm diameter to 600mm diameter
  • Construction of valve pits and associated fittings
  • Construction of special perimeter box drain
  • Roadworks
  • Electrical works
  • Testing and commissioning
  • Landscaping

Kinlyside Reservoir-scope of works:

  • capacity of 6.4 mega litres
  • diameter 29.5m
  • excavation 12,000m3 including 9000m3 of rock
  • post tensioned cast is situ base 190m3 concrete
  • 36 precast post tensioned panels 2.3m wide, 10.66m high, .300m thick, approximately 18 tonne each, connected by 300mm wide stitch joints
  • Structural steel roof structure and cladding including access and ventilation
  • Over 780 lineal metres of pipework, including inlet/outlet pipework, scours and storm water, ranging in size from 100mm diameter to 450mm diameter
  • Construction of valve pits and associated fittings
  • Construction of special perimeter box drain
  • Roadworks
  • Electrical works
  • Testing and commissioning
  • Landscaping

Discussion

Guideline ACT opted to construct the two reservoirs using the post tensioned concrete method, rather than alternative methods, because this technique offered the best possible solution technically and economically.

Given the rising cost of steel at the time, the option of constructing steel tanks was seen as higher risk commercially, making the concrete a safer option. Post tensioned concrete tanks are more durable than steel tanks and can be made just as water tight due to the tensioning technique which pulls the components in together

The post tensioning method uses precast concrete panels, which are positioned in a ring beam at the base, and the steel tendons are then threaded through the panels. Between each panel a stitch joint is formed and poured to “stitch” the precast panels together.

The detail and subsequent construction of the stich joints was a critical activity of the project. Each panel face had to be prepared and fitted with a water stop, the ducts for the stressing tendons extended through the joint, and reinforcing placed and formed, and finally the concrete poured to join the precast elements together.

The erection of the precast panels also posed a challenge, because there was not sufficient access around most of the tank to allow crane access. A solution was engineered in modifying the design of the slab, which allowed the crane to work from within the reservoir while at the same time shortening the installation duration.

Other areas of technical complexity that required specific input:

  • Excavation in rock requiring blasting
  • Floor penetrations for inlet and outlet pipework, that required sealing, through a post tensioned slab
  • Sealing the join between the walls and the base to withstand approximately 10m head.
  • Ensuring that all the ducts for the stressing tendons were in the right place and not damaged during concrete placing, both for the slab and wall panels
  • The logistics in transporting the precast elements
  • Installing the heavy precast elements to a tight tolerance
  • Tying in the roof design, specifically the footings with the post tensioned cast in situ slab and penetrations
  • Dealing with the construction issues of working inside the tank once the walls were erected.

Environmental controls were also important.

Establishing a pre-cast yard on site to make the panels greatly reduced truck movements and transport costs, and environmentally meant less of a carbon footprint. In keeping with this practice, much of the fill generated by excavating the two sites was of such good quality it could be used as road base on other Guideline ACT projects. Therefore what would have been two truck movements, to remove material from one site and to import material on another site, became one truck movement.

Noise pollution was controlled by ensuring working hours were kept to 7am to 5pm weekdays and 7am to 1pm Saturday and dust was controlled by watering the site with water trucks, and by covering loads of material entering and exiting the site.