THOUGHT LEADERSHIP

What is the best dam type for my dam site?

Asking ‘what is the best dam type for my dam site?’ is a key question in the early stages of planning a dam project.

A number of dam types are being used around the world, such as concrete-faced rockfill dams (CFRD), roller-compacted concrete (RCC) dams, and asphalt-core rockfill dams, amongst others. Consultants that specialise in a particular dam type may offer a biased answer to the question, which is not in the best interests of the project.

The most appropriate response is that the best dam will depend on the unique characteristics of the dam site – available materials, climate, geology, topography, river flows, seismic risk – just to name a few.  This may sound obvious, yet it is surprisingly often ignored.

Although it is important that we take our experience and learnings from other projects and apply them to any new dam project, it is equally important that we do not force a past solution to fit a new dam site without considering that particular site’s uniqueness.

So, for any dam site, it is important to undertake an unbiased dam-type assessment that takes into account the range of site-specific conditions balanced against the broader project constraints (such as cost optimisation, and the availability of local design and construction expertise).

The following examples of dams with which I’ve been involved show how particular characteristics of each dam site determine the appropriate dam type.

Responding to local climate, geology and materials in Tasmania

On the west coast of Tasmania, Australia, where the rainfall is around four metres per year, Hydro Tasmania, Australia’s largest renewable energy producer and water manager, has constructed nine CFRDs.  Good rockfill was available at all of these sites.  Key reasons why CFRDs were used at these sites, rather than central-core earth and rockfill dams, included both the lack of quality clay material and the high rainfall in this region. High rainfall would significantly disrupt and delay the construction of clay-core dams due to issues with the compaction of the clay material, whereas construction of CFRDs could continue during rain. Asphalt-core dams might also have been possible in these local climate conditions, but would have presented different challenges for river diversion.

One of these CFRDs, the 82m-high Crotty Dam, faced potential instability should a traditional side-channel spillway be cut into the abutment slope, due to the steep abutment slopes and identified geological defects.  As a result, for the first time in the world, a spillway was created over the top of a high CFRD wall, combined with a large-capacity bottom outlet gate installed in the diversion tunnel.  This solution included an articulated concrete slab that would allow the spillway to move without cracking as the rockfill settled over time.

The solution at Crotty Dam was unique, but it was heavily influenced by the characteristics of the dam site and the construction materials available, which allowed a very stiff rockfill to be constructed, minimising the amount of settlement. The solution has proven a good choice, operating well for more than 20 years.

However, the particular solution that was successful for Crotty Dam may not be appropriate for other dam sites, where the dam might be significantly higher, where rockfill may be of poorer quality, or at a site with greater flood discharge or higher seismic risk.

A process of elimination in Papua New Guinea

In 2012, Entura undertook a detailed dam-type assessment for a site in Papua New Guinea, and in this case, the ultimate decision about the best dam type emerged from considering many site characteristics and then eliminating inappropriate dam types to reach a subset of viable possibilities.

The site was in an extremely high rainfall area (with approximately ten meters of rain each year), the rock foundation had relatively low strength and low stiffness, river flows were consistently high, there was deep alluvial material that would need to be removed or sealed, and the site was in a region with relatively high seismic risk.  Sources for concrete aggregate and rockfill were available.  However, due to the lack of strength and stiffness of the foundation rock, a conventional concrete gravity dam was not possible; and the extremely high rainfall posed significant challenges for a central clay core and rockfill dam.

As a result, the choices narrowed to a CFRD, an asphalt-core rockfill dam or a thick, trapezoidal RCC (or hard-fill) dam.  Ultimately, an asphalt-core rockfill dam was chosen as the preferred solution, as the core could be placed during high rainfall, the load would be adequately spread over the foundation, and it was considered a resilient design for the seismic loading.

Another important factor influencing the choice was that an asphalt-core rockfill dam would minimise problems and delays should the upstream temporary cofferdam overtop, because asphalt-core rockfill dams are always complete up to the current height of construction with no further work required below this level. Some other dam types require further work on the dam’s upstream face after the dam wall is constructed to its full height, requiring flood water to be pumped away to enable final-stage activities to proceed.

Challenges influencing RCC dam design and construction  

Many design and construction decisions for RCC dams can be significantly influenced by specific site conditions – for example, should the proportion of cementitious material (cement and fly-ash) in the RCC mix be low or high? Should one use conventional facing concrete, grout-enriched RCC or a PVC upstream membrane? Is RCC best delivered by truck or conveyor?

A significant influence on the design of the Wyaralong Dam in Queensland was the benefit of using an onsite, but inferior, sandstone aggregate for the RCC aggregate.  Using the onsite aggregate would reduce costs and benefit the local community through avoiding major aggregate haulage on public roads.

Using the onsite aggregate also influenced the investigations that occurred as part of the normal trial program used to develop an appropriate RCC mix. This in turn influenced the design of the dam because the trial mix program identified an issue with the surface durability of the RCC. Grout enrichment of the facing RCC was not suitable, so a conventional concrete using a high-quality basalt aggregate was used to provide the necessary durability.

The topography of the left abutment provided a natural ramp for an articulated truck delivery system.  This delivery system proved to be very flexible, but it was important to develop a suitable RCC mix for truck delivery.

One size doesn’t fit all

There is no easy or immediate answer to the question posed at the start of this article.  No single dam type offers the best solution for every dam site.  Although lessons learnt from other dam sites can help to rapidly narrow down a set of preferred dam types to consider in more detail, the appropriate dam type must be influenced by the unique conditions of the dam site.

Specialist power and water consulting firm Entura has significant experience with a wide range of dam types across the Asia-Pacific and southern African regions, particularly CFRDs and RCC dams.

You can read more about the way in which we responded to specific site characteristics for three RCC dams and the reasoning underpinning our design decisions in my 2012 ICOLD Conference paper “Unique challenges influencing the design and construction of three recent Australian RCC dams”.

If you would like to discuss how we can assist you with selecting the most appropriate dam type for your dam site, please contact Richard Herweynen on +61 3 6245 4130 or Shekhar Prince on +61 412 402 110.

About the author

Richard Herweynen is Entura’s Technical Director, Water. Richard has three decades of experience in dam and hydropower engineering, and has worked throughout the Indo-Pacific region on both dam and hydropower projects, covering all aspects including investigations, feasibility studies, detailed design, construction liaison, operation and maintenance and risk assessment for both new and existing projects. Richard has been part of a number of recent expert review panels for major water projects. He participated in the ANCOLD working group for concrete gravity dams and is the Chairman of the ICOLD technical committee on engineering activities in the planning process for water resources projects. Richard has won many engineering excellence and innovation awards (including Engineers Australia’s Professional Engineer of the Year 2012 – Tasmanian Division), and has published more than 30 technical papers on dam engineering.

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22 July, 2015