Concrete Face Rockfill Dams (CFRDs) have gained popularity as a dam type over the past five decades, with their construction extending to various regions worldwide, primarily in China and South American countries. In contrast, Asphalt Core Rockfill Dams (ACRDs) have emerged as a remarkably effective, flexible, and ductile solution for more than six decades, serving as a viable alternative to conventional Earth Core Rockfill Dams (ECRDs). Currently, there are approximately 200 ACRDs either already constructed, under construction, or in the planning phase, underscoring their increasing prominence in the realm of dam construction.
In accordance with international design standards and established practices, the choice of a rockfill dam for a local river involves a meticulous consideration of various factors. These factors encompass the finest engineering designs, geological site conditions, topographical features of the valley, and the availability of substantial volumes of rock material resulting from necessary excavations, such as those for the spillway, spillway approach channel, and powerhouse, among others. In the case of Concrete Face Rockfill Dams (CFRDs), the sealing system encompasses a concrete face shielded with transitional rock fill on the upstream slope. Conversely, in Asphalt Core Rockfill Dams (ACRDs), a watertight asphalt wall runs along the central axis of the dam.
The purpose of this article is to provide a comparative analysis of two types of dams constructed using compacted rockfill and employing distinct methods of permeability treatment:
a) Concrete Face Rockfill Dams (CFRD): These are compacted rockfill dams featuring a concrete face on the upstream slope.
b) Asphalt Core Rockfill Dams (ACRD): These dams incorporate an asphalt core positioned within the central axis of the structure.
Both of these dam types offer viable and feasible solutions, presenting excellent technical and economic advantages.
CFRD and ACRD: Options for compact rockfill dams
The evolution of CFRD and ACRD rockfill dams has seen remarkable progress over the past five decades. CFRDs have now reached heights exceeding 250m, while ACRDs have achieved a height of 172m. Notable examples of ACRDs include Storvatn (100m, 1987) and Storglomvatn (128m, 1997) in Norway, Mao Ping Xi (104m, 2003) and Yele (125m, 2006), along with Quxue (132m high asphalt core and a total height of 174m in China. Furthermore, La Romaine in Canada has an ACRD standing at 109m, constructed in 2013, while Zarema May Day boasts a bituminous core of 137m in height and a total rockfill height of 152m in Ethiopia.
In the context of CFRD, the design encompasses several key components, including a concrete face with vertical joints, transition zones on the upstream side, an external plinth, external grouting, as well as various types of water stops and mastics.
For ACRD, the sealing system comprises an asphalt wall that runs along the dam’s length. This system also incorporates fine and coarse transition zones, an internal plinth, internal grouting, and an asphalt core positioned in the centre of the dam.
In CFRDs, the inclusion of an extruded concrete curb is necessary, particularly in front of zone 2B. Meanwhile, in ACRDs, transition zones are positioned both upstream and downstream of the asphalt core. These transition components are installed concurrently with the advancement of the asphalt core.
Foundation Treatments – grouting strategies
In both CFRDs and ACRDs, foundation treatments hold paramount significance in terms of permeability control and leakage prevention. The management of foundation permeability through cement grouting is a standard practice and follows a well-established pattern in both alternatives. A notable method known as the GIN method, as proposed by Lombardi and Deere in 1993, has been widely employed in some of the tallest CFRDs worldwide, such as Aguamilpa in Mexico and Mazar in Ecuador.
For instance, the grouting treatment employed at the Tianshengqiao-I Dam in China, which stands at an impressive height of 178 meters. In this case, the grouting is administered on the plinth, which is situated over the external rock foundation, on the outside of the compacted rockfill.
Conversely, the Storglomvatn Dam, where a typical grouting treatment is carried out from the plinth situated at the central core abutment of the dam in the case of ACRD.
Construction phases
In the case of ACRDs, the initial construction stage involves the early development of the concrete plinth at the central part, conducted alongside the foundational grouting services. Additionally, the treatment of the foundation rock surface follows established international rock standards and necessitates meticulous cleaning with compressed air. Prior to proceeding further, a crucial step involves the application of pre-coating material (primer) to facilitate strong adhesion between the asphalt core and the concrete surface of the plinth. The commencement of the asphalt core construction follows these preparatory measures. It’s imperative to note that all these activities are considered critical components in the primary construction sequence for a rock-fill dam of this nature.
In the CFRD alternative, the foundational treatment, concrete pouring, and grouting services occur upstream of the primary dam. Significantly, these activities can be carried out with a relatively high degree of independence from the progress of the primary rockfill construction.
Primary construction phases for CFRDs
Diversion Phase (CFRD)
- Excludes cofferdams from the main rockfill, resulting in the need for a larger volume of rockfill.
- Involves longer diversion tunnels, requiring more rockfill compared to ACRDs.
Sealing Activities
- External sealing encompasses concrete, external and internal joints, and internal water stops. This method tends to be more expensive and does not involve concerns related to uplift and pore pressure.
Transition Zones
- Transition zones include the extruded curb, employing the Itá Method (Ref.15), and areas like 2B and 3A, which have less elaborate transitions.
Plinth
- External plinth for grouting, involving perimeter joints, and the use of costly water stops.
Concrete Face
- The external concrete face entails joints, various stages of water stops, rebars, and internal water stops.
Parapet Wall
- Upstream and downstream parapets offer a means of achieving rockfill cost-efficiency.
Industrial Facilities
- Necessitates the presence of a concrete batching plant, specialized slip forms, equipment for constructing copper water stops, and a concrete laboratory.
Reservoir Impounding
- Involves the construction of concrete Phase I, facilitating coordination with the reservoir filling process. This can be executed in stages.
- Increased number of slip forms.
Seismic Activity
- Ensures a high level of seismic resistance for both the rockfill and the face slab’s impervious features, as seen in the Zi Ping Pu CFRD.
Primary construction phases for ACRDs
Diversion Phase (ACRD)
- Incorporates cofferdams into the dam’s structure.
- Requires a reduced volume of rockfill.
- Involves shorter diversion tunnels, resulting in cost savings.
Sealing Activities
- Utilizes an internal asphaltic core without the need for joints.
- Requires special attention to weather conditions, placing transitions concurrently with core placement.
- Involves a unique asphalt design and rigorous laboratory control.
- Eliminates the use of water stops and extruded curbs.
Plinth
- Internal plinth construction necessitates effective coordination and meticulous planning with regard to grouting and core placement.
Asphalt Core
- The core is internal, featuring a distinctive design for placement.
- No joints or rebars are employed.
Parapet Walls
- ACRDs typically do not include parapet walls.
- This design requires a higher volume of rockfill.
Industrial Facilities
- A specialized batching plant is utilized for the production of a self-healing asphalt core.
- Equipment is deployed for the passage of trucks over the core and transitions.
- An asphalt laboratory is an essential component.
- Conventional concrete batching plants may be required for certain aspects of construction.
Seismic Activity
- ACRDs are designed to possess high seismic resistance, ensuring their structural integrity in the face of seismic events.
Concluding Remarks
Concrete Face Rockfill Dams (CFRDs) and Asphalt Core Rockfill Dams (ACRDs) emerge as viable choices in regions necessitating extensive rock excavation and where specialized construction equipment is accessible for these types of dam projects.
What’s particularly intriguing is the consideration of asphaltic core dams in the feasibility assessments by Latin American nations, especially when suitable equipment is at their disposal for material placement. This proactive approach highlights a forward-thinking perspective in harnessing advanced dam construction techniques for the region’s benefit.
This article first appeared in International Water Power magazine.
