Asphalt concrete cores for embankment dams

The use of asphalt in construction and for waterproofing structures can be traced back over a period of 5000 years. The oldest application is thought to be a small rockfill reservoir in the Indus Valley.

Built with natural asphalt as mortar, it is still operational today. Other discoveries from Mesopotamia (present-day Iraq), Egypt and from the Inca Indians in Peru show that asphalt was commonly used for building, preservation and waterproofing.

Modern dam building using asphalt as a central core inside the embankment dam was first introduced by the contractor strabag in 1962 and over the next 15 years a number of such dams were built, mostly in Germany. The Chinese developed their knowledge of the structures and built their first asphalt core dams in the 1970s. To date 13 dams of this type have been completed in China.

Until the 1970s, the large majority of Norwegian embankment dams were rockfill dams with a central moraine/clay core. However, as these materials became increasingly difficult to find at new construction sites, asphalt was used as a replacement core material. The first dam with an asphalt core was completed in Norway in 1978, and since then nearly all large Norwegian embankment dams have been built using this method. One reason for this is that asphalt core construction, when compared with its earth core alternative, can still proceed even during rainy or cold weather.

Procedures and equipment

Over the years, asphalt concrete cores have been built by different procedures. The method of filling formworks with clean and dry stone material and then filling the voids with hot pumped

bitumen has been used. But the high bitumen content required (30-40%), the subsequent cost and the difficulties in controlling the void content and permeability, makes the method less attractive today.

A different method was introduced by the Russians for three large embankment dams (up to 140m high) on compressible foundations. A very rich asphalt mix with 10-14% bitumen content was poured inside the 1m high steel shutters on top of the previous layer. The shutters were removed as soon as the asphalt had cooled down and transition zones were placed at either side. This method requires no compaction or specialised machinery for placement of the core, but the asphalt becomes expensive due to the high bitumen content.

The machinery, technique and speed

of construction of asphalt concrete core dams has developed considerably since the 1960s. Modern machinery for placing and compacting asphalt concrete with

6-7% bitumen content, and simultaneous placement of the transition zones, is

the most economical way to ensure fast and reliable construction. Furthermore, this technique allows the best quality assurance and control of the asphalt core in situ.

As demonstrated on the Svartisen project in northern Norway (1995-1997), progress on asphalt core dams is not affected by wet conditions when the appropriate equipment is used and necessary measures are taken. The core can be constructed in weather conditions suitable for construction of the rest of

the embankment. Contractors’ standing and stoppage time is reduced, and so

are the construction time and overall cost.

For the 140m high Yele dam in China, currently under final design, an asphalt core has been considered the only feasible alternative because of the extremely wet weather conditions, the high altitude and the partly compressible foundations.

Asphalt and bitumen are frequently confused. Bitumen is a product from the refinery process of crude oil, while asphalt is the mixture of bitumen and aggregates as used on roads, airfields and in dams. Modern refineries produce a range of bitumens for various applications. Although the bitumen can vary in chemical composition, quality products that remain soft, sticky and flexible can now be obtained worldwide.

The asphalt which is used on roads and airfields, where deterioration becomes evident in potholes etc, has a different composition to the asphalt used in dams. Inside a dam the asphalt is kept under virtually ideal conditions. Fairly constant temperatures without exposure to the sun and the rich, dense asphalt mix means that oxidation or hardening does not occur over time.

The asphalt concrete core wall adjusts to the deformations in the embankment and to differential settlements in the dam foundation. Displacements accumulate during the embankment construction

and filling of the reservoir. By adjusting

the bitumen content or the viscosity penetration), the properties of the core can be tailored to local conditions and the core will remain flexible and impervious.

At the Eberlaste dam, completed in Austria in 1968, an embankment dam with an asphalt core was built on top of a deep compressible and inhomogeneous alluvial deposit. The 28m high dam and the flexible asphalt core were constructed on top of a cut-off slurry wall. During construction the foundations settled about 2.2m in the middle of the valley and in the following two years secondary settlements of 20cm were measured.

No noticeable leakage was recorded, which means that the core was able to adjust to the significant foundation settlements, differential settlements, the large tension stresses and shear strains without cracking or noticeable increase in permeability. The material’s ductility and self-healing properties, as documented in the laboratory, are clearly demonstrated in practice.

For a well-compacted asphalt core embankment dam which has good rockfill resting on bedrock, the slopes may be as steep as 1:1.3, as demonstrated by the Finstertal dam in Austria. Even so, the measured maximum displacements inside this almost 100m high dam were very small during and after construction, and the strains in the asphalt fell well below acceptable levels.

Asphalt concrete has been documented to be virtually impervious when compacted to a void content of less than 3%. The viscoelastic-plastic properties can be tailored to local conditions and climate which makes asphalt core dams specially suited to seismically active areas, and on compressible foundations where stiffer structures like CFRD and RCC may not be suitable.

The asphalt core of the dam is permanently protected against weathering and cannot be damaged by impact or sabotage. The reservoir can be filled during construction, which is not feasible for an upstream facing alternative. Furthermore, overtopping of an asphalt core during construction will not have the dramatic consequences as for a clay core or an upstream facing solution. Stress and strain levels in the core, estimated from finite element design analyses, may be used for modelling the field behaviour of the asphalt concrete in the laboratory.

Cost comparisons for designs

Over the past few years fairly detailed cost comparisons have been made between asphalt concrete core dams and their alternatives at the design stage of projects. Several tenders have also opened for alternative bids.

For the Urar dam, completed in 1997

in Norway, tenders were submitted for an RCC dam and a rockfill dam with asphalt core. When only considering contractor costs and additional spillway expenses, the asphalt core alternative turned out to be approximately 10% cheaper than the RCC option. For

the Greater Ceres dam, completed in 1998 in South Africa, three alternatives were compared at the design stage:

RCC, concrete faced and asphalt concrete core dams. The latter was chosen due to cost and because the dam was located in an earthquake region on a poor sandstone foundation.

For the Mao Pin Xi dam, under construction in China (Three Gorges Project), the alternatives were an earth core and an asphalt concrete core dam. The latter was chosen due to its lower cost.

Tender documents for the 105m high Maguga dam, now under construction in Swaziland, were issued in three alternatives after excluding others

at the design stage. The asphalt core

and clay core alternatives, which were fairly similar in price, were significantly less expensive than the upstream concrete

face alternative. The clay core alternative was chosen. Recent alternative bids in Iceland (1999) have shown that embankment dams with asphalt cores

are still very competitive with upstream concrete face alternatives.

The Norwegian Geotechnical Institute (NGI) in co-operation with Kolo Veidekke, a Norwegian asphalt con-tractor, has carried out considerable research into asphalt concrete cores. Tri-axial tests and finite element analyses have been performed to document the suitability of using asphalt concrete in embankment dams. All dams with built-in asphalt cores are reported to have excellent field performance. Seepage through the core is claimed to be negligible and to date no repairs have been required.

It seems quite probable that embankment dams with asphalt concrete cores are likely to find a more prominent place in future dam construction.

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