Higher and higher29 February 2008
Bayardo Materon examines the slab design of some of the highest CFRDs in the world, and analyses the structures’ performance after impoundment
Although CFRDs have steadily increased in height around the world, the same formulae are still being selected for the slab design. However, the final thicknesses and reinforcement design of slabs can differ. The first dams that were built with non-compacted, dumped rockfill used the empirical form:
T=0.30 + 0.0067H (m)
T = slab thickness in metres;
H = reservoir pressure in metres.
As the slabs were laid on the rock, which was placed onsite by cranes, it resulted in over-excavations of 30-38cm and, consequently, very thick slabs. The introduction of vibratory compactors meant that the excess concrete could be reduced to 10-15cm and the empirical formula changed to:
T= 0.30 + KH
K is a factor ranging from 0.001 to 0.0036 for dams where the transitions (2B) were compacted using a vibratory roller horizontally and on the upward slope. The value depends on the designer.
Following the implementation of extruded curbs, the excess concrete was further reduced to values lower than 5cm, resulting in less thick slabs.
Completed in 1971, the 110m high Cethana dam in Australia was the world’s highest dam at the time of construction. The 2B zone transition under the slab was built with a maximum size of 22.5cm, compacted into 45cm layers with four passes of a 10-tonnes vibratory roller and the addition of water. The transition material was well-graded with a uniformity coefficient equal to 19. However, the over-excavation of additional concrete was reported to be 12.5 cm, which means that the actual formula for the slab was:
T= 0.30+0.002H m, plus over-excavation of 0.125m.
It is important to mention that to calculate the slab reinforcement, designers considered the theoretical slab thickness with an additional 10cm in recognition of the excess concrete.
The 3B main rockfill was compacted into 90cm layers, with four passes of the 10-tonnes vibratory roller. The 3B rockfill, with a maximum size of 60cm, had a uniformity coefficient of 25, and a mean compressibility modulus of 140 MPa.
The slabs were 12m wide; with an area of 30,000m² and a narrow valley shape factor of A/H²=2.48. The perimetric joint and the tension joints had two water stops, one made of copper, and a central one made of rubber. The compression joints had a copper water stop without a V-notch but with anti-spalling reinforcement. Near the abutments, the slabs were divided into 6m wide pieces.
The slab was built in one stage and the maximum deformation upon filling of the reservoir was 11.6cm. Although retraction cracks were displayed every 7m, the general behaviour of the slabs was found to be excellent, with leakage of 35 l/s.
The 140m high Alto Anchicaya dam in Colombia was completed in 1974. To distribute potential movement due to the steepness of the abutments, several protection lines in the perimetric joint were required. High filtrations were observed (1800 l/s) during the filling of the reservoir, particularly on spots concentrated near the abutments. There was greater intensity on the right due to detachment of the loosened water stop. After swift treatment with the placement of mastic, leakage was reduced to 180 l/s and has remained almost constant throughout the life of the project.
The slab was built in two stages, and the maximum deformation after reservoir filling was 12cm. The slab showed only minor cracks at the central portion. An inspection conducted after the reservoir was lowered showed excellent slab behaviour.
Foz do Areia
The 160m high Foz do Areia dam in Brazil was completed in 1980. The 2B zone transition was built with a maximum size of 7.5cm, although the specifications allowed up to 15cm. That material was compacted into 40cm layers with four passes of a 10-tonnes vibratory roller and the addition of water. The transition material was uniform, with a uniformity coefficient equal to 10 due to the lack of fines in the processed basalts.
The main characteristics of the slab were: 16m wide; an area of 140,000m²; and a wide valley shape factor of A/H²= 5.47. The slab was built in two stages, and the maximum deformation after filling was relatively high at 69.2cm.
The initial leakage was 236 l/s but decreased over time. The slab was built down to the parapet level and is described as behaving very well.
At 187m, the Aguamilpa dam in Mexico was completed in 1993 and was the world’s highest dam until 2006. The slab thickness was calculated using the following formula:
T= 0.30 +0.003H m.
The upstream portion of the slab consisted of natural gravel from the Santiago river, followed by a transition of alluvium and rockfill. The downstream portion consisted of rockfill using rock from the structure’s excavation.
To calculate the reinforcement, the theoretical slab thickness was considered at a rate ranging from 0.3% to 0.5%, depending on the location. The reinforcement was placed at the central part of the slab, with anti-spalling reinforcement near the perimetric joint and the tension joints.
The slab was built in three stages. Deformation after filling was relatively small at 15cm but its crest moved downstream twice as much. The slab displayed a high horizontal crack due to the difference in modulus between the rolled gravel and the rockfill.
The dam behaviour is good with initial leakage being recorded at 258 l/s. However, this maximum rate has been decreasing with time.
Dams in China
Currently one of the highest dams in China, the 178m high Tianshengqiao (TSQ-1) was completed in 2000. It is the country’s largest commercially operational facility.
To compute the reinforcement, designers considered 0.3% of the theoretical slab section horizontally, and 0.4% vertically, located at the central part of the slab. The presence of cracks on the slab indicated the need for placing the reinforcement in the upper and lower parts.
TSQ-1 displayed central compression joint breakage due to the concentration of stresses three years after the first filling.
The Sanbanxi (185.5m high) and the Shuibuya (233m) dams have been recently built in China, and are now in the process of reservoir filling. These dams have a valley shape factor of A/H²= 2.73 and 2.21, respectively. The designers are aware that in order to prevent problems with concentration of stresses on the joints, it is important to have a high compressibility modulus, reducing the rockfill construction layers and allowing a rockfill pre-settling time prior to the construction of slabs.
In Sanbanxi, the upstream and downstream rockfill have been compacted into 80cm layers with eight passes of a 25-tonnes vibratory roller (total weight) and the addition of water in an amount equivalent to 20% of the volume. It was specified that the slabs would only be built when the rockfill had a deformation of 5mm/month, which was obtained 6-8 months after conclusion of the dam rockfill.
In Shuibuya, the compaction was also very strict. Fibres were incorporated into the concrete in order to reduce the frequency of cracks on the slab which were a frequent occurrence during construction.
All high dams in China are built with thick slabs using formulae where the K value is equal to or higher than 0.003. A corrugated joint seal with a specialist mastic protection has been used on the perimetric joints, with the addition of a band consisting of strong EPDM material. All tension and compression joints have such protection.
Excessive compression stresses on the central joints in CFRDs can have detrimental effects on slab behaviour. To prevent this, the following must be taken into consideration:
• The rockfill or compacted gravel must have: a high compressibility modulus, achieved using well-graded materials (Cu >15); reducing layer thicknesses; increasing the roller passes; and utilising compacting rollers featuring heavier drum weights (> 5 tonnes/m).
• A generous use of water during the spreading and compaction.
• Ensuring the thickness of the downstream layer is similar to the upstream layer helps to prevent differential movements that might affect the slab behaviour.
Other important design considerations for the compression joints in high CFRDs include:
• The copper waterstop that supports the mortar bed must be outside the theoretical slab thickness and within the extruded curb.
• The theoretical slab thickness must always be preserved.
• The size of the vertical element of the copper waterstop must be reduced or eliminated.
• Compressible fills consisting of wood or equivalent materials must be used to mitigate compression stresses.
• The upper V notch should be reduced or eliminated on the slabs.
• Anti-spalling reinforcement must be used.
This article is based on the full paper presented by Bayardo Materon at the 3rd Symposium on CFRDs, in Brazil, in October 2007. Conference proceedings can be obtained from: www.cbdb.org.br