In the know - rapid knowledge transfer for CFRD projects29 February 2008
The empirical nature of CFRD development is illustrated through knowledge transfer to construction projects following cracking at Mohale dam, Suzanne Pritchard reports
Concrete spalling at the Mohale CFRD in Lesotho, in February 2006, did not endanger the structure but accompanying leakage was a cause for concern. CFRD development has always taken an empirical approach, being guided by experience and not design theories, and those involved with the design and construction of similar dams sat up and took notice of what was happening. The events at Mohale helped to illustrate how the experience of one CFRD can have a ripple effect on the design and construction of others around the world.
Applying the knowledge
The198m high Kárahnjúkar dam in Iceland is the highest CFRD in Europe, and was designed by montgomery-watson-harza (MWH) with Palmi Associates as sub-consultant. More than 700m long, the dam is constructed of 8.5M m3 of basalt rockfill and located in a relatively narrow valley. The incident at Mohale illustrated how this type of rockfill in tall CFRDs, located in narrow valleys, can generate unprecedented concrete face cracking. Consequently, the original design of the Icelandic dam included mitigation measures to deal with construction in such a valley shape.
At Kárahnjúkar, the modulus of deformation during construction of basalt fill was in the order of 90MPa, which is substantially higher than the basalt fill dams that have recently experienced face slab cracking. The higher modulus was obtained as a result of extra wide (34m) zones 3 and 4, respectively, and heavily compacted in 0.4 m lifts; with finer basalt rockfill gradation in a thinner layer. Face slab thickness is set as 0.3 + 0.002*H, with one middle reinforcement layer amounting to approximately 0.42% of the concrete section on the abutments and 0.65% for the canyon portion.
The slab joint detail was also modified away from the Mohale design which had steel crossing from slab to slab. It is believed that this contributed to Mohale’s spalling under high horizontal compression. Instead, at Kárahnjúkar the reinforcing bars did not cross the vertical slab joints and the steel was recessed at 12cm.
Kárahnjúkar dam has many interesting design features. For example, construction of a toe wall reduced the length of the slabs from 198m to 145m, as the shorter length reduces slab stresses. The deformation of the rockfill and the slabs is complex due to the narrow canyon and rather significant arching. In addition, the fill placed on top of the slab increases the vertical slab strain in the slope direction and thus reduces the horizontal perimetric joint opening.
Additional mitigation measures incorporated into the dam design included:
• Placing 15mm thick bituminous fibre spacers to reduce horizontal slab stresses.
• Using a non-bituminous bond-breaker between face slabs and the extruded curb.
• Filling the upstream zone 1 between the cofferdam and main dam up to El. 505m.
• Constructing a bottom outlet to permit controlled loading of the face slab.
The news that additional face slab cracking had occurred at other dams, such as Campos Novos in Brazil, prompted a meeting between dam designers involved in the Icelandic project in early 2006. Although project construction was well underway, they examined the situation and additional mitigation measures were implemented at Kárahnjúkar. These included:
• Stiffening the crest rockfill from El. 584m to El. 625m, heavily compacted in 0.4m lifts.
• Placing a 3mm thick hot asphalt bond-breaker below thickened slabs.
• ?ncreasing the thickness in the 10 central slabs by 10cm above El. 535m. The middle waterstop was also eliminated and
• Placing 25mm fibre spacers between the central parapet wall vertical joints.
• Placing self-healing material upstream of the face slabs to El. 540m.
• Increasing the surface in contact at the vertical joints. This was achieved by decreasing the mortar pad intrusion and eliminating the V-notch.
The knowledge that slab distress had occurred at similar CFRDs constructed from basalt rockfill had prompted MWH to perform a 3D analysis of Kárahnjúkar dam’s behaviour and estimate the slab stress conditions that could be experienced during impounding.
The model revealed that without the joint fibres the horizontal stresses alone, due to the high compressibility of the basalt rockfill and the steep slopes, would create a maximum horizontal stress to the order of 31MPa. The mitigation measures included in the design, principally the joint fibres within the vertical joints at all the long slabs, reduced the horizontal stress to a maximum of 21.5MPa.
To take advantage of the 3D model, a test was performed using 30mm wide spacers of the same type and characteristics. The model indicated that if 30mm fibre spacers had been included in the vertical joints, the maximum horizontal stress would have been reduced to 17MPa. It was clear that Kárahnjúkar would have experienced serious face slab cracking if these fibre spacers had not been used.
The designers of this dam were in the advantageous position of having prior knowledge about problems encountered at existing facilities. This enabled them to incorporate remedial measures into the original design at Kárahnjúkar dam, as well as respond to further spalling incidents when construction was already underway. Due consideration was given to the effects of construction in a narrow valley, while further stress reduction was achieved through the use of fibre spacers and a bituminous bond-breaker. Other measures to help dissipate stress included a two-season reservoir filling period and controlled reservoir filling which was achieved through construction of a bottom outlet.
Porce III experience
The 660MW Porce III hydroelectric project in Colombia is currently under construction. It comprises a 154m high CFRD with a crest length of 400m and a total rockfill volume of 4.1M m3. The spillway comprises a lateral chute controlled by four radial gates with a maximum discharge capacity of 11,350m3/sec. There also is a 12.45km long power tunnel and an underground powerhouse.
Just after completion of the detailed designs for the Porce III dam, the news arrived about the occurrence of concrete cracking at high CFRDs. However, such undesirable behaviour was not expected at Porce III. Standing at only 154m high with an adequate infill deformation modulus, its characteristics are similar to other dams that have behaved adequately. Nevertheless, design revisions and minor adjustments were still introduced as additional measures to prevent any untoward behaviour.
The cracking of concrete faces in high dams built in narrow canyons is thought to be due to the transfer of stresses generated by rockfill deformation upon filling of the reservoir. If the deformation is small, as in compacted gravel dams, such stress transfer is small and no cracking develops. If the concrete face allows a certain degree of horizontal movement through its vertical joints so that the compressive stresses are not excessive, it reduces the possibility of crack formation.
Therefore, the design of Porce III dam has paid special attention to reducing the compressibility of the upstream shoulder and including vertical joints with a deformable infill in the central portion of the concrete face.
The need to ensure the highest deformation modulus possible led to the adoption of a thinner layer of concrete and a greater number of passes using a heavier compactor.
The best rocky material available was also placed, with a high uniformity grading coefficient being achieved by using an adequate quantity of water during placement and compaction. The study of the construction materials for the dam has been a subject of utmost importance within the design and the first stage of construction because:
• Different types of schist are found in the excavation (quartzitic, sericitic and graphitic), with different weathering levels (IIA, IIB and III). Both have a direct influence on the quality of the rockfills.
• It is essential to ensure the supply of the best quality materials for construction of the upstream shoulder to reduce compressibility.
• The main source of materials for the dam is from the excavation of the spillway, which has restrictions with general slope stability.
In this first stage of construction, detailed field investigations have been performed on the rockfill acquired from the different excavations. Because of the nature of the rock, the design has had to verify the deformation modulus of the rockfill which is required for adequate support of the concrete face.
Taking the strain
To dissipate the high stress levels in the concrete face, five central vertical compressible joints were also incorporated, together with a deformable infill between joints, with a modulus of about 10MPa.
A 3D finite element model was developed to evaluate the stresses and strains of the concrete face during different stages of dam operation. Based on the modulus and deformations obtained from laboratory and field tests performed on the rockfill material, it was established that the maximum deformation of the concrete face due to the reservoir impounding is in the order of 30m and takes place in h/Htotal between 0.43 and 0.45.
Upon completion of the first reservoir fill, the maximum horizontal displacement of the dam crest is calculated to be 22cm, and the maximum vertical displacement is 30cm. The unit deformations of the rockfill increase up to 0.2% and 2.0%, for the horizontal and vertical components, respectively. Under these deformations, the analytical modulus indicates that the behaviour of the concrete slab should be satisfactory.
As the construction of the dam progresses, further refinement of the analyses will be performed. The dam is to be completed by mid-2009, with the whole project scheduled for completion in 2010.