Down in Tasmania1 January 2002
Richard Herweynen from Hydro Tasmania traces the development of CFRDs in Australia's island state
HYDRO Tasmania (the Hydro -Electric Corporation) is the Australian power supply authority for the island State of Tasmania. The company has an installed capacity of 2500MW, of which 2250MW is generated from 27 hydro power stations.
Of the 43 icold large dams owned by Hydro Tasmania, 13 are concrete faced rockfill dams (CFRD) built from 1971 to 1993. The first was the 110m high Cethana dam which took into account lessons learnt from earlier, smaller dams built around the world. In 1991, Crotty dam was constructed with a spillway chute on the downstream face. The last of the Tasmanian CFRDs to be constructed was the 38m high Anthony dam which was commissioned in 1993.
All the CFRDs have instrumentation, the minimum being a leakage measuring weir, inclinometer instrument to measure the deformed shape of the face slab and crest settlement targets. A number of CFRDs have had extensive instrumentation measuring settlements, joint openings and strains in the concrete face. Further details on some of Hydro Tasmania's principal CFRDs are discussed below.
As indicated by J B Cooke (1991), the development of the CFRD moved slowly between 1960 and 1970, and progressed rapidly after Cethana in Australia, Achicaya in Colombia (140m high) and Foz do Areia in Brazil (160m) in the 1970-1980 decade.
Cethana dam, located on the Forth river, was completed in 1971 and is recognised as being one of first high modern CFRDs. Prior to constructing Cethana, Hydro Tasmania built the 34m high Wilmot dam to trial the design and construction methodology.
Cethana dam was originally planned to be an arch dam but adverse rock jointing on one abutment led to the change in dam type. Construction had already commenced and the diversion tunnel had been completed. It was found that a CFRD (with its steep face slopes) was the only embankment that would fit between the two tunnel portals.
At the time, Cethana was the second highest CFRD. The highest CFRD, using compacted rockfill, was the 150m high New Exchequer dam in the US which was used to raise an arch dam. Unfortunately, a large settlement was understood to have occurred at the arch/concrete face boundary on the downstream side of the arch dam, resulting in large leakage. Other dumped rockfill dams over 80m in height had also not been completely successful because of large leakage and/or damage to the concrete faces caused by excessive settlement.
At Cethana a more extensive system of instrumentation was installed than at any previous dam. Construction of the embankment in layers enabled hydrostatic settlement gauges to be buried in the rockfill, thus allowing the rockfill settlement at various levels to be measured and the rockfill modulus to be determined during placing operations.
Joint meters were installed at various positions on the perimetric joint and on the vertical face joints to measure relative movements in three dimensions. Strain meters were placed in the concrete face and in the rockfill to measure tensile and compressive strains. Survey targets, which allowed movements of the embankment as a whole to be measured, were established on the crest and the downstream slope of the embankment.
Finally, in the river at the downstream toe of the dam, a leakage measuring weir was constructed.
Due to the successful performance of Cethana dam, many design and construction engineers throughout the world adopted Cethana details and methods. The information obtained from Cethana and the other early CFRDs assisted in understanding the fundamentals of this type of dam. This understanding allowed Hydro Tasmania to exploit the versatility of the CFRD and apply innovative solutions to specific site conditions.
At the 122m high Reece dam, the upper left abutment was weathered to depths of 30-50m. The deep weathering of the foundation on the left abutment precluded the normal procedure of founding the plinth on sound rock. It was decided to found the plinth in a shallow excavation and to counteract the erodibility of the foundation by increasing the leakage path.
A downstream blanket was adopted because it was the cheapest option and had the additional advantage of utilising the rockfill placed over it to resist the uplift pressures. A reinforced shotcrete blanket extends downstream of the plinth for a distance of approximately one half of the reservoir head following earth dam practice. The blanket was connected to the plinth with a waterstop. The whole of the abutment was covered with layers of fine and coarse filters to prevent piping from the foundation into the fill. This solution shows how CFRDs can be adapted even in situations of poor foundations.
In 1991, Hydro Tasmania commissioned the 82m high Crotty dam which has a concrete chute spillway on the downstream face. The construction of the spillway chute on the downstream slope of a high rockfill dam was a new concept, as there was no known precedent for such a construction elsewhere.
As part of the design process, alternative spillway options were considered which included glory hole spillways and a side channel spillway on the right abutment. The tunnel spillways were discarded early on because of their relative expense. The chute spillway on the right bank would have required significant and expensive stabilisation by rock anchors, because the bedding planes in the rock are parallel to the valley wall. Additionally, excavation of a right-bank spillway would have conflicted seriously with the construction programme of the dam plinth. Therefore, a spillway on the downstream face seemed the most effective answer.
Although it had never been attempted, Hydro Tasmania had confidence in the design because it was based on over 20 years of measuring deformations of rockfill dams during construction, reservoir filling and in service. Hydro Tasmania found that there was a marked similarity in the long term behaviour patterns of all our CFRDs built between 1970 and 1991. Even for the two highest dams (Cethana 110m, Reece 122m) the deflections were small. These measurements indicated that the envelope of downstream face movements predicted for Crotty dam would be small. Thus a concrete slab on the downstream face, if properly articulated, would be able to cope with any differential movements that might occur.
Since filling of the lake, the spillway has operated on some six occasions. On two of these the spill lasted for about one month with a maximum surcharge of 0.7m. The maximum discharge that the spillway has experienced is 14m3/sec, which is approximately 5.5% of the design discharge. Judging by the results of all measurements the spillway is performing satisfactorily. While the maximum flow experienced represents a small proportion of the design discharge, there is no reason, based upon the current observations, to suppose that the spillway will operate other than as intended.
In the early stage of the design of Crotty dam, it was evident that there were abundant alluvial gravels near the dam site which, after being processed, could be used as dam fill. The gravels offered a significant cost-saving compared with quarried rock. As these gravels had a significant proportion of fines, the embankment design required regular horizontal drainage layers constructed from tunnel spoil material or from gravels whose fines had been removed. Thus, judicious use of available material with appropriate drainage layers resulted in a satisfactory high density dam. The settlement of the dam after 10 years of operation has been very small at 44mm or 0.053% of the dam height.
Upgrades at Cethana Dam
By 1990 a generalised method had been fully developed for estimating extreme rainfalls for southeast Australia. Using these estimates, flood estimates were updated for all dams owned by Hydro Tasmania. These estimates indicated that the spillway capacity of the three Forth river dams, including Cethana dam, no longer complied with current practice.
For this reason, it was decided to increase the spillway capacity of Cethana dam from approximately 2000m3/sec to 4000m3/sec, corresponding to an annual exceedence probability of 1:100,000. Following a detailed options study it was concluded that the most economical means of achieving this increase in spillway capacity was to raise the dam crest and increase the height of the spillway chute walls.
Constructing a 5.5m high L-shaped wall behind the existing parapet wall will raise the dam crest. The rockfill level will then be raised behind this wall to provide a stable cross-section. This crest wall arrangement is a similar design to Hydro Tasmania's Reece and Crotty dams which were constructed as part of the second stage rockfill.
Once raised, the width of the Cethana dam crest will still be sufficient for single lane vehicle access. This is due to the fact that the existing crest width at the Cethana dam is considerably wider than the more recent CFRDs constructed by Hydro Tasmania. The existing crest width is approximately 9.5m, which was needed for the concrete faced slip forming equipment.
In order to cope with a spillway flow approximately double the original design flow the height of the spillway chute walls needed to be dramatically increased. The chute wall needed to be raised from a 6.6m high wall to a 12m high wall. Raising the chute walls externally was impossible due to the narrow bench that the spillway chute was constructed on. Therefore the only real solution was to raise the walls from inside the chute. There are three main components to the design:
•Anchoring of the wall base using epoxy coated, high tensile steel bars grouted into the rock foundation.
•Thickening and raising of the chute wall on the inside with substantial tensile reinforcement.
•Utilising shear keys between the old wall and the new wall to ensure they act together as a composite section.
The spillway upgrade for Cethana dam was approved in 2000 and detailed design was completed in 2001. Construction for the spillway upgrades of all three Forth river dams, including Cethana, commenced in December 2001 and completion is planned for 2003.
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