Owen Falls Movements

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<p>The effect of concrete expansion at Owen Falls power station, UgandaP. J. Mason, BSc , MPhil , PhD , CEng , FICE and J. D. Molyneux, BEng , CEng ,MICECracking of concrete at Owen Falls power station, Uganda, was diagnosed as being due to concrete expansion caused by alkaliaggregate reaction. Resulting structural movements had caused local overstressing and also deections of installed plant and equipment. The degree of expansion varied according to the dierent cements used during construction. The processes of detecting and clarifying the implications of the various movements are explained, together with measures taken to mitigate immediate problems and provide adequate monitoring to areas of longer-term concern. Lessons are drawn for the guidance of others investigating similar phenomena. Keywords: dams, barrages &amp; reservoirs; power stations (non-fossil fuel); rehabilitation, reclamation &amp; renovation levels. The level rises increased both the loading and the uplift on the power station and other works. The fact that the cracks were rst noticed in the same year that the rises occurred, led to the inevitable suspicion that some form of structural distress due to increased loading had taken place. As this seemed, at the time, to be limited to machine No. 4, no changes were made to arrangements for the subsequent installation of machines No. 9 and 10. 3. The power station and associated works deteriorated rapidly during the 1970s as Uganda passed through a period of political instability, rstly through the Amin regime (197179) and then through the Obote regime (198085). The works were not inspected in detail again by the original designers until 1983. By this time most turbines were operable only at reduced load. This was largely due to lack of spares and maintenance, although there were also some machine misalignments and clearance losses. The cracking in the power station had also increased signicantly to include a major crack up to 25 mm wide through the machine hall oor and running the length of the power station (see Fig. 4). The cracking at machine No. 4 had increased signicantly and was mirrored at all other machines. It should be noted that when the inspections took place in 1983, general social and security conditions in Uganda were still quite dire with no local hotels available and little infrastructure. Visiting engineers slept in the power station. 4. Refurbishment works started in 1988 and these included rewaterproong the roof, making underwater repairs throughout the works, carrying out considerable amounts of stressed anchoring in the power station to stabilize the civil structure and generally refurbishing all the mechanical and electrical equipment. As part of this refurbishment the generators were uprated from 15 to 18 MW, increasing the total capacity of the station to 180 MW. Details of the stressed anchoring which was carried out are given elsewhere. 1 5. During 1990, continued monitoring of the cracks indicated that movement had not ceased and that the underlying cause might be other than simple structural overload. Indeed, the crack monitoring results showed a broadly linear trend of crack opening since 1973 which did not, for example, seem to vary with changes</p> <p>Proc. Instn Civ. Engrs Wat., Marit. &amp; Energy, 1998, 130, Dec., 226237 Paper 11726 Written discussion closes 15 April 1999</p> <p>&amp;</p> <p>Introduction</p> <p>Construction of the Owen Falls dam and power station complex in Uganda started on site in 1951. The works comprised the damming of the Victoria Nile a short distance downstream from its source at Lake Victoria. A right ank main dam and sluice structure is separated by high ground from a left ank power station with ten Kaplan turbines. The turbines were commissioned in stages, the rst two in 1954 and the last in 1968, giving a total installed capacity of 150 MW. The power station has provided the overwhelming majority of the power in Uganda, plus additional power for export, right up to the present day. The location of the works is shown in Fig. 1. A downstream view of the power station is shown in Fig. 2 and an internal view along the machine hall in Fig. 3. A chronology of events and principal characteristics of the schemes are given in Tables 1 and 2 respectively. 2. In 1964 cracks were noticed in the concrete around the generator housing on machine No. 4. Also in 1964, Lake Victoria, by then eectively impounded by the dam and power station, reached record levels with a rise of more than 2 m above the levels which had been carefully monitored since 1896. The Nile Waters Agreement required additional releases to be made through the sluices to reect the level rise, leading also, therefore, to higher tailwater</p> <p>Peter J. Mason, Director, Binnie, Black &amp; Veatch, Redhill (formerly Director, GIBB Ltd)</p> <p>J. Dominic Molyneux, Senior Project Engineer, GIBB Ltd, Reading</p> <p>226</p> <p>CONCRETE EXPANSION AT OWEN POWER STATION</p> <p>MediterraneanCairo Egypt</p> <p>Victoria NileJ To inja</p> <p>Re dS eaTailrace n Power statio Headrace</p> <p>Sudan</p> <p>Khartoum</p> <p>Sluices</p> <p>R. Nile</p> <p>Road bridge</p> <p>Main dam</p> <p>Kampala</p> <p>Owen Falls Lake Victoria</p> <p>K To</p> <p>am</p> <p>pal</p> <p>a</p> <p>Victoria Nile</p> <p>0</p> <p>50</p> <p>100</p> <p>150</p> <p>Scale: m</p> <p>in lake level. The present lead author became involved at this stage and the following paper broadly outlines the review and work that was subsequently carried out to clarify and diagnose the cause of distress and to put in place appropriate mitigating and monitoring measures. 6. During the initial inspections in 1983, samples of spalled concrete had been obtained and were examined for potential distress such as that caused by alkaliaggregate reaction (AAR). This included analysis by thin section. At that time no such distress could be detected. The review in 1990 therefore focused on taking a broad overview of what visible signs of movement had occurred in order to visualize overall patterns and see if this could shed further light on underlying mechanisms. 7. It was noted that the patterns of cracking and movement were very similar at most machines, although focused more heavily on machines No. 5 to 10. The pattern was therefore initially viewed from a two-dimensional perspective as superimposed on a cross-section through the power station, arbitrarily taken on the centreline of machine No. 8 (see Fig. 5). 8. Monitoring the absolute and vector directions of crack movements in various parts of the power station indicated that the downstream wall of the station was rotating downstream about a hinge point immediately above the draft tube (see Fig. 6). It should be noted</p> <p>Analysis of movements</p> <p>that the power station was initially cast with just the upstream and downstream walls as rst-stage concrete and with the latter heavily reinforced to resist tailwater levels. This permitted the machines to be erected and concreted at a subsequent, second, stage. 9. It should also be noted that this downstream rotational movement was compatible with two other observations. One was the main longitudinal crack along the machine hall oor. This was up to 25 mm wide and, together with other minor cracks, indicated a downstream movement at that level of 32 mm. Secondly, the overhead gantry crane rails were also known to</p> <p>Fig. 1. Location plans</p> <p>Fig. 2. Downstream view of Owen Falls power station</p> <p>227</p> <p>MASON AND MOLYNEUXTable 1. Owen Falls dam and hydropower complexa brief chronology Date 1935 1947 1948 1949 1951 1954 1955 1957 1958 1959 1964 1966 1968 1971 1973 1978 1979 Event River Nile examined for hydroelectric potential Further survey of hydropower potential by Ugandan government Uganda Electricity Board formed Owen Falls hydroelectric complex planned First concrete placed at Owen Falls Machine No. 1 commissioned in January and No. 2 in April Inauguration ceremony by HM the Queen on 29 April Machine No. 3 commissioned in January and No. 4 in August Machine No. 5 commissioned in January and No. 6 in February 50-year power agreement reached with Kenya Machine No. 7 commissioned in May Machine No. 8 commissioned in July Lake Victoria reaches unprecedented levels in May First awareness of concrete cracking around machine No. 4 Machine No. 9 commissioned in May Machine No. 10 commissioned in July Idi Amin seizes power in Uganda in a military coup Crack gauging commenced in power station by local sta Targets installed to monitor downstream wall movements Idi Amin ousted by rebel forces Milton Obote returned to power Reinspection of power complex by team of UK engineers Milton Obote overthrown Start of refurbishment works on site</p> <p>Fig. 3. View down the machine hall</p> <p>1980 1983 1985 1988</p> <p>Table 2. Owen Falls dam and hydropower complexprincipal characteristics Description Lake Victoria: Catchment area Lake area Lake mean depth Turbine generators: Total number Design head range Originally installed output per machine Original ow per turbine (now uprated to 18 MW with corresponding ow increase) Sluices: Total number Size per sluice Design discharge per sluice Principal dimensions: Dam crest road level Upstream max. water level Upstream min. storage level Max. tailwater level Min. tailwater level Nominal min. dam foundation level Nominal min. power station foundation level Length of gravity dam Length of machine hall (excluding loading bay) Width of machine hall asl = above sea level. Dimensions 267 000 km 2 67 000 km 2 40 m 10 17522 m 15 MW 96 m 3 /s</p> <p>6 3 m 6 51 m high 212 m 3 /s 113615 m asl 113500 m asl 113190 m asl 111435 m asl 111280 m asl 110800 m asl 110000 m asl 726 m 1676 m 165 m</p> <p>Fig. 4. View of the longitudinal crack in the machine hall oor</p> <p>228</p> <p>CONCRETE EXPANSION AT OWEN POWER STATIONPower station Roof truss</p> <p>Overhead crane</p> <p>Generator Lower bracket Draft tube deck</p> <p>Columns Turbine runner Spiral casing Intake dam Suction cone Tailrace</p> <p>Draft tube</p> <p>be moving apart. Some years earlier, the overhead crane had in fact jammed and crane movement was reinstated by machining 95 mm o the bosses on one set of wheels to allow them greater axle oat. 10. It was also known that the power station oor had risen in a number of locations. Again these were concentrated between machines No. 6 and 10 but rises were as much as 76 mm (see Fig. 7). Another indication of movement at several locations was diagonal cracks in the draft tube side walls (see Fig. 6). 11. After a reassessment of the evidence, the 1990 review concluded that all these eects could be broadly explained by an expansion of the concrete around the machines. This would exert a load on the downstream wall which was one part of the structure relatively free to move. It would also tend to cause cracking in the original rst-stage concrete. Diagonal shear cracking in the draft tube side walls would be due to the force couple developed by the expansive thrust downstream being resisted by the draft tube foundations. 12. At this stage a simple two-dimensional nite element model was undertaken to assess</p> <p>Initial modelling</p> <p>the eects of second-stage concrete expansion around the turbines and their spiral casings when viewed in the plan. The results are shown in Fig. 8. It can be noted that for any single machine, the concrete is unable to move laterally as it is restrained by neighbouring blocks, nor can it move upstream. Movements are therefore concentrated vertically and downstream. The downstream movement is amplied on the centrelines of the machines due to arch action around the spiral casing. The model therefore indicated the potential for the development of voids downstream of the spirals and also for vertical cracks between the rst- and second-stage concrete in the stair wells between the machines. Such vertical cracks were in fact present on site. When the steel spiral casings were subsequently drilled through downstream, gaps between the steel and concrete of up to 15 mm were found. 13. The model also predicted that the magnication of upstream/downstream movement, coupled with the lateral restraint against expansion, would tend to produce ovality in the water passages. This was also conrmed by clearance measurements around the tips of the turbine blades, to the surrounding suction cone.</p> <p>Fig. 5. Typical crosssection through the power station</p> <p>229</p> <p>MASON AND MOLYNEUX</p> <p>14. A remaining concern was why dierent areas of the station had developed movements in dierent ways. This applied not only to the power station proper but also to other parts of the associated works such as the main dam and intake structures. 15. In order to clarify this issue, the historic records of concrete pours were examined for various parts of the work. It was noted that midway through construction the cement type changed from imported Rugby cement from the UK, to Tororo cement from Uganda's rst cement factory which was commissioned in 1953. Fig. 9 shows patterns of level rise with areas where Tororo cement was used. The relationship between level rise and cement type is clearly apparent. 16. An analysis of the Tororo cement indicated that it had been produced from the local volcanic material carbonatite which is very rich in both alkalis and potash. This was certainly a contributory factor to the accelerated eects of alkali-aggregate reaction in those areas where the local cement was used. Another factor was that the coarse aggregate, though originally classied by the Uganda Geological Survey under the broad generic name amphibolite, was in fact amphibol-schist. This contains small particles of reactive materials such as strained quartz in a non-reactive matrix. 17. Typically, with this form of reaction, symptoms are not seen in the early days after construction. Gradually, alkali pore water seeps from the cement paste into the large aggregate. This aects any reactive particles, producing silica gel and leading to internal splits and expansion. The resultant local cracks eventually join, giving the appearance of structural cracking, rather than random crazing. This process is known as alkalisilicate reaction (ASR) or `slowlate' reaction. It was discovered much later than the more common AAR, generally associated with reactive sand. 2 The eects of ASR, or slowlate reaction, are typically not seen until ve to ten years after construction. 18. The rst- and second-stage concretes in the power station are described on record drawings as 5/1 mixes. These in turn were originally specied as ratios of 5 cwt (254 kg) of cement to 12 cu. ft (034 m 3 ) of sand to 20 cu. ft (057 m 3 ) of coarse aggregate of maximum size 1 in. (38 mm). This is dicult to interpret precisely in modern terms without knowing original bulking factors and densities. An analysis of concrete core samples from the power station, however, revealed cement contents averaging 300 kg/m 3 of concrete, plus or minus about 50 kg/m 3 . Water/cement ratios were back-analysed as between 052 and 060. More importantly, the analyses revealed exceptionally high, equivalent alkali (sodium oxide)</p> <p>Dif...</p>