Cool under pressure21 April 2004
Construction work on China’s Longshou power station took only 14 months in the face of extreme continental climate conditions, writes Su Yong
LONGSHOU power project, located about 30km south west of Zhangye city in the Gansu province, in the outlet of the Heihe river, has an installed capacity of 52MW, and a mean annual discharge of 50m3/sec. The area has a continental climate, with temperatures ranging from –33 to 37.2ºC. While mean annual rainfall is low at 171.6mm, evaporation is high at 1378.7mm. These factors made construction of the project difficult. Work began at the site in April 1999, with the dam concrete compacted in March 2000. Lowering of the brake and impounding was carried out in April 2001, and generation began in May of the same year. Roller compaction around the entire dam was completed in June and all four units were generating electricity by July 2001.
Longshou consists of a gravity dam on the left bank, a double curvature thin arch dam in the master river bed, a thrust pier in the right bank, a short apron downstream, scour slope protection and the second dam was rebuilt by downstream cofferdam. The total volume of dam concrete is 208,000m3, of which RCC makes up 176,000m3.
The gravity dam is 54.5m high with a diversion penstock buried within two-grade RCC, used as anti-saturation in the upstream of the dam, and 2m thick normal concrete set as align-based cushion layer in the dam foundation. The horizontal depth of the cushion layer concrete in the banks is 1m, and the total concrete volume of the dam is 108,000m3.
The thrust pier is set above 1720m on the right bank. It is 31.5m high, the top is 14.5m wide, and the dam base is 30.25m wide.
The crest is 29.32m long, and 2.1m thick normal concrete is set as align-based cushion layer in the dam foundation. The horizontal depth of the cushion layer is 1m, and the total concrete volume of the thrust pier is 21,000m3.
The arch dam has a maximum height of 80m. The crest has a maximum arc length of 140.84m and a thickness of 5–7m while the maximum thickness of the dam base is 13.5m. The horizontal depth of the cushion layer concrete is 1m, and the total concrete volume of the arch dam is 68,300m3.
The two crest overflowing orifices are set in the two sides of the three mid discharge orifices. It is WES overflow weir, and the elevation of the crest is 1741m. The clear width of each orifice is 10m and a ski-jump energy dissipator is used. Plane bulkhead gate and plane main gate are set in the entry.
The three mid discharge orifices are set symmetrically along the centre line of the dam. The elevation of the bed plate is 1710m and the outlet measures 5m x 5.5m. The plane emergency bulkhead gate in the inlet measures 5m x 6.6m, and the plane main gate in the outlet 5m x 6.6m. Ski-jump, slit-type and drop energy dissipators are used.
To simplify construction, 2-grade RCC was used as an anti-permeation layer for the double-curvature arch dam. The RCC layer is 1.5–6.5m thick, with a permeation resistance index of W8, intensity index of C20, and frost resistance index of F300. The upstream anti-permeation layer of the thrust pier and the gravity dam are similar to the arch dam. At the same time the plaster mixed by cement and fly ash is laid in each roller compacted lift in the range of the upstream 2m surface.
An induction joint is set in the arch dam, and the peripheral joint is set at the interface between the arch dam, gravity dam and thrust pier. The induction joint is laid outside the surface orifice. Two induction joints divide the arch dam into three segments; 40m, 60m and 41m. Computation results showed that the principal tensile stress of the two arch-abutments in the upstream surface above 1694m was quite great. To release the stress, the peripheral joint is set from the elevation, key is set in the joints and joint grouting system, dowel is set in the downstream 1/2 joint width and the range interval is 50cm x 50cm. Such measures were taken to improve anti-sliding stabilisation and anti-seism along the arch abutment surface.
Action of induction joint
The structure joints include peripheral short joints and two induction joints. Generally, construction of concrete around the joint was influenced by the structure configuration, so the degree of compaction is smaller than other parts. The modulus of deformation is also small and deformation must be produced after stressing.
Compressive deformation is within 2–3cm, which partly influences the stress of the upstream surface two-abut of the arch dam. The stress is about 0.1–0.3MPa and the deflection along the downstream of the medial part of the arch dam is about 2–4mm. From the total body, the increase of stress and deflection of the dam body is much smaller.
RCC construction technology and placing without longitudinal joints was adopted at Longshou. The period of compaction was only 14 months. Though the temperature control measures of tube cooling were used from 1685m to 1705m of the dam body, the body is thin, and heat elimination was not too difficult. Great temperature stress is still likely to appear, so a semi-opened peripheral joint was set above 1694.6m of the dam’s two sides, and the induction joint was respectively set outside the surface of the left and the right from 1700m to the crest.
Thermal control design
All concrete constructed below 0ºC had to be intruded with frost-resistance agent (max. 4% of total cementitious material). The prescriptive temperature of the agent should not be more than -15ºC and it should not have any erosion effect on the bar inside concrete.
Concrete aggregate is produced using the wet method. At the bottom of the finished product, stockpile aggregate is heated by steam tube and in the bottom of the feed barn, batching and mixing plant and each kind of belt up to the building, the aggregate is heated by steam tube, heat preservation quilt (board), etc. Concrete mixing water is heated by steam, and additive is also thawed by heat water in order to improve the concrete exit temperature.
The concrete placing surrounding design temperature cannot be below –5ºC for the arch dam or below –10ºC for the gravity dam and thrust buttress. During the construction process, the cooling water pipe was buried in the RCC dam to ensure temperature reduction. In each water cooling area, the pipe length is controlled within 200m, and natural river water is used for cooling. Monopipe water feed quantity was not less than 20l/min, the import director was changed every 24hr, and the importing time was at least three months.
The project used about 30,000t of mark Qilian 525# Portland cement produced by the Yongden cement plant. The original fly ash produced by the Yongchang thermal power plant was of bad quality, and could not be used directly. A wind-choice machine was added to the ash collection device so that elements larger than 45µm in the original fly ash were separated, producing suitable fly ash.
The indoor mix proportion review and RCC test on site were done in November 1999. The test showed that for some materials the mix proportion had to be further adjusted. The major changes were water, sand and additive volume. The value of Vc was changed to about 5min.
The RCC mix proportion after adjustment was: water cement ratio = 0.43–0.48, water volume = 82–88kg/m3, fly ash volume = 50–65%.
RCC production and placing
The mixing devices were three HZS75-1500 type compelling mixing stations, and every station was configured with a 400t cement silo and a 300t fly ash silo.
There were three concrete placing methods were used truck direct placing, cable placing and sub atmospheric pressure slide placing.
Two RCC layer paving paste options were used; a 1.0–1.5cm thick cement mortar layer was paved when the new and old layer combined; and a 1–1.5mm thick cement and fly ash pure mortar layer was paved between the RCC and 30cm layer within 23m before the dam to ensure anti-permeation.
Two BW-202D and two BW-75S compaction plants were used. The former was used extensively for compaction on the work face, while the BW-75S was used for corner section compaction, such as near the pattern plate, reinforcing bar and pedestal rock.
Roller-compaction belt direction is consistent with the spreading and levelling belt. They are all parallel to the dam axis.
For the concrete feed with paved thickness of 34–35cm, compaction run speed was controlled at 1–1.5km/hr. The vibration frequency was over 43Hz. The compaction belt combining length was 10–20cm, and the belt end section combining length was 1–1.8m.
Spray curing was used for the RCC placing face. The effect showed that spray work was effective at keeping the concrete surface wet. Compared to the method of no-spray/no-covering, this technique comprehensively decreased the concrete placing temperature (uncovered for about six hours) 7ºC–9ºC.
A heat preservation method was used during the upstream placing of cofferdam anti-permeation RCC. Winter construction of the main dam RCC began at the end of October 2000, and lasted for more than two months. The construction sections were as follows: gravity dam 1730.5m–1750.5m, arch dam 1716.0m–1732.6m, thrust pier 1722.1m–1738.9m, and the total concrete volume was 60,000m3. During the construction period, the highest temperature was 12ºC, the lowest was –13ºC and the average was about –2 to –4ºC.
The test proved that RCC can be ensured not to freeze under –10ºC conditions with about 4% concrete frost-resistance agent.
The power station experiences hot seasons lasting 70 to 80 days, during which temperatures typically reach 25–35ºC. Evaporation is intense and the climate is dry. The mean humidity during the hot season is about 30%. The temperature difference between day and night is also large.
Some RCC construction during the summer was inevitable. Up to the end of July 2000, the finished RCC volume was about 20,000m³, including an arch dam concrete volume 1698.7m–1709.0m and a gravity dam volume of 1711.6m–1716.1m.
From the preparation of construction to the commissioning of the first unit, the project took two years. The main dam RCC compaction also took one year.
Su Yong is with the Guiyang Hydroelectric Investigation Design & Research Institute of China, Guiyang, China, 550002. For more information email: [email protected]
This is an edited version of an article reprinted with permission from: RCC Dams, L. Berga, J.M. Buil, C. Jofre & S Chonggang (eds>), ISBN 90 5809 5649, A A Balkema Publishers, 2003 Lisse, The Netherlands<