Scholarly article on topic 'Effect of Climate Anomaly on Xixi Roller Compacted Concrete Gravity Dam'

Effect of Climate Anomaly on Xixi Roller Compacted Concrete Gravity Dam Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Procedia Earth and Planetary Science
OECD Field of science
Keywords
{"climate anomaly" / "RCC gravity dam" / "critical temperature" / "statistical model" / "uplift pressure coefficient"}

Abstract of research paper on Materials engineering, author of scientific article — Yan Xiang, Zhimin Fu, Hui Yuan, Zhanjun Wang, Yunjie Guan

Abstract Taking the event of low temperature faced by Xixi reservoir dam due to climate anomaly for example, the effect of climate anomaly on Xixi RCC gravity dam was systematically studied. Through analyzing observational data of uplift pressure of dam foundation, the conclusion that low temperature and increase of reservoir water level lead to the increase in uplift pressure was drawled qualitatively. To further analyze the relationship between uplift pressure coefficients and the changes of temperature or water level, the statistical regression model of uplift pressure coefficient was established. Then, the effect of low temperature and reservoir water level on the dam safety was analyzed qualitatively. The results show that in high water level conditions, extreme low temperatures resulting in a larger temperature difference will affect the safety operation of RCC gravity dam. By calculating the strength and stability of gravity dam under high uplift pressure, the approach of determining the critical temperature of dam damage is obtained and the critical temperature of Xixi reservoir dam due to climate anomaly is derived.

Academic research paper on topic "Effect of Climate Anomaly on Xixi Roller Compacted Concrete Gravity Dam"

Available online at www.sciencedirect.com

Procedía

Earth and Planetary Science

ELSEVIER

Procedía Earth and Planetary Science 5 (2012) 13 - 18

2012 International Conference on Structural Computation and Geotechnical Mechanics

Effect of Climate Anomaly on Xixi Roller Compacted

Concrete Gravity Dam

Yan Xianga, Zhimin Fub, Hui Yuana, Zhanjun Wanga, Yunjie Guanb, a*

aDepartment of Dam Safety Management, Nanjing Hydraulic Research Institute, Nanjing 210029, China bCollege of Hydrology and Water Resources, Hohai University, Nanjing 210098, China

Abstract

Taking the event of low temperature faced by Xixi reservoir dam due to climate anomaly for example, the effect of climate anomaly on Xixi RCC gravity dam was systematically studied. Through analyzing observational data of uplift pressure of dam foundation, the conclusion that low temperature and increase of reservoir water level lead to the increase in uplift pressure was drawled qualitatively. To further analyze the relationship between uplift pressure coefficients and the changes of temperature or water level, the statistical regression model of uplift pressure coefficient was established. Then, the effect of low temperature and reservoir water level on the dam safety was analyzed qualitatively. The results show that in high water level conditions, extreme low temperatures resulting in a larger temperature difference will affect the safety operation of RCC gravity dam. By calculating the strength and stability of gravity dam under high uplift pressure, the approach of determining the critical temperature of dam damage is obtained and the critical temperature of Xixi reservoir dam due to climate anomaly is derived.

©©2011 Publlshed by Elsevier Ltd. Selection and/or peer-review under responsibility of Society for Resources, Environment and Engineering.

Keywords: climate anomaly, RCC gravity dam, critical temperature, statistical model, uplift pressure coefficient

1. Introduction

Climate change is bringing huge influence on human environment and becoming a major issue that the world pays close attention to. China is one of the countries that are the most seriously affected by climate change[1]. People pay attention to the impact of many aspects of climate fluctuation, but the specific details of effect of the changes on dam lack studying. Zhang Jianyun, et al. [2] have done preliminary

* Corresponding author: Yan XIANG. Tel.: +86-025-85828145.

E-mail address: yxiang@nhri.cn

1878-5220 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Society for Resources, Environment and Engineering. doi:10.1016/j.proeps.2012.01.003

analysis of impact of climate change on reservoir dam. Climate change may have important impacts relating to reservoir dam including [3]: 1) the changes of spatial and temporal distribution of water resources; 2) the probable intensity of magnitude and frequency of occurrence of extreme weather, which leads to frequent flooding; 3) snow melting due to climate change, which increase the intensity of flood. Under the context of climate change, dam safety behavior relates to [4]: 1) the changes of rainfall and runoff of basin due to climate change, thereby affecting the design storm and flood of the basin; 2) the possibility of exacerbating the frequency, scope and extent of droughts occurrence , thereby affecting the guarantee rate of water supply; 3) the increase in intensity and frequency of storm, leading to geological disasters and increasing the impact of sediment on the safety and life of projects; 4) the variability of frequency and intensity of extreme hydro-climatic events, leading to super-standard flood. Researches show that[5], under the context of global climate change, drought, frequency and intensity of flood, typhoon, earthquake and other natural disasters will increase, meanwhile, the impact on the reservoir dam will be more serious. Aiming at the potential impact on reservoir operation due to climate change, Kelcy Adamec1 et al.[6] evaluated the impact of climate change on reservoir operation of Connecticut River area. Taking Xixi reservoirs for example, the impact of climate fluctuation on RCC dam is studied.

2. Project case

Xixi reservoir is located in Ninghai County, Zhejiang Province. The main project was completed in August 2006. The reservoir is a multi-year regulating hydraulic engineering with main objectives of flood control and water supply as well as irrigation, electricity generation. The dam is a RCC gravity dam with a total of 13 dam blocks and maximum height of 71.0m. It includes 10 non-spillway dam blocks, a diversion dam block, 2 spillway dam blocks. The geographical position and typical dam section are shown in Fig.1. It is with normal water level of 147.00m, design flood level of 152.02m, check flood level of 152.45m. The heavy rainfall caused by typhoon 16 "Rosa" made the reservoir reached its highest level 151.29m on October 7, 2007. 11 uplift pressure test holes are arranged in 3#~12# dam block to monitior the uplift pressure of dam foundation. Depth of the holes are below the base surface to 1.0m. Meanwhile, for monitoring temperature of the bedrock, bedrock thermometers are also laid.

Fig.1. The geographical position and typical section of Xixi reservoir dam

3. Analysis of observational seepage data of Xixi reservoir

The period of monitoring data is from June 27, 2007 to November 22, 2010. In the view of observed sequences of uplift pressure coefficients, before April 12, 2010, UP4 and UP5 uplift pressure coefficient was below 0.042 and 0.065 and the amplitude was small. From April 13, 2010 to April 28, 2010, UP4 and

UP5 uplift pressure coefficients surged to their historical maximum. In addition, the uplift pressure coefficients of other dam blocks did not have obvious phenomenon of abnormal increase. In order to analyze the sudden increase in uplift pressure coefficients, process lines of measured value of UP4 and UP5 uplift pressure vs. water level (see Fig.2) and process lines of UP4 and UP5 uplift pressure coefficient and the corresponding rock temperature (see Fig.3) were drawn respectively. Changes of various of environmental factors when the reservoir running high water levels are indicated in Table 1.

uplift coefficient 0.40

water level/m 160.00

120.00

20070607 20071025 20080313 20080731 20081218 20090507 20090924 20100211 20100701 20101118 date

Fig. 2. Process lines of measured value of UP4 and UP5 uplift pressure coefficients vs. water level

uplift coefficient temperature/°C 0.40 |--------- 18.00

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00

lit y

—■— UP4 uplift coefficient —"—UP5 uplift coefficient —•— UP4 rock temperature —*— UP5 rock temperature M*

17.00 16.00 15.00 14.00 13.00 12.00 11.00 10.00

20070607 20071025 20080313 20080731 20081218 20090507 20090924 20100211 20100701 20101118 date

Fig.3. Process lines of UP4 and UP5 uplift pressure coefficients and the corresponding rock temperature Table 1. The statistical analysis of each environmental factor change situation at high reservoir level

duration/d water level/m temperature/C

contact surface

water temperature of bedrock/°C

Bedrock temperature at measuring point/C

uplift pressure coefficient variation

(UP4/UP5)

2007.10.7~10.28 2010.3.10—5.1

147.00—151.29 147.00—149.86

24.3—15.3 -0.4—18.6

21.9^21.7 10.3^10.2

15.9 14.7^10.5

0.025/0.096 0.353/0.276

Fig.2 and Fig.3 and Table 1 show that the water level of 147.00m on October 7, 2007 as well as Oct.28, 2007, and the highest water level of 151.29m during the period which lasted 22 days. Moreover, air temperature change from 24.3C to 15.3C and water temperature of bedrock surface maintained at about

21.9°C. UP4 and UP5 uplift pressure coefficients were near 0.025 and 0.096 without obvious change. From March 10, 2010 to May 1, 2010, water level raised from 147.00m to 149.86m, sustained 51 days of high water level. During the period rainfall were 51mm and 36.6mm on April 11 and April 16 respectively; air temperature was from -0.4 C to 18.6C (including the temperature plummeted from 17.4C to 7C on April 1); water temperature of bedrock surface maintained at 10.2 C; UP4 and UP5 uplift pressure coefficients were up to 0.353 and 0.276. Therefore, during the high water level (147.00m) experienced in July 2007, the high water level lasted short and the temperatures were relative high. From April to May 2010, the high water level (up to 148.90m) lasted longer and the temperatures of bedrock were lower. The uplift pressure increase anomaly of UP4 and UP5 occurred on April 15, 2010, can be considered under low temperature, bedrock fracture opened, weak flow channels of bedrock opened and low temperature water through the flow channels leaded to reduce the temperature of cracks which would open further. With drawdown of water level and rise of temperature, uplift pressure gradually reduced. As on November 12, 2010, uplift pressure coefficients were 0.041 and 0.079 respectively.

4. Calculation and analysis of the intensity and stability of gravity dam under uplift pressure

Through analysis of seepage observed data, it can be obtained qualitatively that low temperature and increase in reservoir water level lead to increase in dam foundation uplift pressure. To further analyzing the relationship of uplift pressure coefficients with temperature change and water level, statistical regression model of uplift pressure coefficients will be set up for quantitatively analyzing impact of low temperature and water level on dam.

4.1. Statistical mo/el

According to experience the function relation of pressure coefficient a with temperature (T-T0) and water level changes (H-H0) has been built, namely:

a=Ai(H-Ho)+A2(T-To)+Bi(H-Ho)2 +B2(T-To)2+C (1)

There: H-water level, m; H0-initial water level, m; T-temperature, C; T0-initial temperature, C; a-uplift pressure coefficient; A1, A2, B1, B2, C-return factors.

By using seepage observed data and the regression calculation, results of regression coefficient (Ai, A2, B1, B2, C) are obtained in Table 2. The final function is as follows:

a=0.0284(H-H0)-0.0825(T-T0)+0.0028(H-H0)2-0.0017(T-T0)2+0.0388 (2)

Table 2. The regression coefficient of the statistical model

model parameters regression coefficient standard error

C 0.0388 0.0065

A1 0.0284 0.0011

A2 -0.0825 0.0094

B1 0.0028 0.0002

B2 -0.0017 0.0028

4.2. Stresses cn/ stability calculation of /cm by mechanics of materials metho/ 4.2.1. Solutions to the uplift pressure reduction factor

At normal water level, assuming the temperature difference (T-T0) are on six kinds of temperature conditions( 1°C, 2°C, 3°C, 4°C, 5°C, 6°C, respectively) , then the corresponding uplift pressure coefficients are calculated by the formula (2). The results are shown in Table 3.

Table 3. The calculated value of uplift pressure coefficient a under temperature change at normal water level

parameter serial number Ho H-H0 T-T0 (H-H0)A2 (T-T))A2 a

® 147.00 0.18 -1 0.0324 1 0.124747

® 147.00 0.18 -2 0.0324 4 0.201982

normal ® 147.00 0.18 -3 0.0324 9 0.275731

level ® 147.00 0.18 -4 0.0324 16 0.345993

© 147.00 0.18 -5 0.0324 25 0.412768

® 147.00 0.18 -6 0.0324 36 0.476057

4.2.2. Stress an/ stability calculations in case of different a values

The stability calculation formula of dam foundation and surface stress calculation formula are drawn from Design specification for roller compacte/ concrete /ams(SL314-2004) and Design specification for concrete gravity /ams(SL319-2005). According to on-site test of volume density of RCC, the average of second and third grade are 23.31kN/m3 and 23.35kN/m3 respectively; the shearing strength indicators of base surface are f=0.9 and c'=0.8MPa. The uplift pressure coefficients are calculated in case of combined with different temperatures. The corresponding calculation results of dam stability and stress, indicated in Table 4, show that the anti-sliding stability factor K' and the stress of dam heel 7 meet the specifications.

Table 4. Results of dam stability and stress at normal water level

temperature difference /C characteristic water level a normal water level K' a

-2 0.202 3.993 0.310

-4 0.346 3.809 0.250

-6 0.476 3.643 0.195

Note: a> K\ 7 represent respectively the uplift pressure coefficient, sliding stability factor and dam heel surface stress.

4.3. Determination metho/ of critical temperature of /am /amage

Calculation steps of threshold are divided into two steps. First, obtain the critical value of a where justly does not meet the requirements when calculating the dam stress and stability by trial calculation. Second, make a substituted into formula (2) and find the critical temperature.

First, estimate a value of a, such as a = 0.600, and calculate anti-sliding stability factor K' and normal stress of dam heel in accordance the above steps. The results meet the stability and stress conditions. Then, increasing a value (a = 0.800), K' meets the stability requirement, but tensile stress appears, namely less than zero, which indicates the dam cannot operate safely and reliably. Therefore, the values are obtained between 0.600 and 0.800 and with analogizing, when either K' or just does not meet the safety conditions, namely K' is just less than 3.0 or when tensile stress just appear, the spreadsheet is shown in Table 5.

The calculation results show when a is 0.739, the dam is in a critical safety state, that is the critical uplift pressure coefficient of dam damage. Make the value of a substituted into formula (2) and find that the corresponding critical value of temperature difference (T-T0) is 10.98 °C. It is demonstrated when the

temperature is lower than initial temperature of 10.98°C, tensile stress appears in dam heel. From the perspective of dam operation management, the case that temperature difference is higher than 10.98 C should be avoided and if there is sudden drop in temperature, water level should be properly controlled.

Table 5. Results of value of a calculated by trial calculation

trial time parameteF^^^^ 1 2 N

a 0.600 0.800 . ..... 0.739

K' 3.484 2.919 . 3.049

a 0.143 -0.022 . ..... -0.001

5. Conclusion

Based on the analysis of seepage observed data, it was obtained qualitatively that low temperature and increase in reservoir water level lead to increase in uplift pressure of Xixi RCC dam foundation. The statistical regression model of uplift pressure coefficients was set up and the effect of low temperature and water level on dam was quantitatively analyzed by mechanics of materials method. It was derived that in case of normal water level with gradually temperature decrease, when it reached 10.98C the tensile stress firstly appeared in dam heel, which indicated dam did not meet safety requirements. The results show that climate fluctuation has adverse effect on dam safe operation. To respond to climate fluctuation, some safety measures should be taken to reduce the risk of dam operation if a sudden drop in temperature occurs, such as properly controlling water level, and to strengthen the monitoring of dam seepage and security check.

Acknowledgements

The paper was supported by the National Natural Science Foundation of China (Grant No. 51179108, 50909066, 41102144) and the Fundamental Research Funds of Nanjing Hydraulic Research Institute (Grant No. Y711007, Y711005).

References

[1] Guo Huadong. Space observation and cognition of global change sensitive factors. Bulletin of Chinese Academy of Sciences,2010,25(2):167-169 (in Chinese)

[2] Zhang Jianyun, Wang Guoqing. Climate change and dam safety[J]. China water resources,2008(20):17-19 (in Chinese)

[3] Jiao Yong. Global warming and water security in China[J]. China water resources,2008(2):10-13 (in Chinese)

[4] He Ruimin, Wang Guoqing, ZhangJianyun, etc. Impacts of climate change on large-size water projects in China[J]. China water resources,2008(2):52-54 (in Chinese)

[5] Yan Xiang, Fuheng Ma, Hui Yuan. Preliminary study on the disaster vulnerability and resilience of the dam under the extreme event[C]. International Conference on Dam Safety Management, 0ct.22-24,2008 Nanjing, P.R.C

[6] Adamec1 K., Palmer,R. N., Polebitski A. et al. Climate Change Evaluation of Climate Change Impacts to Reservoir Operations within the Connecticut River Basin[C]. World Environmental and Water Resources Congress 2010: Challenges of Change:92-100