Mechanism of uplift deformation of the dam foundation of Jiangya Water Power Station, Hunan Province, P.R. China Wu Faquan · Qi Shengwen · Lan Hengxing
Abstract Uplift deformation of Jiangya Dam in Loushui River, Hunan Province, P.R. China reached 28.4– 32.6 mm in 2 years and 5 months (December 1998–April 2001), which is unusual for dams worldwide. A perfectly confined aquifer, Yuntaiguan hot water aquifer, in the Jiangya syncline is recharged at a remote wing and outflows in front of the dam in the reservoir which is 875 m lower than the recharge point. The rise of the reservoir water level increases the head in the aquifer. The uplift deformation of the dam foundation corresponds spatially with the outcrop of the top interface of the aquifer and shows a direct time response to the fluctuation of the reservoir water level, indicating that the fluctuation of the reservoir water level is the dominant influence on the deformation. Numerical simulation shows spatial and dynamic similarities to the monitoring data. Numerical simulation of the cyclic reservoir water level confirms that the uplift is due to elastic deformation. Rsum Pendant deux ans et cinq mois ( Dcembre 1998–Avril 2001) les dplacements verticaux du barrage Jangya, situ sur la rivire Loushui, dans la province Hunan de la R.P. de Chine ont atteint des valeurs peu communes, de 28.4–32.6 mm. L’aquifre thermal captive de Yuntaiguan est recharg sur le flanc lointain du synclinal de Jiangya, o il est cantonn et se dcharge dans le rservoir du barrage dont le niveau et avec 875 m au dessous du point de recharge. Le niveau pizometrique de l’aquifre est augment par la housse de niveau dans le rservoir. Les dplacements verticaux correspondent au l’affleurement du toit de l’aquifre et prsentent une rponse temporelle directe aux fluctuations de niveaux du rservoir, en indiquant que celles-ci ont une influence dominante sur les dplacements. Les simulations numriques montrent des similarits spatiales et dynamiques Received: 29 January 2002 / Accepted: 8 July 2004 Published online: 15 January 2005 Springer-Verlag 2005 W. Faquan · Q. Shengwen ()) · L. Hengxing Key Lab. of Engineering Geomechanics, Institute of Geology and Geophysics, 100029 Beijing, P.R. China e-mail:
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avec les donnes du systme de surveillance. Les simulations numriques de variations cycliques des niveaux du rservoir ont confirm que les dplacements verticaux sont engendres par des dformations lastiques. Resumen En la presa de Jiangya en el rio Loushui, Provincia de Hunan, R. P. De China, la deformacin por levantamiento alcanz 28.4–32.6 mm en dos aos y cinco meses (Diciembre 1998–Abril 2001), lo cual no es comffln para las presas alrededor del mundo. El acufero de Yuntaiguan, con agua caliente, es perfectamente confinado, y es recargado en un flanco lejano del sinclinal de Jiangya, descargando su flujo enfrente de la presa, dentro del embalse, sitio que est 875 m ms bajo que el punto de recarga. El aumento en el nivel del embalse, incrementa la cabeza en el acufero. La deformacin por levantamiento en la cimentacin de la presa tiene una correspondencia espacial con el afloramiento de la interfase superior del acufero y muestra una respuesta directa en el tiempo con la fluctuacin del nivel del agua en el embalse, indicando que la fluctuacin del nivel del agua en el embalse es la influencia dominante sobre la deformacin. La simulacin numrica muestra similitudes espaciales y dinmicas con los datos de monitoreo. La simulacin numrica del nivel de agua cclico del embalse, confirma que el levantamiento se debe a deformacin elstica. Keywords Jiangya Dam · Uplift deformation · Hot confined aquifer · P.R. China
Introduction The Jiangya water power station is situated at the middle reach of the Loushui River, Hunan Province, P.R. China, where the river’s natural water level was 125 m (see Fig. 1). It has an installed capacity of 300 MW, normal pool water level 236 m (from sea level) and dead water level 188 m. It is a gravity dam with full section rollercompact concrete. The dam is 131 m high with bottom elevation 114 m (from sea level) and crest elevation 245 m, and the total crest length is 368 m including spillways, in dam sections No.5–No.7 as shown in Fig. 2. Construction of the dam commenced in July 1995 and was completed by the end of 1999. DOI 10.1007/s10040-004-0374-9
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Fig. 1 Regional map of the Jiangya hydropower station Fig. 2 Dam sections and safety monitoring network of El120 aisle
The water level of the reservoir rose to 170 m in August 1998, and reached the designed level of 236 m in November 2000. The dam monitoring network detected an uplift of the adjacent hills of 3.63–19.08 mm from August 1998 to December 2000. A monitoring line was set up along “El120 aisle”, which is built for drainage at the bottom (elevation 120 m) of the middle part (sections No.4–No.8) of the dam (Fig. 2). Observational data at “El120 aisle” shows that the uplift of the dam was 28.4– 32.6 mm from December 1998 to April 2001. Such a large uplift deformation of the dam foundation is rare in the world-wide history of hydro-power projects. Therefore, the uplift deformation, its developing tendency and its influence on the safety of the dam attracted serious attention. In this paper, the phenomenon of the uplift deformation of the dam foundation is introduced first, and then the mechanism is analyzed.
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Geology
General Conditions The dam site is located at the northwest side of the Jiangya syncline, which is open and gentle, and the strata in this area are monoclinal. The strike of the strata is almost perpendicular to the direction of river flow, dipping downstream at an average dip angle of 38 (Fig. 3). According to the geological survey (Hunan Institute of Investigation and Design of Water Conservation, 1991), the strata in this area are mainly sandy shale formations of the Devonian system and carbonate rock formations of the Permian system as described below: 1. Xiaoxi formation (Fm) (D2x): sandy shale, sandstone and quartz sandstone, thickness >200 m; 2. Yuntaiguan Fm (D2y): quartz sandstone with intercalated sandy-muddy shale, thickness 173 m. The outcrop of its top is located at 160–310 m upstream of the DOI 10.1007/s10040-004-0374-9
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Fig. 3 Sketch of geological structure near the dam site (Hunan Institute of Investigation and Design of Water Conservation, 1991) Table 1 Parameter values of the rock mass at the dam site and dam body Groups of strata
Moduli E;
Yuntaiguan(D2y) Huangjiadeng(D3h) Xiejingshi(D2x) Qixia-1(P1q 1) Qixia 2–6(P1q 2–6) Maokou (P1m) Wujiaping(P2w) Concrete of the dam
sort Gpa 11.0 9.6 9.6 15.5 15.0 9.7 9.5 40
Poisson’s ratio v
Friction angle f;
Cohesion C;
Tensile strength st
Densityr
Permeability K;
0.25 0.35 0.35 0.30 0.25 0.25 0.30 0.25
sort degree 31 31 31 33 33 33 31 45
sort Mpa 0.8 0.7 0.7 1.1 1.1 1.1 1.1 1.1
sort Mpa 1.5 1.0 1.0 1.8 2.5 2.5 2.0 2.0
sort kg/m3 2,640 2,630 2,630 2,640 2,670 2,680 2,650 2,450
sort 10 7m/s 115.7 0.46 0.46 0.46 11.57 11.57 0.57 0
dam. With well-developed fractures, it is the main aquifer in this area, and it is also a confined hot water aquifer. Because it is upstream and close to the dam, the changing seepage condition plays a significant role in the deformation of the dam foundation and adjacent hills. Therefore it is a main object of the research report here; 3. Huangjiadeng Fm (D3h) and lower member of Qixia Fm of the lower Permian system (P1q 1): sandstone, shale and muddy carbonate rock; 67 m thick. It is an impermeable stratum over unit D2y; The Qixia Fm consists of six members. For the sake of clarity, the Qixia Fm is noted as P1q . In other words P1q =P1q 1-6· The lower member of the Qixia Fm is noted as P1q 1 and P1q 2-6stands for the Middle to Upper member of the Qixia Fm; 4. Middle to Upper member of Qixia Fm (P1q 2-6): thick carbonate rock and dolomitic limestone, 376 m thick, fractured karst aquifer. It is located under the dam foundation and a wide area downstream of the dam. Owing to the good drainage system of the foundation, the influence on the deformation and stability of the foundation by groundwater seepage is not obvious; Hydrogeol J (2005) 13:451–466
5. Wujiaping Fm (P2W) and Dalong Fm (P2d): siliceous, dolomitic limestone; shale; stone coal seams. According to the Guangzhou Department, National Bureau of Seismology (1975), the basic earthquake intensity of this area is VI degree [corresponding peak ground acceleration (PGA) about 45–89 cm/s2], and the area is part of a tectonically stable plate.
Characteristics of the Confined Hot Aquifer According to the geological investigation, the Yuntaiguan Fm (D2y) is a confined hot aquifer, and has the following characteristics: (1) Occurrence of the groundwater The Yuntaiguan formation is a fractured confined aquifer. Permeability values of all the strata at the dam site are listed in Table 1. It is shown that the permeability of the Yuntaiguan formation is 3–5.8 times higher than that of the other strata. According to the geological structural analysis made in the preliminary design stage, the hot aquifer, Yuntaiguan formation (D2y), is confined in the “U” shaped Jiangya DOI 10.1007/s10040-004-0374-9
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Fig. 4 Topography and locations of boreholes and safety monitoring network
syncline in the section parallel to the river (Fig. 3). It is recharged at the elevation of 1,000 m at the southeast wing of the syncline, and the water moves 20–24 km along the aquifer and discharges in the reservoir where it is 875 m lower than that at the recharged point before the dam was built. (2) Water-pressure distribution By observation of boreholes ZK12, ZK20, ZK36, ZK99 (Fig. 4) at the dam site, the slope of the water table of the hot aquifer fluctuates between 0.58-0.98%, rising upstream. Observations from ZK132 and ZK133 (Fig. 4), which are 320 and 500 m downstream from the outcrop of the hot aquifer, respectively, show that the hydraulic head of the borehole is sensitive to the change of the reservoir water level, and is always 2–3 m higher than that of the reservoir. On the basis of the hydraulic-dynamic model illustrated in Fig. 3, considering the head of the recharge area to be 1,000 m, the discharge area 125 m, and the length of seepage passage 20,000 m, the initial hydraulic gradient of the hot aquifer can be calculated as I 0=0.04375. Once the water level of the reservoir reaches 236 m, the hydraulic gradient will reduces to I =0.0382. Hydrogeol J (2005) 13:451–466
The uplift pressure of the hot water aquifer acting on the above impermeable strata is one of the main causes for the uplift deformation of the rock mass. (3) Temperature of the aquifer According to analysis and observations, water temperature at the core of the syncline can be higher than 70 C, and reduces gradually to 53–36 C toward the drainage area upstream of the dam. Data from the boreholes show that:
1. The confined hot aquifer drains along the stratum continuously, and the flow is mainly concentrated in the middle-upper layers; 2. The stratum above the hot aquifer has good adiabatic properties and a low permeability. The hydrochemical characteristics of the hot aquifer also indicate that it is an independent hydrologic system. From the above observation and analysis it can be concluded that the dam site has the following engineering geological characteristics:
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– The dam was built on one wing of the syncline, and the strata dip downstream at a moderate angle; – Under the dam is a confined aquifer, recharged far from the dam, which drains the reservoir near the dam. The uplift pressure of the confined aquifer acting on the roof stratum may affect the stability of the rock mass of the dam foundation; – The confined aquifer is a hot water aquifer, and its temperature near the ground surface reaches 36–53 C.
about 5–6 mm. Moreover, the uplift at the top of the dam has a similar deformation pattern as at the El120 aisle of the dam. This indicates that the deformation distribution is integrated and differential deformation can not induce dam failure. – Uplift deformations of all the dam sections have synchronous response related to the water level of the reservoir. The dam displacement is sensitive to the rising water level but less sensitive to the falling level.
Characteristics of the Uplift Deformation of the Dam Foundation and Adjacent Hills
Deformational Pattern of Adjacent Hills
A safety monitoring network was designed in August 1998 in the critical area of the adjacent hills of the dam, and a leveling line was installed in December 1998 along the “El120 aisle” at the bottom of the dam when the reservoir water-level elevation was 170 m.
Characteristics of the Dam Deformation Monitoring points LD4-2, LD4-1, LD5-1, LD6-1, LD7-1, LD8-1, LD8-2 are distributed at the El120m aisle, LD means aisle, the number 4-8 stand for dam sections, and 1 and 2 are the monitoring point numbers for each dam section (see Fig. 5). Figure 5a shows the uplift deformation curves for different monitoring times, each curve related to a corresponding water level. The figure shows that all the dam sections present a similar regular uplift deformation, and the uplift displacement of the 7th section near the left side of the river is 32.6 mm, that is 4.2 mm larger than that of the 4th dam section. Figure 5b shows the uplift displacement of each monitoring point at El120 aisle vs. water level of the reservoir. The uplift displacement increases as the reservoir water level rises. However, the uplift displacement does not reach the topmost point when the water level goes up to the highest point, and it does not drop down immediately when the water level falls. In other words, the deformation has obviously a delayed response. According to observations and analysis, the delayed time is about 2–3 months. In order to monitor the uplift deformation along the dam, 13 monitoring points were established at the top of the dam during June 2000, and labeled as BD-01-BD-12 from the right side to the left. Figure 6a shows the uplift displacement distribution at different monitoring time, and Fig. 6b shows uplift displacement of each monitoring point vs. water level of the reservoir. They show similar characteristics as the displacement of points at El120 aisle, Fig. 5. By analyzing Figs. 5 and 6, one can see some trends of uplift deformation of the dam and its foundation: – With the rising of the reservoir water level, all sections of the dam are uplifted synchronously. The uplift deformation at left side of the dam is just slightly larger than that of the right side, and the largest difference is Hydrogeol J (2005) 13:451–466
The safety monitoring network around the dam site is shown in Fig. 4. The farthest point upstream is 820 m from the dam axis, and the farthest point downstream is 1,300 m from the dam. The initial monitoring was done in August–September, 1998 when the water level of the reservoir was 125 m, and six monitoring campaigns were performed from August 1999 through March 2001. Figure 7a, b represent the displacements at the 1st to 4th monitoring events for the left and right bank respectively. From the two figures one can see: In the direction parallel to the river, at the farthest monitoring points upstream, i.e. point 01JY at left bank (550 m from the dam) and 02JY at right bank (580 m from the dam), the uplift displacements are almost zero. The uplift deformation increases rapidly and reaches a peak at the top of the hot water aquifer (D2y), i.e. 19.08 mm for point 05JY at left bank (340 m from the dam) and 11.04 mm for point 06JY at the right bank (360 m from the dam). The uplift displacements decrease gradually to nearly zero at points 13JY-15JY of the left bank and 14JY-18JY of the right bank far downstream. The uplift displacement at the left bank is obviously larger than the right bank with the exclusion of some exceptional points. For instance, the largest vertical displacement of the left bank reaches 19.08 mm, but the corresponding displacement of the right bank is only 11.04 mm. Similar to the uplift of the dam, the vertical displacements at the points of the monitoring network along the river also correspond to the change of the reservoir water level. This indicates that the uplift deformation of the adjacent hills is closely consistent with that of the dam. Thus, one can infer that the dam and the adjacent hills deform simultaneously for the same cause and, therefore, differential deformation can not induce failure of the dam.
Mechanism of Uplift Deformation of the Dam and Foundation The uplift of the dam foundation and the adjacent hills is a rare engineering geological phenomenon. Not only does the mechanism provide a new subject for engineering geology, but also it may affect the safety of the dam. A series of investigations has been made, and five possible factors for the cause and effect have been suggested: 1. The reservoir impoundment causes a change of heat transfer conditions in the hot water aquifer in the DOI 10.1007/s10040-004-0374-9
456 Fig. 5 Displacement of points at El120 aisle; a for different monitoring time vs. point position; b for each point vs. water level of the reservoir
2. 3. 4. 5.
drainage area. The increase of temperature of the rock mass may induce thermal expansion; Mineral expansion may take place as a result of the change in the hydrological condition of the rock mass at the dam foundation; Simultaneous action of tectonic stresses may create deformation; Increasing uplift pressure on the foundation surface may cause uplift of the dam; Uplift pressure acting on the roof strata increases with the rising of the water level of the reservoir and causes uplift of the roof strata; on the other hand, increase of pore pressure in the confined aquifer induces the de-
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crease of effective stress, thus the aquifer expands by unloading. Following is a brief analysis for factors No. 2, 3 and 4, after which No. 1 and 5 are discussed in detail. Factor 2: The reservoir impoundment does not change the saturation state of the rock mass of the dam foundation because it was under the water table before the dam was built. Also the main expansive mineral, hydro-mica, has a weak swelling ability and the total thickness of the strata which contain hydro-mica is only 9.2 m. Therefore, the expansion caused by this expansive mineral is negligible compared with the recorded uplift of the dam.
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457 Fig. 6 Vertical deformation distribution of the dam top; a for different monitoring time; b for each section of the dam vs. water level of the reservoir
Factor 3: The uplift reached 32.6 mm during 2.3 years with a speed of 14.2 mm/a, and the intensity and direction of the uplift deformation of the dam changes much more rapidly than that induced by tectonic stresses. This is incompatible with the tectonic condition of the area (Hunan Institute of Investigation and Design of Water Conservation, 1991; Guangzhou Department, National Bureau of Seismology, 1975). Factor 4: A continuous impervious curtain was built and extended into the water-tight strata and has been working perfectly. Figure 8 shows the curves of the uplift Hydrogeol J (2005) 13:451–466
pressure at the shallow dam foundation of section 5 vs. the reservoir water level. It is obvious that the head of the groundwater in front of the curtain fluctuates consistently with the reservoir water level; however, the uplift pressure behind the curtain has almost no relationship with the change of reservoir water level. This is to say that the impervious curtain is well sealed, and the drainage system behind the curtain is effective. Therefore, one can affirm that the uplift pressure of the shallow dam foundation is not the main cause of the uplift of the dam foundation.
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458 Fig. 7 Vertical displacements of monitoring points at four water levels; a at the left bank; b at the right bank
Thermal Expansion of Rock Mass According to this model, three possible situations may lead to rock mass thermal expansion: 1. Temperature of the aquifer and its roof strata may increase because the water exchange rate slows down as the water level of the reservoir rises. 2 Seepage of hot water towards the overlying rock mass may increase because of the increase in pore pressure
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of the hot water aquifer, thus the temperature of the roof layer may increase. 3. Temperature at the drainage area of the hot water aquifer may rise because the water cover thickens with the increase of the reservoir water level. Following are some analyses of these inferences. 1. General analysis of the thermal expansion
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Fig. 8 Uplift pressure at the shallow dam foundation of section 5 and the reservoir water level vs. time of impounding
Coefficients of thermal expansion for the strata at the dam site are listed in Table 2. According to the data, to create a thermal expansion of a few centimeters, a thickness of hundreds of meters of rock masses and a temperature increment of more than 10 C are needed. However it is not the case here. 2. Thermal expansion induced by slowing down of hot water exchange The permeability of the Yuntaiguan Formation, measured by field tests, is K =1.157 10 5m/s, and as mentioned above, the initial hydraulic gradient is I 0=0.04375. Taking the length of the seepage passage as 20,000 m, then the flow time of water in the aquifer from the recharge area to the drainage area, can be calculated by Darcy law: s s 20000 t¼ ¼ ¼ ¼ 1250years v KI 1:157 105 0:04375 where s is the length of seepage passage and v is the seepage velocity. When the water level of the reservoir reaches 236 m, I=0.0382 and the flow time will be 1,432 years. Therefore, even in the initial case there is enough time to heat the water in the aquifer, and the change of reservoir water level does not affect the flow time and the water temperature significantly. 3. Thermal expansion of roof strata induced by hot water leakage Four temperature monitoring boreholes, DK1, DK2 on the left bank and DK3, DK4 on the right bank (Fig. 4) have been used near the dam for comparison of temperature development since previous measurements were made in borehole ZK36, ZK20, ZK16, and ZK99, respectively. As an example, Fig. 9 shows that the temperature of the water-tight roof strata at DK4 decreased after the reserTable 2 Coefficient of rock thermal expansion ( 10 6K 1) (Material Research and Testing Center, Wuhan University of Technology, 2001)
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voir impounding. The temperature was reduced by about 5 C. Obviously, the temperature decrease cannot lead to the uplift of the dam foundation, but may induce contraction of the rock masses. To monitor the temperature change of the dam foundation, thermometers were installed into the dam foundation at depths of 0, 1.5, 3.0 and 5.0 m, respectively. The monitoring curves from October 1996 to March 2000 (Fig. 10) show that the temperature of the base rock decreases from about 27 C in October 1996 to about 26 C by March 2000; and the temperature at the surface of the base is 1 C lower than that at the depth of 5 m. This decrease is caused mainly by drainage from holes behind the leakage-proof curtain. All these observations indicate that there was no thermal expansion induced by hot water leakage. Additionally, the water temperature at various depths in the reservoir was measured to investigate whether the water prevents thermal dispersion and leads to an increment of temperature at the outcrop area of the aquifer. Three monitoring lines were placed at distance of 20, 150, and 350 m away from the dam axis, respectively, and the water temperature at different depths, i.e. 0.5 m below the water surface of the reservoir at an altitude of 170 m, and 0.5 m above the bottom of the reservoir, were measured. Results showed that the water temperature is about 12– 14 C, and it is 1–2 C higher at the near surface than that at the bottom. In fact, according to thermal physics, water is a good thermal convection and dispersion medium. This means that there is no thermal effect by the water cover and no related thermal expansion. From the analysis presented above, it is concluded that the thermal expansion and the related rock mass deformation are negligible.
Influence of the Hydraulic Head of Confined Aquifer It can be seen from the observational data that the uplift of the dam and foundation is closely related to the reservoir water level. As discussed above, the uplift deformation is characterized by the following features: 1. Uplift is concentrated toward the roof of the hot water aquifer, and decreases sharply upstream and gradually downstream from the interface; 2. In the transverse river direction, the uplift decreases gradually from the middle of the dam (6th–8th dam sections) to the abutments; 3. Also, the uplift response is closely correlated with the fluctuation of the reservoir water level. Thus, it is certain that the reservoir impoundment is the predominant cause of the uplift.
Temperature ( C)
30
40
50
60
70
Carbonate rock Sandstone Shale
0.55–13.4 0.55–15.2 2.5–11.1
0.55–8.9 1.0–14.5 4.1–11.5
0.55–6.0 1.6–13.0 5.1–11.5
0.55–7.2 2.3–11.9 5.5–11.0
0.55–5.1 1.7–11.5
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460 Fig. 9 Temperature in borehole DK4 ( a) and ZK99 ( b)
Fig. 12 A differential element in the aquifer
Fig. 10 Rock temperature versus time in dam foundation at El120 aisle
Fig. 13 Uplift mechanism along transverse direction
a=
Fig. 11 Uplift mechanism along the river
Following is an analysis of the hydro-mechanical features of the confined hot water aquifer. According to the simplified geological model shown in Fig. 3, point C stands for the roof-line outcrop of the aquifer and point B stands for the interface point right beneath the dam. Thus the total uplift force in the unit width acting on the roof of the aquifer can be calculated as: 1 F ¼ rgðhB þ hC ÞLBC 1 2 1 ¼ rgð2hC þ I LBC þ LBC sin aÞLBC ð1Þ 2 where F = the total uplift force, MN; Hydrogeol J (2005) 13:451–466
38 , the slope of the confined hot aquifer; LBC ¼ 320=cos a ¼ 406:08m; the distance between point B and point C; hB, hC = the piezometric head of point B and point C respectively, m; r= 1,000 kg/m 3, mass density of the water; g= 10 m/s 2, the gravity acceleration; and I = the hydraulic gradient of the confined aquifer. As the water level of the reservoir rises to 236 m from 125 m, the hydraulic gradient of the confined aquifer I reduces from 0.04375 to 0.0382; meanwhile h C changes from 11m to 122 m, so the increment of the uplift force DF=446.2 MN. This is the force component for the uplift deformation of the dam. According to the analysis above, the uplift pressure of the confined hot water aquifer affects the uplift deformation of the overlay rock mass in two ways: DOI 10.1007/s10040-004-0374-9
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Fig. 14 Numerical simulation model Fig.15 Hydraulic boundary condition of FLAC3D model
1. Raising the reservoir water level induces the increase of the uplift pressure of the confined hot water aquifer, thereby causing an uplift deformation of the overlying rock mass. This situation corresponds to the mechanics of a variable thickness cantilever beam model under unevenly distributed forces (Fig. 11). Although the Hydrogeol J (2005) 13:451–466
uplift pressure perpendicular to the roof surface of the confined aquifer increases with increasing depth, the structural stiffness of the roof will increase too as the thickness of the roof strata increase in the downstream direction. Therefore, uplift deformation should be
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Fig. 16 Pore pressure distribution as the water level rises to 236 m
larger upstream for a relatively thinner cover and diminish downstream. 2. Rebound deformation induced by the decreasing effective stress of the confined aquifer is another component to be considered. Assume a differential element in the aquifer where one of its surfaces is parallel with the surface of the roof stratum as illustrated in Fig. 12. The water pressure difference caused by the reservoir impoundment, Dp equals the total stress difference at the point, Ds , i.e., Ds= Dp. If the modulus of the rock mass is E and the thickness of the deformed rock mass is H, then, according to the effective stress principle, rebound deformation u caused by the reservoir impoundment can be roughly estimated as: Ds Dp u¼ H¼ H E E If E =11GPa as tested, Dp=1.1 MPa and H =173 m, which is the thickness of the Yuntaiguan aquifer, the displacement will be u =17.3 mm. Moreover, a beam Hydrogeol J (2005) 13:451–466
model with two ends fixed can be applied also in the transverse direction. The beam is the roof strata above the confined hot aquifer. An evenly distributed uplift pressure acts on the roof surface, and an uneven pressure by the weight of the rock mass of the hills on both sides acts on the ends of the beam. This force system will lead to uplift deformation which is larger at the middle part of the dam than that the two ends (Fig. 13). This is consistent with the monitoring data. In conclusion, the predominant reason for the uplift of the dam and its adjacent hills is the increase of pore pressure of the confined hot aquifer, which is caused by rising of the reservoir water level.
Numerical Simulation of the Uplift Deformation of Dam Foundation by Seepage A numerical simulation has been used to study the effect of reservoir water level on the uplift deformation of the dam foundation. Software FLAC3D 2.0 by Itasca Consulting Group, Inc. (1997) is used. DOI 10.1007/s10040-004-0374-9
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Fig. 17 Uplift deformation distribution as the water level rises to 236 m
A model has been designed with the dimension of 800 m both up and downstream from the dam, 600 m to the two sides from the center of the dam, and 920 m vertically from the riverbed. In total, 6,864 units and 7,934 nodes including the dam are used in the model as shown in Fig. 14. The positive directions of the x, y , z axes point downstream, to the left bank and upward respectively. An elastic-plastic material model and MohrCoulomb failure criterion were used and coupled stressfluid flow was considered. (Block property bulk refers to the modulus of groups of strata and the dam body) The hydraulic boundary condition is shown in Fig. 15. The influence of hydraulic gradient has been taken into account along the hot water aquifer, i.e. the initial hydraulic gradient (water level of the reservoir being 125 m) corresponding to I 0=0.04375 and I =0.0382 for the 236 m water level of the reservoir, respectively, and changing in proportion to other water levels. Additional parameter values of the rock mass for the numerical simulation are listed in Table 1 according to the ‘Engineering geological investigation report’ (Hunan InHydrogeol J (2005) 13:451–466
stitute of Investigation and Design of Water Conservation, 1991). The following cases have been simulated: 1. The water level in the reservoir rises gradually from 125 to 236 m; 2. The water level in the reservoir fluctuates in the following sequence: 125–236–178–236–188–236 m. For the initial case without the dam, there is an uplift deformation which is concentrated to a core at the top surface of the hot water aquifer, decreasing rapidly upstream and gradually downstream. The maximum uplift reaches about 16 cm. Figures 16, 17, 18, 19 show parts of the simulation results. As the water level of the reservoir rises to elevation 236 m, pore pressure in the hot water aquifer increases and is distributed evenly. As shown in Fig. 16, isolines of the pore pressure turn sharply downstream and tend to parallel the top surfaces of the hot water aquifer. It can be inferred from this that the pore pressure decreases rapidly and the increment of the
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Fig. 18 Horizontal displacement as the water level rises to 236 m
pressure, about 1.1 MPa, acts directly on the interface of the roof. The uplift deformation of the foundation also concentrates at the top of the hot water aquifer near the ground surface (Fig. 17); and the horizontal displacement reaches its maximum in the downstream direction near the interface of the hot water aquifer (Fig. 18). Figures 17 and 19a show that the uplift displacement of the dam is about 45 mm. This is the total deformation value corresponding to the water level change from 125 to 236 m. Deducting the displacement value for the initial monitoring of 15 mm (related to the water level of 170 m), the uplift displacement increment for the monitoring period is about 30 mm, which agrees well with the monitoring data. In conclusion, results of the FLAC-3D simulations show patterns similar to the monitoring data. This confirms that the change of water level of the reservoir is the main factor affecting the uplift deformation of the dam foundation. Figure 19a, b show the calculated uplift displacements at monitoring points on top of the dam (Fig. 2) and the left Hydrogeol J (2005) 13:451–466
bank (Fig. 4) versus time steps respectively; each of the curves stands for one monitoring point. The reservoir water level fluctuates in the following sequence: 125– 236–178–236–188–236 m, reaching the designed water level three times to examine the residual deformation with the long-term-cyclic fluctuation of the reservoir water level. Each of the displacement curves reaches approximately the same value each time the water level rises to 236 m. This means that the residual plastic component is minimal and the uplift is mainly an elastic deformation.
Conclusions 1. The special hydrogeological conditions are the cause of the uplift deformation of the dam foundation. A perfectly confined aquifer which is part of a syncline,
Fig. 19 a Uplift displacements of monitoring points on top of dam vs. time steps. b Uplift displacements of monitoring points at the left bank vs. time steps DOI 10.1007/s10040-004-0374-9
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dips downstream at a moderate angle, and crops out in front of the dam. The aquifer is recharged at the other wing of the syncline where it is much higher than at the reservoir, and the increase of the reservoir water level dams up the head in the aquifer. 2. The uplift deformation of the dam foundation corresponds spatially with the outcrop of the top interface of Yuntaiguan hot water aquifer and shows a transient correlation with the fluctuation of the reservoir water level. This indicates that the fluctuation of the reservoir water level is the dominant influence on the uplift deformation. 3. Numerical simulation of the relation of the reservoir water-level fluctuations to the spatial and dynamic pattern of the calculated deformation is similar to the monitoring data, and that the uplift is dominated by elastic deformation. This confirms that the change of the reservoir water level is the dominant cause of the rock deformation.
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Acknowledgements This research is sponsored by the Special Funds for major state Basic Research Project under Grant No.2002CB412701. The paper is based on a research project supported by Lishui Exploitation Company, Hunan Province, P.R. China. The authors are grateful for the help from senior engineers Wang Shitong, Zhang Ruqiang and Luo Shidong. The authors thank Prof. Ove Stephansson who carefully reviewed and revised the manuscript. The authors acknowledge other anonymous reviewers whose comments substantially improved this manuscript.
References Guangzhou Department, National Bureau of Seismology (1975) Certification of basic earthquake intensity for Jiangya Reservoir in Cili county, Hunan province (in Chinese) Hunan Institute of Investigation and Design of Water Conservation (1991) Engineering geological investigation report for Jiangya Reservoir (in Chinese) Itasca Consulting Group, Inc., (1997) FLAC-3D (Fast Lagrangian analysis of continua in 3 dimensions), Version 2.00, User’s Manual (Volume V) USA Material Research and Testing Center, Wuhan University of Technology (2001) Testing Report no. 2001–05 and 2001–102 (in Chinese) Wu Faquan, Qi Shengwen (2001) On mechanism of uplift deformation of the dam foundation of Jiangya Water Power Station, Research Rep (in Chinese)
DOI 10.1007/s10040-004-0374-9
