Evaluation of Groundwater Leakage into a Drainage

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Nov 14, 2001 - Rao SVN, Thandaveswara BS, Bhallamudi SM, Srinivasulu V (2003). Optimal groundwater management in deltaic regions using simulated ...

Rock Mech Rock Eng DOI 10.1007/s00603-015-0786-y

ORIGINAL PAPER

Evaluation of Groundwater Leakage into a Drainage Tunnel in Jinping-I Arch Dam Foundation in Southwestern China: A Case Study Yi-Feng Chen1 • Jia-Min Hong1 • Hua-Kang Zheng1 • Yi Li2 • Ran Hu1 Chuang-Bing Zhou1,3



Received: 16 December 2014 / Accepted: 13 June 2015  Springer-Verlag Wien 2015

Abstract The Jinping-I double-curvature arch dam, located in the middle reach of Yalong River and with a maximum height of 305 m, is the world’s highest dam of this type that has been completed. Since the second stage of reservoir impounding, after which the reservoir water level was gradually raised by about 232 m, a significant amount of leakage was observed from the drainage holes drilled in the lowest drainage tunnel at the left bank abutment at an elevation of 1595 m a.s.l. (above sea level), with an observed maximum pressure of about 0.3 MPa. A number of investigations, including water quality analysis, digital borehole imaging, tunnel geological mapping, and in situ groundwater monitoring, were performed to examine the source of leaking, the groundwater flow paths, and the performance of the grouting curtains. By defining two objective functions using the in situ time series measurements of flow rate and hydraulic head, respectively, a multiobjective inverse modeling procedure was proposed to evaluate the permeability of the foundation rocks that was underestimated in the design stage. This procedure takes advantage of the orthogonal design, finite element forward modeling of the transient groundwater flow, artificial neural network, and non-dominated sorting genetic algorithm, hence significantly reducing the computational

& Yi-Feng Chen [email protected] 1

State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China

2

School of Hydraulic Engineering, Changsha University of Science and Technology, Changsha 410114, China

3

School of Civil Engineering and Architecture, Nanchang University, Nanchang 330031, China

cost and improving the reliability of the inversed results. The geological structures that lead to the leakage were identified and the seepage flow behaviors in the dam foundation and the left bank abutment were assessed. Based on the field measurements and the inverse modeling results, the effects of the engineering treatments of the leakage event on the dam safety were analyzed. It has been demonstrated that the seepage control system is effective in lowering the groundwater level and limiting the amount of seepage in the dam foundation, and the leakage event does not pose a threat to the safety of the dam. Keywords Jinping-I arch dam  Groundwater flow  Fractured rocks  Seepage control  Inverse modeling

1 Introduction Located in the middle reach of Yalong River in Southwestern China (Fig. 1), the Jinping-I Hydropower Station consists of a double-curvature arch dam 305 m in height (Fig. 2), which is the world’s highest dam of this type that has been completed. The construction site is characteristic of a deeply cut valley environment and poor geological conditions. Geological structures, such as bedding planes, shear zones, fractures, dikes, and faults, are highly developed, which provide for potential groundwater flow paths through the dam foundation and the abutments. Seepage control, therefore, becomes a critical issue in limiting the amount of seepage flow, reducing the pore water pressure and improving the stability of the dam foundation. A seepage control system consisting of grouting curtains, drainage tunnels, and boreholes was designed and constructed in the dam foundation and abutments. Since the second stage of reservoir impounding, during which the

123

Y.-F. Chen et al. Fig. 1 Location of the Jinping-I Hydropower Project

0

300 600 km

Beijing

Yellow River

r

Riv er

Jinsha River

r

ve

Yalong River

Wuhan

Chengdu

ve

Ri

Jinping-I Hydropower Project

Ya lo ng

Shanghai

Ri

Jinsha

Yangtze Chongqing

Xichang

Panzhihua

Left bank Right bank Jinping-I arch dam

Fig. 2 Photo of the Jinping-I arch dam

reservoir water level was gradually raised by 61–232 m, a significant amount of leakage has been observed from the drainage holes drilled in the lowest drainage tunnel at the left bank abutment at an elevation of 1595 m a.s.l. (above sea level), with an observed maximum pressure of about 0.3 MPa at one typical borehole when the valve installed at

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its collar at the tunnel floor was closed. Owing to this, there was an urgent need to detect the origin of leaks and the potential concentrated flow paths, to reassess the permeability of foundation rocks and the effectiveness of the seepage-proof system, and to examine the influences of the leakage on the stability and safety of the dam.

Evaluation of Groundwater Leakage into a Drainage Tunnel in Jinping-I Arch Dam Foundation in…

Although a large number of borehole packer tests were performed during site characterization, erroneous estimation of the rock permeability may still occur due to improper layout of the test boreholes in the rock units and improper consideration of the anisotropic nature of the tested rocks (Coli and Pinzani 2014; Nilsen 2014). The inverse modeling approaches become an alternative effective tool for reassessment of the rock permeability using the field observations obtained by the groundwater monitoring system (Garzonio et al. 2014; Perello et al. 2014). In hydrology or hydrogeology, numerous inverse models (e.g., Neuman 1973; Woodbury and Ulrych 2000; Valstar et al. 2004) have been proposed using various optimization techniques (e.g., Karpouzos et al. 2001; Rao et al. 2003; Gill et al. 2006) for back calculations of the aquifer parameters with an acceptable computational cost. Most of the models, however, relied only on the hydraulic head observations for construction of the objective function (e.g., Shigidi and Garcia 2003; Garcia and Shigidi 2006; Karahan and Ayvaz 2008; Virbulis et al. 2013), making the inverse solution plagued with the non-uniqueness problem (Mao et al. 2013). Furthermore, quite a number of the models (e.g., Yang et al. 2004; Garcia and Shigidi 2006; Hernandez et al. 2006; Virbulis et al. 2013) assumed steady-state flow conditions for the inverse calculations, disregarding the transient nature of groundwater flow induced by fluctuation of the reservoir water level and change of the boundary condition. To improve the reliability of the inverse solution and better address the nonuniqueness problem, therefore, transient flow models are preferable for the back calculations (e.g., Alcolea et al. 2006; Bastani et al. 2010; Dai et al. 2010; Perello et al. 2014; Zhou et al. 2015), in which both the hydraulic head and discharge time series measurements are utilized for the construction of a couple of the objective functions. In this case study, on the basis of the site characterization results obtained at the design and construction stages, the geological features of the dam foundation were reassessed using the borehole packer tests and the borehole TV images performed at the grouting curtain section, together with the fracture mapping data from the drainage and grouting tunnels. The source of the leakage was evaluated using the field groundwater monitoring data and the water chemical analysis results, and the preferential groundwater flow paths were identified using the field investigation data. An inverse modeling procedure combining the orthogonal design (OD), finite element (FE) forward modeling, artificial neural network (ANN), and non-dominated sorting genetic algorithm (NSGA2) was adopted for determination of the hydraulic conductivities of the rock units that were underestimated in the design stage, such that two error functions constructed respectively with the field time series observations of the hydraulic head and discharge were

simultaneously minimized. The effectiveness of the seepage control system consisting of grouting curtains, drainage tunnels, and drainage hole arrays was then assessed with the back calculated parameters. Treatments of the leakage event were presented and their effects were analyzed based on the site characterization and inverse modeling results.

2 Site Characterization 2.1 Project Description Located on the border between Muli and Yanyuan County (Sichuan Province, China), the Jinping-I Hydropower Station is the first level of the cascade of dams in the middle reach of Yalong River (Fig. 1), mainly for power generation as well as sediment trapping and flood control purposes. It consists of a double-curvature arch dam 305 m in height (Fig. 2), a diversion tunnel 1.2 km long in the left bank, and an underground powerhouse cavern system located in the right bank of the mountain, 350 m downstream of the dam axis. It has a total installed capacity of 3600 MW and a reservoir capacity of 7.76 9 109 m3 at the normal pool level of 1880 m a.s.l. The river course at the construction site was blocked on December 4, 2006 for construction of the dam. Excavation of the dam foundation was completed in September 2009, and the first two turbine generators were put into operation on August 30, 2013. 2.2 Geological Settings As shown in Fig. 3, the dam site is located in a typical deeply cut V-shaped valley, with the left bank slope being steep and over 1000 m in height. Along the river direction (N28E), the slope has a uniform shape below 1810 m a.s.l., with a dipping angle of about 50 on the upstream side of section V–V and 60–70 on the downstream side (Fig. 4). Above 1810 m a.s.l., the gradient of the slope becomes gentle, and gullies are developed with a depth of about 50 m. As shown in Figs. 3 and 4, the main strata outcropping at the dam site consist of a series of epizonal metamorphic rocks of the Zagunao group of upper to middle Triassic system (T2–3z), corresponding to the second 2 3 member (T23z ) and the third member (T23z ) of the regional strata (Qi et al. 2004). The second member, outcropping between 1690 m and 1900 m a.s.l. in the left bank slope, is about 600 m in thickness and consists of marble, breccia marble, lens of calcareous tuff phyllite and green schist, and thin layers of crystalline limestone. This 2ð1Þ

member is subdivided into eight layers (from T23z to 2ð8Þ

T23z ) according to the mineralogical composition and structural characteristics of the strata. The third member,

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Y.-F. Chen et al.

Fig. 3 Layout of the Jinping-I Hydropower Project

400 m in thickness and outcropping between 1900 m and 2300 m a.s.l., is grouped into six layers (from

3ð1Þ T23z

to

3ð6Þ T23z

) and mainly consists of slate and metamorphosed sandstone, among which the metamorphosed sandstone accounts for about 51.6 % and the slate about 48.4 %. The strata strike almost parallel to the river (N0–30E), dipping 25–45 towards NW. As shown in Fig. 4, the main structures in the left bank slope consist of the Santan overturned tight syncline (with 3ð6Þ

T23z at the center), a lamprophyre dike (X), deep cracks, a shear zone, and three large-scale faults (F2, F5, and F8). The lamprophyre dike X strikes N60–80E and dips to SE with a dipping angle of 70–80. It extends over 1000 m in the left bank slope with a thickness of 2–3 m. The deep cracks are frequently developed in the left bank slope at horizontal depths of about 50–330 m (Qi et al. 2004), and

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this phenomenon rarely occurs in other valley conditions. The shear zone is developed in the sixth layer of marble 2ð6Þ

(T23z ) and is composed of fault F2 and several minor faults of about 5–30 cm in width. Fault F2, outcropping between 1650 m and 1700 m a.s.l. in the second member 2ð6Þ

(T23z ) of the regional strata (N30–40E/NW \40–56), is about 0.2–0.8 m in thickness and mainly composed of schistose, cataclastic, or mylonitic rocks. Fault F5, about 1.8 km long, extends through the middle and lower part of the slope (N40–50E/SE\70–80). It is a strike–slip thrust fault with a displacement of 70–90 m and a thickness of about 0.5–6.0 m, and mainly contains breccia, cataclasite, and fault clay. Fault F8 is developed in the hanging wall block of fault F5, oriented almost parallel to fault F5, and with a thickness of 0.5–1.0 m. Besides the geological structures mentioned above, four groups of critically

Evaluation of Groundwater Leakage into a Drainage Tunnel in Jinping-I Arch Dam Foundation in… m a.s.l.

Left bank

2100

Legend

Lamprophyre dike X Boundary of drainage hole arrays P207

Sa n tig tan ht ov sy er nc tur lin ne e d

2000

1900

Boundary of grouting curtain

Original ground surface

P217

Fault or dike 33 Lu P231

Dam crest

Boundary between marble and sandstone

1885

P206

Boundary of strata 43 Lu

1800

Initial groundwater level 73 Lu

Boundary of grouting curtain

Lower boundary of weakly weathered zone

PL G -2

1

P208

1700

Shear zone

6 Lu

5

Initial groundwater level

G

-1

Grouting tunnels

PL

1600

Boundary of drainage hole arrays

7 Lu

P208

6 G- -4 PL PLG

1525

1500

-3

G PL

1

LG

P

Borehole packer test data

1481

2 tF ul Fa

Piezometers at the upstream side of grouting curtain

1430 Fault F5

1400

1000

900

800

700

600

500

400

300

200

Borehole and its number

73 Lu

100

Piezometers at the downstream side of grouting curtain

0

m

Fig. 4 Geological cross-section G–G along the grouting curtain of the arch dam (with its trace shown in Fig. 3)

oriented joints are developed in the left bank slope, with their preferential orientations being N40–65E/NW\30– 45, N50–70E/SE\50–80, N20–40E/SE\60–85, and N50–70W/NE\60–80, respectively. Long-term groundwater observations showed that the groundwater level in the left bank slope was relatively low before impounding, only slightly higher than the riverbed water level, and with a gradient of about 4 % discharging to the Yalong River, as shown in Fig. 4. This phenomenon results mainly from the relatively low permeability of the 1 first member of the regional strata (T23z ) and the high 2 and water conductivity of the overlying strata (T23z 3 T23z ). To obtain the hydraulic properties of the rock masses, a total number of 2231 borehole packer tests were conducted at the dam site, among which 774 packer tests were performed in the left bank slope. The borehole packer test results showed that 46.7 % of the packer test intervals are of low permeability (q \ 3 Lu), 28.6 % being of moderate permeability (3–10 Lu), 22.6 % being of medium permeability (10–100 Lu), and only 2.1 % being of high permeability (q [ 100 Lu), where q is the permeability rate in Lugeon units (Lu). Rocks of high permeability (q [ 100 Lu) mainly occur in the fourth layer of metamorphosed 3ð4Þ

sandstone (T23z ) and in the fifth to eighth layers of mar2ð58Þ

bles (T23z ) at high elevation. Figure 4 shows the variations in hydraulic conductivity along some boreholes at the grouting curtain section. In general, the hydraulic conductivity reduces with the increase of both vertical and

horizontal distances from the slope surface (except in deep crack zones), mainly because of sparser release fractures, lower weathering degree, and higher in situ geostress at greater depths. 2.3 Seepage Control and Monitoring Systems Given the poor geological conditions at the dam site, a seepage control system consisting of grouting curtains, drainage tunnels, and drainage hole arrays was constructed in the dam foundation and the abutments, for reducing the pore water pressure and limiting the amount of leakage from the reservoir. Figures 3 and 4 show the layout of the seepage control system in the left bank. The grouting curtain in the left bank abutment was linked to the grouting curtain in the dam foundation to form a seepage-proof system of good integrity. It was constructed from six layers of grouting tunnels at elevations of 1885, 1829, 1778, 1730, 1670, and 1601 m a.s.l., respectively. As shown in Fig. 4, the curtain was extended downwards to about 1430 m a.s.l. in the dam foundation, and with the increase of the horizontal depth from the slope surface, the depth of the grouting curtain in the dam abutment was gradually reduced. The design criterion was to construct the grouting curtains onto the bedrocks with a permeability rate smaller than 1 Lu so that the groundwater flow through the weak structural planes, including the lamprophyre dike, faults, shear zone, and fracture zones, was effectively cut off.

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Y.-F. Chen et al.

2.4 Leakage Event in the Drainage Tunnel at 1595 m a.s.l.

Drainage holes of 110 mm in diameter and 3 m in spacing were drilled from five layers of drainage tunnels excavated at elevations of 1829, 1785, 1730, 1670, and 1595 m a.s.l., respectively, about 11.5 m downstream of the seepage-proof system. Besides, five layers of drainage tunnels were constructed at elevations of 1829, 1785, 1730, 1670, and 1618 m a.s.l., respectively, in the massive concrete block for stabilizing the left bank abutment, from which drainage holes were also drilled for reducing the water pressure in the dam abutment (see Fig. 3). Furthermore, a groundwater monitoring system consisting of 36 piezometers and six weirs was installed in the drainage and grouting tunnels in the left bank, for monitoring the realtime operation of the seepage control system and groundwater flow behaviors in the dam foundation and the left abutment. The piezometers, numbered from PLG-1 to PLG-36, were installed in the boreholes drilled downwards from the grouting tunnels with depths of 30.0–95.0 m, as shown in Fig. 4. Among the 36 piezometers, 12 of them were located on the upstream side of the grouting curtain and the rest were located on the downstream side. The weirs were installed at the middle positions or outer ends of the drainage tunnels for collecting the seepage flow rates out of the drainage system. Particularly, the discharge to the drainage tunnel at 1595 m a.s.l. from chainage k0 ? 000 to k0 ? 226 m was measured by weir WEDB-3, and the amount of groundwater from chainage k0 ? 226 to k0 ? 447 m was collected by weirs WEDB-27(28) (two weirs at the same position), as shown in Fig. 5. The flow rate to the drainage tunnel at 1670 m a.s.l. was measured by weir WEDB-8.

X

4.0

+0

ike pro phy r

Flow rate (L/s)

F5 k0

76

Drainage tunnel at 1595 m a.s.l. k0 +

15

1

k0 +

Drainage hole at k0+267 m

25

22

50 m N

3.0

Lam

Grouting tunnel at 1601m a.s.l. 000

0 3.5

ed

F2

WEDB-27(28)

WEDB-3 k0+

The impounding of the reservoir started on November 30, 2012, and it involved four stages, as shown in Fig. 6. The first stage lasted from November 2012 to May 2013, during which the reservoir water level rose from 1648 m to 1706 m a.s.l. and was maintained at 1706 m a.s.l. for about 157 days. The second stage started in June 2013 and ended in August 2013, during which the reservoir water level rose from 1706 m to 1800 m a.s.l. and was maintained at 1800 m a.s.l. for about 47 days. From September 2013 to May 2014, the reservoir water level fluctuated between 1800 m and 1840 m a.s.l., which was defined as the third stage of impounding. In the fourth stage (from May 30 to August 23, 2014), the reservoir water level was gradually raised to the normal pool level (1880 m a.s.l.). Since the second and third stages of reservoir impounding, a significant amount of seepage was continuously observed out of the drainage tunnel at the left bank abutment at an elevation of 1595 m a.s.l. (Fig. 6). For example, on July 23, 2013, when the corresponding reservoir water level was about 1800 m a.s.l., the seepage flow rate out of the drainage tunnel amounted to 37.3 L/s (7.7 L/s of the discharge originated from the section between chainage k0 ? 000 and k0 ? 226 m, and 29.6 L/s from the rest section between k0 ? 226 and k0 ? 447 m), which was much larger than the expected value in the design stage. Site investigation showed that the leakage was mostly contributed by the drainage holes drilled downwards from the tunnel, rather than the boreholes

Drainage hole blocked on Dec 29, 2013 Drainage hole grouted on Jun 7, 2014

2.5 2.0 1.5 1.0 0.5 0.0 252 258 264 270

342 348 354 360

411 417 423 429 435 441

Chainage (m) 6

k0+276

k0+363

k0+447

Fig. 5 The measured flow rates out of the drainage holes in the drainage tunnel at 1595 m above sea level (a.s.l.) on July 23, 2013, when the reservoir water level was about 1800 m a.s.l.

123

Evaluation of Groundwater Leakage into a Drainage Tunnel in Jinping-I Arch Dam Foundation in…

1.5 1650

1850

Reservoir water level

1800 1750 1700

10 5

Aug 23, 2014

Sep 1, 2013

15

1900

IV

WEDB-8

WEDB-3

1650

1.0 12/11/01 13/03/01 13/07/01 13/11/01 14/03/01 14/07/01 14/11/01

0 12/11/01 13/03/01 13/07/01 13/11/01 14/03/01 14/07/01 14/11/01

Time (yy/mm/dd)

Time (yy/mm/dd)

Reservoir water level (m a.s.l.)

1700

20

Jun 1, 2013

1750

25

III

II

WEDB-27(28)

30 Nov 30, 2012

1800

35

I

b

May 30, 2014

2.0

Aug 23, 2014

Reservoir water level

May 30, 2014

Sep 1, 2013

2.5

Jun 1, 2013

3.5 3.0

40

1900

IV

Flow rate (L/s)

III

II

1850

Nov 30, 2012

Flow rate (L/s)

4.0

I

a

Reservoir water level (m a.s.l.)

4.5

Fig. 6 The reservoir impounding process and the variations of the measured discharges: a out of the drainage hole at chainage k0 ? 267 m and b through weirs WEDB-3, WEDB-8, and WEDB-27(28)

drilled upwards. The measured flow rates out of each of the drainage holes with relatively larger amount of leakage are illustrated in Fig. 5. It can be observed that, along the tunnel axis, there are three segments at chainage k0 ? 249–273, k0 ? 342–363, and k0 ? 411–435 m in which the drainage holes display much larger leakages. Among these drainage holes, the one located at chainage k0 ? 267 m dominates with the largest amount of leakage, reaching 3.7 L/s at the reservoir water level of 1800 m a.s.l. and with a pressure of about 0.3 MPa when the valve installed at its collar at the tunnel floor was closed. No seepage erosion, however, was found to occur in the foundation rocks, faults, and weak zones. 2.5 Treatments of the Foundation Leakage The considerable amount of leakage out of the drainage tunnel at 1595 m a.s.l. raised a concern on the effectiveness of the seepage control system and the safety of the dam. After a series of tentative experiments and assessments, two engineering measures were taken to treat the leakage event. The first one was simply to block 29 adjacent drainage holes of relatively larger leakage amount (see Fig. 5 for their locations) by closing the valves installed at the collars. As shown in Fig. 6, this treatment led to a sudden decrease of the measured flow rate through wears WEDB-27(28) by 4.3 L/s (from 30.4 to 26.1 L/s) on December 29, 2013. The second treatment was to perform cement grouting at three drainage holes (see Fig. 5 for their locations) at the beginning of the fourth impounding stage. The grout was prepared with a water:cement ratio of 0.45:1, and it was injected with a pressure of up to 3.0–6.5 MPa, until an injection rate lower than 1.0 L/min was obtained and maintained over 30 min. This treatment further resulted in a decrease in the measured flow rate

through wears WEDB-27(28) to about 19.5 L/s on June 7, 2014, as shown in Fig. 6.

3 Analysis of the Leakage 3.1 Origin of Leaks The leakage event in the drainage tunnel at 1595 m a.s.l. engendered a controversy about the origin of the leaks from the reservoir or from the groundwater in the left bank slope. Figure 6 plots the variations of the measured discharges out of the drainage hole at chainage k0 ? 267 m and through weirs WEDB-3, WEDB-8, and WEDB-27(28). It can be observed that the measurements overall varied consistently with the reservoir water level. Besides, water chemical analyses were performed on August 21, 2013, when the reservoir water level was about 1800 m a.s.l. Water samples collected from the reservoir, from the drainage hole at k0 ? 267 m, from the weirs WEDB27(28), and from the inner end of the drainage tunnel at 1670 m a.s.l. (which represents the groundwater in the left bank slope) were analyzed. The determined pH value, the contents of Ca, Mg, Na, K, Cl, SO4, HCO3, CO3 ions, and the total dissolved solids (TDS) are listed in Table 1. Figure 7 shows a piper diagram of the water samples. The results show that the groundwater in the left bank slope has an Mg-HCO3-SO4 hydrochemical facies and a relatively high TDS value (668–783 mg/L) for its long pathway through the fractured aquifers. The groundwater out of the drainage hole at k0 ? 267 m and through the weirs WEDB-27(28), on the contrary, has lower contents of Mg, HCO3, SO4 ions, and a much lower TDS value, with its chemical composition closer to that of the reservoir water. The content of SO4 ion rises in the groundwater as it moves

123

Y.-F. Chen et al. Table 1 Results of water chemical analysis Source of water samples

pH

Ion content (mg/L) Ca

Mg

TDS (mg/L) Na

K

Cl

HCO3

SO4

Drainage hole at k0 ? 267 m

8.3

25.61

13.04

6.36

4.13

4

39.75

105.84

Weirs WEDB-27(28)

9.6

11.08

14.80

10.12

10.83

2

51.5

56.04

Reservoir

8.4

25.37

10.85

3.48

0.86

2.41

16.94

115.53

Groundwater in the left bank no. 1

9.0

11.46

62.96

41.82

18.92

Groundwater in the left bank no. 2



39.57

89.24

33.89

4.34

0 10

667.89 783.29

80 4

40

60

60

20

20

10 20

3

CO

40

+H CO

3

80 60

40

60

80 0 10 0

0

0

20

0

(T23z ) in the hanging wall block of fault F2, though deeply seated, have much higher permeability than the expected values in the design stage, with a considerable

0

SO 4 40

10

Before construction of the grouting curtain, a large number of borehole packer tests were conducted at the grouting curtain section from the grouting tunnel at 1601 m a.s.l. The results, plotted in Fig. 8 and summarized in Table 2, show that the zoning of rock mass permeability in the design stage is remarkably different from the borehole packer test observations. Conventionally, the rock permeability is expected to decrease with the increase of vertical or horizontal depths from the river slope surface. At the grouting curtain section, however, the rock formations

60

80

3.2 The Potential Leakage Paths

20

80

20

0

Mg

10

0

0

0

0

Drainage hole at k0+267 m Weirs WEDB-27(28) Reservoir Chainage k0+530 m of the drainage tunnel at 1670 m a.s.l. (Groundwater in the left bank No.1) Chainage k0+508 m of the drainage tunnel at 1670 m a.s.l. (Groundwater in the left bank No.2)

g + M 40 Ca

Cl+ SO

0 0

80

+K 60 40

through the rocks rich in pyrite. The above results demonstrate that the leakage into the drainage tunnel at 1595 m a.s.l. is mainly originating from the reservoir.

123

119.24

295.24

0

Na

40

60

Ca

2ð68Þ

142.70

366

20

80

0

148.00

229

0

100

0 12.25

187

10

Fig. 7 Piper diagram of water samples collected from different positions

4.46 32

CO3

20

40

60

80

100

Cl

number of the test intervals displaying a permeability rate q [ 10 Lu. This phenomenon was not given enough attention in design, and the depth of the grouting curtain was not adjusted to effectively cut off the high-permeability rocks in the sixth to eighth layers of the second member of the regional strata. As a result, the lower boundary of the curtain was gradually raised from 1430 m a.s.l. at the river side to 1525 m a.s.l. at the mountain side. 2ð68Þ

Therefore, the high permeability of the rocks in T23z and the improper design of the grouting curtain depth could be the major reasons for the leakage into the drainage tunnel. The performance of the grouting curtain, on the other hand, plays an important role in seepage control, and may significantly influence the amount of groundwater flow out of the drainage tunnels. The variability of the geological and hydrogeological conditions and the difficulty in uniform construction quality control may make the hydraulic conductivity of the grouting curtain deviate from its original design value (Li et al. 2014). However, the packer tests

Evaluation of Groundwater Leakage into a Drainage Tunnel in Jinping-I Arch Dam Foundation in…

F ult Fa

Symmetry axis of the arch dam 2

k0+000.00

k0+264.00

lt F 5 u Fau L 00

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