Investigation of the Geological and Geotechnical Characteristics of

Investigation of the Geological and Geotechnical Characteristics of Daroongar Dam, Northeast Iran

M. Ghafoori, G. R. Lashkaripour & S. Tarigh Azali

Geotechnical and Geological Engineering An International Journal ISSN 0960-3182 Geotech Geol Eng DOI 10.1007/ s10706-011-9429-6

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Author's personal copy Geotech Geol Eng DOI 10.1007/s10706-011-9429-6

ORIGINAL PAPER

Investigation of the Geological and Geotechnical Characteristics of Daroongar Dam, Northeast Iran M. Ghafoori • G. R. Lashkaripour S. Tarigh Azali

•

Received: 30 June 2008 / Accepted: 4 July 2011 Ó Springer Science+Business Media B.V. 2011

Abstract This paper discusses the results of the engineering geological and geotechnical investigations that have been carried out at the Daroongar dam site. According to the geomorphology and geological conditions and economic reason, the dam has been designed as an earth dam with a clay core. The dam foundation is composed of a sequence of sandy limestone and limy marl of the Upper Cretaceous period. This study is based on field and laboratory investigations, surface discontinuity surveying, drilled borehole data and permeability of dam foundation. The present studies include the evaluation of the dam foundation by water pressure tests. The water pressure tests indicate the necessity to provide a grout curtain below the dam foundation.The geology of the Daroongar dam foundation has a significant influence on the permeability and groutability characteristics. The permeability of jointed rock masses is strongly depended on joint characteristics; degree of jointing, opening, continuity and presence of filling materials. The laboratory tests included tests for unit weight, porosity, uniaxial, triaxial, tensile strength and deformation parameters. The strength and modulus of elasticity of rock masses were determined using the Hoek–Brown empirical strength criterion. The rock mass qualities and M. Ghafoori (&) G. R. Lashkaripour S. Tarigh Azali Department of Geology, Ferdowsi University of Mashhad, Mashhad, Iran e-mail: [email protected]

classifications of the dam site is assigned using the rock mass rating (RMR), the rock quality (Q) and the geological strength index (GSI) classification systems. Keywords Dam Geological Geotechnical Rock mass Classification Discontinuity Permeability

1 Introduction The under construction Daroongar dam with a crest length of about 470 m, a maximum height above the river bed level of 40 m, and a total storage capacity of about 20 million m3 will be built on the Daroongar river, about 79 km northeast of Quchan city in the northeast of Iran (Fig. 1). The watershed of Daroongar river in Emamgholi mountains at west of DarrehGaz city is part of Gharaghum main watershed with an area of 1,005 km2. The Daroongar dam is located on a sedimentary rock with rugged terrain and complicated geological properties. The dam and reservoir sites are located in an active seismic region of rugged mountainous terrain with steep slopes. Three types of dam were considered, roller compacted concrete dam, concrete gravity dam and earth dam. Based on geological conditions, lower costs and lower construction risks, the earth dam with a clay core option was found to be the best solution (Fig. 2). The dam will store water for domestic and irrigation purposes for developing agricultural land for the

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Author's personal copy Geotech Geol Eng

Fig. 1 Location map of the Daroongar dam site

Darreh-Gaz area, which is located about 35 km northeast of dam site. The bedrock inherently has weakness planes such as faults, beddings, joints, and fractures, which are the major factors that affect the engineering properties of rock foundations such as permeability, shear strength, and deformation. This study describes an engineering geological assessment of the dam foundation and focuses on the geotechnical behavior of the rock masses that affect the stability of dam abutments and the ability to control seepage that may occur through the fractured rock masses. Geological and geomechanical surveys and various laboratory and in situ tests were carried out to evaluate the characteristics of the rock masses and intact rocks at the dam site and reservoir area. Water pressure test is the most common and appropriate method in order to determine rock mass permeability due to the presence of weak planes. The results of rock mass permeability test are strongly related to the geometric characteristics and weathering degree of the water paths (Ewert 1997; Karagu¨zel and Kiliç 2000). The results of the water pressure tests can be used to delimit the zones of the dam foundation that show different rock mass quality (Foyo et al. 2005). At present, it is broadly accepted

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that the water pressure test can induce modifications in the joints characteristics (Shibata et al. 1981; Kutzner 1996; Foyo and Sańchez 2002). The rock mass quality of the test section obtained from water pressure test completed with the degree of jointing of the drill core acts as a useful reference for ground treatment design (Foyo et al. 2005). The underlying foundation with unknown discontinuities needs to be improved to raise its engineering properties and ensure a watertight reservoir. Using cement grouting to improve bedrock has been quite common (Verfel 1989; Deere1982; Houlsby 1992), and there are numerous examples of its application to the engineering of dam foundation improvement (Ewert 1985; Weaver 1991; Warner 2004). Detailed discontinuity surveying was also performed to provide the basic parameters for classification of the rock masses. The Bieniawski rock mass rating (RMR) and Barton (Q) classification system have been used to provide tools for the designer during construction. In this study, rock masses of the left and right banks were classified according to the RMR method and Geological Strength System (GSI). In addition the most widespread empirical strength criterion that proposed by Hoek and Brown (1980) was used to obtain the rock mass strength parameters.


Fig. 2 The dam under construction, a a profile of the dam that showing construction progress, b a photo of the dam under construction

2 Site Geology This study focuses on the geomechanical and hyrogeological problems based on the local geological model of the dam site and reservoir area. The regional and local engineering geology have played a major role in the planning, design, construction and preference of the dam in Kopet-Dagh basin (Lashkaripour and Ghafoori 2002).

Daroongar dam and its reservoir are located in a part of the Kopet-Dogh mountain physiographic province termed the Kopet-Dogh basin. The KopetDogh formed as an intercontinental basin in NE Iran and SW Turkmenistan (Berberian and King 1981; Alavi et al. 1997), and contains more than 6,000 m of Mesozoic and Cenozoic marine and fluvial sedimentary rocks (Afshar-Harb 1994). The general stratigraphy of the Kopet-Dogh comprises 15 formations

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Author's personal copy Geotech Geol Eng Fig. 3 General stratigraphy of the Kopet-Dogh basin

from mid Jurassic to Oligocene age is shown in Fig. 3. The valley walls at the dam site are moderately steep, with slopes of 40–50° in the left abutment and 25–35° in the right abutment. Bedrocks exposed near the dam axis and reservoir consists of a sequence of Cretaceous to Tertiary aged marine and fluvial sedimentary rocks (Fig. 4). A generalized geological map of the dam site and reservoir area, exhibiting geological units, location of boreholes, dam and diversion tunnel axis, is presented in Fig. 5. On this map, the outcrops of the geological bedrock units and the various types of surficial deposits are shown. The dam site is situated in an area underlain by the following stratigraphic formations: Quaternary deposits (Q) Plio-Pleistocene Conglomerate (PLQ) Abderaz formation (Kad) Atamir formation (Kat) Quaternary deposits are mainly composed of alluvium, terrace, slopewash and colluvial materials. The Plio-Pleistocene Conglomerate unit is

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composed of conglomerate with an interlayer of claystone and red sandstone, dating to the Pliocene–Pleistocene, covers a large area above the reservoir level. The conglomerate is limy, cemented and consists of rounded coarse pieces of limestone. This unit unconformably overlies the oldest formation. It is exposed on both the right and the left banks of the dam site with a thickness of about 20–25 m. The Abderaz formation of Upper Cretaceous age consists of grey shale, limey marl and white chalky limestone. This formation exposes in a large part of the right abutment, in the riverbed and considerable areas of the reservoir of the dam. Thin layers of the Abderaz formation are usually overlaid by alluvium at the dam site. The dips and dip directions of these layers in the right banks and the reservoir are 80–85°/180–215° (dip/dip direction). The Atamir formation of early Upper Cretaceous age consists of sandy limestone layers. This formation exposes in a large part of the left abutment. The dips and dip directions of these layers in the left and right banks are 80–85°/ 200–204° (dip/dip direction).


Fig. 4 A photo of dam site with a view of left bank

3 Site Investigation The site investigations took place in two stages. The preliminary feasibility study of the dam site ended in 2002. The second stage began from 2003 and ended in 2005 and the construction stage began from 2006 (Fig. 2). Twenty five boreholes with total depth of 980 m were drilled on the dam site in conjunction with packer tests in two stages (Toos Ab Engineering Company 2005). The depth of these boreholes varied from 16 to 70 m. Engineering geological investigations and geotechnical studies of dam site carried out in detail include discontinuity surveying, core drilled data and in situ testing. A geotechnical cross section was constructed based on the exploratory boreholes drilled along the dam axis during the site investigation. Figure 6 shows the rock units and fault system across the dam axis. 3.1 Discontinuity Surveying The discontinuity data were interpreted statistically to define the rock-mass conditions of the dam site and reservoir area. The discontinuity surveying was undertaken according to the suggested method of the International Society of Rock Mechanics

(ISRM 1978) at dam site in order to provide basic parameters for classification of the rock mass and to determine engineering characteristics of the rock mass. A total of 259 structural discontinuities, 98 joints on the right bank and 161 on the left bank, have been measured. Once the geological data have been collected, computer processing of this data can be of great assistance in plotting the information and in the interpretation of statistically significant trends. The discontinuity data were interpreted statistically to define the rock-mass conditions of the dam site and reservoir area. The basic orientation data were analyzed using a computer program based on equalarea stereographic projection (DIPS 2.2) (Diederichs and Hoek 1989). The stereographic projection can give an overall view of the number of dominant discontinuity sets of the joint systems and bedding directions within the rock mass. Figure 7a and b illustrates a plot of contoured pole concentrations and corresponding great circles. Three representative joints sets are identified in the three main formations on the right bank (Fig. 7a): J1:78/291; J2:85/203; J3:31/296. Three dominant discontinuity sets are also determined on the left bank (Fig. 7b): J1:83/112; J2:85/20; J3:33/301.

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Author's personal copy Geotech Geol Eng Fig. 5 Engineering geological map of the dam site

The quantitative descriptions and statistical distributions of discontinuities of rock units derived from boreholes and those obtained from the geomechanical mapping through scan lines at the dam site according to ISRM (1981) are summarized in Table 1. 3.2 Drilling At the dam site a total of 25 boreholes with total depth of 980 m were drilled to assess the condition of the dam foundations and to obtain rock samples for laboratory testing. However, only 17 drill holes,

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located within the dam foundation have been studied in detail (Fig. 5). The details of the drill holes examined are given in Table 2. The higher rock quality obtained from boreholes can be related to the relatively good rock masses and the lower rock quality can be related entirely to the presence of weak rocks such as shale and marls. During core drilling, permeability tests were performed and the permeability of the rock masses is expressed in terms of Lugeon values. The degree of jointing and the waterabsorbed quantity should be in a direct relation but the RQD index is not an adequate reference to test


Fig. 6 Geological cross-section of dam axis

section behaviour because it can not evaluate the joint continuity and presence of filling materials. After analysing the data obtained from drill holes, the following results are made: The drill holes in the river bed area were proved to have a maximum depth of fluvial fill material of 48.50 m (BH-6). The maximum depth of the Quaternary alluvium in the river valley under the dam axis is 43.30 m (BH-2). The drill holes on the right aboutment encountered Plio-Pleistocene Conglomerate (PLQ), shale, limey marl and chalky limestone of Abderaz formation (Kad). The majority of these rock units covered with colluvium materials ranges from 1 to 15 m thickness. The drill holes on the left aboutment encountered sandy limestone layers of Atamir formation (Kat). This formation exposes in a large part of the left abutment. 3.3 Selection of the Type of Dam The type and size of dam constructed depends on the need for and the amount of water available, the topography and geology of the site, and the construction materials that are readily obtainable. The final choice of type of dam is made after consideration of these factors (Emiroglu 2008). As far as dam

construction is concerned, safety must be the primary concern, this coming before cost. Safety requires that the foundations and abutments be adequate for the type of dam selected. Of the various natural factors that directly influence the design of dams, none is more important than the geological, not only do they control the character of the foundation but they also govern the materials available for construction. The major questions that need answering include the depth at which adequate foundations exist, the strengths of the rock masses involved, the likelihood of water loss and any special features that have a bearing on excavation (Bell 2007). As shown in Fig. 6, the dam site is quite wide valley. Therefore, this site is more favorable to the design of an earthfill dam. According to the borehole results (Table 2), sound bedrock is not reasonably close to the surface and quaternary alluvium at the dam site has a thickness of about 43 m (depth of the alluvium approximately near the dam height) and overlies the Cretaceous grey shale and limey marl basement (Kad rock units). The quaternary alluvium is mostly composed clay soils (thickness of about 30 m) with clayey gravel. Furthermore, the surrounding rock mass of the foundation and right abutment is composed by grey shale and limey marl basement (Kad rock units) and conglomerate (PLQ rock unit).

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Author's personal copy Geotech Geol Eng Fig. 7 Dominant joint sets: a right bank, b left bank

Because, PLQ rock unit has the low deformation modulus and rock strength parameters, this site is unfavorable to the design of a concrete dam. Therefore, on the basis of topographic, geological and geotechnical conditions, the earthfill dam option was found to be the best solution. 3.4 Permeability During core drilling, the packer permeability tests were carried out in the Daroongar dam foundation directly in the vertical borehols at descending sequence. The main objective of these tests was to

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determine the permeability of the each section of rock masses of the dam foundation and its abutments, which allows for the determination of different zones that require a seperate treatment. A total of 184 permeability tests or water pressure tests were carried out during the two stages of studies (93 tests in the Quaternary alluvium, named constant and falling-head permeability tests, and 91 tests in the rock masses, named Lugeon tests). Constant and falling-head permeability tests were performed in boreholes to determine the permeability of the alluvium. According to the test results, alluvium is ‘‘medium permeable’’ (k = 5 9 10-4 cm/s) so it is

Author's personal copy Geotech Geol Eng Table 1 Quantitative descriptions and statistical distributions of discontinuities of rock units Range

Description

Distribution (%) Right bank

Spacing (cm)

Persistence (m)

Aperture (mm)

Infilling

Roughness (JRC)

Left bank

PLQ

Kad

Kat

Kat

\2

Extremely close

100

33

13

22

2–6

Very close

–

13

57

61

6–20

Close

–

30

8

7

20–60

Moderate

–

23

14

10

1–3

Low

–

22

–

4

3–10

Medium

–

42

–

9

10–20

High

–

8

16

6

[20

Very high

100

28

84

81

0.25–0.5

Partly open

–

–

–

–

0.5–2.5

Open

–

15

–

–

2.5–10 Gouge

Moderately wide

100 75

85 –

100 6

100 20

Oxide

17

–

–

–

Calcite

8

60

60

52

Clay and loam

–

35

20

12

Clean

–

5

14

16

4–6

–

–

–

–

6–8

–

–

–

–

6–8

8

–

3

–

8–10

5

–

7

8

10–12

87

23

26

7

12–14

–

77

64

85

proposed to cut-off and remove or treatment the alluvium before construction of dam. Because the dam foundation consists of a relatively thick deposit of pervious alluvium (43 m), seepage control is necessary to prevent excessive uplift pressures, instability of the downstream slope, piping through the foundation, and erosion of material by migration into open joints in the foundation and abutments. Generally, the slurry excavation method is suggested for cutoff trenches to create a barrier to seepage (Fig. 2a). From the results of each of the water pressure tests, the pressure-flow relationship was plotted and the lugeon value was calculated. The results of permeability tests on rock units are shown in Table 2. Table 2 shows the geotechnical parameters such as RQD (rock quality designation) and the Lugeon test results, for both sides of the valley, the right and left abutments, and riverbed boreholes. As this table shows, permeability in rock units is measured in the

Lugeon scale (0–3 Lugeon impervious, 3–10 Lugeon low permeability, 10–30 Lugeon medium permeability, 30–60 Lugeon high permeability, and [60 Lugeon very high permeability). The average Lugeon values of the three main formations are summarized and presented in Table 3. Table 3 shows that permeability in the rock units of the dam foundation and left bank are medium to high. The results of these tests indicated that the permeability of the conglomerate unit (PLQ) formation and Atamir formation (Kat), which form the bedrock of the dam site, range between 0.5 and [100 Lugeons. The high Lugeon values especially in BH-22 and BH-8A boreholes in left abutment are because of the shear zone and highly jointed rock masses. This high permeability is one of main geological engineering problems of the Daroongar dam. The Lugeon values, determined from the Lugeon tests, indirectly expressed the status of the

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Author's personal copy Geotech Geol Eng Table 2 Geotechnical parameters of the right abutment (R), the left abutment (L) and the riverbed (RB) boreholes Borehole number

Borehole depth (m)

RQD (%)

Permeability (Lugeon)

Total

Quaternary deposit

Rock

Min

Max

Average

Min

Max

Average

BH-3R

50.20

03.60

46.60

0

100

85.5

0.4

50

12

BH-4R

29.50

03.70

25.80

0

91

44.5

11

[100

[100

BH-15R

33.02

23.85

09.17

0

100

73

\1

4

2

BH-16R

33.10

01.70

31.40

0

84

23.5

5.6

66

29

BH-17R

33.50

14.50

18.70

72

98

92

1

55

4

BH-5L

59.80

–

59.80

0

100

78

1

[100

33

BH-8AL

16.00

–

16.00

0

20

5

61

[100

[100

BH-8BL

71.07

–

71.07

0

100

79

1

7

3

BH-22L

70.15

–

70.15

0

100

66

69

[100

[100

BH-1RB

50.05

39.95

10.1

0

96

76

10

46

28

BH-2RB

53.90

43.30

10.6

27

100

92

0.4

0.5

0.47

BH-6RB

59.10

48.50

10.6

49

91

62

35

40

38

BH-7RB

50.50

40.30

10.2

55

97

90

50

61

58

BH-18RB

45.00

32.25

12.75

48

100

87

35

48

40

BH-19RB BH-20RB

50.55 48.25

18.63 40.27

31.92 7.98

61 50

100 100

87 98

1 35

43 40

18 36

BH-21RB

50.55

31.00

19.55

0

93

6

3

46

29

Table 3 Results of the permeability tests Location

Rock unit

Average minimum (Lugeon)

Average maximum (Lugeon)

Average (Lugeon)

Right bank

PLQ

5.6

[100

79

Kad

0.4

55

10

Kad

0.4

58

25

Kat

1

46

30

Kat

0.5

[100

38

Riverbed Left bank

discontinuities in the dam foundation. The water pressure test indicated the necessity of providing a grout curtain below the dam foundation. The results of water pressure tests and the degree of jointing also employed as a first aid to design the water to cement ratio and the injection pressure in the grouting process for foundation treatment. The Daroongar dam foundation consists of different rock layers that have more hidden discontinuities. Shallow bedrock especially in the left abutment tends to have a high density of cracks or openings and will be subjected to grout leakage and hole collapse It was found that the

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geology of the dam foundation has a significant influence on the permeability and groutability characteristics. The correlation between permeability and depth for the rock units in the dam site shows a reduction in permeability with increasing depth. In addition, analysis of drilling data shows that permeability rises in low quality rock masses. After dam site investigation and considering the relevant factors such as the permeability of surrounding rock units in left abutment (Kat) are medium to high and furthermore in BH-22 and BH8A boreholes (in left abutment) are observed the shear zone and highly jointed rock masses with high Lugeon values, therefore a grout curtain 50–60 m deep was proposed at two rows in left bank. As shown in Fig. 6, the right bank is composed of the conglomerate PLQ unit (thickness of about 10–15 m) with high Lugeon values and grey shale and limey marl basement (Kad rock units) with low Lugeon values. Therefore, considering the relevant factors such as permeability, discontinuities, and lithological properties of the rock foundation, it was proposed that a grout curtain 30 m deep at one row in right bank was needed.

Author's personal copy Geotech Geol Eng Table 4 Comparison of the rock mass quality defines by SPI and the degree of jointing of the left abutment (L) Borehole no.

Dept. From

BH-19

BH-22

GL-2

SPI class

SPI (l/s*m2)

RQD

LU

Dominant flow

To

19.95

24.95

C

1.2341E-12

79

65.8

T

24.5

29.5

C

3.1524E-13

98

21.8

D

29.65

34.65

C

3.7406E-13

98

20.95

T

34.15

39.15

C

7.038E-13

99

43.45

T

38.75

43.75

A

2.0807E-14

99

0.93

D

43.3

48.3

A

1.1442E-14

98

0.61

D

16.55

21.55

C

1.43E-12

74

65

L

25.75

30.75

C

4.29E-13

67

17.5

L

45

50

C

8.74E-13

52

31.1

W

50.05

55

C

4.03E-13

70

18.52

L

55

60

C

3.80E-13

68

17.1

W

60

65

B

9.21E-14

84

4.4

W

65.15

70.15

C

4.75E-13

65

23.7

W

8

13

C

8.50E-13

65

60.2

W

13 18

18 23

C C

3.32E-13 9.0037E-13

65

14.56 41.11

T T

28.3

33

A

4.51E-15

98

0.21

L

33

38.2

A

1.57E-14

75

0.9

W

43.8

49.2

A

2.16E-14

85

0.92

D

48

53

B

9.29E-14

60

4.3

L

55.6

61.35

A

1.66E-14

75

0.79

W

61.35

65.45

A

8.81E-15

78

0.32

V

T turbulent, L laminar, V void filling, W wash out, D dilation

The contact injections were suggested at two rows along both sides of the grout curtain. The grout curtain must be carried out by split spacing, and the injection at holes with good quality will be carried out in stages using down-up method, at 5 m sections and in the holes with poor quality (shear zone in left bank) will be carried out in stages using up-down method at 1–5 m sections. The ratio of cement to water supposes to be 1:3 at the beginning of the injection process. In order to build up the stability of the suspension, 5% bentonite suggested to be added. 3.5 Secondary Permeability Index To describe and estimate the permeability of jointed rock, the result of water pressure test should be transferred to k-value instead of lugeon value. Much more effort had been done to find correlation between the result of water pressure test and k-value. This problem was solved by using Secondary Permeability

Index method (SPI). The Secondary Permeability Index (SPI) usually, expressed from the conversion of the take of water pressure test into a permeability coefficient analogous to porous mass (Foyo et al. 2005). Usually, the grouting of the dam foundation requires that the rock mass be previously divided in zones with different ground treatment. The Secondary Permeability Index (SPI), based on water flow trough fissures, allows zoning the dam foundation regarding different quality classes. The importance of the SPI method is possibility of distinguishing difference between dilation and hydraulic fracturing. The dilation is occurred at elastic manner, but the hydraulic fracturing is occurred at plastic manner. The rock mass classification defined by SPI and the depth of the test has been investigated. As an example comparison of the rock mass quality defines by SPI and the degree of jointing for three boreholes in left aboutment reveal the following aspect (Table 4; Fig. 8):

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Author's personal copy Geotech Geol Eng Fig. 8 Comparison between degree of jointing (RQD) and rock mass classification obtained from SPI (SPI with logarithmic scale)

1.00E-11

100

SPI RQD

CLASS

90 80

1.00E-12

70 60

1.00E-13

50 40

CLASS B

RQD

SPI

CLASS C

30 1.00E-14

20

CLASS A BH-22

10

GL-2

BH-19

1.00E-15

48 55 . 61 6 .3 5

33 43 .8

18 28 .3

8 13

6 65 0 .1 5 19 .9 5 24 . 29 5 .6 34 5 .5 1 38 .7 5 43 .3

55

4 50 5 .0 5

16

.5 5 25 .7 5

0

DEPTH

Less than 40% of tests (10 test from 22 in Fig. 8) show an accurate correlation which indicates the important differences between rock mass quality definition from SPI and degree of jointing as defined using RQD index. In BH 19, BH 22 and GL 2 in left aboutment, the SPI shows a very low rock mass quality for the surface area at depth of less than 30 m. The SPI also shows class C and the degree of jointing is high. Regarding to deep zone (depth [ 30 m), still more or less C classes in some boreholes exist. In borehole Number 22 in left aboutment of the dam(depth [ 30 m) in spite of class B that appear, the dominant classes are C yet. In BH 19 and Gl-2 an improvement of the rock mass quality is confirmed. This interval has been considered long enough to ensure that the rock mass quality in some parts of the dam site is not too increasing. It is possible that discontinuities of high permeable activity may exist. 3.6 Laboratory Studies Laboratory studies were performed on core specimens obtained from the boreholes, in order to determine physical and mechanical properties of the Table 5 Summary of the laboratory testing results on different rock units

rock units. Density, uniaxial compressive strength (UCS), deformability and triaxial tests were conducted according to ISRM (1981) standards. The deformability parameters, Poisson’s ratio (t) and modulus of elasticity (E) were obtained from triaxial test. The test results are shown in Table 5. The standard method of assessing the strength of a geomaterial is to recover representative core samples (length of core in pieces [ 100 mm) and to test them in the laboratory. However, in the case of the conglomerate unit (PLQ), it was not always possible to recover core samples that were large enough to represent the rock mass.

4 Rock Mass Classifications Three empirical rock mass classification systems, namely the rock mass rating (RMR) (Bieniawski 1989), Q-system (Barton et al. 1974) and Geological Strength Index (GSI) (Hoek 1994; Hoek and Brown 1997) methods have been used to summarise the geological and geotechnical data, and to provide tools for the designer during construction. These rock mass

Parameters

Kad rock unit Min.

Max.

Unit weight (KN/m )

24.1

2.62

Uniaxial compressive strength, UCS (MPa)

20.2

3

Modulus of elasticity, E (GPa) Poisson’s ratio (t) Internal friction angle (u) Cohesion (Mpa)

123

5.42 0.26

54.9 9.43 0.34

Kat rock unit Average

Min.

Max.

Average

25.7

24.9

25.6

25.1

33.3 7.00 0.3

37.4

62.9

54.7

1.3

8.3

7.3

34.4

61.4

46.3

10.0

12.9

11.03

44.8

62.9

53.8

8.3

5.6

3

Author's personal copy Geotech Geol Eng Fig. 9 Rock mass classification of the dam site based on GSI system

classification systems were used for the left and right banks. 4.1 Results of Rock Mass Classification Systems at the Dam Site The rock mass qualities of two formations (Kat and Kad) of the dam site were assessed using three

empirical rock mass classification systems, which is dependent on the joint characteristics. The results show, Kat formation on the both the right and left bank are classified as fair rock mass (RMR = 50), as well as the Kad unit at right bank and riverbed is classified as fair rock (RMR = 55). The Geological Strength Index (GSI) provides a system for estimating the reduction in rock mass

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strength for different geological conditions as identified by field observations (Hoek et al. 1998). This system was developed by Cai et al. (2004). For this study, the GSI system was used to classify the rock masses of both banks. The results are shown in Fig. 9. According to the Geological Strength Index the Kat unit (sandy limestone) is classified as blocky to very blocky (GSI = 42–53). The Kad unit (marly limestone) is classified as blocky to very blocky (GSI = 45–55). The PLQ unit (conglomerate) is classified as blocky/disturbed (GSI = 25–35) (Fig. 9). 5 Estimation of Rock Mass Strength Parameters The rock masses as the foundation of dam are sedimentary rocks. The rock mass constants, uniaxial compression strength was calculated by using Hoek et al. equation (Hoek et al. 2002) and the in situ deformation modulus of each rock units evaluated by Hoek and Diederichs equation (Hoek and Diederichs 2006). The results show that the deformation modulus ranges from 4.33 for Kat at left bank to 9.07 GPa for Kad at right bank and the uniaxial compressive strength ranges from 1.5 for Kat formation to 3.7 MPa for Kad formation at dam site.

6 Conclusions Based on the topography, local geology and ground conditions of the dam reservoir area, an earth rockfill dam with a clay core option was found to be the best solution to control and store water for domestic and irrigation purposes. It is necessary to consider that the geological section of the dam foundation is prepared as a more or less substantiated geological and geotechnical hypothesis which is refined as data are accumulated. Therefore, the geological model was proposed for Daroongar dam site is a dynamic tool that will be changed as more information will expose during the various stages of excavation and construction process. Because the dam foundation consists of a relatively thick deposit of pervious alluvium (43 m), seepage control is necessary to prevent excessive uplift pressures, instability of the downstream slope,

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piping through the foundation, and erosion of material by migration into open joints in the foundation and abutments. Generally, the slurry excavation method is suggested for cutoff trenches to create a barrier to seepage. The Daroongar dam foundation consists of different rock layers that have more hidden discontinuities. Shallow bedrock especially in the left abutment tends to have a high density of cracks or openings and will be subjected to grout leakage and hole collapse. The water pressure test indicated the necessity of providing a grout curtain below the dam foundation. The results of water pressure tests and the degree of jointing also employed as a first aid to design the water to cement ratio and the injection pressure in the grouting process for foundation treatment. Based on the estimated GSI values and intact rock strength properties at Daroongar dam site, equivalent Mohr–Coulomb strength parameters and elastic modulus of the jointed rock mass were calculated. Acknowledgments The authors express their sincere thanks to the Water Organization of Khorasan Razavi Province and to the staff of Toos-Ab Engineering Company, Mashhad Office, for access to the data of the dam.

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