isrm-arms5-2008-055_rock mass characterization at kangir dam site in iran

8/12/2019 ISRM-ARMS5-2008-055_Rock Mass Characterization at Kangir Dam Site in Iran

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ISRM International Symposium 20085th Asian Rock Mechanics Symposium (ARMS5), 24-26 November 2008 Tehran, Iran

483

ROCK MASS CHARACTERIZATION AT KANGIR DAM SITE IN IRAN

A. SHAFIEI and M.B. DUSSEAULTEarth & Environmental Sciences, University of Waterloo, Waterloo, ON, Canada, N2L 3G1

(e-mail of corresponding author: [email protected])

Abstract

The proposed Kangir dam site is located in western Iran, Elam province, and will impound flow of the Kangir River. The

foundation rocks comprise some limestones (Asamri Formation) and, to a greater extent, the marls, gypsum-bearing-marls, and

gypsum strata of the Gachsaran Formation. To characterize the rock masses investigated at the dam site, rock mass classification

systems such as rock mass rating, Q-tunneling index, rock quality designation, dam mass rating, geological strength index and

rock mass index were all used. These classifications provide the basis for estimating deformation and strength properties, for

supplying quantitative data to the design engineer, and they also present a platform for communication between exploration,

design and construction groups. The results obtained from the comprehensive rock mass studies at the proposed dam site are

presented and discussed in this article.

Keywords: Iran, Kangir dam site, rock mass characterization.

1. Introduction

Water is a rare and vital commodity in Iran; the

west of Iran can be described as semi-arid

mount-ainous region that suffers from water

shortages, periodic droughts, and a limited water

supply which constrains development activities.

Owing to the increasing demand for water in thewestern provinces of Iran, the Ministry of

Energy has selected a number of sites that may

be suitable for the construction of storage dams

to be used mainly for the domestic recharge of

groundwater and for irrigation. This paper

highlights the engineering geological

characterization of the rock masses exposed at

the dam abutments and foundation area at the

proposed Kangir dam site. This site is located in

western Iran in Elam province, and will impound

flow from the Kangir River. The study area ispart of the folded Zagros area, a well-known

tectonic belt which stretches from NW to S and

SE of Iran. The foundation rocks comprise some

limestones and to a greater extent strata

comprising marl, gypsiferous marl, and gypsum.

Karstification is widespread in the limestone and

gypsum units. In order to achieve a geotechnical

characterization of the rock masses, a

comprehensive engineering geology and rock

mechanics study was conducted in the study

area. The physical and geomechanical propertiesof the different rock types exposed in the study

area were determined in the laboratory based on

ASTM standards and ISRM suggested methods.

To characterize the rock masses investigated at

the dam site, rock mass classification systems

such as rock mass rating (RMR), Q-tunneling

index, rock quality designation (RQD), dam

mass rating (DMR), geological strength index

(GSI) and rock mass index (RMi) were used.These classifications provide the basis for

estimating deformation and strength properties,

for supplying qualitative and quantitative data to

the design engineering team, and constitute a

platform for clear communication between

exploration, design and construction groups.

The rock mass constants mb, s and a, GSI

values, UCS and tensile strength, cohesion,

internal friction angle, global strength and the

Youngs modulus of each rock unit at the damsite were determined from GSI values using

RocLab freeware. Results obtained from

different rock mass classification systems for

evaluation of various rock types in different parts

of the dam site are shown and interpreted. Also,

some recommendations and considerations are

presented to further investigate the effect of

gypsum dissolution and karstification on seepage

potential and dam stability in case building a

dam on such a poor foundation is more seriously

considered.


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2. Geological Setting

The main purpose for considering the design and

construction of the proposed Kangir dam was tosupply drinking and irrigation water to the area

of Eivan city, located 25 km SE of the Kangir

river in Elam province (Fig. 1).

Fig. 1. The proposed Kangir dam site study area in Iran.

The study area is located at 467042W

longitude and 335423E latitude, situated in

the Zagros Mountains [1,2] which extend from

southern to northwestern Iran with a general

NW-SE trend. The bedrock at the site is formedof sedimentary rocks including the Asmari

Formation limestone (Oligo-Miocene) and to a

greater extent the Gachsaran Formation

evaporites (Lower Miocene). The major

stratified lithotypes exposed in the proposed dam

site, based on geological field studies and

petrographical studies of thin sections taken from

core samples, are limestone, marl (shaley

limestone), gypsum, and intermediate rock types.

A geological cross-section along the proposed

dam axis is shown in Fig. 2. On this cross-section, the different rock masses, the sacrificial

deposits, and the proposed dam axis are

indicated.

The Asmari Formation is Oligo-Miocene in

age and is a member of the Fars Group in the

stratigraphical classification of the Zagros

Mountains. It mainly consists of limestone,

dolostone and sandstone, with lesser amounts of

anhydrite and shale, and it outcrops in the

foundation and abutments of the proposed

Kangir dam site. This formation is divided into

three sections at the proposed dam site. The

Lower Asmari consists mainly of crystalline

limestone, dolomitic limestone and grey to light

cream dolostone with interlayers of marlylimestone. The thickness of this section is

estimated at 200-250 meters. The Middle

Asmari consists mainly of crystalline limestone,

dolomitized limestone, marly limestone with thin

interlayers of grey to brown marl, and brecciated

limestone. This section is exposed in the

spillway, culvert and some parts of the proposed

dam foundation; about 20% of the overall area.

The Upper Asmari limestone, outcropping in

most parts of the proposed dams foundation and

its abutments, over 70% in area, consists ofmedium bedded to massive, light- to medium-

grey limestone and marly limestone, locally

dolomitized with a few marlstone interbeds, with

a thickness of ~10 meters.

The Gachsaran Formation, which

stratigraphically is the Lower Miocene member

of the Fars Group, consists mainly of intensely

karstified evaporitic rocks; it also contains marls,

limestones, and shales. The Gachsaran

Formation is underlain by the Asmari limestone

and underlies over 90% of the impoundment area

of the proposed dam, as well as a very small

portion of the dam foundation and its abutments.

The Gachsaran Formation at the right abutment

consists of red marl, gypsum-bearing marl and

gypsum, whereas in the left abutment it is

composed of red marl and clayey sandstone.

During field investigations several sinkholes as

well as large solution cavities and fissures were

identified on the left abutment and left bank of

the proposed dam site. These are indicators ofthe karstic nature of the rock masses identified at


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the proposed dam site.

Quaternary alluvial deposits extend along the

river course and cover the reservoir area of the

proposed Kangir dam site (Gachsaran

formation). They consist of gravel, sand and silt,

with some fine-grained deposits and boulders

mostly composed of limestone and sandstone.

Late Quaternary river terraces composed of

clayey silt and sandy silt form parts of the

reservoir flanks at the proposed dam site. The

engineering geology and geomechanical

properties of the proposed Kangir dam site wereinvestigated in great detail, and the interested

reader is encouraged to see reference number 3

for more information.

Fig. 2. The proposed Kangir dam site study area in Iran.

3. EG Investigations

A comprehensive Engineering Geological and

rock mechanics study was conducted at the

proposed dam site, mainly involving

discontinuity surveying, core drilling, in situ

evaluations and laboratory testing.

The comprehensive quantitative discontinuity

survey was executed at the proposed damlocation, and orientation, spacing, persistence,

roughness, aperture and filling of discontinuities

were determined by exposure logging in

accordance to ISRM (1978) [4]. Three dominant

discontinuity sets are identified both on the left

and right abutments of the proposed Kangir dam.

The first dominant discontinuity is the layering

(bedding) of the Asmari limestone (J1). The

other two dominant discontinuities are persistent

joint sets (J2 and J3). J2 and J3 are found to beperpendicular to and parallel with the layering of

the Asmari limestone (J1), respectively. The

spacing of the joint sets decreases in the right

abutment of the proposed dam site where the

Middle Asmari is exposed. Note that drilling

reports were also used to help in discontinuity

characterization. Filling materials between the

joint walls are composed mainly of clay, calcite

precipitates and silt. Joint surfaces are rough and

undulating. Discontinuity orientations were

processed with a computer program based on

equal-area stereographic projection, DIPS 2.2

[5]. The dominant discontinuity sets

distinguished on the left and right bank are

shown in Figs. 3 and 4. No major fault was

discovered in the study area although a modest

number of minor faults was found. The strike

direction of these small faults is perpendicular to

the axis of the major structural elements in the

area, namely the oriented megafolds in the strata

(anticline-syncline array). The dominant faultstrike direction in the study area is N-S and NE-


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SW, and these are respectively the major and

minor water path-ways in the study area.

Fig. 3. Contour diagram of discontinuities, left abutment

of the proposed Kangir dam site.

Fig. 4. Contour diagram of discontinuities, right

abutment of the proposed Kangir dam site.

The Asmari Formation (AS) limestone at the

foundation of the proposed dam dips upstreamby 10 to 30. This acts in favor of retaining

water seal as well as improving the general

stability of the dam. Drilling was carried out at

the site to verify foundation conditions and to

obtain rock samples for laboratory testing

purposes. A total of 47 boreholes with 2500 m

aggregate length were drilled using rotary

drilling equipment on the foundation, abutments

and the reservoir area of the proposed dam site.

Rock quality designation (RQD) values of the

rock masses were determined.

A group of rocks with similar

lithostratigraphical and structural features in a

rock mass are often defined as a single

Geomechanical Unit or GMU. For engineeringanalysis, each rock mass must be divided into a

tractable number of GMUs in order to ease the

problem of rock mass characterization. Each

GMU is distinct from other GMUs in a rock

mass (e.g. through layering, fault separation or

lithomechanical differences) and has its own

geomechanical characteristics. Only two GMUs

have been identified at the proposed dam site:

Asmari limestone (AS) and Gachsaran evaporitic

rocks (GS) and we refer to them using

abbreviations for the rest of the article.

According to drilling reports, the thickness of

the GS geomechanical unit which underlies

~95% of the reservoir and ~20% of the dam

foundation in the proposed Kangir dam varies

from a few meters up to 60 meters. Large,

crystallized and dense blocks composed of

gypsum were identified along with marls in the

middle of the proposed reservoir area. The AS

geomechanical unit forms over 95% of the

bedrock in the foundation and abutments of theproposed Kangir dam foundation. Based on

drilling reports, RQD values in the left abutment,

foundation and right abutment in the proposed

Kangir dam site vary between 85-97%, >90%

and 71-99%, respectively. In the right abutment,

where the Lower Asmari section outcrops, there

is a decrease in RQD values.

Laboratory experiments were carried out to

determine the physical and geomechanical

properties of rock samples from the AS and theGS GMUs. These units were tested for dry

density, porosity, water absorption ratio, uniaxial

compressive strength, compressional and shear

wave propagation velocity, and deformation

parameters. Test results are summarized and

presented in Table 1. Laboratory tests performed

to determine the geotechnical parameters of

different rock units were based on ASTM (1981)

standards [6] and ISRM suggested methods [7].

Uniaxial compressive strength tests, intended to

measure the strength of a regular cylindricalgeometry rock sample, were carried out on


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prepared whole core specimens. The UCS data

are mainly intended for strength classification

and characterization of intact rock; alone, the

UCS value is considered inadequate for rockmass strength assessment because specimens

tend to be taken from intact zones, not the

broken and weakened zones, and the role of

fractures is minimized (hence the development

of the Hoek-Brown rock mass criteria and

related methods). A mean value of UCS in the

saturated state for the AS and GS geomechanical

units is between 8-54 MPa and 16-32 MPa,

respectively. These are wide ranges of values,

reflecting the heterogeneity that is typical of

these types of rocks. Physical properties such as

specific gravity, dry and saturated densities and

porosity were determined for each

geomechanical unit. For each physical property

the value quoted is the mean over 10 specimens.

Table 1. Physical and geomechanical properties of AS

and GS units at the proposed Kangir dam site.

4. Gypsum Karstification

According to James and Kirckpatrick (1980) [8]

and Johnson (2005, 2008) [9, 10] dissolution of

rocks such as gypsum can cause a potential risk

of settlements and seepage within the

foundations of dams founded on soluble

foundation rocks. Hence, in the case of a

proposed dam on karstic rocks containing

gypsum, study of rock solubility is vital to an in-

depth engineering geological appraisal.

The obvious karstic nature of the study area

and the specific and wide-spread presence of

solution features at the proposed Kangir dam site

are directly linked to the presence of limestone

and gypsum-bearing rocks. Rock solubility will

play a crucial role in seepage management

through the dam foundation and abutments, as

well as in dam foundation stability in both short-

term (first two years post-impoundment) and

long-term assessment periods. If a high

dissolution rate is allowed to develop locally

because of inadequate impermeation grouting, asolution cavity can develop, leading to collapse

and rubble generation, massively weakening and

altering the rock behavior. Gypsum is a highly

soluble rock material and forms karstic features

such as caves, sinkholes, and other solution

characteristics if exposed to flowing water.

Gypsum solubility at 20 C is 2.531 g/l, about

two orders of magnitude greater than the

solubility of calcite in pure water. The

dependence of the solubility of gypsum on

temperature is non-linear, reaching a maximumat 43C. Pressure has little effect on the

solubility of gypsum, although gypsum solubility

increases somewhat at pressures greater than 10

MPa [11].

Gypsum and gypsiferous rock materials in a

proposed dam area may cause a number of

serious problems, such as seepage and dam

failure. Johnson (2005) [9] reported that

gypsum-dissolution features normally have a

linear orientation, and these appear to becontrolled by structural features in the rock mass

(e.g. joints and fractures). Waters unsaturated in

gypsum can easily attack and dissolve gypsum

rocks in the dam foundation and create karstic

paths for water flow, leading to potentially

serious seepage and settlement problems. Also,

karstification processes can enlarge existing

geomechanical weakness planes in the rock mass

such as fractures, joints and sinkholes. Gypsum

karst in the abutments or foundation of a dam

can facilitate water flow around or under thestructure, and solution channels can enlarge

Geomechanical unitProperties

AS GS

Dry 2.40-2.67 2.25-2.28Density

(g/cm3

) Saturated 2.45-2.70 2.29-2.31

Water absorption (%) 0.50-5.50 1.30-2.70

Porosity (%) 1.10-12 2.70-7.20

Dry 10-79 9-31UCS (MPa)

Saturated 8-54 16-30

Dry 5-53 7-33Elasticity

Modulus

(GPa)

Saturated3-30 16-19

Dry 2800-4800 4200-5400Primary wave

velocity

(m/sec)Saturated 2400-5700 4700-5100

Dry 1400-3300 2100-2450Shear wave

velocity(m/sec)

Saturated 1300-2500 2180-2500


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quickly once water starts flowing through such a

karst system. Because of the very high hydraulic

gradients induced by water collected in the dam

reservoir in the vicinity of the dam site, thegypsum dissolution rate will be much faster, as

indicated by Romanov et al. (2003) [12] using

model simulations of karstification process

below dams by coupling equations of

dissolutional widening to hydrodynamic flow.

Gypsum dissolution rates of as high as 1 cm/year

under the high hydraulic gradient created by

dams is implied using mathematical models [12].

Gypsum karst is widespread around the world

and has caused problems at several dam orproposed dam sites. Compared with limestone

and dolomite karst, gypsum karst features form

much more rapidly. A number of dams founded

on gypsiferous strata around the world

[8,9,10,13,14,15,16] have faced serious

instability and seepage problems and most of

them were abandoned or considered failures.

Mitigation measures were not successful where

the extent of gypsum karst was widespread and

deep in the dam site. Johnson (2008) [10]

reported abandonment of a number of dam siteor proposed dam sites on soluble rocks due to

failure, collapse or seepage problem in the

United States. James and Kirkpatrick (1980) [8]

described a quantitative assessment method for

the purpose of potential risk assessment of

soluble minerals in dam foundations and

necessary mitigation measures.

Solution rate constants (K) of 0.36-0.38 m/sec

10-5 were obtained for the soluble rock

materials from the proposed Kangir dam site.

The experiments were carried out under

controlled temperature and water acidity similar

to in situ conditions in the study area. In the

laboratory, water was circulated in a closed

system through a sawed gypsum sample taken

from the proposed dam site as an analogue to

flowing water through the joints in the gypsum

rock mass leading to dissolution and widening of

the fractures and joints. Moreover, water taken

from Kangir River was used in the experiments,

and it is expected that the chemistry of thedissolutioning water will be similar to the river

water. In addition to widening the preexisting

weakness planes in the rock samples, the

measured dissolution rate within the rock matrix

of the tested sample was very high. Based onlaboratory studies on soluble rock samples taken

from the proposed Kangir dam site, it is clear

that the carbonate and gypsiferous strata are

prone to very high solution potentials which

increase with an increase in the acidity of the

water flowing through the fractured soluble

media.

5. Rock Mass Appraisal

In this section, the results from engineering rockmass evaluation of different geomechanical units

exposed at the proposed Kangir dam site using

various rock mass classification systems are

presented. As discussed above, two GMUs are

identified and defined at the proposed dam site.

5.1. RMR Geomechanics Classification

The RMR geomechanics classification was

originally proposed by Bieniawski [18,19] for

use in tunnels, slopes and foundations and thenmodified around 1989. The data collected for

the GMUs AS and GS were processed in

accordance with the RMR classification system

shown in Table 2.


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Table 2. RMR values for the different geomechanical units at the proposed Kangir dam site.

GMU

AS GSParameter

Value Rating Value Rating

1Uniaxial compression

strength (MPa)8-54 2-7 16-30 2-4

2 RQD 90-97 20 71-85 17

3Discontinuity

Spacing> 0.20 and < 2 10-15 > 0.20 and < 2 10-15

Discontinuity conditions:

Discontinuity

Persistence (m)1-20 1-6 1-20 1-6

Aperture (mm) 1.0-2.0 1 1.0-2.0 1Roughness Rough 5 Slightly rough to rough 3-5

Infilling Soft filling < 5 mm 2 Soft filling < 5 mm 2

4

Weathering Slightly weathered 5Slightly weathered to

moderately weathered3-5

5 Groundwater Wet to dripping 4-10 Wet to dripping 4-10

6 Rating adjustment Favorable -2 Favorable -2

RMR = 1+2+3+4+5-6 48-69 41-63

5.2. RMi Rock Mass Index

Palmstrom [21,22] proposed the Rock Mass

Index (RMi) classification system to characterize

rock mass strength for civil and construction

purposes. The selected input parameters are

based on earlier research in the area of rock mass

classification (e.g. Q system) and his experience

in this field. The main focus of the RMi is on

the effects of the weakness planes in a rock mass

that reduce the strength of the intact rock. The

data presented in Table 3 were processed in

accordance with the RMi classification system.

Table 3. RMi values for the different geomechanical units at the proposed Kangir dam site.

Parameter GMU

AS GS

qc50 8.11-54.76 16.23-30.42RQD 90-97 71-85

JL 0.75-1.0 0.75-1.0

JR 4-6 3-4

JA 8 8

JC 0.375-0.75 0.50-0.75

D 0.392-0.469 0.392-0.425

Vb 0.40-0.70 0.10-0.20

JP= 0.20(JC)0.50VbD 0.01920-0.0568 0.0055-0.0147

RMi = qc50 JP 0.155-3.11 0.089-0.447

RMi = Rock Mass index (MPa)

qc50= Intact rock uniaxial compressive strength (MPa) = qc54/0.986 [14]


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5.3. Q Tunneling Index

Barton et al. (1974) [20], using a large numberof case histories of underground excavations,

proposed a Tunneling Quality Index (Q) for the

determination of rock mass characteristics and

tunnel support requirements. The data were

processed in accordance with the Q classification

system and results at the proposed Kangir dam

site are presented in Table 4.

Table 4: Q values for the different geomechanical at the proposed Kangir dam site.

5.4. RQD Rock Quality Designation

In 1963, Deere et al. [23] introduced the Rock

Quality Designation or RQD index for

quantitative rock quality appraisal. The RQDisa core recovery percentage based on the number

of fractures in the rock mass observed in drill

cores. For calculation of the RQD, intact pieces

with length greater than 4 are summed and

divided by the total length of the core run.

Because of simplicity, relatively low cost and

quick determination of RQD, this index is widely

used for rock mass appraisal in different rock

mass classification systems such as Q and RMR.

A correlation between RQD and rock massquality proposed by Deere [24] is presented in

Table 5. Results from application of the RQD

system for evaluation of the different GMUs in

proposed Kangir dam site are shown in Table 6.

Table 5. Rock mass classification based on RQD [13].

Table 6: RQD values for the different geomechanical units at

the proposed Kangir dam site and their description.

GMU

AS GSParameter

Value Rating Value Rating

RQD90-100 90-97 71-85 71-85

Joint set number (Jn) Three joint sets 9 Three joint sets plus random 12

Joint roughness number (Jr)

Rough and irregular,

undulating to

discontinuous joints

3-4Smooth undulating to rough

and irregular, undulating2-3

Joint alteration number (Ja)Softening or low friction

clay minerals4

Softening or low friction clay

minerals4

Joint water reduction factor

(Jw)

Dry excavation or minor

inflow1.0 Dry excavation or minor inflow 1.0

Stress reduction

factor (SRF)

Low stress,near surface, to

medium stress

1.0-2.25Low stress,near surface, to

medium stress

1.0-2.25

SRF

Jw

J

J

J

RQDQ

a

r

n

=

4.79-7.50 2.36-2.96

RQD Rock Quality Classification


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5.5. GSI Geological Strength Index

The Geological Strength Index (GSI) is a rock

mass classification system proposed by Hoek etal. (1995) [25] for characterizing jointed rock

masses and inferring rock mass deformability

and strength parameters using intact rock

properties and jointing data. The GSI index

values can be estimated based on the geological

features of the studied rock mass such as

structure and block surface conditions. The GSI

rock mass classification system is the only rock

mass classification system that provides a set of

mechanical properties such as HoekBrown

strength parameters (mb, a and s) or theequivalent MohrCoulomb strength parameters

(C and ) and Youngs modulus for design

purposes. Results from application of the GSI

system for evaluation of the different GMUs at

the proposed Kangir dam site are presented in

Fig. 5.

Fig. 5. GSI values for the different geomechanical units at the proposed Kangir dam site.

AS

GS


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5.6. DMR Dam Mass Rating

Several authors have referred to the use of RMRas a useful tool for the description of rock

masses for foundation engineering [26,27] but

there are some difficulties in using RMR for dam

foundations. These difficulties mainly derive

from several points such as poor consideration of

the water injectivity, inappropriate rules for

quantifying the adjusting factor for joint

orientation, and inadequate incorporation of

changes in properties of the rock, the rock mass,

as well as the joints induced by weathering. In

order to address some of these issues for the

description of rock mass foundations, a new

geomechanics classification system, DMR (Dam

Mass Rating), is proposed by Romana (2003b)

[28], giving tentative guidelines for several

practical aspects in dam engineering and

appraisal of dam foundations. The collected data

were processed in accordance with the DMR

classification system and are presented in Table

7.

Table 7. DMRSTAvalues for the different geomechanical units at the proposed Kangir dam site.

a- RMRBDis basic dry RMR defined by Romana (2003b) as the addition of the first four parameters of RMR plus 15:

b- RSAT is the adjusting factor for dam stability obtained from Table 3 of Romana (2003b).

c- CF is the geometric correction factor calculated based on the formula proposed by Romana (2003b) as below:

CF = (1-sin (d- j))2 (d>j) (1)

CF = (1-sin (j- d))2 (j>d) (2)

where dis the upstream-downstream direction of the dam axis and jis the dip direction of the dominant joint. In this

study j= 0 and d= 45.

d- DMRSTA is the Dam Mass Rating value, related to the dam stability against sliding, proposed by Romana (2004) as following:

DMRSTA= RMRBD+ (CF x RSTA) (3)

6. Discussion

The foundation and abutments of the proposed

Kangir dam site are composed of different

soluble rocks, divided into two GMUs, theAsmari Formation limestones (AS) and the

Gachsaran Formation gypsiferous evaporitic

rocks (GS). These two geomechanical units

(GMUs) have been identified at the proposed

Kangir dam site and have been extensively tested

in situ and in the laboratory. Rock solubility

under in situ conditions (controlled temperature

and water acidity) was investigated as well.

Physical and geomechanical tests were carried

out on specimens obtained from boreholes

drilled in the proposed dam site foundation.

Based on results from uniaxial compression tests,

the AS and GS GMUs are classified as very low

to low strength (Class E and D) and very low

strength (Class E) rock groups according to the

classification for rock material strength proposedby Deere & Miller (1966) [17]. Three dominant

discontinuity sets exist both on the left and right

abutments of the proposed Kangir dam site.

Water injectivity tests show that the foundation

rock units at the proposed site are highly

permeable.

Impermeation grouting and/or cut-off wall

construction will be necessary. Solution rate

constants (K) of 0.36-0.38 m/sec 10-5 were

obtained for materials from the proposed Kangirdam site. This study showed that the studied

Parameters AS GS

RMRBDa 61-76 54-70

RSTAb -2 -2

CFc 0.0857 0.0857

DMRSTAd 60.82-75.82 53.82-69.72


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rocks are prone to very high solution potential.

The in-situ deformation moduli of rock units at

the Kangir dam site were estimated using GSI

values from Hoek et al. (1995). In-situdeformation modulus for the AS and GS units is

4.607 and 2.554 GPa, respectively. The rock

mass strength of the rock units at the Kangir dam

site were expressed using the HoekBrown

empirical failure criteria (Hoek et al. 2002).

Uniaxial compressive strengths of rock masses

AS and GS using the Hoek classification criteria

are 1.81 and 0.53 MPa, respectively.

Rock mass classifications provide a semi-

quantitative measure of rock mass condition and

hence tend to reduce judgmental bias. The rock

mass rating (RMR), Q-system, GSI, RQD, rockmass index (RMi) and dam mass rating, DMR,

were all applied to the rock masses at the

proposed Kangir dam site, and the data are

presented in Tables 2, 3, 4, 6 and 7 and Figure 5.

Also, results from application of different rock

mass classification systems to characterize the

rock masses exposed at the proposed Kangir dam

site and their description are summarized in

Table 8.

Table 8. Summary of rock mass classification values for the different geomechanical units at Kangir dam site and theirdescription.

In general, results obtained from the Q and RMR

classification systems are similar and more

conservative, as compared to other rock mass

classification systems such as RMi and GSI, but

there is considerable scatter among them. It

should be noted here that the nature of the RMi

and GSI classification systems is close to thenature of the Q and RMR classification systems.

However, RMi does not account for any

parameter related to the stress state or

groundwater conditions, and these factors are

important in tunneling and underground

structures. The GSI rock mass classification was

the most useful classification system for the dam

site; it provided the engineering geologist and

the design engineer with more quantitative data

such as rock mass parameters and parameters

related to the failure criteria. These quantitative

data can be used for design purposes directly.

Furthermore, the in situstress state and nature of

the discontinuities are well-defined in the GSI

system. However, this system does not account

for groundwater conditions.

Rock mass properties for each GMU were

analyzed using RocLab 1.0 free software [29].Results from this analysis are summarized in

Table 9. It should be noted here that these

analyses were carried out based on laboratory

and field data. Input parameters for each

analyzed geomechanical unit using the RocLab

1.0 software include intact rock uniaxial

compressive strength (ci), Hoek-Brown constant

(mi), GSI, intact rock modulus (Ei) and

disturbance factor (D). Hoek [30] proposed a set

of typical rock mass strength parameters for rock

masses with different characteristics. Comparing

Geomechanical at the Kangir dam site

AS GS

Rock mass

classification

systemValue Description Value Description

RMR 48-69 Fair to Good Rock 41-63 Fair Rock

Q 4.79-7.50 Fair 2.36-2.96 Poor

RMi 0.155-3.11 Weak to Strong 0.089-0.447 Weak to Medium

RQD 90-97 Excellent 71-85 Good

GSI 50-60 Very Blocky/ Good (VB/G) 40-50

Very Blocky/ Fair

(VB/F)

DMR 60.82-75-82 No Primary Concern 53.82-69.72 Some Concern


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the results obtained from this study and the

guidelines provided by Hoek [30] revealed that

the AS and GS GMUs can be classified as rock

masses with qualities that are average.

Table 9. Rock mass parameters obtained from analyzing AS and GS rock masses using RocLab 1.0 free software.

RQD index for each rock mass is determined

based on the work of Deere et al. (1967) [24].

From the RQD index data, the AS and GS rock

units at Kangir dam are classified as having

Excellent and Good rock mass quality,

respectively. Evaluation of rock units for rock

mass classification systems indicated that in the

RMR classification system the AS and GS unitsshould be classified as Fair to Good and Fair

rock masses, respectively. Based on Q index

classification of the rock units at the Kangir dam

site, the AS and GS units are classified as Fair

and Poor quality rock masses, respectively.

Based on RMi values for the AS and GS rock

units, they are classified as Weak to Strong and

Weak to Medium rock masses, respectively.

From GSI values for each rock unit, the AS and

GS units are classified as Very Blocky and Good

(VB/G) and Very Blocky and Fair (VB/F) rockmasses, respectively. It seems that in general

results obtained from the Q-system are more

conservative, as compared to other rock mass

classification systems, but there is considerable

scatter among them. The resulting final DMRSTA

values (Table 7) are used to examine overall

stability of the Kangir dam against sliding, for

determining the required excavation depth for

the foundation [31]. As shown in Table 7, thereis no concern for the level of dam safety against

sliding on the AS and GS rock units, based on

empirical guidelines proposed by Romana [28].

Based on his tentative guidelines, the minimum

desirable excavation depth for a rock-fill dam

should be until rock mass classes with DMRSTA

> 30 are reached. In the case of the Kangir dam,

such excavations are not required considering

the DMRSTA values obtained (Table 7), but in

any case, removal of the alluvial materials that

cover the rock masses at the dam isrecommended. Romana [28] recommended

GMUs at the Kangir dam siteParameter

AS GS

Input parameters

Geological strength index (GSI) 50-60 40-50

Intact rock strength (ci) 8-54 16-32

Hoek-Brown constant (mi) 8 12

Intact rock deformation modulus (Ei) GPa 3-30 16-19

Disturbance factor (D) 0 0

Rock mass properties

Rock mass compressive strength (cm) MPa 1.807 0.529

Rock mass tensile strength (tm

) MPa -0.086 -0.014

Deformation modulus (Em

) GPa 4.607 2.554

Hoek-Brown failure criterion

mb Rock mass parameter 1.341 1.408

s Rock mass parameter 0.0039 0.0013

a Rock mass parameter 0.536 0.511

Mohr-Coulomb fit

Friction angle () Degree 28.64 29.09

Cohesive strength (c) MPa 1.397 0.719


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495

systematic and local consolidation grouting in

rock-fill dams with DMRSTAbetween 20-30 and

30-40, respectively. In any point in a dam

foundation with DMRSTA > 50, consolidationgrouting is not recommended; hence in the

foundation of Kangir dam, systematic

consolidation grouting is not recommended.

Engineering geological investigations, field

and laboratory test results and computations

indicate that the Kangir dam, as currently

proposed, CAN NOT be safely constructed on

the site studied. Additional detailed geotechnical

investigations and mitigation measures will be

required if it is decided to found a dam on afoundation with very high solubility rocks. The

integrity of the reservoir with respect to possible

leakage through karstic channels must be

examined as well before a construction decision

is undertaken. Furthermore, during the design

phase, a more detailed Engineering Geology

investigation focusing on particular details of

foundations and abutments is needed, as well as

a sound plan to cope with highly soluble gypsum

strata in the foundations and abutments.

Finally, we note that by several measures, the

foundation and abutment rocks would be deemed

satisfactory for dam construction. However,

with soluble gypsum present, the issues change.

The importance of geological detail and of a

broad assessment of all relevant mechanisms in

site selection for hydraulic structures is

paramount.

Despite the fact that the geomechanical

characteristics of the proposed Kangir dam site

seemed good enough to build the dam, the maincontrolling Engineering Geological factor in this

site is gypsum and limestone dissolution and

karstification rather than the general rock mass

properties. Referring to the literature and

studying similar cases around the world,

especially in the United States, suggested that the

dam within less than two years from

impoundment would have serious seepage and

structural instability problems caused by forming

of sinkholes and large cavities at the dam

foundation and reservoir.

The authorities decided to build the Kangir

dam and hoped to prevent the seepage and

karstification problem with implementation of

mitigative measures, but after foundationtreatment a number of sinkholes along with

many large cavities appeared at the dam

foundation. Remedial measures such as building

an impermeable blanket, constructing a cut-off

wall and massive impermeation grouting had

been underway since the beginning of dam

construction in 2005, but satisfactory results

have not been obtained to date. The Kangir dam

remains incomplete with spending now at a level

exceeding 20 million dollars.

7. Conclusions

The Kangir dam, an earth-fill dam with upstream

concrete slab, was proposed for a site in the west

of Iran, to be constructed on different rocks units

(lithotypes) described as AS and GS

geomechanical units. These rock units are

classified using RQD, RMR, Q, RMi, DMR and

GSI rock mass classification systems and results

interpreted and discussed. Results obtained from

the Q-system are more conservative compared to

the other rock mass classification systems.

Physical and geomechanical tests were carried

out on core samples obtained from boreholes

drilled in the proposed Kangir dam site

foundation. Based on results from uniaxial

compression tests, the AS and GS rock units are

classified as very low to low strength (Class E

and D) and very low strength (Class E) rock

groups according to the classification for rock

material strength proposed by Deere & Miller(1966) [17].

Results are summarized here.

Three dominant discontinuity sets exist bothon the left and right banks of the proposed

dam.

RQD index for each rock mass wasdetermined based on Deere et al.(1967) [24].

From the RQD index data, the AS and GS

rock units at Kangir dam are classified as

having Excellent and Good rock mass


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496

quality, respectively. Evaluation of rock

units for rock mass classification systems

indicated that in the RMR classification

system the AS and GS units should beclassified as Fair to Good and Fair rock

masses, respectively. Based on Q index

classification of the rock units at the Kangir

dam site, the AS and GS units are classified

as Fair and Poor quality rock masses,

respectively. Based on RMi values for the

AS and GS rock units, they are classified as

Weak to Strong and Weak to Medium rock

masses, respectively. From GSI values for

each rock unit, the AS and GS units are

classified as Very Blocky and Good (VB/G)

and Very Blocky and Fair (VB/F) rock

masses, respectively.

Application of the DMR rock massclassification system indicates that rock mass

excavation in the dam foundation will not be

required, based on calculated DMRSTA

values. Removal of alluvial materials is

considered necessary. Based on DMR

values, there is no concern for dam safety

against sliding on AS and GS rock units.The engineering design phase must focus on

the foundation and sub-slab pore pressures

and the mobilized shear strength of the

dominant joint sets in both AS and GT rock

units in order to re-assess dam safety against

sliding. According to the empirical

guidelines recommended by the DMR rock

mass classification system, systematic

consolidation grouting is not required, and

only local consolidation grouting is

recommended where the dam will be

constructed on the AS and GS rock units.

In situ rock mass deformation modulus is4.61 and 2.55 GPa for the AS and GS rock

units, based on Hoek et al. 1995. Rock mass

uniaxial compression strengths of 1.81 and

0.53 MPa are calculated for the AS and GS

rock units, using the HoekBrown empirical

failure criteria.

Engineering geological investigations, field

and laboratory test results and computations

indicate that despite favorable geomechanical

properties in the examined dam site, the

proposed Kangir earth-fill dam CAN NOT be

safely constructed on the proposed site mainlydue to the high risk of gypsum karstification in

the site.

This article is an example of systematic

application of Engineering Geology principles to

the pre-design phase of an important proposed

dam site. The use of all well-known

classification systems shows that there are

substantial differences among them, and we

recommend that in such cases, all rock mass

classification systems be computed forcomparative purposes. Furthermore, in this case,

despite favorable classifications using all

systems, geological detail, the presence of

gypsiferous strata, leads to a recommendation

that a dam not be built at this site at this time.

Failure to rectify the seepage and dissolution

issues to date suggests that this recommendation

remains fully valid for the site.

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Nomenclature

Acronyms

d = dry

GSI = Geological Strength Index

RMi = Rock Mass indexRMR = Rock Mass Rating

RQD = Rock Quality Designation

s = saturated

SRF = Stress Reduction Factor (a parameter

in Q)

Symbols: Latin, then Greek

a = Rock mass parameterC = Cohesion (MPa) for rock

D = a parameter in RMiE = Young modulus in static state (MPa)

Edynamic = Elasticity modulus in dynamic state

(GPa)

Em = Deformation modulus (GPa)

Estatic = Elasticity modulus in static state

(GPa)

Ja = Joint alteration number (a parameter

in Q)

JA = Joint alteration number (parameter in

RMi)

JC = Joint condition factor (parameter in

RMi) = JL (JR/JA)

JL = Joint size factor (parameter in RMi)

Jn = Joint set number (a parameter in Q)JP = Jointing parameter (parameter in

RMi)

Jr = Joint roughness number (a parameter

in Q)

JR = Joint roughness number (parameter in

RMi)

Jw = Joint water reduction factor (a

parameter in Q)

mb = Rock mass parameterq

c50 = Intact rock uniaxial compressive

strength (MPa) for rock samples 50 mm in

diameter (a parameter in RMi)

s = Rock mass parameterVb = Block volume (parameter in RMi)

Vp = Primary (compressional) wave

velocity (km/s)

Vs = Secondary (shear) wave velocity

(km/s)

Wa = Water absorption (%)

= Density (g/cm3)

c = Uniaxial compressive strength (MPa)for intact rock

cm = Rock mass compressive strength,

MPa

tm= Rock mass tensile strength, MPa

= internal friction angle (degree)

isrm-arms5-2008-055_rock mass characterization at kangir dam site in iran

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