10th Asian Regional Conference of IAEG (2015)

Mechanical Properties of Weak Rock for Tunnel Construction in Korea
Hyun-Seok Yun (1), Seong-Woo Moon (1), Gyu-Jin Song(1) and Yong-Seok Seo(1)*(1) Dept. of Earth and Environmental Sciences, Chungbuk National University, Korea
E-mail:ysseo@cbu.ac.kr
Abstract
This study analyzed the mechanical properties of rock mass classified into class Ⅴ
by the RMR system and fault zone distributed in Korea to understand the geotechnicalparameters of weak rocks. The mechanical properties data of weak rocks were collectedfrom established design cases and literature. The mechanical properties of fault zone are
distributed in their ranges as follows; unit weight 17.3~26.3 kN/㎥, cohesion 0.036~260
kPa, internal friction angles 14.7~44.1°, moduli of deformation 3.5~800 MPa, and Poisson's
ratios 0.23~0.40. The average values of the mechanical properties of fault zone were
calculated as follows; unit weight 21.5 kN/㎥, cohesion 48.275 kPa, internal friction angle
29.9°, moduli of deformation 170.9 MPa, and Poisson's ratio 0.3. Whereas unit weights andPoisson's ratios of rocks in fault zone and Class Ⅴ rock mass are almost the same, theircohesion, internal friction angles, and the moduli of deformation show differences as thesevalues in fault zone are 39.1 %, 94.9 %, and 34.4 % respectively of these values in Class Ⅴrock mass. On reviewing distributions by rock type, it can be seen that the unit weights andPoisson's ratios of rock mass are distributed in similar ranges in fault zone and in Class Ⅴrock mass regardless of rock types. The cohesion and internal friction angles are distributed
in similar ranges in the case of only sedimentary rocks and the values of the moduli of
deformation in fault zone are distributed in lower ranges than those in Class Ⅴ rock mass
for all rock types.
Keywords: mechanical property, class V by the RMR system, fault zone, weak rock1.

Introduction

the mechanical properties for RMR Ⅴ rock mass are
unsuitably applied as they are to weak rocks in manycases. In addition, in some cases, collapses occur
during tunneling even when the mechanicalproperties of weak rocks have been cautiously
considered and applied to the design. Such cases
occur because the heterogeneity of weak rocks was
not sufficiently considered in the investigation stage
or inappropriate results of in-situ tests were reflected.
Furthermore, laboratory tests of weak rocks are
hardly conducted because of their difficulties insampling and specimen shaping. However, studies
intended to establish the mechanical properties offault cataclastic zone and fault gouge have beensteadily conducted because of the importance of suchstudies (Sinha and Singh, 2000; Sulem et al., 2004;
Heo et al., 2007; Lee et al., 2007; Chung et al., 2009;
Moon et al., 2014).
In the present study, to understand thegeotechnical characteristics of weak rocks, themechanical properties of RMR Ⅴ rock mass andfault zone that are typical weak rocks were collected
and analyzed. The mechanical properties used in the

Since weak rocks such as fault zone, shear zone,
and differential weathering zone remarkably reduce
the strength of rock mass, they act as geological risk
factors that degrade the stability of constructions
during tunneling. In particular, since fault zone
becomes a cause of shear strength reduction, decreaseof arching effects, the expansion of plastic and stressrelaxation zones, the swelling of fault gouge, anddecreases in confining pressure of the ground due togroundwater inflows, excessive displacement orcollapse may occur during tunneling in these grounds.
Therefore, the mechanical properties of weak rocks
should be accurately figured out in the design stage toestablish excavation and reinforcement methods.
When a tunnel is designed, the mechanicalproperties of the ground are determined by rock mass
rating (RMR) based on the results of in-situ andlaboratory tests considering empirical formulas,design cases and references. However, in theplanning stage, due to insufficient understanding of
weak rocks and temporal and economic restrictions,1

10th Asian Regional Conference of IAEG (2015)

analysis are unit weights, cohesion, internal friction
angles, the moduli of deformation, and Poisson's
ratios determined during tunnel design stage. The
mechanical properties of RMR V rock mass wereanalyzed by collecting data from 64 tunnel designcases and the geotechnical parameters of fault zone
were analyzed through various in-situ and laboratory
tests and numerical analyses for 21 tunnels.
2.

structural units in such a way that each type of rockmass is represented by a separate geological structuralunit (Singh and Goel, 1999). The following six
parameters are used to classify a rock mass using the
RMR system.
-

Rock Mass Rating (RMR) system

The rock mass rating (RMR) system was initiallydeveloped by Bieniawski (1973) on the basis of his
experiences in shallow tunnels in sedimentary rocks.
Since then the classification has undergone several
significant changes. To apply the RMR, a given siteshould be divided into a number of geological

Uniaxial compressive strength
material
Rock Quality Designation (RQD)
Spacing of discontinuities
Condition of discontinuities
Groundwater conditions
Orientation of discontinuities

of

rock

The rock mass rating system is shown in Table 1,
giving the ratings for each of the six parameters listed
above.

Table 1 Rock mass rating system
A. Classification parameters and ratings
Parameter
Point load
> 10
(MPa)
Strengthof rock Uniaxial
1 material compressive
> 250
(MPa)

2

3

4

Range of values
4-10

2-4

1-2

100-250

50-10025-50

5-25 1-5 <1Rating

15

12

7

4

RQD (%)

90-100

75-90

50-75

25-50

< 25

Rating

20

17

13

8

3

Spacing of joint (m)

>2

0.6-2

0.2-0.6

0.06-0.2< 0.06

Rating

20

15

10

8

5

Condition of
discontinuities

RatingInflow per
10 m tunnel
lengthRatio joint
water
Groundpressurewater
5/ majorstressGeneralconditions
RatingVery rough,Slightly rough,Slightly rough,Slickenside,
not continuous,separation
separation
gouge < 5 mm,
no separation,< 0.1 mm,
< 1 mm,separation
unweathered slightly weathered highly weathered
1-5mm,
wall rockwall rockwall rockcontinuous

2

1

Slickenside,
gouge > 5 mm,
separation> 5 mm,continuous

30

25

20

10

0

None

< 10 ℓ10-25 ℓ20-125 ℓ> 125 ℓ0

< 0.1

0.1-0.2

0.2-0.5> 0.5

Completely
dry

Damp

Moist only

Dripping

Flowing

15

10

7

4

0

2

0

10th Asian Regional Conference of IAEG (2015)

Table 1 (Continued)
B. Rating adjustment for discontinuity orientations (see F)
Strike and dip orientation
Very
Favourableof joint
Favourable
Tunnels
0-2
RatingFoundations
0-2
Slopes0-5

Fair

Unfavourable

-5
-7
-25

-10
-15
-50

Very
unfavourable
-12
-25
-60

C. Rock mass classes determined from total ratings
Rating100 ~ 8180 ~ 61

60 ～ 4140 ～ 2120 ～ 0Class No.

Ⅰ

Ⅱ

Ⅲ

Ⅳ

Ⅴ

Description

Very goodrock

Good rock

Fair rock

Poor rock

Very poor rockD. Meaning of rock mass classes
Class No.

Ⅰ

Ⅱ

Ⅲ

Ⅳ

Ⅴ

Average stand-up time

20 years/ 15 m span

1 years/ 10 m span

1 week/ 5 m span

10 hours/ 2.5 m span

30min
/ 1 m span

> 400

300-400

200-300

100-200

< 100

> 45°

35-45°

25-35°

15-25°< 15°

Cohesion of rock mass(kPa)
Friction angle ofrock mass

E. Guidelines for classifying discontinuity condition
Discontinuity length(persistence) rating

<1m
6

1-3 m
4

3-10 m2

10-20 m1

> 20 m0

Seperation (aperture)
ratingNone
6< 0.1mm50.1-1mm
4

1-5mm
1

> 5mm
0

Very rough

Rough

Smooth6

5Slightly
Rough
3

1

Slicken
Slide
0

None

Hard filling< 5 mm4

Hard filling> 5 mm2

Soft filling< 5 mm2

Soft filling> 5 mm0

Slightly
weathered5Moderately
weathered3

Highlyweathered1

Decomposed

Roughness rating

Infilling (gouge)rating6
Fresh

Weathering rating
6

F. The effect of joint strike and dip orientation in tunneling
Strike perpendicular to tunnel axis
Drive with dip
Drive with dip(Dip 45-90°)(Dip 20-45°)
Very
Favourable
favourable
Drive against dip
Drive against dip(Dip 45-90°)(Dip 20-45°)
Fair
Unfavourable

30

Strike parallel to tunnel axis
Dip 45-90°

Dip 20-45°

Very
unfavourable

Fair

Dip 0-20° – Irrespective of strike
Fair10th Asian Regional Conference of IAEG (2015)3.

Mechanical properties of RMR
Ⅴ rock mass

while that of granite shows a range of distribution of
30~210 kPa and that of sedimentary rocks shows arange of distribution of 30~200 kPa.
Fig. 3 shows the distribution of internal friction
angles of RMR Ⅴ rock mass. The internal friction
angles are in a range of 25.0~37.0 ° andapproximately 95% of the data are distributed in arange of 29.5~34.5 ° . The average value of theinternal friction angles of RMR Ⅴ rock mass is
31.5° and differences in ranges among rock types
are not large.
Fig. 4 shows the distribution of the moduli of
deformation of RMR Ⅴ rock mass. The moduli of
deformation are quite widely distributed showing a
range of 200~1,000 MPa. In particular, wide ranges
were shown in granite, gneiss and sedimentary rocks.
The values are mainly distributed in a range of
350~550 MPa and high strength rock masses such as
granite and gneiss show values in a range of950~1,000 MPa. The average value of the moduli of
deformation of RMR Ⅴ rock mass is 496.4 MPa.
Fig. 5 shows the Poisson's ratios of RMR Ⅴ
rock mass indicating that most are intensively
distributed in a range of 0.28~0.32 except for only
one case that shows a value of 0.33. In addition, theranges of distribution of Poisson's ratios of different

Mechanical properties of RMR Ⅴ rock mass
were analyzed using geotechnical parameters applied
to 64 tunnel construction in Korea. Fig. 1 (a) shows
the unit weights of RMR Ⅴ rock mass as
frequencies according to the ranges of the values andFig. 1 (b) shows unit weights according to rock types.
Unit weights are distributed in range of 20.0~24.5
kN/㎥. The interval in which the largest number ofunit weights are distributed is 20.5~21.5 kN/ ㎥
where 33 out of 64 data are distributed and 94 % of
the data are distributed in a range of 20.0~22.5 kN/㎥.
The average value is 21.3 kN/㎥. The values are themost widely distributed in the case of sedimentaryrocks.
Fig. 2 shows the distribution of cohesion of RMR
Ⅴ rock mass indicating that the values are
distributed in a range of 30~400 kPa. The interval inwhich the largest number of data is 100~140 kPa
where 22 out of 64 are distributed and the averagevalue of cohesion was calculated as 123.4 kPa.
According to the results of analysis of cohesion
according to rock types, the cohesion of gneiss is
distributed in the widest range which is 30~300 kPa(a) Frequency of unit weights(b) Range of unit weights of rocks
Fig. 1 Frequency and range of unit weights of rock mass class Ⅴ by RMR system(a) Frequency of cohesion(b) Range of cohesion of rocks
Fig. 2 Frequency and range of cohesion of rock mass class Ⅴ by RMR system4

10th Asian Regional Conference of IAEG (2015)(a) Frequency of friction angles(b) Range of friction angles of rocks
Fig. 3 Frequency and range of friction angles of rock mass class Ⅴ by RMR system(a) Frequency of moduli of deformation(b) Range of moduli of deformation of rocks
Fig. 4 Frequency and range of moduli of deformation of rock mass class Ⅴ by RMR system(a) Frequency of Poisson’s ratios(b) Range of Poisson’s ratios of rocks
Fig. 5 Frequency and range of Poisson’s ratios of rock mass class Ⅴ by RMR system

rock types were shown to be similar to each other.
4.

fault zone in Korea. The mechanical properties in theprevious study and RMR Ⅴ rock mass werecomparatively analyzed as Table 2. The ranges of
distribution of the values for RMR Ⅴ rock mass are
generally narrower compared to fault zone. The
reason is considered to be attributable to the methodsof determining mechanical properties in the design
stage. In general, geological characteristics are notsufficiently reflected when mechanical properties ofComparative analysis of themechanical properties of RMR Ⅴ
rock mass and fault zone

3.1 Numerals
Yun et al. (2015) analyzed the mechanicalproperties of collapsed tunnel due to the effects of5

10th Asian Regional Conference of IAEG (2015)

Table 2 Range, average and ratio of mechanical properties of fault zone and RMR Ⅴ rock mass
Mechanicalproperty
Rock type

Unit weight(kN/㎥)

Cohesion(kPa)

Frictionangle (°)

Moduli ofdeformation(MPa)

Poisson’s
ratio

Fault zone

Range
Average17.3~26.3
21.5

0.036~260
48.3

14.7~44.1
29.9

3.5~800.0
170.9

0.23~0.4
0.3

RMR Ⅴ

Range
Average

20.0~24.5
21.3

30~300123.4

25.0~37.0
31.5

200~1,000
496.4

0.28~0.33
0.3

1.01

0.39

0.95

0.34

1.00

Ratio of average

RMR Ⅴ rock mass are determined in the design
stage because diverse data such as in-situ andlaboratory test results, design data, and references arecomprehensively analyzed and the average values areused. On the other hand, the mechanical properties offault zones are determined using the test results forsamples collected from various fault zones andnumerical analytical methods so that theheterogeneity of faults is sufficiently reflected. The
average value of unit weights of fault zone is similar
to that of RMR Ⅴ rock mass and the average valueof Poisson's ratios of fault zone was calculated to be
the same as that of RMR Ⅴ rock mass. The averagevalues of cohesion, internal friction angles, and themoduli of deformation of fault zone are shown to belower by 39.1 %, 94.9 % and 34.4 % respectively
compared to those of RMR Ⅴ rock mass.
Fig. 6 shows an analysis of the average values ofthe mechanical properties of fault zone and RMR Ⅴ
rock mass by rock type. The bar graphs show theaverage values of the mechanical properties of fault
zone and RMR Ⅴ rock mass by rock type and thelinear graphs shows the ratios of the average valuesof the mechanical properties of fault zone to those of
the mechanical properties of RMR Ⅴ rock mass byrock type. According to the results of the analysis,
whereas the average unit weights of RMR Ⅴ rockmass are in a range of 20.0~21.5 kN/㎥ indicating
not much differences among rock types, the average
unit weights of fault zone are distributed in a range of
18.7~22.0 kN/㎥ which is wider compared to RMR
Ⅴ rock mass. In addition, whereas the average unit
weights of fault zone are a little lower compared to
RMR Ⅴ rock mass amounting to 87.5~98.8 % of
those of RMR Ⅴ rock mass in the case of granite,
gneiss, andesite and schist, they are higher comparedto RMR Ⅴ rock mass in the case of sedimentaryrocks and phyllite. When the cohesion and internal
friction angles that determine the characteristics ofshear strength of fault zones were compared withthose of RMR Ⅴ rock mass, those of sedimentaryrocks were shown to have the least difference and in
the case of internal friction angles, the average valuesof fault zone were shown to be higher. It shows thatsince most of collected sedimentary rocks are shaleand mudstone which are swelling rocks, clay minerals
reduced shear strength. On the other hand, on
reviewing the ratios of the average values of themechanical properties of fault zone to the RMR Ⅴ
rock mass by rock type, it can be seen that andesiteshows the largest differences in all mechanicalproperties except for cohesion. However, thereliability of this result is low because the number ofdata on andesite is not larger than 5. The average
moduli of deformation of fault zone shows
remarkably lower values amounting to 21.8~83.4 %of those of RMR Ⅴ rock mass. Since the moduli of
deformation are importantly used during continuous
numerical analyses in the tunnel design stage forstability analysis and the establishment ofreinforcement methods, applying the mechanicalproperties of RMR Ⅴ rock mass to fault zone may
cause serious problems to the stability of tunnel
constructions. Therefore, when weak rocks are
distributed, the mechanical properties of the weakrocks should be accurately analyzed and reflect thoseon design and excavation work.
5.

Conclusions

The mechanical properties of weak rocks are
remarkably poorer compared to surrounding grounds
and not only cause the instability of constructions but
also their values are distributed in a quite wide range
even within the same site. Although such states may
result from site conditions, technicians' skill levels or
the reliability of test results, a primary cause is highlevels of heterogeneity resulting from different
degrees of damage by fault movements. Therefore, toaccurately understand the mechanical properties of
weak rocks, geometry of fault zone should be
accurately analyzed, the area of weak rocks should be
subdivided into fault gouge zone, cataclastic zones,
etc., and diverse data collected from in-situ andlaboratory tests, case analysis and references should
be comprehensively reviewed and evaluated.

6

10th Asian Regional Conference of IAEG (2015)(a) Unit weight(b) Cohesion(c) Friction angle(d) Moduli of deformation(e) Poisson’s ratio
Fig. 6 Average values and ratios of mechanical properties of fault zone and RMR Ⅴ rock mass by rock type

Acknowledgements

References

This research was supported by a grant(13SCIP-C069074-01) from the Ministry of Land,
Infrastructure and Transport of Korean government.

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10th Asian Regional Conference of IAEG (2015)

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