[technical notes] geotechnical description and … description and jgs engineering classification...

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Volume 1, Number 1, October 2005, pp.7-17 [TECHNICAL NOTES]

Geotechnical description and JGS engineering classification system for rock mass

Masahiko OSADA*, Akio FUNATO**, Ryunoshin YOSHINAKA*, Hiroshi ITO***, Takashi KITAGAWA****, Katsuji SASAKI*****, Kenji AOKI******, Omer AYDAN*7,

Shinji AKUTAGAWA*8, Hideo KIYA*9, Keizo KUWAHARA*10, Masahiro SETO*11, Soichi TANAKA*12, Kazuo TANI*13, Toshiaki MIMURO*14 & Takayuki MORI*15

* Member of ISRM：Geosphere Research Institute, Saitama University, Sakura-ku, Saitama, 338-8570, Japan ** Member of ISRM : Core Lab, OYO Corporation, Kita-ku, Saitama,331-0812, Japan

*** Member of ISRM : Central Research Institute of Electric Power Industry, 1646 Abiko,Chiba, 270-1194, Japan **** Member of ISRM : Nishimatsu Construction Co.,Ltd., Minato-ku, Tokyo, 105-8401, Japan

***** Member of ISRM : Suncoh Consultants Co.,Ltd., Koto-ku, Tokyo, 136-8522, Japan ****** Member of ISRM : Kyoto University, Sakyo-ku, Kyoto, 606-8501, Japan *7 Member of ISRM : Tokai University, Shimizu-ku, Shizuoka, 424-8601, Japan

*8 Member of ISRM : Kobe University, Nada-ku, Kobe, 657-8501, Japan *9 Member of ISRM : Japan Railway Technical Research Institute, Kokubunji, Tokyo, 185-8540, Japan

*10 Member of ISRM : Fukken Co.,Ltd., Chiyoda-ku, Tokyo, 101-0032, Japan *11 Member of ISRM : National Institute of Advanced Industrial Science & Technology, Tsukuba, Ibaragi, 305-8567, Japan

*12 Member of ISRM : Tanaka Geotechnical and Geophysical Consultant Office, Nerima-ku, Tokyo, 176-0012, Japan *13 Member of ISRM : Yokohama National University, Hodogaya-ku, Yokohama, 240-0067, Japan

*14 Member of ISRM : A-Tic Corporation, Nishi-ku ,Sapporo, 063-0801, Japan *15 Member of ISRM : Kajima Corporation, Minato-ku, Tokyo, 107-0051, Japan

Received 3 September 2005; accepted 20 September 2005

ABSTRACT

This paper presents the new classification system which identifies and designates rock masses based on their fundamental engineering characteristics. The system encompasses the stepwise procedural classification with three steps and a sub-step. The first-step is to classify rock mass into two types; (1) hard rock mass and/or its weathered or altered rock mass, and (2) soft rock mass that is not lithified enough. The second and third steps are to further classify the rock mass based on their inherent structural and physical parameters such as discontinuities, strength of rock material and others that govern the engineering properties. Most of identification items and description terms for classification parameters are adopted from internationally recognized standard (ISO14689-1), and expressed quantitatively. Since, examples on the applicability of this classification system to the actual rock mass of typical infrastructures showed well that this system is suitable for the engineering classification of rock mass.

Keywords: Rock mass, Engineering classification system, Identification and description of rock mass

1. INTRODUCTION

Rock mass classification, which is a useful tool for rock engineering, was initiated in Europe in 1940s. Terzaghi (1946) proposed nine categories of rock mass associated with rock load on tunnel supports. After long time passes, Q-system (Barton et al.1974), RMR (Bieniawski, 1976), and others were proposed in 1970s, in which the classes are expressed as numerical values like ratings through the research on the relation between rock condition and tunnel support design. In Japan, rock mass classification started as one of the

evaluation techniques for large dam foundations in 1960s, and extended for tunnels. Since then, they were advanced with time by taking into account the objective approaches as compared with the subjective at the early stages. These methods have been applied to various fields such as rock slopes, underground excavations, rock foundations of nuclear power plants and long-span bridges to the most.

Though these classification methods have developed through such process described above, but there are many problems of the present situation, which are summarized as follows: (1) Many rock classification methods have too much subdivision depending upon the enterprises and kinds of

8 M. OSADA et al. / International Journal of the JCRM vol.1 (2005) pp.7-17

structure. This is a result of “easy to use”, however, it deviates from the common utilization of rock mass information.

(2) Unified soil classifications make the ground information as a common property, but there is a lack of common system or measure for rock mass classification.

(3) The classification systems using numerical value are widely used, but they are assumed to be based mainly on hard rock masses and thus their evaluation items and classes are not adequate for soft/weak, rudaceous, foliated, and thinly interbedded rock masses.

(4) It is very difficult to discriminate the reproducibility of the actual states of rock masses and the identity of their physical conditions for the same number in the numerical value. Therefore, it is necessary to clarify the common terms.

(5) It is important to utilize the valuable experiences and test / measured data accumulated up to now, by the development of unified classification.

With the recognition of above mentioned factors, this paper introduces the new classification system (JGS: 3811-2004) on the basis of constitutive relations between physical properties of rock mass, and internationally recognized "identification and description of rock and rock mass (ISO14689-1: 2003)".

2. IDENTIFICATION AND DESCRIPTION OF FUNDAMENTAL PROPERTIES OF ROCK MASS

The purpose of this classification system is to develop the engineering classification of rock mass that is compatible with all rock types, but based on fundamental properties of rock mass. Therefore, most relevant test and investigation schemes are necessary to determine the appropriate classes and classification parameters governing the engineering properties of rock mass. In order to establish the common terminology system， care has been taken to select the

internationally recognized terms and classes, after ISO14689-1: 2003 for identification and description.

2.1 Strength of rock material

The strength is one of the most important characteristics of rocks, in which uniaxial compressive strength is commonly used to represent the strength of rock material. Besides the uniaxial compressive strength, tensile strength, elastic wave velocity, Schmidt hammer rebound number, or geological hammer sound is also used for characterization of rocks. In these techniques some empirical relations are used for inferring the strength of rock material. Examples for such empirical relations are shown in Figures 1 to 5. However the parameters of these relations may obviously be different from one particular rock type to another. Therefore, most appropriate method should be decided according to the desire level of accuracy.

一軸圧縮強さ　(MN/m2)

引張り

強さ　

(MN

/m2 )

1000 200 300 400

5

10

15

20

25

30

凡例

花崗岩閃緑岩斑岩玄武岩輝緑岩安山岩石英粗面岩凝灰岩砂岩礫岩チャート泥岩シルト岩石灰岩頁岩

Tens

ile s

tren

gth

(MPa

)

Uniaxial compressive strength (MPa)

Legend

GraniteDiolitePorphyryBasaltDiabaseAndesiteLipariteTuffSandstoneConglomerateChertMudstoneSiltstoneLimestoneShale

一軸圧縮強さ　(MN/m2)

引張り

強さ　

(MN

/m2 )

1000 200 300 400

5

10

15

20

25

30

凡例

花崗岩閃緑岩斑岩玄武岩輝緑岩安山岩石英粗面岩凝灰岩砂岩礫岩チャート泥岩シルト岩石灰岩頁岩

Tens

ile s

tren

gth

(MPa

)


Legend

GraniteDiolitePorphyryBasaltDiabaseAndesiteLipariteTuffSandstoneConglomerateChertMudstoneSiltstoneLimestoneShale

Figure 2. Relation between uniaxial compressive strength and

tensile strength (after Kasuya, 1978.)

0

1000

2000

3000

4000

5000

6000

7000

0.01 0.1 1 10 100 1000


P-w

ave

velo

city

(m/s

)GraniteGranodioriteAndesiteBasaltTuff brecciaLaplli tuffTuffConglomerateSandstoneShaleSlateSandy mudstoneMudstoneGreen schist

Figure 1. Relation between uniaxial compressive strength and P-wave velocity. (after JGS, 2002.)

M. OSADA et al. / International Journal of the JCRM vol.1 (2005) pp.7-17 9

1

10

100

1000

100 1000Equotip hardness

Unia

xial

com

press

ive s

trengt

h （M

Pa)

◆ artificial 1

● andesite lava

＋ artificial 2

○ clastic limestone

□ crystalline

△ sandstone

×metamorphic rocks

■andesite

▲pyroclastic rock

＊mudstone

Figure 3. Relation between Equotip hardness and uniaxial

compressive strength (after Okawa et al., 1999)

Figure 4. Relation between needle penetration index and

uniaxial compressive strength. (after Takahashi et al., 1998)

2.2 Foliation

Foliation is a planar or thinly layered structure of crystals constituting the rock material. This structure can be observed in any rock type. However, it is a common structure of metamorphic rocks such as schist, gneiss and phyllite. The importance of foliation is that it has a strong influence on the anisotropy of mechanical properties of rocks. In addition, they can be open up quite easily by uplifting, and/or stress release mechanism initiated by excavations, resulting in anisotropy.

The existence of the foliation can be investigated through the flaking and spacing of planes, and the petrography of rock material.

2.3 Discontinuities

The characteristics of discontinuities considered in this classification system are the spacing, number of discontinuity sets, aperture, roughness and filling materials.

In order to determine the discontinuity spacing, firstly, a particular size of measurement area for observation is set on

Figure 5. Relation between hammer sound index and uniaxial

compressive strength.

the surface of the rock mass. Then the spacings of two successive discontinuities are measured using a scanline or a two-dimensional investigation frame. Three mutually orthogonal measurement lines are desirable. When the discontinuities are orientated clearly parallel to each other on outcrops, it is easy to determine the number of discontinuity sets. But it is not usually the case, and when the orientation data is scattered or a number of observation surfaces are exposed, it is better to examine by stereographic projection technique.

As for the discontinuity aperture, distance perpendicular to the both sides of discontinuity walls, measurement can be made by using either a crack scale or a gap gauge. Another important characteristic is discontinuity roughness that varies depending on the scale of discontinuity being studied. The roughness appears as a combination of the superimposed secondary roughness on the first order roughness. Therefore, two different scales, namely, large scale (length is about 1-2 m) and small scale (length is 10 cm) are used. The large scale roughness is determined through naked-eye observations, and the small scale roughness is commonly determined by either a needle-type profilometer, or non-contact displacement gauge, or digital photographing. The filling material is defined as the filling of soft ground material such as clay or seams between the walls of discontinuities. However, the hard filling materials such as quartz veins or calcite veins are omitted.

2.4 Grain size of constituting rock material

The dominant grain size is an important property of third-step classification in the case of massive soft rock mass. The dominant grain size is defined as the size of particles having the largest volume ratio.

Although it is always desirable to use the results from particle size distribution test, it can also be determined by naked-eye or touching test of rock surface as an alternative and denote as clayey, silty, sandy, or coarser than sand, if it is difficult to perform the particle size distribution test.

2.5 Content of large fragments

The volumetric ratio of large fragments is also an important property in the third-step classification of rudaceous soft rock mass. It is generally difficult to determine it using the conventional procedure for particle size distribution tests of soils. Therefore, it is determined either on


Massive ｛M｝

Foliated ｛F｝

Hard Rock Mass

[H]

First-step Classification

Second-step Classification

Sub-stepClassification

Massive ｛M｝

Rudaceous ｛R｝

Interbedded ｛B｝

Soft Rock Mass

[S]

No.Weathering

degree

Averagethickness ofweak layer

1 w1 t 1

2 w2 t 2

3 w3 t 3

4 w4 t 4

5 w5 t 5

6 w6

*If necessary, the symbol SM classis designated

No. W eathering degree

Num ber of discontinuity

sets

Discontinuity aperture

Discontinuity roughness

Existence of filling m aterial

1 w1 n1 a1 rsr f1

2 w2 n2 a2 rsm f2

3 w3 n3 a3 rss f3

4 w4 n4 a4 rwr

5 w5 n5 a5 rwm

6 w6 a6 rws

7 rpr

8 rpm

9 rps

No. Weatheringdegree

Discont inuityspacing

1 w1 s1

2 w2 s2

3 w3 s3

4 w4 s4

5 w5 s5

6 w6 s6

No. Weathering degree

Dom inant size of m atrix

particles

Large fragm ents

content

Dom inant fragm ent

size

Fragm ent strength

1 w1 p1 b1 g1 h1

2 w2 p2 b2 g2 h2

3 w3 p3 g3 h3

4 w4 p4 g4 h4

5 w5 h5

6 w6 h6

7 h7

Third-step Classification

Ⅰ Ⅱ Ⅲ Ⅳ

D DⅠ DⅡ DⅢ DⅣ

10 E EⅠ EⅡ EⅢ EⅣ

5 F FⅠ FⅡ FⅢ FⅣ

1 G GⅠ GⅡ GⅢ GⅣ

Fragment Content ％ 50 20 10

Mat

rix S

treng

th M

pa

Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ

Example Example Example Example Example

DDⅠ DDⅡ DDⅢ DDⅣ DDⅤ


DEⅠ DEⅡ DEⅢ DEⅣ DEⅤ


DFⅠ DFⅡ DFⅢ DFⅣ DFⅤ

Level 3 DGⅠ DGⅡ DGⅢ DGⅣ DGⅤ

P ercentage of Weak Layer ％ 10 30 50 80

Diff

eren

ce o

f lay

erstr

engt

h cl

ass

Same

Level 1

Level 2

Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ

A AⅠ AⅡ AⅢ AⅣ AⅤ AⅥ

100 B BⅠ BⅡ BⅢ BⅣ BⅤ BⅥ

50 C CⅠ CⅡ CⅢ CⅣ CⅤ CⅥ

25 D DⅠ DⅡ DⅢ DⅣ DⅤ DⅥ

10 E EⅠ EⅡ EⅢ EⅣ EⅤ EⅥ

5 F FⅠ FⅡ FⅢ FⅣ FⅤ FⅥ

Discont inuity Spacing mm

2000 600 200 60 20

Rock

Mat

eria

l Stre

ngth

M

Pa

Ⅰ Ⅱ Ⅲ Ⅳ





Dominant Grain Size mm

2 0.063 0 .002

Rock

Mat

eria

lSt

reng

th

MPa

Massive ｛M｝

Foliated ｛F｝

Hard Rock Mass

[H]

Massive ｛M｝

Foliated ｛F｝

Hard Rock Mass

[H]

First-step Classification

Second-step Classification

Sub-stepClassification

Massive ｛M｝

Rudaceous ｛R｝

Interbedded ｛B｝

Soft Rock Mass

[S]

Massive ｛M｝

Rudaceous ｛R｝

Interbedded ｛B｝

Soft Rock Mass

[S]

No.Weathering

degree

Averagethickness ofweak layer

1 w1 t 1

2 w2 t 2

3 w3 t 3

4 w4 t 4

5 w5 t 5

6 w6

*If necessary, the symbol SM classis designated

No. W eathering degree

Num ber of discontinuity

sets

Discontinuity aperture

Discontinuity roughness

Existence of filling m aterial

1 w1 n1 a1 rsr f1

2 w2 n2 a2 rsm f2

3 w3 n3 a3 rss f3

4 w4 n4 a4 rwr

5 w5 n5 a5 rwm

6 w6 a6 rws

7 rpr

8 rpm

9 rps

No. Weatheringdegree

Discont inuityspacing

1 w1 s1

2 w2 s2

3 w3 s3

4 w4 s4

5 w5 s5

6 w6 s6

No. Weathering degree

Dom inant size of m atrix

particles

Large fragm ents

content

Dom inant fragm ent

size

Fragm ent strength

1 w1 p1 b1 g1 h1

2 w2 p2 b2 g2 h2

3 w3 p3 g3 h3

4 w4 p4 g4 h4

5 w5 h5

6 w6 h6

7 h7

Third-step Classification

Ⅰ Ⅱ Ⅲ Ⅳ





Fragment Content ％ 50 20 10

Mat

rix S

treng

th M

pa

Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ


DDⅠ DDⅡ DDⅢ DDⅣ DDⅤ


DEⅠ DEⅡ DEⅢ DEⅣ DEⅤ


DFⅠ DFⅡ DFⅢ DFⅣ DFⅤ

Level 3 DGⅠ DGⅡ DGⅢ DGⅣ DGⅤ

P ercentage of Weak Layer ％ 10 30 50 80

Diff

eren

ce o

f lay

erstr

engt

h cl

ass

Same

Level 1

Level 2

Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ

A AⅠ AⅡ AⅢ AⅣ AⅤ AⅥ

100 B BⅠ BⅡ BⅢ BⅣ BⅤ BⅥ

50 C CⅠ CⅡ CⅢ CⅣ CⅤ CⅥ

25 D DⅠ DⅡ DⅢ DⅣ DⅤ DⅥ

10 E EⅠ EⅡ EⅢ EⅣ EⅤ EⅥ

5 F FⅠ FⅡ FⅢ FⅣ FⅤ FⅥ

Discont inuity Spacing mm

2000 600 200 60 20

Rock

Mat

eria

l Stre

ngth

M

Pa

Ⅰ Ⅱ Ⅲ Ⅳ





Dominant Grain Size mm

2 0.063 0 .002

Rock

Mat

eria

lSt

reng

th

MPa

Figure 6. The system for engineering classification of rock mass. (after JGS, 2004)


two-dimensional rock outcrops or on the surfaces of boring cores.

2.6 Layer thickness

Another important parameter for third-step classification of soft layered rock mass is the layer thickness. The layer thickness is defined as the distance perpendicular to the bedding planes of a given layer.

2.7 Weathering / alteration state

Rock may be degraded as a result of physical or chemical weathering or the combined effects. The effect of weathering reflects the strength of rock material and characteristics of discontinuities. Weathering states are defined using colour change, the change of granular structure and formation of rock material as indices. Since weathering never occur uniformly over the rock mass, the investigation on weathering

Table 1. Classification parameters and classes of hard rock mass. (after JGS, 2004)

Classification parameter Class

A B C D E F Rock Material Strength （MPa） More than 100 100～50 50～25 25～10 10～5 Less than 5

Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Ⅵ Discontinuity Spacing （mm） More than 2000 2000～600 600～200 200～60 60～20 Less than 20

w1 w2 w3 w4 w5 w6 Weathering degree

Table 3

n1 n2 n3 n4 n5 Number of discontinuity sets

1 set 2 sets 3 sets 4 sets and above Random

a1 a2 a3 a4 a5 a6 Discontinuity aperture （mm） Less than 0.1 0.1～0.25 0.25～0.5 0.5～2.5 2.5～10 More than 10

rsr rsm rss rwr rwm rws rpr rpm rps Discontinuity roughness

Figure 7

f1 f2 f3 Existence of filling material None Partially filled Fully filled

Table 2. Classification parameters and classes of soft rock mass. (after JGS, 2004)

Classification parameters Class

D E F G Rock material strength （MPa） 25～10 10～5 5～1 Less than 1

D E F G Matrix strength （MPa） 25～10 10～5 5～1 Less than 1

DD/EE/FF/GG DE/EF/FG DF/EG DG Difference of layer strength class Same level 1 level 2 level 3

Ⅰ Ⅱ Ⅲ Ⅳ Dominant grain size （mm） More than 2 2～0.063 0.063～0.002 Less than 0.002

Ⅰ Ⅱ Ⅲ Ⅳ Fragment content （％） More than 50 50～20 20～10 Less than 10

Ⅰ Ⅱ Ⅲ Ⅳ Ⅴ Percentage of weak layer （％） Less than 10 10～30 30～50 50～80 More than 80

w1 w2 w3 w4 w5 w6 Weathering degree Table 3

s1 s2 s3 s4 s5 s6 Discontinuity spacing （mm） More than 2000 2000～600 600～200 200～60 60～20 Less than 20

p1 p2 p3 p4 Dominant size of matrix particles （mm） More than 2 2～0.063 0.063～0.002 Less than 0.002

b1 b2 Large fragment content （％） More than or equal to 10 Less than 10

g1 g2 g3 g4 Dominant fragment size （mm） More than 630 630～200 200～63 Less than 63

h1 h2 h3 h4 h5 h6 h7 Fragment strength （MPa） More than100 100～50 50～25 25～10 10～5 5～1 Less than 1

t1 t2 t3 t4 t5 Average thickness of weak layer（mm） More than 600 600～200 200～60 60～20 Less than 20


is necessary for rock mass classification. Weathering is induced by atmospheric agents but the hydrothermal waters may also induce the similar effects on rock known as alteration. Therefore, the alteration is included within the terminology of weathering except in some special cases. When the investigation results are reported, weathering and alteration must be distinguished in descriptions.

3. CONCEPT AND OUTLINE OF NEW ROCK MASS CLASSIFICATION SYSTEM (JGS: 3811-2004)

The engineering classification of rock mass is sequentially performed on the basis of investigations and tests in the order of first-step, second-step, third-step, and sub-step classifications. The system of the engineering classification of rock mass is shown in Figure 6.

3.1 First-step Classification

First-step classification entirely relies on the uniaxial compressive strength of rock material. The rock mass is classified as Hard rock mass [H] if the uniaxial compressive strength is equal to or greater than 25 MPa and the weathered rock mass also is considered the same only if the strength of its original rock material can be assumed to have the approximately same value. Except the hard rock mass as defined above, that is, if the uniaxial strength of rock material even for fresh-unweathered rock mass is less than 25 MPa, the rock mass is classified as Soft rock mass [S].

The overall mechanical behavior of the rock mass classified as hard rock mass presumably depends on the existence of discontinuities in rock mass rather than rock material itself. On the other hand, the overall mechanical behavior of the rock mass designated as soft rock mass depends on the rock material itself.

3.2 Second-step Classification

Second-step classification is based on the rock structure or the grain size. For hard rock mass, it is further classified into massive {M} and foliated {F} depending upon the rock structure. On the other hand, soft rock mass [S] is classified as massive {M} if the constituting materials are homogenous, rudaceous {R} if constituting materials are fragments and matrix, and interbedded {B} if constituting materials are thinly layered.

3.3 Third-step and Sub-step Classifications

Third-step classification is fundamental and indispensable, in which rock mass is classified by considering the indices of classes from the combination of two classification parameters governing the engineering properties of rock mass. On the other hand, sub-step classification is performed by selecting the required parameters for classification according to need.

The parameters and their classes of third-step and sub-step classifications are given in Table 1 and Table 2. In addition, the detail description of roughness classes and weathering classes are described separately in Figure 7 and Table 3 respectively.

3.4 Classification of Fracture Zone

Fracture zones consist of fault breccia and gouge. Since

rsr rsm rss

rwr rwm rws

rpr rpm rps

Rough：ｒSlightly

rough： m Smooth:s

Stepped：s

Wavy：w

Planar：p

Small scale(10cm)

Large scale(1～2m）

rsr rsm rss

rwr rwm rws

rpr rpm rps

Rough：ｒSlightly

rough： m Smooth:s

Stepped：s

Wavy：w

Planar：p

Small scale(10cm)

Large scale(1～2m）

Figure 7. The classes of discontinuity roughness. (after JGS,

2004)

Table 3. Classes of weathering degree of rock mass. (after JGS, 2004)

Term State of weathering or alteration ClassFresh No weathering or alteration is visible on rock material.

Mainly none or slight colour change of discontinuity wallsmay be observed.

w1

Slightlyweathered

Colour change of rock material and/or discontinuity walls isvisible.

w2

Weathered Colour change of rock material and discolouration of rockmass is less than half. The structure or formation of rockmaterial and discontinuity walls is still visible.

w3

Highlyweathered

Colour change of rock material and discolouration of rockmass is more than half. In the fresh parts or color changedparts, the structure or formation of rock material anddiscontinuity walls is still visible.

w4

Fullyweathered

Whole rock material weathered and underwent colour changeor altered. However, the original formation and structure ofrock mass still remain.

w5

Residual soil /alteredmaterial

Rock material is fully weathered or altered. The structure orformation of rock mass is totally destroyed. The dissolvedmaterial greatly changed. The dislocation of weatheredmaterial is not distinct.

w6

the structure and formation of fracture zone are very complex, it is very difficult to classify with current classification procedures. Therefore, the classifications for both hard rock mass and soft rock mass are used for the classification of fracture zone while considering its special characteristics. If such a classification is performed, the letter “f” should be added in front of the symbol for the first-step classification.

4. APPLICATION EXAMPLES OF JGS ROCK MASS CLASSIFICATION SYSTEM

In this section, some examples on the application of this classification system are presented for representative rock masses. Through these examples, one would clearly understand how this classification system can be applied for practical use.

4.1 Hard Massive Rock Mass: HM

Hard massive rock masses are widely distributed in the earth’s crust and therefore many rock types are included in this category. Many large structures such as dams, tunnels and underground caverns have been constructed in/on such rock masses and there is huge accumulation of information and experiences associated with hard massive rocks.

The following is an example of Seto Bridge founded on weathered granite, which is one of the Honshu-Shikoku Strait Crossing Bridges. In this classification system, weathered granite is classified as HM class on the basis of its original strength regardless of the present strength. Weathering effect is well represented by the indices of uniaxial compressive


strength and porosity. Figure 8 shows the relation between porosity, and tensile strength and compressive strength. When the porosity is 3%, the granite is classified as C. But it is designated as D-F when the porosity is 6-40% (JSSFE, 1979).

Photo 1 shows a view of the Seto Bridge, and the anchorage seen in the lower right is a large-scale foundation BB7A. Figure 9 shows a geotechnical cross-section along the longitudinal axis of the bridge along BB7A anchorage (Honshu-Shikoku Bridge Authority, 1993). If this classification method is applied to the foundation surface, the rock mass is classified as HM-DIII~IV/w4n4.

Figure 8. Relation between porosity (n) and uniaxial

compressive and tensile strength of granite. (after JSSFE, 1979)

4.2 Hard Foliated Rock Mass: HF

An example for hard foliated rock mass is the foundation rock of the most famous dam in our field, that is, Malpasset arch dam in France. The failure of this dam in 1959 taught an undesirable but an important lessons to the engineers for how to treat hard foliated and discontinuous nature of rock masses from rock engineering point of view. This event was one of the initiators for the development of rock mechanics to present level.

The dam is founded on gneiss. Photo 2 shows a view of the lowest part of the wedge-like failed portion of foundation rock. The schistosity plane is seen from left towards the person in the center of the picture. Photo 3 shows a detailed view of the rock mass in the failed zone. The ball-point pen seen as white shining object in the left side of the central part of the picture is 14cm long. The schistosity plane dips toward the reader and discontinuous nature of rock mass is well seen. The discontinuity set number of rock mass including schistosity plane is 3 (Bernaix, 1969).

Photo 1. Seto-Ohashi Bridges including Minami-Bisan-Seto Bridge. The lower right anchorage is BB7A, depth of foundation of TP-50m, base of 75×62m, height of 131m. (after Honshu-Shikoku Bridge Authority)

Kojima side Sakaide side

legend

Sand

Silt/Clay

Gravel

grade

grade granite

Kojima side Sakaide side

legend

Sand

Silt/Clay

Gravel

grade

grade granite

Figure 9. Geological cross section at the anchorage BB7A of

Minami-Bisan-Seto Bridge. D, CL, CM: Honshu- Shikoku Bridge Authority’s rock mass classification. (after Honshu-Shikoku Bridge Authority, 1993)

Photo 2. Lowest part of wedge-shape collapsed rock mass at

left bank of Malpasset dam.


Photo 3. Gneiss at bottom end of left bank. Rock classification for this outcrop is HF-CIV/w2n2a3rpm.

The uniaxial strength of intact rock samples having 70mm length and 35mm diameter ranged between 25 and 68 MPa for left bank and 85-97 MPa for right bank. All tests were carried out perpendicular to the foliation. If this classification method is applied to the foundation rock on the basis of uniaxial compressive strength, the views of rock mass in Photos 2 and 3 and reports on discontinuities(Bernaix, 1969), the rock masses are classified as HF-CIII~CIV/w2n3~w3n3 for left bank and HF-BIII~BIV/w2n3 for right bank.

4.3 Soft Massive Rock Mass: SM

The representative feature of soft massive rock mass is to have both characteristics of soil-like ground (continuous) and hard rock mass (discontinuous). The dominance of one of these characteristics depends upon its physical environment.

The Akashi Strait bridge with a total length of 3911 m is the longest span suspension bridge in the world, and it has four foundations. The anchorage 1A and pier 3P on Awaji Island side are founded on the Tertiary Kobe formation. Kobe formation consists of sandstone and mudstone layers, whose thickness varies from location to location. Photo 4 shows a view of anchorage 1A at the final stage of excavation. The excavation level is at T.P.-61m. Figure 10 shows the geological cross-section of anchorage 1A perpendicular to the axis of the bridge (Honshu-Shikoku Bridge Authority, 1998). Each layer gently dips towards NW. The foundation of anchorage 1A is situated on fine sandstone layer at T.P.-61m.

Photo 4. Anchorage 1A of Akashi Strait Bridge at last stage of

excavation. Inside diameter is 81m.

Figure 10. Geological cross section of anchorage 1A of

Akashi Strait Bridge. (normal to bridge axis) Diameter of foundation is 85m. (after Honshu-Shikoku Bridge Authority, 1998)

up lane down laneup lane down lane

Figure 11. Geological cross section of Oranda-Zaka Tunnel.

(after Kawachino et al., 2004)

Photo 5. Boulders in cutting face of tunnel (after Nagasaki

Prefecture)

The foundation rock of anchorage 1A and pier 3P has a uniaxial compressive strength of 1-3 MPa when its water content is about 10-15%. If this classification method is applied to the foundation rock, it is classified as SM-FII~FIV/w1s2~s3.


Table 4. Report of rock classification results. (after Nagasaki Prefecture)

Name : Rudaceous soft rockSymbol : SR-GⅡ/w4p2b2g3h3

① Matrix strength : equal to orless than 1 MPa. Judged byblows of geological hammer.Easily penetrated by a blow.② Fragment content : 20-30%.Judged by visual observation andphoto of outcrop.③Weathering degree : semi-consolidated residual soil.Judged by visual observation.④ Dominant grain size of matrix: Rank as sand or silt. Judged byvisual observation.

⑥ Dominant grain size : 200-300mm. Judged by visualobservation and photo of outcrop.

⑦ Fragment strength : 50MPa.Judged by blows of geologicalhammer. High-metallic sound.Some uniaxial compressiontests.

⑧ Large Fragment contents(sub-classification) : half ofcontent of fragment.LargeFragment content is defined ascontent of fragment thatdiameter is more than 200mm.

① Geology : Pleistocene, volcanicconglomerate, debris flowsediment, gravelly sand, non- tosemi-consolidated matrix.

② Outcrop : tunnel face.Excavation by machinery.③ Attached material : Photo anddrawings of tunnel face.

(3) Contents and methods incase of employing methodspartly different from thisstandard

(4) Other special notes

(1) Name and symbol ofClassification

(2) Methodand results ofrockclassification

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4.4 Soft Rudaceous Rock Mass: SR

The engineering properties of soft rudaceous rock mass are largely governed by the strength of matrix and characteristics of fragments such as fragment strength, diameter, and volumetric content. Oranda-Zaka Tunnel is a 2960 m long by-path tunnel constructed in Nagasaki Prefecture. The geology consists of Quaternary volcano-clastic rocks. The overburden thickness is about 130 m. However it becomes very shallow at several locations as seen in Figure 11 (Kawachino et al., 2004). Photo 5 shows a view of volcano-clastic rock at tunnel face, in which large boulders with a size exceeding 1m are observed. The results of the application of this classification method to the rock mass of this tunnel are given in Table 4. The classification symbol is SR-GII/w4p2b2g3h3.

4.5 Soft Interbedded Rock Mass: SB

Typical rock mass belonging to SB rock mass is the alternating beds of mudstone and sandstone. When investigation, design, and construction of rock structures involves SB rock mass, it is quite important to evaluate the mechanical properties of each individual rock layer as well as

Photo 6. Geological structure of interbedded rock mass of Neogene Aira-Group.

Table 5. Physical properties of mudstone obtained from rock material tests. (after Chubu Electric Power Co. Inc., 1997)

Physical property Average Standarddeviation

Wet density (g/cm3) 1.98 0.02Water content (%) 25.1 2.2

Density of soil particle (g/cm3) 2.66 0.02Effective porosity (%) 40 1.6P wave velocity (km/s) 2.09 0.05S wave velocity (km/s) 0.93 0.04

Dynamic Young’s modulus (×103MPa) 4.74 0.4Dynamic poisson’s ratio 0.38 0.01

Table 6. Physical properties of sandstone obtained from rock material tests. (after Chubu Electric Power Co. Inc., 1997)

Physical property Average Standarddeviation

Wet density (g/cm3) 2.12 0.04Water content (%) 16.9 2.1

Density of soil particle (g/cm3) 2.67 0.01Effective porosity (%) 32.2 1.3P wave velocity (km/s) 1.91 0.1S wave velocity (km/s) 0.89 0.05

Dynamic Young’s modulus (×103MPa) 4.57 0.48Dynamic poisson’s ratio 0.36 0.02

of interbedded rock layers with the consideration of anisotropy and scale effect from tests and explorations. Furthermore, it may also be necessary to determine the properties of interfaces between layers.

Photo 6 depicts the alternating beds of sandstone and mudstone. The discontinuities in each layer of mudstone and sandstone are very few and the whole properties are homogenous. The thicknesses of mudstone and sandstone layers at horizontal adits ranges between 8-30cm and 2-20cm, respectively and the ratio of layer thickness is 4:1. The uniaxial compressive strengths of mudstone and sandstone are 9.6-10.6 MPa and 2.7-3.5 MPa, respectively. The whitish


colored layer is mudstone layer whereas the dark grayish colored layer is sandstone layer. But the weathering effect was observed neither in mudstone nor sandstone layers. The average properties of mudstone and sandstone as individual rock unit are summarized in Tables 5 and 6 (Chubu Electric Power Co., Inc., 1997). The rock mass shown in this photograph is designated as SB-EFII/w1t3.

4.6 Fracture Zones

Fracture zones are mostly formed as a result of shearing of original rock mass. Consequently, their properties are much lower than the original rock mass. The mechanical actions, which caused the fracture zones, have different sizes and forms. As a result, the size and form of fracture zone differ from each other. Therefore, it is necessary to utilize rock mass classification representing the characteristics of fracture zones. The present rock mass classification selects the most appropriate rock class to represent fracture zones and differentiates with a letter ‘f’ in designation. When the rock classification is applied, it is important to carry out geological investigation on the structure of sheared zone beforehand.

Okukawanami fault is running in the NNE mountainous region of Lake Biwa. Photo 7 shows an outcrop of the fracture zone associated with this fault. Figure 12 shows the structure of fracture zone observed in this outcrop, in which the fracture zone of 2m thick is crossing fine sandstone. The fracture zone consists of fault gouge and brecciated zone, and several layers with various particle sizes are observed in the form of bands.

Photo 7. Outcrop of fracture zone of Okukawanami fault.

(1.0m by 2.5m) (after Japan Water Agency)

Figure 12. Drawing of fracture zone of Photograph 7. (after

Japan Water Agency)

It is the most appropriate that the fracture zones is classified as soft rudaceous rock mass SR. The classification results are fSR-GIV/p4g4 for clay, fSR-GIV/p3g4 for clayey sand, fSR-GIII/p2g4 for sand with fragment, and fSR-GI for fragments on the basis of naked-eye observation and hammer rebound number.

5. UTILIZATION OF JGS CLASSIFICATION

The aim of JGS classification is to assign the class of rock masses on the basis of its fundamental characteristics which specifically determine the essential behavior of rock masses. Thus, the application results of JGS classification can be used for (1) understanding the actual state of rock mass, (2)sharing the information about rock mass among the various fields of engineers, (3)estimating the geotechnical properties in preliminary investigation stage, as well as for (4)planning the methods of investigation / testing to determine the design parameters, (5)making the analysis model, and for (6)basic information to assess the appropriateness of them, in advanced stages of project; the design, construction and monitoring.

In order to make sure the utilization of the classification mentioned above, it is very important, based on the classification system of the common standard, systematically to collect and analysis the case history of rock engineering projects and data of in-situ tests such as rock shear test, rock deformability test, etc., that have been accumulated so far. The Japanese Geotechnical Society has already begun this research project.

ACKNOWLEDGEMENTS

This paper is written based on the research outcome from a series of study on “Identification and Classification of Rock Mass” over several years. Authors wish to express their sincere gratitude to the following member who contributed to the research；M.Aoki & H.Namiki of Japan Water Agency, T.Ikuma, S.Endo & Dr. T.Nagai of Dia Consultants Co.,Ltd., K.Sasaki of OYO Corporation, H.Matsuyama of Japan Highway Public Corporation, and Dr. Y.Wakizaka & Y.Sasaki of Public Works Research Institute. Authors are also indebted to M.Eng. H.Hinata, a secretariat of The Japanese Geotechnical Society, for helpful assistance.

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Bernaix, J., 1969. New laboratory methods of studying the mechanical properties of rocks, Int. J. Rock Mech. Min. Sci., 16, pp.43-90.

Bieniawski, Z. T., 1976. Rock mass classification in rock engineering, Proc. of the Symposium on Exploration for Rock Engineering, 1, pp.97-106, Balkema.

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Okawa, S., Ohoka, M. & Funato, A., 1999. Application of hardness tester to rock specimen, Proc. of the 29th Symposium of Rock Mechanics, JSCE, pp.256-260. (in Japanese).

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characteristics of Kobe Formation in Akashi-Strata (No.1), Proc. of 10th Japan National Conference on Geotechnical Engineering, The Japanese Geotechnical Society, pp.1231-1232. (in Japanese).

Terzaghi, K., 1946. Rock defects and loads on tunnel support, In Rock Tunneling with Steel Supports, Section 1, eds. Proctor, R. V. and White, T. L., Commercial Shearing and Stamping Co., Youngstown, OH, pp.15-99.

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