ESTIMATING THE INFLUENCE OF SURFACE CHARACTERISTICS OF ROCK

JOINTS ON SHEAR BEHAVIOR

Graduate school of Science and Technology, Nagasaki University

Y.Tasaku, Y.Jiang, Y.Tanahashi, B.Li

・ The dependency of mechanical behavior on boundary conditions. ・ The relationship between the mechanical behavior and the surface characteristics are evaluated.

The deformation behavior and stability of underground structures depend principally on

the shear strength of discontinuities in rock masses.

Direct shear testMeasurement and evaluation of joint surface

Introduction

The shear strength is generally dominated by surface characteristics of rock joints.

The development of deep underground space has received high attention

Rock slope (non- reinforcement)

Constant normal load condition

Freeσn ＝ Constant

τ

Constant normal load (CNL) condition

Constant normal stiffness (CNS) condition

Joint

Joint

Constraint from rock masses

Deep underground

Change of normal stress

σn≠Constant

τ

Constant normal stiffness (CNS)

condition

　 Digital-controlled shear test apparatus 　

Horizontal Jack

Vertical Jack

Specimen

Structure of surface measurement system

Laser displacement meter

Computer for control and record

X-Y positioning table

・ Mix ratio(weight ratio)

　 plaster ： water ：retardant

　 　 =1 ： 0.2 ： 0.005

・ Unconfined 　compressive strength

　　 σn0=32.0MPa

　　 (middle hard rock)

Specimen

Natural surfaces

Possible to analyze the substantial fractal dimension

・ Dimension (mm)

200×100×100

Test cases

J １ J2

J3

(Large asperity in the center and smooth surface)

(Many small asperities)

(Specimen of previous study with very smooth surface)

Test cases

・ Initial normal stress　　 σn0: 1MPa, 2MPa (50 ～ 100 ｍ ), 5MPa (about 250 ｍ )

Each of J1 and J2 has 9 cases; 18 cases at all

・ Normal stiffness: kn 　

J1 is 1GPa/m and 7GPa/m J2 is 1GPa/m and 3GPa/m,

・ Control condition 　　

　　 Constant normal load (CNL) condition and constant

normal stiffness (CNS) condition

0

0.5

1

1.5

2

0 5 10 15 20Shear displacement(mm)

Nor

mal

dis

plac

emen

t(m

m)

0

1

2

3

4

5

0 5 10 15 20Shear displacement(mm)

She

ar s

tres

s(M

Pa)

Result of shear test (J1,σn0=2MPa)

CNL condition

CNS condition(kn=3GPa/m)

CNS condition(kn=7GPa/m)

Surface of specimen before and after shear (J1,σn0=2MPa)

Before shear

After shear on CNL condition

After shear on CNS condition ( kn=3

GPa/m)

(mm)1 3 . 5

1 4

1 4 . 5

1 5

1 5 . 5

1 6

1 6 . 5

1 7

1 7 . 5

1 8

1 8 . 5

1 9

1 9 . 5

2 0

2 0 . 5

2 1

Evaluation method of surface roughness by projective covering method

Projective covering cell

Fracture surfacea

b c

d

akh

dkh

ckhbkh

δ : length of a mesh

AT(δ): Total areaAT0(δ): Apparent area

δδloglogA

)(logA

T0 T

sD2 (2 ＜ Ds ＜3)

Transition of fractal dimension

CNL

CNS(kn=1GPa/m ）

Before shearing

CNS （ kn=3GPa/m ）

J2

2

1

1

1

21

2 Δ1

1

N

i

ii

x

yy

Nz

Statistical parameter of two dimensions

N: Total number of measuring points along the profiles

　 yi ： ith value of height

⊿x ： Minute distance to direction x

Z2 is the average slope of asperity

Transition of Z2

CNL

CNS(kn=3GPa/m ）

Before shearing

CNS （ kn=7GPa/m ）

J2

Maximum shear stress

τ ： Maximum shear stress

σn0 ： Initial normal stress

JRC ： Joint roughness coefficient

JCS ： Joint wall compressive strength

φb ： Inter friction of joint

)φ)log(JCS/σtan(JRCστ bn0n0 Maximum shear stress: (Barton 、 1977)

Two dimensional indicator

Suggestion equation

))log(JCS/b)-(tan(a00 bnn

σDστ s ‘a’ and ‘b’ are obtained from the relationship between JRC and Ds on experience.

Actual rock joint is three dimensional

Comparison of the Barton’s empirical equation with proposed equation

Barton’s empirical equation σn0(MPa) 1 2 5

J1

J2

J3

σn0(MPa) 1 2 5

J1

J2

J3

Proposed equation

Proposed equation

))log(JCS/2)-(tan(50000 bnn

σDστ s

Base line

Conclusion

Shear behavior of rock joint

Constraint from rock blocks

Surface characteristics of joint

The relation of Initial normal stress

and surface roughness （ about CNL ）

Influence

Confirmation of validity

Proposed equation

It is possible to predict the shear behavior by 　 Ds

END

自然の岩石を供試体として用いる圧裂試験

表面のケースの充実Ds を用いて、せん断特性を正確に予測

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6初期垂直応力σ n0(MPa)

τ(MPa)

ピー

クせ

ん断

応力

Ds とピークせん断応力との関係

))log(JCS/)-(tan(50000 bnn

σDστ s 2

表面2 （ Ds=2.0279)

表面1 （ Ds=2.0235 ）

表面3 （ Ds=2.008 ）

破線：理論値

2Z32.47log32.2JRC

JRC 値は以下の式 (Tse 、 1979 ）

JRC 値と Z2

00.5

11.5

22.5

3

0 10 20(mm)せん断変位

(MPa)

せん

断応

力

00.5

11.5

22.5

3

0 10 20(mm)せん断変位

(MPa)

せん

断応

力

せん断応力の比較 (σno=2MPa)

1 4

1 4 . 5

1 5

1 5 . 5

1 6

1 6 . 5

1 7

1 7 . 5

1 8

1 8 . 5

1 9

1 9 . 5

2 0

2 0 . 5

2 1

2 1 . 5

表面1

表面2

CNL 制御CNS 制御(kn=3GPa/m)

表面1

表面2