Can tilt tests provide correct insight regarding frictional behavior of sandstone under seismic...

Can tilt tests provide correct insight Can tilt tests provide correct insight regarding frictional behavior of sandstone regarding frictional behavior of sandstone

under seismic excitation?under seismic excitation?

Y.M. Hsieh, Y.M. Hsieh, K. C. Lee*K. C. Lee*, F.S. Jeng and T.H. Huang, F.S. Jeng and T.H. HuangNational Taiwan UniversityNational Taiwan University

National Taiwan University of Science and TechnologyNational Taiwan University of Science and Technology

September 22, 2009September 22, 2009An International Conference in Commemoration of the 10th Anniversary

of the Chi-Chi Earthquake

OutlinesOutlines Introduction Purposes Methodology Differences on μt and μc

Factors affecting μc

Observed μi(t) after initiation of sliding

Effect of excitation frequency on μi(t)

Discussion Conclusion

2

3

IntroductionIntroduction (1/2) (1/2)

(Chen, 1999 )

Wedge failureWedge failure

(Lee, 2004 )

• Friction angle or friction coefficient obtained from tilt test is questionable when applied to dynamic problems.

• For slope stability analyses considering only the gravity effect, tilt test is a convenient and reasonable test method for measuring friction coefficient or friction angle.

4

IntroductionIntroduction (2/2) (2/2)

the applicability of tilt test measured friction coefficients on determining the sliding threshold of earthquake-induced block slides is discussed.

The discussion is facilitated by comparing the sliding thresholds obtained from measured using the proposed small-scale

laboratory tests versus calculated using tilt test measured friction

coefficient.

5

Purposes

Methodology (1/3)Methodology (1/3)

• Tilt test μt (Barton and Choubey (1977) )

• Shaking table test

6

•Uniaxial accelerometers at A1, A2, A3 positions•High precision laser displacement sensors at L1 and L2 positions

Methodology (2/3)Methodology (2/3)• Instantaneous Friction Coefficient μi(t)

7

Note: when t=ti

μc= μi(ti)(instantaneous critical friction

coefficient)

Methodology (3/3)Methodology (3/3)

8

• Laboratory test program

11

22

33

1. Differences on 1. Differences on μμtt and and μμcc

9

73.39%

50.25%41.91%

2. Factors affecting 2. Factors affecting μμcc (1/2) (1/2)

10

The measured were affected by excitation frequency of 4Hz, 6Hz, and 8Hz. (Synthetic sandstone)

Frequency of excitationFrequency of excitation

0o 5o 10o

2. Factors affecting 2. Factors affecting μμcc (2/2) (2/2)

11

Affects of normal stressesAffects of normal stresses

4Hz 6Hz 8Hz

3. Observed 3. Observed μμii(t)(t) after initiation of sliding after initiation of sliding (1/2) (1/2)

12(Olsson et al, 1998)

Point Aμc=0.25

3. Observed 3. Observed μμii(t)(t) after initiation of sliding after initiation of sliding (2/2) (2/2)

13(Olsson et al, 1998)

3. Effect of excitation frequency on 3. Effect of excitation frequency on μμii(t)(t)

14

-1.0

-0.5

0.0

0.5

1.0

-20 -10 0 10 20 30 40 50

Vr (mm/sec)

μi(Vr)

4Hz_synthetic sandstone6Hz_synthetic sandstone

-1.0

-0.5

0.0

0.5

1.0

-2 0 2 4 6 8 10

s (mm)

μi(s)

4Hz_synthetic sandstone6Hz_synthetic sandstone

Discussion Discussion (1/2)(1/2)

15

under static conditionunder static condition under seismic excitation conditionunder seismic excitation condition

Discussion Discussion (2/2)(2/2)

16

ConclusionConclusion

• The sliding threshold should be measured using dynamic tests such as shaking table tests used in this study.

• During excitation, the inertial forces of the sliding block cause non-uniformity of normal stresses, and indirectly cause reductions to friction coefficients, thus, larger displacements.

• In order to establish rock block or rock slope sliding models during excitation, it is necessary to use frictional model based measured behaviors on real rock materials or synthetic rock materials.

17

Thanks for your attentionsThanks for your attentions

K.C. LeeK.C. Leef93521113@ntu.edu.twf93521113@ntu.edu.tw