Can tilt tests provide correct insight regarding frictional behavior of sandstone under seismic excitation? Can tilt tests provide correct insight regarding frictional behavior of sandstone under seismic excitation? Y.M. Hsieh, K. C. Lee*, F.S. Jeng and T.H. Huang National Taiwan University National Taiwan University of Science and Technology September 22, 2009 An International Conference in Commemoration of the 10th Anniversary of the Chi-Chi Earthquake

Outlines  Introduction  Purposes  Methodology  Differences on μ t and μ c  Factors affecting μ c  Observed μ i (t) after initiation of sliding  Effect of excitation frequency on μ i (t)  Discussion  Conclusion 2

3 Introduction (1/2) (Chen, 1999 ) Wedge 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 Introduction (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) Tilt test  μ t (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) Instantaneous Friction Coefficient μ i (t) 7 Note: when t=t i μ c = μ i (t i ) (instantaneous critical friction coefficient)

Methodology (3/3) 8 Laboratory test program 1 2 3

1. Differences on μ t and μ c % 50.25% 41.91%

2. Factors affecting μ c (1/2) 10 The measured were affected by excitation frequency of 4Hz, 6Hz, and 8Hz. (Synthetic sandstone) Frequency of excitation 0o0o 5o5o 10 o

2. Factors affecting μ c (2/2) 11 Affects of normal stresses 4Hz6Hz8Hz

3. Observed μ i (t) after initiation of sliding (1/2) 12 (Olsson et al, 1998) Point A μ c =0.25

3. Observed μ i (t) after initiation of sliding (2/2) 13 (Olsson et al, 1998)

3. Effect of excitation frequency on μ i (t) 14

Discussion (1/2) 15 under static condition under seismic excitation condition

Discussion (2/2) 16

Conclusion 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 attentions K.C. Lee