ANALYSIS ON SLIDING MECHANISM OF TSAOLING ROCKSLIDE IN CHI-CHI EARTHQUAKE EVENT Tien Chien Chen National Pingtung University of Science & Technology Meei Ling Lin National Taiwan University Wang, Tau-Teh Wang, Tau-Teh National Pingtung University of Science & Technology

2 Contents  Introduction  Earthquake G-M Records  Ground Surface Profile& Material Properties  Newmark's Method of Sliding Block  Results and Discussions  Conclusions

3 Introduction  During the 1999 Chi-Chi earthquake, extensive slope failures were triggered by the earthquake in central Taiwan.  A large-scale dip-slope slide in Tsao-Ling occurred, which involved mass movement of 120 million cubic meters.  only 25-million cubic meters (20 %) of the sliding mass dropped into the valley of the Ching-Shui River at the toe of the slope. The remaining sliding mass of about 100-million cubic meters slid pass the Ching-Shui River, and landed on the remains of old landslide dam. Photo by Lien,2000

4  Residents who lived near the crest of the slope, flied with their house on the sliding rock block and landed on top of the landslide dam through a distance of 3100 m away. 32 people were killed and 7 survived after the “sliding-landing” process. Lansilde dam Landslide lake 3.5 km 4.5 km Bird’s eye view of landslide lake, Tsao-Ling village, and the landslide area,.

5 View of landslide dam, 2002

6  Seismic records of Taso-Ling area Ground acceleration records of strong motion station CHY 080, CWB Earthquake G-M Records

7  The strong ground motion records of station CHY080 of the Central Weather Bureau located just north to the crest of Tsaoling,  In order to consider properly the effects of ground motion, the short-time Fourier transform of seismic record was conducted to illustrate the landslide initiation time. Figure 2 shows the seismic wave arrived at 29 sec., the main shock was at 32 sec. Fig. 2 The short-time Fourier transform of N-S ground motion

8 Ground Surface Profile & Material Properties Average slope angle is 12 o Before landslide After landslide Distance (m) Elevation(m) Ground surface profile for seismic analysis  A large dip slope  The dip angles of slope ranged from 12 to 14 degree, and the main sliding occurred in the shale formation

9  Material properties The photograph was taken in 17 th November 2002, it can be seen in the front of photo that water seeping down slope along the interface of the two formations.

10  Material parameters 

11 Newmark's Method of Sliding Block a V D a cp Elapsed time a cr asas

12 Results and Discussions  a s vs. a cp slidingstatic  Depending on the acceleration a N of earthquake the critical acceleration a cp is not a constant.  The down-slope acceleration could be determined from formula: a s +g.sin12o- a cr  The down-slope acceleration a d could be determined from formula: a s +g.sin12o- a cr Down-slope a d = a s +g.sin12 O - a cr

13  Newmark’s acceleration increased as  p increasing （ Case of  p =15 o -30 o ；  r =2.8 o ） Down-slope acceleration vs. displacement

14 The short-time Fourier transform of Z-direction ground motion （ Case of  p =15 o ；  r =2.8 o ）  Result of the short-time Fourier transform of vertical direction (Z) seismic record shows the initiation time of rockslide about at 37 secoend.  The result coincides with the result from Newmark’s method.

15 Sliding Velocity and Distance under various residual friction angle  As a result, the critical acceleration decreased from peak to residual state, the slid rock block gained velocity and displacement.  The sliding velocity of rock mass could be larger than 200 km/hr (56 m/s) at the “ jumping point” as  r was lower than 6 o.

16 Does air cushion exist?  4 air densities (i.e. 1.2, 100, 500, and 1000 kg/m 3 ) were tested to illustrate the effect of “air cushion”.

17 Does air cushion exist?  Results show the rock block will fall into valley until the density of 1000 kg/m 3 is reached. Air, in the circumstance, is acted a incompressible-like medium (such as water), since the rock block was flied over valley In reality.

18 Conclusions  In the result of short-time Fourier transform on ground motion record shows the initiation time of Tsaoling rockslide about at 37 second of record. Which is coincidence with the time calculated by Newmark’s method analysis.  Result shows the shear strength of Tsaoling rockslide plane degraded far from peak strength, the shear strength is close to residual strength and significant to the velocity of sliding block.  Under this circumstance of  r lower than 6 o, the sliding velocity of rock block could be higher than 200 km/hr (56 m/s), which dimmed the occurrence of “sliding-flying-landing” process for the sliding rock block possible with such high velocity.

19  From the comparison in the location of deposition and the result of analysis, air cushion effect could be existed in Tsaoling rockslide case.  Having well representative material parameters and good quality ground motion record, a simple prediction model, e.g.. Newmark’s method, also could achieve a well estimation on rockslide sliding behavior.

20 Thanks for your Attention……