an international conference in commemoration of the 10th anniversary of the chi-chi earthquake...

An International Conference in Commemoration of the 10th Anniversary of the Chi-Chi Earthquake

Cheng-Jie Liao, Der-Her Lee, Jian-Hong Wu, Chia-Ze, Lai

Department of Civil Engineering, National Cheng Kung University, Taiwan

Investigating the Shear Mechanisms of Tsaoling Landslide in Chi-Chi Earthquake Using Dynamic Ring-Shear Test

1112/04/18


2

CONTENTS

INTRODUCTION TEST METHOD

Ring-shear test apparatus Specimen selection and making Test conditions and procedures

TEST RESULTS Shear behaviours and failure criteria Effects of shear surface condition Effects of seismic inertia force

SUMMARY AND CONCLUSIONS

112/04/18


3

Tsaoling landslide is the largest slope failure triggered by the Chi-Chi earthquake. The collapsing volume of the landslide reached 125 million m3, the maximum thickness of the sliding rock mass is 180 m and the run-out distance exceeded 3km.

INTRODUCTION(1/3)

112/04/18


INTRODUCTION(2/3)

4

Fig.1 and 2 show that the Tsaoling landslide is a dip-slope with a sliding surface whose inclination angle is similar to the local rock mass. The sliding surface passed through the Cholan Formation rocks generated in the Pliocene and Pleistocene epoch, and entered the Chinshui Shale of Pliocene epoch rock at the lower part. Fig.1 Tsaoling landslide on the basic image created

from aerial photographs

Fig.2 Geological cross section of the Tsaoling landslide

112/04/18


INTRODUCTION(3/3)

In order to understand the shear mechanics of the sliding surface, we first use the static ring-shear tests to obtain static failure criterion of the rocks at the residual state.

Besides, dynamic ring-shear tests were preceded to specimens with various degrees of shear damage as different sliding conditions probably happened during the Chi-Chi earthquake.

In addition, we investigated the accumulated shear displacement to define the impact of Chi-Chi earthquake on the landslide and considered the existence of the seismic inertia forces in the aspect of tests loading.

5112/04/18

Axis

Normal stress

Sliding Surface

Shear direction


TEST METHOD

The ring-shear test can proceed the shear tests in a large shear displacements and has been widely used to obtain the residual shear strength of clay and sand.

Therefore, we developed a ring-shear test apparatus for rocks to deal with the shear mechanics of the massive rock avalanche with long run-out distance such as Tsaoling landslide.

6

Experimental reproduction of sliding surface liquefaction by means of ring shear device (after Sassa et al., 2004)

112/04/18


Ring-shear test apparatus

The ring-shear test apparatus is divided into several parts as shown in Fig.3, including (1) Control system, (2) Hydraulic loading equipment, (3) Ring-shear box, and (4) Rotating motor.

The higher stiffness of the new ring-shear test apparatus than the conventional one for soils is the main difference.

The new apparatus utilizes dual-axial actuator together with the control system to provide feedback in controlling axial load and torque increment, and proceeds dynamic ring-shear test to involve earthquake effect.

7

Control unit for rotating

motor

Multi-directional

control unit

PC for monitorin

g

Dual-directional Actuator

Ring-shear box

Rotating

motor

Load Frame

112/04/18


Specimen selection and making

According to the geological profile of Tsaoling landslide, the sliding surface formed due to the impact of Chi-Chi earthquake passed through Cholan Formation.

Cholan Formation is mainly formed by greyish silty sandstone interbedded by grey shale or argillaceous sandstone, which often turns to yellow or brown after weathering.

Cholan sandstone from the broadest area where the sliding surface passed through is used as preliminary test specimens.

8Fig.2 Geological cross section of the Tsaoling landslide112/04/18


Physical and mechanical properties of Cholan sandstone

The void ratio of Cholan sandstone is 0.21, and the saturation is 62% after being immersed in water for 2 days, which shows that Cholan sandstone absorbs water easily.

The slaking index Id2=93.9% indicates that the sandstone is a high durability material.

The uniaxial compressive strength of the air-dried Cholan sandstone is 40MPa, but for specimen which is immersed in water for 2 days, its uniaxial compressive strength is halved to 20MPa, which shows the weakening effect since the bonds has been damaged by water.

Properties ValueAir dried unit weight, t (kN/m3) 21.58

Specific gravity, Gs 2.61

Void ratio, e 0.21

Particle analysis

Sand (%) 63.7

Silt (%) 32.4

Clay (%) 3.9

Slake durability index, Id2(%) 93.9

Uniaxial compressive strength on air dried specimen, qu-dired(MPa) 39.5-41.5

Uniaxial compressive strength on saturated specimen, qu-saturated(MPa) 19.4-22.2

Young’s modulus on air dried specimen, E(MPa) 5970-6341

P-wave velocity on air dried specimen, vp(m/sec) 1900-2000

9112/04/18


Specimen making

To precede the ring-shear test, the Cholan sandstone must be shaped to a cannular sample.

The boring sequence of the specimen was inside-out.

The specimen was cut into suitable height, and the cutting surface was then grinded to complete a specimen as shown in Fig. 4.

In addition, a 2mm-deep cutting was made along the middle part of specimen to produce a weak plane as the actual sliding surface, as shown in Fig.4(b), the specimen will be stretched to produce tensile joint in the initial state.

10

Ri=11.0cm

Cutting

Ro=14.9cm

H=7.7cm

Fig.4 Cannular-shaped specimen

(a)

(b)

112/04/18


Test conditions and procedures

A series of ring-shear test was preceded in this research to understand the shear mechanics of the sliding surface of Tsaoling landslide under the impact of Chi-Chi earthquake. The tests are mainly divided to static and dynamic ring-shear tests

1. Static ring-shear test The main purpose of static ring-shear test is to obtain the residual shear

strength of Cholan sandstone under large shear displacement, and then to obtain the failure criterion in peak and residual states.

2. Dynamic ring-shear test Dynamic ring-shear test is aimed at probing the sliding mechanism of

Tsaoling landslide caused by the Chi-Chi earthquake.a. To check the existence of the sliding surface at the Tsaoling landslide before Chi-

Chi earthquake and whether the sliding surface had reached its residual state.

b. To check the existence of the seismic inertia force to the sliding blocks when the earthquake occured.

11112/04/18


Static ring-shear test(1/2)

In this test, we mainly work with cannular specimen with tensile joints.1. The cannular specimen was fixed to the ring-

shear box, and then applied the normal force of 0.5kN for 1day.

2. In the next day, the specimen was pulled with 15 kN/min rate until it broke off to have a specimen with tensile joints (Fig. 5(b)).

In static ring-shear test, lower part of cannular specimen revolved around the rotating shaft, and the shear resistance of the shear surface was checked.

12

sandstone sample

tensile joint

upper part of ring-shear box connect with actuator

O-ringgap

connect with servo motorlower part of ring-shear box

(a) Ring-shear box structure and the specimen installation

(b) A specimen with tensile joints

Fig.5 Cannular specimen and its installation processFig.6 Shear condition of the cannular

specimen and its stress distribution

112/04/18


Static ring-shear test(2/2)

The cannular specimen associated with tensile joints was loaded again by normal stress with rate of 0.2MPa/min until the normal stress reached the designed value.

The loaded normal stress was between 1.0-4.0MPa and was determined according to the maximum depth of the sliding surface of Tsaoling landslide.

After the vertical displacement was stable, the rotation was one-directional with a constant rate of 1.5mm/min (1.3degree/min).

Normal and shear stresses can be obtained through equations 1 and 2 by the axial load and torque measured during the test

13

2 2 (Eq.1)

( )no i

W

r r

3 3

3M (Eq.2)

2 o ir r

W:Normal force loaded on the top of the specimen

M:Resistance torque on the shear surface of the specimen

ro :Outer radius of the specimen

ri :Inner radius of the specimen

112/04/18


Dynamic ring-shear test(1/3)

14(3)The specimen which has reached residual state

Dynamic ring-shear test is aimed at probing the sliding mechanism of Tsaoling landslide caused by the Chi-Chi earthquake.

Specimens in this research were divided to (1) Intact rock specimen, (2) Specimen with tensile joint, and (3) Specimen with joint under residual state to check the existence of the sliding surface at the Tsaoling landslide before Chi-Chi earthquake and whether the sliding surface had reached its residual state.

In the residual state, the specimen was sheared 200mm in one direction, and fines were produced by shearing in the joints.

(2)Specimen with tensile joint

(1)Intact rock specimen

112/04/18



A computer controlled system was used in dynamic ring-shear test to control the dual-directional actuator, while the loadings of the normal and shear stresses were controlled by stress-control method.

Stresses in the test were determined according to the gravity and earthquake forces calculated by the unit weight of sliding blocks and its depth.

Earthquake force is referred from three-directional earthquake accelerations measured at CHY080, which is the nearest accelerograph station at Tsaoling landslide area.

15112/04/18Three-directional earthquake accelerations(CHY080)



16

(a)Without seismic inertia force

(b) With seismic inertia force

Fig.10 Historic chart of dynamic load under different terms

The dynamic stresses on sliding surface were obtained through calculations after baseline correction and vector transformation.

After the acceleration value is corrected, we can obtain the accelerations of the directions parallel and perpendicular to the sliding surface through vector transformation.

Considering whether or not the seismic inertia force exists to the sliding blocks, we calculated the changes of normal and shear stress in dynamic ring-shear test as shown in Fig.10.

112/04/18


TEST RESULTS

Shear behaviours and failure criteria

From static ring-shear test we obtained the complete curve of shear stress-shear displacement of Cholan sandstone with tensile joints.

1.The peak shear stress appears when the displacement was 2-3mm.

2.Specimen height increased when the shear stress approached its peak value, and now shear contraction turns to shear dilatation.

17

n=1.0MPa3. In this Figure, it is obvious that

when the normal stress (n=1.0MPa) was small, the specimen height kept increasing along with the increment of shear displacement before being static and after the shear displacement reached 80mm.

n=1.0MPa

112/04/18


18

(b) n=3.51MPa

4. On the other hand, when the normal stress (n=3.51MPa) was large, the specimen reached its maximum dilatation amount promptly after the shear displacement reached 10mm, then the sample height started to decrease until the shear displacement past 130mm.

5. Therefore, when the normal stress is small, although ripples in joints surface will be sheared or shattered, the main source of shear strength comes from the friction within the climbing process of joints.

6. Oppositely, when the normal stress is large, the main shear strength is provided by the sheared or shattered ripples in joints surface until the shear surface is full of fines, which is the real residual state.

112/04/18


As shown in Fig. 12, the Mohr-Coulomb failure criterion of Cholan sandstone with tensile joints was obtained by proceeding linear regression on the results of static ring-shear tests.

The peak shear strength parameters are cp=0.512MPa and p=40.7∘; while, the residual shear strength parameters are cr=0.024MPa and r=33.3∘.

Cohesion under peak state should be provided by shattered ripples on joint surface.

In residual state, fines covered the shear surface and decreased the cohesion to 0.024MPa.

19

Fig.12 Failure envelopes of the Cholan Formation sandstone on static ring-shear test

112/04/18


Effects of shear surface condition(1/2)

In order to probe the existence of the sliding surface of Tsaoling landslide before the Chi-Chi earthquake, possible conditions of sliding surface were represented by different shear surfaces in dynamic ring-shear tests.

Since the stress path of specimen with tensile joints under earthquake impact didn't touch the peak failure envelope, insignificant shear displacement was obtained.

For the specimen under residual state, obvious shear displacement was measured after the stress path touched and moved along the residual failure envelope.

20

(a)Stress path of tensile joint (b) Stress path of residual condition tensile joint

(c)Variation of shear displacement

Fig13. Results of dynamic ring-shear tests on the different shear surface condition112/04/18


Effects of shear surface condition(2/2)

It indicated that failure criterion of static ring-shear test is suitable for dynamic ring-shear test. In another words, the failure criterion for static and dynamic state are similar to each other.

Finally, according to the shear displacements of shear surfaces under different conditions, the sliding surface should existed in the Cholan sandstone before the Chi-Chi earthquake and also reached residual state, so that, the slope slid during the earthquake.

21112/04/18


Effects of seismic inertia force(1/2)

To probe the effects of earthquake impact on sliding surface, dynamic shear tests were conducted on the specimen under residual state with dynamic loading with/without seismic inertia force.

In Fig.14(a), the stress path touched lower failure envelope which represents negative shear stress and shearing direction without considering seismic inertia force.

Oppositely, the stress path touched only the upper failure envelope (Fig.14(b)) with seismic inertia force.

22

(b)with seismic inertia force

(a)without seismic inertia force

Fig14. Results of dynamic ring-shear tests with/without considering the seismic inertia forces112/04/18


Effects of seismic inertia force(2/2)

According to shear displacement variation graph shown in Fig.14(c), when the seismic inertia forces of the sliding blocks were not exist, displacement underwent to the opposite direction with the actual landslide although there is irrational changes in shear displacements.

When the seismic inertia force exists to the sliding blocks, shear displacement accumulated up to 40mm because the stress path only collided with upper failure envelope.

The displacement direction agrees well with the actual landslide, so it is clear that the seismic inertia force exists to the sliding blocks must be concerned in proceeding dynamic analysis to acquire an accurate analysis results.

23

(c)Variation of shear displacement

Fig14. Results of dynamic ring-shear tests with/without considering the seismic inertia forces

112/04/18


SUMMARY AND CONCLUSIONS(1/2)

1. The residual shear strength of Cholan sandstone with tensile joints under large shear displacement and its static failure criterion can be obtained by static ring-shear test. From the stress paths in dynamic ring-shear test, static failure criterion is also suitable for the one of dynamic ring-shear test, which represents the failure criterion for static and dynamic state as similar to each other.

2. From dynamic ring-shear test results for shear surfaces with different initial conditions, only specimens under residual state have an obvious changes in shear displacements, thus it can be speculated that the sliding surface within Cholan sandstone should reached residual state before the occurrence of the Chi-Chi earthquake.

24112/04/18


25

SUMMARY AND CONCLUSIONS(2/2)

3. Shear displacement obviously accumulated only when the seismic inertia force exists. At this moment, the displacement direction also agrees well with the actual landslide and reveals that blocks above the sliding surface together with slope discontinuities should be considered after the specimen reaches residual state. Hence, the seismic inertia force existed to the sliding blocks when the earthquake occurred.

4. In this research, only air-dried Cholan sandstone is used to precede shear tests, which are not enough to probe the true shear mechanics of Tsaoling landslide. Hence, in the future, another type of rocks, such as Cholan shale and Chinshui shale, from the landslide area will be considered to be used as test specimen to conclude more ideas about the failure of the Tsaoling landslide.

112/04/18


26

Thank you for Your Attention

112/04/18

an international conference in commemoration of the 10th anniversary of the chi-chi earthquake...

Documents

ring shear test apparatus

shear mechanics

ring shear device

static ringshear tests

dynamic ringshear tests

earthquake test method

impact of chichi earthquake

accumulated shear displacement