D07

On the Pre-failure Shear Behavior of Clayey Soil:
Shear Banding and Corresponding Changeable Physical Precursors〇Chengrui CHANG, Gonghui Wang

1. Introduction

2. Methods

The prediction of landslides is of great social andWe conducted a series of ring shear tests to mimic

economic significance and the related problems are

the creep behavior of slopes due to the increasing

widely encountered in recent decades. The preceding

ground water table during rainfall with different

accelerating behavior is widely noticed and perceived

intensities. The clayey soil is taken from a natural

as a precursor for the prediction of time to failure

landslide site. We employed two connected ring-shear[Voight., 1988, 1989]. Since Saito and Uezawa [1966]

apparatuses as one being a pore-pressure controlling

pioneered an empirical attempt to predict the failure

system to induce failure in another being undertime of a landslide successfully, a phenomenologicalconstant

approach has been established gradually. Using slope

applied/monitored pore pressures were measured

models, Fukuzono [1985] found that the surface

separately and simultaneously. All the samples werevelocity is proportional to the acceleration in

pre-consolidated at constant normal stress (σ) of 200

logarithmic coordinates in the terminal stage of slopekPa, and we varied the shear stresses (τ) to simulate twofailure. Further, Voight [1988, 1989] generalized

different slope angles of 20°and 25°.

stressconditions

(Fig.

1).

The

Fukuzono’s finding as a fundamental law that governs
diverse forms of material failure:𝛺̈ = 𝐴𝛺̇𝛼(1)where A and α are empirical constants and Ω denotes ameasurablequantity.

Thedotreferstothedifferentiation of the quantity to time. This approach
had been widely applied for the retrospective analysis
of volcano eruptions, landslides, and laboratory data.

Fig. 1 Sketch of the testing system

Nevertheless, since the approach did not consider theAdditionally, the ring-shear apparatus enables us toeffect of physical properties of the target materials, it

carry out such tests repeatedly to investigate the shear

might introduce uncertainty even great error into

behavior of pre-existing shear zones. Then thepredictions. Besides, the validation of the approach isparameters (A and α) of the equation (Eq. 1) arequestionable without taking adequate account ofdetermined by a retrospective analysis of the kinematicmechanical behavior. Thus, we are devoted tofeature in the pre-failure stage with linear regression.

examining the possible factors on this empiricalapproach, which attempts to deliver us a better
understanding of the underlying physical process andbackground.

3. Results
As a selected result shows in Fig. 2, the pore pressure
in soil mass increases with the monotonic increase ofthe applied pore pressure, which reduces the effective

positive-rate-dependence.

normal stress and leads to failure eventually. The

In closing, our observation highlights a hitherto

increase of the shear displacement ahead of the failure

little-studied aspect of an empirical approach that is

illustrates an acceleration process. We find that: (1)competent in the retrospective analysis of theunlike pore pressure increases before the first failure, it

accelerating behavior preceding material failure. We

doesn’t increase prior to the second failure (Fig. 2b anddetected the variability of key parameters in thisFig. 2f); (2) an outstanding dilation appears in theapproach, which enables us to look further into thevicinity of the second failure (Fig. 2g).

possible physical factors on such kinematic precursors
and elevate the precision of prediction.

Fig. 3 Values of α in different failure counts
Acknowledgments The first author would like toexpress his special thanks of gratitude to Dr. Issei DOI
for his insightful discussions and suggestions.
References
Saito, M., Uezawa, H. 1966 Forecasting the time ofFig.2 Testing results in two counts of failure
Fig. 3 shows that α (slope of the regression line)
decreases with failure count(i.e., pre-sheareddisplacement).
4. Discussion and summary
The value of α determines the kinematic precursors(velocity, acceleration) of an impending landslide andfailure time. The reduction of it with longer pre-sheared
displacement indicates the failure is prone to be lesscatastrophic. We infer that a pre-existing shear bandexerts a fundamental control on such behavior as well
the post-failure mobility. Wang et al. [2010] suggest
catastrophic failure may not occur in some reactivatedlandslides with the shear resistance of soil exhibiting

occurrence of a slope failure. Journal of Japan
Landslide Society, 4(4): 1-8
Fukuzono, T., 1985. A method to predict the time ofslope failure caused by rainfall using the inverse
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Voight, B., 1988. A method for prediction of volcanic
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Voight, B., 1989. A relation to describe rate-dependent
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Wang, G., et al., 2010. Shear-rate-dependent strength
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