Inverse Analysis for Stiffness Factor of Landslide and its Application toFailure Prediction
Student: Zaidatul Azmi
Advisor: Satoru Ohtsuka
I. Introduction
The increment of groundwater level by rainfall or melted snow can be said one of the factors that generates
landslide. Landslide slides because of water migration by the increment of groundwater level and formed slip
surface in the inner part of landslide. Pore water pressure loading test was conducted as a model of landslide
slope. The specimen used in this test is Fujinomori clay. The material parameters of the specimen are λ=0.113,
κ=0.016 and M=1.41. This paper will discuss the behaviour of landslide and the failure prediction using thestiffness factor.
120

III. Time Dependency Behaviour
Figure 2 shows the result of the pore water pressure loading
test where OCR=1 and the deviator stress varied. In this test,
specimens were loaded every 2kPa of pore water pressureloading from the bottom to the top of the specimens. As
shown in the figure, the measured pore water pressure
formed a convex relationship with time at each stage ofloading. This is because a certain finite time is necessary
for pore water pressure to transmit from the bottom to the
top of the specimens.
IV. Stiffness Factor and Failure Prediction
Stiffness factor is a ratio between load and displacement.
But in this study, stiffness factor is defined as ratio between
the pore water pressure and the axial strain. The stiffness
factor is divided into two that are the tangent modulus andthe secant modulus. From these modulus, landslides failure
prediction is carried out. Fig.3 shows the relationshipbetween tangent modulus and time. As shown in the figure,the stiffness modulus shows a constant behaviour, but after
reaching the yield point, it declined. Near the failure point,

Deviator stress , q (kPa)

8040
Critical s tate line

004080Mean effective stress , p' (kPa)

1200.92
Void ratio , e

Figure 1 shows the stress path (upper figure) and the void
ratio path (lower figure) for the case of over consolidated
ratio OCR = 2 and shear stress q = 80 kPa. This study
defines yield point at the point where e – p’ line deviatesfrom elastic swelling line. Pore water pressure loading testshows that due to pore water pressure loading, thespecimen becomes over consolidated, and dilatancy (plasticdeformation) occurred after reaching the yield stress. In thestress path, the yield point exists on the critical state line
When the pore water pressure is loaded more, the stressdisplacement increased rapidly and the deviator stresscould not be maintained at constant anymore. The failure
point is defined at the point where deviator stress declined.
Deviator stress declines after reaching the failure pointshows a similar phenomenon in drained test of overconsolidated clay where after reaching the peak strength,
shear stress decreases.

Initial point
Yield point
Failure point

0.88
Swelling line

0.8404080120

Fig.1 Stress path and void ratio path

60
Pore water pressure , uu (kPa)

II. Yield Point and Failure Point

q=50kPa

Initial point
Yield point
Failure point40

20
q=80kPa

000.5

51Time , t (× 10 sec)

Fig.2 Measured pore water pressure

1.5

400

Secant modulus , u/ε (kPa/%)

Tangent modulus , du/dε (kPa/%)

the stiffness factor declining to zero and expressing the shear behaviour almost accurately.
The secant modulus is also considered as the stiffness factor (ref. Fig4). This modulus shows almost thesame behaviour with the tangent modulus. The dispersion in the secant modulus is smaller than the tangent
modulus and seems to be more stable. After reaching the yield point, the secant modulus declining but thefailure point not necessarily reach zero. This is the characteristic of the secant modulus.

Yield point
Failure point

2000025Time , t ( ×10 sec)

400
Yield point
Failure point

20004Fig.3 Tangent modules of stiffness factor

Tangent modulus at yield point , du/dε(kPa/%)

Tangent modulus at yield point , du/dε(kPa/%)300
2502001501005002325Time , t ( × 10 sec)

4Fig.4 Secant modulus of stiffness factor

350

104Over consolidated ratio, OCR

Fig.5 Tangent modulus at yield point against OCR

140
120
10080
60
40
20050607080

Deviator stress , q (kPa)

Fig. 6 Tangent modulus at yield point against q

As the value of the over consolidation ratio increases, the materials are stiffer. As shown in Fig.5, the bigger
value of over consolidation ratio properly, the bigger value of the stiffness factor at yield point. By this, it canbe said that the stiffness factor expresses soils stiffness. In Fig.6, the value of stiffness factor at yield point
decreases along with the increment of deviator stress. This means that the larger stress is loaded to a specimen,
soil stress greatly inclined.
V. Conclusion
The pore water pressure loading test was carried out to investigate the application of the stiffness factor
regarding the effective stress. The stiffness factor regarding the effective stress is defined as the ratio between
the strain and the pore water pressure. When the stiffness factor is zero, the specimen is considered failed. Thispredicts the landslides failure.
ReferenceKeiichiro Saito (2002) Experimental examination on landslide time dependency behaviour due to pore waterpressure loading test, Master thesis, Nagaoka University of Technology (in Japanese)