thThe 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

AN INVESTIGATION INTO THE EFFECT OF DRAINAGE ON LARGESLOPE FAILURE DURING EARTHQUAKES(MECHANISM OF A LARGE LANDSLIDE COMPOSED OF MUDSTONE DEBRIS INDUCED BY THE
2004 MID NIIGATA PREFECTURE EARTHQUAKE)
1

Atsushi NAKAMURA
1andFei CAI

2andKeizo UGAI

3

Structural Engineering , FORUM8 Co. Ltd., 2-1-1, Nakameguro GT Tower 15F Kamimeguro, Meguro-ku,
Tokyo, Japan
2Assistant Professor, Dept. of Structural Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan3Professor, Dept. of Structural Engineering, Gunma University, Kiryu, Gunma 376-8515, Japan
Email: atsushi@forum8.co.jp, cai@ce.gunma-u.ac.jp, cai@ugai.gunma-u.ac.jp

ABSTRACT :A lot of large, long-distance landslides occurred due to a big earthquake in Japan, called the 2004 Mid Niigata
Prefecture Earthquake. One of these landslides occurred when a slope composed of mudstone debris failed.
Slopes composed of mudstone debris have been thought to be seriously damaged by rainfall but not so seriously
by earthquakes. Sampling of several intact soil blocks was done at the landslide site and the cyclic tri-axial tests
on the specimens were performed under undrained conditions in a laboratory. These experimental results
suggested that the cause of the failure was possibly a rapid increase of the pore water pressure inside themudstone due to the cyclic shear during the earthquake. That is, this rapid increase of the pore water pressure
made the safety factor of the slope drop considerably below 1.0, which triggered the large, long-distancelandslide.
We assumed the groundwater level at the time of the earthquake from the field investigation and calculated thesafety factor of the large slope failure by limit equilibrium method. The groundwater level was calculated byseepage analysis (finite element method) after the drains were installed and their effect on the stability
evaluated by limit equilibrium method.
KEYWORDS:

Excess pore water pressure, Mid Niigata Prefecture earthquake,
Limit equilibrium method

thThe 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

1. INTRODUCTION
A lot of large, long-distance landslides occurred due to a big earthquake in Japan, called the 2004 Mid Niigata
Prefecture Earthquake. One of these landslides occurred when a slope composed of mudstone debris failed.
Slopes composed of mudstone debris have been thought to be seriously damaged by rainfall but not so seriously
by earthquakes. Sampling of several intact soil blocks was done at the landslide site and the cyclic tri-axial tests
on the specimens were performed under undrained conditions in a laboratory. The size of the landslide was
250m long and 150m wide with a depth of 25m, which traveled a distance of 30-40m downhill due to theearthquake.
These experimental results suggested that the cause of the failure was possibly a rapid increase of the porewater pressure inside the mudstone due to the cyclic shear during the earthquake. That is, this rapid increase of
the pore water pressure made the safety factor of the slope drop considerably below 1.0, which triggered thelarge, long-distance landslide.
The topography before the landslide was assumed to be a model case. The groundwater level at the time of theearthquake was assumed following the field investigation and the safety factor of the large slope failure wascalculated by limit equilibrium method. The groundwater level was calculated by three-dimensional seepage
analysis (finite element method) after the drains were installed and their effect on the stability evaluated bylimit equilibrium method. Regarding the earthquake case, inertia was ignored and the ∆U method was used,which considered only the influence of excess pore water pressure.

2. THE INFLUENCE OF EXCESS PORE WATER PRESSURE
2.1. Undrained cycle triaxial tests on the undisturbed soil samples
In order to investigate the effect of the strong earthquake motion on the landslide, undrained cyclic triaxial tests
have been done on the undisturbed samples. The initial consolidation pressures were 200 and 375kN/m2. As aresult, excess pore water pressure was suddenly generated and is understood to have remained behind in large
amounts in unloading. The residual pore water pressure in the sample after the cyclic triaxiality tests can beseen in Table 2.1. Regardless of how much greater the effective overburden pressure was than this, it can besupposed that about 60% excess pore water pressure was generated.
Table 2.1 The residual pore water pressure of the soils after cyclic loading
22
Initial consolidation pressure(A) 375 kN/m
Initial consolidation pressure(A) 200 kN/m
Principal stress
Residual excess pore Principal stress
Residual excess poredifference
water pressure after
difference
water pressure after
the cyclic loading(B)the cyclic loading(B)
155
109
299
215
140
155
277
166
127
118
254
241
115
151
231
264
B/A(Average)
0.67
B/A(Average)
0.59

2.2. Drainage effect and its relationship with excess pore water pressure
It can be understood from the results of the cyclic triaxiality tests that the big excess pore water pressure occurs
as a result of the repetitive vibrations of the earthquake and it is thought that the shear strength of the soil may
have fallen. This is thought to have contributed to the increased length of the landslide.
Therefore, in this report, the influence of excess pore water pressure (∆U) at the time of the earthquake has
been examined by using the ∆U method. The ∆U method is illustrated in the diagram below. Figure 1(a) showsa slice division piece before the drainage, And Figure 1(b) shows after the drainage.

thThe 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, ChinaΔhσv'

ΔU'

σv''

ΔU''

(a)Before the drainage (b)After the drainage
Figure 1 Effective overburden and excess pore water pressure in cross-sectional view
2.2.1 Effective overburden pressure before the drainage Σσv'
If the effective overburden pressure is σv' and the excess pore water pressure ratio is α, the excess pore waterpressure is ∆U' = α / σv' at the time of the earthquake before drainage. Therefore, effective overburdenpressure Σ σv' before the drainage becomes Eqn.2.1.
Σσv'=σv'‑ΔU'=(1‑α)σv'(2.1)
2.2.2 Effective overburden pressure after the drainage Σσv''
Stress increases (∆v) as water level falls (∆h) as shown in Eqn.2.2.
Δv=Δh(γt‑γ')=Δh・γw(2.2)
The increase in excess pore water pressure as a result of the earthquake can be calculated by Eqn.2.3.
Δu''=α・Δv=α・Δh・γw(2.3)
Therefore, incremental effective overburden pressure becomes Eqn.2.4.
Δv‑Δu''=Δh・γw‑α・Δh・γw = (1‑α)Δh・γw=(1‑α)Δv(2.4)
Thus, effective overburden pressure Σσv'' after the drainage becomes Eqn.2.5.
Σσv'' = Σσv'+(1‑α)Δv = (1‑α)(σv'+Δv)(2.5)
2.2.3 Drainage effect and its relationship with excess pore water pressure
It is found that the substantial effective earth artesian increment which occurs at the time of earthquake, considers theexcessive pore water pressure excluding hydraulic ratio of (1-α)∆v. Therefore, it is possible that the maximum effectof drainage decreases by(1-α)times.
The factor of safety in this area is under 1.0 due to this effect. When water pressure rises, drainage occurs. Therefore,
due to the dispersion effect, αbecomes smaller. However, the interval between drainage works is relatively huge andthe dispersal effect is not taken into consideration.

3. A METHOD OF ANALYSIS
3.1. The estimate of the groundwater level
Groundwater level before and after the drainage was calculated using three-dimensional FEM seepage analysis.
Figure 2 shows the analysis mesh. Figures 3(a) & (b) show the results of the analysis before drainage. Figures
4(a) & (b) show the results after drainage. Section B was taken where it is thought that the landslide was thedeepest. As can be seen, water levels fell by around 3.0m after drainage.

thThe 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

Figure 2 Three-dimensional seepage analysis mesh

Figure 3(a) Seepage analysis result before drainage (3D)

Figure 3(b) Seepage analysis result before drainage (Section B)

thThe 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, ChinaFigure 4(a) Seepage analysis result after drainage (3D)

Figure 4(b) Seepage analysis result after drainage (Section B)
3.2. Slope stability by limit equilibrium method
3.2.1 Soil properties
In section B, the first layer is a cohesive silt layer of colluvial soil, and the second layer is strongly weatheredmudstone. These two layers become moving stratum. The angle of shear resistance for the strongly weatheredmudstone is 39°. The maximum depth of the landslide is about 25m and average depth is thought to be about
20m. Therefore the adhesive strength of the moving stratum was adjusted to 20kN/m2. The wet unit weight ofthe soil was assumed to be 18kN/m3. An analytical model was created by two-dimensional limit equilibrium
method and is shown in Figure-5. The slide surface was made of the two layers.

thThe 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

Figure 5 Landslide slope stability analytical model
3.2.2 Slice method
The safety factor can be calculated by Eqn.3.1

Fs =∑ (N − U ) × tan φ + c × ∑ L∑S(3.1)

Fs: Safety factor
ΣN: Levee normal power (kN/m) applied by the weight of the slice (including buoyancy)
ΣU: Excess pore water pressure (kN/m) in the slice during the earthquake
ΣS: Sliding power (kN/m) applied by the weight of the slice (disregarding buoyancy)
ΣL: The extension of the slide surface of the slice. (m)c: Adhesive strength of the slide surface(kN/m2)
φ:The angle of shear resistance for the slide surface(°)

4. Slope stability analysis result
4.1. Before drainage
The resistance power against earthquake and safety factors that occurs before drainage are shown in Tables 4.1(a) and (b) respectively.
Table 4.1(a) Resistance before drainage
Case
ΣN'ΣU'ｃΣL Στ(kN/m)
36103.9
0.0
5966.942070.8
Normal20408.5
Earthquake 36103.9 21662.3 5966.9∑τ = ∑ (N × tan φ − α × N × tan φ ) + c × ∑ L∑τ = ∑ N '−∑ U ' + c × ∑ L

(4.1)
(4.2)

Table 4.1(b) Safety factor before drainage
Case
ΣτΣS
Fs=Στ/ΣS
42070.8 25892.1
1.62
Normal0.79
Earthquake 20408.5 25892.1

4.2. After drainage
The resistance power against earthquake and safety factors that occurs after drainage are shown in Tables 4.2(a) and (b) respectively.

thThe 14 World Conference on Earthquake Engineering
October 12-17, 2008, Beijing, China

Table 4.2(a) Resistance after drainage
Case
ΣN'ΣU'ｃΣL Στ(kN/m)
40693.6
0.05966.9
46660.5
Normal22244.3
Earthquake 40693.6 24416.2 5966.9
Table 4.2(b) Safety factor after drainage
CaseΣτΣS
Fs=Στ/ΣS
46660.5 25355.5
1.84
Normal0.88
Earthquake 22244.3 25355.5
4.3. The comparison of the safety factor
How the safety factor changed before and after drainage for both normal and earthquake cases is shown in
Table 4.3.
Table 4.3 The improvement rate of the safety factor
Before After
Fs
Case
1.62
1.84 ↑ 1.135(Δ0.22)
Normal0.88 ↑ 1.114(Δ0.09)
Earthquake 0.79
In the normal case, there was an improvement of about 14% (1.135 times) because of drain. In the earthquake
case, there was an improvement of about 11% (1.114 times). That is, it appears that the effect of the drain
diminishes as a result of the generation of excess pore water pressure.
5. CONCLUSION
The safety factor was calculated by the limit equilibrium method, as is presently considered to be the best
design method, whilst considering excess pore water pressure caused by the earthquake. It was found that thesafety factor becomes 0.88 after drainage and it is understood that it is considerably smaller than 1.0. Thus, thesafety factor of the slope was able to be confirmed when assuming that significant excess pore water pressure
was generated by the repetitive motion of the earthquake. It has been found that the effect of drain decreased byabout 3% as a result of the excess pore water pressure caused by the earthquake.
This calculation is a trial calculation using a two-dimensional section (B section) where it is thought that thelandslide side was the deepest. Therefore, it is thought that the safety factor is a little small compared with thethree-dimensional landslide. It is not suggested to evaluate drain works solely from the safety factor in thisreport.
In the future, calculation of a three-dimensional model to analyse slope stability by the limit equilibrium
method considering the excess pore water pressure of the entire landslide will provide more accurate
calculation of the safety factor. The relationship between the effectiveness of the drain and the excess porewater pressure can then be reexamined.
REFERENCES
K. Ugai, A. Wakai. (2007). Mechanism of a large landslide composed of mudstone debris induced by the 2004
mid niigata prefecture earthquake. In Proceedings of the 46nd Annual Conference of Japan Landslide Society,Mie, Japan,H. BOLTON. SEED (1979).Geotechnique.29, No.3,215-263.Consideration the earthquake-resistant design ofearth and rockfill dams.