Chapter 4 Influential Factors on Landslide Mobility and Travel Distance
4.4 Qualitative analysis of influential factors on landslide mobility
4.4.1 The effects of topographical factors
Topoghraphical factors play an improtant role in landslide mobility(Okura et al., 2003), herein, three parameters would be discussed, such as slope ange (θ), slope transition angle(ε) and slope height (h).
Figure 4.7 Equivalent coefficient of friction related with tangent of slope angle
Statistical result, shown in Figure 4.7, suggests that equivalent coefficient of friction of landslide induced by Wenchuan earthquake had a positive, but weak, correlation with tangent of slope angle. It implied that landslide mobility (1/μ) decreased with the increment of slope angle(θ), which is attributable to the positive correlation between internal friction coefficient and
0.25 0.50 0.75 1.00 1.25 1.50
0.0 0.2 0.4 0.6 0.8 1.0
-''tan+0.362 (R2=0.038)
Equivalent coefficient of friction (-.max/Lmax)
Tangent of slope angle (tan)
positive correlation with slope angle; meanwhile, the steeper the slope is, the higher consumption of kinetic energy due to impact at the foot of upper slope. Therefore, the likelihood of high mobilization landslide was relatively low to ocurr on the steep or very steep slope. This general tendency related with the tangent of slope angle of earthquake-induced landslides is consistent with previous studies on non-seismic landslide (Okura et al., 2003; Hunter and Fell, 2003;
Hattanji and Moriwaki, 2009, 2011).
Figure 4.8 Equivalent coefficient of friction related Figure 4.9 Statistical histogram of with sine of slope transition angle slope transition angle
The changing degree of slope inclination was represnted by slope transition angle (ε). The relationship between equivalent coefficient of friction and sine of slope transition angle was illustrated in Figure 4.8, which suggests that there was no statistical correlation and tendency.
However, the histogram in Figure 4.9 shown that 45% landslides in this data set concentrated in the range of 160o -170o slope transition angle. It may be explained by that when slope transition angle was relatively small, a large amount of kinetic energy was dissipated by serious impact due to the large inclination change btween upper slope and lower slope. With the increment of slope transition angle, energy consumed by impact at slope foot decreased, and the falling mass was crushed and resulted in the transform of mobile motion from sliding to rolling or flowing, then residual energy drived rolling or flowing mass to travel relatively longer. Howerver, when slope transition angle was large enough to ignore the topographical slope change, the motion of failure mass was highly probable to slide as a relatively intact quasi-rigid body, then kenetic energy
0.0 0.2 0.4 0.6 0.8
0.0 0.2 0.4 0.6 0.8 1.0
Equivalent coefficient of friction (-.max/Lmax)
Sine of slope transition angle (sin/)
140~150 150~160 160~170 170~180 0
10 20 30 40 50
Precentage out of the total (%)
Slope transition angle (/)
would be consumed by sliding friction. As a result of sliding motion generally consumed more kinetic energy than rolling or flowing motion, then sliding mass would be decelerated faster than rolling or flowing mass, therefore, landslides within the group of 170o180o slope transition angle became fewer than those in 160o170o.
Figure 4.10 Equivalent coefficient of friction related with slope height
The relation between equivalent coefficient of friction and slope height was shown in Figure 4.10. Although statistical correlation is very weak, the general tendency suggests that equivalent coefficient of friction increased with the increment of slope height. It may be explained by that slope height implies the potential energy of failed mass and governs the available space to accelerate the failed mass, the higher slope height was, the larger velocity was, resulting in the loss of kinetic energy by impact increased with slope height. When larger kenetic energy was consumed, landslide mobility (1/μ) would be lower, namely, it caused equivalent coefficient of friction (μ) had positive correlation with slope height.
4.4.2 The effect of seismic acceleration
In order to explore the trend between landsldie mobility and seismic ground motion, the
0 100 200 300 400 500 600 700
0.0 0.2 0.4 0.6 0.8 1.0
-0× 10-4h+0.418 (R2=0.017)
Equivalent coefficient of friction (-.max/Lmax)
Slope height (h/m)
landslide. The result was illustrated in Figure 4.11, which suggests that equivalent coefficient of friction had no correlation with peak ground acceleration. It implies that seismic acceleration had little effect on landslide movement. Backward analyzing the scale of 46 landslides, it was found that the volumes of these landslides were in the range of 4.5×1042.75×107m3, 65% landslide volumes are larger than 106 m3, and 39 volumes out of the total were larger than 5.0×105 m3. From the viewpoint of earthquake energy, Kokusho et al. (2009) proposed that the potential energy of very large landslide would be big enough to ignore the effectiveness of earthquake energy on landslide movement; the effect of earthquake was playing a trigger role rather than making landslide have higher mobility and drive sliding mass to travel long away. Herein, the results of these 46 landslides gave an evidence to the statement of Kokushao et al.(2009).
Figure 4.11 Equivalent coefficient of friction related with seismic acceleration
4.4.3 The effect of rock type
Rock type is another influential factor on landslide mobility. Because landslide usually had very changeable rock type along travel path, herein, discussed lithology was limited within the sliding source range for the typical rock type. According to rock strength and weathered degree, rock materials were classified into two types and four sub-classes, as shown in Table 3.1. The statistical result was illustrated in Figure 4.12, which suggests that equivalent coefficient of friction of hard rock was within a smaller range than that of soft rock, indicating that landslides
2 4 6 8 10 12
0.0 0.2 0.4 0.6 0.8 1.0
Equivalent coefficient of friction (-. max/L max)
Horizontal PGA (m/s2)
consisting of hard rock had higher mobility (smaller μ) than those consisting of soft rock. The reason might be caused by that the sliding friction coefficient between soft rock and travel path was larger than that of hard rock; besides, it might be caused by the difference of mobile mechanics. The behavior of soft rock was possible to be viscoplasticity, while the behavior of hard rock was probable to be plasticity; hence, soft rock consumed more kinetic energy than hard rock along travel path, resulting in equivalent coefficient of friction of soft rock landslide distributed within a larger range.
Figure 4.12 Equivalent coefficient of friction related with rock type
4.4.4 The effect of landslide volume
There are lots of previours studies on the relationship between landslide mobility and sliding volume induced by non-seismic causes (Scheidegger, 1973; Hsü, 1975; Corominas, 1996; Legros, 2002; Okura et al. 2003). Scheidegger (1973) stated a log-log linear correlation between landslide volume and equivalent coefficient of friction when volume beyond 105m3; Hsü (1975) stated the threshold value of landslide volume was 5×105m3. However, there are few studies on the relationship btween landslide volume and its mobility induced by earthquake. Based on these 46 landslides induced by the 2008 Wenchuan earthquake, an emprical formula with highly statistical
0.0 0.2 0.4 0.6 0.8 1.0
Equivalent coefficient of friction (-. max/L max)
hard rock soft rock
RT4 RT3 RT2 RT1
decreasing tendency between equivalent coefficient of friction and sliding source volume, especially, excluding four landslides smaller than 2.55×105m3.
Figure 4.13 Equivalent coefficient of friction related with landslide source volume