11.3 Results and Discussion
181
sensitivity parameters such as slope height, slope gradient, rainfall duration, and
rainfall intensity for the scars Tf, Dc, Uc, Ia, Ps, and Co ranged from −6.1 to 3.7.
In particular, the rainfall duration had significant effects on the number of failures
with the scar Tf (S NTf = 4.0). The frequency of mass failures with the scars Dc, Ps,
and Ia were greatly influenced by the change of slope gradient, and their sensitivity
parameters were −4.0, −3.5, and −6.1, respectively. In other words, the number of
mass failures with the scar Uc (S NUc = 2.1) was highly susceptible to slope height.
Meanwhile, for the volume of gravity erosion in the experiments, the results of
sensitivity analysis (Fig. 11.5b) demonstrated that the sensitivity parameters of slope
height, slope gradient, rainfall duration, and rainfall intensity on the scars Tf, Dc,
Uc, Ia, Ps and Co varied dramatically, with values ranging from −5.9 to 12.4. It is
worth mentioning that rainfall duration was the most influential element on the size
of mass failures, for the sensitivity parameter of the rainfall duration on the total
volume of the scar Uc was 12.4. The slope gradient was the second most prominent
trigger factor on the size of the Uc-shaped failure, and the value of the sensitivity
parameter was 9.7. Additionally, the sensitivity parameters of the slope gradient on
the total volume of failures with the scars Tf, Co, Ps and Ia were comparable, at 4.5,
5.9, 7.8 and 5.0, respectively. In addition, the sensitivity parameters of slope gradient
on the total volume and number of gravity erosions were 2.8 and 4.1, respectively.
11.3.4 Formation Mechanism of Scar Morphologies
Loess is a special type of geological material which has soil structure homogeneity and peculiar mechanical properties, leading to the failure scar characterized by
obvious geometrical morphology (Gan et al. 1999). In this study, the scar pattern
also shows a distinct geometric appearance, which could be divided into six types
(i.e., Tf, Uc, Dc, Ps, Co, and Ia). Generally, the mass failure occurs along an inclined
plane when the shear strength is primarily provided by the inter-particle frictional
resistance (Wang and Li 2009). Although the loess is sticky in the experiment
(d 50 = 0.05 mm), the cohesion rapidly decreases to zero while the internal friction angle normally reaches a stable state if the moisture content exceeds the plastic
limit (Derbyshire et al. 1994). Thus, the soil mass falls along the scar Tf. As we
know, the mass failure strongly depends on the shear strength of the slope material, which is not only related to the internal friction angle but also the cohesion.
Change of the apparent cohesion due to variations in the soil water content results in
a re-distribution of shear strength during the process of rainwater infiltration. Owing
to the phenomenon that the strength diminishes as shear stress increases, the mass
failure process is characterized by a non-uniform distribution of the shear strength
along the potential slip surface (Chen et al. 2016). Consequently, rainfall-induced
failures also bring about different forms of scars. For instance, the failure will also
produce an arc-shaped scar, including Uc, Dc, and Co, when the cohesion plays an
important role in the shear strength of the soil (Wang and Li 2009). However, the
proportion of cohesion in the shear strength is dynamic in the study. As a result,
181
sensitivity parameters such as slope height, slope gradient, rainfall duration, and
rainfall intensity for the scars Tf, Dc, Uc, Ia, Ps, and Co ranged from −6.1 to 3.7.
In particular, the rainfall duration had significant effects on the number of failures
with the scar Tf (S NTf = 4.0). The frequency of mass failures with the scars Dc, Ps,
and Ia were greatly influenced by the change of slope gradient, and their sensitivity
parameters were −4.0, −3.5, and −6.1, respectively. In other words, the number of
mass failures with the scar Uc (S NUc = 2.1) was highly susceptible to slope height.
Meanwhile, for the volume of gravity erosion in the experiments, the results of
sensitivity analysis (Fig. 11.5b) demonstrated that the sensitivity parameters of slope
height, slope gradient, rainfall duration, and rainfall intensity on the scars Tf, Dc,
Uc, Ia, Ps and Co varied dramatically, with values ranging from −5.9 to 12.4. It is
worth mentioning that rainfall duration was the most influential element on the size
of mass failures, for the sensitivity parameter of the rainfall duration on the total
volume of the scar Uc was 12.4. The slope gradient was the second most prominent
trigger factor on the size of the Uc-shaped failure, and the value of the sensitivity
parameter was 9.7. Additionally, the sensitivity parameters of the slope gradient on
the total volume of failures with the scars Tf, Co, Ps and Ia were comparable, at 4.5,
5.9, 7.8 and 5.0, respectively. In addition, the sensitivity parameters of slope gradient
on the total volume and number of gravity erosions were 2.8 and 4.1, respectively.
11.3.4 Formation Mechanism of Scar Morphologies
Loess is a special type of geological material which has soil structure homogeneity and peculiar mechanical properties, leading to the failure scar characterized by
obvious geometrical morphology (Gan et al. 1999). In this study, the scar pattern
also shows a distinct geometric appearance, which could be divided into six types
(i.e., Tf, Uc, Dc, Ps, Co, and Ia). Generally, the mass failure occurs along an inclined
plane when the shear strength is primarily provided by the inter-particle frictional
resistance (Wang and Li 2009). Although the loess is sticky in the experiment
(d 50 = 0.05 mm), the cohesion rapidly decreases to zero while the internal friction angle normally reaches a stable state if the moisture content exceeds the plastic
limit (Derbyshire et al. 1994). Thus, the soil mass falls along the scar Tf. As we
know, the mass failure strongly depends on the shear strength of the slope material, which is not only related to the internal friction angle but also the cohesion.
Change of the apparent cohesion due to variations in the soil water content results in
a re-distribution of shear strength during the process of rainwater infiltration. Owing
to the phenomenon that the strength diminishes as shear stress increases, the mass
failure process is characterized by a non-uniform distribution of the shear strength
along the potential slip surface (Chen et al. 2016). Consequently, rainfall-induced
failures also bring about different forms of scars. For instance, the failure will also
produce an arc-shaped scar, including Uc, Dc, and Co, when the cohesion plays an
important role in the shear strength of the soil (Wang and Li 2009). However, the
proportion of cohesion in the shear strength is dynamic in the study. As a result,