144
9 Gravity Erosions on the Loess Gully Bank …
turns on a conceptual landform with the gentle upper slope of 3° and steep lower
slope of 70°–80°. An equal period, 12 h or so, was kept after each rainfall to ensure
the approximate value of the initial water content. We compacted loess in layers with
hands to form the conceptual slope as what we designed before. The 50% diameter
of soil particles, d 50 , was 52.2 µm, and the specific gravity, γ s , was 2.56.
During and 20 min after the rainfall, slope failure occurrence time, slip mode, type
of failure scar, location, and slope failure retrogression behavior were recorded by
direct observation and the topography meter. The instrument based on the advanced
technology of 3D visualization and stereo video stream display, was designed by the
authors themselves, and its performance has been confirmed in the calibration tests
and the landslide experiments (Xu et al. 2015). In contrast to the conventional contact observation instruments, the topography meter could quantitatively measure the
random mass failures on the steep slope in dynamic environments. The topography
meter emitted a group of parallel laser to the slope surface and recorded the dynamic
variation of the steep slope relief under rainfall simulation with a video camera. By
comparing the slope geometries in the moments before and after the erosion incident
on the snapshot images, we could obtain the volume of any individual slide mass.
9.3 Results and Discussion
9.3.1 Types of Observed Mass Failures
The failure style is classified such that it reflects the mechanism responsible for failure
movement style, failure block moisture, debris distribution at which the movement
occurs. Representative failure modes of all the experiments are illustrated schematically in Fig. 9.2. The phenomenon that soil suddenly topples, fragments and rolls
down fully apart from a sloped face is termed avalanche, fall, or collapse, while that
soil on the slope slips down as a whole along a certain weak belt is called a landslide.
Mudslide, i.e., earth flow or mudflow, is defined here as the failure occurring with
distorting shape and involving full saturation.
The maximum volume of the individual failure mass indicates the possible harm
on both local constructions and lives. In the experiments shown in Table 9.1, the peak
amounts of the individual avalanche, landslide and mudslide are 369.9 × 10
3 , 177.6 ×
10
3 and 24.6 × 10
3 cm
3 , respectively. Their average maximum amounts from every
experiment are 145.1 × 10
3 , 98.4 × 10
3 , 9.9 × 10
3 cm
3 , respectively. Consequently,
on the conditions of this experiment, i.e., the steep slope made of homogeneous loess
and under intense rainstorms, hazards caused by the avalanche and landslide may
be much more serious than that caused by the mudslide. Particularly, the bulk of
the individual avalanche was the largest in all types of gravity erosion. As we know,
rapid landslides pose lethal threats, whereas slow landslides damage properties but
seldom cause fatalities (Iverson et al. 2000). Thus avalanche, which moved quite
9 Gravity Erosions on the Loess Gully Bank …
turns on a conceptual landform with the gentle upper slope of 3° and steep lower
slope of 70°–80°. An equal period, 12 h or so, was kept after each rainfall to ensure
the approximate value of the initial water content. We compacted loess in layers with
hands to form the conceptual slope as what we designed before. The 50% diameter
of soil particles, d 50 , was 52.2 µm, and the specific gravity, γ s , was 2.56.
During and 20 min after the rainfall, slope failure occurrence time, slip mode, type
of failure scar, location, and slope failure retrogression behavior were recorded by
direct observation and the topography meter. The instrument based on the advanced
technology of 3D visualization and stereo video stream display, was designed by the
authors themselves, and its performance has been confirmed in the calibration tests
and the landslide experiments (Xu et al. 2015). In contrast to the conventional contact observation instruments, the topography meter could quantitatively measure the
random mass failures on the steep slope in dynamic environments. The topography
meter emitted a group of parallel laser to the slope surface and recorded the dynamic
variation of the steep slope relief under rainfall simulation with a video camera. By
comparing the slope geometries in the moments before and after the erosion incident
on the snapshot images, we could obtain the volume of any individual slide mass.
9.3 Results and Discussion
9.3.1 Types of Observed Mass Failures
The failure style is classified such that it reflects the mechanism responsible for failure
movement style, failure block moisture, debris distribution at which the movement
occurs. Representative failure modes of all the experiments are illustrated schematically in Fig. 9.2. The phenomenon that soil suddenly topples, fragments and rolls
down fully apart from a sloped face is termed avalanche, fall, or collapse, while that
soil on the slope slips down as a whole along a certain weak belt is called a landslide.
Mudslide, i.e., earth flow or mudflow, is defined here as the failure occurring with
distorting shape and involving full saturation.
The maximum volume of the individual failure mass indicates the possible harm
on both local constructions and lives. In the experiments shown in Table 9.1, the peak
amounts of the individual avalanche, landslide and mudslide are 369.9 × 10
3 , 177.6 ×
10
3 and 24.6 × 10
3 cm
3 , respectively. Their average maximum amounts from every
experiment are 145.1 × 10
3 , 98.4 × 10
3 , 9.9 × 10
3 cm
3 , respectively. Consequently,
on the conditions of this experiment, i.e., the steep slope made of homogeneous loess
and under intense rainstorms, hazards caused by the avalanche and landslide may
be much more serious than that caused by the mudslide. Particularly, the bulk of
the individual avalanche was the largest in all types of gravity erosion. As we know,
rapid landslides pose lethal threats, whereas slow landslides damage properties but
seldom cause fatalities (Iverson et al. 2000). Thus avalanche, which moved quite