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Abstract

On the mountainous island of Taiwan, where earthquakes and typhoons are common, many landslide disasters occur. The Tsao-Ling rockslides are perhaps the best-known landslides in Taiwan. Between 1862 and 1979, four large catastrophic rockslides occurred on the southwest slope of Mount Tsao-Ling. These multimillion cubic meter mass movements created substantial landslide dams on the Ching-Shui River, which flows at the toe of the Tsao-Ling slope. The failures of these landslide dams resulted in additional catastrophes, which, in three cases, occurred years after the rockslide event. On December 17, 1941, a rockslide involving a mass movement of more than 80 × 106 m3 occurred on the dip slope forming the southwest flank of Mount Tsao-Ling, triggered by a strong earthquake. On August 10, 1942, heavy rain caused another rockslide on the same slope, and more than 100 × 106 m3 of rock slid down the Tsao-Ling dip slope. The Ching-Shui River was dammed with rock debris. The landslide dam (140–200 m high, 4800 m wide at base) was overtopped on May 18, 1951 and 120 × 106 m3 of impounded water was suddenly released. More than 100 human lives were lost. On August 15, 1979, heavy rain caused a breakaway failure from the lower part of the remaining slope. The slide mass collided with the original debris dam and the Ching-Shui River was once again dammed. The landslide dam was overtopped on August 24, 1979. From 1980 to 1993, intensive investigations in the greater Tsao-Ling area were undertaken. We conclude that a further rockslide involving a mass movement of 50 × 106 m3 or so is possible.

Introduction

Many landslide disasters occur every year on the mountainous island of Taiwan, where earthquakes and typhoons are common. The well-known Tsao-Ling rockslides occurred between 1862 and 1979 on the southwest slope of Mount Tsao-Ling, a dip slope in interbedded Pliocene sandstones, mudstones, and shales. Four large catastrophic rockslides, multimillion cubic meter mass movements, created substantial landslide dams on the Ching-Shui River, which flows at the toe of the Tsao-Ling dip slope. The failures of these landslide dams resulted in additional catastrophes, three of which occurred years after the rockslide event.

Nearly 70% of the island of Taiwan, which has an area of 36 000 km2, is mountains and foothills. There are more than 100 summits higher than 3000 m above sea level in the Central Mountain Ranges; the highest peak, Yu-Shan (Mount Jade), is 3952 m above sea level. The collision between the Philippine Sea plate and the Eurasian continental plate is active (e.g., Wang and Burnett, 1990) and results in rates of uplift (~5 mm/yr; Lundberg and Dorsey, 1990) considered to be some of the highest in the world, if not the highest. Landsliding is one of the denudation processes acting to balance uplift (Li, 1976).

Taiwan is in the subtropics, where annual precipitation is >2000 mm and where typhoons often occur. Landslides occur frequently during and after rainstorms associated with the typhoons. Taiwan has also undergone destructive earthquakes throughout its history (Hsu, 1971): many landslides have been induced by earthquakes (Chang, 1996), and are therefore common in Taiwan.

The Tsao-Ling rockslide, the most noted in Taiwan, is an excellent illustration of the role of major geological factors in landslide occurrence and of the influence of a landslide on the water resources development of a river.

The Tsao-Ling area is a famous scenic spot in central Taiwan, located on the north side of the Ching-Shui River, which is a major tributary of Cho-Shui River (Fig. 1). Table 1 lists the known historical events at the Tsao-Ling rockslide. It covers an area of ~5 km3, of which 1.5 km3 is covered by landslide debris.

Figure 1.

Location map of site of Tsao-Ling rockslides, central Taiwan.

Figure 1.

Location map of site of Tsao-Ling rockslides, central Taiwan.

The scale of the dramatic landslide events at Tsao-Ling attracted the interest of one of us (Hung Ju-Jiang), who was invited by Sinotech Engineering Consultants to inspect the Tsao-Ling area in December 1976. Hung visited the site again on August 11 and 12, 1979; on August 15, 26 × 106 m3 of rock slid down the Tsao-Ling dip slope. The Ching-Shui River was once again dammed. Hung examined the ~70-m-deep dammed lake, the slope that had been the source of the rockslides, and the landslide dam; after leaving Tsao-Ling, a section of the landslide dam was overtopped and breached on August 24. Two bridges downstream were destroyed by the flood of released water as thousands of people watched from high terraces on both banks; more than 1000 tourists were prevented from leaving Tsao-Ling for about 2 weeks due to the destruction of the bridges.

Since 1980, intensive investigations in the greater Tsao-Ling area have been undertaken (Chang and Lee, 1989; Huang et al., 1983; Hung, 1980, 1981, 1982, 1983; Lee et al., 1993; Shieh and Wu, 1988, 1989, 1990, 1991; Sinotech Engineering Consultants, 1977), including remote sensing, air-photo interpretation, ground surveying, borings, sampling, in situ monitoring, and laboratory testing. A better understanding of the engineering geology of the Tsao-Ling area and the slip surfaces of previous events was gained (Lee et al., 1994). The stress distribution and stability of the remaining slope were also analyzed (Lee et al., 1993; Hung et al., 1994). This chapter summarizes the work carried out at the Tsao-Ling rockslides to date.

Geology of the Tsao-Ling Slopes

The Tsao-Ling slopes consist of Tertiary sedimentary rocks. Stratigraphic formations that crop out in the Tsao-Ling area, in ascending order, are the late Miocene Shih-Liu-Feng shale and the Ta-Wuo sandstone, the Pliocene Ching-shui shale and the Cho-Lan formation, and recent terrace deposits and alluvium. A geological map of the study area based on previous studies (Tai-Pei Observatory, 1942; Hsu, 1951; Hsu and Leung, 1977; Hung, 1980, 1981, 1982; Chang and Lee, 1989) and our recent investigation is shown in Figure 2 (Lee et al., 1993). The lithology and thickness of each formation are summarized as Table 2. Structurally, the study area is located at the eastern limb of the Feng-Tzu-Lun syncline (Fig. 2). The rock formations strike northwest-southeast and dip ~12° southwest. These strata form a conspicuous dip slope on the northeast side of the Ching-Shui River that is being actively undercut by the river.

Figure 2.

Generalized geologic map of Tsao-Ling rockslide area. A-A' is central profile line in Figure 4.

Figure 2.

Generalized geologic map of Tsao-Ling rockslide area. A-A' is central profile line in Figure 4.

Table 1.

Historical Events at Tsao-Ling Rockslide, Taiwan, 1862–1979

DateTriggerProcessEffectsReference§
June 6, 1862Earthquake (M = 6.0–7.0)Landslide: formation of a landslide dam.*1, 2, 4, 5, 6
1898UnknownBreach of the landslide dam.*5, 6
December 17, 1941Earthquake (M = 7.1)Landslide—84 × 106 m3 in volume; formation of a landslide dam 70–200 m in height, formation of a dammed up lake containing 12.8 M m3 of water.36 persons killed; 59 houses damaged.1, 2, 3, 4, 5, 6, 7
August 10, 1942Rainfall: 3 day cumulative precipitation of 770 mmLandslide—100 × 106 m3 in volume; height of landslide dam increased from ~140–217 m in height; formation of a larger dammed up lake containing 157 × 106 m3 of water.1 person buried; 1 house damaged.2, 3, 4, 5, 6, 7
May 18, 1951Rainfall: 5 day cumulative precipitation of 776 mmBreak of the landslide dam. Release of 120 × 106 m3 of water.Flooding of 3000 ha of arable land; 137 persons killed; 1200 houses damaged.2, 3, 4, 5, 6, 7
August 15, 1979Rainfall: 2 day cumulative precipitation of 327 mmLandslide—26 × 106 m3 in volume; formation of a landslide dam, 90 m in height; formation of a dammed up lake containing 40 × 106 m3 of water.4, 5, 6, 7
August 24, 1979Rainfall: 2 day cumulative precipitation of 624 mmBreach of the landslide dam. Release of 40 × 106 m3 of water.Two bridges destroyed.4, 5, 6, 7
DateTriggerProcessEffectsReference§
June 6, 1862Earthquake (M = 6.0–7.0)Landslide: formation of a landslide dam.*1, 2, 4, 5, 6
1898UnknownBreach of the landslide dam.*5, 6
December 17, 1941Earthquake (M = 7.1)Landslide—84 × 106 m3 in volume; formation of a landslide dam 70–200 m in height, formation of a dammed up lake containing 12.8 M m3 of water.36 persons killed; 59 houses damaged.1, 2, 3, 4, 5, 6, 7
August 10, 1942Rainfall: 3 day cumulative precipitation of 770 mmLandslide—100 × 106 m3 in volume; height of landslide dam increased from ~140–217 m in height; formation of a larger dammed up lake containing 157 × 106 m3 of water.1 person buried; 1 house damaged.2, 3, 4, 5, 6, 7
May 18, 1951Rainfall: 5 day cumulative precipitation of 776 mmBreak of the landslide dam. Release of 120 × 106 m3 of water.Flooding of 3000 ha of arable land; 137 persons killed; 1200 houses damaged.2, 3, 4, 5, 6, 7
August 15, 1979Rainfall: 2 day cumulative precipitation of 327 mmLandslide—26 × 106 m3 in volume; formation of a landslide dam, 90 m in height; formation of a dammed up lake containing 40 × 106 m3 of water.4, 5, 6, 7
August 24, 1979Rainfall: 2 day cumulative precipitation of 624 mmBreach of the landslide dam. Release of 40 × 106 m3 of water.Two bridges destroyed.4, 5, 6, 7

* Interview of Tsao-Ling villagers by Hung in 1976.

† Authors were unable to obtain significant data.

Table 2.

Stratigraphy and Lithology in the Tsao-Ling Area

EpochStratigraphyLithologyThickness (m)
HoloceneAlluviumClay, sand, gravel1–50
New debrisClay, sand, rock block1–20
Old debrisClay, sand, rock block1–170
PleistoceneTerraceClay, sand, gravel1–10
PlioceneCho-Lan formationGreenish-gray to pale yellowish fine-grained sandstone with a variable amount of shale and sandstone alternation1000
Chin-Shui shale formationDark-gray shale and sandy shale 80–150
MioceneTa-Wuo sandstone memberMassive-gray to greenish-gray muddy sandstone with shale lamination1100
Shih-Liu-Feng shale memberDark-gray shale100
EpochStratigraphyLithologyThickness (m)
HoloceneAlluviumClay, sand, gravel1–50
New debrisClay, sand, rock block1–20
Old debrisClay, sand, rock block1–170
PleistoceneTerraceClay, sand, gravel1–10
PlioceneCho-Lan formationGreenish-gray to pale yellowish fine-grained sandstone with a variable amount of shale and sandstone alternation1000
Chin-Shui shale formationDark-gray shale and sandy shale 80–150
MioceneTa-Wuo sandstone memberMassive-gray to greenish-gray muddy sandstone with shale lamination1100
Shih-Liu-Feng shale memberDark-gray shale100

PRE-1979 Rockslides at Tsao-Ling

Tsao-Ling area before December 17, 1941

Figure 3 is a topographic map of the Tsao-Ling rockslide area before December 17, 1941; the map is used to roughly approximate the pre-1941 slope profile in Figure 4A. The scarp in m formed after a rockslide triggered by a strong earthquake (Tai-Pei Observatory, 1942; estimated magnitude of 6.0–7.0, Hsu, 1971) on June 6, 1862, that occurred in the Chia-Yi and Tai-Nan region. A number of published reports, as well as historical review articles, reported the 1862 event (e.g., Tai-Pei Observatory, 1942; Cheng and Yeh, 1989). Hung interviewed old people in the Tsao-Ling Village in December 1976. They related the story of the 1862 event told by their older relatives: the Ching-Shui River was dammed with rock debris, and the landslide dam was overtopped in 1898 and the retained water released.

Figure 3.

Topographic map of Tsao-Ling rockslide area before December 17, 1941 (modified from Tai-Pei Observatory, 1942, Figure 34a). Contour interval is 50 m. A–A' is central profile line of slide area. Refer to Figure 4A for profile.

Figure 3.

Topographic map of Tsao-Ling rockslide area before December 17, 1941 (modified from Tai-Pei Observatory, 1942, Figure 34a). Contour interval is 50 m. A–A' is central profile line of slide area. Refer to Figure 4A for profile.

Figure 4.

Cross sections through center of Tsao-Ling rockslide area (see Figs. 3, 14, and 15 for location of cross section). A: Reconstructed profiles of 1941 and 1942 slides. B: Reconstructed profiles of 1979 slide. C: Reconstructed profile of slope in 1993; a–f are referred to in the text.

Figure 4.

Cross sections through center of Tsao-Ling rockslide area (see Figs. 3, 14, and 15 for location of cross section). A: Reconstructed profiles of 1941 and 1942 slides. B: Reconstructed profiles of 1979 slide. C: Reconstructed profile of slope in 1993; a–f are referred to in the text.

1941 Tsao-Ling rockslide event

On December 17, 1941, Taiwan was shaken by the strong Chia-Yi earthquake (M = 7.1) (Tai-Pei Observatory, 1942). The epicenter was located at 23.40°N, 120.50°E, 10 km southeast of Chia-Yi (Fig. 5). Focal depth was ~10 km. Figure 5 shows the intensity contours of the earthquake, which killed more than 350 people. Many landslides were caused by the earthquake (Tai-Pei Observatory, 1942, Fig. 33), the Tsao-Ling rockslide being the most noted.

Figure 5.

Intensity contour map of 1942 Chia-Yi earthquake. Numbers on map are intensities according to Japanese Meteorological Agency system (Tai-Pei Observatory, 1942). Asterisk indicates location of Tsao-Ling rockslide. Pound sign indicates A-Li-Shan rainfall gage station.

Figure 5.

Intensity contour map of 1942 Chia-Yi earthquake. Numbers on map are intensities according to Japanese Meteorological Agency system (Tai-Pei Observatory, 1942). Asterisk indicates location of Tsao-Ling rockslide. Pound sign indicates A-Li-Shan rainfall gage station.

Tsao-Ling is ~27.5 km northeast of the epicenter (Fig. 5). More than 80 × 106 m3 of rock slid down the dip slope during the earthquake (Figs. 4A and 6), damming the Ching-Shui River with rock debris. The crest of the landslide dam was estimated to be 70–200 m above the riverbed. Figure 7 is a simplified map of the rockslide area after December 17, 1941 and before March 30, 1942. Although the sliding plane was located at the boundary between the Cho-Lan formation and the Chin-Shui shale formation, the actual sliding plane was probably very near the top of the Chin-Shui formation. This judgment is based on our observations of many similar dip-slope failure cases in Taiwan.

Figure 6.

Tsao-Ling rockslide area after 1941 rockslide event caused by Chia-Yi earthquake (M 7.1. December 17, 1941). Debris at lower left corner was house near head scarp before earthquake (Tai-Pei Observatory, 1942).

Figure 6.

Tsao-Ling rockslide area after 1941 rockslide event caused by Chia-Yi earthquake (M 7.1. December 17, 1941). Debris at lower left corner was house near head scarp before earthquake (Tai-Pei Observatory, 1942).

Figure 7.

Simplified map of Tsao-Ling rockslide area after December 17, 1941, and before March 30, 1942 (modified after Hung. 1980, Fig. 2). Heights refer to height of landslide dam.

Figure 7.

Simplified map of Tsao-Ling rockslide area after December 17, 1941, and before March 30, 1942 (modified after Hung. 1980, Fig. 2). Heights refer to height of landslide dam.

1942 Tsao-Ling rockslide event

On August 10, 1942, heavy rain caused another massive slope failure at the same site (Fig. 4A). More than 100 × 106 m3 of rock slid down the Tsao-Ling dip slope. The sliding plane was located in the middle shale part of the Cho-Lan formation. The landslide dam after the 1942 slide was estimated to be ~170 m high on average, 1200 m long, 100–300 m wide at the top, and 4800 m wide at the base (Hung, 1980). A dammed lake was formed, retaining 157 × 106 m3 of water. The lake was used as a free reservoir for irrigation and water supply for downstream villages from 1942 to May 1951. When the landslide dam (140–210 m high) was overtopped on May 18, 1951, 120 × 106 m3 of the retained water was released. From 6 a.m. to noon on May 18, the drawdown of the dammed lake was 70 m. The average discharge was estimated as 5360 m3/s. Army engineers who were trying to stop the leakage of the landslide dam were swept away: 56 were rescued and 74 were lost. An estimated 63 people in the downstream lowlands were killed and as many as 1200 houses were reported damaged (Chang and Lee, 1989). Downstream, ~3000 hectares of land were inundated, and more than 11 000 people were affected by the flood (Hsu, 1951).

In December 1976, when Hung visited the site, a tension crack above the head scarp was observed. Figure 8 is a general view of the head scarp and the remaining slope of the rockslide area looking from the landslide dam. Figure 9 shows the remaining part of the landslide dam looking from the top of the landslide scarp. Landslide debris is noted on the opposite side of the Ching-Shui River. Opposite the rockslide source area on the south side of the Ching-Shui River is a triangular-shaped tract of flat land (Fig. 9), a relic of the landslide dam that retained more than 100 × 106 m3 of water from 1942 to 1951. The villagers in Tsao-Ling call it Inversely Hooked Mountain. The head scarp has been at 1100 m elevation since the 1942 rockslide event (Fig. 4). Small-scale local failures of the head scarp happen from time to time; the local people in Tsao-Ling Village name the head scarp Cliff Forever.

Figure 8.

General view of head scarp and remaining slope, looking from landslide dam toward remaining slope and head scarp (photograph taken December 1976). Rocks forming head scarp are alternating sandstones and shales of Cho-Lan formation.

Figure 8.

General view of head scarp and remaining slope, looking from landslide dam toward remaining slope and head scarp (photograph taken December 1976). Rocks forming head scarp are alternating sandstones and shales of Cho-Lan formation.

Figure 9.

Remote view of remaining part of landslide dam, looking from top of head scarp (photograph taken December 1976). Landslide debris is noted (⋆) on opposite side of Ching-Shui River. View is downstream.

Figure 9.

Remote view of remaining part of landslide dam, looking from top of head scarp (photograph taken December 1976). Landslide debris is noted (⋆) on opposite side of Ching-Shui River. View is downstream.

1979 Events at Tsao-Ling Rockslide

On August 15, 1979, heavy rain caused a breakaway failure from the lower part of the remaining slope (Fig. 4). The sliding surface cut into the upper part of the Chin-Shui shale formation. which had not been penetrated by previous sliding (Fig. 4). The sliding mass (estimated volume ~26 × 106 m3), consisting of previous landslide debris as well as new material, collided with the original landslide dam, and again dammed the Ching-Shui River. Figure 10 shows a close-up view of a small part of the sliding surface on August 22, 1979, 1 week after the slide. Slickensides were clearly visible at the site (Fig. 10). Figure 11 is a photograph showing the dammed lake and the landslide dam on August 22, 1979. The water surface of the dammed lake was on the verge of overtopping.

Figure 10.

Close-up view of small part of sliding surface, August 22, 1979, 1 week after sliding. Slide surface is in Chin-Shui shale formation. Note slickensides in central part of photograph and hammer (32 cm) for scale on sliding surface.

Figure 10.

Close-up view of small part of sliding surface, August 22, 1979, 1 week after sliding. Slide surface is in Chin-Shui shale formation. Note slickensides in central part of photograph and hammer (32 cm) for scale on sliding surface.

Figure 11.

View upstream of dammed lake and landslide dam, looking from toe of remaining slope. Water surface of dammed lake was on verge of overtopping (photo taken August 22, 1979, 36 h before overtopping).

Figure 11.

View upstream of dammed lake and landslide dam, looking from toe of remaining slope. Water surface of dammed lake was on verge of overtopping (photo taken August 22, 1979, 36 h before overtopping).

From the evening of August 23, 1979, the rain in the greater Tsao-Ling area was even heavier. On August 24 the total 24 h rainfall was ~400 mm, and the newly deposited section of the landslide dam was overtopped and breached. Two bridges downstream were destroyed by the flood resulting from the sudden release of the retained water in the dammed lake. Fortunately, there were no casualties due to continuous monitoring and public-radio warnings.

Figure 12 is an aerial photograph of the slide area. The photo was taken on August 19, 1979, 4 days after the slide. The main scarp, the remaining slope, the contact between the old and new landslide dam, and the dammed lake are noted in this photo. Figure 13 is an aerial photo of the greater Tsao-Ling area in 1980; the Tsao-Ling rockslide area is shown in central part of this photo. Figure 14 is a topographic map of the Tsao-Ling rockslide area in 1977, and Figure 15 is a topographic map of the same site in 1980, showing locations of holes drilled in 1981 through 1982 and block samples. The outcrop of Chin-Shui shale under the landslide dam, noted in Figure 4C, was first observed during 1993 field investigation. It had been covered with landslide debris since the 1979 slide.

Figure 12.

A: Aerial photograph of 1979 rockslide event on August 19, 1979, 4 days after slide and 4 days before overtopping. B: Map of area shown in A. Features include new scarp, sliding area, dammed lake, and landslide dam.

Figure 12.

A: Aerial photograph of 1979 rockslide event on August 19, 1979, 4 days after slide and 4 days before overtopping. B: Map of area shown in A. Features include new scarp, sliding area, dammed lake, and landslide dam.

Figure 13.

Aerial photograph, taken in 1980. of greater Tsao-Ling area. A: Head scarp. B: 1979 scarp. C: Ching-Shui River. D: Ching-Shui River to Cho-Shui River. E: Old landslide dam F: Area of Tsao-Ling village. Breakaway valley is at left of B (arrow). Scale bar at top left is ~1000 m.

Figure 13.

Aerial photograph, taken in 1980. of greater Tsao-Ling area. A: Head scarp. B: 1979 scarp. C: Ching-Shui River. D: Ching-Shui River to Cho-Shui River. E: Old landslide dam F: Area of Tsao-Ling village. Breakaway valley is at left of B (arrow). Scale bar at top left is ~1000 m.

Figure 14.

Topographic map of Tsao-Ling rockslide area in 1977. A–A' is central profile line of slide area. Refer to Figure 4B for profile. Contour interval is 20 m.

Figure 14.

Topographic map of Tsao-Ling rockslide area in 1977. A–A' is central profile line of slide area. Refer to Figure 4B for profile. Contour interval is 20 m.

Figure 15.

Topographic map of Tsao-Ling rockslide area, 1980. Contour interval is 20 m. A–A' is central profile line of slide area. Refer to Figure 4B for profile. S1-S5 indicate locations of block samples taken in 1993. BH1-BH3 indicate locations of drill holes drilled in 1981 and 1982.

Figure 15.

Topographic map of Tsao-Ling rockslide area, 1980. Contour interval is 20 m. A–A' is central profile line of slide area. Refer to Figure 4B for profile. S1-S5 indicate locations of block samples taken in 1993. BH1-BH3 indicate locations of drill holes drilled in 1981 and 1982.

The channel of the Ching-Shui River had been straight before the 1941 rockslide event (Fig. 3). Since then, successive blockages of the channel have changed it into a meandering course (Fig. 14). After the 1979 rockslide event, a new scarp was formed between 650 and 700 m elevation. It is located just southwest of the breakaway valley. The area of the 1979 rockslide event is ~0.75 km3. No clear indication of further movement of the ground above the new scarp has been observed.

The preceding observations are summarized in cross sections through the Tsao-Ling dip slope (Fig. 4) that indicate the sliding planes and the slide blocks of the rockslide. The estimated volumes of the slides are listed in Table 3 for comparison with previous estimates given by Hsu and Leung (1977), Chang and Lee (1989), and Hung (1980).

Table 3.

Geometry of 1941, 1942, and 1979 Rockslide Events

YearElevation (m)Dam Height (m)Volume of Rockslide debris (106m3)Reference
River bedTop of landslide dam
194114048Hsu and Leung (1977)
70–200100–150Hung (1980)
42049070Chang and Lee (1989)
39084Lee et al. (1993)
1942217120Hsu and Leung (1977)
140–170150–200Hung (1980)
590170Chang and Lee (1989)
600210100Lee et al. (1993)
1979526>5Hung (1980)
46056010026Lee et al. (1993)
YearElevation (m)Dam Height (m)Volume of Rockslide debris (106m3)Reference
River bedTop of landslide dam
194114048Hsu and Leung (1977)
70–200100–150Hung (1980)
42049070Chang and Lee (1989)
39084Lee et al. (1993)
1942217120Hsu and Leung (1977)
140–170150–200Hung (1980)
590170Chang and Lee (1989)
600210100Lee et al. (1993)
1979526>5Hung (1980)
46056010026Lee et al. (1993)

Triggers for the Tsao-Ling Events

Earthquake

The 1941 slide was triggered by the 1941 magnitude 7.1 Chia-Yi earthquake (Tai-Pei Observatory, 1942). The peak ground acceleration at the Tsao-Ling slope may be estimated by local attenuation equations (Tsai et al., 1987), as follows: 

formula
and 
formula

In equations 1 and 2, a is the peak ground acceleration; M is the local magnitude; and R is the distance to hypocenter of the earthquake. The peak ground accelerations at the Tsao-Ling site are calculated to be 0.175 g and 0.19 g, respectively.

Precipitation

Tsao-Ling is in the A-Li-Shan region, the area of highest precipitation in Taiwan. From 1949 to 1960, the annual mean rainfall in the Tsao-Ling area was 3000 mm, while that in the A-Li-Shan station (see Fig. 5) was 4000 mm. Histograms of monthly and daily precipitation in the year and month of rockslides and landslide dam breaks are shown in Figure 16. The 1941 dip slope failure was caused by a strong earthquake and is not related to precipitation. The 1942 slide, the 1951 landslide dam break, and the 1979 events were all induced by heavy rain.

Figure 16.

Histograms of monthly and daily precipitation in year and month of rockslide and landslide dam break. Solid horizontal lines indicate yearly (and monthly) means; ▼ is time of rockslide event; X is time of landslide dam break event.

Figure 16.

Histograms of monthly and daily precipitation in year and month of rockslide and landslide dam break. Solid horizontal lines indicate yearly (and monthly) means; ▼ is time of rockslide event; X is time of landslide dam break event.

The monthly mean precipitations for these events since 1942 are all higher than 300 mm and should be considered excessive.

Relationship of Landslides to Geology

The landslide area is bounded by a northeast-southwest-trending lineament on its northwest side. This lineament is a zone of fractured rock that is exposed at a saddle at the north corner of the slide area. This weak zone may provide a lateral release surface for sliding. The main scarp on the northeast side of the slide is formed along two sets of diagonal joints. The southeast side of the slide is limited by the nature of the valley topography. Two main joint sets occur in the slide area (Fig. 17). One set extends north-south, and the other set strikes east-northeast–west-southwest.

Figure 17.

Pole plots and contoured stereonets of joints (boxes with ticks) and bedding plane (arrows) in Tsao-Ling rockslide area. A: Discontinuities at crown of scarp. B: Discontinuities at crown of northern (upslope) cliff of breakaway valley. C: Discontinuities on southern (downslope) cliff of breakaway valley.

Figure 17.

Pole plots and contoured stereonets of joints (boxes with ticks) and bedding plane (arrows) in Tsao-Ling rockslide area. A: Discontinuities at crown of scarp. B: Discontinuities at crown of northern (upslope) cliff of breakaway valley. C: Discontinuities on southern (downslope) cliff of breakaway valley.

There are also two minor sets of joints; one set strikes northeast-southwest, and the other set west-northwest-east-southeast. These joint sets together with southwest-dipping bedding planes cut the rock formation to form blocks of various sizes. Detailed orientations of joint sets in the study area are given in Table 4. Based on the regional geological investigations of the study area, the most representative orientation (strike/dip) of rock formations in the rockslide area is N48°W/12°SW. Therefore, the sliding direction in the dip slope is S42°W, parallel to the dip direction. Detailed field observations have been made on the relationship of the various sliding surfaces to the geology of the Tsao-Ling slope in the area of the rockslide. Along the south bank of the Ching-Shui River, a section of the 1941 and 1942 slide deposits and underlying undisturbed bedrock of Chin-Shui shale formation can be observed (a in Fig. 4C). The contact between the deposits and bedrock here is undoubtedly the 1941 and 1942 slide surface (Fig. 18). The downcutting of the Ching-Shui River bed from 1951 to 1979 was ~100 m (Lee et al., 1993). The Ching-Shui River channel has cut through the Chin-Shui shale and penetrated into the topmost layer of the Ta-Wuo sandstone (b in Fig. 4C). On the north bank of the river channel, the 1979 landslide deposits directly overlie the undisturbed topmost layer of the Ta-Wuo sandstone. This indicates that the sliding took place in the overlying Chin-Shui shale. The Ta-Wuo sandstone was not involved in any of the rockslide events (Fig. 4C).

Table 4.

Orientation of Joint Sets in the Tsao-Ling Rockslide Area

LocationSetNumber of measurements
ABCD
Crown of head scarpN6°E/80°E N12°W/78°EN72°E/83° E28
Crown of northern cliff of breakaway valleyN5°E/82°EN76°E/83°NN49°E/89°N110
Toe-wall of breakaway valleyN18°W/89°EN82°E/83°NN56°W/79°N36
LocationSetNumber of measurements
ABCD
Crown of head scarpN6°E/80°E N12°W/78°EN72°E/83° E28
Crown of northern cliff of breakaway valleyN5°E/82°EN76°E/83°NN49°E/89°N110
Toe-wall of breakaway valleyN18°W/89°EN82°E/83°NN56°W/79°N36
Figure 18.

Section at river level of landslide dam materials (photo taken in July, 1993; location a in Fig. 4C). Base of landslide debris is indicated by dashed line. Below dashed line, B is top of Chin-Shui shale formation. Above dashed line is debris material of 1941 and 1942 rockslide events. Sandstone block at lower right is ~15 m high.

Figure 18.

Section at river level of landslide dam materials (photo taken in July, 1993; location a in Fig. 4C). Base of landslide debris is indicated by dashed line. Below dashed line, B is top of Chin-Shui shale formation. Above dashed line is debris material of 1941 and 1942 rockslide events. Sandstone block at lower right is ~15 m high.

In the area between the new 1979 scarp and the Ching-Shui River channel, there is a small channel flowing southwestward (c in Fig. 4C). In this small channel, bedrock has been exposed. We found that this bedrock is Chin-Shui shale and is undisturbed. Above this undisturbed bedrock, all the materials are 1979 and post-1979 slide deposits. The 1979 sliding plane must therefore be located in the middle part of Chin-Shui shale formation (c in Fig. 4C). The 1979 scarp (between 650 and 700 m elevation) is ~40 m high (d in Fig. 4C). Sandstone beds of the lower most Cho-Lan formation are exposed in the upper part of the scarp. These sandstone beds contain many large open fractures, indicating disturbance probably during the 1941 and 1942 slides. At the lower part of the 1979 scarp, massive undisturbed shale of the uppermost Chin-Shui shale is exposed. We suggest that the sliding plane of the 1941 slide is located at or near the Cho-Lan-Chin-Shui boundary. Above the contact, a 3-m-thick sandstone layer is clearly iron stained and has water seeping out of it. Groundwater is prevented from penetrating deep into the Chin-Shui shale due to its low permeability.

Breakaway valley

Figure 19 shows the breakaway valley. On the south-facing (upslope) cliff of the breakaway valley (e in Fig. 4), further breakaway sliding motion is indicated by the vertical openings of the cliff (Fig. 20). From the north side of the breakaway valley (Fig. 4C) to the base of the main head scarp, a sandstone dip slope of large extent is exposed (f in Fig. 4C). The slab-like rocks forming the surface are not disturbed and are not jointed. We conclude that the 1941 and 1942 slide events did not involve these rocks. There are remnants of thin-bedded sandstone and shale above the sandstone slab; these weak layers in the Cho-Lan formation may have served as the sliding zone for the 1942 slide.

Figure 19.

Breakaway valley (photo taken in April 1987). Fine debris fills in floor of valley, which is about level. Dip of rock layers on cliff wall is 12° on average.

Figure 19.

Breakaway valley (photo taken in April 1987). Fine debris fills in floor of valley, which is about level. Dip of rock layers on cliff wall is 12° on average.

Figure 20.

South-facing (upslope) cliff of breakaway valley (photo taken in July 1993). Vertical open cracks indicate breakaway movement within mass of southern cliff.

Figure 20.

South-facing (upslope) cliff of breakaway valley (photo taken in July 1993). Vertical open cracks indicate breakaway movement within mass of southern cliff.

Laboratory Analysis

Boring and sampling

Three vertical holes in the rockslide area and two holes in Tsao-Ling Village were drilled (Fig. 15): BH1 (60 m), BH2 (60 m), BH3 (120 m), BH4 (55 m), and BH5 (50 m). Continuous NX cores were taken for geological identification and laboratory investigation. Undisturbed block samples (S1, S2, S3, S4, and S5) were also taken from fresh exposures in the side-walls of gullies (Fig. 15). S1 and S2 were recovered to represent the Chin-Shui formation, S3 represents the Cho-Lan formation, and S4 and S5 represent the Ta-Wuo sandstone member. The stratigraphic locations of block samples and boreholes are shown in Figure 21.

Figure 21.

Generalized stratigraphic section showing rock formations in landslide area and locations of drill holes (BH) and block samples (S).

Figure 21.

Generalized stratigraphic section showing rock formations in landslide area and locations of drill holes (BH) and block samples (S).

Laboratory testing on block samples

General physical property testing was performed on five block samples of shale and muddy sandstone. Test results are summarized in Table 5 and Table 6. They are nonplastic or low-plastic silty or clayey materials in nature. Quartz and clay minerals are the two most abundant minerals in these shaley formations (Table 7). X-ray diffractometer analyses show that the clay fraction of the shale contains very little or no montmorillonite (Table 8). Expansion tests also show that the shale has very little (0.25%) expansion upon saturation. Slake durability test results are shown in Table 9; sample S1 (Chin-Shui shale), with a two-cycle slake durability index of 4.8%, is the least durable rock tested.

Table 5.

Summary of Index Test Results on Block Samples

Sample number*Nature moisture content (%)Dry density (Mg/m3)Specific gravityVoid ratioLiquid limit (%)Plastic limit (%)Plastic index (%)
S14.02.322.660.15271710
S22.52.352.680.1421NPNP
S32.22.512.660.0625NPNP
S41.52.552.670.0524168
S52.12.532.690.0623149
Sample number*Nature moisture content (%)Dry density (Mg/m3)Specific gravityVoid ratioLiquid limit (%)Plastic limit (%)Plastic index (%)
S14.02.322.660.15271710
S22.52.352.680.1421NPNP
S32.22.512.660.0625NPNP
S41.52.552.670.0524168
S52.12.532.690.0623149

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

*S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

Nonplastic

Table 6.

Particle Size Distribution of Block Samples

Sample number*Grain Size
Sand(%) >0.075mmSilt (%) 0.075–0.005mmClay (%) <0.005mm
S111782
S2331750
S3253243
S471182
S543165
Sample number*Grain Size
Sand(%) >0.075mmSilt (%) 0.075–0.005mmClay (%) <0.005mm
S111782
S2331750
S3253243
S471182
S543165

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

* S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

Table 7.

Optical Mineralogy of Block Samples

Sample number*Quartz (%)Clay mineral (%)Calcite (%)Opaque mineral (%)
S1524152
S2356131
S3643231
S4682633
S5633241
Sample number*Quartz (%)Clay mineral (%)Calcite (%)Opaque mineral (%)
S1524152
S2356131
S3643231
S4682633
S5633241

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

* S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

Table 8.

Clay Mineralogy of Block Samples

Sample number*Illite (%)Kaolinite (%)Chlorite (%)Montmorillonite (%)
S15619214
S26516190
S36615190
S46616180
S55816240
Sample number*Illite (%)Kaolinite (%)Chlorite (%)Montmorillonite (%)
S15619214
S26516190
S36615190
S46616180
S55816240

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

* S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

Table 9.

Results of Slake Durability Test on Block Samples

Sample number*Slake durability index (Id2) (%)Classification†
S14.8very low
S267.5medium
S369.8medium
S440.0low
S528.6very low
Sample number*Slake durability index (Id2) (%)Classification†
S14.8very low
S267.5medium
S369.8medium
S440.0low
S528.6very low

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

*S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

GROUNDWATER

In boreholes BH1 and BH2, which were drilled into Chin-Shui shale formation, the water table was encountered at depth of 51 m (or 729 m elevation) and 42 m (or 583 m elevation), respectively. In all the gullies in and below the breakaway valley, abundant water flow can be observed even in dry season. The gullies are all located at elevations below the groundwater table. Although a certain amount of water flow can be found in erosion gullies, no groundwater table is found in the area from the crest of the breakaway valley to the head scarp. Borehole BH3 was drilled to a depth of 120 m into the Cho-Lan formation in this place, yet no groundwater was encountered.

Discussion and Conclusions

The downstream region of Ching-Shui River has undergone a great shortage of water since May 18, 1951, when the landslide dam burst and the impounded water of the dammed lake was released. The pressure to build a replacement reservoir at the landslide site has always been high, and feasibility studies and planning have been done; however, all attempts to proceed were held up due to the fear of reactivation of the Tsao-Ling rockslide. We, together with colleague Tsai (Lee et al., 1993), were asked to look into the stability of the Tsao-Ling slope once again. The stress distribution and stability of the remaining slope (including the slope above the head scarp) were analyzed (Lee et al., 1993; Hung et al., 1994). Analysis and mapping of open joints indicated that a tensile zone exists in the slope between 1050 and 1100 m elevation. It is concluded that a rockslide involving a mass movement of ~50 × 106 m is possible.

Researchers in various disciplines and practitioners of different fields have contributed very much to the understanding of the mechanisms of the Tsao-Ling rockslides. In our opinion, the Achilles heel of the Tsao-Ling rockslide research has been the insufficient data on the shear strength and compressibility of Chin-Shui shale formation for stability analysis. To move heavy boring equipment to the site has been difficult and very expensive due to the weather, rough terrain, and the scale of the site. Nevertheless, it was thought worthwhile to record the work that has been carried out at Tsao-Ling to this date.

ADDITIONAL NOTE

On September 21, 1999. another massive landslide (~125 × 106 m3; Figs. 21 and 22) occurred on the Tsao-Ling dip slope, triggered by the Chi-Chi earthquake (M 7.3) at an epicentral distance of ~35 km. The Ching-Shui River was dammed again, forming a 5-km-long lake. An overflow channel was excavated to stabilize the water level in the landslide-dammed lake. The most dramatic element of the 1999 Tsao-Ling slide was that only 20% (or ~25 × 106 m3) of the sliding mass dropped into the valley of the Ching-Shui River; the rest of the sliding mass (of ~100 × 106 m3) jumped over the Ching-Shui River before making a landing on the remaining part of the former landslide dam to the south of the river. People who lived behind the crest (at an elevation of ~1000 m) of the dip slope were moved with the sliding mass; 7 people survived, but unfortunately 29 lives were lost. Some of the debris of their houses and a van can be seen in Figure 22.

Figure 22.

View up sliding surface of September 1999 rockslide on Tsao-Ling dip slope, triggered by Chi-Chi earthquake (M 7.3). Note debris in foreground. Remains of houses at left are visible at left of photograph, as is van. These were transported vertical distance of ~600 m during landslide.

Figure 22.

View up sliding surface of September 1999 rockslide on Tsao-Ling dip slope, triggered by Chi-Chi earthquake (M 7.3). Note debris in foreground. Remains of houses at left are visible at left of photograph, as is van. These were transported vertical distance of ~600 m during landslide.

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Acknowledgments

We thank S.G. Evans, C.F. Watts, and J.V. DeGraff for their critical review and constructive comments on the manuscript.

Figures & Tables

Figure 1.

Location map of site of Tsao-Ling rockslides, central Taiwan.

Figure 1.

Location map of site of Tsao-Ling rockslides, central Taiwan.

Figure 2.

Generalized geologic map of Tsao-Ling rockslide area. A-A' is central profile line in Figure 4.

Figure 2.

Generalized geologic map of Tsao-Ling rockslide area. A-A' is central profile line in Figure 4.

Figure 3.

Topographic map of Tsao-Ling rockslide area before December 17, 1941 (modified from Tai-Pei Observatory, 1942, Figure 34a). Contour interval is 50 m. A–A' is central profile line of slide area. Refer to Figure 4A for profile.

Figure 3.

Topographic map of Tsao-Ling rockslide area before December 17, 1941 (modified from Tai-Pei Observatory, 1942, Figure 34a). Contour interval is 50 m. A–A' is central profile line of slide area. Refer to Figure 4A for profile.

Figure 4.

Cross sections through center of Tsao-Ling rockslide area (see Figs. 3, 14, and 15 for location of cross section). A: Reconstructed profiles of 1941 and 1942 slides. B: Reconstructed profiles of 1979 slide. C: Reconstructed profile of slope in 1993; a–f are referred to in the text.

Figure 4.

Cross sections through center of Tsao-Ling rockslide area (see Figs. 3, 14, and 15 for location of cross section). A: Reconstructed profiles of 1941 and 1942 slides. B: Reconstructed profiles of 1979 slide. C: Reconstructed profile of slope in 1993; a–f are referred to in the text.

Figure 5.

Intensity contour map of 1942 Chia-Yi earthquake. Numbers on map are intensities according to Japanese Meteorological Agency system (Tai-Pei Observatory, 1942). Asterisk indicates location of Tsao-Ling rockslide. Pound sign indicates A-Li-Shan rainfall gage station.

Figure 5.

Intensity contour map of 1942 Chia-Yi earthquake. Numbers on map are intensities according to Japanese Meteorological Agency system (Tai-Pei Observatory, 1942). Asterisk indicates location of Tsao-Ling rockslide. Pound sign indicates A-Li-Shan rainfall gage station.

Figure 6.

Tsao-Ling rockslide area after 1941 rockslide event caused by Chia-Yi earthquake (M 7.1. December 17, 1941). Debris at lower left corner was house near head scarp before earthquake (Tai-Pei Observatory, 1942).

Figure 6.

Tsao-Ling rockslide area after 1941 rockslide event caused by Chia-Yi earthquake (M 7.1. December 17, 1941). Debris at lower left corner was house near head scarp before earthquake (Tai-Pei Observatory, 1942).

Figure 7.

Simplified map of Tsao-Ling rockslide area after December 17, 1941, and before March 30, 1942 (modified after Hung. 1980, Fig. 2). Heights refer to height of landslide dam.

Figure 7.

Simplified map of Tsao-Ling rockslide area after December 17, 1941, and before March 30, 1942 (modified after Hung. 1980, Fig. 2). Heights refer to height of landslide dam.

Figure 8.

General view of head scarp and remaining slope, looking from landslide dam toward remaining slope and head scarp (photograph taken December 1976). Rocks forming head scarp are alternating sandstones and shales of Cho-Lan formation.

Figure 8.

General view of head scarp and remaining slope, looking from landslide dam toward remaining slope and head scarp (photograph taken December 1976). Rocks forming head scarp are alternating sandstones and shales of Cho-Lan formation.

Figure 9.

Remote view of remaining part of landslide dam, looking from top of head scarp (photograph taken December 1976). Landslide debris is noted (⋆) on opposite side of Ching-Shui River. View is downstream.

Figure 9.

Remote view of remaining part of landslide dam, looking from top of head scarp (photograph taken December 1976). Landslide debris is noted (⋆) on opposite side of Ching-Shui River. View is downstream.

Figure 10.

Close-up view of small part of sliding surface, August 22, 1979, 1 week after sliding. Slide surface is in Chin-Shui shale formation. Note slickensides in central part of photograph and hammer (32 cm) for scale on sliding surface.

Figure 10.

Close-up view of small part of sliding surface, August 22, 1979, 1 week after sliding. Slide surface is in Chin-Shui shale formation. Note slickensides in central part of photograph and hammer (32 cm) for scale on sliding surface.

Figure 11.

View upstream of dammed lake and landslide dam, looking from toe of remaining slope. Water surface of dammed lake was on verge of overtopping (photo taken August 22, 1979, 36 h before overtopping).

Figure 11.

View upstream of dammed lake and landslide dam, looking from toe of remaining slope. Water surface of dammed lake was on verge of overtopping (photo taken August 22, 1979, 36 h before overtopping).

Figure 12.

A: Aerial photograph of 1979 rockslide event on August 19, 1979, 4 days after slide and 4 days before overtopping. B: Map of area shown in A. Features include new scarp, sliding area, dammed lake, and landslide dam.

Figure 12.

A: Aerial photograph of 1979 rockslide event on August 19, 1979, 4 days after slide and 4 days before overtopping. B: Map of area shown in A. Features include new scarp, sliding area, dammed lake, and landslide dam.

Figure 13.

Aerial photograph, taken in 1980. of greater Tsao-Ling area. A: Head scarp. B: 1979 scarp. C: Ching-Shui River. D: Ching-Shui River to Cho-Shui River. E: Old landslide dam F: Area of Tsao-Ling village. Breakaway valley is at left of B (arrow). Scale bar at top left is ~1000 m.

Figure 13.

Aerial photograph, taken in 1980. of greater Tsao-Ling area. A: Head scarp. B: 1979 scarp. C: Ching-Shui River. D: Ching-Shui River to Cho-Shui River. E: Old landslide dam F: Area of Tsao-Ling village. Breakaway valley is at left of B (arrow). Scale bar at top left is ~1000 m.

Figure 14.

Topographic map of Tsao-Ling rockslide area in 1977. A–A' is central profile line of slide area. Refer to Figure 4B for profile. Contour interval is 20 m.

Figure 14.

Topographic map of Tsao-Ling rockslide area in 1977. A–A' is central profile line of slide area. Refer to Figure 4B for profile. Contour interval is 20 m.

Figure 15.

Topographic map of Tsao-Ling rockslide area, 1980. Contour interval is 20 m. A–A' is central profile line of slide area. Refer to Figure 4B for profile. S1-S5 indicate locations of block samples taken in 1993. BH1-BH3 indicate locations of drill holes drilled in 1981 and 1982.

Figure 15.

Topographic map of Tsao-Ling rockslide area, 1980. Contour interval is 20 m. A–A' is central profile line of slide area. Refer to Figure 4B for profile. S1-S5 indicate locations of block samples taken in 1993. BH1-BH3 indicate locations of drill holes drilled in 1981 and 1982.

Figure 16.

Histograms of monthly and daily precipitation in year and month of rockslide and landslide dam break. Solid horizontal lines indicate yearly (and monthly) means; ▼ is time of rockslide event; X is time of landslide dam break event.

Figure 16.

Histograms of monthly and daily precipitation in year and month of rockslide and landslide dam break. Solid horizontal lines indicate yearly (and monthly) means; ▼ is time of rockslide event; X is time of landslide dam break event.

Figure 17.

Pole plots and contoured stereonets of joints (boxes with ticks) and bedding plane (arrows) in Tsao-Ling rockslide area. A: Discontinuities at crown of scarp. B: Discontinuities at crown of northern (upslope) cliff of breakaway valley. C: Discontinuities on southern (downslope) cliff of breakaway valley.

Figure 17.

Pole plots and contoured stereonets of joints (boxes with ticks) and bedding plane (arrows) in Tsao-Ling rockslide area. A: Discontinuities at crown of scarp. B: Discontinuities at crown of northern (upslope) cliff of breakaway valley. C: Discontinuities on southern (downslope) cliff of breakaway valley.

Figure 18.

Section at river level of landslide dam materials (photo taken in July, 1993; location a in Fig. 4C). Base of landslide debris is indicated by dashed line. Below dashed line, B is top of Chin-Shui shale formation. Above dashed line is debris material of 1941 and 1942 rockslide events. Sandstone block at lower right is ~15 m high.

Figure 18.

Section at river level of landslide dam materials (photo taken in July, 1993; location a in Fig. 4C). Base of landslide debris is indicated by dashed line. Below dashed line, B is top of Chin-Shui shale formation. Above dashed line is debris material of 1941 and 1942 rockslide events. Sandstone block at lower right is ~15 m high.

Figure 19.

Breakaway valley (photo taken in April 1987). Fine debris fills in floor of valley, which is about level. Dip of rock layers on cliff wall is 12° on average.

Figure 19.

Breakaway valley (photo taken in April 1987). Fine debris fills in floor of valley, which is about level. Dip of rock layers on cliff wall is 12° on average.

Figure 20.

South-facing (upslope) cliff of breakaway valley (photo taken in July 1993). Vertical open cracks indicate breakaway movement within mass of southern cliff.

Figure 20.

South-facing (upslope) cliff of breakaway valley (photo taken in July 1993). Vertical open cracks indicate breakaway movement within mass of southern cliff.

Figure 21.

Generalized stratigraphic section showing rock formations in landslide area and locations of drill holes (BH) and block samples (S).

Figure 21.

Generalized stratigraphic section showing rock formations in landslide area and locations of drill holes (BH) and block samples (S).

Figure 22.

View up sliding surface of September 1999 rockslide on Tsao-Ling dip slope, triggered by Chi-Chi earthquake (M 7.3). Note debris in foreground. Remains of houses at left are visible at left of photograph, as is van. These were transported vertical distance of ~600 m during landslide.

Figure 22.

View up sliding surface of September 1999 rockslide on Tsao-Ling dip slope, triggered by Chi-Chi earthquake (M 7.3). Note debris in foreground. Remains of houses at left are visible at left of photograph, as is van. These were transported vertical distance of ~600 m during landslide.

Table 1.

Historical Events at Tsao-Ling Rockslide, Taiwan, 1862–1979

DateTriggerProcessEffectsReference§
June 6, 1862Earthquake (M = 6.0–7.0)Landslide: formation of a landslide dam.*1, 2, 4, 5, 6
1898UnknownBreach of the landslide dam.*5, 6
December 17, 1941Earthquake (M = 7.1)Landslide—84 × 106 m3 in volume; formation of a landslide dam 70–200 m in height, formation of a dammed up lake containing 12.8 M m3 of water.36 persons killed; 59 houses damaged.1, 2, 3, 4, 5, 6, 7
August 10, 1942Rainfall: 3 day cumulative precipitation of 770 mmLandslide—100 × 106 m3 in volume; height of landslide dam increased from ~140–217 m in height; formation of a larger dammed up lake containing 157 × 106 m3 of water.1 person buried; 1 house damaged.2, 3, 4, 5, 6, 7
May 18, 1951Rainfall: 5 day cumulative precipitation of 776 mmBreak of the landslide dam. Release of 120 × 106 m3 of water.Flooding of 3000 ha of arable land; 137 persons killed; 1200 houses damaged.2, 3, 4, 5, 6, 7
August 15, 1979Rainfall: 2 day cumulative precipitation of 327 mmLandslide—26 × 106 m3 in volume; formation of a landslide dam, 90 m in height; formation of a dammed up lake containing 40 × 106 m3 of water.4, 5, 6, 7
August 24, 1979Rainfall: 2 day cumulative precipitation of 624 mmBreach of the landslide dam. Release of 40 × 106 m3 of water.Two bridges destroyed.4, 5, 6, 7
DateTriggerProcessEffectsReference§
June 6, 1862Earthquake (M = 6.0–7.0)Landslide: formation of a landslide dam.*1, 2, 4, 5, 6
1898UnknownBreach of the landslide dam.*5, 6
December 17, 1941Earthquake (M = 7.1)Landslide—84 × 106 m3 in volume; formation of a landslide dam 70–200 m in height, formation of a dammed up lake containing 12.8 M m3 of water.36 persons killed; 59 houses damaged.1, 2, 3, 4, 5, 6, 7
August 10, 1942Rainfall: 3 day cumulative precipitation of 770 mmLandslide—100 × 106 m3 in volume; height of landslide dam increased from ~140–217 m in height; formation of a larger dammed up lake containing 157 × 106 m3 of water.1 person buried; 1 house damaged.2, 3, 4, 5, 6, 7
May 18, 1951Rainfall: 5 day cumulative precipitation of 776 mmBreak of the landslide dam. Release of 120 × 106 m3 of water.Flooding of 3000 ha of arable land; 137 persons killed; 1200 houses damaged.2, 3, 4, 5, 6, 7
August 15, 1979Rainfall: 2 day cumulative precipitation of 327 mmLandslide—26 × 106 m3 in volume; formation of a landslide dam, 90 m in height; formation of a dammed up lake containing 40 × 106 m3 of water.4, 5, 6, 7
August 24, 1979Rainfall: 2 day cumulative precipitation of 624 mmBreach of the landslide dam. Release of 40 × 106 m3 of water.Two bridges destroyed.4, 5, 6, 7

* Interview of Tsao-Ling villagers by Hung in 1976.

† Authors were unable to obtain significant data.

Table 2.

Stratigraphy and Lithology in the Tsao-Ling Area

EpochStratigraphyLithologyThickness (m)
HoloceneAlluviumClay, sand, gravel1–50
New debrisClay, sand, rock block1–20
Old debrisClay, sand, rock block1–170
PleistoceneTerraceClay, sand, gravel1–10
PlioceneCho-Lan formationGreenish-gray to pale yellowish fine-grained sandstone with a variable amount of shale and sandstone alternation1000
Chin-Shui shale formationDark-gray shale and sandy shale 80–150
MioceneTa-Wuo sandstone memberMassive-gray to greenish-gray muddy sandstone with shale lamination1100
Shih-Liu-Feng shale memberDark-gray shale100
EpochStratigraphyLithologyThickness (m)
HoloceneAlluviumClay, sand, gravel1–50
New debrisClay, sand, rock block1–20
Old debrisClay, sand, rock block1–170
PleistoceneTerraceClay, sand, gravel1–10
PlioceneCho-Lan formationGreenish-gray to pale yellowish fine-grained sandstone with a variable amount of shale and sandstone alternation1000
Chin-Shui shale formationDark-gray shale and sandy shale 80–150
MioceneTa-Wuo sandstone memberMassive-gray to greenish-gray muddy sandstone with shale lamination1100
Shih-Liu-Feng shale memberDark-gray shale100
Table 3.

Geometry of 1941, 1942, and 1979 Rockslide Events

YearElevation (m)Dam Height (m)Volume of Rockslide debris (106m3)Reference
River bedTop of landslide dam
194114048Hsu and Leung (1977)
70–200100–150Hung (1980)
42049070Chang and Lee (1989)
39084Lee et al. (1993)
1942217120Hsu and Leung (1977)
140–170150–200Hung (1980)
590170Chang and Lee (1989)
600210100Lee et al. (1993)
1979526>5Hung (1980)
46056010026Lee et al. (1993)
YearElevation (m)Dam Height (m)Volume of Rockslide debris (106m3)Reference
River bedTop of landslide dam
194114048Hsu and Leung (1977)
70–200100–150Hung (1980)
42049070Chang and Lee (1989)
39084Lee et al. (1993)
1942217120Hsu and Leung (1977)
140–170150–200Hung (1980)
590170Chang and Lee (1989)
600210100Lee et al. (1993)
1979526>5Hung (1980)
46056010026Lee et al. (1993)
Table 4.

Orientation of Joint Sets in the Tsao-Ling Rockslide Area

LocationSetNumber of measurements
ABCD
Crown of head scarpN6°E/80°E N12°W/78°EN72°E/83° E28
Crown of northern cliff of breakaway valleyN5°E/82°EN76°E/83°NN49°E/89°N110
Toe-wall of breakaway valleyN18°W/89°EN82°E/83°NN56°W/79°N36
LocationSetNumber of measurements
ABCD
Crown of head scarpN6°E/80°E N12°W/78°EN72°E/83° E28
Crown of northern cliff of breakaway valleyN5°E/82°EN76°E/83°NN49°E/89°N110
Toe-wall of breakaway valleyN18°W/89°EN82°E/83°NN56°W/79°N36
Table 5.

Summary of Index Test Results on Block Samples

Sample number*Nature moisture content (%)Dry density (Mg/m3)Specific gravityVoid ratioLiquid limit (%)Plastic limit (%)Plastic index (%)
S14.02.322.660.15271710
S22.52.352.680.1421NPNP
S32.22.512.660.0625NPNP
S41.52.552.670.0524168
S52.12.532.690.0623149
Sample number*Nature moisture content (%)Dry density (Mg/m3)Specific gravityVoid ratioLiquid limit (%)Plastic limit (%)Plastic index (%)
S14.02.322.660.15271710
S22.52.352.680.1421NPNP
S32.22.512.660.0625NPNP
S41.52.552.670.0524168
S52.12.532.690.0623149

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

*S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

Nonplastic

Table 6.

Particle Size Distribution of Block Samples

Sample number*Grain Size
Sand(%) >0.075mmSilt (%) 0.075–0.005mmClay (%) <0.005mm
S111782
S2331750
S3253243
S471182
S543165
Sample number*Grain Size
Sand(%) >0.075mmSilt (%) 0.075–0.005mmClay (%) <0.005mm
S111782
S2331750
S3253243
S471182
S543165

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

* S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

Table 7.

Optical Mineralogy of Block Samples

Sample number*Quartz (%)Clay mineral (%)Calcite (%)Opaque mineral (%)
S1524152
S2356131
S3643231
S4682633
S5633241
Sample number*Quartz (%)Clay mineral (%)Calcite (%)Opaque mineral (%)
S1524152
S2356131
S3643231
S4682633
S5633241

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

* S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

Table 8.

Clay Mineralogy of Block Samples

Sample number*Illite (%)Kaolinite (%)Chlorite (%)Montmorillonite (%)
S15619214
S26516190
S36615190
S46616180
S55816240
Sample number*Illite (%)Kaolinite (%)Chlorite (%)Montmorillonite (%)
S15619214
S26516190
S36615190
S46616180
S55816240

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

* S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

Table 9.

Results of Slake Durability Test on Block Samples

Sample number*Slake durability index (Id2) (%)Classification†
S14.8very low
S267.5medium
S369.8medium
S440.0low
S528.6very low
Sample number*Slake durability index (Id2) (%)Classification†
S14.8very low
S267.5medium
S369.8medium
S440.0low
S528.6very low

Note: Refer to Figure 15 for sample locations; Figure 27 for stratigraphic locations.

*S1: Sample (shale) collected in lower portion of Cho-Lan formation; S2: Sample (siltstone) collected in lower portion of Cho-Lan formation; S3: Sample collected in upper portion of Chin-Shui shale formation; S4: Sample collected in middle portion of Chin-Shui shale formation; S5: Sample collected in lower portion of Chin-Shui shale formation.

Contents

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