Rainfall-triggered large landslides on 15 December 2005 in Van Canh District, Binh Dinh Province, Vietnam
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Landslides are one of the most dangerous hazards in Vietnam. Most landslides occur at excavated slopes, and natural slope failures are rare in the country. However, the volume of natural slope failures can be very significant and can badly affect large areas. After a long period of heavy rainfall in the fourth quarter of 2005 in Van Canh district, a series of landslides with volumes of 20,000–195,000 m3 occurred on 15 December 2005. The travel distances for the landslides reached over 300–400 m, and the landslides caused some remarkable loud booming noises. The failures took place on natural slopes with unfavorable geological settings and slope angles of 28–31°. The rainfall in the fourth quarter of 2005 is estimated to have a return period of 100 years and was the main triggering factor. Because of the large affected area and low population density, resettling people from the dangerous landslide-prone residential areas to safer sites was the most appropriate solution. In order to do so, a map of landslide susceptibility was produced that took into account slope angle, distance to faults, and slope aspect. The map includes four levels from low to very high susceptibility to landslides.
KeywordsLarge landslide Rainfall Fault Landslide susceptibility Vietnam
Landslides globally cause major disasters every year and rank seventh as a cause of numbers of people killed by natural disasters during the period of 1992–2001 (Nadim et al. 2006). Currently, the number of disastrous landslides appears to be increasing (Schuster and Highland 2007). Landslides are among the most dangerous geohazards in Vietnam, causing annual damage of nearly 100 million dollars (US) (Tam 2001). Extensive landsliding often takes place during tropical cyclones. Most of these landslides occur on excavated slopes, especially along the national highways such as No. 2, No. 3, No. 6, and the Hochiminh route. Natural slope failures are rarely recorded, as they often occur in remote areas and do not come to the attention of the community. A change of climate in recent years has gradually brought increasing problems, as extreme climate events (typhoons, storms, and tropical depressions) happen more often, and with higher intensity. The amount of heavy rainfall in these extreme events also breaks existing records more frequently. The figure of the 10-year, 50-year, or even a century return period in some areas can appear year by year. Many large landslides have taken place on slopes that were for a long time considered as stable ones (Duc 2010).
In this study, the geological and geomorphologic settings, weathering crust, geotechnical properties of residual soils, and their relationships to landslides were investigated. Then a map of landslide susceptibility was created to provide initial information for resettling people from dangerous landslide-prone residential areas to safer locations.
Materials and Methods
The data used in the study included a topographical map at a scale of 1:10,000, a geological map at a scale of 1:50,000, and daily rainfall monitored from 1976 to 2010 at a hydrological station in Van Canh town. Additional data were mainly gathered from site investigations in Van Canh district that were carried out in August 2006 and June 2007. These investigations included the geological settings, characteristics of weathering crust, geotechnical properties of soils and rocks, and landslide properties. Detailed investigations were carried out at over 16 km2 at Lang Chom commune and adjacent areas where the three largest landslides occurred. Electrical resistivity was measured along six sections and a geological map at the scale of 1:10,000 was made.
All maps (topography and geology) were then digitized so that a map of landslide susceptibility could be digitally produced by overlaying factors affecting landslide susceptibility, including slope angle, distance to faults, and slope aspect using ILWIS—a GIS-based software. Thematic maps of slope angle and aspect were created from topographical maps (details are available in ILWIS 3.0 Academic User’s Guide 2001). The distance from faults was determined from geological maps. Slope angles were categorized through stability analysis using the infinite-slope-analysis method (Duncan 1996).
Geographical location was determined using a Garmin GPS (GPS 72) with an accuracy of about 5–10 m.
Angles and heights of slopes were measured, and the description of a landslide included further information such as slope angles of adjacent areas, upper and lower length of landslide, thickness of the sliding mass, and characteristics of the slip surface. These data were then used to calculate areas of cross-sections at various parts of the landslide. The landslide volume was estimated as the product of average area of cross-sections and length of the landslide. The date the landslide occurred was determined by the author after conversations with local authorities and residents.
Geological descriptions included lithological composition, color and initial classification of rocks, bedding surfaces, dip angles, fault, and joint systems.
Residual soil descriptions included the thickness and distribution of the residual soil layers. Each layer was described in terms of soil composition, color, moisture, and consistency.
Surface and groundwater observations included gullies, streams on the slope, and existence and discharge of groundwater at the slope (if any). Groundwater level was measured in adjacent wells of local residents, including information based on conversations with the owners about seasonal discharge and water-level changes.
Vegetation coverage information included types of trees and brush, density of coverage, and comparison with adjacent areas.
Undisturbed soil samples were taken at the landslides; the depths of sampling were 0.2–0.5 m. Thirteen samples were retrieved at three large landslides, with samples taken at the landslide main scarp, body, and foot. At each smaller landslide, one or two samples were also taken. Soil samples subsequently were analyzed in the laboratory to define geotechnical properties. The tests were performed according to the specifications of ASTM (American Society for Testing and Materials). A modification was made for analysis of grain-size distribution, in which all steps followed ASTM D-422 (2001a), but the diameters of sieves were 20, 10, 4.75, 2, 1.0, 0.5, 0.25, and 0.074 mm. Soils are classified by the Unified Soil Classification System (USCS—ASTM D 2487 (2001b)).
Average monthly rainfall in Van Canh district (Huong 2004)
The main purpose of geophysical investigation is to provide more information for assessing landslide susceptibility around current residential areas and tentative sites for resettlement. It was designed to include information on layering of the weathering crust, and especially to define potential slip surfaces, which are tentatively assumed to be fault and/or joint planes, and interfaces between residual soils and/or high fractured rocks with intact bedrock.
Results and Discussion
Geological and geomorphologic settings
Faults of north–south and northeast–southwest oriented are dominant. The northeast–southwest oriented Ong mountain fault is a normal fault. A new fault was discovered during the detail investigation of geological settings, named the Ba mountain fault. It is a normal fault with strike of 165–345° and dip angle of 45° (Fig. 2). The fault system, especially the 165–345° fault leads to many cracked blocks of bedrock which accelerates the weathering process that can make conditions suitable for the sliding of large rock and soil masses. Fault planes even form the slip surfaces of some large landslides (details in “Landslide properties”).
The upper layer is residual soils which are classified as silt (ML), clayey sand (SC), and well-graded sand (SW). The thickness varies from 0.5 to 6.2 m. The layer has resistivity ranging from 174 to 4,136 Ω m. This layer is covered by trees, Acacia mangium, at a medium density. The trees are cut and re-planted every 3 years for the paper industry.
The second layer is fractured and strongly weathered bedrock with a resistivity of 62–6,318 Ω m. The thickness varies over a large range: from 1.2 to 50.6 m (Fig. 4), greater thicknesses occur at fault zones and above vein rocks.
The lower layer is intact bedrock with a resistivity of 1,168–50,175 Ω m.
Geotechnical properties of residual soils and stability analysis
Geotechnical properties of residual soils
Residual soils from different bedrocks
SC (Xa Lam Co formation)
SW (Van Canh complex)
ML (Deo Ca complex)
Number of test
Grain sizes (%)
Water content (%)
Wet density (g/cm3)
Dry density (g/cm3)
Specific gravity (g/cm3)
Saturated degree (%)
Liquid limit (%)
Plastic limit (%)
−0.61 (−1.12 ÷ −0.08)
−0.63 (−0.73 ÷ −0.37)
Hydraulic conductivity (m/s)
2 × 10−5
5 × 10−5
3 × 10−6
Angle of internal friction (deg.)
As can be seen in Table 2, residual soils of clayey sand and silt have a rather high natural degree of saturation (almost above 70 %), although samples were taken in the dry season. The main reason for this is the frequently high atmospheric moisture. Such permanent saturation may reduce the effect of rainfall as a trigger of slides. Saturated hydraulic conductivities of the soils ranged from 3 × 10−6 to 5 × 10−5 m/s in the most torrential rains, which occurred on 23 and 27 October 2005, the maximum rain intensity was 12 mm/h (equivalent to about 3 × 10−6 m/s). Thus conductivities are rather high in comparison to rain intensity in the study area, and rainwater can easily infiltrate into the slopes, increasing the degree of saturation of the soils, and reducing slope stability.
Determination of the factor of safety (Fs) using the following equation:
Determination of parameters A and B from the following equations:
Determination of ru, the pore pressure ratio, as follows:
Based upon field observations made during this study, seepage was considered to occur parallel to the slope face. In the analysis, the saturated fraction of soil mantle (m = X/T) was considered to range from 0.5 to 1. Three types of residual soils (clayey sand, well-graded sand, and silt) are taken into the calculation. As observed at the recorded landslides, depths from the slope surface to the surface of sliding (H) in the soils of clayey sand, well-graded sand, and silt are 3.5, 1.5, and 4.0 m, respectively. Strength parameters are average values of φ′ and c′ (Table 2).
Landslides triggered by heavy rainfall
Monthly rainfall in Van Canh with different return frequency (mm) (Huong 2004)
P = 5 %
The above results show that the landslides occurred after 3 months of heavy rain from September to December 2005. Heavy rainfall at the beginning of wet season (September and October) increased the water content of the soils but could not trigger landslides. In fact, the rainfall in October 2005 was even larger than in December 2005. As the results of slope stability analysis, we can predict that landslides were triggered when the slopes were almost saturated. The antecedent rainfall in October and November made soils and rock joint surfaces wetter, conditions that are more favorable for later showers to induce landslides. The total rainfall that triggered landslides was 1,260 mm, which occurred after about 1 month (17 November–15 December 2005) of heavy rains.
Comments on loud noises
Loud booms accompanied some of the large landslides in Van Canh district and caused a lot of anxiety among local people. According to local eyewitnesses, when a large landslide occurred there was a loud noise following by weaker ones. On hearing these noises some local people considered that a severe earthquake would come or “the angel of mountain was angry”. Natural slope failures in Vietnam often occur in remote areas where the local knowledge is limited, and the people are unaware of possible early indications of a landslide. It increases risk in landslide-prone areas. Local authorities therefore need a scientific explanation of the phenomenon to enhance residents’ awareness.
At the slip surfaces of landslides, some fractures were recognized in the intact bedrock (Fig. 7d). Although the initial loud noises emanated from the sliding areas, they were not followed by rolling rocks or dust clouds. These sounds may have been the result of intact brittle rock fracturing in the slope. A similar event was recorded in the Afternoon Creek rockslide in the state of Washington, USA (Strouth et al. 2006) and in a catastrophic rockslide–debris avalanche at St. Bernard, Southern Leyte, the Philippines (Catane et al. 2007). The noises that followed may be due to the falling and rolling of rock boulders.
Proposals for counter measures
The population density in Van Canh district is very low. The total land area with potential to slide is 6 km2 which is 0.7 % of the whole district (Ha 2011). Therefore, expensive structural measures are not necessary. Instead, the measure of resettling vulnerable communes to safer places is much more effective. Detailed investigation in Lang Chom commune showed that the east side of Lang Chom commune was less vulnerable to landslides than the west side, especially downslope of geophysical sections T5 and T6. In this place, residual soils are which weathered from rocks of the Van Canh complex and can be classified as SW. The thicknesses of residual soils is 1–5 m. Slopes are gentle, with angles of 5–10° at the T6 section and 5–15° at the T5 section. A part of Lang Chom commune was resettled in a location downslope of the T6 section.
Affecting factors and scores of landslide susceptibility
Distance to fault (m)
Natural slope failures in Van Canh district took place as a series of large landslides (20,000–195,000 m3) that occurred due to a prolonged period of rainfall from early September to the middle of December 2005. Landslides occurred where the slopes were at angles of 28–31° and the thickness of residual soils was more than 3 m. The main slip surface is a fault plane.
The travel distance of landslides reached over several hundreds meters, seriously threatening residential areas and fields near the slopes. The effective counter measure is to resettle local people to safe sites and disseminate the early warning signs of potential landslides (such as tension cracks and scarps) in the area to the local people.
The author would like to thank very much two anonymous reviewers for their valuable comments. Many thanks to Drs. Mauri and Eileen McSaveney (GNS Science, New Zealand) for English editing and comments. All these contributions have helped a lot to improve the quality of the paper.
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