GEOTECHNICAL HAZARDS AND DISASTER MITIGATION TECHNOLOGIES Jiro KUWANO1, Mairaing WARAKORN2 , Mark ZARCO3, and Mary Ann ADAJAR4 1
Geosphere Research Institute-Saitama University Department of Civil Engineering – Kasetsart University 3 Department of Civil Engineering, University of the Philippines-Diliman 4 Department of Civil Engineering, De La Salle University – Manila 2
Abstract : Asian countries like Japan, Philippines and Thailand are one of the most disaster-prone regions in the world. Each year, different types of natural disasters cause countless deaths, disruption of commerce, and destruction of homes, critical infrastructure and the environment. Because of these tremendous losses of life and damage to property, there is a critical need for increase efforts in understanding the causes of disasters, evaluating their risk, and developing procedures for mitigating their effects. By effective mitigation techniques, we can reduce the damage, reduce the severity of its effects and reduce human sufferings that result from disasters. This paper describes some major geotechnical hazards occurrence in Japan, Philippines and Thailand. It also includes some mitigation techniques that can be used to reduce the impact of geotechnical hazards before, during and after their occurrence. Keywords: geotechnical hazards, earthquakes, tsunamis, landslides, mitigation
1. INTRODUCTION Natural hazard is unexpected or uncontrollable natural event of unusual magnitude that threatens the activities of people or people themselves (Orense, 2003). Natural hazard may lead to natural disaster if it resulted to a widespread destruction of property and caused injury and/or death. Those natural events that directly affect the ground or cause ground movements are called geotechnical hazards. Some geotechnical hazards are: earthquakes and earthquake related hazards like soil liquefaction, lateral spreading and tsunami; and landslides or sloping failures. Human activities can increase the occurrence and severity of a geotechnical hazard like building on top of unstable slope will increase the possibility of slope collapsing, steepened slope due to cutting into a hillside or embankment and too much logging operations may initiate landslides. Although natural geotechnical hazards cannot be prevented, there is greater possibility that we can control human activities that can cause disasters. By effective mitigation techniques, we can reduce the damage, reduce the severity of its effects and reduce human sufferings that result from disasters.
2. GEOTECHNICAL HAZARDS 2.1 Earthquakes and Earthquake Related Hazards
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An earthquake is the result of a sudden release of energy in the earth’s crust that creates seismic waves. At the earth’s surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground. Earthquake shaking or other rapid loading can reduce shear resistance of soil and cause the soil to behave like liquid, the event called soil liquefaction. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquake can also trigger landslides and occasionally volcanic activity. Some major earthquakes and earthquake hazards occurrence in Japan, Philippines and Thailand are as follows: The 1983 Nihonkai-chubu Earthquake On 26 May 1983, a major earthquake named “Nihonkai-chubu (Japan sea) earthquake of 1983” occurred in the central sea of Japan. The earthquake generated a major local tsunami which was destructive in Japan and Korea. The earthquake and tsunami waves caused extensive damaged to dwellings, roads, and vessels along the Japan sea coast. Roads in Wakami town in Akita prefecture was totally destroyed (Fig. 1) but severe structural damage on the bridge structure was not found. It is because the damage was not caused by the direct impact of seismic inertia force but by the loss of shear strength of the foundation due to soil liquefaction. A lot of sand volcanoes were found in the fields as evidence of ground liquefaction (Fig. 2).
Fig. 1 Roads damaged after the 1983 Nihon-kai Chubu earthquake
Fig. 2 Sand volcano due to soil liquefaction
The 1995 Hyogo-gen Nanbu (Kobe) Earthquake One of the worst earthquake catastrophes in Japan occurred on 17 January 1995 at western Honshu Island, called the Hyogo-ken Nanbu (Kobe) earthquake. More than 5000 people perished in southern Hyogo prefecture, most in the city of Kobe, Japan’s most important port. Many quay walls in this areas moved by as much as several meters toward the sea as a result of liquefaction of the foundation soil and/or the backfill. As a result, several buildings, including those supported by pile foundation settled and tilted without significant damage to the superstructure. Fig. 3 showed the building affected by this earthquake. The damage on the building is concentrated at a particular story due to building’s vertical irregularities. The 18-span bridge of Kobe Line in the Hanshin Metropolitan Expressway collapsed due to strong shaking (Fig. 4). Columns failed due to insufficient shear strength.
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Fig. 3 Damaged building in the 1995 Hyogo-ken Nanbu Earthquake
Fig. 4 Damage on bridge due to strong shaking
The 2004 Chuetsu Earthquake The Chuetsu earthquake struck Japan in 23 October 2004 and was named The Mid Niigata Prefecture Earthquake of 2004. Niigata prefecture is located in the Hokuriku region of Honshu, the largest island of Japan. The strong ground motion caused extensive damage to buildings and engineering structures. Failure of natural slopes and embankment leads to malfunction of roads and railways. For the first time in its history, a Japanese bullet train (Shinkansen super express) derailed while in service (Fig. 5). Because of liquefaction, manhole on the side street of Nagaoka City was lifted (Fig. 6). Density of liquefied mud was twice as large as that of water, large enough to lift up the manhole.
Fig. 5 Derailed Shinkansen train
Fig. 6 Lifted manhole at Nagaoka City as a result of soil liquefaction
The 1990 Luzon Earthquake One of the deadliest and costliest natural disasters in the Philippines was the Luzon earthquake which occurred on 16 July 1990. The earthquake caused damage in an area of about 20,000 square kilometers, from Northwest of Manila through the Central Luzon and into the mountains of the Cordillera Administrative Region. Baguio City, a popular tourist destination, was one of the hardest hit with number of deaths estimated at around 1000. One of the buildings destroyed was a five-star Hyatt Hotel (Fig. 7), its 12 story section
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collapsed over the lobby. For the first 48 hours after the earthquake, the city was isolated from the rest of the country. Electric, water and communication lines were destroyed. The city was inaccessible by land because of landslides and inaccessible by air, except helicopters, because of damage at the airport. Another city that suffered the most was Dagupan City. Most damage was due to the liquefaction of loose saturated sand deposit. Some buildings sink by as much as one (1) meter.
Before the earthquake
After the earthquake
Fig. 7 The Hyatt Hotel in Baguio City after the 1990 Luzon earthquake The 2004 Indian Ocean Earthquake The 2004 Indian Ocean earthquake was one of the deadliest natural disasters in history. An undersea earthquake occurred in 26 December 2004 at the Indian Ocean off the northwestern coast of Sumatra. The disaster is known as the Great Sumatra-Andaman earthquake and is also known as the Asian Tsunami. The earthquake triggered a series of tsunamis along the cost of landmasses bordering the Indian Ocean which caused tremendous devastation in several countries and killed hundred thousands of people. In Thailand, all provinces facing Andaman Sea were seriously attacked by tsunami waves, where the total death toll including missing of more that 8000 was reported (Warnitchai, P., 2005). Based from the damage found after the tsunami (Fig.8), evidence showed that the wave reached a height of 24 m when coming ashore along large stretches of the coastline, rising to 30 m (100 ft) in some areas when travelling inland.
Fig. 8 The aftermath of tsunami disaster in Thailand
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2.2 Landslides and Slope Failures Landslide is a general term used to describe the down-slope movement of the soil, rock and organic materials under the influence of gravity. It is a normal landscape process in mountainous areas, but becomes a problem when it results in serious damage that oftentimes approach disaster proportions. As cities and towns grow, roads and highways and other amenities progressively encroach onto steeper slopes and mountainsides. Subsequently, these infrastructures attract further built-up environments. Landslide hazards become an increasingly serious threat to life and property. Catastrophic landslides have recently been increasing in the Philippines even surpassing the combined effects of volcanic eruptions and earthquakes. The triggers usually take the form of an earthquake, heavy rainfall and human activities like quarrying and logging. Listed below are some landslide occurrences in the Philippines: The Cherry Hills landslides, Antipolo City, Luzon Island On 3 August 1999, after several days of continuous heavy rainfall, a landslide occurred in Cherry Hills Subdivision, San Luis Village in Antipolo City, 32 Km. east of Manila, Philippines. It destroyed about 379 houses resulting in the death of at least 58 people. The subdivision was developed on the moderately sloping terrain in Antipolo City (Fig. 9). The landslide occurred very quickly, according to eyewitness reports. Two loud noises were heard, and the movement was over in about five seconds. A subsequent field investigation by Maglambayan et al. (1999) showed that excavation related to the construction of the subdivision led to over steeping of slopes. Heavy rainfall may have accelerated the creep and triggered the landslides (Orense, 2003). Hydrostatic pressures developed along fractures may have made the slope unstable. The Panaon Island landslides, Southern Leyte From 17 to 20 December 2003, numerous landslides and flashfloods occurred in Southern Philippines, especially in the province of Southern Leyte, Surigao and Agusan (Cabria and Catane, 2003). The most catastrophic of them occurred on 19 December in Panaon Island, Southern Leyte (Fig. 10). Hundreds of people were killed and injured while more were left homeless. The landslides originated from a moderately steep slope (between 30 to 40 degrees) with thick soil cover. Most of the landslides involved debris and earth materials rather than rocks. The mechanism is dominated by rapid soil slide that transformed into debris flow, signifying the saturated nature of the slope materials.
Fig. 9 The Cherry Hills Subdivision in Antipolo City Photo by Punongbayan (1999).
Fig. 10 Landslides in Panaon Island, Southern Leyte Photo by Philippine Star (2003)
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The Quezon landslides, Luzon Island From mid-November to early December 2004, three typhoons and tropical storms struck Luzon Island, Philippines in two weeks; this resulted in massive landslides that caused thousands of deaths and damage to properties. The hardest hit were the towns of Real, Infanta and General Nakar, all in Quezon Province. Damage to engineering structures was also extensive, with numerous houses and five bridges washed away by mudflows and flashfloods. An aerial survey showed that in the southeastern part of Sierra Madre Mountain Range where Quezon Province lies, numerous landslides occurred even in heavily forested areas (Fig. 11). Trees, together with huge masses of soil, slid down the slopes. Most of the landslides involved only the soil mantle and were not deep-seated; but minor rockslides and rockfalls also occurred along the streams. Mayana landslide, Jagna, Bohol Island On 11 July 2005, large limestone blocks slid along a steep NW-trending scarp in Mayana village, Jagna, Bohol Island, Philippines (Catane et.al., 2005). This initiated down slope movement of debris to the east. The landslide reached a distance of 2.3 km affecting about 75 hectares of residential areas and farmlands. The landslide was characterized by observed movements as high as 29m/day despite the absence of heavy rainfall. Earlier, on 31 March 2005, a surface-wave magnitude 4.9 earthquake with epicenter in Sierra Bullones (about 46 km east of the capital Tabiliran City) had occurred. The epicenter is roughly 10 km from the site of the landslide. However, the role of the earthquake as a contributory factor for the landslide is not clear yet. The very large landslide originated as a rock fall along a very steep NW-trending scarp at the Sierra Bullones in Sito Balikbayan. Local residents claimed to have heard loud sounds and seen large chunks of limestone outcrop toppling down to the toe of the slope. The debris collected at the base of the slope began to move at an alarming rate. The creeping landslide blocked a national highway, destroyed 70 houses and productive farmlands, caused heavy siltation of rivers, and dammed two rivers. Fig. 12 shows the source area of the Mayana landslide.
Fig. 11 Numerous landslides on forested areas along Gumian River, Quezon Province
Fig. 12 The source area of the Mayana landslide in Bohol
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The Guinsaugon landslide, St. Bernard, Southern Leyte On 17 February 2006, a catastrophic rock-slide avalanche buried the entire village of Guinsaugon in St. Bernard Southern Leyte, Philippines (Catane et.al. 2006). The landslide originated at the ridgeline of Mt. Can-abag, a 800-m high mountain range formed by repeated movements along the Leyte Segment of the Philippine Fault Zone (PFZ). Fig. 13 shows the Guinsaugon landslides. It started as a block slide that transformed into an avalanche and the entire event lasted for only a few minutes. The rock-slide avalanche claimed 1119 lives, destroyed millions worth of properties and dammed four streams. Preceding heavy rainfall and low magnitude earthquakes are potential triggers. A rain gauge located 7 km west of Guinsaugon measured cumulative rainfall of 751 mm from 1 to 16 February; this is 2.6 times higher than the average February rainfall. Two low magnitude earthquakes shook the village and surrounding areas on the day of the massive landslide. Slope stability analyses were conducted after the incident and the results revealed that saturation of discontinuities resulting in high pore pressures played a significant role in the initiation of the slope failure.
Fig. 13 The Guinsaugon landslide Photo by D. Batnag, (2006)
3. MITIGATION TECHNIQUES It is not possible to predict the exact time and location of the next big natural hazard like earthquake and landslide but by understanding when, where, why and how it occur, we may be able to intervene on time and avoid high risk situations thereby lessens its impacts to our lives. Mitigation is the process of lessening the impact of natural hazards before, during and after their occurrence. Engineering solutions can be used to temporarily reduce the impact of natural hazard but each hazard requires specific type of mitigation. General awareness and having an effective preparedness plan of the impending disaster are mitigation forms that work to all kinds of natural hazards. Information is the key in a crisis. Information is power when it is credible, timely, locally, relevant and widely accessible to the population. This section briefly describes some mitigation measures that can be adopted to reduce risk from various geotechnical hazards: 3.1 Zoning, Mapping and Monitoring Observations from previous earthquakes provide a great deal of information about a particular area susceptible to geotechnical hazards. It is important to identify and map
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areas prone to earthquake hazards of liquefaction, earthquake-induced landslides and amplified ground shaking. The outcome of this observation and assessment is best presented in a zoning map where locations or zones of different levels of hazard potentials are identified. Cities and municipalities especially those highly populated areas are advise to come up with zoning maps. If you are building a structure and want to find out if the site is susceptible to liquefaction or landslide, the zoning map will be very useful for this purpose. Engineering geology and geotechnical hazard assessment should be required prior to any development projects especially in landslide-prone areas. With a deeper understanding and monitoring of the movements of unstable slopes, one can timely intervene and apply the necessary mitigation measures. 3.2 Strengthening of Structures It is always advisable to avoid areas susceptible to earthquake hazards like soil liquefaction; however, for certain reasons like space restrictions and favorable locations, construction on these areas can not be avoided. It is therefore a must to design the structure earthquake resistant and its foundation elements resistant to the effects of liquefaction and ground settlement. Emphasis of design should always be on safety over aesthetics and functionality. Odd shaped structures, if possible, should be avoided. Soft story building failures can be prevented by proper planning of architectural form of the building and by emphasizing ductility design of the columns, walls and beams. To decrease the amount of damage a structure may suffer in case of an earthquake, a structure must possess ductility in order to accommodate large deformations, adjustable supports for corrections to differential settlements and having foundation design that can span soft soils. 3.3 Soil improvement technology Another way of mitigating earthquake related hazards like liquefaction are by improving the strength, density and/or drainage characteristics of soil. This can be done through various ground improvement techniques. Table 1 summarizes the liquefaction hazard mitigation techniques. Table 1 Examples of liquefaction hazard mitigation techniques Type of technique Densification
Liquefaction hazard mitigation techniques Sand compaction pile, Vibroflotation, Dynamic compaction, Compaction grouting Soil improvement Grouting, Replacement Lowering degree of saturation Well point Rapid dissipation of pore Gravel drain water pressure Deformation control Sheet pile wall, Soil cement column wall
3.4 Slope Protection and Stabilization Engineering countermeasures for reducing landslides generally involve the use of slope stabilization methods such as benching, improvement of subsurface drainage, construction of retaining structures, and reinforcement of slopes. Benching is the practice of
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transforming one high slope into a series of lower slopes with horizontal surfaces in between slopes referred to as benches. The purpose of benching is to reduce the overall gradient of the slope. Installing proper drainage minimizes the destabilizing effects of hydrostatic and seepage forces on a slope, as well as reduces the risk of erosion and piping (Abramson, 1996). In the Philippines, the most widely used drainage technique is the installation of surface drains to carry away surface runoff and prevent it from seeping into the slope. Vegetation like Vetiver grass is also widely used for steep slope stabilization and rehabilitation of degraded and disturbed lands. In the last 50 years, attention has been focused on vetiver’s unique soil conservation properties. It grows both in highly acidic and alkaline soils and its roots can grow to depths of 3 to 4 meters. When planted in single lines along the contour, hedges of vetiver grass are found to be very effective in soil and moisture conservation. Table 2 summarizes some engineering practices for stabilizing and/or protecting precarious slopes. Fig. 14 shows some slope protection and stabilization techniques. Table 2 Examples of slope hazard mitigation techniques Type of technique Slope protection and stabilization techniques Control works Soil removal (Unloading), Counterweight fill, Benching, Drainage, Slope protection (e.g. grating crib, vegetation, gabion, mortar spraying) Prevention works Pile, Shaft work, Soil nailing, Rock anchoring Others Rockfall barrier, Rockfall shelter
a.) Grouted rip rap
b) Soil nailing
c.) Gabion walls d.) Vetiver grass in road projects Fig. 14 Examples of slope protection and stabilzation techniques
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REFERENCES Abramson, L.W. et.al (1996) Slope Stability and Stabilization Methods, John Wiley and Sons. Cabria, H.B and Catane, S.G. (2003) The 19 December landslide in Panaon Isaland, Southern Leyte, Philippines. QRT Report, National Institute of Geological Sciences, University of the Philippines-Diliman, Quezon City. Catane, S.G. et.al. (2005) Assessment of hazards resulting from the July 11, 2005 landslide, Barangay Mayana, Jagna Bohol. Technical Report prepared for the Local Govenrment Unit, Jagna, Bohol, National Institute of Geological Sciences, University of the Philippines-Diliman, Quezon City. Catane, S.G. et.al (2006) Catastrophic rockslide-debris avalanche at St. Bernard, Southern Leyte, Philippines. Landslides DOI 10.1007/s10346-006-005-3. Maglambayan, V.B. et.al (1999) A proposed model for the 03 August 1999 Cherry Hills landslide, Antipolo City. Proceeding of the 12th Annual Geological Convention, Galleria, Mandaluyong City, Philippines. Orense, RP (2003) Geotechnical Hazards-nature, assessment and mitigation. The University of the Philippines Press, Diliman, Quezon City, Philippines. Warnitchai, P. (2005) The 26 December 2004 tsunami disaster in Thailand: experience and lessons learned, Proceedings of the 5th Workshop on Safety and Stability of Infrastructures against Environmental Impacts, De La Salle University-Manila, 5-6 December 2005. Zarco, M.H. et.al (2005) July 11, 2005 landslide, Barangay Mayana, Jagna, Bohol, Proceedings of the 5th Workshop on Safety and Stability of Infrastructures against Environmental Impacts, De La Salle University-Manila, 5-6 December 2005. ACKNOWLEDGMENT Sandra G. Catane, Associate Professor, National Institute of Geological Sciences, University of the Philippines-Diliman, Quezon City, Philippines.
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