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GEOTECHNICAL HAZARDS AND DISASTER MITIGATION TECHNOLOGIES

Jiro KUWANO1, Mairaing WARAKORN

2 ,

Mark ZARCO

3,

and Mary Ann ADAJAR4

1Geosphere Research Institute-Saitama University 2Department of Civil Engineering – Kasetsart University 3Department of Civil Engineering, University of the Philippines-Diliman 4Department of Civil Engineering, De La Salle University – Manila

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 Fig. 2 Sand volcano due to soil

1983 Nihon-kai Chubu earthquake 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 Fig. 4 Damage on bridge due to

1995 Hyogo-ken Nanbu Earthquake 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 Fig. 10 Landslides in Panaon Island, Southern

Antipolo City Photo by Punongbayan (1999). 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 Fig. 12 The source area of the

areas along Gumian River, Quezon Province 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

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

Type of technique Liquefaction hazard mitigation techniques

Densification Sand compaction pile, Vibroflotation, Dynamic

compaction, Compaction grouting

Soil improvement Grouting, Replacement

Lowering degree of saturation Well point

Rapid dissipation of pore

water pressure

Gravel drain

Deformation control Sheet pile wall, Soil cement column wall

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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.