Tsunami devastations and reconstruction with geosynthetics.

Great attention is directed to rebuild livelihoods and rehabilitate coastal communities affected by the Tsunami in the Indian Ocean in South Asia. It takes years of effort of different engineering disciplines to recover from recent devastations caused by the Tsunami. Geosynthetics can play important and vital roles in the protection, mitigation and rehabilitation efforts in affected coastal areas. The use of geosynthetics has advantages such as speed of construction, flexibility and durability, use of local soil materials rather than imported quarry product, and its cost effectiveness. Geosynthetics can be applied for reinforcement, filtration, drainage, protection, lining, and containment. More important, geosynthetics can be used effectively for erosion protection and for strengthening the earth embankments to resist failure during the occurrence of earthquakes associated with Tsunami. This paper demonstrates the various geosynthetic applications and the related engineering solutions to mitigate such Tsunami devastations.

INTRODUCTION

The Asian Tsunami of 26 December 2004, which struck the Indian Ocean Basin, affected hundreds of thousand of people in countries including Thailand, Sri Lanka, Indonesia and India. Its death toll has risen to over 260,000 victims. Many survivors had their lives disrupted since coastal tourism, fisheries, and agriculture have been seriously affected. Housing and public infrastructures have been destroyed Warnitchai (2005). There is urgent need to restore, rehabilitate and repair the damages of the affected people and the area.

Geosynthetics can play important and vital roles in the protection, mitigation and rehabilitation efforts in affected coastal areas. Geosynthetics have been used in hydraulic and geotechnical engineering for about the past two or three decades. Their use is well established for the purposes of material separation, filters, and reinforcement. In addition, all kind of bags are made now from synthetic fabric. The use of geosynthetics has advantages such as speed of construction, flexibility and durability, use of local soil materials rather than imported quarry product, and its cost effectiveness. Therefore, it is strongly recommended to use the geosynthetics engineering applications for restoring and rehabilitation of the recent devastations caused by the Tsunami. In the following, the Tsunami devastation effects in Thailand are presented briefly. Then, an overview for the many applications of geosynthetics and geosystems engineering that can be used for reconstructions of Tsunami devastations are highlighted and discussed.

TSUNAMI AND DEVASTATIONS IN THAILAND

The Tsunami wave height distributions in Thailand are shown in Fig. 1. The wave heights were greater at flat shorelines and shallow seawater depths. Consequently, beach resorts with deeper seawater depths and steeper shorelines only experienced slight damages (Warnitchai, 2005). Moreover, wide vegetated areas protect the shorelines and obstruct drifting debris that have destructive power when combined with water flow (Fig. 2).

[FIGURES 1-2 OMITTED]

In the aftermath of the large-scale disaster, destructions related to coastal areas and waterways as well as infrastructures and buildings have been identified. Figure 3 shows the airphoto before and after tsunami at Khao Lak, Phanga, Thailand. The erosion and scouring in the coastlines and waterways can be observed. More details of coastal erosion at Khao Lak are shown in Fig. 4. Small and weak buildings directly open to the coastline were completely destroyed while large and strong buildings remain standing. The foundations of buildings are damaged by scouring and erosion. Erosion damage also occurs on seawalls and earth structures.

[FIGURES 3-4 OMITTED]

Ground elevation is a key factor. Even a small hill of 2 m height has saved houses on it. In fact, the natural sand dune deposits at Karon Beach in Phuket, Thailand has reduced the destructive effects of Tsunami. There were selective damages of the beaches in Phuket depending on the morphology, topography and depth of seawater of the coastal areas.

OVERVIEW OF GEOSYNTHETICS ENGINEERING

Geosynthetics are man-made synthetic materials utilized in geotechnical engineering applications. The earliest user of geosynthetics was Karl Terzaghi. In the late 1950s, Terzaghi made use of filter fabrics (today, geotextiles) as flexible forms. They were filled with a cement grout, thereby making closure between steel sheetpiling and rock abutments at the Mission Dam (now Terzaghi Dam) in British Columbia, Canada. During this same project, Terzaghi used pond liners (today, geomembranes) to keep an upstream clay seepage-control liner from desiccating.

There are various types of geosynthetics such as geotextiles (GT), geogrids (GG), geomembranes (GM), geonets (GN), geocomposites (GC), geosynthetic clay liners (GCL), geopipes (GP), geofoam (GF), geotubes, geocells, etc. Geosynthetics can be used for reinforcement, filtration, drainage, protection, lining and containment. The functions and geosynthetic types are tabulated in Table 1. The description of the varies geosynthetics types were given by Koerner and Soong (1995) and Bergado et al. (2005) as follows:

Geotextiles usually consisted of polypropylene (PP) or polyester (PET) polymers are made similar to standard textile manufacturing in either woven and nonwoven types. The yarns of woven geotextiles are arranged perpendicular to each other and may consist of slit film, monofilament or multifilament. The nonwoven geotextiles can be either made by mechanical needle punching or by heat bonding. Geotextiles are thin fabrics with open and porous structure. The mechanical and hydraulic properties vary widely depending on the weight per unit area. Geotextiles are primarily used for separation, filtration and reinforcements and secondarily for protection and drainage.

Geogrids are unitized woven yarns or bonded straps with either directional or unidirectional strength characteristics. This type of geosynthetics are mainly used for reinforcement applications such as reinforced vertical walls and steep slopes as well as reinforced foundations.

Geonets are usually made from high density polyethelene with parallel ribs as an integral unit. The primary function is in-plane drainage in either biplanar and triplanar directions.

Geomembranes are impermeable geosynthetics which are mainly used as barrier lining and containment of liquids and gases. It mainly consists of high density (HDPE) or low density (LDPE) polyethelene or PVC polymers. This product is required by regulations for waste containment as well as new applications in hydraulic and private developments.

Geosynthetic clay liners (GCL) consist of bentonite sandwiched between geotextiles. The bentonite infill is usually reinforced by needle-punching or stitching. Many other variations exist such as bentonite product bonded to geomembrane. This product is mainly function as barrier lining or containment as competition or in combination with geomembranes.

Geopipes (GP) are really buried polymer pipe for drainage purposes. The high density polyethelene (HDPE) and polyvinyl chloride (PVC) are most common. The geopipes can be smooth walled or corrugated.

Geofoam (GF) consists of expanded polyesterene (EPS) in block form. This product is mainly used as lightweight fill on soft ground. It can effectively relieve lateral pressure on walls.

TSUNAMI RECONSTRUCTION WITH GEOSYNTHETICS

Geosynthetics can play important and vital role in the protection, mitigation, rehabilitation and reconstruction of affected coastal areas. Traditional construction techniques utilizing rock, concrete and steel for erosion protection are increasingly challenged by alternatives using geosynthetics for revetments, scour protection, groynes, berms, artificial reefs, reclamation, slope, protection, etc. The use of geosynthetics has advantages such as speed of construction, flexibility, durability, use of local materials rather than imported quarry products and its cost effectiveness. Koerner (2000) shows the emerging and future developments of selected geosynthetics applications. Moreover, Pilarczyk (2000) gave comprehensive review for the use of geosynthetics and geosystems in hydraulic and coastal engineering. In the following, some of geosynthetics engineering applications that can be used to restore the Tsunami destruction are presented.

Geotubes and geobags are made of geotextiles filled with dredged sand/silt/clay slurry using a hopper method with density of 1.6 tons per cubic meter and height of 1.8 to 2.0 m. Geotubes and geobags can be used as core for artificial sand dunes for shoreline protection. Heerten et al. (2000) reported that, based on case history in Island of Sylt, Germany, the geotextile "sand cushions" shows superior effectiveness in protecting cliff area against erosion as shown in Fig 5. Geotubes and geobags can be also utilized for construction of shoreline protection and containment dykes to create artificial islands (Fig. 6). Restall et al (2002) gave a comprehensive review for the historical development and the types of geotextile containers used for marine structures in Australia and conclude that geotextiles containers structures can be successfully used to solve conventional coastal problems and non-conventional challenges.

[FIGURES 5-6 OMITTED]

Geocells are cellular confinement system for erosion control (Fig. 7). The geocells can be stacked together for slope erosion protection. These geocells are usually infilled with gravel, sand, silt or clay. Grasses and shrubs can grow at the infills. The geocells are made of ultrasonically-welded polyethelene sheets.

[FIGURE 7 OMITTED]

Gabions and mattresses are rock-filled baskets made of twisted hexagonal wires for erosion control. The wires can be coated by either zinc or PVC. Gabions have thicker dimensions than mattresses (Fig. 8). Gabions, geocells, and mattresses can be applied as revetments.

[FIGURE 8 OMITTED]

Revetments consisting of concrete facing, gabions, and mattresses or rock armour are effective for erosion control and scouring prevention (Fig. 9). Geotextiles underneath revetments serve as separators and filters.

[FIGURE 9 OMITTED]

Dykes and breakwaters protect harbors and recreation areas. Geotubes and geobags can be incorporated as components of breakwaters. Geotextiles at the base serve as separators, filters and membrane reinforcements (Fig. 10).

[FIGURE 10 OMITTED]

Geotextiles and geogrids can be applied as filters, wall reinforcements, slope reinforcements, erosion control and drainage functions in road and railroad embankments especially at approaches to bridges as shown in Fig. 11 (Bergado et al. 1996). In these structures, drainage can be provided by geocomposites and geopipes.

[FIGURE 11 OMITTED]

Geosynthetics are also utilized in geoenvironmental applications such as landfill liners, landfill covers, vertical cut off barriers, etc. In geoenvironmental applications, as shown in Fig. 12, almost all types of geosynthetics are utilized such as geomembranes and geosynthetic clay liners for liners, geotextiles for separation and protection, geonets, geocomposites and geopipes for drainage, etc.

[FIGURE 12 OMITTED]

For private development, geosynthetics have been used in sport fields, artificial lakes, golf courses, airfields, parks, playground, etc. For agriculture and aquaculture applications, geotextiles and geomembranes have been used as lining of shrimp farms, waste containment, fish baskets, and others.

PROPOSALS FOR MITIGATION AND REHABILIZATION OF COASTAL AREAS

Abednego (2005) presented the proposals by the Indonesian Engineering Association (IEA) for the contruction of buffer zone and canal (Fig. 13) as well as "escape mountain" and buildings (Fig. 14). The buffer zone serves to dissipate the impact of strong waves generated by Tsunami. Consequently, the buffer zones are located close to the seashore. These zones may consist of natural barrier such as mangrove forest. Man-made high road embankments and artificial elevated sand dunes can also function as buffer zones. The road embankments should have at least 2.0 m high and 6.0 m wide that can also function as coastal road. The road embankment should be reinforced and erosion resistant through the incorporation of geosynthetics (Fig. 15). The artificial sand dunes can be constructed using geotubes or geobags which are also made of geosynthetics.

[FIGURES 13-15 OMITTED]

Geosynthetic can be also incorporated in the design of the "escape mountain". The escape mountain or hill is constructed similar to a pyramid with geosynthetic reinforcements and erosion protection. The escape mountain or hill should be from 3.0 m to 5.0 m high with 30.0 m by 15.0 m rectangular area at the top in order to accommodate at least 3,000 people. Unlike the escape tower, the escape mountain or hill allows the access of people in all four sides (Fig. 16).

[FIGURE 16 OMITTED]

CONCLUSIONS

Based on the aforementioned presentations and discussions, the following conclusions can be made:

1) Erosion and scouring resulted from tsunami devastations of coastal areas and waterways.

2) Geosynthetics can be incorporated in the reconstructions of infrastructures to be erosion resistant and for flexibility and durability.

3) Geosynthetics can be utilized as filters, reinforcements, drainage, containment and separators.

4) Geosynthetics are lightweight and easy to install with low handling and overall costs.

5) Geosynthetics can be utilized effectively for the construction of "escape mountain or hill" near the coast and earthen berms at buffer zones.

REFERENCES

Abednego, L.G. (2005). "The contribution of Indonesian Engineers Association to Aceh Province after the Earthquake and Tsunami", Proc. of Scientific Forum on Tsunami, its Impact and Recovery, 6 to 7 June 2005, AIT Conference Center, Bangkok, Thailand.

Bergado, D.T., Sadlier, M., Lawson, C. and Piyaboon, S. (2005). "Tsunami reconstruction with geosynthetics in coastal areas and waterways", Proc. 3rd International Conference on Geotechnical Engineering, Semarang , Indonesia.

Bergado, D.T., Anderson, L.R., Miura, N. and Balasubramaniam, A.S. (1996). Soft Ground Improvement in Lowland and other Environment, ASCE press, USA.

Heerten, G., Jackson, L.A, Restall, S.J and Stelljes, K. (2000). "Environmental benefits of sand filled geotextile structures for coastal applications", Proceeding of Geo-Eng 2000, Melbourne, Australia.

Koerner, R.M. and Soong, T.Y. (1995). "Use of geosynthetics in infrastructure remediation", Journal of Infrastructure System, ASCE, Vol. 1, No. 1, 66-75.

Koerner, R.M. (2000). "Emerging and future developments of selected geosynthetics application", Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 125, No. 4, 293-306.

Pilarczyk, K. (2000). "Geosynthetics and Geosystems in Hydraulic and Coastal Engineering", A. A. Balkema Publishers, Rotterdam, The Netherlands.

Restall, S.J., Jackson, L.A. Heerten, G. and Hornsey, W.P. (2002). "Case studies showing the growth and development of geotextile sand containers: an Australian perspective", Geotextile & Geomembranes, Vol. 20, 321-342.

Warnitchai, P. (2005). "Lessons learned from the 26 December 2004 Tsunami Disaster in Thailand", Proc. of Scientific Forum on Tsunami, its Impact and Recovery, 6 to 7 June 2005 AIT Conference Center, Bangkok, Thailand.

D. T. BERGADO and H. M. ABUEL-NAGA

School of Engineering and Technology, Asian Institute of Technology, Bangkok, 12120, Thailand

Table 1. Function vs. Geosynthetic Type.

Type of
geosynthetics              Separation   Reinforcement   Filtration

geotextile                  [check]        [check]       [check]
geogrid                                    [check]
geonet
geomembrane
geosynthetic clay liners
geopipes
Geofoam                     [check]
geocomposites               [check]        [check]       [check]

Type of
geosynthetics              Drainage      Containment

geotextile                  [check]
geogrid
geonet                      [check]
geomembrane                                [check]
geosynthetic clay liners                   [check]
geopipes                    [check]
Geofoam
geocomposites               [check]        [check]