The Study For Assessment Of Susceptibility To Soil Liquefaction In Taiwan

Horng-Yuh GUO, Chih-Feng CHIANG, Tsang-Sen LIU, Jiang-Liung CHU, Jen-Chyi Liu
Taiwan Agricultural Research Institute, Wufeng
Taichung Hsien, Taiwan

Key Words: Susceptibility of Soil Liquefaction, Pedotransfer Fuctions, GIS

Abstract:
Soil liquefaction has been observed in many places after 921 Taiwan Chichi Earthquake. Soil liquefaction resulted in disastrous effects such as tilted buildings, foundation failure and sand volcanoes or sand boils in the fields. It was suggested that government should provide revealing of information regarding the potential hazard of soil liquefaction and teach citizens how to evade soil liquefaction hazard. Responding to this, it is the fact that there is no graphic information (regional map) available about potential hazard of soil liquefaction in disastrous region currently. Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. There are a number of different ways to evaluate the liquefaction susceptibility of a soil deposit. Generally, fine-grained soils are susceptible to liquefaction if they satisfy the criteria (Wang.1979) shown in the followings: Fraction finer than 0.005mm.15%.Liquid Limit(LL).35%.Natural water content .0.9LL.Liquidity Index.0.75. However, there is no quantitative parameter in the soil information here at this moment for applying the criteria to evaluate the susceptibility of soil liquefaction. Pedotransfer fuctions (PTFs) are used to transfer the quantitative criteria to the easily measured soil properties we have in the soil information system. In this study, we evaluated, classified and mapped the in disastrous regions based on three categories in soil information. The map was further verified the reliability in earthquake areas.

Introduction
Soil liquefaction has been observed in many places after 921 Taiwan Chichi Earthquake. Soil liquefaction resulted in disastrous effects such as tilted buildings, foundation failure and sand volcanoes or sand boils in the fields. Because soil liquefaction occurred places did not coincided with the geometry relationship of the fault, so it is assumed that soil liquefaction is a mysterious phenomenon by some people. It was suggested that government should provide revealing of information regarding the potential hazard of soil liquefaction and teach citizens how to evade soil liquefaction hazard. Responding to this, it is the fact that there is no graphic information (regional map) available about potential hazard of soil liquefaction in disastrous region currently. It is a very difficult task to complete the survey of the potential hazard of soil liquefaction in a short time.

Soil surveys provide information that is useful in multiple disciplines. Soil information system can provide the information of the pattern of the soil cover and its characteristics for us to analyze and display the topics of soil resources management. To evaluate the soil liquefaction hazard completely, a number of necessary and sufficient conditions must be successively examined. However, there is no available parameter in the soil information here at this moment for applying the criteria to evaluate the susceptibility of soil liquefaction. In this study, we transfered and applied the soil information system databases to evaluate, classify and map the susceptibility of soil liquefaction in Taichung, Nantou, Changhua and Yunlin's Plain regions in a short time, and the maps were further verified their reliability in the fields.

Review Of Soil Liquefaction
Plasticity parameters, which have been expressed in terms of moisture content of the soil (that is, Atterberg limits), have been defined, and standard methods for making the measurements have been established. (ASTM, 1958) Plastic limit, PL, is defined as that the water content on a dry weight basis corresponding to the arbitrary limit between plastic and brittle states of consistency of a soil. Liquid limit, LL, is defined as that the water content on a dry weight basis corresponding to the arbitrary limit between the liquid and plastic states of consistency of a soil. Plasticity index, PI, is the difference between the liquid and plastic limits and represents the range of moisture within which the soil is plastic. Silts and sands have slight or no plasticity indices, which clays have higher indices. Plasticity index in combination with the liquid limit indicates how sensitive the soil is to changes in moisture. (USDI, 1990) Field observations and experimental investigations have pointed out that the consistency of soils varies with texture, organic matter, the amount and nature of the colloidal material, and especially with moisture content. (Hillel, 1980) Liquidity index, LI, is defined as that the ratio of the difference between the nature soil water content and plastic limit to the soil plastic index (PI) of the same soil. If a soil liquidity index is less than 1, that means the soil water content is less than liquid limit. A soil liquidity index is very low or close to 0, that means the soil water content is near to plastic limit and the soil have a high cohesion. (McCarthy, 1997)

Soil liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. In soil mechanics, liquefaction is the passage from a solid to a liquid state. Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. The fluidification of a soil results in a diminution, or disappearance of its shear resistance, which in turn leads to instability problems. The liquefaction of a soil under seismic solicitation requires two types of conditions: (1) Permanent factors: soil characteristics and parameters describing the state of the soil. Soil is known to have susceptibility to liquefaction when it is relatively pulverulent and uncompacted,.25 under little stress and water-logged. (2) Aggravating factors: the first is an earthquake acting as a trigger. The opportunity to liquefy, when the energy of emitted waves is sufficient. (Monge et al, 1998)