The Study For Assessment Of Susceptibility To Soil Liquefaction In Taiwan

There are a number of different ways to evaluate the liquefaction susceptibility of a soil deposit. Here they are organized as follows (Kramer, 1996), (1) Historical criteria: Soils that have liquefied in the past can liquefy again in future earthquakes. (2) Geological criteria: Saturated soil deposits that have been created by sedimentation in rivers and lakes, deposition of debris or eroded material, or deposits formed by wind action can be very liquefaction susceptible. (3) Compositional criteria: Liquefaction susceptibility depends on the soil type. Soils composed of particles that are all about the same size are more susceptible to liquefaction than soils with a wide range of particle sizes. (4) State criteria: At a given effective stress level, looser soils are more susceptible to liquefaction than dense soils. For a given density, soils at high effective stresses are generally more susceptible to liquefaction than soils at low effective stresses.

Liqefiable soils can be identified by a combination of several criteria. A simple model to evaluate soil liquefaction was developed by Wang, that is, fine-grained soils are susceptible to liquefaction if they satisfy the criteria shown in the followings: Fraction finer than 0.005mm.Í 15%.Liquid Limit(LL) Í 35%.Natural water content.0.9LL.Liquidity Index Ê 0.75. (Wang.1979)

Mterials And Methods

1. Assessment procedures:
To evaluate, classify and map the susceptibility of soil liquefaction in Taichung, Nantou, Changhua and Yunlin's Plain regions, we use the soil information system of Taiwan, which was constructed 10 years ago by Taiwan Agricultural Research Institute, as a bases. Soil texture, soil drainage classes and available soil depth (90cm to 150cm) are the parameters available to evaluate the permanent factors of soil liquefaction susceptibility. We had to apply pedotransfer function to modify these parameters of the soil information system and meet Wang's model requirement. (Wang, 1979) The map of active faults in Taiwan (CGS, 1998) is another useful information to be used as a aggravate factor of soil liquefaction susceptibility. The nearer the fault lines site is, the more opportunity to liquefy, when the energy of emitted waves is sufficient. MAPINFO geographic information system software was used as an analysis tool. We map the susceptibility of soil liquefaction in Taichung, Nantou, Changhua and Yunlin's Plain regions, and the maps were verified their reliability in the soil liquefaction areas..

2. Pedotransfer of soil data:
The objectives of the identification of liquefiable soil are granulometry and Atterberg limits etc, which have some relationship with soil texture and soil water content characteristics. But these two factors are very difficult to measure. Pedotransfer fuctions (PTFs) are used to transfer the quantitative criteria to the easily measured soil properties we have in the soil information system. So we had to transfer the Wang's criteria of the soil liquefaction into the parameters of the soil information system for further analysis. The followings are pedotransfer Functions (PTFs) for soil liquefaction:



Because the effective depth of the shallow foundation construction is about one meter to five meters, we chose the soil texture of the deepest layer (90cm to 150cm) which the soil information system of Taiwan available.

3. Rating the soil liquefaction susceptibility:
Liquefiable soils are rated and classified by the criteria of soil texture and drainage class as following table:

Table 1. Rating table of soil liquefaction susceptibility (for shallow foundation construction)

Soil Liquefaction Susceptibility Rating Class	Coarse texture (sand, loamy sand, etc)	Medium-coarse texture (sandy loam, very fine sandy loam, etc)	Medium texture (silt loam, loam, etc)	Medium- fine texture(clay loam, silty clay loam, stc)	Fine texture(silty clay, clay, etc)	Gravelly
Well to moderate-well drainage	2	3	4	5	5	5
Imperfect drainage	1	2	3	4	5	5
Poor to very poor drainage	0	1	2	3	4	4
Submerged	0	0	1	2	3	3

 

1. Class 1:more than 90% liquefaction vulnerability.Class 2:more than 70% liquefaction vulnerability.Class 3:more than 50% liquefaction vulnerability.Class 4:more than 30% liquefaction vulnerability.Class 5:no liquefaction vulnerability.

2. If the site in the faults buffer zones(10Km wide) increases 1 class..27

Results And Conclusions
Because liquefaction only occurs in saturated soil, its effects are most commonly observed in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans. Parts of Taichung, Nantou, Changhua and Yunlin's Plain regions belong to these kinds of landscapes. Tzo-Sui Alluvial Fan covers the most area in this region. Tzo-Sui River had changed its water way several times in the past two hundreds years, so many abandoned channels spread around this region.