Taiwan situates in Circum-Pacific Seismic Belt, where earthquakes occur frequently. How to protect against and reduce disasters is an important strategy of government officials. On 6 February 2016, a strong earthquake of Richter local magnitude 6.6 occurred in the Tainan area of Taiwan, resulting in many serious disasters. Soil liquefaction is then becomes one of the government’s concern. Consequently, soil liquefaction issue is taken seriously in order to achieve safety, prevention, early warning, etc. The soil liquefaction potential map must be improved for the accuracy at this critical moment. Especially, in densely populated areas, low floor houses, old buildings, and complex geological condition areas, the destruction comes after soil liquefaction severely. Geologically, Ilan area is deposited mostly with fine particles contenting high content of weak soil, coupling with groundwater level very close to the surface. Once the earthquake occurs, the possibility of soil liquefaction is quite high. Therefore, we need to add drilling holes in order to draw a more representative soil liquefaction potential map of intermediate level. As a result, the Ilan County Government can be more prepared for disaster prevention and early warning operations. Accordingly, people in Ilan area can get a safe and comfortable living condition. It is reason that we are obligatory to explore the soil liquefaction related problems in Ilan area.

۲ Story of Soil Liquefaction

Soil liquefaction is the process of changing the soil from solid to liquid just as the name. For the period of liquefaction, the soil behavior is similar to the liquid. Due to the liquid cannot resist shear force and maintain its own shape, there is mobility and buoyancy, thus the structure will be subsidence, tilt or damage. The reason of resulting in this phenomenon is mainly caused by vibration, which may come from construction or earthquake, while most of the soil liquefaction is caused by the earthquake. The study of liquefaction of granular soils using the concept of critical void ratio was first introduced by Casagrande in 1938. When a natural deposit of saturated sand that has a void greater than the critical void ratio is subjected to a sudden shearing stress (due to an earthquake, for example), the sand will undergo a decrease in volume and lead to the condition of soil liquefaction. After that, many researchers kept on working in this topic. For example, Japanese scientists Mogami and Kubo (1953) suggested that the soil will be deformed as “liquefied” when cohesionless soil is disturbed. Following the 2 earthquakes of Alaska in US and Niigata in Japan in 1964, an intensive research has been carried out. Seed (1979) mentioned that high pore water pressure would reduce the effective confining pressure of soil. At this point in time, the soil residual shear strength becomes lower and the soil mass is likely to produce deformation. As soon as the earthquakes occur, it is possible that the soil cannot afford the original stress all of a sudden. Consequently, the soil pore water increases and shear stress intensity becomes smaller. This condition provides the opportunity to produce soil liquefaction, when the excess pore water pressure continues to rise to the original effective stress (when r = l). That is to say, when the earthquake induces the local average shear stress is greater than the local soil shear strength for resisting liquefaction, the soil is liquefied.