Soil liquefaction is not a new term, nor is it something we do not know how to deal with. There have been many serious cases of this phenomenon over the years, including in the 921 Earthquake in 1999, where we saw many cases of damage caused by liquefaction: In Yuanlin (員林), Shetou (社頭), Nantou (南投市) and Wufeng (霧峰), for example.
The magnitude 6.4 earthquake that hit southern Taiwan on Feb. 6 caused structural collapses and 117 fatalities. With such high casualties, there is a danger that the issue of soil liquefaction might be relegated to a secondary role and not taken as seriously as it should be.
In the March 2010 Taoyuan earthquake — originally named the Jiasian (甲仙) earthquake — soil liquefaction was limited to Beishi (北勢), Taiping (太平), Dongrong (東榮) and Shanjiao (山腳) hamlets in Tainan’s Sinhua District (新化區) beside the Yanshui River (鹽水溪). In February’s earthquake, sand boil was seen in the same places. These locations, and especially Beishi, are very close to the High Speed Rail (HSR) route, with all the safety concerns this entails.
In February’s quake, the most serious damage due to soil liquefaction occurred in Tainan’s Siding (溪頂) in Annan District (安南區), Sanmin Street in Sinshi District (新市區), Wenho street in Jhongsi District (中西區), and Jhengjue Street and Gongyuan Road in North District (北區). These areas have all been slated for reconstruction, and the sense of hopelessness and helplessness is visible on the faces of residents.
People whose houses were less seriously damaged and who were able to react more quickly have laid down a new concrete floor. Those whose apartments’ walls were warped or are leaning are unsure whether they should continue living in their homes.
Soil liquefaction is a regional geological problem and as a solution, leveling the ground in the wake of an earthquake is purely cosmetic — it is not a long-term strategy. The buildings damaged in the 921 Earthquake in areas affected by soil liquefaction had their foundations reinforced, predominantly to make sure they were straightened. Little thought was given to increasing the buildings’ resistance to soil liquefaction, which is a major source of concern for buildings being reconstructed in regions prone to this phenomenon.
Premier Simon Chang (張善政) has pledged Tainan will serve as a trial zone for urban renewal, and to help those whose homes have been damaged with reconstruction. Given the likelihood of soil liquefaction, this disaster area reconstruction plan should incorporate building reconstruction and reinforcement of the geological basin.
Figures are soon to be published on which areas are vulnerable to soil liquefaction. It is important to understand that this information is gleaned from random borehole samples and that areas determined to have a higher potential for liquefaction might be so determined simply because borehole samples have been taken there, and that areas said to be low-risk are described as such because no borehole samples have been taken.
Therefore, before urban renewal and the construction of new buildings are mooted, proper geological surveys must be undertaken. First, we need to know where the risks lie, so that we can address the problem — then there would be no need to fear soil liquefaction.
The sixth naptha cracker, which was built on sand-pumped and backfilled land, is an excellent example of this, as it has weathered several major earthquakes without experiencing any damage from soil liquefaction.
Huang Fu-kuo is an associate professor in the department of water resources and environmental engineering at Tamkang University.
Translated by Paul Cooper
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