Liquefaction is a unique phenomenon in which geo-material behave more-like liquid than solid, thus completely lost its resistance as shown in Fig. 1. This phenomenon generally found when geo-material is subjected to the cyclic forces in undrained condition, such as earthquake. During rapid undrained cyclic loading, pore water pressure could not dissipate easily. As a result, the effective stress is decreasing towards zero, in which virtually there is no contact between each particle. The simple illustration of this phenomenon can be seen in the Fig. 2.
There are vast numbers of liquefaction study that has been discussed by previous researchers. They investigated the various aspects that may trigger the liquefaction or the behavior of soil in post liquefaction. Among all of those studies, there was limited number of studies that has been dealing with re-liquefaction phenomenon. Re-liquefaction is phenomenon in which soil can liquefy repeatedly from one time to another. One possible reason why there was limited number of studies on re-liquefaction is possibly due to the limitation of the apparatus itself. General apparatus such as triaxial and hollow cylinder torsional shear apparatuses can hardly able to maintain its shape once it is liquefied as shown in Fig. 3. Therefore, most of previous studies that investigated the repeated/multi-stage liquefaction tests only able to perform up to two stages. The newly developed machine so called stacked-rings shear apparatus at University of Tokyo (Fig. 4) is the answer to the drawback we had on the previous studies. This apparatus is capable of maintaining the surface of the specimen to remain constant even under very large deformation. By taking the advantage of this apparatus, the author is able to perform not just few number of re-liquefaction stages, but virtually unlimited number of re-liquefaction stages with a single specimen. The ability to perform unlimited number of stages in re-liquefaction test is essential in projecting potential disaster of another liquefaction that may occur in the future.
There are also assumptions among engineers that once soil liquefied, than it may not liquefy again in another time. That assumption was due to the densification process of soil that take place during post liquefaction, thus soil is expected to behave stronger. However, that argument was found to be factually not correct. Previous researchers found that the re-liquefied soil could be more vulnerable than the soil that liquefied for the first time even when soil getting denser and denser in the following re-liquefaction. The illustration of this can be seen in the Fig. 5.
In Japan, it was reported that liquefaction had re-occurred at 150 sites during the period from year of 745 to 2008 from the map shown in Fig. 6. It was also reported that in last 2011 Great East Japan Earthquake alone there were at least 60 cases of re-liquefaction were found. At the same event, it was reported that some areas around ToneRiver in Northern part of Tokyo (Capital city of Japan) were also liquefied repeatedly as shown by dotted mark in Fig. 7(a). A year later, it was found later that the re-liquefied sites were actually former abandoned natural ponds which have been reclaimed for the construction of residential houses. In his report, Ishihara (2012) also found that re-liquefaction also appeared in the man made island around Tokyo bay as shown in Fig. 7(b). The structures build on man made islands are very important because many of them are vital infrastructures and strategic industrial factories. Generally, the area that is prone to liquefy repeatedly is the area where large amount of young sandy soils deposited such as reclaimed land, areas along river and river delta. These evidences raised the concern among researchers about the potential danger of re-liquefaction in the future big earthquake.
Why this research becomes so important for the author personally because Indonesia, the country where the author belongs, has similar geological characteristics as Japan. Similarly, Indonesia has been struck frequently by big magnitude of earthquake overtime as shown in Fig. 8(a) and Fig. 8(b). The most recent one which still fresh in our memory was the Great Sumatra Earthquake struck on December 2004 with the Magnitude of 9.1. Even though Indonesian government agency and researchers didn’t fully record the number of liquefaction occurrence during this earthquake, however, local people have spotted the occurrence of liquefaction-like phenomenon in many sites. Another reason of why the government agencies and researchers didn’t put much attention to it, because there was not much knowledge on the disaster induced by liquefaction. In addition, the liquefaction itself does not pose direct threat on human life if it occurred. In other areas of Indonesia, this liquefiable soil can be found easily. Indonesia is composed by many islands, this means Indonesia have vast numbers of shore and river, which large amount of liquefiable sandy soils are deposited.
As the Indonesian economic growing steadily of about 6 to 7% in recent years, we would expect that more and more infrastructure will be build in near future. However, some of these infrastructures will be inevitably build on the areas which are prone to liquefy or to liquefy repeatedly. Therefore, the important part of my study is to educate the author himself as well as sharing to the other Indonesian researchers to be concerned on the potential threat of liquefaction and re-liquefaction issues that might happen in the future. Hopefully, the results of this study will be used for mitigating the disaster related liquefaction, so stay tune for the next post…. cheers.