Estimation method for liquefaction of reclaimed soil including wide range of particle size considering gravel content

Hirofumi TOYOTA

The phenomena of Liquefaction was observed in reclaimed ground of decomposed granite including gravels, which is referred to as Masado, during the 1995 Hyogoken Nambu Earthquake. Those gravelly soils have been classified as hardly liquefiable soil and excluded from examination materials because they have larger uniformity coefficient and dry densities than those of sandy soils. Liquefaction of the Masado prompted to re-exanimate the current liquefaction estimation and the study about liquefaction concerning various soils came to be carried out actively. Although plenty of research has been carried out on liquefaction of sandy soils, it is important to clarify the liquefaction strengths on gravelly soils include a wide range of particle size because little is known about them.
In the present study, field investigation was carried out to examine the characteristics of the ground, which was made by reclaiming the inshore in Hirokawa town of Wakayama Prefecture. Moreover, various laboratory tests were executed to examine the static and dynamic strength using the reclaimed soil with a wide range of particle size including gravels. Finally considering the results of field investigation and laboratory tests synthetically, possibility of liquefaction on the reclaimed ground was discussed.
The results obtained in the research are follows:
1. In uniform sandy soil, the density can be widely changed by the wet tamping, which is sample preparation method, and the liquefaction strength increases with the density. However, in the soils that contain a lot of fine-grained fractions, the density is mainly controlled by saturation and consolidation process. Therefore the compaction control is more effective to restrain the subsidence induced by consolidation than to increase liquefaction resistance.
2. In the soil with a wide range of particle size including gravels, when the specimens were stratified by the gravel and other soil under constant dry density, the liquefaction strength slightly decreases. The reason for this is that cohesion-less gravel layer might have small liquefaction strength. This effect should be considered in the real reclaimed ground because the stratified structure is easily formed during reclamation of coastal area.
3. Liquefaction strength doesn't change too much even if gravel content increases up to about 20% in mass in the soil with a wide range of particle size.
4. Although the penetration resistance becomes large by the existence of gravel in the field penetration tests, the liquefaction resistance doesn¡Çt necessarily increase through the gravel content.
5. From the results of experiments, it is appropriate that the soil with a wide range of particle size including gravels like Hirokawa soil should be judged as gravelly soils because the penetration resistance certainly increases with gravel content in spite of small value of D50. Therefore, a new regulation, which is provided by not only D50 but also gravel content, should be establish for soils with gravel on liquefaction in the specifications for highway bridges.

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