On the previous post, I already discuss in detail about the stacked-ring shear apparatus itself. In the current post, the author would like to focus on the examples of some of the test results and to conclude some of the tentative findings.
As discussed on the previous post, the author would like to investigate the effects of pre-shearing history on re-liquefaction of sand. The pre-shearing history experienced by the soil from previous Earthquake is highly suspected to determine the resistance of next liquefaction on the future earthquake. Different pre-shearing histories experienced by the soil mean different soil behavior due to the changes of soil arrangement, fabrics and etc.
Two sets of test correspond to loose and dense specimens were sheared under different pre-fixed shear strain double amplitudes (gDA(max)) of 2.0%, 3.0%, 4.0%, 5.0%, 7.0% and 10.0%.
Figure 1 shows typical results of a re-liquefaction test. In each stage, the changes in number of cycle needed to liquefy and the changes in specimen’s density were evaluated. The results on re-liquefaction tests for both loose and dense sand will be discussed in detail as follows:
1. Repeated liquefaction behavior of loose sand
Four tests having similar initial relative density of about Dr0= 55.0% (e0=0.793) were sheared with different maximum shear strain double amplitudes (gDA(max.)) of 2.0%, 4.0%, 7.0% and 10.0%, respectively. Each of them was subjected to the cyclic shear stress (tcy.) of ±10 kPa.
Figure 2(a) shows the relationship between the number of cycle to liquefy and the liquefaction stage of loose sand. This figure indicates no significant increase in the re-liquefaction resistance of sand at least up to 5th stage in all tests. However, the re-liquefaction resistances started to increase exponentially from the 6th stage up to the last one. Figure 2(b) shows the relationship between the change in specimen’s relative density and liquefaction stage. All specimens showed almost linear increase in their relative densities in each of the liquefaction stages due to re-consolidation process in post-liquefaction. It can be noticed that the larger the shear strain applied, the larger the increase of the specimen’s relative density. By combining the results in Fig. 2(a) and 2(b), the relationship between number of cycle to liquefy and the change in specimen’s relative density is plotted in Fig. 3(a). This figure clearly shows that the increase of the specimen’s relative density did not directly translate to the increase of re-liquefaction resistance during early liquefaction stages. However, the liquefaction resistance did start to increase exponentially when the liquefaction stage continued further. It can be noticed that, the larger the shear strain applied, the weaker the soil resistance against re-liquefaction. This observation is in contrast with the afore-mentioned observation that specimens sheared with larger shear strains gained larger increase in their densities. Therefore, this may suggest that other factors could play more important role than the increase of the density alone in determining soil resistance against re-liquefaction.
In order to compare more in detail the re-liquefaction resistances in the early liquefaction stages, their change is shown in Fig. 3(b). From this figure, it can be seen that there was no unique correlation between the re-liquefaction resistance and the specimen’s relative density in the early liquefaction stages. Except the test sheared with 4% shear strain double amplitude, all tests showed the 2nd stage is the weakest stage against re-liquefaction.
Figure 3(a) also shows the comparison between the liquefaction resistance of re-liquefied soils and reference soils that liquefied for the first time. It can be seen that in the latter stages, the resistance of the re-liquefied soils were always larger than the reference ones. This may suggest that pre-shearing history also contributes to the increase in the resistance of re-liquefied soils except for the early liquefaction stages.
2. Repeated liquefaction behavior of dense sand
In order to verify the behaviors found earlier in the loose specimen tests, a series of test on dense specimens having initial relative densities of about Dr0= 80% (e0=0.702) were conducted. Six specimens were sheared under different maximum shear strain double amplitudes (gDA(max.)) of 2.0%, 3.0%, 4.0%, 5.0% (2 tests) and 10.0%, respectively. Each of them was subjected to the cyclic shear stress (tcy.) of ±20 kPa.
Figure 4(a) shows the relationship between number of cycle to liquefy and the liquefaction stage. Significant increase in the re-liquefaction resistance can be found only in the specimen sheared with lowest shear strain double amplitude of 2.0%, while others showed minor increase in their resistances. Figure 4(b) shows the relationship between the changes in specimen’s relative density and the liquefaction stage. Need to be noted that there were slight variations of about ±2.0% in the initial relative density among different specimens. However, the behaviors shown in dense specimen tests were similar with the ones in loose specimen tests, in which specimens sheared with larger shear strain showed larger increase in their relative densities. Figure 5(a) shows the relationship between number of cycle to liquefy and the changes in specimen’s relative density. It can be seen that the specimen sheared with the lowest shear strain double amplitude of 2.0% showed the strongest response, and subsequently followed by specimens sheared at 3.0%, 4.0%, 5.0% and 10.0%. These patterns also confirm the previous results which were found in the loose specimen tests. In the early stages, the irregular behavior that previously appeared in the loose specimen tests was also found in the dense specimen tests as shown in Fig. 5(b). Similarly, it was found that the 2nd stage of liquefaction test exhibited the smallest soil resistance against re-liquefaction in all tests.
Figure 5(a) also shows the comparison between the resistance of re-liquefied soils and the reference soils that are liquefied for the first time. Unlike the property of loose sands in which the resistance of re-liquefied soils is always higher than the reference ones, the results from dense sand might suggest it may not be always the case. After 10 stages of re-liquefaction, specimens sheared with the shear strain double amplitudes of 4.0% to 10.0% showed weaker responses than the reference soils. Under that condition, the re-liquefied soil could be more vulnerable than the soil which liquefied for the first time. However, it is expected that soil re-liquefaction resistance will increase exponentially when re-liquefaction stage continues further.
Both findings in loose and dense specimen tests appeared to be consistent to each other. It emphasizes the significance of pre-shearing history on the re-liquefaction resistance of soil. The larger the pre-shearing deformation history applied during previous liquefaction, the weaker the soil response against re-liquefaction. These behaviors possibly appear because larger deformation means greater changes in soil particle structure. It may also affect the re-liquefaction resistance during early stages in both loose and dense specimen tests as shown in Figs. 3(b) and 5(b). During these early stages, it was found that there was no unique correlation between the re-liquefaction resistance and the specimen’s density. The change on soil particle structure seems to become the predominant factor than the increase of soil’s density in determining the re-liquefaction resistance of sand. It was also found that the 2nd re-liquefaction stage exhibited the smallest soil response in almost all tests. However, it is expected when the specimens becomes denser and denser in the further stages, the soil resistance against re-liquefaction will become larger.
The investigation on the effects of pre-shearing history in the re-liquefaction behavior of sand revealed several observations, which are:
- Tests on loose and dense sand show re-liquefaction resistance of sand significantly affected by the pre-shearing history. The larger the shear strain history applied, the weaker the soil resistance against re-liquefaction.
- Inconsistent patterns on the re-liquefaction resistance of sand during several of early stages (e.g 1-4 stages) may imply that the increase of density alone is not the primary factor in determining the soil resistance against re-liquefaction, perhaps the changes in soil structure plays more important role.
- In addition to the point no. 2. It was found that the second re-liquefaction resistances of soil appeared to be always the weakest among all. This finding is consistent with the previous studies conducted by the researchers mentioned before.
- However, it is expected that as liquefaction stage goes further, the liquefaction resistance will increase exponentially. The density of soil becomes more predominant factor than others.
From some of the test results discussed above, we may realize that the soil re-liquefaction resistance on the future great earthquake may appear to be weaker than their first liquefaction resistance. This phenomenon is very important to the areas where large sandy soils were deposited e.g. reclaimed land as previously discussed in my first post.
My next post will share about the author experiences from the last 2011 Great East Japan Earthquake, lesson learned and sharing knowledge….so stay tune with Resultan Engineering.
(Including from previous posts)
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