In the previous post, we already discuss about the meaning of re-liquefied soil, their significance and their effects on human’s life. The behaviors of re-liquefied soil and the potential damage caused by it are still widely unknown even among researchers. The limited number of experimental studies in this particular topic was possibly due to the limitation of the test apparatus itself. General triaxial and hollow cylinder torsional shear apparatuses are hardly able to maintain the shape of the specimen once it liquefies. Therefore, in the previous experimental studies, the investigation of re-liquefaction (multi-stage liquefaction) could be made mostly up to two stages. In the current post, I would like to talk about specifically on one apparatus specifically design to simulate the re-liquefaction test so called stacked-ring shear apparatus. Unlike the tests conducted in triaxial or hollow cylinder torsional shear apparatuses, the stacked-rings shear apparatus is capable of maintaining the shape of the specimen to remain constant even under very large deformation. By taking such advantage it can perform not just a few number of re-liquefaction stages, but virtually unlimited number of re-liquefaction stages with a single specimen.
2. STACKED-RINGS SHEAR APPARATUS
The outline of the newly developed machine so called stacked-ring shear apparatus is shown in Fig. 1. The vertical load is applied to the specimen through a pneumatic system with bellofram cylinder while the torque is applied to the specimen through a direct motor system. This direct motor system allows the apparatus to virtually apply endless rotation in both clockwise and anti-clockwise directions. Both vertical stress and torque are measured at the top cap connected to the load cell. In the current setting of the apparatus, the capacity of vertical load and torque are 30 kN and 1500 N.m, respectively.
In the stacked-ring shear apparatus, an annular specimen is placed in between two parts of stacked-rings, which are inner and outer parts as shown in Fig. 2(a). Each inner and outer part is composed of 31 pieces of vertically-stacked annular rings having a thickness of 5 mm as shown in Fig. 2(b). It needs to be mentioned that there is no direct contact between the rings. Six pieces of metal bearings with a thickness of 0.1 mm were inserted in between the rings, so that friction can be reduced as minimum as possible. This 0.1 mm gap between the rings is small enough to ensure the sand particles with a mean diameter larger than 0.1 mm (D50 > 0.1mm) will not extrude during shearing. Each ring is allowed to move in circumferential direction, while it is restrained in the vertical direction. The inner and outer diameters of the specimen are 90 mm and 150 mm, respectively, and the height of the specimen is 155 mm.
Standard testing sand, so called Toyoura sand, was used as the test material. Its particles have an angular or sub-angular shape with the following physical properties: specific gravity, Gs=2.656; mean diameter, D50=0.162mm; fines content, Fc=0.1%; max. void ratio, emax=0.992; min. void ratio, emin=0.632. Specimens were prepared by pluviation of air-dried sand particles into a mold through air.
Figures 3(a) and Fig. 3(b) show the sequences to conduct a re-liquefaction test in this study. Prior to the application of cyclic shear loading, each specimen was consolidated one-dimensionally up to vertical stress of 200 kPa as shown on state B. Then, the specimen was subjected to cyclic shear stress (±t) under constant volume condition. The liquefaction was defined as the state whenever the double amplitude of shear strain reached e.g. 2.0%, while the cyclic loading was continued to achieve the pre-fixed gDA.max value (state C). Then, the stage of liquefaction was completed by adding another half cycle of shear loading from state C to state C’, where the shear strain (g) is equal to zero as shown in Fig. 3(c) and 3(d). State C’ in this study was set to be the starting and the ending states of each stage during re-liquefaction test. The next liquefaction stage was started by re-consolidating the liquefied specimen into their original effective vertical stress (sv’) of 200 kPa at state D. Then, the liquefaction test continued following the same procedure as the one described in the first liquefaction stage.
The liquefaction resistance was evaluated by evaluating the number of cycles needed to reach the double amplitude of shear strain 2.0%, using the equation shown below.
where, gDA is the target shear strain double amplitude, which in this study is 2.0%, gDA(Ni) and gDA(Ni+0.5) are the shear strain double amplitude measured during the loading cycle just before and half cycle after the target of shear strain double amplitude, and Ni is the number of cycle to liquefy just before the target of shear strain amplitude reached.
In the next post we will discuss some of the example of test results and finally draws the conclusions….so stay tuned.
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