Characterization of low placement ratio SCP and deformation of SCP ground under backfilled

通过一系列离心模型试验，研究了低置换率砂压桩加固软土在沉箱回填荷载作用下的短期和长期性能。在模型试验中，模拟了从沉箱安装到回填的施工过程。研究了置换比、改进宽度、SCP位置和加载速率对沉箱性能的影响。通过模型试验得出以下结论：(1)无论是在回填过程中还是回填后，较高的充填率都能有效降低沉箱的沉降。沉箱回填后的沉降受SCP段后地面沉降的影响较大。(2)较宽的填土改善面积对减小沉箱在填土过程中及填土后的侧向位移更有效，SCP地基中粘土部分的固结程度对沉箱的短期侧向变形有显著影响。(3)采用轻质材料作为回填体，可有效改善短期稳定性和长期沉降行为。在此基础上，利用空腔膨胀理论，推导出了考虑空腔替换率对弹簧刚度影响的锥尖阻力和轴阻力理论方程。通过压力计试验研究了替代率对刚度的影响，并在不同替代率的SCP中进行了离心锥突试验，验证了推导方程的适用性。结果表明：(4)SCP砂的路基反力k_h远小于同密度均匀砂，且k_h随置换比的减小而减小。结果表明，锥尖阻力和轴阻力均随A_s的减小而减小。(5)将计算得到的锥尖和轴阻力与观测值进行比较，证实了由锥尖阻力确定SCP虚角的可能性。少

Short and long term behavior of soft clay improved by sand compaction piles with low replacement ratios, which is subjected to backfilled caisson loading, have been investigated through a series of centrifuge model tests. In the model tests the construction sequence from the installation of the caisson to the backfilling was simulated in-flight. Effects of replacement ratio, A_s, improvement width, SCP location and loading rate on the behavior of the caisson were studied. From the model tests the following were derived :(1)Both during and after back filling, settlement of the caisson can be effectively reduced by higher replacement ratio. The settlement of caisson after backfilling is highly influenced by the settlement of the ground behind the SCP portion.(2)Wider improvement area towards the fill is more effective in reducing the lateral displacement of the caisson both during and after backfilling and the degree of consolidation of clay part in SCP ground has a significant effect on … More the short-term lateral deformation.(3)Using light-weight material as the back fill, both short-term stability and long-term settlement behavior can be effectively improved.Furthermore, by using cavity expansion theory, theoretical equations on the cone tip resistance and shaft resistance were derived, in which the effect of replacement ratio of the SCP is taken into account as the stiffness of spring assumed surrounding a SCP. The effect of replacement ratio on the stiffness was investigated by pressure-meter tests and applicability of the derived equation was examined by cone penetration tests in the SCP with different replacement ratio in the centrifuge. From these studies, the followings were obtained :(4)Subgrade reaction, k_h, of SCP is much smaller than the uniform sand with same density and the k_h, decreases with decreasing replacement ratio. As a result, the cone tip resistance and shaft resistance decrease with decreasing A_s.(5)Comparing the calculated cone tip and shaft resistance with the observations, the possibility of the determination of fiction angle of SCP from the cone resistances was confirmed. Less