钙质砂中群桩模型试验研究

 针对取自南沙群岛永暑礁的钙质砂，利用室内模型试验研究钙质砂中群桩的承载和变形性能及其影响因素，并进行石英砂中的对比试验。试验结果表明，钙质砂由于颗粒破碎作用，导致其群桩承载和变形性能与石英砂有着显著差异。相对密度对钙质砂中群桩承载力影响较大，闭口群桩的承载力比开口群桩高17%～20%，但与石英砂相比很小，仅为石英砂的56%～71%。钙质砂中桩身轴力衰减速率缓慢，桩侧阻力值仅为石英砂的21%～30%，但具有深度效应，而且钙质砂中桩侧阻力对相对密度的变化没有石英砂敏感，受相对密度影响较小。钙质砂的承载力群桩效应系数在Dr = 46%，75%时均小于1，且随着相对密度的增加而增加，与石英砂有着本质的区别。该研究成果可为钙质砂桩基工程设计提供有益的参考。

MODEL TESTS OF PILE GROUPS IN CALCAREOUS SANDS

Calcareous sand is a special marine geomaterial medium，which has unexpected physico-mechanical properties due to its origin and fabric. The traditional practice，successfully used for other types of materials，is proved to be ineffective when used for pile foundations driven into calcareous sands. Model pile tests are carried out in a cylindrical testing chamber to investigate the bearing and deformation characteristics of pile groups in calcareous sands，which are taken from the Yongshu coral reefs in Nansha Islands in South China Sea. In contrast to the tests with silica sands，it is shown that calcareous sands behave in a completely different manner than silica sands due to grain crushing. Relative density has effect on the bearing capacity of pile group in calcareous，and the bearing capacity of close-ended pile group is 17%–20% larger than that of open-ended pile group，but it is less than that in silica sands. The axial force of pile group in calcareous sands has a slow decay rate，and the ultimate shaft friction of pile group in calcareous sands is only 21%–30% of that in silica sands，which is effected by depth，and it is very sensitive to density in silica sands but less in calcareous sands. In addition，when Dr = 46% and 75%，the bearing efficiency of pile group in calcareous sands is less than 1，and it increases with relative density，which is completely different from silica sands. The conclusion drawn from the study can provide valuable references for the practice.