【作者】 刁钰；

【导师】 郑刚；

【作者基本信息】 天津大学 ， 岩土工程， 2011， 博士

【摘要】 高层建筑和大型地下结构的发展促进了超深和超大基坑开挖的兴起,超深开挖强烈的卸荷和回弹作用,必然影响坑底抗压工程桩的承载力和沉降特性。首先进行了四组离心机模型试验,不但考察了超深基坑开挖对坑底抗压桩的竖向承载力特性和沉降特性的影响。而且,对比了具有不同表面粗糙度的桩在深开挖条件下,不同的响应。结果表明:对于光滑桩,超深开挖降低影响桩的承载力和刚度。对于粗糙桩,由于强烈的剪胀效应,模型试验的结果反而是超深开挖提高的桩的承载力和刚度。此外,超深开挖都造成了桩身产生拉力。采用ABAQUS中无剪胀效应接触面模型和摩尔库伦模型,对光滑桩展开有限元数值分析。结果表明,超深开挖卸荷效应降低了光滑桩法向应力,从而降低了承载力和刚度,而回弹效应不会造成承载力损失,但会减小桩的刚度。进而,改变桩的长度,开挖半径,桩的模量等,对超深开挖条件下的光滑桩进行了参数分析。结果表明,随着相对开挖深度和相对开挖宽度的增大,光滑桩的承载力和刚度损失值增大,而桩身拉力先大后小,出现峰值。此外,随着桩土模量比的增大,拉力越大,而且在开挖过程中出现的越早。改进了考虑剪胀效应的Norsand模型,使其弹性模量与围压和孔隙相关。引入了修正摩尔库伦模型,考虑桩土接触面剪胀效应,并应用Bolton公式对模型进行改进,使剪胀和强度可以反映围压的影响。在此基础上,模拟了粗糙桩的离心机试验。分析了超深开挖条件下的粗糙桩竖向传力机制,并与离心机试验结果进行了对比验证。结果表明,开挖增大了土的剪胀性,在加载过程中增大了桩的法向应力,提高了接触面剪切强度,从而使粗糙桩承载力和刚度提高。更多还原

【Abstract】 The high-rise buildings and large underground structures promote the development of super-deep excavations. The intense stress relief and basal heave will influence the capacity and settlement behavior of the compression piles beneath excavations.Firstly, four centrifuge modeling tests are carried out. The effect of deep excavation on the capacity and settlement behavior of piles is investigated. Also, the responses of piles with different surface roughness are compared. The results show that the deep excavation could reduce the capacity and stiffness of smooth piles. However, for the rough piles the capacity and stiffness are increased due to excavation. Beside, for both cases the excavation could lead to the tension in piles. The finite element analysis is conducted on the smooth piles using the interface model without dilation and Mohr-Coulomb model in ABAQUS. It is found that the stress relief during excavations reduces the normal stress of piles. Hence, the capacity and stiffness decrease. The basal heave would not lead to a reduction of capacity, but it could reduce the stiffness of pile.Furthermore, the parametric study on smooth piles under the condition of deep excavation is conducted by changing the length of pile, the radius of excavation and the modulus of pile. It is shown that the reduction of smooth pile increase with the increasing of normalized excavation depth and radius. The heave-induced tension in pile increases at the first and then decreased. Moreover, as the the modulus ratio of pile to soil increases, the tension in pile increases and appears earlier during excavation.The Norsand model with considering dilation is modified to make the modulus as a function of confining pressure and void ratio. The Modified Mohr-Coulomb model with dilation is introduced to capture the dilation of the pile-soil interface. The Bolton’s equation is adopted in the moidified interface model, which could consider the effect of confining pressure on the dilation and strength.On this basis, the centrifuge tests of rough pile are simulated. The mechanism of vertical load transfer of the rough pile is analyzed and the results of numerical simulation are compared with that of the centrifuge tests. It is found that the excavation enhance the dilation of the soil. This leads to an increase of normal stress and shear strength of the interface. Therefore, the capacity and stiffness of rough pile is increased.更多还原