盾构施工对桩基的影响及桩基近邻度划分

隧道盾构施工过程中因盾构对土体的扰动和盾构脱出而没有及时注浆或注浆量不够,通常会对隧道邻近的桩基造成影响,如桩基内力发生改变从而使桩基变形、移位等,影响了桩基的承载力和正常使用。正交试验是一种高效、快速的找出某种指标的主要和次要影响因素的方法。通过建立盾构施工过程中近邻桩基的弹塑性有限元模型,结合正交试验和方差分析,得到了盾构施工对近邻桩基的影响因素大小。结果表明,盾构隧道施工过程中对近邻桩基沉降影响大小的因素依次为:桩与隧道的距离、桩顶荷载、应力损失和土体物理力学性质;桩基在(2.5³.0)D0(D0为隧道直径)以外,则桩基基本上不受盾构开挖的影响;桩顶沉降与桩顶原来的荷载相关性显著,桩顶荷载的大小反映了土体应力水平的高低。根据方差分析的显著性检验结果提出桩基近邻度的概念和计算公式,把盾构隧道周围的桩基分为非常近、近邻、远邻和非常远等4类桩,表明Ⅰ类桩和Ⅱ类桩受到盾构施工的影响较大,施工前应对桩基采取必要的保护和加固措施,Ⅲ类桩视情况进行处理,Ⅳ类桩一般不需要处理。

Shield construction influences on piles and classification of adjacent pile degree

Shield constructions usually have significant influences on adjacent piles due to soil disturbing, not grouting in time and shortfall grouting, which changes inner force and induces deformation and displacement. Orthogonal test is an effective and fast method to find major and minor factors on certain parameters. Based on the mode of elastoplastic FEM induced in the process of shield construction, combined with orthogonal tests and variance analysis, magnitude of shield construction influence factors on adjacent piles is got. The results show that magnitude of influence factors from large to small is distance between pile and tunnel, load applied on pile top, stress loss and properties of soil. Piles are not affected by shield construction beyond 2.5-3.0 times diameter of tunnel; and there is obvious relationship between pile settlement and pile loading which reflects state of initial stresses. Adjacent pile degree and formula are proposed according to results of significance testing of variance analysis, based on which, piles are classified into four categories: very near pile, adjacent pile, distant pile and very far away pile. The results show that there will have major impacts on first and second category piles that will be protected and strengthened, while third piles are taken measures regarding situations and the fourth piles needn′t be dealt with.