基坑的尺寸效应及考虑开挖宽度的抗隆起稳定安全系数计算方法

相比于宽基坑，狭窄基坑具有更好的稳定性和更小的变形，表现出明显的尺寸效应，相同挖深时窄基坑围护结构的入土深度可适当减小。目前设计采用的基坑稳定性分析方法基本上都没有考虑开挖宽度对安全系数的影响，导致狭窄基坑设计偏于保守。为保证窄基坑滑裂面也能够形成，通过移动滑动圆弧圆心方法，使滑裂圆弧通过围护墙底和基坑底与对侧围护墙的交点，以该滑裂面计算的抗隆起稳定安全系数 值可以考虑基坑宽度的影响。基于该理念，根据宽窄不同，把基坑分为不同种类，分别对应不同的Ks值计算方法。该方法可以在基坑足够宽时过渡到现有规范方法，保证与既有规范的统一，在狭窄基坑时， 。计算分析表明，在基坑设计时忽视尺寸效应，片面强调插入比是不科学的。由于仍然采用传统的抗隆起稳定安全系数的滑裂面形式，计算方法没有过大变化，适合在基坑设计和施工中推广应用，文中方法为狭窄基坑减小围护插入深度提供了理论依据。

Size effect of foundation pits and calculation method of safety factor of heave-resistant stability considering excavation width

Project practices show that the narrow foundation pits, behaving a significant size effect, have better stability and smaller deformation than wide ones; so the embedded depth of envelope structure for narrow pits can be properly reduced at same excavation depth. However, most present stability analysis method of foundation pits design did not consider the influence of the excavation width on the safety factors, which leads to conservative design for a narrow pit. In order to make slip surface come into being for narrow pits, by moving the center of the slip arc, let the slip arc through the bottom of the enclosure wall and the intersection of excavation bottom and contralateral enclosure wall. The safety factor of stability calculated based on this slip surface can consider influence of excavation width. According to this idea, foundation pits are divided into different types by different widths, corresponding to the different calculation methods of This method can be transformed to the calculation method in the existing norms at wide pit, which can ensure the its unity with the existing specifications; for narrow pit, . The analysis shows that ignoring the size effect in the excavation design and one-sided emphasis on the insertion ratio is unscientific. Because applying traditional slip surface form of safety factor of heave-resistant stability and no excessive change on the calculation method; the calculation method are suitable for use in the design and construction of foundation pits. This calculation method of provides a theoretical basis for the narrow pit to reduce insertion depth.