基坑围护结构的抗冲剪稳定安全系数的计算与应用

基坑最下道支撑设置过高会造成围护结构中剪力过大，导致围护结构发生冲剪破坏。这种破坏属于脆性破坏，具有突发性，可能造成灾难性的后果。目前常用的各类安全系数只能控制围护结构的插入深度，不能控制最下道支撑的设置高度，所以不能避免基坑发生这种破坏。为此，提出基坑围护结构抗冲剪稳定安全系数，描述围护结构冲剪破坏形式和特征，给出抗冲剪稳定安全系数的计算方法。研究表明，在其他安全系数满足要求时还应验算围护结构抗冲剪稳定性，分析最下道支撑设置高度的合理性。应用抗冲剪稳定安全系数可以解释国内某城市地铁基坑坍塌事故的破坏机制。计算分析表明，采用该安全系数控制最下道支撑高度可以使基坑的各种安全系数与插入比关系线性化。该安全系数形式简单、计算方便，在基坑的设计和施工中有重要的应用价值。

CALCULATION OF SAFETY FACTOR OF PUNCHING SHEAR-RESISTANT STABILITY FOR RETAINING STRUCTURE OF FOUNDATION PITS AND ITS APPLICATION

The shear force in retaining structure of foundation pits will be too large while the lowest support is set too high，which may lead to punching shear failure of the retaining structure. Such failure is a kind of brittle fracture and it usually occurs suddenly，which may result in disastrous consequences. The safety factor commonly used can only control the embedment depth of retaining structures，but they can not control the height of the lowest support. So，such failure can not be avoided. Therefore，the safety factor of the punching shear-resistant stability is proposed. The failure modes and features of punching shear failure of the retaining structure have been described. And then，the calculation method of the safety factor of the punching shear-resistant stability is put forward. The study results show that even all other safety factors are satisfied，it is still necessary to check the safety factor of the punching shear-resistant stability and to analyze the rationality of the lowest support height. The safety factor of the punching shear-resistant stability is applied to explain the failure mechanism of collapse accident of a domestic metro foundation pit. Calculation analysis also shows that using the safety factor to control the height of the lowest support can achieve a linear relationship between various safety factors and embedment ratio. The safety factor is simple in form and easy to calculate；so it has important applications to design and construction of the foundation pits.