考虑软土空间变异性之基坑隆起分析

研究旨在探讨于软土地盘进行基坑工程时,考虑空间变异性对于基坑隆起分析的影响程度。空间变异性对于基坑工程隆起分析的影响研究相当新颖,采用台湾建筑技术规则之隆起破坏分析法和随机场理论,透过蒙特卡罗模拟,可以同时考虑岩土材料性质的不确定性以及空间变异性。采用一个台北盆地实际的深基坑工桯隆起破坏案例,利用研究发展的可考虑空间变异性分析模式进行分析,和传统分析方法相较,分析结果显示考虑空间变异性其隆起破坏机率会降低。

基于一个上限分析方法的深基坑抗隆起稳定分析

提出了一个以塑性力学的上限分析理论为基础的深基坑基底抗隆起稳定分析方法;该法从地基极限承载力的普朗德尔-瑞斯纳解答的滑裂面出发,建立基本破坏模式。引入莫尔-库伦屈服准则得到其流动法则后,求得塑性区的协调速度场,从而通过虚功率原理求得深基坑基底的极限承载力,最终求得深基坑的基底抗隆起稳定安全系数;将该方法应用于上海浦东张杨路商业购物中心第一商厦的深基坑分析,揭示了该商厦施工期间基坑基底发生严重隆起破坏的内在原因。

软土地区基坑墙底抗隆起稳定性Prandtl计算式的改进

软土地区,应用Prandtl地基承载力计算式验算墙底抗隆起稳定性安全系数存在两个问题:一是不能反映随着围护体插入深度的增加,墙底抗隆起稳定性安全系数也随之增大;二是不能体现基坑宽度对墙底抗隆起稳定性安全系数的影响。本文在分析基坑开挖卸荷与应用Prandtl公式计算地基承载力的力学边界条件差异的基础上,认为现有的Prandtl计算式不足以反映基坑隆起破坏滑动模式;依据极限平衡计算原理,提出考虑围护体墙底以上基坑内外两侧土体抗剪作用的改进计算模式。按基坑宽深比为3的标准划分宽、窄基坑,并分别给出宽窄基坑抗隆起稳定性安全系数计算式。通过工程实例计算表明:本文提出的改进公式与成功实施的工程实例安全状况相符,能解释在软土地区随着围护体插入比的增大,基坑抗隆起稳定性得到提高的规律;按照宽窄基坑划分,可以合理计算窄基坑的抗隆起稳定性安全系数。本文研究结果可有助于软土地区基坑抗隆起稳定性计算评价和基坑围护结构的优化设计,也有待于进一步理论研究的补充完善和工程实践的验证。

软土地区超深沉井抗隆起计算分析

随着城市土地资源越来越稀缺,沉井在城市地下空间开发中的应用越来越广泛,沉井的深度和尺寸也随之加深、加大。目前规范对沉井设计未有抗突涌破坏的验算,施工单位理论水平认知相对较低,沉井下沉时突涌破坏案例时有发生。结合宁波软土地区某沉井破坏案例,参照基坑工程目前研究成果,以太沙基极限承载力假定为基础,考虑沉井外侧土体抗力,尝试对沉井抗隆起计算进行分析,并采用有限元分析软件MIDAS GTS对工程案例进行模拟分析,通过变形趋势对沉井抗隆起计算公式进行佐证,通过不同安全系数、不同平面尺寸对沉井的坑内外土体变形趋势分析结果,为宁波等软土地区其他超深沉井提供了有益的参考。

基于强度折减法的圆形深基坑坑底抗隆起稳定性分析

用于一般形状基坑的坑底抗隆起稳定分析方法因不能考虑圆形基坑环向土体挤压作用而不适合于圆形基坑的分析。基于非线性有限元提出了轴对称条件下基坑坑底抗隆起稳定分析的强度折减方法。研究表明:圆形基坑强度折减到破坏时的滑裂面形状与一般形状基坑的滑裂面形状相似,但圆形基坑的滑裂面并不通过围护墙墙底,而是通过墙底下方的土体。采用平面轴对称有限元的强度折减法分析得到的坑底抗隆起稳定系数较平面应变有限元强度折减分析得到的稳定系数高得多。当开挖深度很小时,圆形基坑具有较大的坑底抗隆起稳定系数;但其值随着开挖深度的增大而逐渐减小。圆形基坑的坑底抗隆起稳定系数随着围护墙插入比的增大呈线性增加。最后采用基于平面轴对称有限元的强度折减法分析了上海世博500kV地下变电站圆形基坑的坑底抗隆起稳定性。

颍河特大桥深水基础钢板桩围堰设计

文章对阜六铁路颍河特大桥主桥深水基础钢板桩围堰的设计过程进行了介绍,对于大型的钢板桩围堰的应力分析、基坑抗隆起分析提出了一整套设计思路,为相同类型的围堰结构的设计提供实践参考。

Differential uplift and settlement between inner column and diaphragm wall in top-down excavation

Top structure and basement will confront the risk of being damaged on account of large stress and strain fields incurred by differential uplift and settlement between inner column and diaphragm wall in top-down method. Top-down excavation of the Metro Line 10 in Shanghai was modeled with finite element analysis software ABAQUS and parameters of subsoil were obtained by inverse analysis. Based on the finite element model and parameters, changes in the following factors were made to find more effective methods to restrain differential uplift and settlement: length of diaphragm wall, thickness of jet-grouting reinforcement layer, ways of subsoil reinforcement, sequence of pit excavation, connection between slabs and diaphragm wall or column and width of pit. Several significant results are acquired. The longer the diaphragm wall is, the greater the differential uplift between column and diaphragm wall is. Rigidity of roof slab is in general not strong enough to keep diaphragm wall and column undergoing the same uplift during excavation; Uplift at head of column and differential uplift between column and diaphragm wall decrease when subsoil from-16.6 to-43 m in pit is reinforced through jet-grouting. But, as excavation proceeds to a lower level, benefit from soil reinforcement diminishes. During the process applying vertical load, the larger the depth of diaphragm wall is, the smaller the settlement is at head of column and diaphragm wall, and the greater the differential settlement is between column and diaphragm wall. When friction connection is implemented between column, diaphragm wall and floor slabs, uplifts at head of column and diaphragm wall are larger than those of the case when tie connection is implemented, and so does differential uplift between column and diaphragm wall. The maximum deflection of diaphragm wall decreases by 58% on account of soil reinforcement in pit. The maximum deflection of diaphragm wall decreases by 61.2% when friction connection is implemented instead of tie connection.