旋喷搅拌水泥土加劲桩受力机理及其应用研究

Jet mixing cemented anchor is a new kind of supporting technology for foundation pit engineering in soft clay. It is a large composite anchorage structure made of steel strand and cemented soil by swing-injected agitating, inclined in the foundation soil with a very small angle to the horizantal level. The grout is generally more than 500mm in diameter so the anchor is high in uplift stiffness while providing enough space inside the foundation pit for excavation and construction. Although it has been applied to many excavation projects and brought about significant economic and social benefit, the theoretic system of its work mechanism and calculation method quite lag behind its application, which delays the standardization of this technique. This scheduled fund project will find suitable additives to improve the function of Portland cement for soft clay and organic saturated soft soil through in-situ and lab tests. The interaction and optimizm of the internal structure will also be analyzed, as well as the load transfer mechanism of a single composite anchorage, the lateral displacement of the retaining wall, the settlement and the the safety factor of the ground near the excavation. A kind of removable anchor will be designed for the purpose of enviromentel protection. Finally the design and calculation method for jet mixing cemented anchor will be compeleted.

斜打旋喷搅拌水泥土加劲桩是应用于软土深基坑支护的一项新技术，其在沿接近于水平方向对软土用高压旋喷搅拌成直径约500mm的水泥土中设置钢绞索或锚杆，形成抗拔刚度较大的复合式锚固结构，为基坑内部留出宽旷的施工空间。这一新技术已成功地试用于上海等地的深厚软土基坑工程并产生了良好的技术和经济效益，但其机理和计算方法的理论研究滞后，影响了规范化的推广应用。本申请项目拟通过室内外试验和理论分析，探索提高淤泥质、高含有机质等饱和软土用硅酸盐水泥改良效果的添加剂，研究水泥土加劲桩的内部优化结构及其相互作用、单桩抗拔的荷载传递机理，建立基坑坑壁围护结构水平位移、坑边地基整体稳定性和地面沉降计算方法，并提出钢绞索工后回收技术，以最终形成旋喷搅拌水泥土加劲桩基坑支护技术的一套计算与设计方法。

为更好地应用斜打旋喷搅拌水泥土加劲桩（水泥土复合锚杆）新技术，本项目研究提出了高效水泥基的饱和软粘土固化剂及其强度估算模型；推导了水泥土复合锚杆的拉拔力-位移关系解析算法，并通过数值模拟和现场试验研究了锚杆-水泥土接触面形式和钢绞线根数及其安装方式对复合锚杆力学性能的影响，表明对常用直径和长度的水泥土搅拌桩，钢绞线不宜超过3根且以分散式安装为宜，而锚锭板对提高复合锚杆整体性有益；在现有基坑工程设计规范的受力模式下，对坑底上、下区域的桩身，分别利用麦考利奇异函数法和级数解法，建立了设有复合锚杆或内支撑的围护桩变形和内力的解析算法，以解决常用有限元等数值算法在单元划分等方面因人而异影响计算结果的问题；对设有复合锚杆或支撑的基坑围护体系，考虑支护桩身截面内力，提出了穿越桩身的潜在滑动面的整体稳定安全系数计算公式，安全系数最小的潜在滑动面并不一定是现行规范规定核算的经过围护桩底的那个滑动面；对坑边地表沉降预测，提出了基于水文学广泛应用的皮尔逊Ⅲ型概率密度函数的计算模型。项目还拓展研究了h型双排支护桩体系、液压高频振拔可回收性支护桩技术、坑底防承压水的降压降水优化方法；初步调查分析了居民房屋安全认知状况，为改善基坑工程环境保护提供参考。