【作者】 郑卫锋；

【导师】 贾金青； 邵龙潭；

【作者基本信息】 大连理工大学 ， 结构工程， 2007， 博士

【摘要】 深基坑预应力锚杆柔性支护技术作为一种安全可靠、经济可行、快速简便的基坑支护新技术，已在深基坑开挖支护过程中得到了广泛的应用。但目前对预应力锚杆柔性支护技术的理论研究远远落后于工程实践，由于设计不当造成的深基坑工程事故还时有发生，因此有必要对此技术的工作原理进行更深入的研究与探讨。本文试图通过试验研究、理论推导、数值计算等三个方面，对预应力锚杆柔性支护的变形与稳定等工作性能进行了较为全面、深入地研究。主要研究成果如下：(1)全面总结归纳了预应力锚杆柔性支护体系的基本构成及其主要特点；对锚下承载结构的各个构件进行有限元数值模拟分析，分析了可能发生的破坏形态，得到了工程中常用的各个构件间的最佳匹配原则；利用传统的极限平衡理论，概括了该技术的实用计算方法；与土钉支护进行了比较，无论在控制基坑变形还是在提高基坑稳定性方面，预应力锚杆柔性支护都比土钉支护具有一定的优越性。(2)基于数字图像测量系统，对基坑开挖过程中土体的卸荷应力路径进行了试验研究。室内试验结果表明，基坑开挖卸荷试验时土体达到破坏时的主应力差明显小于常规三轴加载试验的剪切破坏值；基坑开挖卸荷后，土体的抗剪强度指标有所降低，特别是土体的内摩擦角的变化，在基坑工程设计时应引起重视。(3)基于凯尔文问题的位移解得到了单根预应力锚杆锚固段的剪应力与轴力的分布函数。研究结果表明：不同岩土体介质条件下，预应力锚杆破坏方式不同，要特别注意以最薄弱环节作为锚杆设计控制标准；预应力锚杆在荷载作用下的最大剪应力位置并不是在自由段与锚固段的连接处，而是在该点以下的某个位置上，剪应力分布沿着锚固段杆长从零迅速增大到最大值，然后再逐渐减小到零；预应力锚杆杆体剪应力的分布与岩土体的力学性质有关，岩土体越坚硬，剪应力分布越集中，反之岩土体越松软，剪应力分布越均匀。基于有限差分法中的CABLE单元，对单根预应力锚杆的作用机理进行了数值分析。数值模拟与工程实测数据均表明：注浆压力对预应力锚杆的极限拉拔承载力有着显著影响，通过高压注浆等措施可提高预应力锚杆的极限拉拔承载力；增加锚固段长度可提高预应力锚杆的拉拔力，但锚杆的极限拉拔承载力与锚固段长度呈非线性增长关系，当锚固段长度达到一临界值时，再增加对锚杆极限拉拔承载力提高毫无意义，因此单纯通过增加锚固段的长度来提高锚杆的极限拉拔承载力并不可取。(4)基于有限差分法与滑面应力分析法，对采用预应力锚杆柔性支护的深基坑开挖过程进行了数值模拟分析，对影响基坑变形与稳定的因素进行了敏感性研究。研究结果表明：①有限差分法结合滑面应力分析法，可以得到基坑的变形状况与基坑的稳定安全系数，既能反映土体的受力和变形情况，也可逐步模拟基坑开挖过程，可用来客观评价基坑的安全稳定性；②预应力锚杆柔性支护下基坑坑壁水平位移和地表沉降均呈曲线分布且相互对应，坑壁水平位移最大值常常发生在基坑顶面，随深度的增加而逐渐减小；地表沉降最大值发生在坑壁处，随背离坑壁距离的增大而逐渐变小；提高锚杆预应力值对减小基坑位移有显著作用；③锚杆的预应力对基坑滑移场产生很大影响。随着锚杆预应力的增加，基坑潜在滑移区不断缩小，当预应力达到一定值时滑移区变得很小，甚至消失；④基坑的破坏性态主要表现为基坑上部的拉伸破坏与基坑下部的剪切破坏，且随着预应力的增加，基坑塑性区明显减小；⑤结合有限差分法与滑面应力法，可以很好的得到不同支护条件下基坑的潜在滑动面形状与基坑的稳定安全系数，考虑基坑开挖的应力场分布状态得到的基坑稳定安全系数随着锚杆预应力的增加逐渐增大，但提高的幅度越来越小，因此单纯通过提高锚杆预应力的措施来保证基坑的稳定并不可靠。⑥岩土体参数中容重、内摩擦角、粘聚力三个传统参数的选择和锚杆参数中锚杆预应力、自由段长度、锚杆倾角三个设计参数的选择对基坑稳定与变形至关重要。更多还原

【Abstract】 As a safe and reliable, economical and feasible, and easily operated technique, the prestressed anchor flexible supporting system has been widely used in the deep foundation pit. However, the current theory research on prestressed anchor flexible supporting technique is far lagged behind the engineering practice. There are still accidences, related to deep foundation pit occurring, because of poor design; hence it is necessary to conduct deep research about the theory principle about this technique.This paper conducts comprehensive and in-depth research on the deformation and stability of pre-stressed anchor flexible supporting with respect to the aspect of experiment research, theory research and numerical computation. The research is reduced as the following:(1) The basic structure and characteristics of prestressed anchor flexible supporting system is comprehensively summarized. The components of bearing structure under the anchor is analyzed using finite element numerical method, and the optimal matching principle is obtained. The practical computation method for this technique is generalized in use of traditional limit balance method. The prestressed anchor flexible supporting system has advantage in foundation pit deformation control and stability improvement, compared with soil nail supporting system.(2) Based on digital image measurement system, the experiment about unloading stress path of the soil mass during excavation is conducted. Lab experiment shows that stress deviator of soil mass at damage state during excavation is obviously smaller than shear stress of general tri-axial experiment. After foundation pit excavation unloads, soil shear strength is lowered, and the variation of internal friction angle should be taken into account for the foundation pit design.(3) Based on Kelvin’s displacement solution, the distribution function of anchor sector shear stress and axial stress for a single prestressed anchor bar. The research displays that, different prestressed anchor bar damage mode corresponds to different soil condition, and the anchor bar design criteria should be set to the weakest point. The location of maximum shear stress is not the connection point of free sector and anchor sector, but is somewhere else beneath that point, and the shear stress increases to the maximum and then decreases to zero along the anchor sector. The shear stress distribution is related to soil mass mechanical characteristics, and there will be stress concentration for hard soil and it will be uniform for soft soil.The single prestressed anchor bar mechanism is numerically analyzed through CABLE element in finite difference method FLAC2D. Numerical simulation and engineering practice both show that grout injection pressure has great influence on prestressed anchor bar limit tension capacity, which can be improved through high pressure grout injection technique. The prestressed anchor bar limit tension capacity can be improved by increasing the length of anchor sector, but the relationship of anchor bar limit capacity and anchor sector length is nonlinear. When the anchor sector length arrives to the critical value, it will be useless to increase the limit tension capacity. Hence it is infeasible to simply increase the anchor sector bar length to improve the limit tension capacity of anchor bar.(4) Based on finite difference method FLAC2D and slip surface stress method, the foundation pit with prestressed anchor bar excavation process, along with the factors related to foundation pit deformation and stability, is numerically analyzed. The result illustrates that: (1) The finite difference method, combined with slip surface stress method, can show the deformation and stability safety factor of the foundation pit, which can reflects the stress state and deformation of the soil mass, and can also simulate the excavation process, then the foundation pit stability can objectively evaluated. (2) The horizontal displacement of foundation pit with prestressed anchor bar flexible supporting corresponds to the vertical deformation, both of which are linear distribution. The maximum horizontal displacement usually takes place on the top of the foundation pit, and it decreases with the increasing of the depth. The maximum vertical deformation occurs on the pit wall, which decreases with the increasing of the distance from pit wall. Increasing prestressed value can obviously decrease foundation pit displacement. (3) The anchor bar prestress has great influence on foundation pit slip field. With the increase of prestress, the slip zone shrinks and even disappears when the prestress equals to critical value. (4) The main damage style is tension damage on the top of the foundation pit and shear damage at the bottom, and the plastic zone shrinks when the prestress increases. (5) The finite difference method along with slip surface stress method, can obtain potential slip shape and safety factor for foundation pit with different supporting condition. Taking the stress field during foundation pit excavation into consideration, the safety factor increases with the increase of anchor bar prestress, but the increase amplitude is gradually decreasing, so it is infeasible to guarantee the foundation pit stability simply through increasing the prestress of the anchor bar. (6) The soil mass gravity, internal friction angle and cohesive, and the parameter of anchor bar, prestress, free sector length and inclination angle are of great significance for stability and deformation of the foundation pit.更多还原