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压力分散型锚索锚固性能与设计方法研究
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摘要
压力分散型锚索是在一个钻孔中安装若干个锚索单元,每个单元都有自己的杆体和锚固段,在张拉时分别承受相同的工作荷载;锚索总的锚固力由分散布置于钻孔不同深度处的这些单元锚索的锚固段来共同承担,并利用各单元的承载体将无粘结锚索的拉力转化为对锚固段注浆体的压力,从而将锚索总的锚固力以压力形式分散作用于不同深度的岩土体上。压力分散型锚索主要有两种结构型式,本文是以将不同长度的无粘结钢绞线分别绕过不同的承载体弯曲成“U”形而组成各个单元锚索体这种型式为研究对象,对其承载力、荷载位移特性、锚固段位移特征、应变与粘结应力分布特征以及锚索体的可回收性能展开研究,并借助于理论分析研究其粘结应力分布状态,然后在此基础上提出了压力分散型锚索的设计计算方法。
     本文以现场试验为主要研究手段,试验锚索数量为三根,由两级单元组成,单元锚固段长度为4m。按照常规的锚索施工工艺进行造孔、制作、安装、注浆与张拉等各道工序的施工,并在最后对锚索体进行了回收。试验结果,锚索的最大试验荷载在600kN以上,其中一根的最高达650kN,且锚索仍处于良好的工作状态。作为试验的附属产品,设计和研制出了具有工程实用性的锚索承压板与钢绞线弯曲装置。
     通过对锚索进行循环加、卸载试验,观察锚索的荷载—位移变化关系,根据荷载位移数据分析锚索在不同荷载级别条件下的位移特性,对其评价指标表观自由长度进行了计算。并对锚索锚固段位移的变化特征及其影响因素进行了初步分析。
     试验测试手段通过在锚索承载体上布设应变测点,观测锚索在不同的荷载级别下的应变变化情况,对不同荷载级别下,单元锚索注浆体内应变的影响范围及分布规律展开分析,依据虎克定律计算出了其轴力与粘结应力的分布;并对测试结果进行了拟合,得出了锚固段粘结应力分布的拟合方程。试验结果表明,加载对锚固段内的应变影响范围大致在5~6m左右,4m以外很微弱;锚固段注浆体内的应变与粘结应力分布按照负指数规律衰减。
     在一定的简化假设基础上,根据局部变形假定和Mindlin位移解,分别推导出锚索注浆体内的粘结应力、轴力与应变分布公式,并将其与试验结果进行了对比分析,表明局部变形假定解与试验结果能够较好地吻合。
     最后,综合国内外的相关研究成果以及现场试验成果和理论计算结果,提出了压力分散型锚索的经验设计方法和基于峰值粘结应力的设计计算方法。在经验设计方法中,作者认为,在荷载由锚索锚固段向地层传递过程中,沿各个单元锚固段上的粘结应力分布并不是均匀的,并引入了一个长度有效系数来考虑这种现象,从而建立了各单元锚索极限承载力和锚固长度等参数的设计计算公式。峰值粘结应力方法中,根据锚索在不同的受力条件下其峰值粘结应力所发挥的程度分别给出了锚索极限承载力和单元锚固长度设计计算公式。
Compression dispersion-type anchor involves the installation of a multiple of unit anchors into a single borehole. Each unit anchor has its own individual tendon, its own unit fixed length of borehole, and is loaded with its own unit stressing jack which ensures that the load in all unit anchors is always identical. The total working load of the anchor is beard by each fixed anchor length of all units in staggered depth in the borehole and by means of respective bearing bodies, the tension loads applied to the plastic coated strands are changed into the compression loads on the grout of each fixed anchor length, then the total working load can be divided into several smaller loads and transmitted to the rock mass and soil in different locations in dispersed compression. Compression dispersion-type anchor can be mainly classified into two categories, one of which is that, plastic coated strands in different length are respectively passed through their own bearing bodies and pre-bent as their midpoints to be t
    he shape of "U" , then forming several totally unbonded strand loops, i.e. unit anchor tendon. As the research object in this paper, study is performed on the anchor holding capacity, load-displacement behavior, displacement of the fixed anchor length, strain and bond stress distribution, and the removability of anchor tendon, furthermore, with the aid of analytical solution, the distribution of the bond stress is calculated and analyzed, based on the results of test and analytical solutions, design method is provided in this paper.
    On-site tests are taken as the main research way in this paper, in which total three trail anchors are installed, each consisted of two units with 4m fixed lengths. Conventional construction techniques have been employed in all procedures such as drilling, fabrication, installation, grouting and stressing. All anchor tendons are removed from the borehole in the end. In the tests, the maximum loads of three trail anchors reach above 600kN, one of which is 650kN, and all anchors are still in good working conditions. As products of the tests, anchor bearing body and a device for strand pre-bending practical for engineering application have been developed.
    In the on-site test, with loading and unloading in several cycles, load-displacement data is monitored with which the load-displacement behavior and reproducibility are investigated, and according to related specifications, apparent free tendon length of anchor is calculated for assessment of anchorage. At the same time, behavior and some influence factors of displacement of the fixed anchor are analyzed.
    As the main method of measurement in the tests, some strain measuring points are arranged on the reinforced steel bars of the anchor bearing body. The changes of strain with different loading levels at their locations have been measured, through which range of influence and distribution of strain with loading in grout can be investigated, and corresponding distributions of axial force and bond stress are calculated according to Hooke's Law. Making use of nonlinear curve fitting, distribution equation of bond stress of the fixed unit anchor length is achieved. It is indicated in the
    
    
    
    test that the influence of loading to strain in the grout was about in the range of 5~6m, and very weak beyond 4m. The distributions of strain and bond stress in the grout are attenuated in the minus exponential rule.
    Based on Winkler Assumption and Mindlin's solution of displacement, elastic solutions for distributions of strain, axial force and bond stress are derived, and the influence of mechanical parameters to those are discussed. The elastic solutions are compared with results of testing, which indicate that the solution from Winkler Assumption can be well tallied with the test results.
    In the end, according to achievements from home and foreign countries in related research and those gained in the on-site test, two design methods for compression dispersion-type anchor are provided in this paper, the first one is experiential des
引文
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