让压锚杆让压管长度的解析设计方法

Analytical Design Method for Yielding Pipe Length of Yielding Bolt

为确保让压锚杆在高地应力软岩大变形隧道中起到良好的让压性能,保证围岩释放变形能的同时能有效保持围岩的稳定性,基于锚杆中性点理论,在分析围岩与锚杆的相互作用时考虑让压管的让压效应,假定由让压变形量引起的相对拉伸变形量沿隧道半径符合双曲线的变化规律,通过建立锚杆中性点两侧轴向拉力的平衡方程,推导全长锚固让压锚杆的内力计算公式。结合昆宜高速万溪冲隧道工程,得到让压锚杆的轴力分布情况及其随让压管长度的变化规律:锚杆最大轴力随让压管长度的增加而减小,而轴力的分布形式变化不大。在此基础上得出让压管合理长度范围的设计准则:由锚杆抗拉极限轴力确定让压管长度的下限值,以保证让压锚杆不因拉断而破坏;根据隧道洞壁处锚杆受到的黏结摩阻力确定让压管长度的上限值,以确保锚杆中性点理论假设始终成立。

A favorable yielding performance of yielding bolt in soft rock tunnel with high ground stress and the stability of surrounding rock when release its deformation energy should be guaranteed.As a result,an analytical design method for yielding pipe length of yielding bolt is proposed.Based on the neutral point theory of bolt,the yielding effect of yielding bolt is taken into consideration in analysis of the interaction between surrounding rock and the bolt;the relative tensile deformation caused by yielding deformation along the tunnel radius is supposed to be coincide with the hyperbolic curve rule is assumed;and the calculation formula of internal forces of full length yielding bolt is deduced by establishing the equilibrium equation of axial tension on the two sides of bolt neutral point.The axial force distribution of yielding bolt and its performance with the variation of yielding pipe length are obtained by application to Wanxichong Tunnel on Kunming-Yiliang Highway,and it is described as follow:the maximum axial force of the bolt decreases with the increase of yielding pipe length,while the distribution pattern of the axial force changes little.Based on the above-mentioned results,the design criteria for yielding pipe length is drawn as follows:(1)The lower limit of yielding pipe length is determined by the ultimate tensile force of yielding bolt to ensure that the bolt is not damaged due to pull-off.(2)The upper limit of yielding pipe length is determined by adhesive frictional resistance acting on bolt at the wall of tunnel to guarantee the bolt neutral point theory established.