Research On Effective Length Of Bolt And Its Effect On The Value Of The Bond Strength

Many experimental studies have shown that axial shear of anchor is uneven,however, the parameter of the bond strength still widely adopts the parameter of the single bond strength in engineering design of anchor. As a result, it can cause a waste of anchorage length when used in a large anchor engineering, and even lead to safety problems to a certain degree. Given the situation, the paper explores the effective anchorage length and its effects on the value of bond strength in a rock layer based on the existing researches and the experimental results of the pit of underground engineering of Snow World in Mount King of Changsha, with field pull-out tests of high-tonnage prestressed anchorage conducted. The main research contents and results are as follows:(1)Based on the anlysis of the research on the anchor solid stress strain test technology,a new method of concrete strain gauge to anchor effective anchorage length test has been explored.The numerical model is established and the pull-out process of anchor is simulated, with the mechanical and deformation characteristics of the grout concluded during the load delivery from the steel strand to grout and then to the anchoring layer. Based on this, the location of strain gauge is put forward and applied to on-site tests. Hence the effective anchorage length is deduced through testing the strain data of the grout.(2)Based on the characteristics of the limestone the and the free segment(50 metre)in background engineering,method of the load-displacement curve inversion has been researched.For this method,he derivation of the model of load-displacement relationship is given and the analysis of the free segment length’s effect on the calculation of the displacement at the top of the anchoring body is also put forward by taking the the variability of anchor rod body into account in this paper. Based on the existing instance, the studies of the free segment’s effects on the results of soil anchor and load-displacement curve inversion are given respectively.(3)For the load-displacement curve inversion parameter does not apply to the problem of rock bolt anchorage interface,based on the mechanical system of double broken line model, analysis of the process that how the load transfer to the ground from the anchoring grout has been conducted, and the influence of the anchoring length on the value of bond strength was illustrated.(4)Multiple pull-out tests of high-tonnage prestressed achorage were designed and conducted in a rock layer, from which the setting method of strain gauge was proposed and corresponding load-displacement curve and strain data of grout were obtained.Limit load and residual strength values were substituted into the calculating equation of the influence coefficient of bond strength, with different bond strength values of anchoring length obtained required by background project.