Abstract
Rock bolts are one of the main measures used to reinforce unstable blocks in a rock mass. The embedment length of fully grouted bolts in the stable and competent rock stratum behind the unstable rock blocks is an important parameter in determining overall bolt length. It is required that the bolt section in the stable stratum must be longer than the critical embedment length to ensure the bolt will not slip when loaded. Several series of pull tests were carried out on fully grouted rebar bolts to evaluate the pull-out mechanics of the bolts. Bolt specimens with different embedment lengths and water/cement ratios were installed in either a concrete block of one cubic meter or in steel cylinders. Load displacement was recorded during testing. For some of the bolts loaded beyond the yield load, permanent plastic steel deformation was also recorded. Based on the test results, three types of failure mechanisms were identified, corresponding to three loading conditions: (1) pull-out below the yield strength of the bolt steel; (2) pull-out between the yield and ultimate loads, that is, during strain hardening of the steel; and (3) steel failure at the ultimate load. For failure mechanisms 2 and 3, it was found that the critical embedment length of the bolt included three components: an elastic deformation length, a plastic deformation length and a completely debonded length due to the formation of a failure cone at the borehole collar.
Highlights
Rock bolts are the most crude and basic rock support measure for stabilising rock surfaces
The embedment length of bolt samples used for such tests is usually short, less than 20 cm, and the bolt bar is loaded below its yield strength (Fy) with elastic deformations (Kılıc et al 2002)
The rock bolt used in the tests was a 20 mm diameter rebar bolt made of type B500NC steel, which is often used for rock support in tunnels in Norway
Summary
Rock bolts are the most crude and basic rock support measure for stabilising rock surfaces. The embedment length of bolt samples used for such tests is usually short, less than 20 cm, and the bolt bar is loaded below its yield strength (Fy) with elastic deformations (Kılıc et al 2002). If the encapsulated segment of a rock bolt exceeds the critical embedment length, plastic deformation occurs in the bolt bar before rupture (Li et al 2016b). Most rock bolts on site have extensive embedment lengths and in extreme cases, when loaded above their working capacity, are subjected to plastic deformation and necking before final failure. The main objective is to present a theory, and supporting test data, for the mechanics of grouted rebar bolts subjected to pull forces, with a special focus on bolts pulled with forces above the steel Fy. presented are the equations used to calculate the critical embedment length of rock bolts subjected to such loads
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