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In addressing limitations of conventional ground support measurement, a novel optical strain sensing technique is presented that is capable of measuring strain at increments as low as 0.65 mm along the entirety of an optical sensor affixed to a support element. The technique considers monitoring three sensing lengths along the profile of a fully grouted rock bolt element using a single optical fiber, which, in turn, allows the derivation of both the principle strain and principle strain direction along the bolt. A series of three experiments, which include: symmetric bending, combined axial load and bending, and double shear loading are presented and highlight the potential of the technique to capture bolt behaviour under such generalized rock bolt loading conditions. Analogized as a distributed strain rosette, the optical technique can distinguish both the coaxial and bending induced constituents of the total strain in the bolt regardless of load orientation. [ABSTRACT FROM AUTHOR]

An analytical model is proposed to predict the axial force of grouted rock bolt in the tunnelling design. The interaction mechanism of the rock bolt and the soft rock mass has been described according to their consistent displacement. Coupling and decoupling behaviors of the rock bolt around a circular tunnel have been analyzed. According to case studies, the theoretical prediction of the axial force agrees well with the in situ measured results. The installing time and the length of the rock bolt, and the deformation modulus of the rock mass are taken as study parameters to analyze the supporting behavior of the rock bolt. According to the results of the theoretical analysis, there are some conclusions as followings: (1) a lower axial force is resulted because of the delay of installing rock bolt and its supporting effect may be reduced; (2) the larger displacement is caused by the lower deformation modulus of the rock mass, and a higher axial force is resulted in the rock bolt. If the shear strength of the rock mass is not enough, the decoupling behavior will take place interior the rock mass, and the performance of rock bolt may be reduced; (3) the position of a neutral point is related with the radius of tunnel, the physical properties of the rock bolt and the rock mass. It is found that the position of the neutral point and the maximum axial force of the rock bolt installed in the soft rock may tend to be constant when its length is long enough, which means that the supporting effect of the rock bolt can not be improved significantly only by increasing the length of the rock bolt. By using this model, a way is supplied to analyze the supporting behavior of the rock bolt, and a method is provided for the quantitative evaluation of its supporting effect in NATM tunnelling. [Copyright &y& Elsevier]

• A new energy-absorbing rock bolt was proposed, called the CEF bolt. • A theoretical model was developed to predict the constant resistance of the CEF bolt. • The effects of main parameters on the CEF bolt were investigated. • The applicability and accuracy of the current theoretical model were verified. To provide an effective support device for dealing with the instability problems in geotechnical engineering, this paper proposes a new energy-absorbing rock bolt based on the existing energy-absorbing devices, called compression-expansion-friction bolt (abbreviated as the CEF bolt). The CEF bolt mainly consists of two parts, a variable-diameter tube and a hollow cone attached at the distal end of the bar. Controllable constant resistance and large displacement are the characteristics of the CEF bolt. The constant resistance was generated by the expansion of the tube under the compression of the cone, as well as the friction between the tube and cone. And the large displacement was provided by the sliding of the cone relative to the tube. A theoretical model was developed to predict the constant resistance by taking into account the effect of material strain-hardening. Based on the energy conservation law, the effect of shear deformation was also considered. Further, the current model was formulated as three forms to consider different conditions. The predictions of the current theoretical model were in good agreement with existing experimental data. An experimentally validated finite element model was established to study the effects of main parameters on the constant resistance of the CEF bolt, such as cone angle, tube wall thickness, friction coefficient, expansion ratio and the strain-hardening property of the tube. The results of numerical simulation and theoretical analysis show that the constant resistance was directly proportional to the tube wall thickness, friction coefficient, expansion ratio and the strain-hardening modulus of the tube. But the constant resistance increased first and then decreased with respect to the cone angle, with an inflection point existed between 5° and 10°. The shear stress distribution and the deformation of the tube were investigated as well. According to the shear stress distribution, the zone under expasion of the tube can be divided into three characteristic areas: stress concentration area, smooth transition area, and stress re-concentration area. In addition, compared with the large number of experimentally validated numerical results, the wide applicability and sufficient accuracy of the current theoretical model were verified. By taking the derivative of the current theoretical model, the location of the inflection point can be obtained. These results can provide theoretical basis for the design and field application of the CEF bolt, as well as the similar structure. [ABSTRACT FROM AUTHOR]