A hardening load transfer function for rock bolts and its calibration using distributed fiber optic sensing

Confinement of rock bolts by the surrounding rock formation has long been recognized as a positive contributor to the pull-out behavior,yet only a few experimental works and analytical models have been reported,most of which are based on the global rock bolt response evaluated in pull-out tests.This paper presents a laboratory experimental setup aiming to capture the rock formation effect,while using distributed fiber optic sensing to quantify the effect of the confinement and the reinforcement pull-out behavior on a more local level.It is shown that the behavior along the sample itself varies,with certain points exhibiting stress drops with crack formation.Some edge effects related to the kinematic freedom of the grout to dilate are also observed.Regardless,it was found that the mid-level response is quite similar to the average response along the sample.The ability to characterize the variation of the response along the sample is one of the many advantages high-resolution fiber optic sensing allows in such investigations.The paper also offers a plasticity-based hardening load transfer function,representing a"slice"of the anchor.The paper describes in detail the development of the model and the calibration/determination of its parameters.The suggested model captures well the coupled behavior in which the pull-out process leads to an increase in the confining stress due to dilative behavior.

Direct and relaxation methods for soil-structure interaction due to tunneling

This paper deals with the problem of tunneling effects on existing buildings. The direct solution,using the condensation method,is presented. This method allows the structural and geotechnical engineers to treat the problem separately and then assemble a relatively small matrix that can be solved directly,even within a spreadsheet. There are certain concerns that the resultant matrix may be ill-conditioned when the structure is very stiff. This paper suggests an alternative method that essentially relaxes the system from an infinitely rigid structure solution. As such,it does not encounter the problems associated with stiff systems. The two methods are evaluated for an example problem of tunneling below a framed structure. It is found that while the direct method may fail to predict reasonable values when the structure is extremely rigid,the alternative method is stable. The relaxation method can therefore be used in cases where there are concerns about the reliability of a direct solution.

Upper bound of tunnel face stability using asymmetric yielding

Although significant advancement has been made over recent years with respect to three-dimensional upper bound calculations of tun-nel facing,a considerable difference still exists between analytically and empirically based stability values.The current work suggests that the difference may well be the outcome of the traditional use of Tresca yield criterion for the upper bound calculations,which,by definition,does not distinguish among the shearing modes(compression,extension,plane strain).Consequently,this paper suggests and discusses a new yield function,which allows for asymmetric yielding.Such yielding is only beneficial in the case of three-dimensional and continuous velocity fields,and therefore a numerical procedure that generates relevant kinematically admissible fields for classical upper bound cal-culation is suggested.The procedure involves conversion from a load controlled boundary value problem to a velocity controlled problem at the limit state of collapse.The analysis results in significantly lower upper bound values than those presented earlier(for Tresca mate-rial),and the values are much closer to the stability curves of Kimura and Mair(1981),which are commonly used in design.