Ultrasonic Propagation Inflenced By Defects In Fully Grouted Rockbolt Under External Loads

The fully grouted rockbolt is widely used in mining engineering.In the actual construction of the rockbolt,the quality problems when constructing are inevitable.These problems include the separating of rockbolt and anchoring agent,the insufficient rockbolt length,the rockbolt corrosion etc.In addition,the preexisting natural joints also have obvious influence on bond quality,thus affecting safety of rock mass structure.Therefore,it is significant to test the bond quality and defect of the rockbolt quickly and effectively under different load regimes.In this paper,researched areas on the load transfer,defect detection and bond quality of the fully grouted rockbolt are investigated in various load regimes by methods of laboratory test and numerical simulation,based on the theoretical analysis of the propagation characteristics of ultrasonic guided wave.Here the combined effects of confining pressure and pullout load account for load changes.The main research aspects are as follows:(1)The propagation characteristics of ultrasonic guided waves are theoretically analyzed,and the guided wave dispersion equation is solved.The dispersion curves of group velocity,phase velocity and wave number are obtained,and the optimal frequencies are selected to provide a theoretical basis for numerical simulation.A laboratory apparatus is developed autonomously,and it can be used to detect the defect in grouted rockbolt systems during the process of the rockbolt pullout.The changes of confining pressure can be considered during laboratory test.(2)Fully grouted rockbolt pullout tests are carried out with grouted length 1.5 m and rockbolt diameter 18,20 and 25 mm.The effects of diameter and anchorage agent curing time on the load transfer behavior of the rockbolt are evaluated,with anchorage agent curing time 7,14 and 28 days respectively.The energy dissipation of the rockbolt in the pullout process is analyzed.Numerical simulation method is used to calculate and analyze the failure process of grouted system during the pullout process,which are difficult to record during laboratory test.(3)The pullout and ultrasonic guided wave detection tests of the grouted rockbolt systems with one bond defect are carried out.The stress distribution in rockbolt with bond defects is obtained.The length and position of the defect are determined based on the results of ultrasonic guided wave detection,and the guided wave propagation law in the rockbolt under load is studied.The bond quality of the rockbolt is quantitatively analyzed.Then,using numerical simulation is conducted to deduce the propagation process of guided wave in grouted rockbolt systems with defects,considering three kinds of defects(a bond defect in different location,insufficient length of rockbolt and natural joints in rockmass).The influence mechanism of load on the propagation process of guided wave in the rockbolt is analyzed.(4)Based on Fourier transform,using Wavelet multi-scale decomposition to deal with the ultrasonic guided wave detection signal in the fully grouted rockbolt under different pullout load and determine the bond quality of the rockbolt.The propagation process of guided wave in grouted rockbolt systems is analyzed through numerical simulation,under different bond length scenarios.The bond quality of rockbolt under different loads is studied.(5)In the laboratory,considering the confining pressure,the pullout and ultrasonic guided wave tests of rockbolt are carried out,the propagation of ultrasonic guided wave in the grouted rockbolt systems with the combined action of confining pressure and pullout load are analyzed.The bond quality of rockbolt is determined.Finally,based on the numerical simulation method,the propagation process of the ultrasonic guided wave in the grouted rockbolt systems(with the bond defect while without the bond defect)is analyzed in scenarios of changing only one factors of pullout loads and confining pressures in one test.