This paper characterizes the waves induced in a grouted soil nail from axial excitations in an attempt to develop a non-destructive field procedure for determining the length of in-situ soil nails. The study comprised experimental tests in which a 3.66-m long grouted laboratory soil nail was built and tested and numerical analyses in which a finite element model (FEM) of the nail was developed and calibrated. The FEM clarified the experimental responses of the nail by providing wave velocities and mode shapes for natural frequencies below 5 kHz, which described the actual dynamic behaviour of the soil nail. The results showed that axial excitations of the composite soil nail induced modes, which displayed both shear and longitudinal behaviour. Wave velocities below 3000 m/s suggested shear wave modes of energy transport whereas the nearly uniform axial displacements within the cross sections of the nail were more in keeping with longitudinal modes of behaviour.