*Article* **Non-Destructive Evaluation of Rock Bolt Grouting Quality by Analysis of Its Natural Frequencies**

### **Mario Baˇci´c \*, Meho Saša Kovaˇcevi´c and Danijela Juri´c Ka´cuni´c**

Faculty of Civil Engineering, University of Zagreb, 10000 Zagreb, Croatia; msk@grad.hr (M.S.K.); djk@grad.hr (D.J.K.)

**\*** Correspondence: mbacic@grad.hr; Tel.: +385-1-4639-636

Received: 5 November 2019; Accepted: 6 January 2020; Published: 8 January 2020

**Abstract:** Grouted rock bolts represent one of the most used elements for rock mass stabilization and reinforcement and the grouting quality has a crucial role in the load transfer mechanism. At the same time, the grouting quality as well as the grouting procedures are the least controlled in practice. This paper deals with the non-destructive investigation of grouting percentage through an analysis of the rock bolt's natural frequencies after applying an artificial longitudinal impulse to its head by using a soft-steel hammer as a generator. A series of laboratory models, with different positions and percentages of the grouted section, simulating grouting defects, were tested. A comprehensive statistical analysis was conducted and a high correlation between the grouting percentage and the first three natural frequencies of rock bolt models has been established. After validation of FEM numerical models based on experimentally obtained values, a further analysis includes consideration of grou<sup>t</sup> stiffness variation and its impact on natural frequencies of rock bolt.

**Keywords:** rock bolt; grouting quality; dynamic response; natural frequency; finite element method
