Investigation of Rock Bolts in Karst

Marcic, D. (University of Zagreb) | Kovacevic, M.S. (University of Zagreb) | Bacic, M. (University of Zagreb)



A rock bolt represents the most important element in ensuring stability of rock mass excavations. This paper presents weaknesses and strengths of methods which deal with evaluation of rock bolts. Most widely used is pull-out test, conducted for capacity determination in destructive manner. To overcome ‘destructivity’, a method of acoustic emission shows that the force close to the failure point can be determined, so that the rock bolt will remain usable for strengthening purposes. Another NDT system, called GRANIT, is developed in order to determine load level inside rock bolt by analyzing its natural frequencies. When it comes to evaluation of grouting quality, efforts were made in order to find reliable solution, but existing methods still have weaknesses which limit their applicability. First developed and today still widely used is Boltometer which is based on emission of energy in rock bolt and on analyzing returned energy. However, dissipation of energy, due to karstic phenomena may not yield good results. Some efforts in improvement of weaknesses of Boltometer have been conducted and are presented in paper. A basic concept is given for novel method for NDT evaluation of grouting quality, which is in development at Faculty of Civil Engineering in Zagreb.