Abstract
The stability of tunnels and rock slopes is adversely affected by defects in rock bolts. This study investigates the suitability of the smart sensing method using electromagnetic waves for inspecting defects in rock bolts. Experiments were performed with one fully grouted and eight defective rock bolts, out of which five have non-grouted parts at the ends with different non-grouted ratios, and three have different types of voids. Electromagnetic waves were generated and detected using a time domain reflectometer by configuring two-conductor transmission lines in the rock bolts. Results show that electromagnetic waves are reflected both at defects and ends of rock bolts. The electromagnetic wave velocity increases with an increase in the non-grouted ratio and decreases when rock bolts are embedded in a concrete block simulating rock mass. The estimated locations of defects found by electromagnetic waves are in good agreement with actual defect locations. This study demonstrates that smart sensing using electromagnetic waves is an effective method for inspecting and determining defect locations and the non-grouted ratio of rock bolts.
Highlights
Rock bolts are widely used as permanent or temporary support systems for stabilizing rock slopes and tunnels
They show that electromagnetic waves are reflected at the head, end, and at defects of rock bolts
All round-trip travel times (t0 ) of electromagnetic waves reflected at the heads of all rock bolts in the non-embedded condition are 0.88 ns
Summary
Rock bolts are widely used as permanent or temporary support systems for stabilizing rock slopes and tunnels. A typical rock bolt installation procedure occurs in the following order: first, rock masses are drilled to make holes, rebars are inserted into holes and holes are filled with grout material to bond rebars to the surrounding rock mass. Various defects can be generated in rock bolts during construction. Grout materials occasionally flow out due to gravity, resulting in a non-grouted part at the end of the rock bolt. The collapse of the wall, ground water flow, insufficient insertion depth of tremie pipe, improper withdrawal rate of tremie pipe, and poor consistency of the grout mixture, grout flowing into voids of a fractured rock mass may adversely affect the grout quality and create voids in the grout [3,4,5,6,7]. Lee et al [8] reported that the defect occurrence rate in tunnel construction is 15.8%
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