Issue 66

W. Frenelus et alii, Frattura ed Integrità Strutturale, 66 (2023) 56-87; DOI: 10.3221/IGF-ESIS.66.04

Accuracy Range

Rock bolt type

Sensor capability

Measurement type

Monitoring outcomes

Study

Sensor

By energy leakage at the interfaces rock and grout, guided waves are greatly reduced in grouted rock bolts. The effectiveness of guided waves relies on the low frequency mode. The premature failure of the rock bolts can be alerted thanks to the evolution of the distribution of the axial forces along the bolts.

Inspect the integrity of rock bolt using RBT

Non-grouted and grouted rock bolts

Ultrasonic guided waves

Stepinski [56]

Direct

0.1 MHtz

FBG-Based Instrumented Rock Bolt and FBG temperature sensors

Slotted and grouted rock bolt

Detect the early failure of rock bolts

Tang et al. [19]

Direct

0 – 100 kN

Monitor the continuous strain profile of the rock bolts Monitor rock bolt pre-load based on ultrasonic transducers Evaluate the distributed strain profile in the rock bolts subjected Assess corrosion induced strain in rock bolts Assess the grout quality into rock bolts Assess the degree of delamination to shear at millimeter scale.

Strain profiles are irregular and decrease from the borehole collar to the end of the embedded section of the rock bolts.

Fully grouted rock bolts

Vlachopoulos et al. [53]

Direct

1 Htz

DOS

Especially before saturation, the magnitude of the relevant signal decreases with the reduction of the preload in the bolt.

Huo et al. [63]

Grouted rock bolt

Direct

54 – 0 MPa

PZT

The strain profile consists of coaxial and bending strains. A limit of 120 mm from the plane of continuity should be considered for nearly 90% of the shear induced strain. Within the framework of accelerated tests, quasi- uniform strain evolutions were detected. Minor echoes in the energy dissipation in the rock indicate that the quality of the grout is good. The increase in wave amplitudes indicates the increase in the percentage of delamination in the rock bolts.

Fully grouted rock bolts

Forbes et al. [55]

Direct

No data

ROFDR

Direct and indirect

Wei et al. [59]

Grouted rock bolt

No data

FOMI

Buys et al. [64]

Grouted rock bolt

Ultrasonic guided waves

Direct and indirect

No data

He et al. [65]

Grouted rock bolt

Ultrasonic guided waves

0.97 – 1.17 MHtz

Indirect

at the rock bolt-grout interfaces.

Table 2: Monitoring of rock bolts with diverse sensors.

Figure 7: Illustration of direct installation and direct measurement of FBG in an instrumented rock bolts: red point traduces FBG sensors; red line is optical fiber

It is crucial to properly monitor the excavation damaged and the rock bolts/cable bolts to ensure the long-term stability of deep rock tunnels. In fact, one factor affecting the durability of rock bolts is the bolt length which should be sufficient and exceed the plastic zone of deep rock tunnels [57]. The plastic zone can generally be thought of as the EDZ [66]. For circular

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