Bond Strength of Plain Strand Cablebolts

Introduction:

Cablebolts are essential in reinforcing rock or concrete structures, providing stability and strength. Understanding the bond strength of plain strand cablebolts is crucial for ensuring their effectiveness in various engineering applications.

Adhesion and Bond Strength:

Adhesion refers to the bonding mechanism between steel and cement grout. This bond is brittle and breaks easily under stress, offering minimal residual strength after rupture. For typical carbon steel and cement grouts with a water-to-cement ratio (W:C) of 0.35 to 0.5, the adhesion or shear resistance ranges from 1 to 3 MPa. For a 15.2 mm diameter cable, this translates to a capacity of 10 kN over a 20 cm length. However, this bond fails after less than 0.2 mm of slip, making it unreliable for significant load transfer.

Slip, Dilation, Friction, and Bond Strength:

When adhesion fails, the cable slips within the grout. The helical structure of the cable creates ridges in the grout, leading to a geometric mismatch as the cable moves. This mismatch causes the grout to compress, generating normal pressure and friction at the interface. This process, known as dilation, is crucial for cablebolt performance. Dilation is limited by the height of the grout ridges, typically less than 0.1 mm for plain strand cables.

Bond Strength and Load Transfer:

Load transfer from the rock to the cable occurs through shear resistance at the cable-grout interface. As the rock moves, shear stresses build up along the cable, increasing tension in the steel strand. The tension peaks at a certain point in the borehole and then decreases as the load is transferred back to the rock. This process ensures that the cable effectively anchors the rock, preventing significant displacement.

Load Transfer Example: Slab Displacement:

Consider a slab of thickness A displacing under gravity. If the bond strength along segment A is insufficient, the shear stress becomes constant as the slab slides off the cable. The tension in the cable increases linearly from the face to the separation plane between segments A and B. Segment A, the pick-up length, transfers the load to segment B, the anchor length, where the shear stresses reverse, transferring the load back to the rock.

Load Transfer Example: Fractured Ground:

In fractured rock, the displacement profile is non-linear, with maximum displacement at the face reducing to zero into the rockmass. The boundary between the loading section (A) and the anchor section (B) becomes undefined. The neutral point, where relative displacement is zero, marks the boundary between the pick-up and anchor lengths. Above this point, the load is transferred back to the rock, and the cable tension drops to zero.

Conclusion:

Understanding the bond strength and load transfer mechanisms of plain strand cablebolts is vital for their effective application in rock and concrete reinforcement. Adhesion, while initially present, quickly fails, making dilation and friction the primary mechanisms for load transfer. Proper design and installation ensure that cablebolts can effectively stabilize structures, even in challenging conditions like fractured rock.

Sources:

• Farmer, I. W. (1975). "Stress distribution along a resin grouted rock anchor." International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 12(11), 347-351.
• Hyett, A. J., Moosavi, M., & Bawden, W. F. (1992). "Load distribution along fully grouted cablebolts with special reference to stress corrosion cracking." In Proceedings of the 11th International Conference on Ground Control in Mining (pp. 1-10).
• Diederichs, M. S., Kaiser, P. K., & Eberhardt, E. (1993). "Damage initiation and propagation in hard rock during tunnelling and the influence of near-face stress rotation." International Journal of Rock Mechanics and Mining Sciences, 41(5), 785-812.

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