Understanding Adhesion and Bond Strength

In rock reinforcement, adhesion refers to a pseudo-chemical bond that forms between steel and cement at their interface. However, this bond is brittle and breaks easily with very little movement—less than 0.2 mm of slip (Fuller & Cox, 1975; Hyett et al., 1992).

This means adhesion provides very limited contribution to long-term bond strength. Once the cable starts slipping inside the grout, adhesion fails progressively along the embedded length and no longer helps in holding the cable.

Friction and Dilation Effects

Cablebolts are made from twisted steel wires. When grouted, these spirals leave an imprint inside the hardened grout, creating a mechanical interlock. As the cable begins to move:

• The ridges of the spiral press against the grout

• This causes dilation or expansion of the grout interface

• Pressure increases between the cable and grout, generating friction

This friction—dependent on normal pressure—is what resists the pullout force. However, once the pressure exceeds the limit of the grout ribs (usually < 0.1 mm displacement), these ribs can break and reduce the bond capacity (Diederichs et al., 1993).

Load Transfer Mechanism

The load from the rock is transferred through the cable to the surrounding grout via shear stress. This process involves:

• Rock moving or deforming slightly around the cable

• Creating stress at the grout-cable interface

• Transferring that stress through the cable over a defined "anchorage length"

In simple terms, when a cable is embedded in grout and the rock starts to shift, tension builds along the cable. This tension gradually increases from the point of loading until it reaches an anchor zone, then reduces back to zero where the load is fully transferred to the rock.

Key Concept – Anchorage Zones

• The "pick-up length" is where tension builds in the cable

• The "anchor length" is where tension is gradually relieved into the rock

• Proper design ensures these regions are long enough to transfer load without cable slippage or failure

Sources:

• Farmer (1975); Littlejohn & Bruce (1975)

• Fuller & Cox (1975); Hyett et al. (1992)

• Diederichs et al. (1993); Nosé (1993)