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Chapter 11 — Rockbolts, Cablebolts & Shotcrete Support Systems

11.1 Fundamental Support Mechanisms

Ground support systems stabilize excavations through three distinct mechanical mechanisms:

  1. Suspension: Suspending dead-weight rock blocks or loose stratified layers across unstable spans into competent overlying rock strata.
  2. Beam Building (Lamination Clamping): Bolting thin, stratified sedimentary roof beds together creates a composite structural beam. Clamping normal stresses generate inter-bed shear friction, suppressing slip and reducing roof deflection by a factor of \(n^2\) (where \(n\) is the number of bolted laminations).
  3. Reinforced Rock Arch (Keying & Confinement): In blocky or heavily fractured rock, pattern bolting with surface shotcrete applies radial confining stress (\(\sigma_3 > 0\)). This mobilizes the Hoek-Brown strength envelope, transforming broken rock into a self-supporting structural compression ring.

11.2 Classification & Selection of Rockbolts

A. MECHANICAL EXPANSION SHELL     B. FULLY GROUTED RESIN REBAR      C. FRICTION BOLT (SPLIT SET)
   [Anchor]=======[Plate]            [====Resin Grout====][Plate]      [===Expanded Tube===][Plate]
   Point-anchored; creeps            Fully coupled; stiff              Frictional; dynamic yield
Bolt Type Anchorage Mechanism Load-Deformation Characteristic Primary Application
Mechanically Anchored Expansion Shell Point anchor wedged into rock by serrated shell Stiff initial elastic response; susceptible to blast vibration loosening Temporary mining support in hard, competent rock
Fully Grouted Resin Rebar Continuous bond along corrugated steel rebar using polyester resin capsules High stiffness (\(> 100\text{ kN/mm}\)); rapid cure (\(30\text{ s}\)); excellent corrosion protection Civil tunnels and permanent underground mining roadways
Friction Stabilizer (Split Set) C-shaped slotted steel tube driven into undersized hole (\(38\text{ mm}\) into \(35\text{ mm}\) hole) Frictional yield along length (\(10 - 15\text{ kN/m}\) embedment); large displacement capacity High-deformation mining stopes; temporary sidewall pinning
Inflatable Tube (Swellex) Folded steel tube expanded hydraulically with high-pressure water (\(30\text{ MPa}\)) Immediate full-column mechanical interlock; high capacity (\(100 - 200\text{ kN}\)) with moderate ductility Rapid installation in tunneling; water-bearing rock masses
Yielding Rockbolt (D-Bolt, Garford) Smooth rebar with engineered anchor wedges or upset points Deforms up to \(150 - 200\text{ mm}\) under constant high load (\(180 - 220\text{ kN}\)) Rockburst-prone and severe squeezing ground environments

11.3 Steel Fiber Reinforced Shotcrete (SFRS)

Shotcrete (sprayed concrete) acts as a flexible, fast-acting structural skin.

Advantages of Steel Fiber Reinforced Shotcrete:

  1. Eliminates Wire Mesh Hazard: Spraying fibers directly eliminates the time-consuming and dangerous manual installation of welded wire mesh against unsupported rock faces.
  2. Post-Crack Toughness: While unreinforced shotcrete fails brittlely immediately upon cracking, ductile steel fibers (\(30 - 50\text{ kg/m}^3\) mix) bridge crack mouths, providing substantial energy absorption capacity (\(Q\)-system energy absorption classes: \(500\text{ J}, 700\text{ J}, 1000\text{ J}\)).
  3. Early Strength Gain: Accelerator admixtures achieve compressive strengths of \(1\text{ MPa}\) within 1 hour and \(10\text{ MPa}\) within 24 hours, providing immediate support behind advancing drill-and-blast or TBM headings.

11.4 Support Interaction & System Compatibility

The most common failure mode of ground support systems is strain incompatibility:

  • Pairing a stiff, brittle support (unreinforced thick shotcrete) with a flexible rockbolt (Split Set) causes the shotcrete to crack and spall at \(2\text{ mm}\) deformation, long before the bolts mobilize their tensile capacity.
  • The Golden Rule of Support Design: Ground support components (surface skin, tendon, anchor) must possess matching displacement capacities and deform harmoniously under the anticipated rock mass dilation.

11.5 Canonical Terminology

English Term Canonical Vietnamese Translation Definition
Resin rebar Neo thanh thép dính kết bằng vữa nhựa Corrugated steel bolt bonded with polyester resin cartridges
Friction stabilizer / Split Set Neo ma sát dạng ống xẻ rãnh Slotted tube driven into undersized hole developing frictional resistance
Steel fiber reinforced shotcrete (SFRS) Bê tông phun gia cường sợi thép (SFRS) Sprayed concrete containing hooked-end steel fibers for post-crack toughness
Energy absorption capacity Khả năng hấp thu năng lượng dẻo Toughness of reinforced shotcrete measured by energy absorbed under deflection
Strain incompatibility Không tương thích về mặt biến dạng Mismatch in deformation capacity causing premature failure of stiff components