2.3 Underground Ground Support Systems & Excavation Design

2.3 Underground Ground Support Systems & Excavation Design

Quick Answer: Ground support systems stabilize underground excavations by reinforcing the rock mass and supporting surface blocks. Support elements are divided into primary (installed at the face) and secondary (installed for long-term or dynamic duty) systems, operating via active (pre-tensioned) or passive mechanisms. Rock reinforcement hardware includes mechanically anchored bolts, resin-grouted rebars, friction bolts (Split Sets, Swellex), cable bolts, fiber-reinforced shotcrete, and steel arches. Mine pillar design can combine Tributary Area Theory for stress screening with empirical strength formulas calibrated to the relevant deposit. Required reliability or Factor of Safety is selected from failure mode, database applicability, geometry, variability, consequence, and current design criteria; 1.5–2.0 is not a universal statutory range.

Primary vs. Secondary Ground Support & Support Mechanics

Underground ground support systems prevent roof falls, rib spalling, and major collapse by modifying rock mass behavior or catching detached rock blocks:

• Primary Ground Support: Installed immediately after blasting and mucking at the active face. Its objective is to preserve workplace safety, prevent unravelling of loose key blocks, and maintain excavation boundary geometry.
• Secondary Ground Support: Heavy-duty support installed later in long-term excavations (shaft stations, main haulage drives, permanent crusher chambers, stope intersections) or in high-stress/dynamic ground. Includes long cable bolts, thick fiber-reinforced shotcrete, and heavy steel arches.

Support Function Mechanics

1. Active Support: Applies an immediate compressive pre-load to the rock mass upon installation. Pre-tensioning compresses joint planes, increasing normal stress and friction resistance along potential shear surfaces (e.g., pre-tensioned resin bolts, expansion shell anchors).
2. Passive Support: Provides no initial load upon installation. Load develops only as the surrounding rock mass undergoes deformation and displacement toward the excavation void (e.g., untensioned grouted rebars, Split Sets, timber props, standing steel sets).

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Rock Reinforcement Hardware

1. Mechanically Anchored Expansion Shell Bolts

• Mechanism: A point-anchor bolt. Rotating the bolt forces a threaded wedge into an expandable serrated shell, gripping the borehole wall at the deep end.
• Characteristics: Provides immediate active pre-tensioning upon installation. However, susceptible to loss of tension under blast vibration or rock anchorage slip; un-grouted steel is highly vulnerable to corrosion.

2. Resin-Grouted Rebar Bolts

• Mechanism: Full-column encapsulated steel rebar. Two resin cartridges (fast-set resin at the anchor end, slow-set resin along the shank) are inserted into the borehole. Spinning the rebar breaks and mixes the chemical resin cartridges.
• Characteristics: Provides stiff, high-capacity, full-column reinforcement and can protect steel from groundwater when encapsulation is complete. Ordinary resin-grouted rebar is generally passive unless installed with an intentional tensioning system; bond and installation quality require testing.

3. Friction Bolts: Split Sets & Swellex

• Split Sets (Friction Stabilizers): High-strength C-shaped steel tube with a longitudinal slot driven into an undersized borehole. Frictional resistance along the full length resists rock movement (1.0–1.5 tonnes per meter of embedment). Highly yieldable under ground convergence.
• Swellex Bolts: Folded steel tube inserted into a drill hole and expanded hydraulically using high-pressure water (~300 bar). The expanded tube presses against the borehole wall to provide immediate frictional load transfer; capacity and deformation performance depend on hole condition, installation pressure, corrosion, and pull testing.

4. Cable Bolts

• High-strength steel wire strands (single or multi-strand, birdcaged or bulbed to increase bond shear strength) installed at project-specific lengths and grouted to a verified design. Essential for stabilizing large open stope roofs, hanging walls, and wide drive intersections.

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Surface Support & Standing Support Systems

Rock bolts reinforce the interior rock mass, but surface elements are required to prevent loose rock fragments from falling between bolt pattern grids:

• Shotcrete: Pneumatically projected concrete applied wet-mix or dry-mix at 50–150 mm thickness. Acts as a continuous structural membrane, filling open joints, sealing rock against air/moisture weathering, and locking key blocks. Fiber-reinforced shotcrete (FRS) incorporates steel or macro-synthetic fibers to provide post-crack flexural toughness and energy absorption (E700–E1000 Joules rating under panel testing).
• Wire Mesh: Welded wire mesh or flexible chain-link mesh secured against the rock surface using bolt bearing plates. Retains fractured rock spalls.
• Steel Sets & Yielding TH Arches: Standing structural support for severely broken, squeezing, or fault-damaged ground. TH (Toussaint-Heintzmann) Yielding Arches consist of overlapping sliding steel segments clamped with friction shackles, allowing controlled yield under heavy convergence without structural buckling.

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Mine Backfill Technologies

Backfilling empty stopes provides regional ground stability, prevents major wall sloughing, reduces surface subsidence, and permits maximum ore extraction by enabling pillar recovery.

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Pillar Design & Stress Analysis

Pillars are unmined rock columns left between excavations to support overlying strata.

1. Tributary Area Theory

Assumes each pillar supports the entire weight of the rock column directly above it plus half the span of adjacent rooms ($w_o$). For a regular grid of square pillars of width $w_p$:

• Extraction Ratio ($R$): $R = 1 - \frac{w_p^2}{(w_p + w_o)^2}$

• Average Pillar Stress ($\sigma_p$): $\sigma_p = \frac{\sigma_v}{1 - R} = \gamma z \left( \frac{w_p + w_o}{w_p} \right)^2$

2. Obert-Duvall Empirical Pillar Strength Formula

Obert and Duvall formulated pillar strength $S_p$ based on pillar width-to-height ratio ($w_p / h_p$):

$S_p = S_0 \left( 0.778 + 0.222 \frac{w_p}{h_p} \right)$ where $S_0$ is the unconfined compressive strength of a cubic rock specimen ($w/h = 1.0$).

3. Factor of Safety ($FoS$)

$FoS = \frac{S_p}{\sigma_p}$ A preliminary design may use a range such as 1.5–2.0 in a stated context, but no universal factor is guaranteed or required by this equation alone. Acceptance criteria must reflect the calibrated pillar-strength method, variability, design life, failure consequence, interaction, monitoring, and controlling standard.