Ground Condition Illustrator

Principle of Rockbursts

A rockburst is a sudden, violent failure of rock in a high-stress environment. It is an energy-driven phenomenon where accumulated elastic strain energy is released explosively. This requires a rock mass that is both strong enough to store immense energy and brittle enough to fail suddenly without significant prior deformation. The Rock Mass Strength (horizontal axis) is typically high, reflecting competent, massive rock with a high UCS.

The Zone is Based on These Rules:

• Critical Stress-to-Strength Ratio: The hazard becomes significant when the maximum induced tangential stress (σ_max) exceeds a critical fraction of the rock mass strength (σ_cm). Empirical criteria (e.g., Russenes, Hoek) place this threshold in the range of 0.55 < σ_max / σ_cm. The zone shown on the plot begins at a conservative ratio of approximately 0.6.
• High-Strength, Brittle Rock: The rock must be competent and massive (e.g., high UCS, GSI > 65) to store large amounts of strain energy before failure.
• Energy-Driven Failure: The rate of energy release during excavation exceeds the rock mass's capacity to dissipate it gradually through plastic deformation.
• Unstable Failure Propagation: The trigger is when the concentrated stress surpasses the rock mass strength, leading to an unstable, self-sustaining fracture process.

Key Academic References

Principle of Squeezing Ground

Squeezing ground is characterized by large, time-dependent deformation occurring when induced stresses significantly exceed the rock mass strength. It is a ductile, creep-like failure common in weak rock masses (e.g., schists, phyllites, fault gouge). Unlike brittle failures, squeezing involves a slow, viscoplastic flow of material into the excavation, which can continue for long periods and exert immense pressure on support systems.

The Zone is Based on These Rules:

• High Stress / Low Strength: The fundamental condition for squeezing is a low "competency factor," meaning the rock mass strength (σ_cm) is significantly lower than the in-situ or induced stress (p_o).
• Weak Rock Mass: The condition is associated with poor quality rock, characterized by low intact strength (UCS), a low Geological Strength Index (GSI), and often containing clay-rich or altered minerals.
• Ductile, Time-Dependent Behavior: The rock mass deforms plastically (creeps) rather than fracturing violently. The deformation is a continuous process over time.
• Volumetric Constancy: The deformation occurs without a perceptible increase in the rock's volume, distinguishing it from swelling ground.

Key Academic References

Principle of Structurally Controlled Unravelling

This failure mode is dominant in low-stress environments where gravity is the main driver. It occurs when the rock mass is divided into blocks by discontinuities (joints, faults). Failure happens when a "key block" is free to fall or slide under its own weight. The Rock Mass Strength (horizontal axis) represents the overall strength of the jointed rock, which is derived from the Uniaxial Compressive Strength (UCS) of the intact rock but is significantly reduced by these discontinuities.

The Zone is Based on These Rules:

• Low-Stress Domain: The induced stress field is insufficient to provide a stabilizing "clamping" force across discontinuity surfaces, leading to a loss of confinement. This typically occurs at shallow depths or in low horizontal stress regimes.
• Gravity-Driven Mechanism: Gravitational body forces acting on discrete rock blocks are the primary driver of instability, rather than excavation-induced stresses.
• Structurally-Defined Failure: The rock mass must be sufficiently jointed (e.g., blocky or very blocky, with a moderate to low GSI) for discrete, removable blocks to be formed. The rock mass strength is therefore a fraction of the intact rock's UCS.
• Kinematic Release: The geometry of intersecting discontinuities must create a "key block" that is kinematically free to be released into the excavation. This is a geometric condition determined by kinematic analysis (e.g., stereonets).

Key Academic References

Principle of Intact Rock

Intact rock represents the theoretical baseline in rock mechanics: a solid, homogeneous, isotropic continuum free of significant discontinuities. Its properties are the upper-bound benchmark for strength and stiffness. The behavior of any real-world rock mass is predicted by systematically reducing these "perfect" properties to account for joints and fractures.

The Zone is Based on These Rules:

• High Strength & Stiffness: The rock exhibits high Uniaxial Compressive Strength (UCS) and a high elastic modulus. It is considered a continuous medium (GSI = 100).
• Stable Stress State: The induced stress is significantly lower than the intact rock's compressive strength (σ_c). The Factor of Safety against failure is high.
• Failure Governed by Intact Properties: If failure were to occur, it would be through the solid rock matrix, governed by criteria like the Hoek-Brown criterion for intact rock, not along pre-existing weaknesses.
• No Kinematic Release: The absence of a pervasive discontinuity network means there are no blocks or wedges that can be released by gravity.

Key Academic References

Principle of Uncertainty

This region represents conditions that do not fall neatly into one of the primary failure mechanisms. The ground behaviour could be transitional or influenced by a complex interplay of factors not captured by this simplified chart. A detailed site-specific analysis, including geological mapping and numerical modeling, is required to properly assess the ground conditions.