Questions Flashcards
1 How does the concept of intact rock differ from rock mass?
- Intact rock: Joint-free sample from core drilling used for testing
- Rock mass: Consists of discontinuities, which include one or more of joints, faults, fissures, or bedding planes which occur in parallel
2 What are the CHILE and DIANE concepts?
- CHILE: Continuous, homogeneous, isotropic, linearly elastic
- DIANE: Discontinuous, inhomogeneous, anisotropic, not elastic
3 Explain the complete stress-strain curve in uniaxial compression.
- Strain increases until the peak strength, where the material breaks, is reached.
- Afterwards the material still retains a residual strength.
4 Explain what is meant by Class I and Class II post peak behavior.
• Class I behavior:
– “Stable” fracture propagation
– Work must be done on the specimen to effect further reduction in load-bearing ability.
– Some strength after the compressive strength has been exceeded
• Class II behavior:
– Unstable or self-sustaining behavior
– The elastic strain energy stored in the sample when the applied stress equals the compressive strength is sufficient to maintain fracture propagation until the specimen has lost virtually all strength.
– To control the fast failure process the surplus strain energy must be removed from the system.
Why are Brazilian test more often used to estimate the tensile strength than direct tensile test?
Direct tensile test difficult to conduct; indirect tensile strength by brazialian test are more comon
6 What is the difference between Mohr-Coulomb and Hoek-Brown?
- Mohr-Coulomb is a classic constitutive model, while the Hoek-Brown is a failure criterion.
- The Hoek-Brown model can not relate stress and strain in a general way as the Mohr-Coulomb model.(Hoek-Brown: intact rock: mb = mi; s=1; a=0,5)
- Hoek-Brown assumes that the rock mass is characterized by an elastic-brittle-plastic behaviour while Mohr- Coulomb assumes that it is characterized by an elastic-perfectly-plastic behaviour.
7 Name and explain Mohr-Coulomb parameters with units and describe how these parameters can be defines in laboratory.
- Coulomb postulated that, as the stresses increase, failure would first occur on the plane that satisfies the condition τ = c + σn tan φ for intact rock strength. These parameters can be defines in a triaxial test (shear test) plus stress-strain curve (UCS)
- c = Cohesion [MPa]
- σn = Normal stress [Mpa]
- φ = Friction angle [°]
9 Describe the displacement related to shear strength in a shear test of rock
• Graph: shear stress t (y) to displacement (x) > first linear expansion to peak strength and then reduce to residual strength // compare to normal stress I ´n the mohr coulomb strength criterion: with cohesion its peak strength [Graph]
10 Explain JRC and JSC
- JRC: joint roughness coefficient; on a scal 1 (for the smoothest) to 20 (for the roughest surface)
- JSC: joint wall compressive strength
11 Explain three roughness components of a discontinuity
- Asperity failure component
- Geometrical component
- Residual component
12 Difference between “constant normal load conditions” and constant normal stiffness conditions
- Constant normal load conditions (CNL): normal load is constant, normal fracture stiffness equals 0
- Constant normal stiffness conditions (CNS): normal load is not constant, normal fracture stiffness unequal 0
13 Show using a diagram the relations of shear stress and displacement in a shear test of a rock joint
Graph: shear stress t (y) to displacement (x) > first linear expansion to peak strength and then reduce to residual strength // compare to normal stress I ´n the mohr coulomb strength criterion: with cohesion its peak strength [Graph]
14 Why rock mass classification?
- Identify critical parameters
- Sectioning the rock mass in similar rock mechanics units
- Numerical data for modelling
- Joint language between specialists (e.g. geologist and engineers)
15 Describe the parameters used in the Q method.
• Q = RQD∗Jr ∗Jw Jn∗Ja∗SRF • RQD = Rock Quality Designation (drill core quality) • Jn = joint set number • Jr = joint roughness number
- Ja = joint alteration number
- Jw = joint water reduction factor
- SRF = stress reduction factor
16 RQD, how is it defined? Where do we apply RQD?
• Rock Quality Designation
Length of core pieces > 10 cm length Total length of core run
• Used for evaluation of drill core qualities
• Is part of the Q method
17 SRF, how is it defined and where to use it?
- Stress Reduction Factor
- Weakness zones intersecting excavation, which may cause loosening of rock mass when tunnel is excavated
- Is used in the Q method
18 Explain how Q-value can be used to evaluate the need of rock support.
- With the help of Equivalent Dimension (De) and Excavation Support Ratio (ESR) the required rock support can be determined in a diagram.
- It also allows to determine the recommended bolt spacing and length.
19 Describe the six parameters in RMR method.
- UCS (Uniaxial compressive strength of intact rock)
- Rock Quality Designation (RQD)
- Spacing of discontinuities
- Condition of discontinuities
- Groundwater conditions
- Orientation of discontinuities
20 How can UCS be estimated without laboratory testing?
• Field estimates with geological hammer, pocket knife or thumbnail (Grades: R0-R6)
21 Explain how the stand-up time and span are related.
- They are used for the “Meaning of Rock Classes” in RMR
- If a small span only stays open for a short amount of time, the cohesion of the rock mass is low and vice versa
- If the stand-up time is long, there is less support required
22 Describe the two main factors the GSI system is focused on.
- Rock structure
* Block surface conditions
23 Fractures and their properties are important in varying rock mass classifications systems. List out the main properties used in Q, RMR, GSI systems.
- Joint sets/number/roughness/alteration/water reduction
- Stress reduction
- Spacing and condition of discontinuities
- Groundwater conditions
- Orientation of discontinuities
- Uniaxial compressive strength of intact rock
24 Describe how rock mass classification is utilized to define the rock mass strength.
- The strength and stiffness of the intact rock is not the same as that of the rock mass.
- Scaling of the intact rock data needed, commonly by empirical relationships based on classification systems.
- Strength and deformation is controlled by discontinuities
- Presence of discontinuities is scale dependent
25 Explain the parameters in H&B criterion and describe how they can be determined.
• Parameters
– σ1r ; σ3r = Max. and min. effective principal stress at failure
– mb = H&B constant m for rock mass
– s and a = Constants for rock mass
– σci = UCS (for intact rock)
• Determination
– UCS = Uniaxial compressive strength
– mi = H&B constant for intact rock for mb
– GSI = Geological Strength Index for s, a
26 Applicability of the H&B criterion at different scales?
• Hoek-Brown is only based on individual samples, therefore it can not easily be scaled.
27 How is GSI utilized in H&B criterion? How can RMR and Q systems be utilized?
• Utilization of GSI
– GSI is used for the determination of s and a
– For RMR89r > 23 →− GSI = RMR89 − 5
– For RMR89r < 23; use Q’ classification value
• Utilization of RMR and Q systems
– For RMR76r > 18 →− GSI = RMR76r
– For RMR76r < 18; use Q’ classification value
28 Explain Young’s modulus, shear modulus and Poisson’s ratio.
- Young’s modulus is a mechanical property that measures the stiffness of a solid material. It defines the relationship between stress (force per unit area) and strain (proportional deformation) in a material in the linear elasticity regime of a uniaxial deformation.
- The shear modulus is defined as the ratio of shear stress to shear strain.
• Poisson’s ratio is a measure of the Poisson effect, the phenomenon in which a material tends to expand in directions perpendicular to the direction of compression.
29 Explain elasticity and plasticity.
- Elasticity is defined as the property which enables a material to get back to (or recover) its original shape, after the removal of applied force.
- Plasticity is defined as the property which enables a material to be deformed continuously and permanently without rupture during the application of force.
30 Explain what is “ In situ stress”” and how does it differ from “induced stress”?
- In situ = pre-mining state of stress. The true state of stress on the earth´s crust is always a superposition of several stress components; essential to know for excavation and rock support design (magnitude and orientation); tectonic and gravitation are the two most important contributions to in situ stress
- Difference to inducted stress is that is naturally exists and induced is made by mining for example
31 Kirsch equations: what can you calculate with them? Describe the parameters.
• With the kirsch equation you can calculate the stresses and displacements around a circular opening in elastic medium Sigma v = vertical stress, r and a are radius of boundries; k is stress ratio; phi is shear angle
32 For a future metro station you need to know approximate in situ state of rock mass at depth of 50 meter. How would you estimate the stress magnitude and orientation without doing stress measurements?
- Calculation of the weight of the overlying strata: σv = γ(weight of overburden) ∗ z(50m)
- Calculation of stresses of tectonic origin: σh = k ∗ σv = k ∗ γ ∗ z or Kirsch equation
33 Explain methods to evaluate and to measure the stress in the earth´s crust (both magnitude and direction)
- Calculation of the weight of the overlying strata: σv = γ(weight of overburden) ∗ z(50m)
- Calculation of stresses of tectonic origin: σh = k ∗ σv = k ∗ γ ∗ z
34 Explain what “In situ stress” is. What are the relevant components and how are they estimated and measured?
• In situ stress is the weight of the overlying strata combined with the stresses of tectonic origin (Pre-mining state of stress)
• Relevant components
– γ = Unit weight of overlying rock
– z = Depth below surface
– Eh = Average deformation modulus, based on stress measures
• Estimation and measurement
– Not direct stress measurements, only the effect can be measured
– Overcoring – Based on determination of strains in the wall of the borehole (stress relief)
– Hydraulic fracturing - Formation of a tensile crack in deep boreholes
– Borehole breakout – Analyses of stress failure of boreholes
35 Describe the rock conditions that increase the risk for structurally controlled failure. What are the conditions for stress induced fail- ure?
• Structurally controlled failure
– Low stress regime, common for low depth underground structures
– Relatively small loads, only key-blocks to be supported
• Stress induced failure
– Typical at great depth
– Induced stress/rock strength ratio important
– Time dependency of failure
