Key points

Definition of Rock strength and diffrent types of rock strength?

Intact rock vs rock mass? Rock vs stone?

Definition of specific gravity, mass density, absorption

Types of discontinuties in rock?

Factors of weak planes that control the behavior of rock mass?

How do the weak planes in rock influence the rock mass properties?

Compressive strength? How to determine?

Elastic moduli (Ei, Es, Et) ? Poisson ratio? How to determine these parameter by uniaxial compression test?

Uniaxial compression test vs Triaxial compression test?

Equation of shear strength (Mohr envelope) and two shear strength parameters (phi and So/c)

and The axial load required to cause failure of a rock increase with increasing confining pressure. --> from 2 or 3 tests at different confining pressures --> shear stress - compressive stress diagram (Mohr diagram) --> shear stress of at failure varies with confining pressure.

Technical terms

Rock strength

maximum stress that rock can withstand before failure

Compressive strength

maximum compressive stress that rock can sustain before failure

Tensile strength

Shear strength

Stress

shear stress

compresive stress

Bedding

Foliation (slaty cleavage, phyllitic cleavage, schistosity, gnessic structure)

Flow banding in lava rock

Joint

Faults

Attitutde of the planes

The extent of continuity

Spacing

The nature of the weakness surface

Infilling material

modulus of elasticity

Tangent modulus

Secant modulus

Strain

Lateral strain

Vertical stran/axial strain

Deflection of rock core

Questions

The following questions are guidelines for quizzes and exam. Student should find the answers to the following questions.

What is the name of the branch or study which deals with behaviour of rocks under applied forces?

What is the density/specific gravity/ absortion of the rock? The formula to determination?

Which type of compressive strength is taken as the most important index property of stones?

How do the discontinuties influence the rock mass?

Excercise to practice

Problem 1. Physical properties of rock

Given the mass of rock specimen in diffrent condition( dried, saturated, and saturated weighted in water), specific gravity of rock (can also be obtained in table 6.1)

Required:

• determine bulk specific gravity (for dry and saturated condition), apparent specific gravity

• determine % absorption

• determine the volume of rock sample?

Problem 2. Uniaxial compression testing.

Given size of rock core and load at failure (can be force or pressure).

Required: Determine the compressive strength of rock.

E.g.

A rock core is 50mm in diameter and 100mm long, is loaded to failure in a uniaxial compression test. Failure load was 276 kN. Determine the unconfined compressive strength of rock?

Problem 3. Rock deformation

Given:

• size of rock core and

• the modulus of elasticity of rock

• Poisson‘s ratio value

• unconfined compressive strenght.

• a value of load or loading pressure

Required:

• Determine load at failure?

• Determine strains (axial and lateral) occur at the given loading pressure?

• Determine deflections (axial and lateral)?

Problem 4. Triaxial compression test

Given:

• confined pressure and responding axial pressure at failure (at least data from two tests)

Required:

• Plot Mohr Circle for each test

• Draw failure envelope

• and determine S_o or c (cohesion) and phi (internal friction angle)

Problem 5. RQD

Given:

• picture or sketch of rock core obtained from drilling

Required:

• Determine RQD and corresponding quality of rock mass.

Problem 6. Engineering properties of rock mass

Given:

• RQD and/or RMR and/or Q and strength of intact rock

Required:

• Determine deformation modulus and compressive strength of rock mass.