12/18/2010

1

Rock and Rockmass Properties

12/18/2010

2

Difference of Rock mass andIntact rock

Stope

Hutchinson and Diederichs, 1996

12/18/2010

3

Why do we need to know physical/mechanical rock

properties?

� In order to anticipate conditions leading to failure, we

need to know the strength (Tensile/Compressive) of the

rock.

Examples:

T is needed to design stability of pressure tunnels, boreholes.

C is needed to design stability of tunnels, shafts, pillars

� Rock is a natural material. Hence, each engineering

site is DIFFERENT and requires knowledge of the

SITE-SPECIFIC properties of the rock material.

Rock MASS (due to joints and other imperfections) may

behave differently from an intact specimen removed

from it.

Laboratory versus Field Tests: DO WE NEED THEM BOTH?

12/18/2010

4

� Hence, large-scale field tests are ideally the best way to determine rock properties for rock engineering.

� However, lab tests give early information, long before field tests can be conducted. Material properties of small samples are a good first approximation of the rock mass properties, much easier to conduct, and a lot cheaper.

Field Tests

• Involve subjecting a large volume of

rock to load & monitoring the

deformation

─more representative results

─ the volume of rock still not big

enough?

─more expensive

─ test location/development

changes?

12/18/2010

5

Laboratory Testing of Intact Rocks

LAB TESTS

On discontinuities

AND

cores of intact rock

Empirical models can then provide an estimate

of the rock mass characteristics

12/18/2010

6

Standards

� ISRM – minm. no. of tests that should be

carried out on a particular rock type?

� AS 4133: Methods of Testing Rocks for

Engineering Purposes

Common Sense Lab Testing of Rocks

� Clear identification of

samples & specimens

� Avoid moisture loss

� Prevent physical

damage to samples

� Consult field records

during specimen

selection

� Maintain equipment in

good working order

� Photo documentation of

test specimens

� Careful alignment of

axes for measurement

by dial gages, load

cells, and displacement

transducers

� Save remnant pieces of

rock after testing.

12/18/2010

7

Objectives: Lab Testing of Rock

� Recognize why and when to test intact rock

� Locate & review standard lab testing

procedures for indexing parameters of

strength, stiffness, and durability.

� Select representative specimens for testing

� Recognize importance of QA/QC for

mitigating common errors during lab testing

of intact rock.

Laboratory Testing of Rocks

� Index testing of intact rock materials

for identification & classification.

� Strength and stiffness characteristics

� Degradation potential; Durability

� Used in assessing the overall Rock Mass

� Purposes of Construction: rockfill, cuts,

slopes, foundations, tunnels

12/18/2010

8

Physical propertiesPorosityDensityPermeabilityDegree of Fissuring

Mechanical properties

Compressive strength Tensile strengthUniaxial Compression Test Brazilian TestSchmidt Hardness 4-Point Beam testPoint load test Uniaxial Tension Test

Basic Rock-Mech. Lab Tests in Intact Rock

(Index Tests)

situation of stress1σ

2σ

3σ

1σ

1σ

1σ

1σ

1σ

1σ

2σ

3σ

3σ

Uni-axial

Bi-axial

Tri-axial

Poly-axial

Hydrostatic

12/18/2010

9

Why Test Rock?