Lecture 1: Intact Rock Strength
(Ref : Chapter 6, Hudson and Harrison 1997
Chapter 4, Wyllie and Mah 2004)
Overview
Overview
Overview
ROCK AS AN ENGINEERING MATERIAL
Continuous
Homogeneous
Isotropic
Linear
Elastic
Versus
Discontinuous
Inhomogeneous
Anisotropic
Non-linear
Elastic
Uniaxial strength
Field estimates of uniaxial compressive strength
Uniaxial
Schmidt
comp
Point Load hardness
Term strength Field estimate of strength Examples
Index (Mpa) (Type L –
(Mpa)
hammer)
Extremely >250 >10 50-60 Rock material only chipped under Fresh basalt, chert, diabase, gneiss,
strong repeated hammer blows, rings when hit granite, quartzite
Very 100-250 4-10 40-50 Requires many blows of a geological Amphibolite, sandstone, basalt,
strong hammer to break intact rock specimens gabbro, gneiss, granodiorite,
limestone, marble, rhyolite, tuff
Strong 50-100 2-4 30-40 Hand held specimens broken by a single Limestone, marble, phyllite,
blow of a geological hammer sandstone, schist, shale
Medium 25-50 1-2 15-30 Firm blow with geological pick indents Claystone, coal, concrete, schist,
strong rock to 5mm, knife just scrapes surface shale, siltstone
Weak 5-25 <15 Knife cuts material but too hard to shape Chalk, rocksalt, potash
into triaxial specimens
Very weak 1-5 Material crumbles under firm blows of Highly weathered or altered rock
geological pick,can be shaped with knife
Exteremel 0.25-1 Indented by thumbnail Clay gouge
y weak
**All rock types exhibit abroad range of uniaxial compressive strengths which reflect the heterogeneity in composition and
anisotropy in structure. Strong rocks are characterised by well interlocked crystal fabric and few voids.
** Rocks with a uniaxial compressive strength below 25 Mpa are likely to yield highly ambiguous results under point load
testing
Standard testing procedure (ISRM- International Society for Rock Mechanics)
1. The test specimen should be right circular cylinders
having a height to diameter ratio of 2.5-3.0 and diameter
preferably of not less than NX core size (approximately
54mm). The specimen diameter should be at least 10
times the size of the largest grain in the rock.
2. The ends of the specimen should be flat to within
0.02mm and should not depart from perpendicularity to
the axis of the specimen by more than 0.001 rad or 0.05
mm in 50 mm.
3. The use of capping materials or end surface treatments
other than machining is not permitted
4. Specimens should be stored, for no longer than 30 days,
in such a way as to preserve the natural water content, as
far as possible, and tested in that condition.
5. Load should be applied to the specimen at a constant
stress rate of 0.5-1.0 Mpas -1
6. Axial load and axial and radial circumferential strains or
displacements should be recorded throughout each test.
Factors influencing the Uniaxial Compressive Strength of Rock
Strength will depend on the nature and composition of the rock. For similar
mineralogy strength will decrease with increasing porosity, increasing degree of
microfissuring. Strength may also decrease with increasing water content.
For the above reasons the strength of samples of rock having the geological name
can vary widely. For example the strength of sandstone will vary with grain size, the
packing density, the nature and extent of cementing between the grains and the
levels of pressure and temperature that the rock has been subjected to throughout
its history. However, the geological name can give some qualitative indication of the
likely mechanical behaviour
Definitions
Stress v strain behaviour
or Force v displacement
The typical complete force (load) – displacement (stress-strain) curve for rock
The complete force-displacement (stress-strain) curve for rock is shown in Figure 1. This curve is obtainable
using servo-controlled testing machines and by making the axial strain (displacement) the independent
variable. Stages I to III describe the pre-peak behaviour and stage IV describes the post peak behaviour of the
sample deformation. The main stages of material behaviour can be classed into four regions. These regions are:
Region I: Closure of microcracks in the sample and/or ‘bedding in’ of the sample under the initial load.
Region II: Linear-elastic region (deformation essentially recoverable). At about 50% of UCS value,
microcracks start to start form. At Yield point, behaviour becomes non-linear elastic.
Region III: Non Linear strain-hardening behaviour. Microcrack intensity incresses and start to coalesce
Region IV: Peak rock strength defines the uniaxial compressive strength of the material. At about half way
down this side of the strain-softening curve, macro-fractures occur and failure occurs on failure planes created
in the sample. This later stages of this curve define the residual strength of the sample
Factors to consider:
1. End effects and the influence of height to diameter ratio
2. Influence of standard of end preparation
3. Influence of specimen volume
4. Influence of strain rate
5. Influence of testing machine stiffness
Pitfalls
Failure on multiple shear planes
Failure by ‘shear’ along
along one distinct plane
Tensile Strength
Direct tensile strength
difficult to achieve therefore
use indirect methods
BRAZILIAN METHOD
Tensile Strength Calculation
Strength Properties of Rocks
Rock type Density Porosity Dry UCS Dry UCS Modulus tensile Shear Friction
dry % range UCS Saturated of Strength Strength angle
t/m3 Mpa mean Mpa elasticity Mpa Mpa φ
Mpa GPa
Granite 2.7 1 50-350 200 75 15 35 55
Basalt 2.9 2 100-350 250 90 15 40 50
Greywacke 2.6 3 100-200 180 160 60 15 30 45
Sandstone- 2.2 12 40-100 70 50 30 5 15 45
Carboniferous 1.9 25 5-40 20 10 4 1 4 40
Sandstone - Triassic
Limestone – 2.6 3 50-150 100 90 60 10 30 35
Carboniferous 2.3 15 15-70 25 15 15 2 5 35
Limestone- Jurassic 1.8 30 5-30 15 5 6 0.3 3 25
Chalk
Mudstone- 2.3 10 10-50 40 20 10 1 30
Carboniferous 2.3 15 5-30 20 5 2 0.5 25
Shale- Carboniferous 1.8 30 1-4 2 0.2 2 0.7 20
Clay- Cretaceous
Coal 1.4 10 2-100 30 10 2
Gypsum 2.2 5 20-30 25 20 1 30
Salt 2.1 5 5-20 12 5
Hornfels 2.7 1 200-350 250 80 40
Marble 2.6 1 60-200 100 60 10 32 35
Gneiss 2.7 1 50-200 150 45 10 30 30
Schist 2.7 3 20-100 60 20 2 25
Slate 2.7 1 20-250 90 30 10 25
