Download CE260-Rock Mechanics

Civil Engineering Department
College of Engineering
Course: Soil and Rock Mechanics
(CE 260)
Lecturer: Dr. Frederick Owusu-Nimo
Introduction
• Engineering Uses of Rocks: Rocks are used for
engineering purposes in two primary ways:
o As a building material : aggregates, cut stones, decorative
panels, etc.
o As a foundation : bedrock determines kind of structure that
can be built.
• Knowledge and understanding of basic rock
properties will
o Enable us assess the usefulness and quality of aggregates
o allow structures to be founded correctly so the required
support will be there
Properties of Rocks
• The most important properties of rocks are those
related to its strength and deformability
• Intact Rock: They are
rock containing no
significant fractures
• Discontinuities : These
are faults and joints
found in insitu rock.
These may be weak
links in the rock mass
and may cause failures
Properties of Rocks
• The strength of rocks is the maximum stress it can
sustain under a given set of conditions
o Uniaxial Compressive Strength: It is stress which will cause a
cylindrical rock sample to fail when it is subjected to two
opposing (compressing forces).
o Tensile Strength: It is stress which will cause a cylindrical
rock sample to fail when it is subjected to two opposing
(outward forces). Tensile strength of rocks is usually much
lower than compressive strength
o Shear Strength: It is the shear stress that acts on the plane
of failure when rock is subjected to compressive or tensile
forces
Properties of Rock
• Strength of rock mass depends on the extent of
discontinuities (fractures) within it
• Rock Quality Designation (RQD): It is used to
quantify degree of fracturing or discontinuities in the
original rock mass.
• RQD is obtained by measuring intact fragments
length of core pieces obtained from drilling
𝑅𝑄𝐷 =
𝑐𝑜𝑟𝑒 𝑙𝑒𝑛𝑔𝑡ℎ𝑠 ≥10𝑐𝑚
𝑏𝑜𝑟𝑒ℎ𝑜𝑙𝑒 (𝑐𝑜𝑟𝑒 𝑟𝑢𝑛) 𝑙𝑒𝑛𝑔𝑡ℎ
× 100
• RQD gives an idea of the rock mass quality
• Values of RQD > 70% generally indicate sound rock
Properties of Rock
Rock Mass Classification based on RQD
RQD (%) Description of Rock Quality
90 – 100
75 – 90
50 – 75
25 – 50
0 - 25
Excellent
Good
Fair
Poor
Very poor
Rock Testing
• Rocks are tested to determine their physical and
mechanical properties
• Tests may be used to provide information for
classification purposes or for engineering design
purposes
• Test for classification is used to group rocks of similar
behavior and provide information on their
geomechanical characteristics
• Test for engineering design purposes is used to
provide detailed information on insitu stresses,
rockmass strength and deformation properties
Rock Testing
• Laboratory Test: Conducted on small rock samples
taken from the field site (“Rock Properties”)
• Field test : Conducted at the field site to determine
the bulk strength properties of rock mass (“Rock
mass properties”). These properties are influenced
by the ‘discontinuities’ or planes of weakness
present in the rock mass
• Mechanical properties of an intact rock sample
may be much different from that of the rock mass
from which sample was obtained.
Rock Testing
Laboratory Testing
For Classification Purposes
For Engineering Design Purposes
Density
Moisture content
Porosity
Water absorption
Direct Shear test
Uniaxial Compressive Test
Triaxial Compressive Test
Deformation characteristics
Rock Testing
Field Testing
For Classification Purposes
For Engineering Design Purposes
Discontinuity orientation
Spacing
Core recovery
Rock Quality Designation
Point load test
Direct Shear test
Field Permability
In Situ Rock Stress determination
Rock Materials
• Reasons for laboratory Testing of Rocks
Materials (Aggregates)
o To assess the usefulness and quality of a new
source of aggregate (quarry site)
o Compare quality of rock materials from different
sources
o Predict performance in service
o Predict durability
Rock Materials
• Various Laboratory Testing of aggregates: To
evaluate how aggregates will perform when in
service
o Aggregate crushing value Test
o The Ten Percent Fines Value Test
• Determine resistance of aggregates to crushing when
subjected to loading
o Aggregate impact value Test
o Los Angeles abrasion test
• Used to indicate the toughness and abrasion
characteristics of aggregates
Effect of water on
Strength of Rocks
• Presence of water can significantly reduce rock
strength in two ways
o Water interrupts the bonding between minerals and allows the
break up of clay cement leading to a reduction in rock strength
and increased deformability
o Water in joints may induce pore water pressure which acts in
opposition to compressive stress, thus reducing effective normal
stress between the rock surface.
• Water greatly reduces strength of porous
sedimentary rock but have minimal effect on intact
rock with low porosity