ROCK SLOPE ENGINEERING
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ROCK MASS
Rock mass is a non-homogeneous, anisotropic
and discontinuous medium ; often it is a prestressed mass
ROCK MECHANICS
Rock mechanics is defined as “ the theoretical and
applied science of mechanical behavior of rock; it is
that branch of mechanics concerned with the
response of rock to the force field of its physical
environment “
- As per ASEG (American Society of
Engineering Geology )
Applications of Rock Mechanics
Rock mechanics is primarily applied in
- Civil Engineering
- Mining Engineering
- Petroleum Engineering
CIVIL ENGINEERING
APPLICATIONS
The Civil Engineer is mainly concerned with
• Competency of the rock mass to carry
the loads of the structures built on it
• Stability of the excavations undertaken
involving a rock mass, whether surface or
underground
STRENGTH OF ROCK
• Rock should be classified and its strength to be
assessed in its simple state of existence i.e
unconfined condition
• Rock can be either “intact “ or “ jointed”
• Parameters for assessing the rock strength/stability
 In-situ stress/confining conditions
 Environmental factors eg. seepage pressure etc.
INTACT ROCK Vs ROCK MASS
INTACT ROCK
• No through going fractures
ROCK MASS
• Intact rock + Discontinuities
INTACT ROCK Vs ROCK MASS
(Contd.)
Discontinuity
• Joints
• Fractures
• Faults
• Shear zones
 Makes the rock discontinuous
 Makes the rock anisotropic
 Makes the rock stress dependent
FACTORS AFFECTING ROCK
STRENGTH
• Nature of discontinuities
• Location of discontinuities
• Orientation of discontinuities
 Deformability
 Strength
 Permeability
ROCK MASS DESCRIPTION
MASSIVE ROCK
• Rock mass with few discontinuities
• Excavation dimension < discontinuity spacing
BLOCKY/JOINTED ROCK
• Rock mass with moderate number of discontinuities
• Excavation dimension > discontinuity spacing
HEAVILY JOINTED ROCK
• Rock mass with a large number of discontinuities
• Excavation dimension >> discontinuity spacing
DISCONTINUITY PARAMETERS






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Spacing & frequency
Orientation & dip/dip direction
Persistence, size & shape
Roughness
Aperture
Discontinuity sets
Block size
AFFECT OF DISCONTINUITIES



Permeability – Grouting
Blast design
Stability of slopes
ROCK CLASSIFICATION
Intact rocks are classified on the basis of
i) Uni axial compressive strength (UCS)
ii) Modulus of deformation
iii) Modulus ratio
Rocks are classified as of very high strength,
high strength, medium strength, low
strength and very low strength based on
the above classifications
ROCK MASS CLASSIFICATIONS

Terzaghi’s in situ rock classification
◦
◦
◦
◦
◦
◦
◦
Intact
Stratified
Moderately jointed
Blocky and seamy
Crushed
Squeezing
Swelling
ROCK MASS CLASSIFICATIONS
(Contd.)

Rock quality designation (RQD)
◦ Very Poor ( 0-25)
◦ Poor
(25-50)
◦ Fair
(50-75)
◦ Good
(75-90)
◦ Excellent
(90-100)
RQD , expressed as % , is the summation of all
the cores larger than 10 cm from the
preferred 150 cm drilled core run.
RQD = 110.4 -3.68 Jn
ROCK MASS CLASSIFICATIONS
(Contd.)
Geomechanics classification (RMR)
Based on the following parameters
i) UCS of Intact material/rock
ii) RQD
iii) Spacing of discontinuities
iv) Condition of discontinuities
v) Ground water condition
vi) Orientation of discontinuities

ROCK MASS CLASSIFICATIONS
(Contd.)
Q-System (Norwegian geo.tech
classification)
Based on i) RQD ii) No.of joint sets
iii) Joint Roughness iv) Degree of alteration
or filling v) Water inflow vi) stress
condition.
Based on these parameters Q is expressed
as (RQD/Js)x (Jr/Ja) x (Jw/SRF)

ROCK MASS CLASSIFICATIONS
(Contd.)
RMR (Rock mass rating ) proposed by
Bieniawsky (1973,19888 & 1993) based
on shear strength parameters (Cohesion
c and Ф (angle of friction) ). The rock
mass is classified at five levels.
ROCK QUALITY
(Massive Rock)
(Massive Rock)
(Jointed Rock)
(Heavily Jointed Rock)
ROCK QUALITY
Q=100(Massive Rock)
•
Rock mass with few discontinuity
•
Excavation dimension< discontinuity
spacing
Q=3 (Jointed Or blocky Rock)
•
Rock mass with moderate nos. of
discontinuity
•
Excavation dimension> discontinuity
spacing
Q=0.1(Heavily Jointed Rock)
•
Rock mass with large nos. of
discontinuity
•
Excavation dimension >> discontinuity
spacing.
SLOPE FAILURE MECHANISIM


CONTINUUM

MANY CONTINUTIES

WEAK ROCK

EFFECTIVELY CONTINUUM
DISCONTINUUM

FEW CONTINUTIES

STRONG ROCK

DISCONTINUUM
TRANSITION FROM INTACT ROCK
TO HEAVILY JOINTED MASS

IDEALISE ILLUSTRATION OF TRANSITION FROM INTACT ROCK TO
HEAVILY JOINTED MASS WITH INCREASING SAMPLE SIZE (AFTER HOEK
AND BRON,1980)
JOINTS PROBLEM IN CIVIL
ENGINEERING

ROAD CUTTING ARE CONSTRUCTED, WHERE POSSIBLE AT RIGHT ANGLE
TO STRIKE OF THE MAIN, GENTLY TO MODERATLY DIPPING PLANES OF
WEAKNESS IN THE ROCK.
JOINTS PROBLEM IN CIVIL
ENGINEERING (Cont.)

RESERVOIR AND DAMS:THE AXIS OF THE DAM SHOULD BE
CONSTRUCTED PARALAL TO MAIN FACTURE SET AND THE LATER
SLOULD DIP UPSTREAM TOWARDS THE RESERVOIR.
ROCK ENGINEERING

Rock “Material”-STRONG, Stiff, Brittle
WEAK ROCK> STRONG CONCRETE.
 STRONG IN COMPRERSSION, WEAK IN TENSION
 POST PEAK STRENGTH IS LOW UNLESS CONFINED.


Rock “Mass”-behavior controlled by
discomfitures.


ROCK MASS STRENGTH IS ½ TO 1/10 OF ROCK STREGTH.
Discontinuities give rock masses scale effects
ROCK ENGINEERING (Contd.)
ROCK STRESSES IN SITU

VERTICAL STRESS ≈ WEIGHT OF OVERLYING ROCK


~27 KPA / M => 35.7 MPA AT 1300 M
HORIZONTAL STRESS CONTROLLED BY TECTONIC FORCES
(BUILDS STRESSES) & CREEP (RELAXES STRESSES)

AT DEPTH, σv ≈ σh UNLESS THERE ARE ACTIVE TECTONIC FORCES
ROCK SLOPES
• SLOPES CAN BE
 NATURAL SLOPES
 MAN-MADE OR CUT SLOPES EXCAVATED
FOR
 ENGINEERING CONSTRUCTION LIKE
BUILDINGS, ROADWAYS, WATER WAYS,
WATER RESOURCES/HYDRO-ELECTRIC
PROJECTS ETC.
 OPEN CAST MINING
FAILURE OF SLOPES
• Failure of natural slopes is common geological
phenomenon
• Reasons for failure are
 Imbalance between shear strength and the shear
stress in the ground/rock mass
 Failure can be either slow-time dependent
process or by extraneous factors in an abrupt
manner.
• The extraneous factors can be
 Increased shear stresses due to surface loadings
 seepage pressure due to built up of hydro static
pressure.
SLOPE FAILURE MECHANISMS
• Rock falls due to dislocation of blocks (Mostly
occur on steep slopes - with out sliding)
• Fractures/weathered rock slope fails along a curved
surface with a rotational slide
• Translational slide to be planar along weak
bedding/shear plane/fault zone
• Movement initiates rotational slides where as
imbalance in forces results in translational
movements.
MODES OF FAILURES

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Rotational or circular/arc failure
Sliding /planar failure
Wedge failure
Toppling failure (without sliding mechanism)
Buckling failure
In any open/surface rock excavation, one mode or a
combination of several modes of failures can occur
FAILURE MECHANISMS HAVE GENERALLY BEEN
DESCRIBED AND ANALYZED IN TWO DIMENSIONS
MAIN LANDSLIP TYPES
SLOPE MASS RATING
A practical approach proposed by
ROMANA (1985) to evaluate the slope
stability. SMR is expressed as
SMR = RMRbasic – (F1,F2,F3) + F4
Where
RMR basic is evaluated according to
Bieniawsky (1979,1989)
F1= Square (1-Sin A)
Where A denotes angle between strikes of
slope face and that of the joints
SLOPE MASS RATING (Contd.)
F2 = Tan (βj) where βj is the joint dip angle
in planar failure mode.
Both F1 and F2 vary from 0.15 to 1.0. For
toppling mode of failure value of F2
becomes 1.0
F3 = Measure of relationship between the
slope face and joint dips.In planar failure
mode F3 refers to the probabilty of joints
day-lighting in the slope face.
SLOPE MASS RATING (Contd.)
F4 pertains to adjustment for the method
of excavation.Values of F4 are as follows:
i) Natural slope
+ 15
ii) Pre splitting
+ 10
iii) Smooth blasting
+ 8
iv) Normal blasting or
mechanical excavation
0
v) Poor blasting
- 8
SMR value ranges from 0 to 100
FOUNDATIONS ON ROCK
STABILITY OF SLIDING BLOCK RELATED
TO DIP OF SLIDING SURFACE.
POTENTIAL FAILURE PATH
ANALYSIS OF ROCK SLOPES


Rock mass or the proposed slope needs to be
analyzed for the possible mode of failure
Rock slopes can be analyzed by
Conventional limit equilibrium methods/closed form
solutions
Numerical approximation methods
• Discrete element methods
• Finite element methods
SLOPE STABILITY / PROTECTION
• Decreasing the seepage pressure
• Flattening the rock slope as much as possible
• Reducing the height of slope/excavation depth (may
not be possible in some situations)
• Rock support measures
Rock bolts/rock anchors/soil nailing
Shotcrete with or without wire mesh (mainly to
improve/protect the surface stability)
SLOPE STABILITY / PROTECTION
(Contd.)




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Toe protection – Retaining walls/butress with weep
holes
Tree plantation/grass turfing
Catch water drains
Nailing wire mesh/geo-grids ( in to steep slopes)
Drainage gallery behind toe ( in special
circumstances)
Rock Slope Stability Problems In
Himalayas
Slope Protection Using Soldier
Piles/Shotcrete Retaining Wall
Slope Protection Using Soldier
Piles/Shotcrete Retaining Wall
Use of Soil Nail in Slope Protection
Use of Soil Nail in Slope Protection
Use of Soil Nail in Slope Protection
Use of Rock Anchors/Shotcrete in Slope
Protection
POINT TO PONDER

“…… Care has to be taken that the
design is driven by sound geological
reasons and rigorous Engineering
Logic rather by the very attractive
images that appear on the Computer
screen.”
by Hoek, 1999
POINT TO PONDER

The innocent rock mass is often blamed for
insufficient stability that is actually the result of
rough and careless blasting. Where no
precaution have been taken to avoid blasting
damage, no knowledge of the real stability of
undisturbed rock can be gained from looking at
the remaining rock wall. What one sees are the
sad remains of what could have been perfectly
safe and stable.
by Holmberg and Persson,1980