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SLOPE STABILITY BY CLASSIFICATION
SSPC RMR GSI
ROBERT HACK
ENGINEERING GEOLOGY, ESA,
ITC, FACULTY OF GEO-INFORMATION SCIENCE AND EARTH
OBSERVATION, UNIVERSITY OF TWENTE,
THE NETHERLANDS. PHONE:+31 (0)6 24505442; EMAIL: [email protected]
TU Delft, The Netherlands, 12 September 2016
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Causes and triggers for in-stability of a slope
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WHAT CAUSES IN-STABILITY OF A SLOPE ?
• Wrong design
(e.g. too steep, too high)
• Decrease of ground mass properties in the future
(e.g. weathering, vegetation)
• Changes in future geometry
(e.g. scouring, erosion, human influence – road cut)
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WHAT IS REQUIRED TO ANALYSETHE STABILITY OF A SLOPE ?
• ground mass properties
• present and future geometry
• present and future geotechnical behaviour of ground mass
• external influences such as earthquakes, rainfall, etcetera
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GROUND MASS PROPERTIES
In virtually all slopes is a considerable variation
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Therefore:
First divide the soil or rock mass in:
homogene “geotechnical units”
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HOMOGENE GEOTECHNICAL UNIT?
Is that possible ?
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VARIATION
Heterogeneity of mass causes:
• variation in mass properties
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GEOTECHNICAL UNIT:
A “geotechnical unit” is a unit in which the geotechnical properties
are the same.
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GEOTECHNICAL UNITS ARE BASED ON THE EXPERIENCE AND EXPERTISE OF THE INTERPRETER
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“No geotechnical unit is really homogene….”
A certain amount of variation has to be allowed as otherwise the
number of units will be unlimited
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“The allowable variation of the properties within one geotechnical unit depends on:
the degree of variability of the properties within a mass,
the influence of the differences on engineering behaviour, and
the context in which the geotechnical unit is used.
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Smaller allowed variability of the properties in a geotechnical unit results in:
higher accuracy of geotechnical calculations
less risk that a calculation or design is wrong
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Smaller allowed variability of the properties in a geotechnical unit:
requires collecting more data and is thus more costly
geotechnical calculations are more complicated and complex, and cost more time (and thus also more money)
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HENCE:
the variations allowed within a geotechnical unit for a slope along a major highway is smaller
the variations allowed within a geotechnical unit for a slope along a farmers road will be larger
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EXAMPLE
What are the implications if the units are wrongly assumed in a
design?
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ORIGINAL SITUATION
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AFTER EXCAVATION OF THE WRONG SLOPE DESIGN
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OPTIONS FOR ANALYSING SLOPE STABILITY
Analytical
Numerical
Classification (empirical)
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SLOPE STABILITY
analytical: only in relatively simple cases possible for a
discontinuous rock mass
numerical: difficult and often cumbersome
(however, possible with discontinuous numerical rock mechanics
programs such as UDEC & 3DEC)
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NUMERICAL SLOPE STABILITY(1)
Extra work for deterministic numerical methods is justified if:
Quantity and quality of input data is high, e.g. available should be:
representative tests of discontinuity (i.e. joint) shear strength of
each discontinuity family
orientations of each discontinuity family and spread in a family
etcetera, etcetera.
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NUMERICAL SLOPE STABILITY(2)
High quality and quantity of data not only of the rock mass at the
slope face but also inside the slope ground mass!
Hence:
excavate the site and rebuilt (then it is exactly known)
or
many large-sized borehole samples required
High quality and quantity of data of rock mass inside a slope rock
mass are virtually never available because far too expensive to
obtain
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NUMERICAL SLOPE STABILITY(4)
Solution often used:
Use a numerical program and estimate or obtain the input
parameters from literature
In particular dangerous because:
Users (i.e. the civil engineers) expect numerical calculation to be
accurate (the result becomes the "truth")
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NUMERICAL SLOPE STABILITY(5)
Alternative:
use rock mass classification for input data in a numerical
calculation
or
use rock mass classification without numerical calculation
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SLOPE CLASSIFICATION SYSTEMS
Classification systems are empirical relations that relate rock mass
properties either directly or via a rating system to an engineering
application, e.g. slope, tunnel
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CLASSIFICATION SYSTEMS:
For underground (tunnel):
• Bieniawski (RMR)
• Barton (Q)
• Laubscher (MRMR)
• etcetera
For slopes:
• Selby
• Bieniawski (RMR)
• Vecchia
• Robertson (RMR)
• Romana (SMR)
• Haines
• SSPC
• etcetera
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ROCK MASS RATING (RMR) (BIENIAWSKI)
one of the oldest still used systems (Bieniawski, 1989).
developed in South Africa for underground mining
but currently widely used in civil engineering as well
excavation and support is determined by the RMR value
and results in five different support classes.
adjustment factors and refinements are possible
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RMR (2)
based on a combination of five parameters
Each parameter is expressed by a point rating
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(from De Mulder et al., 2012)
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RMR(3)
addition of the points results in the RMR rating
reduction factors for: orientation, excavation damage, etc.
related (empirically) to rock mass cohesion, friction angle of the
rock mass, and other rock mass properties
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(from De Mulder et al., 2012)
)(sfactorreductionr)groundwateconditionspacingRQDRMR =(IRS
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RMR - SLOPE MASS RATING (SMR) (Romana)(modified Bieniawski)
RMR rating multiplied with series of compensation factors
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excavation of methodfor factor =
dipity discontinu and face slope between relationfor factor =
angle dipity discontinufor factor =
face slope and itiesdiscontinu of strikes theof mparallelisfor factor =
) si' Bieniawskas (same Rating Rock Mass=
Rating MassSlope =
4321
F
F
F
F
RMRRMR
SMR
F) + F * F * F- (SMR = RMR
4
3
2
1
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RMR(5)
Advantages:
Simple
Disadvantages:
developed for tunneling in (generally) high surrounding stress
environment
Cohesion generally considered (far) too high
for low stress environment (i.e. not suitable in slopes)
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GEOLOGICAL STRENGTH INDEX (GSI)
The Geological Strength Index (GSI) is derived from a matrix
describing the ‘structure’ and the ‘surface condition’ of the rock mass
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GSI(2)
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‘structure’ is related to
the block size and the
interlocking of rock
blocks
‘surface condition’ is
related to weathering,
persistence, and
condition of
discontinuities.
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GSI(3)
The GSI is one of the constituents of the Hoek-Brown failure
criterion.
The failure criterion does not provide excavation or support
recommendations but rather determines rock mass properties, such
as rock mass cohesion and rock mass angle of friction (Hoek et al.,
1998, Marinos & Hoek, 2000, Marinos et al., 2005).
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SLOPE STABILITY PROBABILITY CLASSIFICATION (SSPC)
three step classification system
based on probabilities
independent failure mechanism assessment
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SSPC - THREE STEP CLASSIFICATION SYSTEM (1)
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river
old road
proposed new road cut slightly
weathered
moderately
weathered
1 2
3
Reference
Rock Mass
fresh
1: natural exposure made by scouring of river, moderately weathered; 2: old road, made by excavator, slightly weathered; 3: new to develop
road cut, made by modern blasting, moderately weathered to fresh.
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THREE STEP CLASSIFICATION SYSTEM
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EXPOSURE ROCK MASS (ERM) Exposure rock mass parameters significant for slope stability:
Material properties: strength, susceptibility to weathering
Discontinuities: orientation and sets (spacing) or single Discontinuity properties: roughness, infill, karst
REFERENCE ROCK MASS (RRM) Reference rock mass parameters significant for slope stability:
Material properties: strength, susceptibility to weathering
Discontinuities: orientation and sets (spacing) or single Discontinuity properties: roughness, infill, karst
SLOPE ROCK MASS (SRM) Slope rock mass parameters significant for slope stability:
Material properties: strength, susceptibility to weathering
Discontinuities: orientation and sets (spacing) or single
Discontinuity properties: roughness, infill, karst
Exposure specific parameters:
Method of excavation Degree of weathering
Slope specific parameters:
Method of excavation to be used
Expected degree of weathering at end of engineering life-time of slope
SLOPE GEOMETRY Orientation
Height
SLOPE STABILITY ASSESSMENT
Factor used to remove the influence of the
method excavation and degree of weathering
Factor used to assess the influence of the
method excavation and future weathering
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SSPC
Excavation specific parameters for the excavation which is used to
characterize the rock mass:
Degree of weathering
Method of excavation
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SSPC
Rock mass Parameters:
Intact rock strength
Spacing and persistence discontinuities
Shear strength along discontinuity:
- Roughness - large scale
- small scale
- tactile roughness
- Infill
- Karst
Susceptibility to weathering
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SSPC
Slope specific parameters for the new slope to be made:
Expected degree of weathering at end of lifetime of the slope
Method of excavation to be used for the new slope
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SSPC
Intact rock strength (IRS)
By simple means test:
hammer blows, crushing by hand, etcetera
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SSPC
Spacing and persistence of discontinuities:
Determine block size and block form by:
visual assessment, followed by:
quantification (measurement) of the characteristic spacing and
orientation of each set
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SSPC
Shear strength based on a combination of:
roughness (persistence)
infill
presence of karst
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SSPC
Roughness is a combination of:
large scale roughness (Rl),
small scale roughness & tactile roughness (Rs)
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SSPC
Shear strength – roughness
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SSPC
Shear strength
roughness tactile
Three classes:
rough
smooth
polished
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SSPC
Infill (In):
- cemented
- no infill
- non-softening (3 grain sizes)
- softening (3 grain sizes)
- gauge type (larger or smaller than roughness amplitude)
- flowing material
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SSPC
Karst (Ka):
karst or no karst
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SSPC
Shear strength - condition factor
Discontinuity condition factor (TC) is a multiplication of the ratings
for:
small-scale roughness
large-scale roughness
infill
karst
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SSPC
12/09/2016Slope Stability by Classification - Hack 49
CONDITION OF DISCONTINUITY factor
Roughness
large scale (Rl)
(visual area > 0.2 x 0.2 and <
1 x 1 m2)
wavy
slightly wavy
curved
slightly curved
straight
1.00
0.95
0.85
0.80
0.75
Roughness
small scale (Rs)
(tactile and visual on an area
of
20 x 20 cm2)
rough stepped/irregular
smooth stepped
polished stepped
rough undulating
smooth undulating
polished undulating
rough planar
smooth planar
polished planar
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
Infill
material (Im)
cemented/cemented infill
no infill - surface staining
1.07
1.00
non softening
& sheared
material, e.g.
free of clay,
talc, etc.
coarse
medium
fine
0.95
0.90
0.85
soft sheared
material, e.g.
clay, talc, etc.
coarse
medium
fine
0.75
0.65
0.55
gouge < irregularities
gouge > irregularities
flowing material
0.42
0.17
0.05
Karst (Ka)none
karst
1.00
0.92
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SLIDING CRITERION
TC is related to friction along plane by:
12/09/2016Slope Stability by Classification - Hack 50
0113.0
*KaImRl*Rs*anglesliding
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SLIDING CRITERION (EXAMPLE)
bedding plane description factor
large scale straight 0.75
small scale & tactile rough stepped 0.95
infill fine soft sheared 0.55
karst none 1.00
12/09/2016Slope Stability by Classification - Hack 51
degrees3501130
001550950750
01130
Im
..*.*.*.
.
*KaRl*Rs*
anglesliding
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SSPC
Orientation dependent stability
Stability depending on relation between slope and discontinuity
orientation
For example:
Plane and wedge sliding
Toppling
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SSPC
Orientation dependent stabilityDiscontinuity related shear strength failure
Plane sliding
Conditions:
- discontinuity must daylight
- downward stress > shear strength along discontinuity plane
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SSPC
Orientation dependent stability
Discontinuity related shear strength failure
Wedge sliding
Conditions:
- intersection line must daylight
- downward stress > shear strength along discontinuity planes
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Orientation dependent stability
Sliding if:
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APTC *0113.0
TC = discontinuity condition factor
AP = apparent discontinuity dip in direction of slope dip
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SSPC
Orientation dependent stability
Sliding probability
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SSPC
Orientation dependent stability
Discontinuity related shear strength failure
Toppling
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SSPC
Orientation dependent stability
Toppling criterion
12/09/2016Slope Stability by Classification - Hack 58
itydiscontinudipAPTC 90*0087.0
TC = discontinuity condition factor
AP = apparent discontinuity dip in
direction of slope dip
DIPdiscontinuity = dip of discontinuity
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SSPC
Toppling probability
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Orientation independent stability
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SSPC
Orientation independent stability
Slope instability not dependent on the orientation of discontinuities in relation with the slope orientation
E.g. in situations with:
• No discontinuities
• Too high stress for the ground intact material strength (intact
material breaks) (e.g. slope too high)
• So many discontinuities in so many directions that there is always
a failure plane (comparable to a soil mass)
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SSPC
Orientation independent stability
In SSPC based on:
• Intact rock strength
• Block size and form
• Condition of discontinuities
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SSPC
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Probability orientation independent failure
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SSPCCOMPARISON BETWEEN SSPC AND OTHER CLASSIFICATION
SYSTEMS
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SSPC stability probability (%)
nu
mb
er o
f sl
op
es (
%)
< 5 7.5 15 25 35 45 55 65 75 85 92.5 > 95 0
20
40
60
80 visually estimated stability
stable (class 1) unstable (class 2) unstable (class 3)
Romana's SMR (points)
nu
mb
er o
f sl
op
es (
%)
5 15 25 35 45 55 65 75 85 95 0
20
40
60
80 visually estimated stability
stable (class 1) unstable (class 2) unstable (class 3)
Haines' slope dip - existing slope dip (deg)
nu
mb
er o
f sl
op
es (
%)
-45 -35 -25 -10 -5 5 15 25 35 45 0
20
40
60
80 visually estimated stability
stable (class 1) unstable (class 2) unstable (class 3)
Percentages are from total number of slopes per visually estimated stability class.
visually estimated stability:
class 1 : stable; no signs of present or future slope failures (number of slopes: 109) class 2 : small problems; the slope presently shows signs of active small failures and has the potential for future small failures (number of slopes: 20) class 3 : large problems; The slope presently shows signs of active large failures and has the potential for future large failures (number of slopes: 55)
unstable stable stable unstable
a: SSPC b: Haines
c: SMR
Haines safety factor: 1.2
completely unstable
completely stable
partially stable unstable stable
'tentative' describtion of SMR classes:
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EXAMPLES
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POORLY BLASTED SLOPE
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POORLY BLASTED SLOPE
New cut (in 1990):
Visual assessed: extremely poor; instable.
(SSPC stability < 8% for slope height 13.8 m high, dip 70°, rock
mass weathering: 'moderately' and 'dislodged blocks' due to
blasting).
Forecast in 1996: SSPC final stability: slope dip 45.
In 2002: Slope dip about 55 (visually assessed unstable).
In 2005: Slope dip about 52
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SABA - DUTCH ANTILLES - LANDSLIDE IN HARBOUR
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SABA - GEOTECHNICAL UNITS
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SABA
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Pyroclastic deposits Calculated SSPC Laboratory / field
Rock mass friction 35° 27° (measured) Rock mass cohesion 39kPa 40kPa (measured)
Calculated maximum
possible height on the
slope
13m 15m (observed)
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FAILING SLOPE IN MANILA, PHILIPPINES
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FAILING SLOPE IN MANILA (2)
volcanic tuff layers with near horizontal weathering horizons (about every 2-3 m)
slope height is about 5 m
SSPC non-orientation dependent stability about 50% for 7 m slope height
unfavourable stress configuration due to corner
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BHUTAN
Widening existing
road in Bhutan
(Himalayas)
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BHUTAN
Method of
excavation
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BHUTAN
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BHUTAN
Above road level:
Various units
Joint systems (sub-) vertical
Present slope about 21 m high, about 90° or overhanging (!)
Present situation above road highly unstable (visual assessment)
Below road level:
Inaccessible – seems stable
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BHUTAN
Above road level:
Following SSPC system about 12 – 27 m for a 75° slope (depending on unit) (orientation independent stability 85%)
Below road level:
Inaccessible – different unit ? – and not disturbed by excavation method
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FUTURE DEGRADATION OF SOIL OR ROCK DUE TO WEATHERING, RAVELLING, ETC.
Forecasting
future geotechnical properties of soil or rock mass
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FUTURE DEGRADATION
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FUTURE DEGRADATION
Reduction in
slope angle
due to
weathering,
erosion and
ravelling
(after
Huisman)
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1.0
1.5
2.0
2.5
3.0
3.5
7.0 7.5 8.0 8.5 9.0 9.5
y [m]
z [
m]
Excavated 1999 May 2001 May 2002
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FUTURE DEGRADATION
Main processes involved in degradation:
Loss of structure due to stress release
Weathering (In-situ change by inside or outside influences)
Erosion (Material transport with no chemical or structural
changes)
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12/09/2016Slope Stability by Classification - Hack 82Slope Stability by Classification - Hack 82
CINDARTO SLOPE:VARIATION IN CLAY CONTENT IN INTACT ROCK CAUSES DIFFERENTIAL WEATHERING
bedding planes
Slightly higher clay content
April 1990
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CINDARTO SLOPEVARIATION IN CLAY CONTENT IN INTACT ROCK CAUSES DIFFERENTIAL WEATHERING
April 1992
mass slid
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SIGNIFICANCE IN ENGINEERING
When rock masses degrade in time, slopes and other works that
are stable at present may become unstable
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12/09/2016Slope Stability by Classification - Hack 8585
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IMPACT OF WEATHERING
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From: De Mulder, E.J.F., Hack, H.R.G.K., Van Ree, C.C.D.F., 2012.
Sustainable Development and Management of the Shallow Subsurface.
The Geological Society, London. ISBN: 978-1-86239-343-1. p. 192.
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The susceptibility to weathering is a concept that is frequently
addressed by “the” weathering rate of a rock material or mass.
Weathering rates may be expected to decrease with time, as the
state of the rock mass becomes more and more in equilibrium
with its surroundings.
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log 1app
init WEWE t WE R t
WE(t) = degree of weathering at time t
WEinit = (initial) degree of weathering at time t = 0
RappWE = weathering intensity rate
WE as function of time, initial weathering
and the weathering intensity rate
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WEATHERING RATES
Middle Muschelkalk near Vandellos (Spain)
•Material:
Gypsum layers
Gypsum cemented siltstone layers
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SSPC system with applying weathering intensity rate:
- original slope cut about 50º (1998)
- in 15 years decrease to 35º
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KOTA KINABALU, MALAYSIA
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10 years
old
(after Tating, Hack, & Jetten, 2011)
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KOTA KINABALU
Side road (dip 45°, 5 years old)
sandstone: slightly weathered
SSPC
stability:
Sandstone:
stable (92%)
Shale:
unstable (< 5%)
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KOTA KINABALU
Main road (dip 30°, 10 years old):
sandstone: moderately weathered
SSPC
stability:
Sandstone:
stable (95%)
Shale:
ravelling (<5%)
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10 years
old
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KOTA KINABALU
time
[years]
dip
[degre
es]
SSPC visual SSPC
probability
unit RM friction RM
cohesion
[degrees] [kPa]
shale
slightly 5 45 4 2.4 in stable
moderately 10 30 2 1.1 in stable
sandstone
slightly 5 45 20 10.0 stable
moderately 10 30 11 6.3 stable
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SSPC system in combination with degradation forecasts gives:
reasonable design for slope stability
with minimum of work and
in a short time
(likely a reasonable tool to forecast susceptibility to weathering)
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REFERENCES
De Mulder, E.J.F., Hack, H.R.G.K., Van Ree, C.C.D.F., 2012. Sustainable Development and Management of the Shallow Subsurface. The Geological Society,
London. ISBN: 978-1-86239-343-1. p. 192.
Hack, H.R.G.K., 2002. An evaluation of slope stability classification; Keynote lecture. In: Dinis Da Gama, C., Ribeira E Sousa, L. (Eds) ISRM EUROCK 2002, Funchal,
Madeira, Portugal. Sociedade Portuguesa de Geotecnia, Av. do Brasil, 101, 1700-066 Lisboa, Portugal, pp. 3–32.
Hack, H.R.G.K., Price, D.G., Rengers, N., 2003. A new approach to rock slope stability : a probability classification SSPC. Bulletin of Engineering Geology and the
Environment. 62 (2). DOI: 10.1007/s10064-002-0155-4. pp. 167-184.
Hack, H.R.G.K., Price, D., Rengers, N., 2005. Una nueva aproximación a la clasificación probabilística de estabilidad de taludes (SSPC). In: Proyectos, U.D., Minas,
E.T.S.I. (Eds), Ingeniería del terreno : ingeoter 5 : capítulo 6. Universidad Politécnica de Madrid, Madrid. ISBN: 84-96140-14-8. p. 418. (in Spanish)
Hack, H.R.G.K., Price, D.G. & Rengers, N., 2003. 研究岩质边坡稳定性新方法—概率分级法 (Translation of "A new approach to rock slope stability - A probability
classification (SSPC)"). Original in: Bulletin of Engineering Geology and the Environment. 62 (2). DOI: 10.1007/s10064-002-0155-4. ISSN: 1435-9529; 1435-9537. pp.
167-184. (in Chinese)
Hoek, E., Marinos, P., Benissi, M., 1998. Applicability of the geological strength index (GSI) classification for very weak and sheared rock masses. The case of the
Athens Schist Formation. Bulletin of Engineering Geology and the Environment. 57 (2). DOI: 10.1007/s100640050031. pp. 151-160.
Huisman, M., Hack, H.R.G.K., Nieuwenhuis, J.D., 2006. Predicting Rock Mass Decay in Engineering Lifetimes: The Influence of Slope Aspect and Climate.
Environmental & Engineering Geoscience. 12 (1). DOI: 10.2113/12.1.39. pp. 39-51.
Marinos, P., Hoek, E., 2000. GSI: A geologically friendly tool for rock mass strength estimation. In: Drinan, J., Geom Australian (Eds) GeoEng2000 - International
Conference on Geotechnical & Geological engineering, Melbourne, 19-24 November 2000. Technomic Publishing Co, Lancaster, PA, USA, pp. 1422–1446.
Marinos, V., Marinos, P. & Hoek, E. 2005. The geological strength index: applications and limitations. Bull. of Engineering Geology and the Environment 64/1, doi:
10.1007/s10064-004-0270-5, 55-65.
Price, D.G., De Freitas, M.H., Hack, H.R.G.K., Higginbottom, I.E., Knill, J.L., Maurenbrecher, M., 2009. Engineering geology : principles and practice. De Freitas, M.H.
(Ed.). Springer-Verlag, Berlin, Heidelberg. ISBN: 978-3-540-29249-4. p. 450.
Tating, F.F., Hack, H.R.G.K. & Jetten, V., 2013. Engineering aspects and time effects of rapid deterioration of sandstone in the tropical environment of Sabah,
Malaysia. Engineering Geology. 159. DOI: 10.1016/j.enggeo.2013.03.009. ISSN: 0013-7952. pp. 20-30.
Tating, F.F., Hack, H.R.G.K. & Jetten, V., 2015. Weathering effects on discontinuity properties in sandstone in a tropical environment: case study at Kota Kinabalu,
Sabah Malaysia. Bulletin of Engineering Geology and the Environment. 74 (2). DOI: 10.1007/s10064-014-0625-5. ISSN: 1435-9529. pp. 427-441.
White, A.F., Blum, A.E., Schulz, M.S., Vivit, D.V., Stonestrom, D.A., Larsen, M., Murphy, S.F., Eberl, D., 1998. Chemical Weathering in a Tropical Watershed, Luquillo
Mountains, Puerto Rico: I. Long-Term Versus Short-Term Weathering Fluxes. Geochimica et Cosmochimica Acta. 62 (2). DOI: 10.1016/s0016-7037(97)00335-9. pp.
209-226.
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