slope stability classification · 2010-05-04 · twente. 2010 -04 27 tu delft sspc robert hack 11...
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University Twente. 2010-04-27 - TU Delft - SSPC - Robert Hack 1
SLOPE STABILITY CLASSIFICATION
TU Delft, The Netherlands, 27 April 2010
Robert HackEngineering Geology, ESA,International Institute for Geoinformation Sciences and Earth Observation (ITC), University Twente, The Netherlands
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Slope stability
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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(too steep, too high, etc.)
• Decrease in the future of soil or rock mass properties(weathering, vegetation, etc.)
• Changes in future geometry(scouring, erosion, human influence – road cut, etc.)
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What triggers in-stability of a slope ?
• Earthquakes (extra stress)• Rainfall (changes the properties)
These are not causes (!) because:• Should have been anticipated in design, or• For long standing slopes (e.g. natural slopes): There has
been an earthquake or rainfall before and then it did not collapse
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What is required to analyse the stability of a slope ?
• soil and rock mass properties• present and future geometry• present and future geotechnical behaviour
of soil or rock mass• external influences such as earthquakes,
rainfall, etc.
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soil and rock mass properties
In virtually all slopes is a considerable variation
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 costlygeotechnical calculations are more
complicated and complex, and cost more time
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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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Examples
What are the implications if the units are wrongly assumed in a design?
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Original situation
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design error
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Example 2: Many discontinuity sets with large variation in orientation (too many for the design engineer?)
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Example 3: Many discontinuity sets with large variation in orientation
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Example 4: Variation in clay content in intact rock causes differential weatheringbedding planes
Slightly higher clay content
April 1990
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Example 4: Variation in clay content in intact rock causes differential weathering
April 1992
mass slid
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Apart from variations in a geotechnical unit, also:
• Variation in present and future geometry• Variation in present and future
geotechnical behaviour of soil or rock mass
• Magnitude of external influences such as earthquakes, rainfall, etc. not certain
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Uncertainty Uncertainty in properties Uncertainty (error) in measurements of
properties Uncertainties in geometry Uncertainty (error) in measurements of
geometry (often small) Uncertainty in failure mechanisms applicable Uncertainty in future environment (for
example, weathering)
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Options for analysing slope stability
AnalyticalNumerical
Classification
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Analysing slope stability
analytical: only in relatively simple cases numerical: difficult and often cumbersome
and time consuming (provided the required input data is available)
Hence, classification systems may be a good and simple alternative
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Slope classification systems
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Classification systems are empirical relations that relate rock mass properties either directly or via a rating system to an engineering application,e.g. a slope, tunnel, etc.
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For underground:Bieniawski (RMR)
Barton (Q)Laubscher (MRMR)
etc.For slopes:
SelbyBieniawski (RMR)
VecchiaRobertson (RMR)Romana (SMR)
Hainesetc. etc.
Classification systems:
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Development of many rock mass classification systems First developed for underground excavations Most slope systems are based on
underground systems adjusted to be used for slopes
Therefore a legacy in properties and parameters from underground (read “tunnel”) systems
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Development of many rock mass classification systems
Most systems that are used at present are based on systems developed some 30 years ago At that time “state-of-the-art” and new, but this is
no reason not to investigate whether the systems are still as applicable or that new methodologies (for example, with the use of computers) allow for better systems
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Many rock mass classification systems
Wide variation in rating systems, methodologies, parameters, calculation methods, boundaries, etc. Addition, multiplication, logarithmic, etc. Wide variation in the influence of
parameters on the final result In some un-understandable ratings and
relations
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Strange influence parameters in some systemsFor example:
A slope in a rock mass with a high intact rock strength and one thick clay filled (gauge type) discontinuity set that will lead to sliding failure.
35º
clay-filled discontinuity
UCS = 150 MPa
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Strange influence parameters in some systemsIn some systems the intact rock strength will partially determine the stability rating, while the slope will be unstable due to the presence of the thick clay filled discontinuity and not at all be influenced by the intact rock strength.How valid is such a system?
35º
clay-filled discontinuity
UCS = 150 MPa
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Many more strange procedures in many classification systems
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For example:If RQD is used: Arbitrary length of 10 cm
Orientation of borehole in relation with discontinuity spacing
spacing discontinuities 0.09 m
vertical borehole RQD = 0 %
horizontal borehole
RQD = 100 %
horizontal borehole RQD = 0 %
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No clear differentiation between “as is” and “as will be”
External influences as weathering and method of excavation will have influenced the site characterized but will also (and likely differently) influence the new slope in the future
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Therefore new system developed:
Slope Stability probability Classification (SSPC)
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SSPC
• three step classification system• based on probabilities• independent failure mechanism assessment
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Three step classification system (1)
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Three step classification system (2) 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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Excavation specific parameters for the excavation which is used to characterize the rock mass
• Degree of weathering• Method of excavation
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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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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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Intact rock strength
By simple means test - hammer blows, crushing by hand, etc.
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Spacing and persistence of discontinuities
Based on the block size and block form by:• visual assessment, followed by:• quantification (measurement) of the
characteristic spacing and orientation
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slightly wavy
curved slightly curved straight
(i-angles and dimensions only approximate)
amplitude roughness: wavy
i = 14 - 20°
i = 9 - 14°
i = 2 - 4°
i = 4 - 8°
≈ 5 – 9 cm
≈ 5 – 9 cm
≈ 3.5 – 7 cm
≈ 1.5 – 3.5 cm
≈ 1 m
Shear strength -roughness large scale
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Shear strength -roughness small scale
stepped
undulating
planar
≈ 0.20 m
amplitude roughness > 2 - 3 mm
(dimensions only approximate)
amplitude roughness > 2 - 3 mm
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Shear strength -roughness tactile
Three classes:
rough
smooth
polished
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Shear strength - Infill
Infill:
- 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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Shear strength - karst
Karst or no karst
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Shear strength - condition factorDiscontinuity condition factor (TC) is a multiplication of the rating for small- and large scale roughness, infill and karst
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Orientation dependent stability
Stability depending on relation between slope and discontinuity orientation
For example:• Plane and wedge sliding• Toppling
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Discontinuity related shear strength failurePlane sliding(1)
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Discontinuity related shear strength failurePlane sliding(2)
Conditions:- discontinuity must daylight- downward stress > shear
strength along discontinuity plane
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Discontinuity related shear strength failureWedge sliding (1)
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Discontinuity related shear strength failureWedge sliding(2)
Conditions:- intersection line must daylight- downward stress > shear strength along
discontinuity planes
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Discontinuity related shear strength failureToppling
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How did we develop it? - sliding criterion:
AP (= apparent discontinuity dip in direction slope dip) (deg)
TC (=
disc
ontin
uity
cond
ition
par
amet
er) (
-)
0 20 40 60 80 0
0.2
0.4
0.6
0.8
1
stable unstable
TC = 0.0113 * AP (AP in deg)
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Sliding criterion
APTC *0113.0:if occurs sliding
<
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Sliding probability
AP (deg)
TC (c
ondi
tion
of d
iscon
tinui
ty)
1.00
0.80
0.60
0.40
0.20
0.00 0 10 20 30 40 50 60 70 80 90
5 % 30 % discontinuity stable with respect to sliding
discontinuity unstable with respect to sliding
70 % 50 % 95 %
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Toppling criterion
( )itydiscontinudipAPTC +−°−< 90*0087.0
TC = discontinuity condition factorAP = apparent discontinuity dip in direction
of slope dipDIPdiscontinuity = dip of discontinuity
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Toppling probability
Fig. 9. Toppling criterion.
- 90 - AP + slope dip (deg)
TC (c
ondi
tion
of d
iscon
tinui
ty) (
-)
0 10 20 30 40 50 60 70 80 90
1.00
0.80
0.60
0.40
0.20
0.00
70 %
5 %
95 % discontinuity stable
with respect to toppling
discontinuity unstable with respect to toppling
50 % 30 %
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Orientation independent stability
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Orientation independent stabilityI.e. slope instability not dependent on the
orientation of discontinuities in relation with the slope orientation
E.g. in situations:• No discontinuities• Too high stress for the soil or rock intact
material (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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Orientation independent stability
In SSPC based on:
• Intact rock strength• Block size and form• Condition of discontinuities
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Overall spacing of discontinuity setsBlock size and form relations from Taylor
0.1 1 10 100 1000 0.1
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1
discontinuity spacing (cm)
fact
or
1 discontinuity set
2 discontinuity sets minimum spacing maximum spacing
factor1 factor3
joint2
factor2
3 discontinuity sets minimum spacing intermediate spacing maximum spacing
bedding1 &
joint3
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Overall condition of discontinuity sets
3 2, 1, setsity discontinu of spacings theare and condition, theare
111
3,2,13,2,1 DSTCDSDSDS
DSTC
DSTC
DSTC
CD
321
3
3
2
2
1
1
++
++=
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Shear plane failure following Mohr-Coulomb for rock mass
( ) ( )( )’ - dip -
’ * dip *’ * coh * . = H
H
:’ dip
massslope
massslope
mass-
max
massslope
ϕϕ
ϕ
cos1cossin
1061else
infinite is )(height slope maximum the
theIf
4max
≤
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1
0.1
10
0.0 0.2 0.4 0.6 0.8 1.0
5 % 10 % 30 % 50 %
95 % 90 %
70 % probability to be stable > 95 %
probability to be stable < 5 %
(example)
ϕ’mass / slope dip
Hma
x / H
slop
e Dashed pr obability lines indi cate that the number of sl opes used for the devel opment of the SSPC sys tem for these sec tions of the graph is limited and the pr obability lines may not be as certai n as the pr obability lines dr awn with a conti nuous line.
Probability orientation independent failure
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Comparison between SSPC and other classification systems
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SSPC stability probability (%)
num
ber o
f slo
pes (
%)
< 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)
num
ber o
f slo
pes (
%)
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)
num
ber o
f slo
pes (
%)
-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: Comparison
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Examples
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Poorly blasted slope
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New cut (in 1990):Visual assessed: extremely poor instable.SSPC stability < 8% (13.8 m high, dip 70°, rock mass weathering: 'moderately' and 'dislodged blocks' due to blasting).
Forecast in 1996: SSPC stability: slope dip 45°.
In 2002: Slope dip about 55° (visually assessed unstable).
In 2005: Slope dip about 52° (visually assessed unstable – big blocks in middle photo have fallen).
Poorly blasted slope
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Plane sliding failure 40 year old road cut, Spain
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road
9 m 15 m
37°
bedding planes
162°
Fig. 108. Geometrical cross section of the slope.
Plane sliding failure (2)
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Plane sliding failure (3)
• Laboratory test: φ=45°• SSPC: φ≈35°• Stability assessed using:
- SSPC – 55% stability probability, failure imminent (φ≈35°)
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Slope Stability probability Classification (SSPC)
Saba case - Dutch Antilles
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Landslide in harbour
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Geotechnical zoning
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SSPC results
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)
• 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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Widening existing road in Bhutan (Himalayas)
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Bhutan (5)Method of excavation
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Widening existing road in Bhutan (Himalayas) (2)
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Widening existing road in Bhutan (Himalayas) (3)
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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Widening existing road in Bhutan (Himalayas) (4)
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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SSPC extensions:
measuring discontinuitiesfuture decay of slope material
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Heterogeneity
• even if uncertainty is included this is only up to a certain extend – what extend is to the discretion of the engineer
• can heterogeneity be defined by an automatic procedure , e.g. for example Lidar
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Heterogeneity (2)
(modified after Slob et al, 2002)
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Future degradation of soil or rock due to weathering, ravelling, etc.
no reliable quantitative relations exist to forecast the future geotechnical properties of soil or rock mass
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Future degradation (2)
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Future degradation (3)
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
Reduction in slope angle due to weathering, erosion and ravelling (after Huisman)
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Degradation processes
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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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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Erosion
• Essentially: migration of solid or dissolved material• Weathering occurs usually before and possibly
during erosion• Transporting agents:
- Water- Gravity- Ice- Wind- Man!
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Quantify weathering: SSPC
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Weathering in time
• 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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Weathering rates
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Weathering rates
( ) ( )log 1appinit WEWE t WE R t= − +
WE(t) = degree of weathering at time tWEinit = (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 layersGypsum cemented siltstone layers
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Weathering rates
Middle Muschelkalk near Vandellos (Spain)
- Balance between weathering and erosion(or generally) decay, andexposure orientation dependent features, such as: sunlight, wind, and rain.
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Weathering intensity rate
Weathering intensity rates R(appWE) for Middle Muschelkalk, siltstone,versus slope dip-direction (after Huisman)
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Weathering intensity rate
Weathering intensity rates R(appWE) for Middle Muschelkalk, gypsum,versus slope dip-direction (after Huisman)
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Weathering intensity rate
SSPC system with applying weathering intensity rate:- original slope cut about 50º (1998)- in 15 years decrease to 35º
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Conclusions
SSPC system in combination with degradation forecasts gives:
• reasonable design for slope stability• with minimum of work and• in a short time
University Twente. 2010-04-27 - TU Delft - SSPC - Robert Hack 112
LiteratureHack HRGK (2002) An evaluation of slope stability classification. Keynote Lecture & article. Proc. ISRM
EUROCK’2002, Portugal, Madeira, Funchal, 25-28 November 2002. Editors: C. Dinis da Gama & L. Ribeira e Sousa, Publ. Sociedade Portuguesa de Geotecnia, Av. do Brasil, 101, 1700-066 Lisboa, Portugal. pp. 3 – 32.
Hack HRGK, Price, D & Rengers N (2003) A new approach to rock slope stability - a probability classification (SSPC). Bulletin of Engineering Geology and the Environment. Springer Verlag. Vol. 62: article: DOI 10.1007/s10064-002-0155-4. pp. 167-184 & erratum: DOI 10.1007/s10064-002-0171-4. pp 185-185.
Hack HRGK, Price D & Rengers N (2005) Una nueva aproximación a la clasificación probabilística de estabilidad de taludes (SSPC). In "Ingeniería del Terreno", IngeoTer 5. chapter 6. publ. U.D. Proyectos, E.T.S.I. Minas -Universidad Politécnica de Madrid. ISBN 84-96140-14-8. pp. 418.
Huisman M, Hack HRGK & Nieuwenhuis JD (2006) Predicting rock mass decay in engineering lifetimes: the influence of slope aspect and climate. Environmental & Engineering Geoscience. Vol XII, no. 1, Feb. 2006, pp. 49-61.
Price, DG (2009) Engineering geology : principles and practice. De Freitas (ed), MH. Berlin, Springer, 2009. 450 p. ISBN: 978-3-540-29249-4.
Slob S, Hack HRGK, Knapen B van & Kenemy J (2004) Digital outcrop mapping and determination of rock mass properties using 3D terrestrial laser scanning techniques In: Schubert, W. (ed.): Rock Engineering – Theory and Practice. Proceedings of the ISRM Regional Symposium Eurock 2004 & 53rd Geomechanics Colloquy. Verlag Glückauf, Essen, Germany, ISBN 3-7739-5995-8, pp. 449-452.