SLOPE STABILITY BY CLASSIFICATION SSPC RMR · PDF fileSLOPE STABILITY BY CLASSIFICATION SSPC...

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

Causes and triggers for in-stability of a slope

12/09/2016Slope Stability by Classification - Hack 2

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)


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


GROUND MASS PROPERTIES

In virtually all slopes is a considerable variation


Therefore:

First divide the soil or rock mass in:

homogene “geotechnical units”


HOMOGENE GEOTECHNICAL UNIT?

Is that possible ?

VARIATION

Heterogeneity of mass causes:

• variation in mass properties


GEOTECHNICAL UNIT:

A “geotechnical unit” is a unit in which the geotechnical properties

are the same.


GEOTECHNICAL UNITS ARE BASED ON THE EXPERIENCE AND EXPERTISE OF THE INTERPRETER


“No geotechnical unit is really homogene….”

A certain amount of variation has to be allowed as otherwise the

number of units will be unlimited


“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.


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


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)


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


EXAMPLE

What are the implications if the units are wrongly assumed in a

design?


ORIGINAL SITUATION


AFTER EXCAVATION OF THE WRONG SLOPE DESIGN


OPTIONS FOR ANALYSING SLOPE STABILITY

Analytical

Numerical

Classification (empirical)


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)


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.



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



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")



Alternative:

use rock mass classification for input data in a numerical

calculation

or

use rock mass classification without numerical calculation


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


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


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


RMR (2)

based on a combination of five parameters

Each parameter is expressed by a point rating


(from De Mulder et al., 2012)

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


(from De Mulder et al., 2012)

)(sfactorreductionr)groundwateconditionspacingRQDRMR =(IRS

RMR - SLOPE MASS RATING (SMR) (Romana)(modified Bieniawski)

RMR rating multiplied with series of compensation factors


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

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)


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


GSI(2)

12/09/2016Slope Stability by Classification - Hack 32(from De Mulder et al., 2012)

‘structure’ is related to

the block size and the

interlocking of rock

blocks

‘surface condition’ is

related to weathering,

persistence, and

condition of

discontinuities.

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).


SLOPE STABILITY PROBABILITY CLASSIFICATION (SSPC)

three step classification system

based on probabilities

independent failure mechanism assessment


SSPC - THREE STEP CLASSIFICATION SYSTEM (1)


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.

THREE STEP CLASSIFICATION SYSTEM


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:


Discontinuities: orientation and sets (spacing) or single Discontinuity properties: roughness, infill, karst

SLOPE ROCK MASS (SRM) Slope rock mass parameters significant for slope stability:


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

SSPC

Excavation specific parameters for the excavation which is used to

characterize the rock mass:

Degree of weathering

Method of excavation


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


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


SSPC

Intact rock strength (IRS)

By simple means test:

hammer blows, crushing by hand, etcetera


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


SSPC

Shear strength based on a combination of:

roughness (persistence)

infill

presence of karst


SSPC

Roughness is a combination of:

large scale roughness (Rl),

small scale roughness & tactile roughness (Rs)


SSPC

Shear strength – roughness


SSPC

Shear strength

roughness tactile

Three classes:

rough

smooth

polished


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


SSPC

Karst (Ka):

karst or no karst


SSPC

Shear strength - condition factor

Discontinuity condition factor (TC) is a multiplication of the ratings

for:

small-scale roughness

large-scale roughness

infill

karst


SSPC


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

SLIDING CRITERION

TC is related to friction along plane by:


0113.0

*KaImRl*Rs*anglesliding

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


degrees3501130

001550950750

01130

Im

..*.*.*.

.

*KaRl*Rs*

anglesliding

SSPC

Orientation dependent stability

Stability depending on relation between slope and discontinuity

orientation

For example:

Plane and wedge sliding

Toppling


SSPC

Orientation dependent stabilityDiscontinuity related shear strength failure

Plane sliding

Conditions:

- discontinuity must daylight

- downward stress > shear strength along discontinuity plane


SSPC


Discontinuity related shear strength failure

Wedge sliding

Conditions:

- intersection line must daylight

- downward stress > shear strength along discontinuity planes



Sliding if:


APTC *0113.0

TC = discontinuity condition factor

AP = apparent discontinuity dip in direction of slope dip

SSPC


Sliding probability


SSPC


Discontinuity related shear strength failure

Toppling


SSPC


Toppling criterion


itydiscontinudipAPTC 90*0087.0

TC = discontinuity condition factor

AP = apparent discontinuity dip in

direction of slope dip

DIPdiscontinuity = dip of discontinuity

SSPC

Toppling probability


Orientation independent stability


SSPC


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)


SSPC


In SSPC based on:

• Intact rock strength

• Block size and form

• Condition of discontinuities


SSPC


Probability orientation independent failure

SSPCCOMPARISON BETWEEN SSPC AND OTHER CLASSIFICATION

SYSTEMS


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



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



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:

EXAMPLES



POORLY BLASTED SLOPE

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


SABA - DUTCH ANTILLES - LANDSLIDE IN HARBOUR


SABA - GEOTECHNICAL UNITS


SABA


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)

FAILING SLOPE IN MANILA, PHILIPPINES


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


BHUTAN

Widening existing

road in Bhutan

(Himalayas)


BHUTAN

Method of

excavation


BHUTAN


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


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


FUTURE DEGRADATION OF SOIL OR ROCK DUE TO WEATHERING, RAVELLING, ETC.

Forecasting

future geotechnical properties of soil or rock mass


FUTURE DEGRADATION


FUTURE DEGRADATION

Reduction in

slope angle

due to

weathering,

erosion and

ravelling

(after

Huisman)


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

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)


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


CINDARTO SLOPEVARIATION IN CLAY CONTENT IN INTACT ROCK CAUSES DIFFERENTIAL WEATHERING

April 1992

mass slid

SIGNIFICANCE IN ENGINEERING

When rock masses degrade in time, slopes and other works that

are stable at present may become unstable


IMPACT OF WEATHERING


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.

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.



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


WEATHERING RATES

Middle Muschelkalk near Vandellos (Spain)

•Material:

Gypsum layers

Gypsum cemented siltstone layers

SSPC system with applying weathering intensity rate:

- original slope cut about 50º (1998)

- in 15 years decrease to 35º


KOTA KINABALU, MALAYSIA


10 years

old

(after Tating, Hack, & Jetten, 2011)

KOTA KINABALU

Side road (dip 45°, 5 years old)

sandstone: slightly weathered

SSPC

stability:

Sandstone:

stable (92%)

Shale:

unstable (< 5%)


KOTA KINABALU

Main road (dip 30°, 10 years old):

sandstone: moderately weathered

SSPC

stability:

Sandstone:

stable (95%)

Shale:

ravelling (<5%)


10 years

old

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


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)


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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