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

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: HACK@ITC.NL

TU Delft, The Netherlands, 2 October 2012

Causes and triggers for in-stability of a slope

02/10/2012 Slope Stability by Classification - Hack 2

WHAT CAUSES IN-STABILITY OF A SLOPE ?

• Wrong design (e.g. too steep, too high) • Decrease in the future of ground mass properties (e.g. weathering, vegetation) • Changes in future geometry (e.g. scouring, erosion, human influence – road cut)

WHAT IS REQUIRED TO ANALYSE THE 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

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

EXAMPLES

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

ORIGINAL SITUATION

DESIGN ERROR

OPTIONS FOR ANALYSING SLOPE STABILITY

Analytical Numerical Classification

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.:

representative tests of discontinuity (i.e. joint) shear strength of each discontinuity family orientations of each discontinuity etcetera, etcetera.

High quality and quantity of data not only of the rock mass at the slope face but also in the slope! 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 the 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 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 02/10/2012 Slope Stability by Classification - Hack 28

(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 =

RMRRMRSMR

F) + F * F * F- (SMR = RMR

RMR(5)

Advantages: Simple

Disadvantages: developed for tunneling in (generally) high surrounding stress environment

Cohesion and friction 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)

02/10/2012 Slope 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)

old road

proposed new road cut slightly

weathered

moderately weathered

Reference Rock Mass

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:

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

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

Excavation specific parameters for the excavation which is used to characterize the rock mass:

Degree of weathering Method of excavation

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

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

Intact rock strength (IRS) By simple means test:

hammer blows, crushing by hand, etcetera

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

Shear strength based on a combination of:

roughness (persistence) infill presence of karst

Roughness is a combination of:

large scale roughness (Rl) small scale roughness & tactile roughness (Rs)

Shear strength roughness large scale

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

3.5 – 7 cm

1.5 – 3.5 cm

Shear strength roughness small scale

stepped

undulating

planar

0.20 m

amplitude roughness > 2 - 3 mm

(dimensions only approximate)

amplitude roughness > 2 - 3 mm

Shear strength roughness tactile

Three classes:

smooth

polished

Infill (In):

- cemented

- no infill

- non-softening (3 grain sizes)

- softening (3 grain sizes)

- gauge type (larger or smaller than roughness amplitude)

- flowing material

Karst (Ka):

karst or no karst

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

small-scale roughness large-scale roughness infill karst

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 planarpolished 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 0.75small scale & tactile rough stepped 0.95infill fine soft sheared 0.55karst none 1.00

degrees3501130001550950750

01130Im

..*.*.*.

.*KaRl*Rs*

anglesliding

Orientation dependent stability Stability depending on relation between slope and discontinuity

orientation For example:

Plane and wedge sliding Toppling

Orientation dependent stability Discontinuity related shear strength failure

Plane sliding Conditions: - discontinuity must daylight - downward stress > shear strength along discontinuity plane

Orientation dependent stability Discontinuity related shear strength failure Wedge sliding Conditions: - intersection line must daylight - downward stress > shear strength along discontinuity planes

Orientation dependent stability Sliding if:

APTC *0113.0TC = discontinuity condition factor AP = apparent discontinuity dip in direction of slope dip

Orientation dependent stability Sliding probability

AP (deg)

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 %

Orientation dependent stability Discontinuity related shear strength failure Toppling

Orientation dependent stability Toppling criterion

itydiscontinudipAPTC 90*0087.0

TC = discontinuity condition factor AP = apparent discontinuity dip in

direction of slope dip DIPdiscontinuity = dip of discontinuity

Toppling probability

02/10/2012 Slope Stability by Classification - Hack 60 - 90 - AP + slope dip (deg)

0 10 20 30 40 50 60 70 80 90

95 % discontinuity stable

with respect to toppling

discontinuity unstable with respect to toppling

50 % 30 %

Orientation independent stability

Orientation independent stability 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 strength (e.g.

slope too high) • So many discontinuities in so many directions that there is always

a failure plane (comparable to a soil mass)

Orientation independent stability

In SSPC based on:

• Intact rock strength • Block size and form • Condition of discontinuities

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)

’mass / slope dip

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

SSPCCOMPARISON BETWEEN SSPC AND OTHER CLASSIFICATION SYSTEMS

SSPC stability probability (%)

< 5 7.5 15 25 35 45 55 65 75 85 92.5 > 95 0

80 visually estimated stability

stable (class 1) unstable (class 2) unstable (class 3)

Romana's SMR (points)

5 15 25 35 45 55 65 75 85 95 0

Haines' slope dip - existing slope dip (deg)

-45 -35 -25 -10 -5 5 15 25 35 45 0

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

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

Pyroclastic deposits Calculated SSPC Laboratory / field Rock mass friction 35° 27° (measured)

Rock mass cohesion 39kPa 40kPa (measured) Calculated maximum possible height on the

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

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

Reduction in slope angle due to weathering, erosion and ravelling (after Huisman)

7.0 7.5 8.0 8.5 9.0 9.5

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)

02/10/2012 Slope Stability by Classification - Hack 83 Slope Stability by Classification - Hack 83

CINDARTO SLOPE: VARIATION IN CLAY CONTENT IN INTACT ROCK CAUSES DIFFERENTIAL WEATHERING

bedding planes

Slightly higher clay content

April 1990

CINDARTO SLOPE VARIATION 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

02/10/2012 Slope Stability by Classification - Hack 86 86

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 1appinit WEWE t WE R t

WE(t) = degree of weathering at time t WEinit = (initial) degree of weathering at time t = 0 Rapp

WE = 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

02/10/2012 Slope Stability by Classification - Hack 93 93

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 [degrees]

SSPC visual SSPC probability

unit RM friction RM cohesion

[degrees] [kPa]

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) 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. 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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