Slope stability & weathering by classification; SSPC, RMR, GSI : powerpoint

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SLOPE STABILITY & WEATHERING 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: H.R.G.K.HACK@UTWENTE.NL

Ingeokring, Bad Bentheim, Germany, 25 August 2018

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WHAT CAUSES IN-STABILITY OF A SLOPE ?

• Wrong design

(e.g. too steep, too high)

• Decrease of ground mass propertiesin the future (e.g. weathering, vegetation)

• Changes in future geometry

(e.g. scouring, erosion, human influence – road cut)

25/08/2018 Slope Stability & weathering by Classification - Hack - Bad Bentheim 3

WHAT IS REQUIRED TO ANALYSE

THE STABILITY OF A SLOPE ?

• ground mass properties • present and future geometry

• present and future geotechnical behaviourof ground mass • external influences such as earthquakes, rainfall, etcetera

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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?

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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.

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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

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“The allowable variation of the properties within one

geotechnical unit depends on:

▪ the degree of variabilityof the properties within a mass, ▪ the influenceof the differences on engineering behaviour, and ▪ the contextin 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

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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)

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?

ORIGINAL SITUATION

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AFTER EXCAVATION OF THE WRONG SLOPE DESIGN

OPTIONS FOR ANALYSING SLOPE STABILITY

▪ Analytical ▪ Numerical

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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)

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 insidethe 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

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

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 25/08/2018 Slope Stability & weathering by Classification - Hack - Bad Bentheim 25

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.

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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) ) (s factor reduction r) groundwate condition spacing RQD RMR =(IRS+ + + + +

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RMR - SLOPE MASS RATING (SMR) (Romana)

(modified Bieniawski)

▪ RMR rating multiplied with series of compensation factors

excavation of method for factor = dip ity discontinu and face slope between relation for factor = angle dip ity discontinu for factor = face slope and ities discontinu of strikes the of m parallelis for factor = ) s i' Bieniawsk as (same Rating Rock Mass = Rating Mass Slope = 4 3 2 1 F F F F RMR RMR 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)

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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

GSI(2)

25/08/2018 Slope Stability & weathering by Classification - Hack - Bad Bentheim 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.

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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).

SLOPE STABILITY PROBABILITY CLASSIFICATION (SSPC)

▪ three step classification system ▪ based on probabilities

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SSPC - THREE STEP CLASSIFICATION SYSTEM (1)

25/08/2018 Slope Stability & weathering by Classification - Hack - Bad Bentheim 35 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: • 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

SSPC

Rock mass Parameters:

▪ Intact rock strength

▪ Spacing and persistence discontinuities

▪ Shear strength along discontinuity:

- Roughness - large scale

- small scale

- tactile roughness

- Infill

- Karst

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SSPC

Slope specific parameters for the new slope to be made: ▪ Expected degree of weathering at endof 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

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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

SSPC

Shear strength based on a combination of:

▪ roughness (persistence) ▪ infill

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SSPC

Roughness is a combination of: ▪ largescale roughness (Rl),

▪ smallscale roughness & tactileroughness (Rs)

SSPC

Shear strength – roughness

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SSPC

Shear strength roughness tactile Three classes: ⚫ rough ⚫ smooth ⚫ polished 25/08/2018 Slope Stability & weathering by Classification - Hack - Bad Bentheim 45

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

SSPC

Shear strength - condition factor

Discontinuity condition factor (TC) is a multiplication of the ratings for: ▪ small-scale roughness ▪ large-scale roughness ▪ infill ▪ karst 25/08/2018 Slope Stability & weathering by Classification - Hack - Bad Bentheim 48

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SSPC

Roughness large scale (Rl) (visual area > 0.2 x 0.2 and <

1 x 1 m2) slightly wavy curved slightly curved straight 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 *Ka

Im

RlRs

angle

sliding

=



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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

degrees 35 0113 0 00 1 55 0 95 0 75 0 0113 0 Im ₌ ₌ = . . * . * . * . . *Ka Rl*Rs* angle sliding



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

stability

Discontinuity related shear strength failure Plane sliding

Conditions:

- discontinuity must daylight

- downward stress > shear strength along discontinuity plane

SSPC

Orientation

dependent

stability

Discontinuity related shear strength failure Wedge sliding

Conditions:

- intersection line must daylight

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Orientation

dependent

stability

Sliding if:

( )

AP

TC



0 .

0113

*

TC = discontinuity condition factor

AP = apparent discontinuity dip in direction of slope dip

SSPC

Orientation dependentstability

Sliding probability

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SSPC

Orientation

dependent

stability

Discontinuity related shear strength failure Toppling

SSPC

Orientation dependentstability

Toppling criterion

(

AP

dip

discontinu

ity

)

TC



0 .

0087

*

−

90 

−

+

TC = discontinuity condition factor

AP = apparent discontinuity dip in

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SSPC

Toppling probability

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:

•

Nodiscontinuities

•

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

Orientation

independent

stability

In SSPC based on:

• Intact rock strength

• Block size and form

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SSPC

Probability orientation

independent

failure

SSPCCOMPARISON BETWEEN SSPC AND OTHER CLASSIFICATION SYSTEMS

SSPC stability probability (%) n u m b er o f sl o p 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) n u m b er o f sl o p 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)

n u m b er o f sl o p 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 unstable stable a: SSPC b: Haines

c: SMR

Haines safety factor: 1.2

completely unstable completely stable partially stable unstable stable

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EXAMPLES

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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

SABA - DUTCH ANTILLES - LANDSLIDE IN HARBOUR

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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

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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

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BHUTAN

Widening existing road in Bhutan (Himalayas)

BHUTAN

Method of excavation

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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

FUTURE DEGRADATION OF SOIL OR ROCK DUE TO

WEATHERING, RAVELLING, ETC.

Forecasting

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FUTURE DEGRADATION

Reduction in slope angle due to weathering, erosion and ravelling (after Huisman) 25/08/2018 Slope Stability & weathering by Classification - Hack - Bad Bentheim 80

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 ]

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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)

CINDARTO SLOPE:

VARIATION IN CLAY CONTENT

IN INTACT ROCK CAUSES

DIFFERENTIAL WEATHERING

bedding planes

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25/08/2018 Slope Stability & weathering by Classification - Hack - Bad BentheimSlope Stability by Classification - Hack 8383

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

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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.

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( )

_init

_WE

app

log 1

( )

WE 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

•Material: Gypsum layers

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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

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KOTA KINABALU

Side road (dip 45°, 5 years old)

sandstone: slightly weathered

SSPC stability: Sandstone: stable (92%) Shale: unstable (< 5%) 25/08/2018 Slope Stability & weathering by Classification - Hack - Bad Bentheim 93

KOTA KINABALU

Main road (dip 30°, 10 years old):

sandstone: moderately weathered

SSPC stability: Sandstone: stable (95%) Shale: ravelling (<5%) 10 years

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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 25/08/2018 Slope Stability & weathering by Classification - Hack - Bad Bentheim 95

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.