Rock – Rock Mass · definition of rock types, structural discontinuities and material properties...

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INSTITUTE FOR ROCK MECHANICS AND TUNNELING

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Short Course Singapore 2011

Rock Mass

Klima, Schubert

Rock – Rock Mass


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Rock Mass

Klima, Schubert

Properties appointed by:

• mineral composition

• texture / structure / microtexture

• history of origin

• condition / weathering / alteration

ROCK (Intact rock)

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Properties appointed by:

• Properties of intact rock

• Properties of discontinuities

• History of origin

• Stress condition

Rock Mass


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Fundamental dualistic pairs of material properties

• HOMOGENEITY – HETEROGENEITY

• ISOTROPY – ANISOTROPY

• CONTINUUM - DISCONTINUUM

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Geneity

• Homogeneity means being the same throughout (one piece of rock can be exchanged by an other one without changing the over all properties)

• A heterogeneous rock or rock mass is one that consists of many different items (minerals, rock types), which are often not easily sorted or separated, though they are clearly distinct.

• Geneity is a highly scale dependent property of rock or rock mass!


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Tropy

• Isotropy is the property of being independent of direction; that means having the same physical properties in any direction (strength, electrical conductivity,……)

• Anisotropy is directional dependence of physical properties

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Scale-Dependency of Basic Material properties


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• Continuum means material that is continuous, e.g. without cracks

• Discontinuum means material that is interrupted by discontinuities

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CHILE material, most commonly assumed forthe purposes of modelling of rock material, is:

Continuous

Homogeneous,

Isotropic

Linear-Elastic

DIANE material is the rock mass with whichthe engineer has to deal:

Discontinuous

Inhomogeneous

Anisotropic

Non-Elastic

„ROCK“

„ROCK MASS“

CHILE vs. DIANE Conceptual Models for Geological Material


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Rock Mass

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SOIL:isotropic (seldom anisotropic) homogeneous continuum

INTACT ROCK:anisotropic (seldom isotropic) homogeneous continuum

ROCK MASS:anisotropic (seldom isotropic) heterogeneous discontinuum

GEOMECHANICAL DEFINITIONS

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Sedimentary bedding

Origin of Discontinuities


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Cooling of vulcanicrocks


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Overstressing



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Sheardisplacment: faulting


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• Determine shape and orientation of rock blocks

• Determine for a large part rock mass response to loading

• Determine stability of cut slopes, excavations

• Are crucial for the directions of anchoring, drainage or grouting drillings

Discontinuity Attitudes:


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• “The corner-stone of any practical rock mechanics analysis is the geological data base upon which the definition of rock types, structural discontinuities and material properties is based.

• Even the most sophisticated analysis can become a meaningless exercise if the geological information upon which it is based is inadequate or inaccurate.“

ROCK ENGINEERING Course notes by Evert Hoek, Vancouver 2000

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Strength of rock mass is generally significantly lower than

strength of intact rock

ROCK MASS


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Controlling factors and processes:

•Relief of stress

•Mechanical wear (abrasion – attrition)

•Crystal growth (ice, salt)

•Change in temperature (insolation, frost)

•Change in water saturation (evaporation dewatering - wetting)

•Chemical solution

• Hydrolyses

•Oxidation – reduction

•Hydration

•Chelation

Weathering of rock and rock mass:interaction of physical, chemical and biological processes

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Surface Area and Weathering


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Development of core stones by increased material loss at edges and corners (above)

Exponential increase of surface area and hence sites of weathering activity due to linear decrease in spacing of permeable divisional planes

Total surface (m2)

num

ber

of pie

ces

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Geologcal factors of weathering:Time and type of rock

Marble vs. Time

Granite vs. Time


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1948 1958

1973

Cut slope in sandstone, California:

Less and more resistant sandstone beds influencing morphology of slope

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Weathering and degradation during „engineering“ time


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Geological factors of weathering: Climate

Same rock type; same age, different weathering conditions:

different weathering results!


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Scheme of Weathering, Vegetation and Regolith Development from the Poles to the Equatorial Region



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Typical Weathering Residuals of Massive Igneous (Plutonic) Rocks: Rounded Corestone Boulders. Original Jointing Pattern in Parts Preserved.

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Deep Rock Mass Weathering:

Progress along of Master Joints (Subtropical Taiwan ROC and Domin.Rep.)


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Mass weathering profiles and zonal weathering classification of an exposure, Hong-Kong Granite

example

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Primary geometrical properties of discontinuities

HUDSON 1989


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Rockmass parameters of interest for engineering structures in and on rock

deformation

deformation parameters of intact rock / rock mass

susceptibility to weathering

infill material

strength

roughness (dilatancy)

material friction

surface characteristics of discontinuity wall

condition of discontinuity (shear strength along discontinuity)

persistence per set

spacing per set

number of setsrock block size and form

orientation (with respect to engineering structure)

discontinuities

intact rock strength

rock mass


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• Band, Layer, Bed (sedimentary): one of a sequence of near-parallel, tabular shaped rock bodies

• Foliation: Any repetitively occurring or penetrative planar feature in a rock body

• Cleavage: the property of a rock to split along a regular set of approx. parallel closely spaced surfaces

• Schistosity, Schistose structure: A preferred orientation of minerals or aggregates produced by metamorphic processes. As structure uniformly throughout the rock or in narrowly spaced zones such that the rock will split on a scale of 1 cm or less.

• Gneissossity, Gneissose Structure: Characterized by a schistosity which is either poorly developed, or occurs in broadly spaced zones such that the rock will split on a scale of more than 1 cm

• Kink-zone: Array of tight kink (chevron) folds with short rotated limbs, very narrow hinge zones and with high extend along the axial plane

A few terms concerning (potential) discontinuities not generated by brittle fracturing:

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Orientation of discontinuities


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But before you go, always consider what your task is and how you will be able to achieve it!

Let’s search the outcrop.....

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• Description of the site (coordinates, accessibility)

• Outcrop sketch (size of outcrop, orientation of outcrop)

• Lithological description

• Structural description

• Orientation measurements and description of discontinuities.

• Photographs

• Samples

What outcrop work should include:


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Consider to prepare an outcrop resp. data sheet and a legend before going to the field:

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Compass after Prof. Clar, (two circle compass) mainly used in Europe, measures dip and dip direction in one step.

Brunton and Bergmann compass, mainly used in the anglo-americanregion, measures dip and strike direction in two separate steps.

The different types of geological compasses:

Android App


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45°

strikeline dipline

Brunton compass Bergmann compass

N35E/45 SEor

S35W/45 SEor

035/45 SE

N

W E

S

35°

N

W E

S

35°

(after Meschede, 1994)

Clar compass 125/45

N

W E

S

125°

The orientation readings for the different compasses:

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Dip direction α(azimuth)

Dip angle β

Strikeline: the intersection of a horizontal plane with the fracture surface

NDip angle βs of thestriation

Dip direction αs of the striation

β

αs

βs

α

What can we measure on a fracture surface with a geological compass (“Clar” type)?


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Lower hemisphere

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Plotting data of joint110/60:

Great circle

Dip direction

Pole


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Statistical evaluation of orientation data

bedding 188/32

joint 082/74

joint 001/64

shear 308/81

shear 020/85 shear 346/73

Center of gravity („mean“)

Cone of confidenceSpherical aperture („Standard-deviation“

Assumption:

3-D Normal Distribution of Data in Set

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• Center of Gravity: The plane having the gravity vector S as pole (“mean plane” orientation).

• Cone of Confidence: Small circle, within which the estimate of the mean orientation is situated with a certain probability. Higher levels of confidence accordingly render larger cones of confidence! In cases that the cone of confidence reaches or exceeds the spherical aperture further statistical considerations are meaningless.

• Spherical (circular) Aperture: gives the angular range of a small circle (or great circle section) which would have the same concentration parameter (k) and the same eigenvalues as the actual sample, however with uniformly distributed data points. Depending on the value of k the area presents between 64% and 69% of the dataand hence can be understood as equivalent to the standard deviation (representing 68.27% of data within 2σ)

Principle output parameters of orientation statistics after WALLBRECHER (1986)


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• Confidence: Chosen level of confidence for the probability that the statistical estimate (e.g. the calculated mean orientation, center of gravity) lies within certain angular range. Makes sense only when > 90%.

• Percent Degree of Orientation (0 ≤ R% ≤ 100): Data concentration parameter derived from the sum vector divided by the number of data: R% = Rsn * 100

• Parameter of Concentration (0 ≤ k ≤ ∞): The relation of the data concentration density of the mode and the least concentration density (“anti-mode”) on a sphere is an exponential function with the term “ek”. Orientation statistics makes sense only with k>4 (better k>6) and with symmetrical normal distributions (circular or spherical).

Explanation on the output parameters of orientation statistics after WALLBRECHER (1986)

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• Eigenvectors: An orthogonal axes system of an ellipsoid, for which shape and orientation depends on the pattern of distribution. The length of the axes represent the eigenvalues of the orientation tensor, the longes axis (max eigenvalue λ3) denotes the max. concentration of data. Eigenvalues – eigenvectors make possible the quantitative comparison of fabric orientation diagrams.

• Cylindricity, Small Circle Distribution: Since R% does not characterize the shape of the data point distribution, by means of the eigenvalues it is evaluated wether the distribution resembles a great circle section (partial girdle) or a small circle (cluster) distribution.

Explanation on the output parameters of orientation statistics after WALLBRECHER (1986)

cont.


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Presentation as great circles or as poles?

2450 discontinuity data, Tunnel Semmering, exploratory tunnel

SlickensideJointFoliationfault

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Hierarchical Clustering

agglomerativ

divisiv

x1 x2 x3 x4 x5 x6 x7

d1d2d3

d4

d5

d6

d7

Result:

Distinct attribution of elements to clusters depending on preset discriminatory values (angular distances)

Cluster 1 Cluster 2D

ista

nce

betw

een

clus

ters

G. Winkler 2003


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Views of Discontinuity Systems with differing Set Angles and Spacing

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Consisting of Three Joint Sets of Different Properties and a Set Formed by Bedding Planes

A.Steidl (1996)

Well-Defined Discontinuity System


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z

y

x

Typical joint-bound blockco-ordinates refer to fabric axes of the rock. Block shape/size and hence the rock mass

structure depends on relevant orientation and spacing of joint sets

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Ancient practical use of joint-bound blocks as building and dimension stone


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V. Mencl, 1968

considering block shapes and type of compound interlocking (geometrical relationships, mutual intersection and termination of joints)

Bulk Rock Mass Characterization

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Typical joint-bound blocks


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Overbreakcontrolled byorientation of discontinuities

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Overbreak controlled by orientation of discontinuities


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Estimate of Overbreak controlled by orientation of discontinuities by using Goodman´s Block Theory

(Goodman 1985)

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Estimate of Overbreak controlled by orientation of discontinuities by using Goodman´s Block Theory (Goodman

1985)


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Shape controlled by excavation process

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Influence of discontinuity orientation on failure modes and affected geometry in a rock mass, schematically


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Persistence and spacing

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RQD (%) Rock Quality<25 very poor25-50 poor50-75 fair75-90 good90-100 excellent

RQD: Rock Quality Designation(D. Deere, 1968)

Evaluation principle and according rock mass qualification

RQDLengthofCorePieces cm

TotalCoreRunLength=

>×

∑ 10100%

RQD =+ + +

×38 17 20 43

200100%

RQD FAIR= 59%( )


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Persistence

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Persistence (Continuity) K: A Relative Measure of Discontinuity Size


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Orthogonal joint system withdiffering set persistences

system with shear joint sets(acute angles)

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Termination Indexmeasure for the blockiness, discontinuity-bound strength and

permeability of the rock mass

T NN N N1

1

1 2 3

100[%] =+ +

N1,2,3: Number of respective terminations

Suffix 1: Discontinuity ends in host rock „R“(„blind end“)

2: Ending at another discontinuity „D“

3: End conceiled (air, unknown, ground, vegetation: „A“, „X“, „0“)

High percentage means:

- low intersection density of joints

- low blockiness (few isolated joint blocks, high strength)

- low discontinuity-bound permeability


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R- and D- ends of joints in the

outcrop

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2 cm weatheringmud

100% Smectite(mm)

Tectonic (primary) partial opening, aperture <cm

Discontinuity Aperture -Filling - Permeability


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Distribution patterns of discontinuity data

Skewed distribution for trace length and aperture of discontinuities

Winkler 2003

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Roughness and waviness


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according to ISRM Recommended Methods (1981) as e. g.: „stepped-rough“, „planar-smooth“ etc.

Scale : 1 – 10 m

Template for the standard field

identification and description of rock

discontinuity surfaces

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Joint Roughness CoefficentRoughness profiles and corresponding JRC values

(After Barton and Choubey 1977).

Field estimate of roughness: JRC


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Surface irregularities affect the frictional strength in particular in the initial states of a shear displacement and at comparatively low normal stresses, as it is the case in the crustal regions of geotechnical interest

i = dilation angle

Why do we care about discontinuity roughness?

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Scan-line versus Window-mapping

Scan-line (straight or circular):

• record and characterize each and any discontinuity (trace) that intersects (actually or as projected) the scanning line

• avoid acute angles between SL orientation and important joint set traces – vary orientations

Window-mapping:

• Choose appropriate form (rectangle, circle) as possible

• record and characterize each and any discontinuity (trace) which is contained within the window or intersects/transects it


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a) Scanline 1: normal, scanline 2: at an arbitrary angle

b) Subparallel impersistant traces, spacing (s) and orientation poorly defined

c) Circular scanline with n trace intersections (white dots); circular window with m trace endpoints (black dots)

Mauldon et al. 2001

Scanlines: Sampling fracture traces

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Scan-linediscontinuity characteristics recorded

Spacing

Orientation

Persistence

Topology

Geology

Mechanical

- Distance to intersection on scanline- Spacing to neighbor from same set

- Dip and dip direction

- Trace length- Termination code

- Waviness- Roughness- Aperture (distance between discontinuity walls)

- Type of rock- Filling material- Alteration (Weathering)- Water conditions

- Wall strength- (friction angle)


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Scan-lines

Winkler 2003

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Fieldwork with students of the Addis Ababa University, Ethiopia, April 2007

Scan-line-mapping


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Apparent spacing! Bulk or Set Spacings?

Scan-Line Mapping of Spacing

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Frequency along a scan line must be multiplied by a weighting factor (w) to give the „true“ frequency:

w =1

cosδ

True spacing S is calculated from apparent spacing s´ by:

S = s´cos δδ= acute angle between scanline and normal (pole) to discontinuity

Cutoff at about 80° for w= 5

The „TERZAGHI“ correction for apparent spacing /frequency


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wJd vs. Jv

Palmström 1996)

Weighted joint density (wJd) considers the varying frequency of joint intersections with drill-holes / observation planes according to different intersection angles by using fixed correction factors

Simplified Correction for Orientation Bias:

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Windowmapping

Steidl 1999


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Apparent Density: 7 / 320 m2 = 0.02 m-2

True Density: 4 / 320 m2 = 0.01 m-2

White and black dots: true centers for traces that intersect window

Mauldon 2001

Fracture Density = Σ Trace centers/m²

Effects of sampling biasesHigh effect for density!

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Mauldon 2001

Fracture Intensity = Σ Trace lengths/m²

Intensity: 61.5 m / 320 m2 = 0.2 m-1

Intensity: 174 m / 980 m2 = 0.2 m-1

Dashed lines: trace segments notvisible in window

Effects of sampling biases


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Mauldon 2001

Apparent Mean Trace Length: 61.5 m / 7 = 8.8 m

Uncensored Mean Trace Length : 103 m / 7 = 15 m

Solid lines: true length of traces that intersectwindow

Effects of sampling biases

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intersecting NI transsecting NT

contained NC

N total number of tracesNC contained tracesNT transecting tracesNI intersecting traces

Mauldon 1998

Fracture density

Mean trace length

Circular Windows


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• Probability of intersecting long traces >> intersecting short traces

• Extreme short traces are omitted in sampling (trimming, truncating), greatest trace length is controlled by size of outcrop (curtailment, censoring)

• Estimators for mean trace lengths available, however must be used with utmost prudence

• Fracture shape? (appr. rectangular in stratified, bedded rocks, irregular or elliptical in massive rocks)

• Persistence: Relative measure of fracture size

Fracture Size/Shape – Trace LengthJoint Size Distribution = trace length distribution? = negative exponential pattern of distribution?Trace length distribution as sampled = distribution of the population?

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Exponential Negative Patterns of Distribution

0

5

10

15

20

25

30

0.0 -0.05

0.05-0.10

0.10-0.15

0.15-0.20

0.20-0.25

0.25-0.30

0.30-0.35

0.35-0.40

0.40-0.45

0.45-0.50

0.50-0.55

0.55-0.60

0.60-0.65

0.65-0.70

0.70-0.75

0.75-0.80

0.80-0.85

0.85-0.90

0.90-0.95

0.95-1.00

1.00-1.05

1.05-1.10

1.10-1.15

1.15-1.20

1.20-1.25

1.25-1.30

1.30-1.35

1.35-1.40

Gap length (m)

Frequency (%)

Horizontal discontinuity set Vertical discontinuity set

Discontinuity gap length

0

5

10

15

20

25

30

0.0 -0.05

0.05-0.10

0.10-0.15

0.15-0.20

0.20-0.25

0.25-0.30

0.30-0.35

0.35-0.40

0.40-0.45

0.45-0.50

0.50-0.55

0.55-0.60

0.60-0.65

0.65-0.70

0.70-0.75

0.75-0.80

0.80-0.85

0.85-0.90

0.90-0.95

0.95-1.00

1.00-1.05

1.05-1.10

1.10-1.15

1.15-1.20

1.20-1.25

1.25-1.30

1.30-1.35

1.35-1.40

Gap length (m)

Frequency (%)


Discontinuity gap length

0

2

4

6

8

10

12

14

16

18

20

0.0 -0.05

0.10-0.15

0.20-0.25

0.30-0.35

0.40-0.45

0.50-0.55

0.60-0.65

0.70-0.75

0.80-0.85

0.90-0.95

1.00-1.05

1.10-1.15

1.20-1.25

1.30-1.35

1.40-1.45

1.50-1.55

1.60-1.65

1.70-1.75

1.80-1.85

1.90-1.95

Spacing (m)

Frequency (%)


Discontinuity spacing

0

2

4

6

8

10

12

14

16

18

20

0.0 -0.05

0.10-0.15

0.20-0.25

0.30-0.35

0.40-0.45

0.50-0.55

0.60-0.65

0.70-0.75

0.80-0.85

0.90-0.95

1.00-1.05

1.10-1.15

1.20-1.25

1.30-1.35

1.40-1.45

1.50-1.55

1.60-1.65

1.70-1.75

1.80-1.85

1.90-1.95

Spacing (m)

Frequency (%)


Discontinuity spacing

0

5

10

15

20

25

30

0.0 -0.05

0.10-0.15

0.20-0.25

0.30-0.35

0.40-0.45

0.50-0.55

0.60-0.65

0.70-0.75

0.80-0.85

0.90-0.95

1.00-1.05

1.10-1.15

1.20-1.25

1.30-1.35

1.40-1.45

1.50-1.55

Trace length (m)

Frequency (%)


Discontinuity trace length

0

5

10

15

20

25

30

0.0 -0.05

0.10-0.15

0.20-0.25

0.30-0.35

0.40-0.45

0.50-0.55

0.60-0.65

0.70-0.75

0.80-0.85

0.90-0.95

1.00-1.05

1.10-1.15

1.20-1.25

1.30-1.35

1.40-1.45

1.50-1.55

Trace length (m)

Frequency (%)


Discontinuity trace length


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Limitations and uncertainties

• Efficient and technically sound rock engineering is primarily based on rock mass characterization by site investigations

• Due to limited time and financial resources it is difficult to assess a realistic rock mass model.

• This is in particular true in complex geological environments.

• The synthetic rock mass models, as achieved by site investigations and displayed in geological longitudinal sections, are to a certain degree incomplete and not deterministic.

• All predictions contain uncertainties.

Liu, Brosch & Riedmüller 2004

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Estimate of geometrical properties of discontinuities

• Estimate of geometrical properties of discontinuities in principal is possible

• No difference in quality between „professional experienced“ and „green“ geologists

• Quality depends on specific experience

• Mean-value-estimate difficult with skewed distributions

• Tendency to overestimates compared to mean values

• Use of comparison-charts preferable

Raab and Brosch1996


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Principal errors with data recording

• Sampling errors: unintentional subjective preference or discrimination during recording

• Measurement errors: Inaccurateness of measurement process or instrumental error

• Estimation (“statistical”) errors: statistical fluctuations from one sample to another

EINSTEIN & BAECHER 1983

Rock – Rock Mass · definition of rock types, structural discontinuities and material properties...

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Transcript of Rock – Rock Mass · definition of rock types, structural discontinuities and material properties...