ASCEsem1txt (1)

8/8/2019 ASCEsem1txt (1)

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Rock Engineering Basics

Rock: compact, indurated natural material (composed of oneor more minerals) that requires drilling, blasting, wedging, orother brute force to excavate.

Rock Substance: solid rock material which does not containobvious structural features (discontinuities) and which usuallycan be sampled and tested in the lab; known as intact rock.

Rock Mass: a complex system of natural rock materialcomprised of blocks of intact rock and structural features(discontinuities) that allow for interactions among the blocks;too large and complex to sample and test in the lab


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Geologic Info for Rock Slope Engineering

1. Geologic mapping of formations and units needed togenerate surface-geology maps and cross-sections

2. Site topography and proposed cut-slope geometries (best to

display cross-sections 1:1 with no vertical exaggeration)


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Geologic Info for Rock Slope Engineering

1. Geologic mapping of formations and units needed togenerate surface-geology maps and cross-sections

2. Site topography and proposed cut-slope geometries (best to

display cross-sections 1:1 with no vertical exaggeration)

3. Relevant rock-strength data for the rock substance

4. Engineering properties of rock discontinuities, includingorientation, geometry, shear strength

5. Groundwater regime (water table, piez. head distributions)


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Uniaxial Compressive Strength

A cylinder of rock taken from drill-core is cut square on theends, then the ends are ground smooth, and the specimenloaded to failure in a testing machine. The length-to-diameterratio (L/d) typically ranges between 2 and 3.

UCS = Pf / A (stress units of psi, psf, MPa, tsm)

where: Pf = ultimate failure load (at rupture);

A= cross-sectional area of the cylindrical specimen

= Td2/4


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Reporting of UCS Standardized Results

Empirical corrections of the tested value of UCS tostandardized L:d values are given below:

For L:d of 2:1

UCS2:1 = UCS / [0.88 + 0.24(d/L)]

For L:d of 1:1

UCS1:1 = UCS / [0.778 + 0.222(d/L)]


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Point Load Index

The point load test is conducted on a piece of drill core (withragged ends) with L/d > 1.5 whereby the core piece is loadedperpendicular to the core axis between cone-shaped platensuntil failure occurs and the core is split. The core diameterand instrument gage pressure at failure are recorded. The

Point Load Index then is given by:

PtL = Pg(Ar) / d2

where: d= core diameter, Pg = instrument gage pressure atspecimen failure, and Ar = cross-sectional area of instrument

loading ram.


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Using PtLto Estimate UCS

UCS $ PtL(14 + 0.175d)

for d measured in units of mm

For typical core diameters (47 61 mm), use the

approximation:

UCS $ 23(PtL)


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Estimating UCS Using a SchmidtHammer

A Schmidt Type-L rebound hammer can be used toapproximate the UCS. A reas onable estimate of therock unitweightalso is needed.

Rebound measurements often are qui te variable, s othe field investigation should include at least 10measurements at a given sampling site (for averagingpurposes).


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Brazilian Disk Tension Testing

A small disk of rock core with known diameter (d) andthickness (h) is loaded along its diameterto induce anapparent tensile stress field and cause the disk torupture. T he tensile strength then is given by:

T = 2(Pf) / (Tdh)

where Pf = failure load atwhich the disk ruptured

A general rule-of-thumb: (10 x T) $ UCS


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Mapping & Display of Discontinuity Data

Field mapping methods to obtain information on discontinuityorientations, spacing, length, roughness, etc.:

Scanline mapping detailed mapping of individual discontin-

uities that intersect a designated mapping line or linearwindow


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Fracture-Set mapping (Cell mapping) mapping of fracture-set properties observed within user-defined cells on the rockexposure


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Fracture-Set mapping (Cell mapping) mapping of fracture-set properties observed within user-defined cells on the rockexposure

Oriented core logging mapping of oriented drill core toobtain orientations, fracture spacings, roughness


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Display of Discontinuity Orientations

The orientations of planar discontinuities are best displayedand evaluated by plotting their poles (normals) on lower-hemisphere stereographic projections (known as stereonetplots). A cluster of such poles then represents a fracture sethaving planes in similar orientations.

W

N

E

S

+ + + +

+ + + + + ++ + +

+ +

+

+ + +

+ +++ +

+ +

+ + +

++

Example of lower-hemis. stereonet plot of fracture poles.


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Display of Discontinuity Orientations

Poles near the center of the stereonet are for shallow-dipping(fairly flat) fractures, and poles near the outer edge of thestereonet are for steeply dipping fractures.

Thus, a cluster of fracture poles in the upper-right portion ofthe lower-hemisphere stereonet plot indicates a fracture setwith planes dipping toward the southwest.


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ShearStrength Modeling for Discontinuities

1. Linear Mohr-Coulomb failure envelope with y-intercept(known as cohesion) and slope (known as the coefficientof friction, tanJ):

X = c + Wn tanJ

where: X = shear strength along the discontinuity;

Wn = effective normal stress acting on the discontinuity;c = cohesion (generally equal to zero or a very small

value for clean rock fractures);

J = friction angle.


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2. General nonlinear, power-curve model:

X = c + a(Wn )b


Wn = effective normal stress acting on the discontinuity;

a, b, c = power-curve parameters.

Note that when b = 1.0, this model reduces to a linear modelwith the parameter a = tanJ. Therefore, this general

model also covers the special case of the linear model.


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3. JRC model of shear strength (nonlinear model):

X = Wn tan[(JRC)log10(JCS/Wn) + Jb]


Wn = effective normal stress acting on the discontinuity;

JRC = joint roughness coefficient (typ. values: 2 to 6);

JCS = joint-wall compressive strength (UCS of intact rock);

Jb = base friction angle (i.e., for saw-cut, smooth surfaces).


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4. Back-analysis of a rock-slope failure with well-defined geometry and groundwater conditions:

We set the FOS equal to 1.0, and back-calculate thecorresponding combinations of J and waviness that seemappropriate (linear shear-strength model with zerocohesion). We can follow the same approach with the JRC

model of shear strength (select appropriate values of Jb,JCS, and JRC that give FOS = 1.0).


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Shear Strength

Analysis ofLaboratory Direct-Shear Data

During the laboratory direct-shear test of a natural

rock joint, data are collected to record the shear load

as a function of the applied normal load and the shear

displacement. The graph of shear load vs. shear

displacement for each applied normal load provides

the basis for describing the shear strength of the

specimen.


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Shear

L ad

Shear Dis lacement

1

2

3

4

Residuals

rmal

L ads


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Laboratory Direct-Shear Data

The contact area in shear when the specimen attains either the

peak shear load or the residual shear load is needed to

calculate the corresponding normal stress and shear stress

(strength) for any particular graph trace (trial).

For circular or rectangular specimens, this contact area can be

calculated directly, once the pertinent shear displacement is

identified. For irregularly shaped specimens, a reference tablemust be constructed that displays the contact area as a

function of shear displacement.


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Laboratory Direct-Shear Data

A least-squares regression program (such as Taussm

or the Mathcad sheet entitled TauRegr) then

provides the linear and power models for shear

strength, as shown in the typical plots of shear

strength on the overheads


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OverallShearStrength for Highly FracturedRockMasses

1. Exponential RQD Method

Required input:Average RQD (Rock Quality Designation) of the

rock mass (%)Estimated c (psi) and J for intact rockEstimated c (psi) and J for natural fractures

Intermediate factors (weights):A= .475exp(.007 x RQD) B = .188exp(.013 x RQD)

Then:cm = cr (B

2) + cf (1-B2) in psi

Jm = Jr (A2) + Jf(1-A2) in deg.


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2. Hoek-Brown Rock Mass Strength Model

Required input:mi - Hoek-Brown constant (a material constant

ranging from about 4 to 33)GSI - Geological Strength Index (see handout)

Ci - uniaxial compressive strength of intact rockD - estimated rock-mass disturbance factor (0 for

insitu rock or for carefully designed blastingprograms; 1 for poor blasting practices with

considerable overbreak)

See Mathcad calculation sheet for examples.

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