5.Underground Excavations in Jointed Hard Rocks Aug 2013

8/14/2019 5.Underground Excavations in Jointed Hard Rocks Aug 2013

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Jointed Hard Rocks

1. Rock joints and stereographicprojection technique


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Rock joints in Singaporesedimentary rocks


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no er examp e o roc o n s nJURONG FORMATION


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azimuth 0 o (north)to 360 o

Dip angle :

0o (horizontal) to90 o (vertical)

Lower

em sp er ca projection (beneath

[Strike line is the horizontal line of a plane: angle from 0 o to 360 o]


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Determination of discontinuity orientation

eo og casurvey

Example 331/82

plane dips towardsapproximatelynorthwest (331 o) at82 o (almostver ca


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Boreholecamera to

jointorientation

erna ve y,boreholes (at least 2with differentinclination) are drilledto intersect the samerock joint. This methodis outdated with the

borehole camera.


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Weakzones orcavitiesvo or n-

filled)

Good rock with some

fractures at the lower part


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Borehole Image Processor

(After Kiso-Jiban)


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(after Kiso-Jiban)


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Camera


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Borehole looking from top


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360 degreeview (round

theborehole)Depth

-view ofrockphoto


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- Note: Borehole water needs to be clear!


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Sample log ofBIP (after Kiso-Jiban)


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o n or en a on p o .

How do geologists identify

(a) Number of joint sets(b) Mean orientation of each joint set


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Projection of weakness plane on a hemisphere


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Stereographical projectiontechnique

-

a stereonet (2-dimensional) A line is re resented b a oint on the stereonet

(trend 0 o to 360 o; plunge 0 o to 90 o)

Two lines can form a lane which is re resentedby a great circle on the stereonet.

The pole is a line perpendicular to a plane. Two planes will intersect at a line.


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Stereonet

Theexam le

shown isan equalangleequatorial

(Largescaledrawing

providedin PDF

file)


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Raw data preferred discontinuity orientation(typically 3 to 5 sets at a given site)Note: Major discontinuity such asSee next 2 slides for exam le!fault line must be clearly identified.


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Plots of concentration of poles ofdiscontinuities/weakness planes

Determination of poleconcentration

The pole is perpendicular to theweakness plane


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The pole of a majorfault line should bemarked distinctl

e.g.

Some poles are more concentrated than others.


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Demo of rock orientationanalysis using DIPS6 Program


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Basic constructions using thestereonet

Stereographical projection


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.Rocks

underground excavations in rock)


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(a) Falling block (b) Sliding block along 1 plane orline of intersection of 2 planes

Tunnel excavation with unfavourable discontinuity orientation

(after Hoek and Brown, 1980)


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Summary

ver ang ng ree aces(rock above void) -Roof, overhanging side

Non-overhanging free

Side wall, floor, slopecut

A falling or sliding blockconsists of 3discontinuities and 1

excavation free face(e.g. roof)

h l l


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Stereographical analysis

(b) Sliding block Note shadedregion does not cover centre of circle

(a) Falling blockHoek & Brown Fig. 88 Fig. 89


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determine rock blocks Page 2 - 3 for detailexplanation of the 2 failure modes (note Figs. 88and 89 referred in the above write-up are given

in the previous slide of this PowerPoint handout) Refer to illustrative examples Example 1 Gravity fall

xamp e ng e p ane s ng Example 3 Sliding along line of intersection of 2


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Required combined

bolts = F x Wwhere F = desired safety

W = weight of fallingblock

Refer to attachmentPage 4 5.


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For sliding on 1 plane or lineof intersection of 2 planes,the shear strength of therock joint(s) (c and ) maybe sufficient to present the

block from sliding. Minimumdesired safety factor = 2.

If safety factor is inadequate,

to enhance stability. Refer toEquations (90) and (91) on

.


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Typical rock bolt details


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Vector methodcan be computerised

Please refer to attached paper for details


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computer program. Refer to

compu er emons ra on

.

rock tunnel design (see next 3 slides).


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Optimaltunnelexcavationaxis design

Refer tocomputerdemonstration


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Optimal tunnel support design


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Analysis involving external force e.g. earthquake force, water pressure


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,

non-overhanging) Refer to Hoek and Brown(1980) Inclined tunnel (i.e. non-horizontal tunnel). Use

Inclined Hemisphere projection method. Refer toPriest SD. Hemispherical Projection Methods inRock Mechanics. (1985). George Allen & Unwin


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.Reliability block stability analysis based on (a) K value of weakness

,Refer to Leung CF and Quek ST, "Probabilistic stability analysis of

excavations in ointed rock". Canadian Geotechnical Journal, 32, no.3 (1995): 397-407.

Quek ST and Leung CF, "Reliability-based stability analysis of rockexcavations". International Journal of Rock Mechanics and Mining

-

Block stability analysis involving multiple weakness planes (>3) usingBlock Theory.

Refer toGoodman RE and Shi GH. Block Theory and its Applications to Rock

Engineering. (1984). Prentice Hall.

5.Underground Excavations in Jointed Hard Rocks Aug 2013

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