part 05 - geomorphology (structural geology)

7/26/2019 Part 05 - Geomorphology (Structural Geology)

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PART 5: GEOMORPHOLOGY - STRUCTURAL GEOLOGY Earth surface is still undergoing physical changes, in both small & continental scale.

Resulting from eect of weathering & tectonic forces.

Produce various deformations and changes to the earth-crusts and shape of theearth surface e.g. roc mass that constitutes mountain range.

!n responding to the physical changes "e.g. state of stresses# of the surroundings, aroc mass may deform or change its physical state.

$hanges in roc mass include% epands if con'ning stress reduces( deformsplastically "ruptures# if acting stress is larger than its strength( deforms elastically"reshapes# if acting stress is smaller than its strength.

)s result, roc mass displays certain characteristics, indicating changes in thesurrounding.

EXCAVATION OF SLOPE IN ROCK MASS

*he phrase +solid as a roc is not a correct statement on the properties of roc

mass.

Roc bodies, whether eposed on the surface or buried in the earths crust, are not

free from fracture and weaness planes. !n addition, fresh and strong roc may be degraded and weaened by weathering

processes.

*ae a strong, solid"without discontinuityfeatures#, and fresh "un-weathered# roc mass

) slope cut in a fresh,strong and solid roc massbut, with horiontalbedding planes "a type ofdiscontinuity or weanessplane#.....

or the cut slope ")# in thisroc mass, is the stabilitybeing aected by thehoriontal bedding planesand inclined /oints0 1etpropose another slope "2#

on the other side of thehill...

) near vertical slope isecavated in the rocmass...

)gain, one will 3uestion thestability of the cutslope . . . is it being

aected by the horiontalbedding planes0 !s thereany immediate eect dueto roc mass properties &bedding planes0

*he roc properties and thehoriontal bedding planeshave no immediate eect

on the stability of both415PE ) and 2. 6owever,with respect to theorientation the /oints,415PE 2 is critical.

.....in terms of the roc

mass properties wouldthere be any problem onthe stability of the cutslope0

) cut slope in a fresh,

strong roc mass withhoriontal bedding planesand inclined fractureplanes "e.g. /oints#.


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GEOMORPHOLOGY

*he characteristics include presence of structural discontinuities in roc mass, andthe most important in construction are%a# bedding planesb# folding "lipatan#c# fault "sesar#d# /oint "ear#

*hese discontinuity features induce weaness and defect into roc bodies.

*hey aect roc behaviour when sub/ected to loading "e.g. stability of tunnel

ecavated in highly /ointed roc mass, compared to massive roc#.

FOLD (LIPATAN) olds only occur in clastic sedimentry rocs which ehibit bedding7layers7strata,

metamorphic rocs originating from clastic sedimentary rocs & volcanic rocs.

2eddings7strata are weaness planes in roc relative displacement can occurbetween bedding planes.

olds is formed when bedded7strati'ed roc is sub/ected to lateral compressivestress in the earth crust.

THINGS TO PONDER

Why there is no folding in granite?

For horizontal rock strata, why folding is associated with compressive lateralstresses?

FOLDINGS

*ype of folding% symmetrical, non-symmetrical, over-turned & recumbent fold

"lipatan rebah#.

Recumbent fold can aect se3uence of bedding arrangement "di8culties inverifying the relative age of beddings#.

!mportant geometrical parameters of fold%a#1imbs, syncline & anticlineb#9ip direction7strie of inclined bedding "limbs#.

FOLD GEOMETRY


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:hen roc strata are folded, they are under a state of strain "eterian#, this willinduce remnant or residual stress in the folded roc mass.

or symmetrical fold residual tensile stress occurs in the upper portion of the fold,

& residual compressive stress in the lower portion. ;sually tension cracs can befound at the crest.

or a deformed roc mass "e.g. folded#, its in situstress distribution is no longer ine3uilibrium presence of remnant stress due to deformation.

THINGS TO PONDER

What type of stress usually associated with tension cracks in the crest of a fold?

For an excavation work in a folded rock mass, why it is critical to excavate at theshoulder of the fold?

TENSION CRACKS AT CREST OF A FOLD

olded roc mass creates problems to construction - higher in situstress%

o


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4lope ecavated in folded roc mass may ehibit fre3uent failures inclinedbedding plane.

or petroleum industry, folded strata is the main target area in eploring for

petroleum reservoirs.

Inclin! "!!in# $l%n& in 'l!! &*%% Inclin!"!!in# $l%n&


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Inclin! "!!in# $l%n& in 'l!! &*%%FAULT (SESAR)

ault a fracture between two roc blocs where there has been observable relativedisplacement.

5ne of the bloc can be displaced upwards or downwards or even displaced

laterally.

ault can occur in all ind of rocs "whenever cracs are present#.

!mportant geometry of fault% dip direction of fault, dip angle, strie & amount ofdisplacement.

ASSOCIATED DISPLACEMENTS IN FAULT FORMATIONFAULT STRUCTURE + GEOMETRY

aults are classi'ed based on

their formation% normal fault,reverse fault and strie-slip fault"sesar mendatar#.

) large scale displacement "fewms# may inducemetamorphism in thesurrounding roc, formingmetamorphic rocs e.g. faultbreccia & clay gouge.


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ault traversing a river gives rise to waterfall. 4pringline is a result of groundwater>owing a long a fault line.

1arge faults "?@ A@ m deep# can act as passage for magma to >ow upwards from

inside the mantle.

NORMAL + REVERSE FAULT

STRIKE - SLIP FAULT (,*in%l !i&$l%c.n)

M/.n& ' '%0l


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S%n An!*%& F%0l 1 2 cninn%l $l%& &li! $%& n %n,*

E%*,30%4 i& $*!0c! " % &0!!n &li$ %ln# in*lc4! '%0l lin

ormation of multiple large sie faults "fault one# may induce remnant or residualstress in host roc.

Residual shear stress may occurs in roc bodies at near a fault one.

Ecavation wor in roc displaying fault structure, instability of ecavated surfacesis a common problem, particularly if faults are 'lled with clay gouge.

aults induce discontinuity "etaselarasan# and weaness into roc unevenstresses distribution.

!deally, roc mass without discontinuities "faults & fold#, stress distribution can be

assumed in a state of hydrostatic stress "P = gh#( stresses are e3ual in all

directions.

!f faults are present in roc mass, stress in one direction may not be e3ual to stress

in the opposite direction "e.g. horiontal stresses, h#.6OINT (KEKAR)

Boint is a fracture, separating two roc blocs, where there is no observable "verysmall# displacement.

ormation of /oints is associated with tensile stress acting within a roc body. *he

source of tensile stresses can be from any7all of the following processes%o Epansion & shrining of a massive magma body during cooling process /oint

sets that are perpendicular to each other.


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In*&c ' 7 8 9in && E$&! 9in &0*'%c&6in & (/*ic%l *in%in)

Epansion of roc bodies due to reduction in overburden stress. or instance,decreasing thicness of materials "soils# covering a deep seated bed roc aftercontinuous & intensive weathering at the surface sheet /oints.

*ensile stress acting at crest of a fold tension cracs.

or impermeable roc "e.g. granite# /oint acts as a secondary permeability.

)s other discontinuities in roc, /oints are weaness planes in roc.

SHEET 6OINT (KEKAR ;ERLAPIS)

TENSION CRACKS AT CREST OF A FOLD


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6in !i&$l%in# /* &.%ll !i&$l%c.n Un-!i&$l%c!6in

V%*i0& $& ' !i&cnin0i


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6in S& 1 > & %n! ? && 6in S(&,%l)

6in S (G*%ni)

E=c ' > 6in n UCS ' Rc4 E=c ' 8 6in n UCS ' S*n#,


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E=c ' M%n 6in n UCS ' Rc4: In ,i#,l '*%c0*! *c4 , &*n#,.% %$$*%c, ,% ' &il

*he more is the number of /oint sets, the weaer and more unstable is the roc.

Roc with more /oint sets is easier "cheaper# to ecavate. 2ed roc displaying

multiple /oint sets may not be suitable for foundation of large structures.

Boints in hard roc are passage for weathering agents "water, 5A# to penetrate intoroc thus rocs with more /oint sets are more easily weathered compared to roc

with less /oint sets. Eposed rounded roc boulders are the result of weathering of roc that ehibits G

/oint sets that are perpendicular to each other.

L%*# *0n!! &0"-*0n!! "0l!* (#*%ni)


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;0l!*& %* $*!0c! " 2%,*in# ' 9in! #*%ni 1 &$,*i!%l2%,*in#

Physical weathering "disintegration due to /oints# creates new surfaces for

chemical weathering to tae place on the eposed surfaces, especially at the Hcorners.

THINGS TO PONDER

What are the main dierences between oint and fault?

Why a slope face cut in rock displaying ! oint sets is less stable than in rockdisplaying " oint set ?

#o $ have any other options besides this non-entertaining %ock &echanics?'(es)))))))try %ock * %oll+)

!ntrusive igneous roc "e.g. granite# almost always ehibiting G sets of /oint, typical/oint characteristic for massive and strong roc, with random "rawa# mineralsarrangement.

or rocs displaying structured minerals arrangement in certain orientation

"lamination, slaty & schistosity#, e.g sedimentary & metamorphic roc, the ma/or/oint sets are usually parallel to the mineral arrangement.


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F*%c0* $l%n $%*%lll l%.in%in (&%n!&n)

MEASUREMENT OF 6OINT (+ O,* '*%c0* $l%n&)

Fa/or roc ecavation wor "e.g. slope & tunnel# re3uires a preliminary "initial#investigation before design & construction +Boint survey% 'eld wor to identifytypes of weaness planes "/oint, bedding planes, faults# measurement ongeometrical orientation "dip & dip direction#.

Boint orientation & /oint sets for a roc type are usually consistent thusmeasurement can be undertaen on the eposed portion of the roc.

9ata & info obtained "e.g. geological mapping# are used for preliminary design ofthe structure.

igure ?C shows the important geometrical characteristics of /oint "or otherweaness plane# that need to be measured in the 'eld%o 9!P angle "sudut econdongan#.o 9ip direction "arah econdongan#74trie.

Boint survey is undertaen on eposed roc body "accessibility & cost#.

5ther relevant data & info on /oint% open or close, conditions of /oint surface "rough

or smooth#, in'll "berinti# & sie7length, continuity "eselan/aran#.

G.*ic%l $%*%.*& : %n inclin! 2%4n&& $l%n Fill! 9in(K4%* "*ini)


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MEASUREMENT OF 6OINT (+ ,*'*%c0* $l%n&)

!nstrument used are 2runton compass &tapes.

$ollected data few hundreds to

thousands.

9ata is analysed using computer software

"e.g. Ieorient#% lower hemispherestrereonet pro/ection.

5utput data% dip & dip direction of ma/or& minor /oint sets, probablity of failuredirection and mode of failure.

Fean dip & dip direction of all the measured /oint is compared with the proposedcut slopes.

6in .%&0*.n 0&in#;*0nn C.$%&&

T$ic%l *0#,n&& $*@l + in! (6RC) ' 9in &0*'%c

DATA FROM 6OINT MEASUREMENT


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S*n $*9cin $l :G*% ci*cl&

S*n $*9cin $l : S*n$*9cin 0$0 :

Pl& !n&i G*% ci*cl& ' 9in&& + c0 &l$


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M!& ' Sl$ F%il0* %n! Pl! S*n

M!& ' Sl$ F%il0* %n! Pl! S*n


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M!& ' Sl$ F%il0*

E=c ' 2%4n&& $l%n& (!i$ + !i$ !i*cin):On &%"ili ' c0 &l$ '%c in % !i&cnin00& *c4 .%&&

Di*cin ' 0nnlin# 2i, *&$c 9in *in%in

THE GRAND FINALE*heoretically, a vertical slope face of several thousand meters height "6# can be

ecavated in a massive, continuous & strong roc "based on formula 6 J ;$47#

6owever, in actual condition in the 'eld, this is impossible "the allowable maimumvertical height, 6, is less than ?@ m#0

part 05 - geomorphology (structural geology)

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