Kuliah Fracture 19 09 2007 - · PDF fileFRACTURES • Fracture : a break in rock due to brittle behavior to stress • Joint : A fracture with no displacement –Joint Sets : multiple

FRACTURES• Fracture : a break in rock due to brittle

behavior to stress• Joint : A fracture with no displacement

– Joint Sets : multiple joints oriented parallel to one another, in response to regional stress

– Groundwater flow, hydrothermal ore deposits

• Fault : A fracture with displacement

FRACTURES AND JOINTSFRACTURES AND JOINTS

FRACTURED ROCKSFRACTURED ROCKS

Rekahan dan Sesar• Rekahan (fractures) adalah suatu bidang diskontinuitas dalam

batuan• Jenis-jenis fractures : extension fractures dan shear fractures• Extension fractures (mode )• Shear fractures (mode II)• Shear fractures (mode III)

– Joint atau kekar (sistematik dan non-sistematik)• Kekar kolom• Kekar release dan kekar pembebanan (loading)• Kekar yang terbentuk akibat perlipatan

(cross-joint, longitudinal joint dan stretching joint)– Veins (tension gash)

• Sesar (fault) adalah rekahan yang memperlihatkan adanya buktipergerakan (shear displacement) atau off-set

• Sesar (fault) dapat berbentuk satu bidang diskrit yang planar ataumembentuk suatu zona (fault zone) yang terdiri dari banyak bidang-bidang sesar yang sejajar dan saling berhubungan (net-work)

• Rekahan (fractures) adalah suatu bidangdiskontinuitas dalam batuan

• Jenis-jenis fractures :extension fractures dan shear fractures

• Extension fractures (mode I)• Shear fractures (mode II)• Shear fractures (mode III)

Rekahan (Fractures) FRACTURES FORMATION

B. Shear (Mode II) C. Shear (Mode III)A. Extension (Mode I)

Fracture Types

Rekahan (Fractures)– Joint atau kekar (sistematik dan non-

sistematik)• Kekar kolom• Kekar release dan kekar pembebanan

(loading)• Kekar yang terbentuk akibat

perlipatan(cross-joint, longitudinal joint danstretching joint)

– Veins (tension gash)

SYSTEMATIC JOINT

• Nomenclatures (Joint vs. Fault)

• Shear vs. Extension Fractures

• Hybrid Fractures

FRACTURES & JOINT

SETS

FRACTURES AND JOINTSFRACTURES AND JOINTS

FRACTURED ROCKSFRACTURED ROCKS KEKAR PALING SUKAR DIANALISA KARENA :

PEMBENTUKANNYA TIDAK TERBATAS WAKTUSejak sedimen diendapkanBatuan beku mendinginMengalami diagenesaSampai mengalami gangguan oleh gaya

Kekar KekarNon Tektonik Tektonik

TIDAK MEMPERLIHATKAN ADANYA PERGESERAN

PRINCIPAL STRESS

Stress Tensor Notation

σ11 σ12 σ13

σ = σ21 σ22 σ23σ31 σ32 σ33

STRESS vs. STRAINSTRESS vs. STRAIN

Tensionfracture

D. Physical diagram E.Critical uniaxial tension

Normal tofracture plane

σ = 3 0T

σ = 1 0

σs

σn

α = f 0ºx1

θ = f 90º

2 180ºθ = f

Stable

C. Unstable states of stress

Unstable

B. Critical states of stressA. Stable states of stress

Tension fractureenvelope

σS σS σS

T0σn σn σn

D. Conjugate shear fracture

B. Longitudinal splitting

C. Extension fracture

A. Tension fracture

σ3

σ3

σ3

σ1

σ1

σ1 σ1

Formation of Fractures

Relationship between fracture types and stress from rock experiments

Effect of Anisotropy on Fracture Orientation

σc = σo + tan θ (σn)

The Coulomb Law of Failure

σc = critical shear stressσo = cohesive strengthtan θ = coefficient

of internal frictionσn = normal stress

(Modified from Davis and Reynolds, 1996)

Compressive Fractures

COMPRESSIVE TENSILE

MOHR DIAGRAM FOR BRITTLE –DUCTILE TRANSITION Frictional Sliding Behavior (Byerlee’s law)

(Davis and Reynolds, 1996)

σc = tan θf (σn)

σc = critical shear stress

υf = tan θf

υf = coefficientof sliding friction

σn = normal stress

EFFECT OF PRE-EXISTING FRACTURE

THE CONCEPT OF EFFECTIVE STRESS

σN = normal stress

στ = shear stress (σs)

Pf = pore fluid pressure

σ1 = maximum principal stress

σ3 = minimum principal stress

Coulomb Failure

Envelope

ROLE OF FLUID PRESSURE (Pf)

Effective stress (σ*)

σn* = σn - Pf

σc = σo + tan θ (σn- Pf)

If σn* = 0 σc = σo + tan θ (σn*)

σc = σo

σc = Critical stress

σo = Tensile strength of rock

(Twiss and Moores, 1992)

Relationship between Differential Relationship between Differential Stress and Shear DisplacementStress and Shear Displacement

((TwissTwiss and Moore, 1992)and Moore, 1992)

Mohr diagram showing Mohr diagram showing variation of fracture condition variation of fracture condition

resulting from the effect of resulting from the effect of differential stress and predifferential stress and pre--

existing fracturesexisting fractures

Slip and Fracture ConditionSlip and Fracture Condition

A

T0

σs

σn

Von Mises ductlefailure criterionBrittle-ductile

transition

B

CD

E

Coulumbfracturecriterion

Yield stress

Parabolicfractureenvelope

σ3

n αfθf

σ1

Failure Envelope and Development of Fracture at Different Condition

(Twiss and Moores, 1992)

Griffith Crack

I. Pre-existing crackII. Crack closed III. Crack propagationIV. Crack begin to interactV. Fault forms

Twiss and Moores (1992)

τ2 = 4σt(σt+σ)

σt = tensile strength

The Development of Through Going Fracture (Fault)

TWO TYPES OF FRACTURE MECHANISM

• Fracture strongly dependent on: Confining pressure (σ3) and Fluid Pressure (Pf)

FRACTURES AND FAULTSFRACTURES AND FAULTS

FRACTURES MECHANICS

FRACTURES AND FAULTSFRACTURES AND FAULTSFRACTURES AND FAULTSFRACTURES AND FAULTS

Sesar dan Rekahan Sesar dan Rekahan

• Sesar (fault) adalah rekahan yang memperlihatkan adanya bukti pergerakan(shear displacement) atau off-set

• Sesar dapat berbentuk satu bidangdiskrit yang planar atau membentuksuatu zona (fault zone) yang terdiri daribanyak bidang-bidang sesar yang sejajardan saling berhubungan (net-work)

Extensional Extensional CompressionalCompressional

AndersonAnderson’’s Dynamic Fault Classifications Dynamic Fault Classification

FAULT GEOMETRIES AND CLASSIFICATIONFAULT GEOMETRIES AND CLASSIFICATION

•• AndersonAnderson’’s Dynamic Fault Classifications Dynamic Fault Classification

•• Separation ClassificationSeparation Classification

•• Slip ClassificationSlip Classification

Foot wallblock

Rotationalfaults

Hanging wallblock

F. Sinistral-reverse

Foot wallblock

G.E. Sinistral-normal

Hanging wallblock

Oblique-slipfaults

Dip-slipfaults

Dip-slipfaults

B. Thrust D. Left-lateral, or sinistralA. Normal C. Right-lateral, or dextral

Classification of Fault RocksClassification of Fault Rocks

(Sibson, 1977)

Model of a Shear Zone Model of a Shear Zone

(Scholtz, 1990)

Brittle Brittle –– Ductile Deformation in The Earth CrustDuctile Deformation in The Earth Crust

OceanicOceanic ContinentalContinental

CataclasisCataclasis DeformationDeformation

The strength of brittle rocks increases with The strength of brittle rocks increases with confining pressureconfining pressure, but , but decreases with decreases with temperaturestemperatures. .

LithosphericLithospheric Strength ProfilesStrength Profiles

Myloniteicfaultrocks

Surface trace of fault

Mylonites

Cohesivecataclasites

Fault zone

Cataclasticfaultrocks

Incohesivecataclasites 1-4 km.

4-10 km.

250º-350º CTemperature

FAULT ROCKS AND DEFORMATION MECHANISMFAULT ROCKS AND DEFORMATION MECHANISM

1 m

0.1 m

FAULT AND FAULT ZONES

Fault zone on Pre-Tertiary sandstone, Central Sumatera

Clay Gouge

Fault Gouge

FAULT AND FAULT ZONES

Strike-slip

Fault plane

Oblique-slipDip-slip

Obliq ue-slipDip -slip

Heave

Horizontal component

Str ike-slip

component

Verticalcomponent

Throw

FAULT ATTRIBUTESFAULT ATTRIBUTES

First Motion StudyFirst Motion StudyEarthquake Focal MechanismEarthquake Focal Mechanism