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 JOINTS
Rekahan dan Sesar
FRACTURED ROCKS
• 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 bukti pergerakan (shear displacement) atau off-set • Sesar (fault) dapat berbentuk satu bidang diskrit yang planar atau membentuk suatu zona (fault zone) yang terdiri dari banyak bidangbidang sesar yang sejajar dan saling berhubungan (net-work)
Rekahan (Fractures) • Rekahan (fractures) adalah suatu bidang diskontinuitas dalam batuan
FRACTURES FORMATION
Fracture Types
• Jenis-jenis fractures : extension fractures dan shear fractures • Extension fractures (mode I) • Shear fractures (mode II) • Shear fractures (mode III)
A. Extension (Mode I)
B. Shear (Mode II)
C. Shear (Mode III)
Rekahan (Fractures) – Joint atau kekar (sistematik dan nonsistematik) • Kekar kolom • Kekar release dan kekar pembebanan (loading) • Kekar yang terbentuk akibat perlipatan (cross-joint, longitudinal joint dan stretching joint) – Veins (tension gash)
SYSTEMATIC JOINT
• Nomenclatures (Joint vs. Fault) • Shear vs. Extension Fractures • Hybrid Fractures
FRACTURES AND JOINTS
FRACTURES & JOINT SETS
FRACTURED ROCKS
KEKAR PALING SUKAR DIANALISA KARENA : PEMBENTUKANNYA TIDAK TERBATAS WAKTU
¾ Sejak sedimen diendapkan ¾ Batuan beku mendingin ¾ Mengalami diagenesa ¾ Sampai mengalami gangguan oleh gaya Kekar Non Tektonik
Kekar Tektonik
TIDAK MEMPERLIHATKAN ADANYA PERGESERAN
STRESS vs. STRAIN
PRINCIPAL STRESS
Stress Tensor Notation
σ
σ11 σ12 σ13 =
σ21 σ22 σ23 σ31 σ32 σ33
Unstable
σS
σS
σS
Formation of Fractures
Stable
σn
σn
σn
T0
σ3
σ1
A. Stable states of stress
B. Critical states of stress
σ 3 = T0 Normal to fracture plane Tension fracture
σ1
σ3
Tension fracture envelope
σ1
C. Unstable states of stress
σ3
σ1
σs 2θf = 180º
θf = 90º αf = 0º
D. Physical diagram
x1
σ1 = 0
σn
A. Tension fracture
E.Critical uniaxial tension
Effect of Anisotropy on Fracture Orientation
B. Longitudinal splitting
C. Extension fracture
D. Conjugate shear fracture
Relationship between fracture types and stress from rock experiments
Compressive Fractures The Coulomb Law of Failure
σc = σo + tan θ (σn) σc = critical shear stress = cohesive strength σo tan θ = coefficient of internal friction σn = normal stress
(Modified from Davis and Reynolds, 1996)
COMPRESSIVE
MOHR DIAGRAM FOR BRITTLE –DUCTILE TRANSITION
TENSILE
Frictional Sliding Behavior (Byerlee’s law)
σc = tan θf (σn) σc = critical shear stress υf = tan θf υf = coefficient of sliding friction σn = normal stress
(Davis and Reynolds, 1996)
THE CONCEPT OF EFFECTIVE STRESS EFFECT OF PRE-EXISTING FRACTURE
om ul Co
b
i Fa
re lu
ve En
pe lo
σN = normal stress στ = shear stress (σs) Pf = pore fluid pressure σ1 = maximum principal stress σ3 = minimum principal stress
ROLE OF FLUID PRESSURE (Pf)
Slip and Fracture Condition Effective stress (σ*) σn* = σn - Pf σc = σo + tan θ (σn- Pf) σc = σo + tan θ (σn*) If σn* = 0
σc = σo σc = Critical stress σo = Tensile strength of rock
(Twiss and Moores, 1992)
Relationship between Differential Stress and Shear Displacement (Twiss and Moore, 1992)
Mohr diagram showing variation of fracture condition resulting from the effect of differential stress and prepreexisting fractures
Failure Envelope and Development of Fracture at Different Condition
The Development of Through Going Fracture (Fault)
σs Von Mises ductle failure criterion
Brittle-ductile transition Yield stress
Griffith Crack τ2 = 4σt(σt+σ)
Coulumb fracture criterion
Parabolic fracture envelope
D
σt = tensile strength
E
C B
T0 A
n
θf
σ1
σn
αf σ3
(Twiss and Moores, 1992)
TWO TYPES OF FRACTURE MECHANISM
• Fracture strongly dependent on: Confining pressure (σ3) and Fluid Pressure (Pf)
I. II. III. IV. V.
Pre-existing crack Crack closed Crack propagation Crack begin to interact Fault forms
Twiss and Moores (1992)
FRACTURES AND FAULTS
FRACTURES MECHANICS
FRACTURES AND FAULTS
FRACTURES 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 bidang diskrit yang planar atau membentuk suatu zona (fault zone) yang terdiri dari banyak bidang-bidang sesar yang sejajar dan saling berhubungan (net-work)
Anderson’ Anderson’s Dynamic Fault Classification
Extensional
Compressional
FAULT GEOMETRIES AND CLASSIFICATION • Anderson’ Anderson’s Dynamic Fault Classification • Separation Classification • Slip Classification Dip-slip faults
Hanging wall block
Hanging wall block Dip-slip faults
Foot wall block
A. Normal
Foot wall block
B. Thrust
Obliqueslip faults
E. Sinistral-normal
C. Right-lateral, or dextral D. Left-lateral, or sinistral Rotational faults
F. Sinistral-reverse
G.
Classification of Fault Rocks
(Sibson, 1977)
Brittle – Ductile Deformation in The Earth Crust
Model of a Shear Zone
The strength of brittle rocks increases with confining pressure, pressure, but decreases with temperatures. temperatures. Cataclasis Deformation
Lithospheric Strength Profiles
(Scholtz, 1990)
Oceanic
Continental
FAULT AND FAULT ZONES
FAULT ROCKS AND DEFORMATION MECHANISM
Sur face trace of fault
Incohesive cataclasites
Clay Gouge
1-4 km .
0.1 m ul Fa on tz e
Cataclastic fault rocks
Temperature Myloniteic fault rocks
Cohesive cataclasites 4-10 km.
250º-350º C Mylonites
1m Fault zone on Pre-Tertiary sandstone, Central Sumatera
FAULT AND FAULT ZONES
FAULT ATTRIBUTES Strike
-s lip
Di psli p
Oblique
-s lip lt pla Fa u
ne
Hor izon tal com
Throw
Di psl
He a
Fault Gouge
Earthquake Focal Mechanism First Motion Study
ve
Obliq ue
ponent
-slip
ip
p -s li ik e e nt S tr p on c om
Vertical comp onent
