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PARTPART 5: 5: GGEOMORPHOLOGYEOMORPHOLOGY -- structuralstructural geologgeologyy

Earth surface is still undergoing physical changes, in both small- & continental-scale

Resulting from effect of weathering & tectonicforces.

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

GGEOMORPHOLOGYEOMORPHOLOGY

In responding to the physical changes (e.g. state of stresses) of the surroundings, a rock mass may deform or change its physical state.

Changes in rock mass include: expands if confining stress reduces; deforms plastically (ruptures) if acting stress is larger than its strength; deforms elastically (reshapes) if acting stress is smaller than its strength.

As result, rock mass displays certain characteristics, indicating changes in the surrounding.

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EXCAVATION OF SLOPE IN ROCK MASS

The phrase ‘solid as a rock’ is not a correct statement on the properties of rock mass.

Rock bodies, whether exposed on the surface or buried in the earth’s crust, are not free from fracture and weakness planes.

In addition, fresh and strong rock may be degraded and weakened by weathering processes.

Take a strong, solid (without discontinuity Take a strong, solid (without discontinuity features), and fresh (features), and fresh (unweatheredunweathered) rock mass) rock mass

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A near vertical slope is excavated in the rock A near vertical slope is excavated in the rock mass . . . .mass . . . .

. . . . . . in terms of the rock mass properties . . . . . . in terms of the rock mass properties would there be any problem on the stability of would there be any problem on the stability of the cut slope ?the cut slope ?

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A slope cut in a fresh, strong and solid rock mass A slope cut in a fresh, strong and solid rock mass but, with horizontal bedding planes (a type of but, with horizontal bedding planes (a type of discontinuity or weakness plane) . . . . .discontinuity or weakness plane) . . . . .

Again, one will question the stability of the cut slope . . . is it being affected by the horizontal bedding planes . . . ? Is there any immediate effect due to rock mass properties & bedding planes . . . ?

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A cut slope in a fresh, strong rock mass with A cut slope in a fresh, strong rock mass with horizontal bedding planes and inclined fracture horizontal bedding planes and inclined fracture planes (e.g. joints) planes (e.g. joints)

For the cut slope (A) in this rock mass, is the stability For the cut slope (A) in this rock mass, is the stability being affected by the horizontal bedding planes and being affected by the horizontal bedding planes and inclined joints . . . . ? inclined joints . . . . ? Let propose another slope (B) on the other side of the Let propose another slope (B) on the other side of the hill . . . . . .hill . . . . . .

SLOPE A

SLOPE B

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The rock properties and the horizontal bedding The rock properties and the horizontal bedding planes have no immediate effect on the stability of planes have no immediate effect on the stability of both SLOPE A and B. However, with respect to the both SLOPE A and B. However, with respect to the orientation the joints, SLOPE B is critical.orientation the joints, SLOPE B is critical.

SLOPE A SLOPE B

GGEOMORPHOLOGYEOMORPHOLOGY

The characteristics include presence of structural discontinuities in rock mass, and the most important in construction are:bedding planes, folding (lipatan); fault (sesar) & joint (kekar)

These discontinuity features induce weakness, and defect into rock bodies.

They affect rock behaviour when subjected to loading ( e.g. stability of tunnel excavated in highly jointed rock mass, compared to massive rock)

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FOLD (LIPATAN)FOLD (LIPATAN)

Folds only occur in clastic sedimentry rocks which exhibit bedding/layers/strata, metamorphic rocksoriginating from clastic sedimentary rocks & volcanic rocks.

Beddings/strata are weakness planes in rock –relative displacement can occur between bedding planes.

Folds is formed when bedded/stratified rock is subjected to lateral compressive stress in the earth crust.

THINGS TO PONDER

Why there is no folding in granite ?

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

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FOLDINGSFOLDINGS

Type of folding: symmetrical, non-symmetrical,over-turned & recumbent fold (lipatan rebah) -Rajah 14.

Recumbent fold can affect sequence of bedding arrangement (difficulties in verifying the relative age of beddings).

Important geometrical parameters of fold: Limbs, syncline & anticlineDip direction/strike of inclined bedding (limbs).

TYPES OF FOLDSTYPES OF FOLDS

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FOLD GEOMETRYFOLD GEOMETRY

FOLDFOLD

When rock strata are folded, they are under a stateof strain (keterikan), this will induce remnant or residual stress in the folded rock mass.

For symmetrical fold – residual tensile stressoccurs in the upper portion of the fold, & residualcompressive stress in the lower portion. Usually, tension cracks can be found at the crest.

For a deformed rock mass (e.g. folded), its in situstress distribution is no longer in equilibrium –presence of remnant stress due to deformation.

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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 the shoulder of the fold ?

TENSION CRACKS AT CREST OF A FOLDTENSION CRACKS AT CREST OF A FOLD

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FOLDFOLD

Folded rock mass creates problems to construction - higher in situ stress:

Virgin in-situ stress (≈ ρgh) + residual stress

Slope excavated in folded rock mass may exhibit frequent failures – inclined bedding plane.

For petroleum industry, folded strata is the main target area in exploring for petroleum reservoirs(Figure 15)

Inclined bedding planes in folded strataInclined bedding planes in folded strata

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Inclined bedding planesInclined bedding planes

Inclined bedding planes in folded strataInclined bedding planes in folded strata

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Inclined bedding planes in folded strataInclined bedding planes in folded strata

FAULT (SESAR)FAULT (SESAR)

Fault – a fracture between two rock blocks where there has been observable relative displacement.

One of the block can be displaced upwards or downwards or even displaced laterally (Rajah 16).

Fault can occur in all kind of rocks (whenever cracks are present).

Important geometry of fault: dip direction of fault, dip angle, strike & amount of displacement (Fig. 17)

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ASSOCIATED DISPLACEMENTS IN FAULT ASSOCIATED DISPLACEMENTS IN FAULT FORMATIONFORMATION

FAULT STRUCTURE & GEOMETRYFAULT STRUCTURE & GEOMETRY

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FAULTFAULT

Faults are classified based on their formation: normal fault, reverse fault and strike-slip fault(sesar mendatar).

A large scale displacement (few kms) may induce metamorphism in the surrounding rock, forming metamorphic rocks e.g. fault breccia & clay gouge.

Fault traversing a river gives rise to waterfall. Springline is a result of groundwater flowing a long a fault line.

Large faults (10 – 20 km deep) can act as passage for magma to flow upwards from inside the mantle.

NORMAL & REVERSE FAULTNORMAL & REVERSE FAULT

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STRIKE STRIKE -- SLIP FAULT (horizontal displacement)SLIP FAULT (horizontal displacement)

Movements of faultMovements of fault

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San Andreas Fault San Andreas Fault –– two continental plates two continental plates slide past one anotherslide past one another

Earthquake is produced by a sudden slip Earthquake is produced by a sudden slip along interlocked fault linealong interlocked fault line

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FAULTFAULT

Formation of multiple large size faults (fault zone) may induce remnant or residual stress in host rock.

Residual shear stress may occurs in rock bodies at near a fault zone.

Excavation work in rock displaying fault structure, instability of excavated surfaces is a common problem, particularly if faults are filled with clay gouge.

FAULTFAULT

Faults induce discontinuity (ketakselarasan) and weakness into rock – uneven stresses distribution.

Ideally, rock mass without discontinuities (faults & fold), stress distribution can be assumed in a state of hydrostatic stress (P = ρgh); stresses are equal in all directions.

If faults are present in rock mass, stress in one direction may not be equal to stress in the opposite direction (e.g. horizontal stresses , σh).

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JOINT (KEKAR)JOINT (KEKAR)

Joint – is a fracture, separating two rock blocks, where there is no observable (very small)displacement (Fig. 14).

Formation of joints are associated with tensile stress acting within a rock body. The source of tensile stresses can be from any/all of the following processes:Expansion & shrinking of a massive magma body during cooling process – joint sets that are perpendicular to each other.

JOINT (intersecting of more than 2 joint sets)JOINT (intersecting of more than 2 joint sets)

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EXPOSED JOINT SURFACESEXPOSED JOINT SURFACES

JOINT SET (in almost vertical orientation)JOINT SET (in almost vertical orientation)

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JOINT (KEKAR)JOINT (KEKAR)

Expansion of rock bodies due to reduction in overburden stress. For instance, decreasing thickness of materials (soils) covering a deep seated bed rock after continuous & intensive weathering at the surface – sheet joints.Tensile stress acting at crest of a fold – tensioncracks.

For impermeable rock (e.g. granite) joint acts as asecondary permeability.As other discontinuities in rock, joints are weakness planes in rock.

SHEET JOINT (KEKAR BERLAPIS)SHEET JOINT (KEKAR BERLAPIS)

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TENSION CRACKS AT CREST OF A FOLDTENSION CRACKS AT CREST OF A FOLD

JOINT displaying very small displacementJOINT displaying very small displacement

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UndisplacedUndisplaced JOINTJOINT

Various types of discontinuity/weakness planes Various types of discontinuity/weakness planes & and their formation in different rock type& and their formation in different rock type

Rock types Types of structural discontinuity

Igneous Sedimentary Metamorphic Fault plane √ √ √

Joint plane √ √ √

Bedding plane - √ √

Foliation - √ -

Slaty & schistosity - - √

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JOINTJOINT

Important geometrical parameters: dip angle, dip direction & strike (Figure 17). Joints are relatively smaller in size compared to fault (few m length)

A group of joints of almost similar orientation is called JOINT SET.

Two or more joint sets that are intersecting each other is termed as JOINT SYSTEM (Figure 18).

In a joint system, if one joint set is more dominant than other sets, this set is called primary set.

JOINT SETS JOINT SETS –– 1 set and 3 sets1 set and 3 sets

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JOINT SETJOINT SET (granite)

JOINT SETJOINT SET (granite)

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JOINT SETJOINT SET (shale)

EFFECT OF 1 JOINT ON UCS OF ROCK EFFECT OF 1 JOINT ON UCS OF ROCK

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EFFECT OF 2 JOINT ON UCS OF STRENGTHEFFECT OF 2 JOINT ON UCS OF STRENGTH

EFFECT OF MANY JOINT ON UCS OF ROCK EFFECT OF MANY JOINT ON UCS OF ROCK –– in highly in highly fractured rock the strength may approach that of soil fractured rock the strength may approach that of soil

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JOINTJOINT

The more is the number of joint sets, the weaker and more unstable is the rock.Rock with more joint sets is easier (cheaper) to excavate. Bed rock displaying multiple joint sets may not be suitable for foundation of large structures.Joints in hard rock are passage for weathering agents (water, O2) to penetrate into rock thus, rocks with more joint sets are more easily weathered compared to rock with less joint sets.Exposed rounded rock boulders are the result of weathering of rock that exhibits 3 joint sets that are perpendicular to each other.

Large rounded to subLarge rounded to sub--rounded boulder (granite)rounded boulder (granite)

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Large rounded to subLarge rounded to sub--rounded boulder (granite)rounded boulder (granite)

Boulders are produced by weathering of jointed granite –sphreoidal weathering.

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Physical weathering (disintegration due to joints) Physical weathering (disintegration due to joints) –– creates new creates new surfaces for chemical weathering to take place on the exposed surfaces for chemical weathering to take place on the exposed

surfaces, especially at the 8 cornerssurfaces, especially at the 8 corners

THINGS TO PONDER

What are the main differences between joint and fault ?

Why a slope face cut in rock displaying 3 joint sets is less stable than in rock displaying 1 joint set ?

Do I have any other options besides this non-entertaining Rock Mechanics ?

(Yes . . . . . . . try Rock & Roll)

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JOINTJOINT

Intrusive igneous rock (e.g. granite) almost always exhibiting 3 sets of joint, typical joint characteristic for massive and strong rock, with random (rawak) minerals arrangement.

For rocks displaying structured minerals arrangement in certain orientation (lamination, slaty& schistosity), e.g sedimentary & metamorphic rock, the major joint sets are usually parallel to the mineral arrangement.

Fracture plane parallel to lamination (sandstone)Fracture plane parallel to lamination (sandstone)

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MEASUREMENT OF JOINTMEASUREMENT OF JOINT (& other fracture planes)

Major rock excavation work (e.g. slope & tunnel) requires a preliminary (initial) investigation before design & construction – ‘Joint survey’: field work to identify types of weakness planes (joint, bedding planes, faults) measurement on geometrical orientation (dip & dip direction).

Joint orientation & joint sets for a rock type are usually consistent thus, measurement can be undertaken on the exposed portion of the rock.

Data & info obtained (e.g. geological mapping) are used for preliminary design of the structure.

MEASUREMENT OF JOINTMEASUREMENT OF JOINT

Figure 17 shows the important geometrical characteristics of joint (or other weakness plane)that need to be measured in the field:

DIP angle (sudut kecondongan).

Dip direction (arah kecondongan) / Strike.

Joint survey is undertaken on exposed rock body (accessibility & cost) .

Other relevant data & info on joint: open or close, conditions of joint surface (rough or smooth), infill (berinti) & size/length, continuity (keselanjaran)

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GEOMETRICAL PARAMETERS for an inclined GEOMETRICAL PARAMETERS for an inclined weakness planeweakness plane

FILLED JOINT (KEKAR BERINTI)FILLED JOINT (KEKAR BERINTI)

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MEASUREMENT OF JOINTMEASUREMENT OF JOINT (& other fracture planes)

Instrument used are Brunton compass & tapes.

Collected data – few hundreds to thousands.

Data is analysed using computer software (e.g. GEORIENT) - lower hemisphere strereonet projection.

Output data: dip & dip direction of major & minor joint sets, probablity of failure direction and mode of failure.

Mean dip & dip direction of all the measured joint is compared with the proposed cut slopes.

Joint measurement using BRUNTON COMPASSJoint measurement using BRUNTON COMPASS

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Typical roughness profile & index (JRC) of joint surfaceTypical roughness profile & index (JRC) of joint surface

DATA FROM JOINT MEASUREMENTDATA FROM JOINT MEASUREMENT

Dip angle Dip direction 0N Joint set 55 030 J1 55 025 J1 65 035 J1 65 030 J1 60 000 J2 60 350 J2 60 350 J2 75 000 J2 75 000 J2 65 285 J3 70 280 J3 55 280 J3 65 285 J3 75 285 J3 85 235 J4 80 240 J4 90 230 J4 85 220 J4 65 350 J5 70 355 J5

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StereonetStereonet projection plot projection plot –– great circlesgreat circles

StereonetStereonet projection plot projection plot –– poles densitypoles density

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StereonetStereonet projection output projection output –– great circles of great circles of joint sets & cut slopejoint sets & cut slope

MODES OF SLOPE FAILURE and PLOTTED STEREONET

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MODES OF SLOPE FAILURE

Effect of weakness planes (dip & dip direction) on stability of cut slope face in a discontinuous rock mass

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Direction of tunneling with respect to joint orientationDirection of tunneling with respect to joint orientation

Direction of tunneling with respect to joint orientationDirection of tunneling with respect to joint orientation

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THE GRAND FINALETHE GRAND FINALE

Theoretically, a vertical slope face of several Theoretically, a vertical slope face of several thousand meters height (H) can be excavated in a thousand meters height (H) can be excavated in a massive, continuous & strong rock massive, continuous & strong rock (based on (based on formula H = UCS/formula H = UCS/γγ))

However, in actual condition in the field, this is However, in actual condition in the field, this is impossible (the allowable maximum vertical height, impossible (the allowable maximum vertical height, H, is less than 10 m) ?H, is less than 10 m) ?