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ENGINEERING GEOLOGY Chapter 1.0: Introduction to engineering geology Chapter 2.0: Rock classification Igneous rocks Sedimentary rocks Metamorphic rocks

Chapter 3.0: Weathering & soils Chapter 4.0: Geological structures & discontinuities

in rock. Chapter 5.0: Ground Investigation.

Chapter 4: Geological structures & discontinuities

in rock

Earth surface is still undergoing physical changes, in

both small- & continental-scale.

Changes are result from the effect of geological

processes: weathering, erosion & tectonic forces.

Geological structures & discontinuities in rock

In responding to the physical changes (e.g. state of

stress) of the surroundings, a rock mass 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.

Geological structures & discontinuities in rock

The characteristics include presence of geological structures & discontinuities in rock & the most important in construction activities are: FOLDINGS, FAULTS & JOINTS.

These geological structures & discontinuities induce unfavorable state, weaknesses, & defect into rock bodies.

A number of discontinuities are formed during formation of the rock mass, e.g. bedding plane & joints.

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

Folds:

Folds only occur in clastic sedimentry rocks that

exhibit beddings/strata, in metamorphic rocks

originate from clastic sedimentary rocks, & volcanic

rocks.

Beddings 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.

Folds:

Type of folding: symmetrical, non-symmetrical, over-turned & recumbent fold.

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

Beddings are weakness planes which are originally horizontal. When folded the beddings become inclined – inclined weakness planes.

Important geometrical parameters of fold:

Limbs; syncline & anticline; dip direction & dip of inclined beddings at the limbs.

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Types of fold Geometrical features of fold

Folds:

When rock strata are folded, they are under a state of strain, this will induce remnant or residual stress in the folded rock mass.

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

For a deformed rock mass (e.g. folded), the in situ stress field can become more complex – presence of remnant (geological) stresses.

Folds:

Slope excavated in folded rock mass may exhibit

instability – inclined bedding plane at limbs.

For petroleum industry, folded strata is the main

target area in exploring for an economic petroleum

reservoirs.

Inclined bedding planes in folded strata

Fault:

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.

Fault can occur in all types of rocks (whenever

cracks are present).

Important geometry of fault: dip direction of fault,

dip angle, strike & amount of displacement.

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Associated displacements in formation of fault

Geometrical features of fault

Fault:

Faults are classified based on their formation: normal fault, reverse fault & strike-slip fault

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 the mantle. NORMAL & REVERSE FAULT

STRIKE - SLIP FAULT (horizontal displacement) Movements of fault

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San Andreas Fault – two continental plates

slide past one another

Earthquake is produced by a sudden slip along interlocked fault line

Fault:

Formation of multiple large size faults is called fault

zone, may induce remnant or residual stress in host

rock.

Residual shear stress may occurs in rock bodies at

the vicinity of a fault zone.

Fault:

Ideally, the stress distribution in rock mass without

discontinuities (faults & fold), can be assumed in a

state of hydrostatic stress (P gh); 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 , h).

Joint:

Joint is a fracture, separating two rock blocks,

where there is no observable (very small)

displacement.

Formation of joints are associated with tensile

stress within a rock body. The source the stresses

can be from any/all of the followings:

Expansion & shrinking of a massive magma

body during cooling process – joint sets that are

perpendicular to each other.

Joint: Expansion of rock bodies due to reduction in

overburden stress, e.g. decreasing thickness of materials covering a deep seated bed rock after continuous & intensive weathering at the surface – sheet joints.

Tensile stress acting at crest of a fold – tension

cracks. For impermeable rock (e.g. granite) joint acts as a

secondary permeability. As other fracture discontinuity in rock, joints are

weakness planes in rock.

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Joint in granite, intersecting of more than 2 joint sets Exposed joint surfaces

Joint set in almost vertical in orientation Joint are weakness planes, displacement can

occur along this discontinuity

Joints are potential failure planes when there is

free face created in a rock mass, e.g. a cut slope

Joint:

Important geometrical parameters: dip angle, dip direction, joint spacing & persistency. In terms of scale joints are relatively smaller compared to fault (few m).

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

Two or more joint sets intersecting each other is termed as JOINT SYSTEM.

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

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Joint set: 1 set & 3 sets, due to its nature granite usually exhibit more than 3 sets of joint. Joint set

Joint: Strength of rock mass largely depends on the

density, orientation and extent of fractures (joint) within it.

When the number of joint set increases the rock

mass strength decreases dramatically. For less number of joint set (say 1 set), the

orientation of the joint plays a significant role on the strength of the rock mass.

For an infinite number of joint set, with small joint

spacing, the rock mass properties approching that of soil .

Effect of 1 joint set on uniaxial compressive strength (UCS) of rock

Effect of 2 joint sets on UCS of rock Effect of multiple joints on UCS of rock – in highly fractured

rock the strength may approach that of soil

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Joint:

The more is the number of joint sets in a rock body, 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.

Joint:

Intrusive igneous rock (e.g. granite) usually

exhibiting at least 3 sets of joint, a typical joint

characteristic for massive & strong rock, which

exhibits random 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) Table 4.1: Types of discontinuity/weakness planes & and their

formation in different rock types.

Rock types Types of structural

Discontinuity. Igneous Sedimentary Metamorphic

LARGE-SCALE:

Fault plane

Joint plane

Bedding plane -

SMALL-SCALE:

Foliation - -

Slaty & schistosity - -

Microfractures

Voids -

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