geologi rekayasa

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PERTEMUAN KE - 7

TKS24082

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Struktur Geologi:

Bentuk-bentuk geometri yang terdapat pada kulit bumi yang terbentuk oleh pengaruh

gaya-gaya endogen, baik berupa tekanan

maupun tarikan

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Structural Geology

Rocks below the earth's surface are hot and tend to flow, whereas rocks at the surface are relatively

cool and tend to be more brittle. Thus, rocks at the surface (or near-surface) fracture while rocks deep inside the earth flow.

Deformation: when rocks are subjected to stresses (forces) greater than their own internal strength.

Caused by stress and resulting in strain

Stress -- force acting upon an object to create deformation

Strain -- resultant of the stress applied; end product

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There are several types of stresses that can be applied to a rock unit:

1. Extension or tension (pulling apart) 2. Compression (pushing together)

3. Shearing or twisting (one portion in one direction, the other portion in another direction)

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There can be two (4) resulting responses to stress:

• DUCTILE DEFORMATION: Is a continuous

deformation that produces certain kind of folds, ductile faults, cleavage and foliation.

• BRITTLE DEFORMATION: Is a discontinuous deformation that produces folds, brittle faults and joints.

• PLASTIC DEFORMATION: Permanent change in shape of a solid that does not involve failure by rupture

• ELASTIC DEFORMATION: A nonpermanent

deformation, which disappears when the stress is released

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Ductile deformation produces folds:

1. Anticline -- upwarping of rocks to produce an "A- like" structure

2. Syncline -- downwarping of rocks to produce

"spoon-like" structure

3. Dome -- three-dimensional anticline resembling inverted cereal bowl

4. Basin -- three-dimensional syncline resembling upright cereal bowl

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Brittle Deformation

When brittle deformation occurs and rocks fracture, they can simply crack producing a fracture with no offset, called a joint (kekar)

When brittle deformation occurs and rocks fracture, they can also crack

producing a fracture with offset, called

a fault (sesar).

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Types Of Geological Structures

• Different stresses result in various forms of strain (geologic structures)

1. Joints

2. Faults (Any type of stress may cause brittle strain.

The type of fault depends on the type of stress)

3. Folds / lipatan

(compressive stresses may cause ductile strain)

4. Unconformity (ketidakselarasan).

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Stikes and Dips are used to identify geologic structures

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• Joints are a very common rock structure.

• They are fractures with no offset.

• Result from tectonic

stresses on rock mass.

• Occur in parallel groups.

Joints (Kekar)

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• Joints:

– Rocks are characteristically broken by smooth fractures known as “Joints”.

– They are fractured surfaces along which there has been no movement parallel to the surface.

– There can be movement at right angles to the joint surface producing an open fracture.

– Joints may have any orientation.

– Joints are measured by strike and dip like bedding

– Joints never occur alone, but in sets.

– „Joint sets‟ over a region make up a „joint system‟.

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• When shallow crust is strained rocks tend to exhibit brittle strain

Brittle Strain  Joints

Types of genetic joints:

Tension joint, Shear joint,

Extension joint, Release joints

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type of tension joints

• Sheeting joints; parallel to topography, can form in any rock, but common in igneous-plutonic rocks that are exposed

• Columnar Joints; extension fractures characteristic of tabular extrusive igneous rocks i.e., form in lava flow, sill, dike

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A B C D E

A Extension joints B Shear joints C Release joints D Shear Joints E Release Joints

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Source: AR Geologic Commission 7.5‟ Geologic Quads

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Joints and Geotechnical engineering

– An example is in road cuts. Joints or bedding planes sloping into the opening made by a road cut makes that road cut

unstable.

Which side of the hill do you

recommend the road goes around?

Unstable cut

Stable cut

Joints, with another set  to them (not shown)

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C. Geotechnical engineering

– Another example is in excavations for foundations.

– In cases like this, or in many cases with road cuts in which there is not a choice as to which side of the hill you go

around, rock bolts are used to help stabilize the unstable cut.

Rock bolts

Foundation excavation

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Faults

• Fault: When movement

occurs along a discontinuity

• Fault type

depends on the type of stress

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Faults

 a break or crack in Earth’s crust along which movement has occurred.

Fault

Three parts of a Fault include;

1) Hanging Wall - the top part of the rock above the fault plane.

Hanging Wall

Fault Plane

3) Fault Plane - the surface that separates the two moving pieces.

2) Foot Wall - the bottom part of the rock below the fault plane.

Foot Wall

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Faults

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Faults Different Types of Faults;

 Caused by tensional forces.

 Hanging wall drops in relation to the foot wall.

1) Normal Fault (dip-slip)

Hanging Wall

Foot Wall

2) Reverse Fault (dip-slip)

 Caused by compressional forces.

 Hanging wall moves upward in relation to the foot wall.

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`Faults Horst and Graben:

 An uplifted block of crust

bounded by two normal faults.

 A valley formed by the

downward displacement of a block of crust bounded by two faults.

Horst

Graben

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Normal Faults, Horsts and Grabens

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Horsts and Grabens

• Older Rocks are exposed along the ridges formed by the horsts

• Younger rocks lie beneath the grabens

• Sediment fills in the linear valleys

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Faults Different Types of Faults;

 Caused by Compressional forces.

 Hanging wall moves up over foot wall.

3) Thrust Fault (dip-slip)

 Low angle reverse fault.

Hanging Wall

Foot Wall

 Caused by shearing forces.

 Two plates slide side by side.

 No vertical movement.

4) Transform Fault (strike-slip)

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Reverse and Thrust Faults

• Compressive stress

causes the hanging wall to move upward relative to the foot wall 

Reverse Fault

• At convergent plate boundaries ancient

rocks can be thrust over younger rocks 

Thrust Fault

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Thrust Fault:

Glacier NP, Montana

Old

Younger

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Structures at a Convergent Boundary

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Strike Slip Faults

• Physiographic Features

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Folds

 Caused by Compressional forces.

 Crust moves downward forming a valley.

 Referred to as a down-fold.

 Caused by compressional forces.

 Crust moves upward forming a hill.

 Referred to as an up-fold.

Parts of a Fold Include;

1) Anticline

Anticline

2) Syncline

Syncline

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Anticline (fold)

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Anticline

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Syncline (fold)

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Syncline

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Folds

 Point where limbs change angle of dip.

4) Fold Axis

 side part of a syncline or anticline

3) Limbs

Parts of a Fold Include;

 Direction of fold (axis)

5) Strike

 Angle of limb with the horizontal.

6) Dip

Fold Axis

Strike

Limb

Dip

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Types of Folds

• Monocline:

– A local steepening in otherwise uniformly dipping strata.

• Isoclinal fold:

– Limbs are parallel to the axial plane.

• Recumbent fold:

– Fold with horizontal axial plane. Commonly isoclinal

• Symmetric vs. asymmetric folds

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Plunging

Anticline

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• Monoclines – Large, step-like folds in otherwise horizontal sedimentary strata.

• Domes -Upwarped circular or slightly elongated

structure. Oldest rocks in center, younger rocks outside.

• Basins – Downwarped circular or slightly elongated structure. Youngest rocks are found near the center, oldest rocks on the flanks.

Common Types of Folds

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Domes and Basins

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Engineering properties of faulted or folded rock

• shear strength

– loose materials

– compressive materials – permeable materials

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hydrology of fault zones

• water in fault zones common due to fractured rock

– fault zone may be either an aquifer or an aquiclude

• crushed to gravel

• crushed to clay

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hydrology of fault zones

• water in fault zones common due to fractured rock

– fault zone may be either an aquifer or an aquiclude

• crushed to gravel

• crushed to clay

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Problems due to water in fault zones

• leakage of waste water under a landfill

• leakage of water under a dam

• sudden collapse and inflow of water into a tunnel

• hydrothermal alteration of rocks to clay minerals along faults – variable physical, mechanical and hydrological properties

• soluble rocks - cavities

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Activity of faults?

• Risk for further movement

– active fault – has moved in the last 100 000 to 35 000 years

– dormant fault – no recorded movement in recent history

• Risk potential depends upon:

duration of the quake, intensity of the quake and recurrence of the quake

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Baldwin Hill reservoir – failed 1963

• 1 principle embankment, 47 m

high, and 5 smaller embankments

• excavated hollow in between at the top of a mountain range

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Baldwin Hill reservoir – failed 1963

• geology

– friable deposits of the Pliocene Pico Formation, massive beds of clayey, sandy siltstone

– Pleistocene Ingewood Formation. interbedded layers of sand, silt, and clay, with some thin linestone beds; some of the sand and silt beds are unconsolidated and erodable

– Both formations contain calcareous and limonitic concretions – bedding dips slightly 5 to 7 degrees, striking roughly parallel to

the Inglewood fault

– major active fault, Inglewood, passes just 150 m west of the reservoir

– the fault is a right lateral strike slip with a vertical component – fault acts as a subsurface dam for a major oil field in the hills

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Baldwin

• Excavation phase

– 7 minor faults were mapped – mostly normal faults

– 3 to 100 mm silty gouge

– largest fault had a total displacement of more than 8 m

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Baldwin Dam

• Design

– rock foundation lined with

• asphalt and

• gravel drain layer

• covered with compacted clay

• covered with asphalt

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Baldwin

• Construction phase 1947-51

– fault 1 caused problems

– slide initiated revealing that the fault passed beneath the inlet/outlet tower

– the tower was relocated 48 ft

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Baldwin

• after completion

– liner cracked along the trace of the fault – emptied in 1957

– cracks repaired

– cracks were also observed in the surrounding area of the reservoir

– the cracks dipped steeply

– trend NS parallel to the faults

– some exhibited small sinkholes – indicative of extensional strain

– offset dip slip

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Baldwin Dam

• Failure 1971

– emptied completely in 4 hours

– seepage along the fault had enlarged to a pipe

– then to a tunnel and

– then the collapse of the roof

– a canyon eroded completely through the all of the reservoir

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Baldwin Dam

• Failure 1971

• cracks in the floor

extended across the entire reservoir along the trace of the fault

• 50 mm displacement

• open voids along the fault

• movement along the fault had fractured the lining

• rupture of the asphalt membrane

• water eroded cavities into the foundation rock

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Tim KBK Geoteknik Prodi Teknik Sipil UNS

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