Geology And Engenieri Civil

01 Geology and Civil Engineering

THE GEOLOGICAL ENVIRONMENT

Earth is an active planet in a constant state of change.

Geological processes continually modify the Earth’s

surface, destroy old rocks, create new rocks and add to

the complexity of ground conditions.

Cycle of geology encompasses all major processes,

which are cyclic, or they would grind to an inevitable halt.

Land: mainly erosion and rock destruction.

Sea: mainly deposition, forming new sediments.

Underground: new rocks created and deformed.

Earth movements are vital to the cycle; without them the

land would be eroded down to just below sea level.

Plate tectonics provide the mechanism for nearly all

earth movements (section 09). The hot interior of the

Earth is the ultimate energy source, which drives all

geological processes.

SIGNIFICANCE IN ENGINEERING

Civil engineering works are all carried out on or in the

ground. Its properties and processes are therefore

significant – both the strengths of rocks and soils, and

the erosional and geological processes that subject them

to continual change.

Unstable ground does exist. Some ground is not ‘terra

firma’ and may lead to unstable foundations.

Site investigation is where most civil engineers

encounter geology. This involves the interpretation of

ground conditions (often from minimal evidence), some

3-D thinking, and the recognition of areas of difficult

ground or potential geohazards.

Unforeseen ground conditions can still occur, as

ground geology can be almost infinitely variable, but

they are commonly unforeseen simply due to inadequate

ground investigation.

Civil engineering design can accommodate almost any

ground conditions that are correctly assessed and

understood prior to and during construction.

Geological time is an important concept. Earth is

4000M years old and has evolved continuously towards

its present form.

Most rocks encountered by engineers are 10–500M

years old. They have been displaced and deformed over

time, and some are then exposed at the surface by

erosional removal of rocks that once lay above them.

Underground structures and the ground surface have

evolved steadily through geological time.

Most surface landforms visible today have been

carved out by erosion within the last few million years,

while older landforms have been destroyed.

This time difference is important: the origin of the rocks

at the surface may bear no relationship to the present

environment. The classic example is Mt Everest, whose

summit is limestone, formed in a sea 300M years ago.

Geological time is difficult to comprehend but it must be

accepted as the time gaps account for many of the

contrasts in ground conditions.

Endless horizontal rocks exposed in Canyonlands, USA.

Geology Response

Soft ground and settlement Foundation design to reduce or redistribute loading

Weak ground and potential failure Ground improvement or cavity filling; or identify and avoid hazard zone

Unstable slopes and potential sliding Stabilize or support slopes; or avoid hazard zone

Severe river or coastal erosion Slow down process with rock or concrete defences (limited scope)

Potential earthquake hazard Structural design to withstand vibration; avoid unstable ground

Potential volcanic hazard Delimit and avoid hazard zones; attempt eruption prediction

Rock required as a material Resource assessment and rock testing

Concepts of scale are important in geology:

Beds of rock extending hundreds of kilometres across country.

Rocks uplifted thousands of metres by earth movements.

Rock structures reaching 1000 m below the ground surface.

Strong limestone crumpled like plasticine by plate tectonics.

Landslides with over 100M tons of falling rock.

Earthquakes a million times more powerful than a nuclear bomb.

And the millions of years of geological time.

Components of Engineering Geology

The main field of study: Sections in this book

Ground materials and structures 02–06

Regional characteristics 09–12

Surface processes and materials 13–18

Ground investigations 07, 08, 19–23

Material properties

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