Engineering Geology: An Overview

• Geology is the scientific study of the Earth, focusing on rocks, soils, origins, nature, distribution, and formation processes.
• Engineering geology applies geological knowledge to ensure the safety, efficacy, and economy of engineering projects, including structural designs, landslide prevention and hazard prevention.

Importance of Engineering Geology

• Success of engineering projects relies on understanding geological problems.
• Early stages focus on site suitability.
• Planning stages involve site analysis and subsurface investigations.
• Construction stages involve identifying and rectifying geological defects during civil engineering activities.
• Engineering geologists are responsible for identifying adverse geological features, not their presence.

Role of Engineering Geologists

• Conduct subsurface structure analysis using regional experience, surveys, and borings.
• Geotechnical engineers specialize in measuring mechanical properties of foundation soils as civil engineers.
• Engineering geologists assess foundation conditions for structural sites and evaluate terrain for development.
• Groundwater or geohydrological conditions of a terrain are investigated.

Required Knowledge for Engineering Geologists

• Engineering geology requires familiarity with geology, hydrogeology, geomorphology, civil engineering design, soil mechanics, and rock mechanics.

Branches of Geology

• Crystallography: studies crystal characters with bodies bounded by flat faces, arranged on definite planes with inner atoms.
• Mineralogy: studies mineral characteristics (e.g., quartz, mica, garnet) that are naturally occurring, homogeneous, inorganically formed, and have definite chemical compositions.
• Petrology: studies all aspects of rocks, including mineralogy, textures, structures, and origins.
• Geophysics: examines the structure, physical conditions, and evolutionary history of the Earth.
• Geochemistry: studies the chemical composition of minerals and rocks.
• Structural geology: studies rock structures like folds resulting from Earth's crust movements and deformation.
• Stratigraphy: studies stratified rocks, focusing on their time sequence and rock character.
• Physical geology: examines surface features and geological processes.
• Paleontology: studies ancient life to determine environment and evolution.
• Hydrogeology: studies geological factors related to Earth's water.
• Engineering geology: applies geological knowledge to engineering projects.
• Photo geology: identifies geologic features via aerial/orbital photographs.
• Economic geology: studies earth materials, focusing on economic or industrial use.
• Mining geology: studies methods for mining rocks and mineral deposits.

Earth Structure and Composition

• Two classifications of Earth's structure with spherical layers: chemical/elemental and mechanical/physical.

Chemical/Elemental Compositions:

• Oceanic Crust (Basaltic): 71% Earth, composed of magma creating basalt lava flow.
• Continental Crust (Granitic): 29% Earth, mainly granite, less dense than oceanic crust.
• Mantle: Compromises 82% volume with silicate rocks rich in magnesium and iron.
• Core: Made of 85% iron metal and 15% nickel.
• Inner Core: Solid iron with a radius of approximately 760 miles, hottest layer at 7000°C.
• Outer Core: Liquid nickel-iron alloy, about 1,355 miles thick, temperature at 5000°C.

Mechanical/Physical Properties:

• Layers of Earth divided by behaviors, corresponding to properties.
• Lithosphere: Solid, outer part, including upper mantle and crust; the Earth's outermost layer, bounded by the atmosphere and the asthenosphere.
• Asthenosphere: Soft, upper mantle layer, below the lithosphere, allows plastic flow and convection.
• Mesosphere: Layer below the asthenosphere, above the outer core.
• Outer Core: Liquid metals due to extreme temperature that causes strong magnetic field.
• Inner Core: Solid due to intense pressure, spins differently, believed to influence the Earth's magnetic field.

Continental Drift & Plate Tectonics:

• Continental Drift Theory: Developed by Alfred Wegener, stating that all continents formed Pangaea, which existed 240 million years ago.
• Plate Tectonics: Scientific theory of the large-scale motion of plates since 3.3-3.5 billion years ago, average thickness of 100 km.
• South American Plate: 43,600,000 sq. km.
• Indo-Australian Plate: 58,900,000 sq. km.
• Antarctic Plate: 60,900,000 sq. km.
• African Plate: 61,300,000 sq. km.
• Eurasian Plate: 67,800,000 sq. km.
• North American Plate: 75,900,000 sq. km.
• Pacific Plate: 103,300,000 sq. km.
• Oceanic Plates: Constructed of oceanic crust with magnesium and silicon minerals.
• Continental Plates: Constructed of continental crust including aluminum and silicon minerals.

Plate Boundaries:

• Transform Boundary: occur when plates slide by each other laterally, causing minor damage.
• Divergent Boundary: Plates separate, often resulting in seafloor spreading, mid-ocean ridges, and small earthquakes.
• Convergent Boundary: plates collide, causing subduction and mountain formation.

Movement of Plates in Mantle:

• Hot mantle rises at ridge axes and material sinks and reheats.
• Ocean-Continent Convergence: Oceanic plate subducts, forming continental arcs and causing earthquakes and volcanoes.
• Ocean-Ocean Convergence: Older, denser plate subducts, forming island arcs.
• Continental-Continental Convergence: Thick continental plates collide, forming high mountain ranges.

Earth processes on coastlines

• Erosion happens from destructive waves.
• Hydraulic action: waves smash & trap air in cracks on cliffs.
• Abrasion: pebbles grind against coastal rock platforms and cliffs.
• Attrition: rocks collide and become smoother.
• Solution: minerals dissolve in sea water,
• Suspension: small particles are suspended in water.
• Saltation: larger particles bounce on seabeds.
• Traction: pebbles roll along the seabed.

Land Forms and Earth Processes:

• Earth Process: Dynamic actions that occur inside the Earth.
• Weathering: Breakdown of rocks under physical and chemical forces.

Factors Affecting Weathering:

• Nature of rocks, time, and climate.

Processes of Weathering:

• Disintegration: Breaking rocks into small pieces.
• Decomposition: process of breaking up mineral constituents to form new components.
• Denudation: Term used when the earth’s surface is worn away.

Types of Weathering:

• Physical: Rock mass breakdown via atmospheric agents, most active in cold, dry regions.
• Chemical: Alteration of rock by adding or removing elements.
• Biological: Action of plants and animals.
• Thermal Stress: Results from temperature variations.

Ground Water Hydrology:

• The science of ground water hydrology involves the occurrence, distribution, and movement of water underground, which serves as a significant water supply.
• Ground water origin depends on quantum & speed of rains, temperature, and soil composition

Sources of Ground Water:

• Meteoric Water: Rainwater and snow that infiltrates through pores, fissures, and joints.
• Connate Water: Exists in sedimentary rock pores and cavities.
• Magmatic Water: Converts water after vapor condensation from volcanic action.

Geological Formations:

• Aquifers: Transmit water easily, good examples are sand and gravel.
• Types: Unconfined aquifers are closer to the surface, while confined aquifers are bound by impermeable layers.
• Aquitard: Permits seepage
• Aquiclude: Porous, but not permeable
• Aquifuge: Neither porous nor permeable

Earthquake

• Earthquakes are sudden ground shaking caused by seismic waves along geologic faults, releasing sudden energy from Earth Crust.

Faults:

• Zones where rock masses move.
• Earthquakes occur along these faults.

Types of Faults:

• Normal Slip: Crust is pulled apart, with hanging-wall moving down.
• Reverse Slip: Crust is compressed, with the hanging-wall moving up.
• Strike Slip: Blocks move sideways (sinistral or dextral).
• Oblique Slip: Combination of basic movements.

Earthquake measurements

• Focus occurs beneath the surface.
• Epicenter occurs directly above the focus.
• Intensity: Perceived severity of shaking.
• Magnitude is the quantitative source size of energy released.