Geology and Its Role in Civil Engineering

Geology as a Discipline

• Geology is an interdisciplinary field that studies the Earth's physical structure.
• Two main branches of geology: Physical Geology and Historical Geology.
• Physical Geology focuses on natural processes that change the Earth's surface.
• Historical Geology studies the Earth's geologic past.

Importance of Geology to Civil Engineering

• Geology is crucial for civil engineering projects to understand ground conditions, identify potential hazards, and design safe and sustainable structures.
• Civil engineers rely on geological data to assess: soil stability, rock types, groundwater levels, earthquake risks, and other geological factors.
• Understanding geologic processes helps civil engineers mitigate risks such as landslides, subsidence, flooding, and erosion.
• Applying geology principles ensures structures are built safely and efficiently.

Earth's Systems

• Earth's history is divided into four major divisions in abstract time: Eons, Eras, Periods, and Epochs.
• The largest division is the Eon.
• The four Eras, from oldest to youngest, are the Precambrian, Paleozoic, Mesozoic, and Cenozoic.
• Each Era is further subdivided into Periods, and Periods into Epochs.

The Geologic Time Scale

• The Geologic Time Scale is a standardized timeline that portrays the history of the Earth.

External Structure of the Earth

• Earth consists of several spheres:
  • Atmosphere: The layer above the lithosphere and hydrosphere.
  • Biosphere: The "zone of life" encompassing all living organisms.
  • Cryosphere: Contains large ice masses at the poles and elsewhere.
  • Geosphere/Lithosphere: Also known as "land."
  • Hydrosphere: The layer of water surrounding the lithosphere.

Internal Structure of the Earth

• The Earth has three main layers:
  • Crust: The outermost solid layer.
  • Mantle: A thick layer of mostly solid rock below the crust.
  • Core: The Earth's center, consisting of a solid inner core and a liquid outer core.

Discontinuities in the Earth's Layers

• Discontinuities mark boundaries between Earth's layers:
  • Conrad Discontinuity: Separates the upper and lower crust.
  • Mohorovicic Discontinuity: Transition zone between the crust and the mantle.
  • Repetti Discontinuity: Separates the upper mantle from the lower mantle.

Theories: Uniformitarianism vs. Catastrophism

• Uniformitarianism (James Hutton): "The Present is the Key to the Past."
• Uniformitarianism assumes that natural laws haven't changed throughout Earth's history.
• Catastrophism (Baron Georges Cuvier): Emphasized sudden, catastrophic events as the primary drivers of geologic change.

Plate Tectonics

• Continental Drift Theory: Proposed by Alfred Wegener in 1912, suggesting continents shift positions on Earth's surface.
• Seafloor Spreading: The process where new lithosphere is created at divergent plate boundaries, forming new crust.
• Mid-Ocean Ridge: Large mountain ranges on the ocean floor where seafloor spreading occurs.

Crusts

• Continental Crust: Forms continents and continental shelves.
  • Composed mostly of granite.
  • Lighter than oceanic crust.
• Oceanic Crust: The uppermost layer of the oceanic portion of tectonic plates.
  • Composed mostly of basalt.
  • Denser than continental crust.

Volcanic Eruptions

• Different types of volcanic eruptions:
  • Icelandic: Form lava plateaus.
  • Hawaiian: Form shield volcanoes with gentle slopes.
  • Strombolian: Moderate bursts of gas ejecting lava clots.
  • Vulcanian: Dark, turbulent eruptions with expanding gas clouds.
  • Pelean: Explosive outbursts generating pyroclastic flows.
  • Plinian: Intensely violent eruptions.

Volcanic Hazards

• Potential hazards from volcanic activity:
  • Tephra/Ash: Volcanic debris that can fall as a hazard.
  • Lava Flows: Molten rock that can flow and damage areas.
  • Pyroclastic Density Currents: Hot, high-speed flows of volcanic material.
  • Pyroclastic Falls: Deposition of volcanic material from the air.
  • Lahars: Mudflows made up of volcanic debris.
  • Volcanic Gases: Can be toxic and lethal.
  • Volcanic Landslides: Large-scale collapses of volcanic slopes.
  • Climate Change: Volcanic eruptions can influence global climate.

Mitigating Volcanic Hazards

• Essential steps to mitigate volcanic risks:
  • Understanding volcano behavior.
  • Knowing how volcanic hazards behave.
  • Implementing strategies to avoid or minimize hazards.

Sedimentary Rocks

• Sedimentary rocks form from the accumulation and lithification of sediments.
• Processes involved in their formation: weathering, transport deposition, compaction, and cementation.
• Sedimentary rocks are the only type of rock that typically contain fossils.

Types of Sedimentary Rocks

• Classification based on origin:
  • Clastic/Detrital: Formed from fragments of other rocks (clasts).
  • Chemical: Precipitation of minerals from a fluid.
  • Organic: Formed from accumulated organic debris.

Soil Formation; Weathering Products

• Weathering: The breakdown of rocks at or near the Earth's surface.
• Types of weathering: Mechanical/Physical, Chemical, and Biological.

Factors Influencing Weathering Rates

• Key factors that affect weathering rates:
  • Temperature.
  • Rock type.
  • Moisture availability.

Erosion

• Erosion is the removal and transport of weathered material by agents like water, wind, ice, or gravity.
• Mass wasting: Movement of rock or soil downslope primarily due to gravity.

Types of Erosion

• Fluvial Erosion: Erosion by water.
• Aeolian Erosion: Erosion by wind.
• Glacial and Periglacial Erosion: Erosion by ice.
• Gravitational Erosion (Mass Wasting):
  • Rock falls.
  • Landslides.
  • Debris/Mud flows.
  • Slumps.
  • Creep.

Deposition

• Deposition is the process by which weathered and eroded materials are settled in a new location.

Depositional Environments

• Depositional environments are locations where sediments are deposited.
• Influence characteristics of sedimentary rocks, such as lithology, minerals, texture, structures, and fossils.

Keys to Identifying Depositional Environments

• Identifying features that help determine depositional environments:
  • Lithology: The mineral composition and texture of the rock.
  • Sedimentary Structures: Patterns within sedimentary rocks that provide information about depositional processes.
  • Fossils: Remains or traces of organisms preserved in rocks.

Types of Depositional Environments

• Classification of depositional environments:
  • Continental/Terrestrial.
  • Marginal Marine/Transitional.
  • Marine.

Factors Affecting Soil Formation

• Soil formation is influenced by:
  • CLORPT:
    • Climate: Temperature and precipitation.
    • Organic Matter: Decomposed plant and animal material.
    • Relief/Topography: The slope and shape of the land.
    • Parent Materials: The underlying rock or sediment.
    • Time: The length of time soil has been developing.

Soil Properties

• Characteristics used to describe soil:
  • Color: Reflects composition and mineral content.
  • Structure: The arrangement of soil particles.
  • Texture: The relative proportions of sand, silt, and clay.

Soil Profile

• A soil profile is a vertical section of soil, revealing different horizons with distinct characteristics.