Geological Hazards
Learning Objectives
- Understand the mechanisms and engineering impacts of landslides and mass wasting.
- Identify the causes and effects of land subsidence, sinkholes, and soil liquefaction.
- Recognize the hazards posed by problematic soils, volcanoes, and coastal processes.
- Apply the risk equation to assess geohazards and understand mitigation strategies.
Overview
Understanding and mitigating the risks posed by extreme geological events.
Geological Hazards
Extreme natural events in the crust of the earth that pose a threat to life and property.
Geohazards Overview
Civil engineers play a critical role in identifying, assessing, and mitigating these risks through careful site selection, resilient structural design, and protective infrastructure.
Landslides and Mass Wasting
The gravity-driven failure of natural slopes and engineered cuts.
Landslides and Mass Wasting
While detailed in the Surface Processes section, landslides represent one of the most common and deadly geological hazards globally.
Landslide Mechanisms and Impacts
- Mechanisms: Triggered primarily by heavy rainfall (increasing pore water pressure and reducing friction), rapid snowmelt, earthquakes, or human oversteepening of slopes during highway or housing construction.
- Engineering Impact: Massive volumes of rock and soil can completely obliterate transportation corridors, dam rivers (creating secondary flooding hazards when the landslide dam inevitably breaches), and destroy entire communities.
Land Subsidence and Sinkholes
The sudden or gradual downward settling of the Earth's surface.
Subsidence
The downward settling of the Earth's surface due to the removal of subsurface support.
Causes of Subsidence
Subsidence occurs when the ground sinks due to the removal of subsurface support. This can be gradual or sudden, causing severe structural damage.
Common Causes of Subsidence
- Groundwater Withdrawal: Over-pumping aquifers reduces pore-water pressure, transferring the immense weight of the overlying rock entirely to the soil matrix. In clay-rich aquitards, this causes irreversible compaction and widespread, gradual subsidence (e.g., San Joaquin Valley, California).
- Underground Mining: The collapse of abandoned mine workings (coal, salt) can cause localized, rapid subsidence events at the surface.
- Karst Sinkholes: In regions underlain by soluble rocks like limestone, dolomite, or gypsum, acidic groundwater slowly dissolves the rock, creating vast subterranean caves. Cover-subsidence sinkholes form gradually where the covering sediment is permeable and sandy; cover-collapse sinkholes form abruptly where clayey cover bridges a void until the remaining roof can no longer support itself.
Soil Liquefaction
Interactive Simulation
Interact with the simulator below to observe the rapid loss of strength and rise in pore water pressure during a liquefaction event.
Soil Liquefaction Simulation
Simulate an earthquake to observe the rise in pore water pressure and the resulting loss of effective stress in loose, saturated sand.
Shaking causes water pressure to rise in loose sand.
Soil strength drops as effective stress approaches zero.
When solid ground temporarily behaves like a heavy liquid.
Liquefaction
Effects of Liquefaction
During liquefaction, heavy structures like buildings and bridges can rapidly tilt or sink into the ground, while buried buoyant structures like empty storage tanks, sewer pipes, and manholes can violently float to the surface.
Problematic Soils
Soils that undergo massive volume changes under specific environmental conditions.
Problematic Soils Overview
Certain soils pose inherent geological hazards due to their composition and response to environmental changes.
Types of Problematic Soils
- Expansive Soils (Swelling Clays): Soils containing high percentages of specific clay minerals, primarily Smectite (Montmorillonite). These clays forcefully absorb water, causing massive volume expansion when wet and severe shrinkage when dry. Mitigation includes stiffened raft foundations, deep drilled piers below the active moisture zone, or lime stabilization.
- Collapsible Soils: Wind-blown silts (loess) or arid alluvial deposits with a loose, honeycomb structure held together by water-soluble cements or clay bridges. When saturated, the cement dissolves, causing rapid, massive volume collapse without any increase in load.
- Dispersive Soils: Clays with a high percentage of exchangeable sodium ions. In the presence of slow-moving water, the clay particles physically repel each other and wash away in suspension, leading to rapid internal erosion (piping) within earth dams.
Volcanic Hazards
The destructive forces associated with volcanic eruptions.
Volcanic Hazards Context
While civil engineers rarely design structures to withstand a direct volcanic eruption, understanding the secondary hazards is vital for regional planning and infrastructure resilience.
Common Volcanic Hazards
- Lava Flows: Streams of molten rock. While relatively slow-moving and predictable, they are unstoppable and permanently destroy any infrastructure in their path.
- Ashfall (Tephra): Fine, abrasive glass fragments ejected high into the atmosphere. Ashfall severely disrupts aviation, shorts out electrical grids, and when wet, becomes incredibly heavy—frequently causing the catastrophic collapse of wide-span roofs on warehouses and factories.
- Lahars: Rapid, catastrophic volcanic mudflows formed when pyroclastic material mixes with water (from rapid snowmelt, crater lakes, or heavy rain). They flow down river valleys with the consistency of wet concrete, burying towns and destroying bridges.
- Pyroclastic Flows: Superheated, high-density avalanches of toxic gas, ash, and rock that hug the ground and travel at speeds exceeding . They are 100% lethal and instantly incinerate everything.
Coastal Hazards
Geological threats specific to oceanic and coastal environments.
Coastal Threats
- Coastal Erosion: The permanent loss of land and retreat of the shoreline due to the continuous, powerful action of waves, longshore currents, and tides. It is violently accelerated during severe storms. Coastal erosion relentlessly undermines coastal highways, seawalls, and beachfront properties, requiring incredibly expensive, ongoing mitigation like beach nourishment or hard armoring (groins and breakwaters).
- Storm Surge: An abnormal, rapid rise in sea level generated by intense storm winds (hurricanes, typhoons) driving ocean water ashore. It causes devastating coastal flooding and massive scour around bridge piers.
- Tsunamis: A series of massive ocean waves typically generated by sudden displacement of the seafloor during megathrust subduction earthquakes, or massive underwater landslides. Tsunamis can cross entire oceans and cause total destruction of low-lying coastal infrastructure upon landfall.
Risk Assessment
Quantifying the threat of geohazards to human society.
The Risk Equation
The Risk Equation
Mathematical definition of risk as the combination of hazard probability and resulting consequences.
Variables
| Symbol | Description | Unit |
|---|---|---|
| The natural probability or frequency of the event occurring at a specific severity (e.g., a 1% annual chance of a magnitude 7.0 earthquake). | - | |
| The susceptibility of the built environment to damage if the hazard occurs (e.g., unreinforced masonry buildings have high seismic vulnerability; modern base-isolated hospitals have low vulnerability). | - | |
| The inventory of elements at risk in the hazard zone (e.g., the number of people, the financial value of the buildings, critical infrastructure). | - |
Mitigation Philosophy
Engineers cannot stop the Hazard (we cannot prevent earthquakes or hurricanes). Therefore, engineering mitigation must focus entirely on reducing Vulnerability (better structural design) and reducing Exposure (zoning laws keeping people out of floodplains).
Mitigation Strategies
Engineering and planning solutions to reduce geohazard risks.
Key Mitigation Strategies
- Land-Use Zoning: The most effective mitigation. Strictly prohibiting or limiting new construction in known high-risk zones (active fault traces, 100-year floodplains, steep landslide-prone slopes).
- Geotechnical Engineering: Ground improvement techniques to prevent liquefaction (e.g., deep soil mixing, vibro-compaction), installing deep pile foundations to bypass collapsible soils, and constructing massive retaining walls or rock bolts to stabilize slopes.
- Structural Engineering: Designing ductile steel or reinforced concrete frames that can flex during earthquakes without collapsing, and elevating coastal structures above predicted storm surge elevations on robust pilings.
- Monitoring and Warning Systems: Deploying networks of seismometers, GPS receivers to measure tectonic strain, slope inclinometers to detect early landslide movement, and ocean buoys (DART system) to detect tsunamis, providing critical minutes or hours for public evacuation.
- Geohazards encompass a wide range of destructive earth processes beyond earthquakes, including subsidence, volcanism, and coastal extremes.
- Subsidence is often an irreversible, human-induced hazard caused by over-pumping groundwater from compressible clay aquitards. Sinkholes present sudden, localized subsidence risks in karst terrain.
- Liquefaction turns solid, saturated, sandy soils into a liquid slurry during earthquakes, causing massive foundation failures.
- Risk is defined as . Engineers mitigate risk by reducing vulnerability (stronger designs) and exposure (smarter zoning).
- Because natural hazards cannot be stopped, the absolute most effective mitigation strategy is Avoidance via proper Land-Use Zoning.
- Predicting calculating rates of subsidence allows engineers to accurately project structural deformations over a facility's entire design life.