8.2 Geologic hazards (landslides, subsidence, expansive/liquefiable soils)

Geologic Hazards

Geologic hazards are the natural processes - slope failure, ground subsidence, and problem soils - that threaten structures and life. Recognizing and characterizing them is central to the engineering geologist's work and is heavily tested on the PG exam.

Slope stability and landslides

A slope is stable when the forces resisting movement exceed those driving it. The factor of safety (FS) is:

FS = resisting forces (or moments) / driving forces (or moments)

The driving force is the downslope component of gravity acting on the slope mass; the resisting force is the shear strength (Mohr-Coulomb cohesion plus frictional strength) mobilized along the potential failure surface. When FS > 1 the slope is stable, at FS = 1 it is at limiting equilibrium (incipient failure), and FS < 1 means failure. Conditions that lower FS include rising groundwater and pore pressure (heavy rainfall or snowmelt), undercutting of the toe (by streams, waves, or excavation), adding load at the crest, weathering that reduces strength, and earthquake shaking that adds cyclic driving force.

Types of mass movement

The Varnes classification groups landslides by material and type of movement:

• Falls - free fall of rock or soil from steep faces.
• Topples - forward rotation of blocks about a pivot point.
• Slides - movement along a discrete surface; rotational slides (slumps) fail on a curved surface, while translational slides move on a planar surface (bedding, joint, or fault).
• Spreads - lateral extension, often over a liquefied layer.
• Flows - fluid-like movement, including debris flows, earthflows, and slow creep.

Movement rate ranges from imperceptible creep to catastrophic rockfall, and a single failure may transform from a slide into a flow as it moves downslope.

Reducing landslide risk

Slopes are stabilized by attacking the FS ratio directly: lowering pore pressure with horizontal drains, dewatering wells, and surface-runoff control; removing driving weight by unloading the crest or flattening (regrading) the slope; and adding resistance with buttress fills, retaining walls, soil nails, tiebacks, or rock bolts. Because water is the most common trigger, drainage is usually the first and most cost-effective measure. Recognizing ancient landslide deposits - hummocky ground, tilted "pistol-butted" trees, and arcuate head scarps - is essential, because reactivating an old slide is far easier than initiating a new one.

Subsidence and collapse

Subsidence is the downward settling of the ground surface. Major causes include:

• Karst dissolution and sinkholes: groundwater dissolves soluble carbonate (limestone, dolomite) or evaporite (gypsum, salt) bedrock, creating voids. Cover-collapse sinkholes form suddenly where overlying soil ravels into a cavity; cover-subsidence sinkholes develop gradually.
• Fluid withdrawal: pumping groundwater, oil, or gas lowers pore pressure, raises effective stress, and compacts aquifer and clay layers - as at the San Joaquin Valley, Houston, and Mexico City.
• Mining, thawing permafrost, and oxidation of organic (peat) soils also produce subsidence.

Expansive (shrink-swell) soils

Expansive soils change volume with moisture. They are rich in smectite (montmorillonite) clay minerals, whose expanding lattice adsorbs water between layers, so the soil swells when wetted and shrinks when dried. These soils have a high plasticity index and high activity. Seasonal wetting and drying heaves and cracks slab-on-grade floors, foundations, sidewalks, and pavements; expansive soils cause billions of dollars of damage annually in the United States - more than floods, earthquakes, and tornadoes combined. Mitigation includes moisture control, deep or drilled-pier foundations that reach below the active (seasonal) zone, and lime treatment.

Liquefiable soils

Liquefaction occurs when a saturated, loose, cohesionless soil (clean sand or non-plastic silt) is subjected to rapid cyclic loading - almost always earthquake shaking. The grains momentarily tend to densify, but because water cannot drain quickly enough, pore-water pressure rises, effective stress (sigma' = sigma - u) drops toward zero, and the soil temporarily loses its shear strength and behaves like a heavy liquid. Consequences include sand boils, lateral spreading, flow failures, loss of bearing capacity (buildings tilt or sink), and buoyant uplift of buried tanks and pipelines. Susceptibility is highest in young (Holocene), loose, saturated sands with a shallow water table and low fines content; dense sands and plastic clays are largely immune.

Collapsible (hydrocompactive) soils

Collapsible soils are dry, low-density, metastable deposits - commonly loess (wind-blown silt) and some alluvial-fan and debris-flow sediments - held together by weak clay or carbonate bonds. Under load they seem competent while dry, but wetting destroys the bonds and the structure collapses, producing sudden settlement (hydrocompaction). This is the opposite of expansive behavior: expansive soils swell when wetted, whereas collapsible soils settle.

Comparing the problem soils

Recognizing which hazard a site poses - and mapping its extent - drives the investigation and foundation decisions covered in the next section.