3.1 Soil Formation, Horizons, and Geomorphology

Geomorphology and site landforms

Geomorphology studies landforms and the processes that shape them. Inspectors use it to recognize where runoff naturally converges, where sediment accumulates, and where an apparently stable grading surface may overlie a vulnerable deposit. A ridge divides catchments; a valley concentrates drainage; a floodplain receives overbank deposition; a terrace records an older stream level; an alluvial fan spreads sediment where a steep channel emerges onto a flatter surface. Cutting a ditch across a fan can reconnect concentrated runoff to an area previously receiving dispersed flow.

Processes matter as much as names. Stream incision lowers a channel bed and can undermine banks or propagate a headcut upstream. Lateral migration erodes an outside bend and deposits sediment on an inside bar. Gravity carries colluvium downslope; such deposits may be loose and poorly sorted. Wind deposits fine loess that can erode rapidly after vegetation is removed. Karst dissolution can create sinks and hidden groundwater connections. Glacial till may change texture abruptly over short distances, so one soil sample need not represent a whole slope.

A practical inspection compares the existing-condition contours and soil report with actual cuts, fills, seeps, and drainage. Suppose a contractor diverts a hillside swale onto an old stream terrace. The terrace’s flat appearance does not prove that the new concentrated flow is safe. Record the discharge location, freshly eroding bank, sediment trail, and downstream receptor; compare them with the approved outlet protection and refer the changed drainage arrangement for authorized design review. A geomorphic observation explains a potential failure mechanism without pretending that the inspector has completed geotechnical or hydraulic design.

Why Soil Mechanics and Soil Science Matter to the CESSWI Inspector

Every erosion feature, failed perimeter barrier, and turbid basin discharge on an active construction site is governed by the physical, chemical, and mechanical behavior of disturbed soil. Under Specific Area of Practice (SAOP) 2 (Soils Mechanics and Soil Science Principles) of the CESSWI body of knowledge, an inspector must evaluate how site grading alters native soil profiles, identify high-erodibility textures in the field, interpret soil test reports and the United States Department of Agriculture (USDA) textural triangle, and verify that erosion and sediment controls match the detachment and settling behavior of the exposed soil separates.

Pedogenesis: Jenny's Five Factors of Soil Formation (CLORPT)

Natural soils develop over centuries to millennia through pedogenesis—the combined physical disintegration, chemical weathering, biological enrichment, and vertical translocation of mineral and organic matter. Soil scientist Hans Jenny formulated the classic state-factor equation known by the acronym CLORPT:

1. Climate ($cl$): Precipitation (annual depth, storm intensity, and seasonal distribution) and temperature control the rate of chemical mineral weathering, leaching of soluble salts and carbonates, freeze-thaw physical shattering, and plant growth.
2. Organisms ($o$): Native plant canopies, fibrous root systems, rhizosphere bacteria, mycorrhizal fungi, and burrowing soil fauna contribute organic detritus, secrete biological binding glues (such as glomalin and polysaccharides) that cement mineral grains into water-stable aggregates, and open continuous biopores for infiltration.
3. Relief / Topography ($r$): Slope steepness, slope length, concavity/convexity, and slope aspect regulate microclimate and drainage. In the Northern Hemisphere, south- and west-facing slopes receive higher direct solar radiation, experiencing warmer soil temperatures, higher evapotranspiration, lower soil moisture, thinner topsoil, and slower vegetation establishment than cooler, moister north- and east-facing slopes.
4. Parent Material ($p$): The geological deposit from which the soil profile develops:
   • Residual soil: Weathered in place directly from underlying bedrock.
   • Alluvial deposits: Water-sorted sands, silts, and gravels deposited along stream floodplains, terraces, and alluvial fans.
   • Colluvial deposits: Poorly sorted soil and rock fragments transported downslope by gravity (talus and footslope deposits), which are frequently unstable when undercut during grading.
   • Aeolian deposits: Wind-deposited sediments, including sand dunes and loess (deep deposits of windblown silt). Cut slopes in loess are notoriously susceptible to vertical gullying and piping when exposed to concentrated runoff.
   • Glacial and Lacustrine deposits: Glacial till (unsorted), glacial outwash (stratified sand/gravel), and lacustrine sediments (varved fine silts and high-plasticity clays settled in ancient lakebeds).
5. Time ($t$): The chronological duration over which soil-forming processes act. Forming $1\text{ inch}$ of mature, fertile topsoil naturally requires $100\text{ to }1,000\text{ years}$, whereas a scraper pan removes it in seconds.

Master Soil Horizons (O, A, E, B, C, R) and Construction Grading Impacts

A vertical cross-section of undisturbed soil—a soil profile—exhibits distinct horizontal layers called soil horizons:

• O Horizon (Organic Layer): Surface accumulation of undecomposed and partially decomposed plant litter, leaves, needles, and humus. Intercepts raindrop impact energy and acts like a sponge.
• A Horizon (Topsoil): The uppermost mineral horizon darkened by humified organic matter (typically $2\%\text{ to }8\%$ organic matter in mineral soils). Contains peak biological activity, crumbly granular structure, low bulk density, balanced plant nutrients, and high infiltration capacity.
• E Horizon (Zone of Eluviation): A light-colored, leached mineral horizon found beneath the A horizon in older forest soils where clay, iron, aluminum, and organic colloids have washed downward, leaving concentrated silt and fine sand.
• B Horizon (Subsoil / Zone of Illuviation): The dense subsurface mineral horizon where silicate clays, iron and aluminum oxides, or calcium carbonate accumulate from above. Characterized by blocky, prismatic, or massive structure, minimal organic matter ($< 1\%$), higher clay content or cemented hardpans, elevated bulk density, and much lower permeability.
• C Horizon (Substratum / Parent Material): Partially weathered, unconsolidated geologic material below the biological root zone that lacks pedogenic structure.
• R Horizon (Bedrock): Continuous, unweathered consolidated rock requiring heavy ripping or blasting during excavation.