Surface Processes

Learning Objectives

  • Distinguish weathering in place from erosion, transport and deposition.
  • Interpret soil horizons and depositional history without assuming engineering properties from names.
  • Recognize fall, slide, flow and creep mechanisms and investigate their possible triggers.
  • Calculate effective shear resistance and distinguish force from stress.
  • Explore infinite-slope geometry, basal pore pressure, depth sensitivity and zero-demand states.
  • Apply a consistent force-ratio convention to a horizontally anchored planar block and interpret scour-related bed changes.

Weathering

Physical disaggregation or chemical alteration of rock and minerals at or near the surface, in place.

Mechanical and chemical changes can act together

Mechanical weathering fragments material without requiring a change in mineral composition. Joint opening, unloading-related fractures, thermal stresses, roots and frost action can contribute under appropriate conditions. Frost-related fracture growth needs relevant temperature, water and confinement conditions; there is no universal pressure or daily rockfall schedule.

Chemical weathering changes minerals or removes dissolved constituents. Dissolution affects soluble minerals; oxidation changes susceptible iron-bearing minerals; hydrolysis can alter silicates such as feldspar and form secondary minerals. Clay products are not universally expansive: kaolinite and smectite have different structures and swelling behavior. Surface staining is evidence to investigate, not a strength test.

Climate, mineralogy, fractures, fluid pathways, topography, organisms and time interact. Warm, wet conditions often favor chemical alteration, but a climate label alone does not determine a weathering rate or engineering grade.

Physical fragmentation preserves mineral identity while chemical alteration changes minerals and can release dissolved constituents.

Original conceptual comparison informed by USGS weathering and clay studies. No measured rate, strength reduction or universal expansive-clay product is implied.

Physical fragmentation preserves mineral identity while chemical alteration changes minerals and can release dissolved constituents.

Residual soil and soil horizon

Residual soil forms from weathering of underlying material in place. A soil horizon is a layer distinguished by soil-forming features; its name is not an engineering strength classification.

Read a profile without inventing a bearing stratum

USDA master symbols include O for organic material, A for surface mineral soil commonly enriched in organic matter, E for eluviation, B for subsurface alteration or accumulation, C for relatively little-altered parent material and R for hard bedrock. Not every profile contains every horizon, and buried or repeated horizons can occur.

Engineering descriptions also record grading, fabric, structure, weathering, moisture and test results. An organic-rich layer commonly needs special handling, but a B horizon is not automatically suitable for shallow foundations. Residual profiles can retain relict rock structure; material and water conditions may vary abruptly above and within the weathering front.

An idealized O A E B C R sequence distinguishes soil-forming horizons from hard bedrock without assigning foundation capacity.

Original schematic based on USDA NRCS soil-profile guidance. Thicknesses are conceptual, and the full sequence is not present at every site. No layer is designated an approved bearing horizon.

An idealized O A E B C R sequence distinguishes soil-forming horizons from hard bedrock without assigning foundation capacity.

Erosion, transport and deposition

Erosion removes material from its location; transport moves it, and deposition leaves it where transport conditions permit accumulation.

Depositional history suggests questions, not fixed properties

  • Alluvium: deposits from flowing water may contain channel, bar and floodplain materials with lateral and vertical changes. Layering and lenses can affect drainage and settlement.
  • Aeolian deposits: wind deposits include silt-rich loess and sand dunes. An open loess fabric may collapse on wetting under load; origin alone does not prove collapse or its magnitude.
  • Glacial deposits: till is deposited directly by ice and is commonly poorly sorted. Meltwater outwash can be sorted and stratified. Density and stress history must be established rather than assigned to every glacial deposit.
  • Colluvium: slope-derived material moved mainly by gravity or local slope processes can be heterogeneous and angular. Its current stability depends on geometry, fabric, water and loading; not all colluvium is actively unstable.

Water, wind, ice and gravity operate at different sites and times. Transport history informs the ground model but does not uniquely fix bearing capacity, compressibility or liquefaction susceptibility.

Four schematic deposits contrast water layering, wind-blown silt, mixed till with stratified outwash, and angular slope-derived colluvium.

Original schematic informed by FHWA geotechnical site characterization. Grain symbols are conceptual, not a grading curve, density measurement or stability classification.

Four schematic deposits contrast water layering, wind-blown silt, mixed till with stratified outwash, and angular slope-derived colluvium.

Mass wasting

Gravity-driven downslope movement of rock, debris or earth, classified by material and movement mechanism.

Movement modes and observations

Falls detach and travel by falling, bouncing or rolling. Translational slides move along approximately planar surfaces; rotational slides involve curved surfaces. Flows deform internally, while creep describes slow movement. Real events can combine or change modes. Complete saturation is not a universal prerequisite for all flows.

Scarps, displaced drainage, tilted objects, deposits and survey records guide interpretation. A leaning pole or curved tree is a possible movement indicator requiring corroboration, not a unique diagnosis. Mechanisms and controlling discontinuities matter more than dramatic appearance.

Sections compare detached falling blocks, a planar translation, curved rotational movement and internally deforming flow.

Original conceptual sections following the USGS Landslide Handbook. Arrows describe movement type, not computed displacement, speed or a forecast.

Sections compare detached falling blocks, a planar translation, curved rotational movement and internally deforming flow.

Triggers, resistance and investigation

Rain infiltration may raise pore pressure, reduce suction and add weight. River erosion or excavation can remove toe support; loading can add demand. Earthquakes impose transient loading, and vegetation changes can affect root reinforcement and moisture balance. A single trigger does not identify every controlling mechanism.

Select measures after mapping geometry, material and water conditions. Drainage, regrading, support, catchment systems, erosion control or avoidance can address different mechanisms. Horizontal drains do not guarantee that all joint water disappears; meshes and riprap require geometry, capacity, maintenance and residual-risk assessment. Monitoring should have defined observations, thresholds and accountable responses.

Effective-stress shear resistance and factor of safety

Available shear resistance depends on the selected strength model. Here factor of safety compares available resistance with driving demand using compatible stresses or forces and one declared convention.

Effective-stress resistance on a plane

Mohr–Coulomb strength for a compressive effective normal stress and an appropriate effective-stress parameter set.

σn′=σn−u,τf=c′+σn′tan⁡ϕ′,FS=τf/τd\sigma'_{\mathrm{n}}=\sigma_{\mathrm{n}}-u,\qquad \tau_{\mathrm{f}}=c'+\sigma'_{\mathrm{n}}\tan\phi',\qquad FS=\tau_{\mathrm{f}}/\tau_{\mathrm{d}}

Variables

SymbolDescriptionUnit
σn,u,σn′\sigma_{\mathrm{n}},u,\sigma'_{\mathrm{n}}Total normal stress, gauge pore pressure and effective normal stress on the same plane, kPa.-
c′,ϕ′c',\phi'Effective cohesion intercept, kPa, and effective friction angle; select the appropriate peak/residual or other design strength basis.-
τf,τd\tau_{\mathrm{f}},\tau_{\mathrm{d}}Available shear resistance and driving shear stress, kPa. Multiply both by the same area to compare forces in kN.-

Uniform infinite-slope model

Static, uniform layer with a potential surface parallel to the ground. Depth z is vertical; pore pressure on the plane is prescribed.

σn=γzcos⁡2β,τd=γzsin⁡βcos⁡β,FS=c′+(γzcos⁡2β−u)tan⁡ϕ′γzsin⁡βcos⁡β\sigma_{\mathrm{n}}=\gamma z\cos^2\beta,\qquad \tau_{\mathrm{d}}=\gamma z\sin\beta\cos\beta,\qquad FS=\frac{c'+(\gamma z\cos^2\beta-u)\tan\phi'}{\gamma z\sin\beta\cos\beta}

Variables

SymbolDescriptionUnit
γ,z,β\gamma,z,\betaBulk unit weight, kN/m³; vertical layer depth, m; ground and failure-plane inclination above horizontal.-
rnr_{\mathrm{n}}Simulator pressure ratio u/σn from zero to one, so effective normal stress is σn(1−rn). It is not the hazards lesson's excess-pressure ratio.-

Explore geometry and stress balance

Vary angle, depth and basal pressure separately. Compare the uniformly scaled section with the common-scale stress bars. Test zero angle and zero strength: no gravity shear demand means the ratio is undefined, and zero divided by zero must not be reported as infinity.

Infinite-slope stress balance

Concept and model scope

Explore how inclination, vertical layer depth, effective strength and prescribed basal pore pressure change the ratio of available resistance to gravity shear demand.

Model and limits: A uniform layer with a planar surface parallel to the ground; vertical depth z, positive compression, static gravity and effective-stress Mohr–Coulomb strength. Basal pressure ratio rₙ = u/σₙ is prescribed independently, not a water-table depth or a rainfall forecast. No finite-slope, seismic, progressive-failure or displacement calculation is performed. USACE EM 1110-2-1902 provides slope-analysis context; project criteria must be selected separately.

Controls
30 °

Slope angle

β is measured above horizontal. The failure plane is parallel to this ground surface. At zero angle, gravitational shear demand is zero and a finite resistance/demand ratio cannot be evaluated.

Range: 0–60 °. Step: 1 °.

25 °

Effective friction angle

φ′ controls the frictional part of available shear strength through tan φ′. It is a representative effective-stress parameter for the potential sliding material or surface, not automatically the visible ground angle.

Range: 0–45 °. Step: 1 °.

10 kPa

Effective cohesion

c′ is the intercept of the selected effective-stress strength envelope. It contributes stress resistance independently of friction in this idealization; its use requires representative test evidence and an appropriate strength basis.

Range: 0–50 kPa. Step: 1 kPa.

20 kN/m³

Soil unit weight

γ is weight per volume, not mass density. It determines both gravity normal stress and shear demand. Holding γ fixed while changing pore pressure isolates one effect; actual wetting may also alter weight and strength.

Range: 10–30 kN/m³. Step: 1 kN/m³.

5 m

Vertical layer depth

z is the vertical separation of parallel ground and failure surfaces. Normal separation is z cos β. The section uses one scale in both directions; this depth changes its physical thickness.

Range: 1–20 m. Step: 1 m.

0.00

Basal pressure ratio

rₙ = u/σₙ divides total gauge pore pressure by total normal stress on the plane. This differs from the excess-pressure ratio used in the hazards lesson. Zero means u = 0; one means zero effective normal stress. Ratios above one and tensile separation lie outside this model.

Range: 0–1 . Step: 0.05 .

Parallel ground and failure surfacesThe 20 metre horizontal window is an arbitrary view of an infinite layer. Vertical depth is 5 metres. One scale is used for both coordinates. The dashed lower boundary is the potential sliding surface; no movement is predicted.Parallel layer sectionz = 5 m
β = 30°; 20 m horizontal window, vertical depth z. One scale for both coordinates; dashed: potential sliding surface.
Total normal stress
75.00 kPa
Basal pore pressure
0.00 kPa
Effective normal stress
75.00 kPa
Resistance / demand
1.039
Available resistance and driving stress on the same scaleAvailable resistance: 44.97 kPaDriving stress: 43.30 kPa0200400600kPa

Available resistance exceeds driving stress in this model. Site design also needs applicable criteria, representative strength, pore pressures and loading; this ratio does not predict failure time.

Interpret a model ratio without certifying a site

For positive demand, a ratio below one indicates available resistance below demand in the selected model; one is limiting balance; above one means calculated resistance exceeds demand. None of these statements predicts the failure time. A target depends on the governing method, loading condition, consequence, parameter basis and applicable criteria. A universal 1.5 label cannot certify every slope.

For a cohesionless layer with zero pore pressure and nonzero inclination, the ratio reduces to tan⁡ϕ′/tan⁡β\tan\phi'/\tan\beta and is independent of depth and unit weight. With nonzero cohesion, its contribution scales inversely with γz\gamma z; changing depth can then change the ratio. Infinite-slope analysis does not represent a finite cut, a three-dimensional wedge, seismic inertia or progressive failure.

Horizontally anchored planar block: declared global force ratio

A two-dimensional block on one plane, no uplift or cohesion, and a given horizontal anchor force directed into the slope. This is a force-bookkeeping example, not an anchor design method.

N=Wcos⁡β+Tsin⁡β,FSglobal=Ntan⁡ϕ′+Tcos⁡βWsin⁡βN=W\cos\beta+T\sin\beta,\qquad FS_{\mathrm{global}}=\frac{N\tan\phi'+T\cos\beta}{W\sin\beta}

Variables

SymbolDescriptionUnit
W,T,NW,T,NBlock weight, given horizontal anchor force and compressive normal force, kN.-
β,ϕ′\beta,\phi'Plane inclination above horizontal and plane friction angle. The horizontal anchor contributes both upslope resistance and clamping.-

Do not switch the factor-of-safety denominator

Subtracting anchor resistance from gravity demand and then dividing friction by the remainder gives a different ratio. Do not compare that result directly with the global resisting/driving ratio above. The anchor direction, installed force, capacity, bond, corrosion, load path and water conditions require separate design checks; a force ratio alone does not prove stabilization.

Fluvial erosion and bridge scour

Flow, sediment supply and bed resistance interact; erosion is not simply proportional to water velocity. A pier can create downflow and vortices that promote local sediment removal. Long-term bed degradation and contraction scour are different contributors. A depression near a failed pier is evidence to investigate, not proof of the complete failure sequence.

For a stated compatible scenario, bed lowering can be assembled from separately defined components without double counting. If the foundation does not move, remaining embedment below the new bed equals initial embedment minus total lowering. Capacity depends on the exposed foundation and remaining ground, not embedment alone. Follow applicable scour evaluation and countermeasure procedures; riprap is not a universal guarantee against scour.

A pier interrupts flow above a locally lowered bed; a dashed reference bed distinguishes scour from foundation movement.

Original schematic informed by FHWA HEC-18, fifth edition. Flow arrows are conceptual; no rate, maximum scour depth or collapse outcome is predicted. Countermeasures are deliberately not prescribed without design evidence.

A pier interrupts flow above a locally lowered bed; a dashed reference bed distinguishes scour from foundation movement.
Key Takeaways
  • Weathering changes material in place; erosion and transport move it, and deposition records the transport history.
  • Horizon and deposit names guide investigation but do not assign engineering properties.
  • Distinguish movement mechanisms, triggers and observations before selecting mitigation.
  • Keep effective and total stress, vertical and normal depth, and force and stress separate.
  • Report zero demand explicitly and use consistent resistance/demand conventions for supported slopes.
  • A calculated ratio or remaining embedment is a limited model result; site decisions require representative evidence and applicable criteria.

References