3.2 Soil Texture, Mechanics, and Revegetation

Free CESSWI Certified Erosion, Sediment and Stormwater Inspector Study Guide 2026 Exam

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1 Chapter 1: CESSWI Program Overview, Scope of Practice & Professional Ethics

1.1 CESSWI Scope of Practice, Eligibility, and Exam Structure 1.2 Certification Renewal, PDH Requirements, and Audit Policies 1.3 EnviroCert Code of Ethics and Professional Conduct

2 Chapter 2: Federal, State, and Local Stormwater Regulations

2.1 Federal Water-Pollution History and Stormwater Phases 2.2 CWA Sections, Water Quality, and Related Federal Programs 2.3 EPA Construction Permit Coverage and Operational Clocks 2.4 Industrial MSGP and Municipal MS4 Programs 2.5 Regulatory Compliance, Enforcement Mechanisms, and Penalties

3 Chapter 3: Soil Mechanics, Erosion Processes, and Climatic Factors

3.1 Soil Formation, Horizons, and Geomorphology 3.2 Soil Texture, Mechanics, and Revegetation 3.3 Splash, Sheet, Rill, Gully, Channel Erosion, and Slope Movement 3.4 Wind Erosion, Sediment Transport, Climatic Erosivity, and Resource Impacts

4 Chapter 4: Hydrology, Runoff Characteristics, and Drainage Principles

4.1 The Hydrologic Cycle, Watershed Drainage Planning, and Precipitation 4.2 Hydrologic Soil Groups, Disturbed Profiles, and Time Parameters 4.3 Hydrograph Characteristics, Curve Numbers, and Runoff Estimation Equations

5 Chapter 5: Inspection Fundamentals, Duties, and Field Procedures

5.1 Inspector Roles, Scope of Authority, and Core Professional Skills 5.2 Pre-Inspection Document Review and Inspector Support Equipment 5.3 Systematic Field Inspection Sequence, Post-Inspection Meetings, and Follow-Through

6 Chapter 6: Inspection Elements for Active Construction Sites

6.1 Pre-Construction Meetings, Entrance Posting, and On-Site Document Inspections 6.2 Inspecting Active Construction Controls and Diagnosing BMP Failure Modes 6.3 Final Stabilization Verification, Final Inspections, and Notice of Termination

7 Chapter 7: Stormwater Monitoring, Field Sampling, and Chain of Custody

7.1 Permit Monitoring Triggers and Water-Quality Indicators 7.2 Sampling Preparation and Analytical Parameters 7.3 Sampling Locations, Collection Methods, and Field Meter Calibration 7.4 Sample Preservation, Data Recording, Shipping, and Chain of Custody

8 Chapter 8: Inspection Documentation, Reporting, and Map Management

8.1 Inspection Notes and Evidence Procedures 8.2 Objective Findings and Photographic Evidence 8.3 Proper Reporting Techniques and Corrective Action Log Management 8.4 Site Drawing (Map) Management, Compliance Resolution, and Enforcement Records

9 Chapter 9: Professional Communication and Site Safety for Inspectors

9.1 Communication Protocols in Pre-Construction, Post-Inspection, and O&M Meetings 9.2 Dealing with Difficult People, Problematic Sites, and Unpleasant Findings 9.3 Site Hazard Recognition: Physical, Biological, Trenching, and Confined Spaces 9.4 Hazardous Substances, Safety Data Sheets (SDS), Spill Response, and PPE

10 Chapter 10: Stormwater Plan Reading, Map Interpretation, and Site Calculations

10.1 Stormwater Plan Types, Credentials, and Authority 10.2 SWPPP Components and Permit Notifications 10.3 Civil Plan Sets, Legends, Phased Sequencing, and Living Site Maps 10.4 Topography, Contour Lines, Slope Calculations, and Plan Amendments

11 Chapter 11: BMP Framework, Site Planning, and Erosion Control Practices

11.1 Six Functional BMP Categories, Preservation of Vegetation, Phasing, and Tree Protection 11.2 Seeding and Sod Establishment 11.3 Hydroseeding and Hydromulching Inspection 11.4 Mulch and Rolled Erosion-Control Products 11.5 Tackifiers, Soil Binders, Roughening, and Temporary Cover

12 Chapter 12: Perimeter, Linear, and In-Water Sediment Control Practices

12.1 Perimeter & Sheet-Flow Sediment Barriers: Silt Fence, Fiber Rolls, Straw Bales, and Filter Dikes 12.2 Access & Roadside Sediment/Runoff Controls: Stabilized Entrances, Cut-Back Curbs, and Ditch Checks 12.3 In-Water & Dewatering Controls: Temporary Stream Crossings, Turbidity Curtains, and Sediment Bags

13 Chapter 13: Sediment Basins, Traps, and Runoff Conveyance Controls

13.1 Sediment Basins and Traps 13.2 Porous Baffles and Surface Skimmers 13.3 Runoff Conveyances: Diversions, Swales, Pipe Slope Drains, and Rock Check Dams 13.4 Riprap and Gabion Inspection 13.5 Retaining Walls, Outlet Protection, and Wet Ponds

14 Chapter 14: Storm Drain Inlet Protection and Good Housekeeping BMPs

14.1 Storm Drain Inlet Protection: At-Grade, Curb, Block & Gravel, Excavated, Fabric Drop, Gravel Bag, Stone Collar, and Pre-Manufactured 14.2 Good Housekeeping Part 1: Concrete Washout, Equipment Maintenance, Materials Management, and Secondary Containment 14.3 Good Housekeeping Part 2: Stockpiles, Dust Control, Wind Fence, Street Cleaning, Sanitary Waste, and Trash

15 Chapter 15: Post-Construction Stormwater Management BMPs

15.1 Vegetative & Structural Dispersion BMPs: Final Ground Cover, Headwalls, Level Spreaders, Filter Strips, and Riparian Buffers 15.2 Permeable Surfaces and Construction Protection 15.3 Infiltration, Bioretention, and Media Filters 15.4 Detention, Retention & Proprietary BMPs: Dry/Wet Basins, Wetlands, Inline Storage, Catch Basin Inserts, Vaults, and Manufactured Devices

3.2 Soil Texture, Mechanics, and Revegetation

Key Takeaways Explain This Topic AI

USDA sand, silt, and clay percentages describe the fine-earth fraction and sum to 100%.

Particle size, cohesion, aggregation, and flow conditions affect detachment and settling.

Soil tests and compaction observations guide revegetation review under the approved specification.

Last updated: October 2026

Soil Particle Sizes, Mechanical Properties, and Settling Velocity

Under the USDA textural classification system , only the fine-earth mineral fraction passing a 2.0 mm 2.0\text{ mm} 2.0 mm sieve is used to determine soil texture. Particles larger than 2.0 mm 2.0\text{ mm} 2.0 mm —including gravel ( 2.0 to 75 mm 2.0\text{ to }75\text{ mm} 2.0 to 75 mm ), cobbles ( 75 to 250 mm 75\text{ to }250\text{ mm} 75 to 250 mm ), and stones/boulders ( > 250 mm > 250\text{ mm} > 250 mm )—are classified as coarse rock fragments that modify the texture name (e.g., gravelly sandy loam ) and help armor the soil surface against raindrop splash.

The three USDA fine-earth mineral separates are:

Sand ( 0.05 to 2.0 mm 0.05\text{ to }2.0\text{ mm} 0.05 to 2.0 mm ): Rounded to angular quartz and primary mineral grains visible to the naked eye; feels gritty between the fingers. Because sand grains are relatively large, they pack with large interconnected macropores , providing high permeability and rapid infiltration ( > 0.8 to 6.0 + in/hr > 0.8\text{ to }6.0+\text{ in/hr} > 0.8 to 6.0 + in/hr ) but very low specific surface area, low water-holding capacity, and zero cohesion or plasticity .

Silt ( 0.002 to 0.05 mm 0.002\text{ to }0.05\text{ mm} 0.002 to 0.05 mm ): Microscopic mineral particles that feel smooth and floury when dry and silky (like talcum powder, not sticky) when wet. Silt pores are predominantly medium-sized mesopores that hold abundant plant-available capillary water, but silt grains are predominantly quartz-like fragments that possess little to no electro-chemical cohesion or plasticity .

Clay ( < 0.002 mm < 0.002\text{ mm} < 0.002 mm , or < 2 μ m < 2\text{ }\mu\text{m} < 2 μ m ): Microscopic, plate-like crystalline secondary aluminosilicate minerals (e.g., kaolinite, illite, and swelling smectite/montmorillonite). Clay feels sticky when wet, rolls into long flexible ribbons between the thumb and forefinger, and bakes into hard clods when dry. Clay has an enormous specific surface area (hundreds of square meters per gram) and net negative electrical charges that attract nutrient cations ( Cation Exchange Capacity, CEC ) and water films, creating strong cohesion and plasticity but microscopic micropores with very slow infiltration ( < 0.05 to 0.15 in/hr < 0.05\text{ to }0.15\text{ in/hr} < 0.05 to 0.15 in/hr ).

Atterberg Limits, Compaction, and Stokes' Law

Cohesion and Atterberg Limits: In soil mechanics, fine-grained soils transition from solid to semi-solid to plastic to liquid states as moisture content rises. The Plastic Limit ( P L PL P L ) is the moisture content at which a soil thread begins to crumble at 1 / 8 -inch 1/8\text{-inch} 1/8 -inch diameter, the Liquid Limit ( L L LL LL ) is the moisture content at which soil flows under standard mechanical jarring, and the Plasticity Index ( P I = L L − P L PI = LL - PL P I = LL − P L ) measures the moisture range over which the soil behaves plastically. Clays have high P I PI P I values and strong inter-particle cohesion; sands and clean silts are non-plastic ( P I ≈ 0 PI \approx 0 P I ≈ 0 ).

Compaction and Bulk Density: Bulk density is the dry weight of soil per unit total volume ( g/cm 3 \text{g/cm}^3 g/cm 3 ). Undisturbed topsoil typically has a bulk density of 1.1 to 1.4 g/cm 3 1.1\text{ to }1.4\text{ g/cm}^3 1.1 to 1.4 g/cm 3 with roughly 50 % 50\% 50% pore space. Structural compaction by sheepsfoot rollers and heavy haul trucks crushes macropores, driving bulk density to 1.65 to 1.90 + g/cm 3 1.65\text{ to }1.90+\text{ g/cm}^3 1.65 to 1.90 + g/cm 3 . Compacted soil sheds rainfall almost like pavement and physically blocks seedling root penetration unless ripped or subsoiled prior to stabilization.

Stokes' Law and Settling Velocity: Once soil particles are detached and entrained in stormwater, their gravitational settling velocity ( v s v_s v s ​ ) in quiescent water is governed by Stokes' Law , which demonstrates that settling velocity is directly proportional to the square of the particle diameter ( d 2 d^2 d 2 ) : v s = g ( ρ s − ρ w ) d 2 18 μ v_s = \frac{g (\rho_s - \rho_w) d^2}{18 \mu} v s ​ = 18 μ g ( ρ s ​ − ρ w ​ ) d 2 ​

Because v s v_s v s ​ scales with d 2 d^2 d 2 , reducing particle diameter by a factor of 100 100 100 (from 0.2 mm 0.2\text{ mm} 0.2 mm fine sand to 0.002 mm 0.002\text{ mm} 0.002 mm clay) reduces settling velocity by a factor of 10 , 000 10,000 10 , 000 . Coarse and medium sands settle out in seconds at the toe of a slope or inside a sediment trap, medium silts require hours of hydraulic residence time in a baffled skimming basin, and colloidal clays ( < 0.002 mm < 0.002\text{ mm} < 0.002 mm ) stay suspended for days or weeks due to mutual negative charge repulsion and Brownian motion unless coagulated or flocculated (e.g., with anionic polyacrylamide, PAM).

Important

Why Silt and Very Fine Sand Are the Most Erodible Soil Separates: On the CESSWI exam, candidates are frequently asked which soil particle size is most erodible (highest soil erodibility factor K K K , often 0.40 to 0.65 0.40\text{ to }0.65 0.40 to 0.65 ). The answer is silt and very fine sand ( 0.002 to 0.10 mm 0.002\text{ to }0.10\text{ mm} 0.002 to 0.10 mm )— not coarse sand and not clay:

Coarse sand lacks cohesion and detaches easily, but its heavy individual grain mass resists overland flow transport and settles rapidly.

Clay stays in suspension indefinitely once detached, but its strong electro-chemical cohesion binds particles together so tightly that raindrops and sheet flow struggle to detach it initially.

Silt and very fine sand suffer from the worst of both worlds: they lack the electro-chemical cohesion of clay (so raindrops and thin sheet flow detach them effortlessly), and they lack the gravitational mass of coarse sand (so shallow runoff easily carries them offsite).

Soil Property / Behavior Sand ( 0.05 to 2.0 mm 0.05\text{ to }2.0\text{ mm} 0.05 to 2.0 mm ) Silt ( 0.002 to 0.05 mm 0.002\text{ to }0.05\text{ mm} 0.002 to 0.05 mm ) Clay ( < 0.002 mm < 0.002\text{ mm} < 0.002 mm ) Loam (Balanced Mixture)

Field Ribbon / Feel Test Gritty; will not form a cast or ribbon Floury dry, silky wet; weak flake, no true ribbon Sticky wet, hard dry; forms long flexible ribbon Friable cast; slightly gritty yet smooth

Specific Surface Area & Cohesion Very low; non-cohesive, non-plastic ( P I = 0 PI = 0 P I = 0 ) Low to moderate; negligible cohesion, non-plastic Very high; strong electro-chemical cohesion, high P I PI P I Moderate; forms water-stable granular aggregates

Detachment Resistance Low (individual grains loose) Very low (no cohesion to resist splash/shear) High (strong cohesive bonds resist detachment) Moderate to high when organic matter is present

Transportability in Runoff Low (heavy mass requires high velocity) High (lightweight grains easily entrained) Very high (colloidal suspension once detached) Moderate

Overall Soil Erodibility ( K K K ) Low ( K ≈ 0.02 to 0.10 K \approx 0.02\text{ to }0.10 K ≈ 0.02 to 0.10 ) Highest ( K ≈ 0.40 to 0.65 K \approx 0.40\text{ to }0.65 K ≈ 0.40 to 0.65 ) Low to Moderate ( K ≈ 0.10 to 0.25 K \approx 0.10\text{ to }0.25 K ≈ 0.10 to 0.25 ) Moderate ( K ≈ 0.25 to 0.38 K \approx 0.25\text{ to }0.38 K ≈ 0.25 to 0.38 )

Stokes' Law Settling Velocity Rapid (seconds; settles in small traps/wattles) Slow (minutes to hours; requires long basin detention) Extremely slow (days to weeks; requires flocculants/source cover) Mixed (sand settles fast; clay fraction stays turbid)

Infiltration & Water Holding High infiltration; droughty, low water holding Moderate infiltration (crusts easily); high plant water Very low infiltration (high runoff); high unavailable water Optimal infiltration and plant-available moisture

Reading the USDA Soil Textural Classification Triangle

The USDA Soil Textural Triangle partitions mineral soils into 12 textural classes based on the weight percentages of sand, silt, and clay, which must always total 100 % 100\% 100% : % Sand + % Silt + % Clay = 100 % \%\text{Sand} + \%\text{Silt} + \%\text{Clay} = 100\% % Sand + % Silt + % Clay = 100%

Step-by-Step Plotting Procedure and Worked Examples

Each side of the equilateral triangle serves as the zero-to-100 scale for one separate, and its grid lines angle parallel to the previous counter-clockwise axis:

Locate % Sand \%\text{Sand} % Sand along the bottom axis (reading from 0 % 0\% 0% on the right to 100 % 100\% 100% on the left) and trace the grid line diagonally up and to the left (parallel to the right-hand Silt axis).

Locate % Clay \%\text{Clay} % Clay along the left axis (reading from 0 % 0\% 0% at the bottom to 100 % 100\% 100% at the top) and trace the grid line horizontally to the right (parallel to the bottom Sand axis).

Locate % Silt \%\text{Silt} % Silt along the right axis (reading from 0 % 0\% 0% at the top to 100 % 100\% 100% at the bottom) and trace the grid line diagonally down and to the left (parallel to the left-hand Clay axis) to confirm the three lines intersect at a single point.

Worked Example 1 (Classic Loam): A geotechnical laboratory report shows a topsoil sample contains 40 % sand 40\%\text{ sand} 40% sand , 40 % silt 40\%\text{ silt} 40% silt , and 20 % clay 20\%\text{ clay} 20% clay ( 40 % + 40 % + 20 % = 100 % 40\% + 40\% + 20\% = 100\% 40% + 40% + 20% = 100% ). Tracing the 40 % 40\% 40% sand line up-left and the 20 % 20\% 20% clay line horizontally right intersects squarely on the 40 % 40\% 40% silt line inside the Loam compartment. Notice that a Loam does not contain equal thirds ( 33.3 % 33.3\% 33.3% each)—because clay has such a massive surface area, only 20 % 20\% 20% clay is required to balance 40 % 40\% 40% sand and 40 % 40\% 40% silt, whereas a 33 % sand / 33 % silt / 34 % clay 33\%\text{ sand} / 33\%\text{ silt} / 34\%\text{ clay} 33% sand /33% silt /34% clay soil plots in the Clay Loam class.

Worked Example 2 (High-Erodibility Cut Slope): A cut-slope sample contains 15 % sand 15\%\text{ sand} 15% sand , 70 % silt 70\%\text{ silt} 70% silt , and 15 % clay 15\%\text{ clay} 15% clay . Tracing 15 % 15\% 15% clay horizontally right and 70 % 70\% 70% silt down-left intersects inside the Silt Loam class—alerting the inspector to severe splash crusting, rapid rill incision, and high K K K -factor erosion potential.

Soil Structure, Organic Matter, Soil pH, and Fertility (N-P-K)

Beyond particle-size texture (which cannot be altered without importing new soil), four modifiable soil properties govern infiltration and vegetative stabilization:

Soil Structure: The arrangement of sand, silt, and clay particles into secondary aggregates ( peds ). Granular (crumb) structure in A horizons maximizes porosity and seedbed contact. Blocky and prismatic structures occur in B horizons. Platy structure (horizontal overlapping plates caused by equipment compaction) and massive structureless subsoils block vertical percolation and root growth.

Organic Matter: Increasing organic matter via compost incorporation or topsoil salvage binds silt and clay into water-stable aggregates, lowers bulk density, lowers the RUSLE K K K factor, boosts water-holding capacity, and supports soil microbes.

Soil pH and Chemical Amendments: Soil pH measures active acidity or alkalinity on a logarithmic scale from 0 to 14 0\text{ to }14 0 to 14 ( 7.0 7.0 7.0 is neutral). Most erosion-control grasses and legumes require an optimal pH of 6.0 to 7.0 6.0\text{ to }7.0 6.0 to 7.0 to absorb macronutrients:

Acidic Soils ( pH < 5.5 \text{pH} < 5.5 pH < 5.5 ): Common in leached humid subsoils or pyritic rock cuts; soluble aluminum becomes toxic to roots and phosphorus is locked up in insoluble forms. Remediated by incorporating agricultural limestone (calcium/magnesium carbonate) .

Alkaline ( pH > 7.8 \text{pH} > 7.8 pH > 7.8 ) and Sodic Soils: Common in arid and semi-arid basins. High pH is lowered using elemental sulfur . Sodic/dispersive clays (high exchangeable sodium, N a + Na^+ N a + , which causes clay aggregates to deflocculate and pipe) are remediated with gypsum (calcium sulfate, CaSO 4 ⋅ 2 H 2 O \text{CaSO}_4 \cdot 2\text{H}_2\text{O} CaSO 4 ​ ⋅ 2 H 2 ​ O ) , where C a 2 + Ca^{2+} C a 2 + displaces dispersive N a + Na^+ N a + on clay micelles and restores flocculated soil structure.

Soil Fertility (N-P-K): Fertilizer labels state three weight percentages in order— Nitrogen ( N \text{N} N ) for rapid vegetative shoot and leaf blade growth, Phosphorus ( P 2 O 5 \text{P}_2\text{O}_5 P 2 ​ O 5 ​ ) for seedling root development and establishment, and Potassium ( K 2 O \text{K}_2\text{O} K 2 ​ O ) for cellular strength, drought tolerance, and winter hardiness.