13.3 Sitework, Grading, Soil Mechanics & Foundation Layout

Geotechnical Engineering Reports & Subsurface Exploration

Every commercial building, residential subdivision, and civil infrastructure project relies entirely on the structural capacity of the underlying earth. Under IBC Section 1803, the building official generally requires a geotechnical investigation, and may waive it where satisfactory data exist. The presumptive values in IBC 2018 Table 1806.2 may be used without an investigation only for the soils and conditions the code allows. The geotechnical investigation evaluates subsurface stratigraphy, soil classification, groundwater depth, slope stability, and seismic ground motion parameters to establish allowable foundation design criteria.

Soil Test Borings & The Standard Penetration Test (ASTM D1586)

Geotechnical engineers advance hollow-stem continuous flight augers or rotary wash drill rigs into the subgrade to extract soil samples and determine relative soil density and consistency via the Standard Penetration Test (SPT):

                     Standard Penetration Test (ASTM D1586) Setup

       ┌──────────────────────────────────────────────┐
       │           140-Pound Drive Hammer             │
       └──────────────────────┬───────────────────────┘
                              │  ◄── 30-Inch Free Fall Drop Height
                              ▼
       ═══════════════════════╤════════════════════════  Ground Surface
                              │  Drill Rods
                              │
              ┌───────────────┴───────────────┐
              │                               │
              │   6" Seating Drive            │  (Initial seating drive: BLOWS DISCARDED)
              ├───────────────────────────────┤
              │   6" First Recording Drive    │ ┐
              ├───────────────────────────────┤ ├─► SPT N-VALUE = Sum of Blows in
              │   6" Second Recording Drive   │ ┘   Final Two 6-Inch Drives (Blows/Foot)
              └───────────────────────────────┘
                 2-Inch O.D. Split-Barrel Sampler

• Equipment Mechanics: A standard 2-inch outside diameter (O.D.) split-spoon sampler is lowered to the bottom of the borehole. A 140-pound (63.5 kg) hammer is dropped freely from a height of 30 inches (760 mm) onto the drive head.
• Recording the N-Value: The sampler is driven three successive 6-inch increments (total 18 inches). The number of hammer blows required for the first 6-inch drive is considered a seating drive and is discarded. The sum of the hammer blows required to drive the sampler through the final two 6-inch intervals (from 6 to 18 inches) is recorded as the SPT N-value (expressed in blows per foot).
• Engineering Interpretation: An $N$-value under 4 denotes very loose sand or very soft clay; $N = 10 \text{ to } 30$ denotes medium-dense granular soil or stiff cohesive clay; $N > 50$ (refusal) indicates very dense cemented strata, gravel beds, or bedrock.

Groundwater Table Monitoring

Borehole logs record the depth at which free groundwater is encountered during drilling and after 24 hours of stabilization. Fluctuations in the water table due to seasonal rainfall or urban irrigation directly influence the effective stress of soil, reduce bearing capacity, induce hydrostatic uplift pressure on basement slabs, and increase soil liquefaction potential during seismic events.

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Fundamental Soil Mechanics & Bearing Capacity

Allowable Soil Bearing Capacity ($q_a$)

Bearing Capacity is the capacity of the soil to support the structural loads applied by footings and slabs without undergoing shear failure or unserviceable settlement. Soil bearing capacity is expressed in pounds per square foot (psf) or tons per square foot (tsf, where $1 \text{ tsf} = 2,000 \text{ psf}$).

• Ultimate Bearing Capacity ($q_{ult}$): The theoretical maximum intensity of pressure that soil can resist at failure.
• Allowable Soil Bearing Pressure ($q_a$): The working load capacity permitted for structural design, incorporating a safety factor (FS, typically $\text{FS} = 3.0$):

$q_a = \frac{q_{ult}}{\text{Factor of Safety (FS)}}$

Shear Strength of Soil (Mohr-Coulomb Criterion)

Soil shear strength ($\tau$) represents the internal resistance per unit area that soil offers to resist sliding failure along an internal slip surface, governed by Coulomb's equation:

$\tau = c + \sigma \tan \phi$

• $c$ = Cohesion: The molecular attraction between fine clay/silt particles that holds soil together independently of applied stress (expressed in psf). Granular sands and gravels have zero cohesion ($c = 0$).
• $\sigma$ = Normal Stress: The perpendicular compressive force pressing soil particles together along the shear plane.
• $\phi$ = Internal Friction Angle: The angle of internal friction representing the interlocking resistance and friction between individual mineral grains (typically $28^\circ \text{ to } 40^\circ$ for sands/gravels).

Atterberg Limits & Soil Plasticity (ASTM D4318)

Fine-grained clay and silt soils change consistency based on their moisture content. The Atterberg Limits define the boundary moisture thresholds between solid, semi-solid, plastic, and liquid states:

• Liquid Limit (LL): The moisture content (expressed as a percentage) at which soil passes from a plastic state to a liquid state.
• Plastic Limit (PL): The moisture content at which soil transitions from a semi-solid to a plastic state (the moisture level where a soil sample crumbles when rolled into a 1/8-inch diameter thread).
• Plasticity Index (PI): The numerical difference between the Liquid Limit and the Plastic Limit:

$\text{Plasticity Index (PI)} = \text{Liquid Limit (LL)} - \text{Plastic Limit (PL)}$

• Significance: Soils with a $\text{PI} < 10$ exhibit low plasticity and minimal volumetric change. Soils with a $\text{PI} > 30$ contain heavy concentrations of active clay minerals that exhibit severe volumetric expansion and shrinkage upon wetting and drying.

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Problematic Soils in Nevada & Engineering Mitigation

Nevada's arid and semi-arid desert basins feature some of the most challenging foundation soils in North America, posing severe structural risks to slabs, foundations, and underground civil utilities.

1. Caliche Hardpan Formations

• Physical Nature: Caliche is an indurated, rock-like subterranean formation formed when calcium carbonate ($\text{CaCO}_3$) precipitates out of infiltrating rainwater and cements alluvial desert gravels, sands, and silts into a solid concrete-like matrix.
• Jobsite Challenge: Caliche lenses vary unpredictably in thickness and hardness; well-cemented caliche can be as hard as concrete, and ordinary backhoes may be unable to dig it.
• Contractor Mitigation: Excavation requires heavy crawler dozers (Caterpillar D8/D9) equipped with single-shank hydraulic rippers, massive hydraulic rock breakers (hoe-rams) mounted on 80,000-lb excavators, rock wheel trenchers, or non-explosive chemical expanding grouts.

2. Highly Expansive Soils (Swelling Clays)

• Physical Nature: Fine-grained soils containing high percentages of active smectite or montmorillonite clay minerals. These clays possess an expansive crystalline lattice that absorbs water molecules, expanding up to 10% to 20% in volume and exerting swell pressures exceeding 10,000 psf (easily lifting building foundations).
• Jobsite Challenge: Severe differential heave lifts floor slabs, cracks exterior drywall, twists structural framing, and fractures underground sewer lines during wet winter seasons, followed by shrinkage settlement during dry summer heat.
• Contractor Mitigation:
  • Over-excavation and Recompaction: Undercutting the expansive clay 3 to 5 feet beneath footings/slabs and replacing it with non-expansive engineered structural fill (granular soil with $\text{PI} < 15$).
  • Chemical Stabilization: Incorporating hydrated lime ($\text{Ca(OH)}_2$) or quicklime ($\text{CaO}$) at 3% to 6% by dry weight into the upper 12 to 18 inches of clay subgrade to induce cation exchange, permanently lowering the Plasticity Index.
  • Post-Tensioned (PT) Slabs: Designing monolithic post-tensioned slab-on-ground foundations in accordance with the Post-Tensioning Institute (PTI) standards, utilizing high-strength unbonded steel cable tendons to allow the entire building footprint to float as a rigid structural raft over heaving soils.

3. Hydro-Collapsible Soils

• Physical Nature: Loose, dry, low-density alluvial fan silts and sands with high void ratios, bonded together by moisture-sensitive clay bridges or soluble salt precipitates.
• Jobsite Challenge: While stable when bone-dry, upon sudden saturation with water (from landscape irrigation, roof downspouts, or utility leaks), the internal chemical bonds dissolve instantly. The soil collapses under its own overburden weight, causing catastrophic building settlement of several inches without any increase in structural load.
• Contractor Mitigation: Deep pre-saturation (hydro-collapsing) prior to construction, deep dynamic compaction (dropping heavy steel weights from cranes), or installing driven steel pipe piles / helical piers down to competent underlying strata.

4. Gypsiferous Soils & Sulfate Attack

• Physical Nature: Desert alluvial soils containing substantial deposits of crystalline calcium sulfate (gypsum, $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$), prevalent across Southern Nevada.
• Twofold Engineering Risk:
  1. Subterranean Voids: Gypsum is water-soluble; continuous groundwater movement or irrigation runoff leaches the gypsum out of the soil matrix, creating underground piping voids and sinkholes.
  2. Chemical Sulfate Attack: Water-soluble sulfate ions react chemically with the tricalcium aluminate ($C_3A$) in Portland cement concrete, forming ettringite crystals that expand internally, pulverizing concrete footings from within.
• Contractor Mitigation: Mandatory use of ASTM C150 Type V (Sulfate-Resistant) Portland cement or Type II/V with supplementary cementitious materials (fly ash or slag), strict maximum water-cementitious ratios ($w/c \le 0.45$), and minimum concrete compressive strengths of 4,500 psi.

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Site Clearing, Grubbing & Topsoil Stripping

Before earthmoving machinery can begin mass grading or foundation trenching, the site must be prepared in accordance with project specifications (CSI Division 31):

• Site Clearing: Removing and disposing of all above-ground surface growth, dead trees, standing brush, logs, abandoned surface rubbish, concrete rubble, and miscellaneous surface debris across the construction limits.
• Grubbing: Extracting and removing all subsurface vegetative roots, stumps, buried logs, and root mats to a minimum depth of 12 to 24 inches below original ground surface (or 18 inches below final subgrade level). Leaving vegetative matter buried beneath structural fills is strictly prohibited; organic matter decomposes over time, generating methane gas and leaving subterranean voids that trigger severe building settlement.
• Topsoil Stripping & Stockpiling: The upper 4 to 6 inches of natural topsoil (the A-horizon containing rich organic humus, micro-organisms, and weed seeds) must be stripped across all cut and fill footprints. Topsoil is mechanically unstable and strictly prohibited from use as structural fill under building slabs or pavements. Stripped topsoil is hauled to designated stockpile zones outside the active construction perimeter, stabilized with temporary hydroseed or silt fencing, and preserved for final post-construction landscape dressing.

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Mass Earthwork: BCY, LCY, CCY & Swell/Shrinkage Calculations

Earthwork estimating, mass haul planning, and field billing revolve around tracking soil volume across three distinct physical states:

                         The Three Volumetric Earthwork States

     IN-BANK (BCY)                 LOOSE (LCY)                 COMPACTED (CCY)
  Undisturbed, Natural       Excavated, Disturbed,          Mechanically Compacted,
       Soil Mass                 Fluffed Mass                 Densified Fill Mass
  ┌────────────────┐           ┌──────────────────┐           ┌────────────────┐
  │                │           │ ░░░ ░░░ ░░░ ░░░  │           │▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓│
  │  1.00 cu yd    │  ──► SWELL│ ░░ 1.25 cu yd ░░ │  ──► COMP.│  0.85 cu yd   │
  │                │   (+25%)  │ ░░░ ░░░ ░░░ ░░░  │   (-15%)  │▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓│
  └────────────────┘           └──────────────────┘           └────────────────┘
   Baseline State             Haul Truck Sizing              Engineered Embankment

Definitions of Earthwork States

1. Bank Cubic Yards (BCY): Soil in its natural, undisturbed in-situ state before excavation. Structural cuts and borrow pit excavations are measured, calculated, and paid in BCY.
2. Loose Cubic Yards (LCY): Soil after it has been excavated and disturbed. Mechanical digging breaks soil bonds and introduces air voids, causing the material to swell in volume. Dump truck capacities, scrapers, and hauling cycles are calculated exclusively in LCY.
3. Compacted Cubic Yards (CCY): Soil that has been placed in an engineered fill, moisture-conditioned, and compacted by heavy rollers. Mechanical compaction drives out air voids, causing the material to occupy less volume than in its original bank state (shrinkage). Embankment fills and building pad requirements are specified in CCY.

Mathematical Formulas: Swell & Shrinkage

$\text{Loose Volume (LCY)} = \text{Bank Volume (BCY)} \times \left( 1 + S_w \right)$

$\text{Compacted Volume (CCY)} = \text{Bank Volume (BCY)} \times \left( 1 - S_h \right)$

$\text{Bank Volume Required (BCY)} = \frac{\text{Compacted Volume (CCY)}}{1 - S_h}$

$\text{Swell Factor } (S_w) = \left( \frac{\text{LCY}}{\text{BCY}} - 1 \right) \times 100\%$

$\text{Shrinkage Factor } (S_h) = \left( 1 - \frac{\text{CCY}}{\text{BCY}} \right) \times 100\%$

               Worked Engineering Scenario: Mass Earthwork Haul Problem

  Project Requirements:
  • A commercial warehouse building pad requires 8,500 Compacted Cubic Yards (CCY) of fill.
  • Geotechnical laboratory testing of the borrow pit soil indicates:
    - Swell Factor (Sw) = 20% (0.20)
    - Shrinkage Factor (Sh) = 15% (0.15)
  • Dump trucks have an effective hauling payload volume of 16 Loose Cubic Yards (LCY).

  Step 1: Calculate the Bank Volume (BCY) that must be excavated from the borrow pit:
          BCY = CCY / (1 - Sh)
          BCY = 8,500 CCY / (1 - 0.15) = 8,500 / 0.85 = 10,000 BCY

  Step 2: Calculate the Loose Volume (LCY) that the haul trucks must transport:
          LCY = BCY × (1 + Sw)
          LCY = 10,000 BCY × (1 + 0.20) = 10,000 × 1.20 = 12,000 LCY

  Step 3: Calculate the total number of truck loads required:
          Truck Loads = Total LCY / Truck Capacity
          Truck Loads = 12,000 LCY / 16 LCY per load = 750 truck loads

Average End Area Method for Cut/Fill Volumes

To calculate earthwork volumes between adjacent survey cross-sections along a roadway or building pad baseline, civil engineers utilize the Average End Area Method:

$V = \left( \frac{A_1 + A_2}{2} \right) \times \frac{L}{27}$

• $V$ = Volume of cut or fill in Cubic Yards.
• $A_1$, $A_2$ = Cross-sectional end areas of cut or fill at Station 1 and Station 2 (square feet).
• $L$ = Horizontal distance between stations along the centerline (feet).
• 27 = Conversion factor ($27 \text{ cubic feet} = 1 \text{ cubic yard}$).

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Earthwork Compaction Specifications & Quality Verification

Engineered structural fills supporting foundations, floor slabs, and roadways must be compacted to precise engineering densities. Soil compaction increases shear strength, reduces future settlement, and decreases water permeability.

Standard Proctor (ASTM D698) vs. Modified Proctor (ASTM D1557)

In geotechnical engineering, the baseline maximum density of a soil is determined via laboratory Proctor Tests, which establish the Optimum Moisture Content (OMC) at which soil achieves its Maximum Dry Density (MDD).

                               Proctor Compaction Moisture-Density Curve

  Dry Density (pcf)
       ▲
       │                         ┌─── OPTIMUM MOISTURE CONTENT (OMC = 12.5%)
  125  │                         │    Peak = Maximum Dry Density (MDD = 124.5 pcf)
       │                       ┌─┴─┐
  120  │                      ╱     ╲
       │                     ╱       ╲        ACCEPTABLE COMPACTION WINDOW:
  115  │                    ╱         ╲       • Density: ≥ 95% of MDD (≥ 118.3 pcf)
       │                   ╱           ╲      • Moisture: Within ± 2% of OMC (10.5% - 14.5%)
  110  │                  ╱             ╲
       └─────────────────┴───────────────┴────────────────────────────────────────►
                        8%      12.5%    16%     Moisture Content (%)

Field Compaction & Density Quality Verification

• Lift Thickness: Soil must be placed and compacted in loose, horizontal layers known as lifts. Specifications typically restrict loose lift thickness to 6 to 8 inches (uncompacted) so that the compactive energy of the roller penetrates the entire layer.
• Compaction Percentage Thresholds:
  • Structural building pads and foundation support: 95% of Modified Proctor MDD.
  • Roadway aggregate base course (ABC) and asphalt subgrade: 95% to 100% of Modified Proctor MDD.
  • Non-structural exterior landscaping fills: 90% of Standard or Modified Proctor MDD.
• Nuclear Density Gauge (ASTM D6938): The standard modern instrument for rapid jobsite testing. Uses a radioactive source (Cesium-137 for density via gamma-ray transmission, and Americium-241/Beryllium for moisture via neutron thermalization) to provide instant in-situ wet density, moisture content, and percent compaction.
• Sand Cone Method (ASTM D1556): The primary physical verification test. A hole is dug in the compacted lift, the extracted soil is weighed and oven-dried, and the hole's exact volume is determined by pouring calibrated, free-flowing dry Ottawa sand from a cone apparatus.

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Construction Surveying & Foundation Layout

Accurate structural construction begins with transferring architectural and structural dimensions from drawings onto the physical jobsite terrain.

Establishing Primary Site Control

• Horizontal Control: Surveyors locate boundary property corners (iron pins, brass monuments) and establish an orthogonal grid system (column grid lines A, B, C... and 1, 2, 3...) referenced across the project.
• Vertical Control: The surveyor transfers an elevation from the primary regional Benchmark (BM) onto multiple Temporary Benchmarks (TBMs) placed securely around the perimeter of the jobsite using an optical automatic level, digital level, or electronic total station.

Batter Boards & Layout String Lines

                        Batter Board & String Line Layout Setup

         ┌──────────┐                                                ┌──────────┐
         │  Stake   │                                                │  Stake   │
         │    │     │                                                │    │     │
         │  ┌─┴─────┴─┐ Horizontal Ledger Board                      │  ┌─┴─────┴─┐
         │  │    ║    │ (Set at exact reference elevation)           │  │    ║    │
         └──┤────║────┴──────────────────────────────────────────────┴──┤────║────┘
                 ║ Saw Kerf (Notch)                             Saw Kerf║
                 ╚══════════════════════════════════════════════════════╝
                                  Taut Mason's String Line
                                              │
                                              │  ◄── Plumb Bob Dropped to Ground
                                              ▼
                                     [ Building Corner ]
                                              │
                                     ═════════╧══════════ Trench Bottom

1. Location: Batter boards consist of two vertical wooden stakes driven firmly into the ground with a horizontal wooden board (ledger) nailed across them. Batter boards are erected outside the active excavation boundary (typically set back 4 to 8 feet) so they are not disturbed by backhoes digging foundation trenches.
2. Elevation: The horizontal ledger boards are set at a specific, uniform reference elevation (often matching the Top of Footing or a round elevation relative to FFE) using a builder's level.
3. String Lines: Fine, non-stretching nylon mason's twine is stretched taut between saw kerfs (notches) or nails driven into opposite batter boards. The intersection of crossing string lines defines the exact location of exterior building corners and foundation wall centerlines.
4. Transferring Lines: Plumb bobs suspended from string line intersections transfer building corners directly downward into deep footing excavations.

Squaring Layouts: The 3-4-5 Rule & Diagonal Checking

To ensure that corners form perfect 90-degree right angles, contractors utilize geometric principles derived from the Pythagorean theorem ($a^2 + b^2 = c^2$):

                              The 3-4-5 Squaring Method

                               Corner Stake (90° Angle)
                                        ┌
                                        │ ╲
                            Leg A = 30' │   ╲  Hypotenuse / Diagonal
                            (or 3', 6') │     ╲  C = 50' (or 5', 10')
                                        │       ╲
                                        │         ╲
                                        └───────────
                                         Leg B = 40' (or 4', 8')

• The 3-4-5 Proportional Triangle: Measuring 3 units along one string line from the corner stake, 4 units along the perpendicular string line, and adjusting the string lines until the diagonal hypotenuse measures exactly 5 units guarantees a true $90^\circ$ corner. Multiples of 3-4-5 (such as $6-8-10$, $9-12-15$, or $30-40-50$ feet) are utilized on large commercial building layouts to maximize precision.
• Diagonal Equality for Rectangles: For any rectangular or square building footprint, the opposite sides must be parallel and equal in length, and both diagonal measurements from opposite corners must be exactly equal ($D_1 = D_2$). If the diagonal distances differ, the foundation is out of square (racked into an un-square parallelogram) and string lines must be adjusted before trenching.

$\text{Diagonal } D = \sqrt{L^2 + W^2}$

$\text{For } 45\text{ ft} \times 60\text{ ft Building:} \quad D = \sqrt{45^2 + 60^2} = \sqrt{2,025 + 3,600} = \sqrt{5,625} = 75.0\text{ feet}$

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Stormwater Pollution Prevention Plan (SWPPP) & NDEP BMPs

Under the Federal Clean Water Act and Nevada environmental protection statutes (NRS 445A), construction operations disturbing the earth are legally regulated point sources of stormwater discharge.

NDEP General Construction Permit (NVR100000)

• Threshold Trigger: Any construction project that disturbs one (1) or more acres of total land area, or disturbs less than 1 acre if part of a larger common development plan (such as a commercial subdivision pad), must obtain coverage under the Nevada Division of Environmental Protection (NDEP) Construction General Permit (CGP).
• Notice of Intent (NOI): The contractor must submit a formal NOI to NDEP prior to breaking ground, develop a certified site-specific Stormwater Pollution Prevention Plan (SWPPP), and maintain the SWPPP binder on the jobsite at all times.

                         NDEP Construction SWPPP BMP Layout

                                Active Construction Site
       ┌────────────────────────────────────────────────────────────────────────┐
       │                                                                        │
       │      [ Stockpile ] ──► Surrounded by Silt Fence                        │
       │                                                                        │
       │      [ Catch Basin ] ──► Protected with Drop Inlet Filter Insert       │
       │                                                                        │
       └───────────────────────────────┬────────────────────────────────────────┘
                                       │ Runoff / Vehicle Traffic
                                       ▼
       ┌────────────────────────────────────────────────────────────────────────┐
       │  STABILIZED CONSTRUCTION ENTRANCE / EXIT TRACKOUT PAD                  │
       │  • Typically about 50' long, full exit width (per SWPPP/permit)        │
       │  • 2" to 3" angular rock over geotextile fabric                        │
       └───────────────────────────────┬────────────────────────────────────────┘
                                       │ Clear Vehicle Tires
                                       ▼
                               [ Public Paved Street ]

Core Best Management Practices (BMPs)

1. Silt Fencing (Perimeter Control): Synthetic geotextile filter fabric supported by steel wire mesh and metal T-posts placed along the downslope construction perimeter. Mandatory Installation Rule: The bottom edge of the geotextile fabric must be entrenched a minimum of 6 inches vertically into the ground and backfilled with compacted earth. Silt fences installed without trenching fail immediately via undercutting ("piping").
2. Stabilized Construction Exit (Trackout Pad): A designated gravel pad installed at all points where construction traffic exits the jobsite onto paved public roads:
   • Typical Specification: Commonly about 50 feet long and as wide as the exit, built of clean 2- to 3-inch angular rock about 6 inches deep over geotextile fabric. Use the dimensions in the project's SWPPP or dust permit.
   • Function: Knocks off and dislodges mud, aggregate, and sediment caked onto vehicle tires. Trackout of soil onto public paved roadways must be swept or vacuumed daily; washing sediment down municipal storm drains with a fire hose is strictly illegal.
3. Storm Drain Drop Inlet Protection: Silt fence barriers, woven wire/gravel bag barriers, or manufactured geotextile drop-in basket filters installed around municipal storm drain inlets to filter out suspended sediments before runoff enters the public storm sewer.
4. Sediment Retention Basins & Rock Check Dams: Temporary excavation ponds designed to store stormwater runoff, reducing water velocity to allow suspended soil particles to settle out before discharging. Check dams constructed of 2- to 4-inch riprap are installed across drainage swales to reduce runoff velocity and prevent channel erosion.
5. Site Inspections: The SWPPP inspector must inspect BMPs at the frequency the permit sets, typically weekly and after significant storm events, document deficiencies and make repairs within the permit's deadlines.