13.4 Stabilized Construction Entrances, Tire Washes & Dust Control

13.4 Stabilized Construction Entrances, Tire Washes & Dust Control

Quick Reference: Off-site vehicle tracking is one of the most visible and heavily enforced construction stormwater violations. When heavy haul trucks, scrapers, and concrete delivery vehicles traverse unpaved, wet construction sites, cohesive soil packs between tire lugs and dual-wheel assemblies. Upon entering public paved highways, centrifugal force and road friction dislodge this mud, depositing heavy sediment bedload directly onto public streets. Under EPA CGP Part 2.2.4, contractors must control off-site tracking using a stabilized construction entrance/exit (TC-1) measuring a minimum of 50 feet in length (70 to 100 ft for heavy highway/grading projects), 24 feet in width, and 6 inches in depth, utilizing 2 to 3-inch angular AASHTO #1 stone underlain by an AASHTO M288 geotextile separation fabric. If tracking occurs, it must be cleaned using dry mechanical sweepers; flushing streets with water hoses into gutters is strictly prohibited.

Sediment Track-Out Physics & Regulatory Standards

Vehicle tracking—commonly referred to as "track-out"—poses substantial environmental, public safety, and regulatory hazards:

1. Direct Storm Sewer Contamination: Mud deposited on asphalt roadways is rapidly pulverized by passing civilian traffic into fine dust. During the next rainstorm, runoff washes this concentrated sediment slurry directly into curb catch basins and storm sewer inlets without passing through on-site sediment traps or perimeter silt fences.
2. Severe Traffic Safety Hazards: Wet clay and silt on smooth asphalt roadways drastically reduce tire friction coefficients ($f$), creating slick driving surfaces that lead to skidding, increased vehicular stopping distances, and catastrophic traffic collisions.
3. Air Quality Degradation: Dry track-out pulverized by traffic becomes airborne fugitive dust, contributing heavily to ambient particulate matter ($PM_{10}$ and $PM_{2.5}$) violations.

EPA CGP Part 2.2.4 Anti-Tracking Mandate

The federal Construction General Permit establishes clear, strict performance standards:

• Mandatory Installation: Stabilized entrance/exit points must be established at all points of egress where construction traffic leaves the disturbed job site and enters paved public roadways.
• Access Restriction: Contractors must fence, barricade, or restrict site perimeters to force all exiting equipment to pass through designated stabilized entrance pads; exiting across unprotected curbs or bare soil is illegal.
• Immediate Removal of Track-Out: If sediment is tracked off-site onto paved surfaces, it must be removed immediately (by the end of the same work day, or immediately if tracking poses an imminent traffic hazard).

Stabilized Construction Entrance / Exit (TC-1) Engineering Specifications

A stabilized entrance is not merely a pile of gravel dumped on the ground. It is an engineered geotechnical pad designed to flex tire treads and dislodge mud through aggressive mechanical vibration while bridging soft subgrades.

Dimensional Standards

• Length ($L$):
  • Standard minimum length: 50 feet (15.2 meters).
  • Heavy commercial, mass grading, and highway sites: 70 to 100 feet (21 to 30 meters).
  • Engineering Rationale: Standard commercial truck tires have circumferences of 10 to 11 feet. A 50-foot pad ensures tires rotate roughly 4 to 5 full revolutions across the rough stone, while a 70- to 100-foot pad provides 7 to 10 full revolutions, ensuring complete mechanical expulsion of mud cakes from tire grooves.
• Width ($W$): Minimum 24 feet (7.3 meters) for two-way traffic, or 12 feet (3.7 meters) per travel lane. The entrance must flare out at the highway tie-in with a turning radius of 20 to 25 feet to prevent trailer tires from cutting across the unpaved roadside shoulder.
• Thickness ($T$): Minimum 6 inches (150 mm) of compacted stone. On soft, wet, highly plastic clay subgrades ($CBR < 3$), aggregate depth should be increased to 8 to 12 inches (200 to 300 mm) to support loaded tri-axle dump trucks (80,000 lbs gross vehicle weight).

Material Specifications: AASHTO #1 Stone vs. Prohibited Aggregates

The physical geometry and grading of the stone govern entrance performance:

• Specified Aggregate: AASHTO #1 coarse aggregate (nominal particle size 2.0 to 3.0 inches / 50 to 75 mm), or AASHTO #2 stone (1.5 to 2.5 inches).
• Angular Fractured Faces: The aggregate must consist of 100% crushed, angular quarry stone (such as crushed limestone, granite, or basalt). Sharp, angular corners exert high point pressures against tire treads, prying open rubber tread blocks and dislodging packed clay.
• Clean, Washed Stone: Must contain less than 1% fines or dust passing the #200 sieve.

Strict Aggregate Prohibitions:

1. Rounded River Rock / Pea Gravel: Rounded gravels are strictly prohibited. Smooth stones roll under tires, pack tightly into tire tread voids, and fail to generate the shear friction necessary to scrape off cohesive mud.
2. Small Aggregate (AASHTO #57 / 3/4-inch Stone): Small stone wedges between tire duals and is launched like projectiles into windshields when vehicles reach highway speeds.
3. Recycled Asphalt Pavement (RAP): Asphalt millings soften in summer heat, binder leaches into stormwater, and fine particles bind together into an impermeable, smooth crust that does not clean tires.

Geotextile Underlayment: The Critical Separation Layer

A universal structural failure mode of construction entrances is the omission of geotextile fabric beneath the stone:

The Geotextile Mandate: A heavy-duty woven or non-woven geotextile fabric conforming to AASHTO M288 Class 1 or Class 2 for separation and stabilization must be placed directly over the graded subgrade prior to aggregate placement.

Engineering Function of Geotextile:

• Subgrade Separation: Under high axle loads, point stresses from 3-inch stones force them downward into soft, saturated clay subgrades. Concurrently, saturated mud pumps upward through the voids. Without geotextile, an unlined 6-inch stone entrance will be completely swallowed into the subgrade within days, turning the entire entrance into a muddy quagmire.
• Load Distribution & Tensile Reinforcement: The fabric distributes concentrated wheel loads across a wide subgrade footprint, bridging soft spots and preventing deep rutting.
• Longevity: Installing geotextile increases the functional operational life of the stone pad by 300% to 500%, dramatically reducing aggregate top-dressing maintenance costs.

Drainage & Surface Runoff Control

Runoff from the entrance pad must not flow onto the public highway. The entrance must be graded with an inward slope toward the construction site. Where local topography slopes toward the public road, an engineered mountable diversion berm (minimum 6 to 9 inches high with gentle 3:1 side slopes) must be constructed across the entrance pad to divert sediment-laden water into an adjacent ditch leading to a sediment trap.

Advanced Wheel Wash & Automated Tire Wash Systems

On large mass-earthwork projects, highway construction corridors, and sites characterized by heavy cohesive clays (fat clays, Unified Soil Classification CH), aggregate construction entrances alone cannot dislodge packed clay from deep tire treads.

When Wheel Wash Systems Become Mandatory

A Certified Professional in Erosion and Sediment Control must mandate an engineered tire wash facility when:

• Highly plastic, sticky soils cling to tires despite passage across a 100-foot AASHTO #1 stone entrance.
• Haul operations exceed 50 to 100 truck trips per day onto high-speed arterial highways.
• The construction entrance directly borders sensitive municipal streets, zero-tolerance urban corridors, or impaired receiving waters.

▲ Vehicle Inflow (From Muddy Site)
│
├──► 1. Entry Rumble Grate (Vibrates and dislodges heavy clods)
│
├──► 2. Automated Spray Wash Rack / Drive-Through Bath
│       (High-pressure spray jets hitting tires, undercarriage, & wheel wells)
│
├──► 3. Internal Drainage Trench / Sump
│       (Captures slurry: 5,000 - 20,000 NTU)
│
├──► 4. Solids Settling Basin / Closed-Loop Flocculation Recycling Tank
│
└──► 5. Exit Stabilized Aggregate Pad (Dries tires before asphalt highway)

System Engineering Configurations

1. Drive-Through Tire Baths: Shallow, elongated concrete or heavy steel troughs (typically 30 to 50 feet long) filled with 12 to 18 inches of water. Submerged steel rumble rails agitate tires as vehicles drive through at low speed ($\le 3\text{ mph}$), washing mud from rims and tread grooves.
2. High-Pressure Automated Wash Racks: Elevated steel grate decks equipped with high-pressure water spray nozzles (operating at 40 to 80 psi) triggered automatically by optical motion sensors. Multi-directional spray manifolds blast the outer and inner tire walls, undercarriages, and dual-wheel assemblies in 15 to 30 seconds.

Closed-Loop Recycling & Water Treatment

Tire wash wastewater contains extreme solids concentrations, frequently exceeding 10,000 to 30,000 NTU.

• Zero-Discharge Rule: Wash water must never be allowed to flow into street gutters, roadside ditches, or storm sewers.
• Treatment Train: Runoff drains into an integrated concrete solids pit or weir tank, where heavy sands drop out. Clarification is accelerated by dosing anionic polyacrylamide (PAM) flocculants. Clarified supernatant water is pumped through a secondary media filter and recycled back to the high-pressure spray nozzles in a 100% closed-loop system.
• Sludge Cleanout: Settled sludge must be excavated regularly using a backhoe or vacuum truck and stabilized in upland fill areas.

Street Sweeping Protocols vs. The High-Pressure Hosing Ban

Despite stabilized entrances, minor tracking will periodically occur on public roads during heavy earthmoving phases. SWPPP compliance requires rapid, legally compliant street cleaning.

The Absolute Ban on Washing Track-Out with Water Hoses

Strict Regulatory Prohibition: Operating a water truck, pressure washer, or fire hose to flush or wash tracked mud down street gutters or into storm drain catch basins is an explicit Clean Water Act violation.

Hosing tracked sediment converts manageable soil on the asphalt into a high-turbidity slurry that immediately flows into underground storm drains, depositing sediment into municipal pipes and suffocating downstream creeks.

Approved Dry Mechanical Cleaning Technologies

1. Mechanical Broom Sweepers: Rotating cylindrical wire/nylon brooms that sweep sediment onto a conveyor belt for deposit into an internal collection hopper. Must be equipped with light water misting sprayers solely for dust suppression (misting must dampen dust without generating flowing runoff).
2. Regenerative Air & Vacuum Sweepers: High-suction vacuum systems that blast pressurized air into pavement cracks to loosen embedded silts, vacuuming the suspended particulates into an internal settling hopper equipped with fabric micron filters. Highly recommended for capturing fine silts and $PM_{10}$ dust.
3. Manual Hand Scraping & Shoveling: Immediate physical scraping using flat-edged shovels, skid-steer loader buckets with rubber cutting edges, and manual stiff brooms to lift heavy mud clods before running sweepers.

Dust Control & Fugitive Particulate Emissions ($PM_{10}$ and $PM_{2.5}$)

Earthmoving, scraper operations, rock crushing, and haul traffic pulverize dry soil into airborne fugitive dust. Under the federal Clean Air Act and local air quality management district rules, construction sites must prevent visible dust emissions from drifting across property boundaries.

Particulate Size Dynamics & Environmental Health Hazards

• $PM_{10}$ Particulates ($d \le 10,\mu\text{m}$): Inhalable coarse particles that penetrate the human upper respiratory tract, aggravating asthma and chronic bronchitis.
• $PM_{2.5}$ Particulates ($d \le 2.5,\mu\text{m}$): Fine respirable particles that penetrate deep into pulmonary alveoli and enter the bloodstream.
• Respirable Crystalline Silica ($SiO_2$): Generated during rock crushing, concrete cutting, and grading in quartz-rich soils, causing irreversible silicosis and lung cancer.

Dust Palliatives & Suppression Methodologies

The Operational Rules of Water Suppression

Water spraying is the most universal dust palliative, but improper watering causes secondary stormwater violations:

• Misting vs. Soaking: Water trucks must operate high-pressure misting nozzles that lightly moisten the top $1/4\text{ to }1/2\text{ inch}$ of haul road crust.
• No Runoff, No Puddles: Over-watering saturates the subgrade, creating fluid mud that adheres to haul truck tires, compounding off-site track-out and generating erosive surface runoff that carves rills in unpaved haul roads.