Construction sites are inherently dynamic environments, with vehicles constantly moving in and out, often traversing unpaved areas. This movement can lead to a significant issue known as Vehicle Tracking, where soil and mud cling to vehicle tires and are carried off-site onto public roads and other paved areas. This not only creates a messy and unsightly situation but also poses safety hazards and contributes to environmental pollution by introducing sediment into waterways. Implementing effective vehicle tracking Best Management Practices (BMPs) is crucial for mitigating these problems.
Understanding Vehicle Tracking and Its Impact
Vehicle tracking refers to the transportation of soil, mud, and debris from a construction site onto public or private roads by vehicle tires. When vehicles leave a construction site, they can carry substantial amounts of sediment on their tires and undercarriages. As these vehicles travel on paved surfaces, the accumulated sediment is dislodged and deposited, leading to several negative consequences:
- Safety Hazards: Sediment on roadways can create slippery conditions, especially when wet, increasing the risk of accidents for all vehicles, including cars, motorcycles, and bicycles.
- Environmental Pollution: Sediment is a major pollutant in stormwater runoff. It clouds water, reducing light penetration necessary for aquatic plant life, and can carry other pollutants like nutrients and heavy metals into streams, rivers, and lakes.
- Aesthetic Issues: Mud and debris tracked onto public roads are unsightly and can negatively impact the appearance of neighborhoods and communities.
- Regulatory Non-Compliance: Many jurisdictions have regulations in place to prevent sediment tracking from construction sites. Failure to implement adequate vehicle tracking BMPs can result in fines and project delays.
Purpose and Function of Vehicle Tracking BMPs
The primary purpose of vehicle tracking BMPs is to prevent or minimize the off-site migration of soil and mud from construction vehicles. These BMPs are designed to remove sediment from vehicle tires and undercarriages before they leave the construction site and enter public roadways, parking lots, or other off-site areas. By effectively controlling vehicle tracking, construction sites can significantly reduce sediment pollution and maintain safer, cleaner environments.
Vehicle tracking BMPs function by providing designated exit points equipped with various methods to dislodge and capture sediment. These methods can include physical removal through scraping and shaking, or washing vehicles before they leave the site. The effectiveness of a vehicle tracking BMP depends on its design, installation, maintenance, and the specific site conditions.
Applicability of Vehicle Tracking BMPs
Vehicle tracking BMPs are applicable to virtually all construction sites where there is a potential for vehicles to track sediment off-site. This is particularly important in areas with:
- Disturbed Soils: Sites with exposed soil are more prone to vehicle tracking as mud and soil easily adhere to tires.
- Unpaved Access Roads: Construction sites often have unpaved access roads, increasing the likelihood of vehicles picking up sediment.
- Proximity to Public Roads: Sites located near public roadways require stringent vehicle tracking controls to prevent sediment from being tracked onto those roads.
- Wet Weather Conditions: Rain and snow can exacerbate vehicle tracking by creating muddy conditions and increasing the amount of sediment that sticks to vehicles.
The 2018 Minnesota Construction Stormwater Permit, as an example, mandates the use of vehicle tracking BMPs to minimize sediment track-out from construction sites. Permittees are required to implement these practices and to use street sweeping if BMPs are insufficient. This permit also emphasizes the need to restrict vehicle use in areas where final vegetative stabilization will occur to minimize soil compaction, further highlighting the importance of controlled vehicle tracking.
Effectiveness of Different Vehicle Tracking BMPs
The effectiveness of vehicle tracking BMPs varies depending on the type of BMP implemented and how well it is maintained. Here’s a look at the common types and their effectiveness:
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Rock/Stone Pads: These pads consist of a thick layer of coarse aggregate placed at vehicle exit points. They work by physically scraping mud and soil off tires as vehicles drive over them. Properly designed and maintained rock pads can achieve moderate soil removal effectiveness (30-60%). Effectiveness decreases as the voids between rocks become clogged with sediment, requiring periodic top dressing, replacement, or extension of the pad length.
A schematic illustration of a vehicle tracking rock pad at a construction site entrance/exit, highlighting the rock aggregate and geotextile fabric layers. -
Shaker Racks: Also known as rumble strips or exit grids, shaker racks use vibration and shaking action to dislodge sediment from tires. They are more effective than rock pads alone, particularly for removing caked-on mud. Shaker racks are suitable for sites with moderate to high sediment tracking potential.
An example of a shaker rack installation at a construction site exit, demonstrating its integration into a rock pad for enhanced vehicle cleaning. -
Wheel Washers/Wash Racks: Wheel washer systems are the most effective vehicle tracking BMP, capable of removing over 75% of sediment in most applications. These systems use pressurized water sprays to thoroughly clean vehicle wheels and undercarriages. Wheel washers are ideal for sites with high sediment tracking potential or where stringent sediment control is required.
A commercial wheel wash system in operation, showcasing its effectiveness in removing sediment from vehicle wheels using pressurized water.
The table below summarizes the expected performance of temporary vehicle tracking BMPs across various water quantity and quality parameters:
| Water quantity | Expected Performance |
|---|---|
| Flow attenuation | Little or no design benefit |
| Runoff volume reduction | Little or no design benefit |
| Water quality | |
| Erosion prevention | Little or no design benefit |
| Sediment control | Primary design benefit |
| Nutrient loading | Secondary design benefit |
| Pollutant removal | |
| Total suspended solids | Primary design benefit |
| Total phosphorus | Secondary design benefit |
| Heavy metals | Secondary design benefit |
| Floatables | Little or no design benefit |
| Oil and grease | Little or no design benefit |
Planning and Design Considerations for Vehicle Tracking BMPs
Effective planning and design are essential for maximizing the performance of vehicle tracking BMPs. Key considerations include:
- Early Installation: Install vehicle tracking BMPs before any land-disturbing activities begin to prevent initial sediment track-out.
- Designated Exit Points: Limit all construction traffic to designated exit points equipped with vehicle tracking BMPs.
- Location Selection: Whenever possible, locate exit pads in well-drained areas, such as hilltops or upper slopes, away from streams and wetlands to minimize water quality impacts. Avoid dips and low spots that tend to stay wet.
- Traffic Safety: Ensure traffic safety at exit points by providing adequate visibility and turning radius.
- Setback from Roadways: Exit pads can be set back from public roadways, leading to a stabilized access road, to provide an additional buffer for sediment removal.
- BMP Selection Based on Risk: Choose the appropriate BMP based on the site’s sediment tracking risk. Rock pads are suitable for low-risk sites, while shaker racks or wheel washers may be necessary for moderate to high-risk sites.
- Wash Water Management: If using wheel washers, ensure a water source is available and plan for proper management of wash water runoff, including sediment trapping and potential recycling.
- Sediment Management: Design the BMP area to drain towards a sediment trap or other sediment control device to capture sediment removed by the vehicle tracking BMP.
- Site-Specific Factors: Consider underlying soil types (clayey and silty soils cling more), traffic frequency, vehicle types and sizes, and expected maintenance frequency when designing the BMP.
Design Specifications for Rock Pads:
- Width: Minimum 20 feet, sufficient for the widest vehicles.
- Length: Minimum 50 feet, longer if needed for effective cleaning.
- Rock Size: Coarse aggregate, 2 to 6 inches, or larger depending on site conditions.
- Thickness: 6 to 12 inches of rock.
- Geotextile Fabric: Underlay the pad with nonwoven geotextile fabric to prevent subsidence.
- Turning Radius: 20-foot turning radius at road intersections.
- Drainage: Design drainage to a sediment control BMP.
Design Specifications for Shaker Racks:
- Width: Sufficient for the widest vehicles.
- Length: Long enough to effectively shake off sediment.
- Soil Storage: If less than 4 inches of soil storage below the rack, install over a 6-inch coarse aggregate pad.
- Drainage: Design drainage to a sediment control BMP.
Design Specifications for Wheel Washers:
- Spray Coverage: Direct spray to clean both inside and outside of wheels.
- Drainage: Design drainage to a sediment control BMP.
- Wash Water Management: Implement sediment removal from wash water (e.g., sediment trap, pond) or recycling.
- Cold Weather Considerations: Account for potential ice buildup in cold climates.
Inspection and Maintenance of Vehicle Tracking BMPs
Regular inspection and maintenance are critical to ensure the continued effectiveness of vehicle tracking BMPs. Key inspection and maintenance practices include:
- Continuous Monitoring: Regularly check for mud and soil deposits on off-site roads, especially after rain events and snowmelt.
- Sediment Accumulation Inspection: Inspect sediment deposition areas within the BMP for accumulations that could reduce performance.
- Drainage Inspection: Ensure positive drainage from the exit pad area towards the designated treatment BMP.
- Vegetation/Stabilization Inspection: Inspect drainage ditches or swales for proper vegetation or stabilization to prevent erosion.
- Downgradient BMP Inspection: Verify the effectiveness of downgradient sediment treatment BMPs.
- Cold Weather Monitoring: Monitor for ice buildup on paved surfaces due to dripping from vehicles in cold weather.
Maintenance Activities:
- Immediate Off-Site Cleaning: Promptly remove mud, soil, and debris from off-site areas, especially roadways.
- Sediment Removal within BMP: Remove accumulated sediment from the pad area, ditches, swales, or stormwater facilities if it affects performance.
- Erosion Control Maintenance: Revegetate or stabilize eroding ditches, swales, or sediment traps/basins.
- BMP Adjustments: Adjust BMPs to improve performance, such as adding or cleaning rock in rock pads, extending pad lengths, or regrading for better drainage.
- Wheel Washer Maintenance: Ensure proper functioning of wash water drainage, collection, and treatment systems. Regularly clean out wash water collection areas and maintain spray nozzles.
- Cold Weather Maintenance: Maintain vehicle tracking BMPs throughout winter. Have extra rock available for top dressing. Use salt or sand if ice forms due to wheel washer use. Promptly remove sediment-laden snow and ice from roadways before spring melt.
Costs of Vehicle Tracking BMPs
The cost of implementing vehicle tracking BMPs can vary depending on the type of BMP and site-specific factors. Rock pads are generally the least expensive, while wheel washers are the most costly to install and operate. However, the cost of BMP implementation is often significantly less than the potential costs associated with regulatory fines, environmental damage, and safety hazards resulting from inadequate vehicle tracking controls.
According to MnDOT data from 2014, the average bid price for a stabilized construction exit was approximately $5,763.08. This provides a general benchmark for the potential cost of implementing a robust vehicle tracking BMP.
Conclusion
Effective vehicle tracking BMPs are essential for responsible construction site management. By preventing sediment from being tracked off-site, these practices protect water quality, enhance safety, and maintain community aesthetics. Implementing a combination of planning, proper design, regular inspection, and consistent maintenance of vehicle tracking BMPs is a sound investment in environmental protection and regulatory compliance for any construction project. For further details and specific requirements, consult local and state regulations and resources related to construction stormwater management and vehicle tracking best practices.
