Alaska Stabilized Construction Exit and Vehicle Track-Out BMPs

The vast Alaskan tundra makes up the largest and most sparsely populated state in America. Alaska’s approximately 660,000 square miles of land area is bordered by the Arctic Ocean to the north, the Pacific Ocean to the south, and Canada’s British Columbia to the east. Alaska’s sprawling coastline is longer than all other U.S. states’ coastlines combined. The Yukon River, which is fed by thousands of glacial and snowmelt mountain streams, runs nearly 2,000 miles into the Bering Sea. The stormwater basin of the Koyukuk River spans approximately 32,000 square miles across the heart of Alaska and was used to transport miners during the late 1800s Klondike Gold Rush.

Alaska is home to a vast array of wildlife, including the wolverine, moose, caribou, and five species of Pacific salmon. The purity of the state's stormwater system is pivotal to maintaining the health of this habitat, as well as the fishing and hunting industries that are central to Alaska’s economy and subsistence culture. The health of the environment, the economy, and public safety relies on these water sources; therefore, they must be protected from pollutants. The Yukon River Inter-Tribal Watershed Council was formed with the goal of raising the quality of the Yukon’s waters to a standard that is safe to drink. Together with residents and government authorities, the Council works to restore and defend the purity of Alaska’s watershed system.

Alaska Department of Environmental Conservation (DEC) NPDES Stormwater Permits

The federal government enacted the Federal Water Pollution Control Act in 1948, which was amended in 1972 and became the Clean Water Act (CWA). These initiatives spurred the development of laws and guidelines for industries and construction projects that require procedures to mitigate pollutants introduced into the watershed. Most notable of these requirements is the National Pollutant Discharge Elimination System (NPDES) Permit Program created by the Act. The Permit Program requires industries and construction operators to develop plans to reduce or eliminate the negative impacts of their activities.

The Alaska Department of Environmental Conservation (DEC) Division of Water administers the Permit Program under the Alaska Pollutant Discharge Elimination System (APDES). Projects that disturb one or more acres require APDES permit coverage. Many such projects are eligible for coverage under the Construction General Permit for Storm Water Discharges for Large and Small Construction Activities (2021 CGP, AKR100000). To obtain this General Permit, operators must first develop a Stormwater Pollution Prevention Plan (SWPPP) and then submit a Notice of Intent (NOI) to DEC. For projects disturbing five acres or more, the SWPPP must also be submitted to DEC with the NOI.

The SWPPP includes an outline of planned construction activities that may cause sedimentation or pollution to water sources. The plan also lists strategies and devices that the operator will implement to contain contaminants and sediments on the job site and prevent erosion, sedimentation, and pollution. The Alaska DEC publishes the Alaska Storm Water Guide to help engineers and contractors design SWPPPs for each project. The Storm Water Guide includes commonly used resources and methods to prevent erosion and control sediment in the construction area. Notable and common methods implemented in an effective SWPPP include sediment basins, silt fences, dust control, and stabilized construction exits.

Stabilized Construction Exit and Vehicle Track-Out BMP

Construction site entrances receive a high volume of traffic. The Construction General Permit requires exit points to establish stabilized construction entrance and vehicle track-out BMPs designed to remove sediment from construction vehicle tires and minimize sediment tracked onto public roadways, where it can run into storm drains and cause excessive sedimentation. A stabilized construction exit is usually one of the first practices an operator implements before other construction activities to provide site access for delivery vehicles and heavy machinery into the construction zone.

Vehicle track-out controls include regular cleaning of the public right-of-way to capture sediment and debris that escapes through the stabilized entrance. Vacuuming, sweeping, or shoveling should be conducted on the same day that track-out occurs or on the next business day if the track-out occurs on a non-business day. Cleaning ensures that fugitive debris is captured before it runs into storm sewers and drains or causes roadway hazards.

One type of BMP used to reduce sediment track-out is a Rock Construction Exit. The Alaska DOT Best Management Practices (BMP) Guide for Erosion & Sediment Control describes common types of BMPs used throughout the state and includes an aggregate-style Rock Construction Exit. The guide includes design criteria and considerations. The Rock Construction Exit consists of a 12-inch-deep layer of 2–3 inch stone or quarry spall and is a minimum of 12 feet wide. The minimum entrance length is designed to accommodate three complete tire rotations for the largest vehicles that will use it. Before installing the stone in the Rock Construction Exit, a filter fabric must be installed to separate the native soil from the aggregate.

The rock-stabilized construction entrance must be observed and maintained throughout the project to keep the entrance in working condition. Over time, vehicles passing over the aggregate will compact the rock, and the entrance will become saturated with soil. As the entrance becomes compacted, performance decreases, and the entrance will need to be repaired. Maintenance of the gravel system is completed by top dressing with additional rock and repairing or replacing the filter fabric if it is torn. During rain events, entrances will require more frequent cleaning.

If the aggregate or paved entrance does not sufficiently contain sediment on the job site, additional or alternative BMPs may be required to ensure compliance and effective sediment control.

In some cases, Rock Construction Entrances may not be required to contain soil on the site. The Alaska DOT BMP Guide also includes a Temporary Pavement Construction Exit, which can be used in some cases in lieu of a Rock Construction Entrance, with the length and configuration determined by site conditions and project requirements. Paved surfaces can be swept to remove accumulated sediment and debris.

Metal Plate Device BMP

Another technique mentioned in the guide is a Metal Plate Construction Exit, which is commonly used in conjunction with Rock Construction Exits. This technique uses corrugated steel plates, also called shaker plates or rumble plates, which shake vehicle tires as they exit the site. The shaking motion can help dislodge additional mud and debris from vehicle tires.

Depending on the type of metal plate used, there may be a void below the system that collects sediment, or the spaces between the corrugated ridges may act as a containment area. The system must be cleaned to remove sediment that builds up over time, and it may be necessary to move the plates to clean the voids below.

Wheel Wash Station BMP

Wheel wash stations are an effective way to clean vehicle tires when installed with proper considerations. A wash station can be either passive or mechanical. A passive system consists of a pond with a depth of 12 to 14 inches, through which vehicles drive to rinse off tires. Passive wash stations should be constructed with a slight slope to help sediment collect within the basin. Mechanical wheel wash stations use manually operated or automated pressure washers to spray mud off vehicles. Water from the sprayers must be diverted or collected in a sediment basin to prevent runoff from entering the roadway.

FODS Trackout Control System

A modern and effective technique for reducing vehicle track-out is the FODS Trackout Control System. This modular system uses a series of 12-foot-by-7-foot HDPE mats to form pyramid-shaped structures on the surface. The pyramids deform and flex vehicle tires in multiple directions to dislodge sediment, rocks, and debris lodged between tire treads. In most cases, the system can be installed over existing substrates, including dirt, concrete, and asphalt.

The FODS Construction Entrance Mats are designed to be reused across multiple projects over an expected service life of 10 years or more. The mats can be easily relocated, significantly reducing costs on phased projects and across multiple jobs. Installation, maintenance, and removal can be completed without heavy equipment, and a standard installation can be completed in under 30 minutes.

The pyramids can hold up to 2.5 inches of sediment, as tires only contact the tips of the pyramids. When maintenance is required, a street sweeper or a skid steer with a broom attachment can be used to remove sediment and restore performance. Manual cleaning can also be completed using a FODS shovel designed to fit between rows of pyramids. Contractors can integrate maintenance into regular street sweeping to ensure the entrance remains compliant with permit requirements.

The FODS system outperforms traditional rock construction exits and has been shown to reduce street sweeping by 59 percent in high-traffic applications. Because the system is highly effective, FODS mats are commonly specified at approximately half the length of a traditional stone-stabilized tracking pad to provide comparable performance. In addition, the system is cost-effective, enabling contractors to reduce sedimentation and overall project costs where the mats are used.