Mississippi Stabilized Construction Entrance/Exit Detail
Mississippi is home to the second-longest North American River, which creates the state’s western border and gives the state its iconic name. The Mississippi River collects stormwater runoff from 32 different states, comprising the second-largest drainage system in the world, spanning over 1.1 million square miles. Ecologically, the Mississippi River serves as a migratory flyway for 60% of all North American waterfowl, and over 230 species of fish are present in its waters. Watersheds within Mississippi contribute runoff to the Mississippi River as it flows toward the Mississippi River Delta.
This area is also a crucial economic point for the nation, as it serves as the hub for many large trade ports and is home to a vibrant ecological system. Significant industries, including fishing, hunting, boating, and other recreational ventures, contribute tens of billions of dollars annually to the regional economy. The purity of the waters that flow into this indispensable river can significantly impact the health of its production, both economically and ecologically. Consequently, the quality of Mississippi’s stormwater has a direct impact on these areas of vital significance.
It is the responsibility of residents, industries, and governments to establish and maintain standards to protect and rehabilitate this resource. The Federal government initiated the Clean Water Act (CWA) in 1972, which addressed the impact that industry and construction have on the surrounding stormwater system. This effort to mitigate contaminants has led to the establishment of specific laws and permits required for certain activities. Most notable of these programs is the National Pollutant Discharge Elimination System (NPDES), which requires a permit for any water discharges from municipal, industrial, and construction activities affecting land of one acre or more.
Mississippi Department of Environmental Quality (MDEQ) NPDES Stormwater Permits
The EPA has approved the Mississippi Department of Environmental Quality (MDEQ) to administer the NPDES permit program, which involves both reviewing and enforcing the NPDES stormwater permit program. The department also provides information to contractors and engineers in the form of a Guidance Manual and Field Manual.
Many construction projects can be authorized under a General Permit. Mississippi construction stormwater permits distinguish between small construction activities (1–5 acres) and large construction activities (greater than 5 acres), with differing requirements under the General Permit. To obtain coverage under the General Permit, an operator must comply with the Permit’s provisions and submit a Notice of Intent (NOI) to inform the MDEQ of the proposed activities. Other projects may require an Individual Permit, which is reviewed on a case-by-case basis.
To be granted coverage under a General Stormwater Permit or to apply for an Individual Permit, an operator will need to prepare and submit a Stormwater Pollution Prevention Plan (SWPPP). The SWPPP is a document that outlines all potential sources of pollution and sedimentation, as well as the specific methods that will be used to address these issues. These methods are referred to as Best Management Practices, or BMPs, and include techniques to establish erosion control, dust control, and minimize construction sediment tracking. The Mississippi SWPPP Guidance Manual for Construction Activities includes examples, specific preparations, operations, and maintenance necessary to keep the site within the standards set by the state.
Construction Entrance/Exit BMP
A construction entrance is one of the first BMPs to be installed by an operator when beginning a construction project. The purpose of a construction entrance/exit is to provide a stabilized area at entrance points, reducing sediment, mud, and other contaminants that are tracked onto public roadways. To ensure effective use of a construction entrance BMP, site traffic should be restricted to use only stabilized entrances.
A construction exit is preferably located on high ground to prevent sediment-laden runoff from flowing into the street. An effective egress/ingress area will provide adequate drainage to direct runoff into a sediment basin, thereby containing the runoff on-site. The substrate may need to be prepared before the entrance is installed, which commonly includes excavation.
In addition to the entrance, street sweeping or scraping should be conducted regularly to capture any debris or sediment that is deposited onto public roadways. The MDEQ Guidance Manual states that street sweeping should be performed regularly, and often daily, when sediment is tracked onto public roads. Construction entrances should be inspected regularly to ensure they perform adequately. If sediment is not sufficiently contained, the construction entrance is likely in need of maintenance or repair. If sediment is not adequately contained after repairs, another BMP may be required to increase sediment capture.
There are various techniques used to create construction entrances, each with its own benefits and maintenance requirements.
Stone Stabilized Site Entrances
The most common type of construction entrance is the stone stabilized pad. This method uses large gravel to clean the tires of exiting construction vehicles. As vehicles drive through the aggregate pad, the rough surface of the rocks works to loosen any soil clinging to the exiting vehicles’ tires. The Manual includes specifications for a stone construction site entrance with a minimum of 50’ in length and at least 6” in depth. The length of the entrance may need to be extended to provide adequate sediment containment. The stone or crushed rock required is DOT #1 Coarse Aggregate, which is gravel with a diameter of 1.5” – 3”. A geofabric filter fabric is also necessary to separate the stone pad from the native soil and must be applied before the gravel is introduced.
As vehicles drive over the stone pad, the rough surface can become flattened, and soil can build up between the voids in the rock. When the pad is compacted, soil containment is reduced, and the entrance must be repaired. Stone entrances are repaired by top dressing the pad with an additional layer of rock to restore surface roughness.
Tire Washing Station
Construction entrance aggregate pads are susceptible to falling below acceptable standards of performance due to factors such as soil density, high traffic volume, and rain events. If the operator has observed excessive sediment tracking onto public roadways, it may be necessary to add a wash station to the ingress/egress zone. A Tire Washing Station is an area where a pressure washer, operated by a laborer or an automated pressure washer system, is used to clean any remaining sediment from the vehicle’s wheels after passing over the construction entrance and before entering public roads. This system requires the site to have access to water and to set up a sediment trap to collect the used water.
Wheel wash stations also require maintenance. Many systems recycle the water and collect sediment, which must be emptied periodically. Additionally, the sediment basin and drainage paths may need repairs if water begins to spill outside the sediment basin or onto the roadway.
FODS Trackout Control System
The FODS Trackout Control System is an effective BMP for trackout control. The system consists of a single layer of HDPE matting, which is formed into pyramids on its surface. The pyramids work to flex and deform tires, allowing them to dislodge debris and sediment stuck between the tire treads.
The FODS system is a durable product that can be reused on multiple projects. The mats can be reconfigured as needed and installed over dirt, concrete, or asphalt. Additionally, the system can be relocated in various phases during linear projects, significantly reducing the cost of construction entrance installation, maintenance, and removal.
The pyramids do not degrade from construction vehicle traffic, but will eventually become filled with sediment and require maintenance. The 3” tall pyramids can hold up to 2.5” of sediment, as the tires only come into contact with the tips of the pyramids. Maintenance is commonly completed by using a street sweeper or a skid steer with a broom attachment. In this way, contractors can drive over the mats during their daily street sweeping to maintain performance during the project.
The modular FODS system outperforms traditional stone-stabilized construction entrances and is commonly specified at 50% of the length of a stone-stabilized tracking pad. Additionally, the system is both flexible and cost-effective, allowing contractors to reduce sedimentation and expenses on each project where the mats are used.
Additional Resources
Mississippi Erosion and Sediment Control Field Manual
Mississippi SWPPP Guidance Manual for Construction Activities