Ever wonder what it takes to build a parking lot? All new land developments begin with planning and engineering. Engineers need to first consider local zoning and land development requirements before determining the number of parking spaces, dimensions, flow of traffic and stormwater drainage for their parking lot. Once a design is complete and approved, construction may begin.
Construction starts with a surveyor staking out the proposed layout of the parking lot. Surveyors use the grading plan to locate and mark features such as parking lot corners and grade elevations. These points give contractors a guide to follow during construction. Once the parking lot is laid out, it’s time for the site contractor to begin construction.
Step 1 – Strip Topsoil
Topsoil is the organic surface layer of soil that is found in most undeveloped areas. Topsoil needs to be removed as it is unsuitable for supporting a pavement structure. Topsoil depth can vary from a few inches to more than two feet according to the Pennsylvania housing research center. Did you know it can take 100-500 years to form one inch of topsoil?
Depending on the size and complexity of the parking lot, a variety of different machines could be used to strip the topsoil. Two common earthmoving machines used for smaller scale projects include a dozer and an excavator. A dozer can first be used to scrape off the topsoil and pile it up for an excavator to load trucks and haul away.
Step 2 – Subgrade
Once the area is free of topsoil, the subgrade must be prepared. The subgrade refers to the native material below the parking lot. Cut and fill is the process of adding and/or removing dirt to match proposed grades on the engineers plan. Typically soil that is ‘cut’ from one area of the site is moved to an area of the site that needs a ‘fill’. When filling a site, care must be taken to evenly spread and properly compact the soil. Too thick of lifts or improper compaction can lead to future premature failure of the finished asphalt surface. Again, depending on the size and complexity of the parking lot, a variety of different machines could be used to prepare the subgrade. Excavators, dump trucks, graders, compact skid loaders and dozers are all options when it comes to moving dirt. Compaction throughout the subgrade process is performed with a vibratory soil compactor. Step 3 – Stone Subbase Subbase is the layer of crushed stone between the subgrade and the asphalt base course. The thickness will vary depending on the pavement design, but normally you will see a 6 inch thick subbase. The subbase is typically a natural crushed stone (limestone in the Lehigh valley area) with a maximum stone size of 2” with some finer stone mixed in. This mixture of stone size allows for easy grading and excellent compaction. Compaction is key and the most important part of a long lasting pavement section. The subbase stone is sourced from local quarries and hauled to the job site using dump trucks. Equipment such as a dozer, compact track loader and/or grader is used to grade the stone. Once the stone is graded, the subbase should be compacted using a vibratory roller. Once the subbase is compacted and on grade, it’s time to make the parking lot black.
Step 3 – Asphalt Base Course
The asphalt base course is the layer of asphalt pavement directly above the stone subbase. The purpose of the base course is to provide the majority of the structural capacity for the parking lot. The thickness of base course will often vary between 3 and 6 inches. A 3 inch thick base course is typically sufficient for a parking lot with only passenger car traffic, while a parking lot that primarily sees loaded semi-trucks will require a thicker base course up to 6 inches.
25mm Hot Mixed Asphalt (HMA) is most often used as a base course in parking lot construction. HMA is a combination of stone, sand or gravel bound together by asphalt cement and mixed at temperatures of temperatures of 300°F to 350°F. 25mm refers to the largest size aggregate in the asphalt material. 25mm is favored as a base because it allows for a greater lift thickness when surface smoothness or appearance isn’t necessary. To place the asphalt base course, three main pieces of equipment are required; an asphalt paver, dump truck and a smooth drum vibratory roller. The dump truck hauls the asphalt material from the mix plant to the job site and feeds the asphalt paver. The paver then evenly places the material at the required thickness. The vibratory roller then compacts the loose material to its final thickness.
Step 4 – Asphalt Wearing Course
The asphalt wearing course is the final layer of asphalt pavement. The purpose of the wearing course is to provide additional structural capacity, a smooth driving surface and an attractive appearance. The thickness of the wearing course will often be 1.5 inches. 9.5mm Hot Mixed Asphalt (HMA) is almost always used as a wearing course in parking lot construction. 9.5mm is favored as a wearing course because it allows for a smooth driving surface and produces a nice surface appearance. Placement of the wearing course is similar to Step 3, the asphalt base course.
Step 5 – Line Striping
Line striping and other pavement markings are the last step to completing the asphalt paving process for a new parking lot. Parking stalls, handicap parking spaces, fire lanes, directional arrows are all common markings that may need to be marked prior to opening up the parking lot. Parking space dimensions and even drive aisle widths need to meet local ordinances or approved plans. Parking lot pavement markings are most commonly placed with a specialized walk or ride behind paint sprayer.
Conclusion
New parking lot construction may be a fairly simple process, however it takes an experienced team along with a load of equipment to produce a quality product. When designed and constructed properly, a parking lot can last 20-30+ years before needing to be resurfaced. With experience in both the engineering and construction side of parking lots, and 50+ years in the business, Charles D. Herman, Inc. can deliver a finished product that will exceed your expectations. Contact us today if you have a project for us.
