subgrade soil and compaction

Asphalt Pavement Guide: Subgrade Preparation

When it comes to asphalt paving, there are three absolutely essential structural elements that need to be constructed properly to ensure a long-lasting pavement. If any of these three are lacking or compromised, your driveway or parking lot will likely not meet standard asphalt expectations. These elements are subgrade preparation, pavement thickness, and drainage.

When discussing these three elements we are going to speak in terms of residential paving vs. commercial paving – in other words, your driveways vs. your parking lot.  In this blog, we will focus on subgrade preparation (look for drainage and pavement thickness in the coming two weeks).

Some things to consider when it comes to the structural design of your pavement:

  1. Any pavement needs to have sufficient structural capacity to carry the expected loads.
  2. The thickness of the pavement will depend on both the expected loads as well as the subgrade conditions.
  3. The subgrade is extremely essential, as it is the foundation of the pavement.
  4. It should be free of topsoil and vegetation.
  5. It must be well compacted and free of soft spots.
  6. A subgrade that is not properly installed or compacted will compromise the pavement’s structure in the future.

Residential subgrade preparation:

  • subgrade soil and compactionThe first step of any preparation is an extensive evaluation.
  • During an evaluation, it is important to look for any structural or subgrade problems. If these structural problems are found, the old asphalt will need to be removed to dig out the soft spot of the subgrade and refill it with 2-inches of rock and aggregate.
  • Bad soil is removed and replaced with about 12 inches of 2-inch rock and about 2 inches of dense graded stone.
  • During the preparation phase, leveling and contouring is very important in ensuring the pavement has proper drainage (one of our other essential elements). If there are low spots, or the subgrade in not level, drainage will not work as it should and can lead to water pooling in areas that it should not, causing stress to your pavement.
  • Thoroughly cleaning the surface of the old asphalt, and cutting the grass close to the side of the driveway are other important steps during the preparation phase.
  • During the laying of a new driveway, many work trucks will drive over your base in the process of laying the pavement, adding to the level of compaction.
  • Before paving, the subgrade will need to be stable and compacted with no settling.

Commercial subgrade preparation:

  • subgrade soil and compactionPreparing subgrade for a parking lot it very similar but differs slightly.
  • Parking lot subgrade soft spots are replaced with about 12 inches of 2-inch uniformly-sized rock. In addition, fabric is often placed under the 2-inch rock as a means of further strengthening the base, since a parking lot will likely see much more traffic and heavier loads.
  • During the preparation phase for a parking lot paving job, soil condition evaluation is extremely important in ensuring that a good parking lot is installed. Soil types can vary per location, and because of this it is important to hire a professional with the working knowledge of local soil conditions.
  • Based on the condition of the soil and/or general local soil conditions, certain treatments may need to be performed on your parking lot subgrade soil prior to asphalt installation.

The base of your pavement is extremely important in determining how long your pavement will last, and how well it will withstand the elements, making subgrade preparations and installation one of the most important steps of your pavement project; commercial or residential. Following subgrade assessment and repairs/adjustments you can examine and plan for the proper thickness and draining patterns for your pavement. The thickness of your asphalt will vary for your driveway vs. your commercial parking lot – stay tuned for our next blog where we cover thickness extensively.

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