In Michigan, you should use a gravel base that is compacted in lifts of no more than 4" thick to insure proper density. We have always compacted the subsoil (dirt) with a plate compacter or hand tamper prior to installing the paver base. We also like to take the extra step of installing a geotextile fabric material between the subsoil & gravel base material as a seperator. This will insure that during freeze thaw cycles, the gravel paver base will not be compromised over the years from the "churning" action of frost. This way your paver base will not be "polluted" from dirt below, thus allowing for proper drainage under your brick pavers.
Always rake out your paver base material close to the final contour of your finished brick pavers before compacting. Make sure your finished grade of paver base slopes away from your house or follows closely the same pitches you want to achieve with your pavers. The height of your paver gravel base should be approx. 3.5" below your desired brick paver height. This will allow for your 1" bedding course and the thickness of the brick paver. If your paver base material is really dry, wet it gently prior to compacting so that the fines (dust particles) will settle and bond during compaction.
Small or tight areas can be compacted with a plate hand tamper but for large areas like a paver patio or paver driveway, a gas powered plate compactor should be used. Rake out any humps or depressions and recompact before installing your bedding course (1" sand layer) over the top of the compacted gravel paver base. Keep in mind that your paver base is the foundation for your brick paver installation and the top (surface) will mimic the foundation (gravel base) over time.
Please consult with your local brick paver dealer or brick paver manufacturer in your area to advise on the proper or acceptable base materials to use in your interlocking brick paver installation. Proper drainage below your brick pavers will insure long-term success and functionality.
