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www.trentonpaving.com | Frequently Asked Questions > Aggregate Base: How deep?

Please view the cut-away picture on the right side of your page.  It demonstrates inadequate aggregate base (crushed limestone gravel) placed on soil that appears to contain clay--an unacceptable construction technique. 

The base of aggregate (crushed limestone) works with the asphalt layer(s) to support loads (cars and trucks) on the driveway.  The driveway will be designed according to the anticipated load --cars & pickups require less gravel base and asphalt thickness versus dump trucks and heavy equipment that require up to 80 mm of base gravel and two coats of asphalt.  That said, a residential driveway laid on stable sub-soil requires only 50 to 100 mm of new crushed limestone.  Stable sub-soil can be either the temporary drive placed after your house was constructed and comprised of 30 mm and finer gravel or undisturbed native soil that is 'gravelly' and drains well.  The new top layer of limestone gravel should be 18 mm and finer as frost may push (heave) larger stones through the surface of your new driveway.  This practice requires that rocks, clay and organic material are NOT present in the sub soil below the gravel foundation.

If the sub-soil contains rocks, clay and organics, deeper excavation is warranted.  Clay and organic substances by their nature retains water.  In winter the excess water in the sub-soil will freeze and expand, causing the asphalt mat riding on it to flex excessively and eventually crack.  In summer, the presence of poor sub-soils is like a sponge absorbing and releasing water, again causing the asphalt mat to rise and fall, eventually cracking.  Rocks will work upwards with frost, eventually popping through the driveway surface.  Once small cracks develop, the process accelerates with surface water able to enter the compromised area.

To avoid premature cracking on poor sub-soils, we routinely remove 8 to 10 inches of sub-soil and replace with aggregate base.  This extra thickness over any remaining soft sub-soil improves the frost barrier and provides the strength to carry design loads for residential driveways.  At times we will recommend geo-textile fabrics (see below).

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