Whenever we get called in to do an interlock repair the cause is almost always the same: the gravel base. This part of a patio or driveway that no one sees is crucial to the longevity of the project. As landscapers In the Kemptville and Winchester area, we are fortunate to have numerous gravel quarries within minutes of town. The proper selection of the type of gravel is just as critical as the thickness.
The Granular Base
Most interlock driveways and patios we see that have heaved or settled are a result of stone dust (limestone screenings) being used as the only base material. The current manufacturer’s guidelines for interlocking stone require a 6” layer of Granular ‘A’ (3/4”<0) size material at least. Some places where heavy clay is present under a driveway, it is a good idea to use a second layer of 2” Granular B under the Granular ‘A’. This can make the base material as much as 24” thick overall. The most important thing is: DO NOT USE STONE DUST. The stone dust does not protect the paving surface from frost heave and settling over time. Stone dust does not drain as well as Granular ‘A’ and holds water against the paving stones. This can cause efflorescence (white chalky residue on the surface) and lead to premature stone failure due to de-icing salt getting trapped under the stone.
The Bedding Layer
The other place we see stone dust used is under the pavers as a bedding layer. The bedding layer is the 1” layer that is not compacted prior to installing the pavers. The paving stones press down into the bedding sand helping to lock them in place and not have any voids at the bottom of the joints between stones. All manufacturers call for free draining sand to be used for the bedding layer. This is the place where stone dust can cause the most damage with accumulation of salt against the bottom of the stone.
Unfortunately, the only fix for improper base is removal of the stone dust and installation of new Granular ‘A’ base. Other important parts of a quality base are the use of geotextile fabric under the granular base (especially on clay soil) and the proper compaction of the gravel.
So, for your next interlock patio or driveway project, make sure the base is not stone dust and be certain that whoever is doing your project uses the proper base preparation and you will be free to enjoy your new interlock patio for years to come.