The Paving Blog

I want to take a moment and deal with the single most important issue of all regarding driveways, patios, sidewalks and any other item in the paving sphere: the sub grade.

This decidedly unglamorous topic has a relevance that could conceivably ruin any of the work that gets done above it. I am sure we have all seen roads where the pavement dips or cracks. I assume many feel this is somehow due to poor maintenance issues or maybe to an inferior product installed as a surface. And, indeed, many of you would be right. This also happens. In fact, every manufacturer has “bad runs” of stock or material. It just plain happens at the manufacturing level, from human or mechanical errors we all suffer from.

But by far in the overwhelming majority of cases, roads, streets, patios and driveways fail when they do because of ill-prepared bases. It’s an infrastructure issue of the utmost importance. Its failure has ruined many a project.

Strict preparation goes into any estimates made for paving installations. Among the strictest and most demanding preparation deals with the strata of materials laid in just prior to finishing. What needs to happen in all cases is an assessment of the strength and sustaining structural integrity of the soil beneath the work.

Many “clayier” soils bend over time, especially those recently excavated and re-filled after the construction of a home. Sandy soils do this as well, unless they are seriously flooded with water to ascertain the limits of their sinkage after back filling. The topsoil I use in landscaping projects will sink up to 12% of its original size, even after foot traffic. Sand is very similar. Four season climates in particular need to have a sub-base composed of substantial enough material, at a sufficient depth, to counter the effects of climate and weather.

The typical rule of thumb has been roads and such require at least 12″ of base material. Now, base material is a mixture of gravel and “fines” (those finer particles which act like cement in the binding together or larger pieces, and they come from the rocks themselves). Driveways, all else being equal, require a minimum of 6 inches of material laid in over a “compacted” earthen base. OK, so what the heck is term “compacted”?

“Compacted” material is that which has been run over numerous times by some sort of compacting device. A steamroller, for example, as many call them, or drum rollers which are enormously heavy and which actually vibrate, allowing the fine particles to further embed themselves snugly between the larger gravel components. Water is often used to insure further compaction, as it takes the finer particles and distributes them even further in between the rocks. In fact, any compacted material is typically asked to have 6%-12% water content for serious compaction.

Streets and roads are tested by radioactive isotope testing kits which can produce sonic and other waves to ascertain the degree of density and compaction. They then produce a mathematical percentage: 98% compaction is generally required before any material can be laid atop a sub base on the street or highway. For driveways, a 96% rating is often good enough, with so little comparative passage. Likewise, sidewalks and patios are ideally in the 94%-98% range, depending on what is even possible. Anyway, when these figures are met, the result will be that you stand a far, far better chance of this structure not moving whatsoever over ensuing years or even decades.

My advice for anyone hiring contractors to prep and install paving at their home would be two-fold:

1: Determine if your contractor is familiar with compaction issues. Ask what material and how much he is planning to install. Ask him what criterion he used to come up with the depth he may be using. Now, it could be that there was an abundant amount of base material already in place if you’re redoing an existing driveway. Naturally, that would also assume the proper depth and type of material was correctly installed at that time. It could be the sub grade was the failing point of your original piece.

2: Ask him how he intends to compact it. Compaction should never be for over 6″ of material with a hand held plate compactor, nor over 8-12″ with a double drum roller. If he has a riding roller, then he will get an excellent compaction for any depth not over 12″ thick.

The other issues, things like grading (everything needs to slope, somewhere.) and drainage, side hills and the water traffic onto your edifice, gutter behavior and the likes, (including tree roots nearby!) are all other factors to consider and which a smart buyer will ask about.

But for those of you who have seen your back yard patio sink and puddle repeatedly. For those who have watched the same in a driveway, which catches water and deteriorates, sinking in spots and heaving in others, all these things tend to be compaction and sub-base related. A smart buyer will make sure he knows what the heck is planned and is occurring is such an expensive and complex operation as any of these projects.