A sinking asphalt driveway shouldn't be something you have to live with as a homeowner. A permanent repair is possible, but pinpointing the cause is the key to making sure it doesn't return. There are several possible reasons why an asphalt driveway would subside, and some of the most common causes are explained below.
Poorly Compacted or Unprepared Base
As with any building project, a driveway must have an adequate base of support to maintain its integrity for the long-term. Even though the asphalt itself is only a couple of inches deep, a properly prepared base of support should be no fewer than six inches deep. In some cases, eight inches is the minimum requirement, especially if you expect heavy vehicles to use the driveway.
Depth itself isn't the only requirement for a subsidence-free driveway. The base must also be properly graded, then compacted, and it must extend well past the edge of the asphalt surface above it. A failure to build the base to proper specifications will lead to eventual sinking and collapse of the asphalt.
Broken Water, Sewer, or Other Utility Line
Another cause of sinking driveways is the presence of a broken, decayed, or rusted underground utility line. This problem is more common with wider diameter structures, such as water lines and sewer lines, and can occur with lines that are no longer in service.
As these structures collapse, the new void is filled by the soil above; a chain reaction ensues that ends with the driveway surface also dropping. Of course, in the case of water and sewer lines, underground erosion also becomes a problem and can lead to significant damage to a driveway.
Soil Erosion Due to Drainage Problems
Other types of soil erosion can also be a source of a subsiding asphalt driveway. These problems can occur due to poor existing drainage conditions, or they can also be the result of improperly placed drainage features.
For example, if a driveway is bordered on one side by a bar ditch that provides drainage relief for the blacktop surface, then water can scour beneath the driveway during heavy rainfall. Or, as another example, a downspout from a gutter can direct water flow into the vicinity of the driveway. This flow can undercut the driveway or create too much localized saturation, thus resulting in instability and erosion.
Rotting Stump or Other Decomposing Bio-Structure
A common cause of asphalt subsidence is the presence of an underground bio-structure. Such structures include all types of living and nonliving plant materials, including stumps, root clusters or even lumber. As these structures rot or decompose, the end result is much the same as what occurs when a utility line collapses. The soil and driveway above it slowly sink into the ever-increasing void.
The major difference between this type of decomposition and utility line collapse is that plant materials can take years, or even decades, to complete the decomposition process. As such, the process of subsidence can continue on for a long time without detection.
Hidden Geological Feature or Instability
Another cause of a sinking asphalt driveway is some type of unknown geological feature or existing instability. This is rarer than some of the more commonplace causes above, but the end result can be dramatic and result in catastrophic damage to the driveway and nearby structures in some cases.
Some of the specific features that might cause this type of subsidence include sinkhole formation, earthquakes, riparian streams, inherently unstable soil or other wide-scale factors. Determining the exact cause may require the assistance of a civil engineer and other experts who possess the expertise to analyze the underlying conditions. Talk with a contractor about asphalt maintenance so you can determine what to do to prevent such issues from occurring in the future.