Subsidence is often where the surface below the asphalt is weakened by:
Erosion.
Or, the ground (substrate) under the asphalt covering, will not support the load. I.e. The ground does not have sufficient, suitable material to support the load, and the supporting material may not have been sufficiently compacted to prevent settling.
Typically the area will sink, the asphalt surface will stretch, which in turn causes cracking. The asphalt surface can then break up, leading to potholes.
How to Repair a Driveway – Subsidence & Potholes
Overloading:
Can be caused by overuse, such as repeatedly parking in the same spot. Or, placing something heavy on top of the asphalt, (such as an axle stand, where the feet penetrate the asphalt, as the load has not been evenly distributed – A point load).
Changes in temperature can also have an affect. E.g. heat causing asphalt to become more pliable and break up. Or, where heavy items sink into the asphalt.
Remove the source of erosion – Build a wall to support the ground, or divert water flow. Plants and bushes can sometimes be used to stabilise a large soil bank.
Spread the load of heavy items across a larger area. E.g. Placing axle stands on a plywood board, (note: some materials can split under load making it unsafe).
There are three choices of repair, dependant on the state and size of the hole (dip in the surface):
Holes – Or where the surface is breaking up:
Dig out the hole, backfill and compact with a suitable material that will support the load. You may want to fill with concrete, mixed with aggregate to provide a more robust repair?
Fill the hole just below the level of the driveway (if you want to provide a matching surface to the original asphalt).
When dry, it can be overpainted with bitumen paint.
The bitumen paint can be mixed with small stones, as used in the original asphalt if you need to match the texture.
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