Drainage
What causes a dip in a parking lot?
A dip means material under the pavement has gone somewhere, usually a settled utility trench, a failing drain structure, or water washing the base out.
The numbers
- A dip is the asphalt following something that moved under it.
- Straight-sided dips often trace an old utility trench.
- A dip around a grate points at the drain structure rather than at the asphalt.
- A dip that is getting deeper season to season means material is still moving.
- Which of the three it is comes out when somebody opens it up, and the fix follows from that.
A trench that was never compacted
When a pipe crew opens a lot, the trench gets backfilled. Backfill placed all at once instead of in layers, each one compacted, keeps settling for years afterward. The surface over it follows it down, and the dip has straight parallel sides because it is the shape of the trench.
It is the most common cause on a lot that has had any utility work done in it, and it is why a trench patch is backfilled in lifts rather than in one go.
A drain structure coming apart
A catch basin is a structure with joints, and joints open. Once one does, every rain washes a little of the surrounding base into the pipe. Nobody sees the material leave. What shows up is the ring of pavement around the grate sinking, then cracking, then breaking up.
The give-away is where the low point is. If the water runs to the grate and the pavement is lowest right at it, the structure is the thing to look at first.
Water taking the base away
Anywhere water concentrates and can find a route out, it carries fine material with it. A downspout onto the lot, a curb that has broken, a cracked pipe under the surface. Over a few seasons that hollows out a void, and the pavement over the void sags and then breaks through.
Repairing the surface without finding the route the water is taking gives you the same dip in the same place, which is the sequence behind most repairs that get called failures.
None of the three can be told apart from the top with any confidence, so the useful next step is to walk the lot with somebody after a hard rain and, where a structure is in question, have it looked at before the surface work gets priced.