Sod getting taller!

A few years ago, we had our old lawn removed, and a new sod installed. Looked great, but over time, crabgrass returned to a corner of the new lawn, where it once existed in the old lawn. What I can't figure out is why the height of the dirt in that area has increased. It is like the dirt is multiplying! Maybe crabgrass roots are "swelling" the soil? Still, this is happening, to a lesser degree, along other parts of the lawn, all on the lowest part, which is against a sidewalk. This is evident when the lawn is edged, as the soil must be 2-1/2" high or more. The soil used to be even with the sidewalk. I have to be really careful not to scalp the lawn when mowing, whereas this was not a problem before. It is like my entire lawn is slowly sliding to the street, and building up in size at the sidewalk edge. Also, the pop-up sprinklers can no longer clear the sod/lawn. The slope to the sidewalk is slight (18" over 30 feet maybe). What is happening, and how do I stop it? And correct it.
Thanks,
- Phil
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Incline you mentioned is 1/2" on 1 foot. A bit steep for drain plumbing, but okay for the lawn. Soil is moving from some uphill location to the bottom of your lawn. Observe, reflect, and provide that. Dave
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Get out a nice, sharp spade and cut and lift a square of sod. Bet you're going to find a thick layer of thatch that looks like this: http://www.ppdl.purdue.edu/PPDL/weeklypics/Weekly_Picture6-10-02-3.html
Follow the instructions on the web pages mentioned for lawn management.
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