Installing a new landscape often involves the use of heavy equipment. Although this can speed the work and reduce the amount of time the site is a mess, it can also compact soils.
This site has heavy clay soil, so the equipment didn’t do the soil any favors.
Luckily, there are ways to fix the problem. The traditional solution was to rototill organic matter into the clay – although the tines of the tiller can themselves cause compaction. There is now a bit of controversy over this approach. An alternate method involves chopping the compacted soil into small chunks and mixing in organic matter. This way, the organic matter creates a path for roots to penetrate the soil – but supposedly with less risk of the soil turning anaerobic, caused by insufficient oxygen in the soil. Since the organic matter fills in cracks, it can allow air and water into the soil.
Another factor in this case is that the plants in this area were chosen for their ability to grow even in waterlogged clay soil. In time, their roots will penetrate the soil, breaking it up and allowing more water to infiltrate.