Millennium redux? San Mateo Rite Aid says dewatering sank floor

By Kathleen Pender : sfchronicle – excerpt

The owner of the Rite Aid Pharmacy in San Mateo is suing the developer of a large office complex under construction nearby, saying that dewatering at the project caused the store’s floor to sink up to 9 inches in some places, resulting in extensive damage.

If this sounds familiar, it’s because the developer of the Millennium Tower in downtown San Francisco has said the 58-story condominium building sank more than expected because of dewatering at the neighboring Transbay Transit Center. The Transbay Joint Powers Authority rejected those claims, saying the sinking began before the dewatering started and that other nearby buildings were not affected.
Dewatering involves temporarily removing water from the ground around a construction site. If you dig a hole deeper than the water table, the hole will fill up with water, like it would if you dug a hole in the sand near the ocean.

When builders excavate below the groundwater table, they often first sink small wells around the perimeter of the hole to lift water out of the soil. That lowers the water table in the construction site and prevents the excavation from filling up with water. Depending on the type of soil and geology under the sites, it can also cause settling under neighboring buildings…

“It’s a real phenomenon, and it has caused real damage to buildings in the past,” said Alan Kropp, a geotechnical engineer in Berkeley.

Property owners sometimes blame damage on construction dewatering, but it can be hard to prove because other things can cause settling, such as pile driving. If a building sits on what’s known as expansive soil, it could subside during a drought… (more)

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