How digging next door can make a building collapse

Digging next door can make a building collapse when the excavation removes the soil that holds up or braces a neighbor's foundation. Nacionale recently reported How can digging next door make a building collapse? What NYC requires to protect neighbors. The piece looks at the risk and at the rules New York City puts on projects to protect the buildings around them.

What was reported

The story asks something a lot of city homeowners wonder about. How can a hole on one lot put the building on the next lot in danger? It also covers what NYC requires from whoever's doing the digging.

All we have is the headline and a short feed summary. Neither says whether the story was tied to a specific collapse, and neither lists the exact NYC rules. For those details, read the original article. This is still a developing topic, so below we stick to the general picture.

Why digging next door is risky

A foundation doesn't just sit on the dirt directly under it. The soil around and below the footing helps carry the building's weight and keeps it from sliding or settling. Dig a deep hole close by and you can take some of that support away.

The risk goes up with these:

• Shallow or old foundations. Plenty of older rowhouses and attached homes in New York, New Jersey and Pennsylvania sit on shallow footings or rubble stone walls.
• Shared or party walls. When two buildings share a wall, work on one side can affect both.
• Digging below the neighbor's footing. If the new hole goes deeper than the foundation next door, the soil under that foundation can shift toward the hole.
• Water. Pumping out groundwater, or letting rain wash into an open hole, can loosen soil and cause settling.
• Vibration. Heavy equipment, pile driving and demolition can shake older masonry.

When things go wrong, the first signs are usually small. You'll see new cracks, doors or windows that start to stick, or floors that seem to slope. In the worst cases a wall or a whole building can fail.