Sinkholes, Voids & Old Foundations

The ground isn’t always solid. Hidden voids, old foundations, buried tanks, cisterns, abandoned wells and natural sinkholes can open under a machine or a worker with almost no warning. Excavation work disturbs the ground and the water in it, which can trigger collapses. Knowing the signs and checking before you dig can prevent a disaster.

• Construction
• Excavation
• Utilities

What causes voids and sinkholes

• Natural sinkholes (karst): in areas with limestone, gypsum, salt or other soluble rock, water dissolves the rock and creates caves and voids. The ground above can collapse suddenly. Large parts of the Southeast, the Appalachian region and the south-central U.S. are known for this, though it can happen elsewhere.
• Water leaks: broken water mains, sewer lines and storm drains wash soil away and leave voids under pavement and slabs. Heavy rain and flooding add to it.
• Dewatering and groundwater changes: pumping water out of an excavation, or a long drought, can lower the water table and trigger collapses in weak ground. See Dewatering.
• Old mines, tunnels and quarries, and shallow underground workings that are forgotten or poorly mapped.
• Man-made voids: old foundations, basements, cisterns, wells, septic tanks, buried storage tanks, abandoned utilities, backfilled trenches and poorly compacted fill.
• Vibration and heavy loads from equipment, traffic and pile driving can collapse a weak roof over a void.

Warning signs on a site

• Depressions, dips or small circular sags in the ground or pavement, and cracks that form rings or arcs.
• New cracks in walls, floors and slabs, or doors and windows that stick.
• Cloudy or muddy water in wells and ponds after rain, or water suddenly draining away.
• Trees, fences, poles or buildings that tilt or lean, and exposed foundations or pipes.
• Soil that falls into the trench from below, or a hole that opens in the floor of an excavation.
• Unexpected soft ground, a rod or probe that drops through, or machines that sink or tilt.
• Old maps, site history or records that show prior buildings, tanks, wells or mines.

Before you dig

• Study the site. Review old maps, aerial photos, records, geotechnical reports and utility records. Ask the owner and neighbors about previous buildings, fuel tanks, wells and cisterns. See Private Utility Locating and Call 811.
• Get a geotechnical investigation where risk is high: borings, test pits and tests that look for voids and weak layers. Ground-penetrating radar (GPR), electrical resistivity and other geophysical surveys can detect voids.
• Probe and pothole in suspect areas before moving equipment in, and keep heavy machines off areas that haven’t been checked.
• Plan the dig with a competent person and, when needed, an engineer, including protective systems, equipment setup, access and a plan if a void is found. See Protective Systems and Soil Classification.
• Check weather and water. Heavy rain, flooding and thaw increase the risk. See Frozen Ground & Winter Excavation.
• Make sure crews know the signs and the stop-work rule. See Safety Culture & Stop Work.

Old foundations, tanks, wells and cisterns

• Old foundations and basements may be backfilled with rubble, loose soil and trash, which settles unevenly. Slabs can have voids beneath.
• Buried tanks, such as old heating oil or fuel tanks, can leak, contain explosive vapors or collapse. Don’t cut or move them. See Contaminated Soil & Groundwater and Drums & Tanks: Hot Work.
• Wells and cisterns are often covered by rotted boards or buried under soil. A person or machine can fall in. If you find one, stop, fence it off and report it to the owner and the engineer. Don’t enter. They may be confined spaces with bad air. See Manholes & Underground Vaults and Confined Spaces.
• Septic tanks and sewers may collapse under weight. See Sewer & Septic Excavation.
• Abandoned utilities can contain old hazards, such as asbestos pipe or live lines. See Supporting Exposed Utilities.

If a void or sinkhole appears

1. Stop work and get people away from the hole and the surrounding ground. Move equipment back carefully, if it can be done safely. The ground around a hole is unstable and the opening can grow.
2. Keep everyone back at a safe distance, such as at least the depth of the hole or more, and set up barriers. See Barriers & Fall Protection at Excavation Edges.
3. Call 911 if anyone has fallen in or is trapped. Don’t send untrained people in to rescue them. See Trench Emergencies.
4. Check for utility damage: broken water, gas or electric lines can make things worse. Call the utilities. See Utility Strikes.
5. Notify the owner, the engineer and the authorities. A geotechnical engineer should look at it and decide how to investigate and repair.
6. Don’t just fill it with whatever is nearby. Voids usually need engineered fixes, such as flowable fill, grout, compaction and re-designed foundations.
7. Document the event with photos, notes and measurements.

Protecting people and the public

• Sinkholes and voids can damage buildings, roads, pipes and utilities, and injure the public. Barricade the area, post warnings and watch for changes. See Work Zones.
• Keep heavy equipment, crane outriggers, stockpiles and concrete trucks away from edges and suspect ground. See Surcharge Loads and Slopes & Unstable Ground.
• Use mats, cribbing or engineered platforms to spread loads over weak ground. See Blocking & Cribbing.
• Monitor buildings and ground nearby for movement during and after work, with survey points or gauges if recommended.

Void and sinkhole checklist

• Site history, maps, records and geotechnical information reviewed
• Suspect areas probed or scanned before heavy equipment is moved in
• Crew knows the warning signs and will stop work and report them
• Old tanks, wells, cisterns and foundations found are fenced off and reported, not entered
• If a void opens: stop, get everyone and equipment back, call for help, notify the engineer
• Voids are repaired with an engineered fix, not loose fill

Last updated: October 4, 2026