Sand safety: What to know before digging tunnels or building sandcastles at the beach

This story from The Conversation is by Stephen P. Leatherman, Professor of Coastal Science at Florida International University.

While millions of Americans vacation on beaches every year to seek out sun, sand and the sea, many might not realize how dangerous digging holes in the sand can be. In February 2024, a 7-year-old girl died after an approximately 5-foot (1.5-meter) hole she and her brother dug in the sand collapsed in on her, burying her alive.

As a coastal science researcher who’s been studying beaches for many years, I was called in to help investigate the girl’s death. While many people nearby stepped in to try to free the girl after the sand hole collapsed, local firefighters couldn’t arrive until several minutes after the incident – too late to resuscitate the victim.

Digging holes in sand might seem innocent, but if the hole is deep enough and collapses on a person, it is extremely difficult to escape. In fact, research suggests more people die from sand burial suffocation than from shark attacks.

Sand basics

Sand isn’t actually a type of material. It’s a category of material size, ranging from 0.0025 to 0.08 inches (0.06 to 2 millimeters) in diameter. The type of sand is determined by the materials making it up. Quartz sand, made up of silicon dioxide, is the most common sand found on beaches, except at tropical coasts where coral sand beaches, made up of calcium carbonate, are found.

Material coarser than sand is not soft to the touch – it doesn’t make sturdy sandcastles. Silt and clay, which are finer than sand, make water murky and are commonly called mud.

Sand’s weight depends on the materials it’s made of. Pure quartz sand beaches, which have very white sand, weigh around 90 pounds per cubic foot when dry.

But most beaches contain a mixture of minerals, creating a tan or brown appearance. The minerals that darken the sand are much heavier – sand on most beaches would weigh up to 130 pounds per cubic foot when dry.

Dry, loose grains of sand will form a pile with a slope angle of about 33 degrees, termed its angle of repose. The angle of repose is the steepest angle at which a pile of grains remains stable, and the force of friction between each grain determines that stability.

Sand is more stable when it’s wet because the surface tension between water and sand grains can hold the pile of sand in place vertically. But once it dries, the pile will collapse, as there’s no more surface tension.

So if you dig a hole in the beach, it’ll stay stable for as long as the sand stays moist. Once it dries, the hole collapses.

Sand is unstable

When either the sand forming the hole dries out or someone stands near the edge of the hole, adding extra weight, the sand hole collapses in, and the heavy grains fill all open spaces in the hole. This leaves no air available for a trapped person to breathe.

While skiers trapped in avalanches can cup their hands to form an air pocket because snow is light, but that’s not the case when sand collapses.

Rescuing someone from a collapsed sand hole is very difficult because sand is both heavy and unstable. As rescuers scoop away sand to free the victim, the hole will continue to collapse under the rescuers’ weight and refill with sand. Rescuers have only about three to five minutes to save a person who is trapped in a sand hole before they suffocate.

Professionals like firefighters will place boards across the hole when rescuing someone from a sand hole collapse. This way, they can reach down and use tools to remove the sand without putting any weight directly on the edge of the hole.

Experts recommend never digging a hole deeper than the knee height of the shortest person in your group – with 2 feet (0.6 meters) being the maximum depth.

To rescue someone in a collapsing sand hole, focus on exposing their mouth and removing sand from on top of their chest. If you expose their mouth, you can administer rescue breathing while other rescuers continue digging out their chest.

Too many people crowding a sand hole rescue can cause more harm than good. Just two or three rescuers should work in the victim’s immediate area while others work on clearing sand away from the wider excavation area, which makes it easier for those in the center to remove sand. The people on the outer perimeter can clear sand away from the central area using anything available, from buckets and shovels to beach chairs and boogie boards.

Case studies

Collapsing sand holes led to 31 deaths, mostly kids and 87% male, from 1997 to 2007 in the U.S. During that period, 21 others were in a reported sand hole collapse but survived, though many required CPR.

Victims of sand hole collapse have ranged in age from 3 to 21 years. The holes were generally 2 to 15 feet (0.6 to 4.6 meters) in diameter and 2 to 12 feet (0.6 to 3.7 meters) deep. Digging, tunneling, jumping and falling into the hole have all inadvertently triggered collapse.

These collapses can happen suddenly, and in situations that don’t seem dangerous to most. During your next trip to the beach, make sure to keep an eye out for sand holes and fill all holes as soon as possible. Even a shallow hole can injure someone who stumbles into it.