A trench collapse or similar accident where a person is buried under soil presents a danger far more immediate than slow suffocation. The phenomenon at play is traumatic or compression asphyxia, where the sheer weight of the external material mechanically prevents the body from breathing. This occurs because the chest cavity is physically immobilized by the tremendous force, not because the lungs are pierced or damaged. To determine how much soil is required, it is necessary to understand how the weight of the earth translates into pressure on the human body.
Understanding Pressure and Soil Density
The weight of soil translates into pressure, which is a measure of force distributed over an area, calculated using its density and depth. Soil is not a uniform material, and its density varies widely, making the resulting pressure highly dependent on the conditions of the collapse. For instance, dry, loose soil, such as sand or topsoil, typically weighs approximately 85 pounds per cubic foot (pcf).
The presence of water drastically increases this density because the water fills the small spaces between soil particles, adding mass. Wet or saturated soil, such as that found after rain, can weigh between 120 and 130 pcf. This means a given volume of wet soil can exert up to 50% more pressure than the same volume of dry soil.
The load applied to the body is a direct function of soil density and the depth of the burial. As the depth of the soil increases, the pressure applied to the chest and torso increases linearly. This conversion from a three-dimensional mass (volume of soil) to a two-dimensional force (pressure on the chest) is the first step in calculating the danger.
The Physiological Limits of Chest Compression
The mechanics of breathing rely on the ability of the chest wall to expand against external resistance. Inhalation is an active process driven by the diaphragm and the external intercostal muscles, which increase the volume of the thoracic cavity. This volume increase creates a negative pressure inside the lungs, which draws air inward.
The danger of compression asphyxia is the inability of the respiratory muscles to overcome the external pressure and expand the chest. This is known as restrictive breathing, where the external load physically prevents the volume change necessary for a breath. The maximum force the respiratory muscles can generate to inhale, known as the Maximum Inspiratory Pressure, provides the threshold for respiratory failure.
For an average healthy adult, the maximum inspiratory pressure the muscles can exert is approximately 1.5 pounds per square inch (psi). An external pressure that exceeds this capability will essentially immobilize the chest. Once the pressure surpasses this threshold, the muscles cannot expand the ribcage, and the person is physically unable to draw a life-sustaining breath.
Calculating the Critical Depth of Soil
Combining the physiological limit with the physics of soil weight allows for the calculation of the critical depth required to restrict breathing. The target pressure for respiratory immobilization is approximately 1.5 psi, which converts to 216 pounds of force distributed over a single square foot of the body. This is the minimum external load the chest muscles cannot overcome to inhale.
Using the density of dry, loose soil (85 pcf), the critical depth required to generate 216 pounds per square foot is about 2.5 feet. This means that two and a half feet of this lighter material provides just enough weight to overcome a person’s ability to breathe.
The danger is greatly amplified by soil saturation. For heavy, wet soil with a density closer to 125 pcf, the critical depth drops significantly to only about 1.7 feet, or approximately 21 inches. Less than two feet of saturated soil is sufficient to generate the pressure required to cause compression asphyxia.
The load does not have to be uniformly distributed across the entire body. If the weight is concentrated directly over the chest and abdomen, a much smaller total volume of soil can be immediately fatal. Therefore, the critical depth is a measure of the minimum vertical column of soil required, and the danger in any collapse is often measured in inches, especially in densely packed or saturated earth.