Soil – aka “dirt”is a particulate material. This means it is composed of an assortment of individual particles which can be sorted into
4 basic groups: sand, silt, clay and rock (or gravel). Each one is defined by its individual size. Each also varies greatly in size, shape and color. The composition of any soil and its strength is determined by the quantity of each particle in the soil. In PART ONE of a series on soil, this graphic will go through what makes up soil the interaction between particles.

Soil – can be a very predictable material once you understand the things that influence it, like water, air or various loads.

Clay Particles

Clay requires a serious microscope to even see the individual particles. It is made from organic material as well as metal oxides, which often give it a distinctive color.





Silt Particles

Silt is sized between sand and clay particles; silt acts as glue for soil. It’s formed from rock and sand being ground and crushed into a flour-like substance.





Sand is the most common part of all soil. It gives soil its gritty rough feel and is primarily made from silica.








Water is a serious concern for soil. Because water cannot be compressed, it forces the soil to expand and increases pressures placed on shoring systems. It washes out smaller particles which would otherwise hold soil together. It occurs naturally (rain or high water tables) or through artificial sources such as pipe failures.






Gravel or small rocks can often be found as a component of soil. Geography plays a huge role in what type of rock, and how much is found.





This is what takes up space when no other physical particle is present.

Where there is no soil particle, or water – there is air.









Inter-Particle Attraction. Glues like silt and clay and the jagged edges of the sand pull the soil together.




Silt and Clays will act as agents in inter-particle attraction. Because they can get in between all the other larger particles, they help hold everything together.




Dry Soil

DRY: (25 psf)
A condition where the soil does not exhibit visible signs of moisture†. Water has little to no presence.





Moist Soil

MOIST: (35 psf)
When the soil looks and feels damp†. In this scenario, any voids are filled with a combination of water and air. In this condition some soils will exhibit better cohesion.





Wet Soil

WET: (45 psf)
When the soil contains significantly more moisture than moist soil†. Voids are now filled with more water than air. Usually at this point you can start to actually see the water mixed with the soil.





Saturated Soil

SATURATED: (65 psf)
This is a condition when all the voids are filled with water†. This does not mean that water is flowing. Testing tools often require this condition to produce accurate results.




Submerged Soil

SUBMERGED: (85 psf)
This is soil that is underwater or seeping water†. In this case, you’re literally swimming in it. This is soil at its weakest state and highest pressure as the water tries to expand and push the soil particles outward.




Gravel and sand can’t bond aside from their jagged edges. Larger particles act like ball bearings and roll around until they stick to or get stopped by something. This is not an OSHA term.

While also not an OSHA term, the glue is the elements in the soil that cause it to stick together and increases strength. The more glue, the stronger the soil.


Some topics are a little more complicated to cover in a couple sentences. For more information, follow the links below.

LATERAL EARTH PRESSURE: Earth pressures can be a complex subject.  Go here for more.

SURCHARGE LOADS:Surcharge is covered in detail here.

ANGLE OF INTERNAL FRICTION:Go here for a detailed explanation.

What is Shoring: Everything You Need to Know

Read More

The Basic Types of Shoring

Read More

Trench Failures & How You Can Avoid Them

Read More

Daily Excavation Inspections & How They Can Transform Your Builds

Read More

OSHA Trench Box Rules and Fines For Violations

Read More

Soil Pressure and the Angle of Internal Friction

Read More
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