Soil Classification

From strongest to weakest, soils can be listed in order as:

Rock > Gravel > Sand > Silt > Clay > Organic Soils

The word “strong” here, means shear stress supporting ability, which in turn means bearing capacity of the soil, which basically means, load carrying capacity of the soil. These will be talked about later.

Usually in nature, unless we are dealing with just a solid piece of rock, most soils can contain a mixture of gravel, sand, silt and clay, but almost always one portion is dominant. Sometimes the percentage of minor portions are so little that a soil can entirely be classified as solely of a single type above. So a soil can be classified as only sand or silt or clay, where another can be classified as sandy clay or silty sand and so on…

Any soil, which is not considered as rock or very soft organic soil, can generally be classified into two basic groups, such as clays and sands. In general sands are stronger than clay soils. Sands also permit water to flow easily, while clays do not, which has many foundation engineering consequences and that is why sand is usually preferred and even used to replace clays. Clays, has particle sizes of less than 0.002 mm, then comes silt, with particle sizes between 0.002 mm to 0.05 mm, then sands between 0.05 to 2 mm, and larger particles are called gravels. There are of course different soil classification methods. Clays can also be called as cohesive soils, while sands are called cohesionless soils.

So in general soils (except rock) can be classified very generally into two groups and their characteristics are:

SANDS : Cohesionless. Coarse Grained / Granular. Highly permeable for water. Not Plastic. Stronger. Not Expanding (Swelling) or Shrinking.  Settles only elastically.

CLAYS : Cohesive. Fine Grained. Very low water permeability. Plastic. Weaker. Expanding (Sweeling) or Shrinking. Settles both elastically and with consolidation.

Again, to emphasize: Sands are coarse grained and cohesinless. They are also permeable and let water to flow easily. They are not plastic. They have generally better load carrying capacity. Gravels have same characteristics as sands. Clays are fine grained and cohesive and plastic. And they also have very low permeability, and flow of water is extermely slow through them. They have less load carrying capacity than sands. (If you are a not an engineer and will remember only 1 sentence from this subsection, or even from entire soil mechanics, remember the sentences in bold above). We will cover what elastic and consolidation settlement mean later.

Particle sizes are measured with sieves but for very fine particles sieves are not suitable and particle sizes are determined by suspending the soil in water and looking at what portion is descending in what speed, which is an indication of particle size, as larger particles descend faster than smaller particles. 

Clays are weaker and necessitate more foundation work, in some cases even deep foundations as piles, while for sands only regular shallow footings may be enough, for the same load from structure above. Rocks of course are the strongest foundation materials.

Clay soils also have great potential for swell or shrink, depending on presence or absence of water, which is another reason that they require more ground improvement or foundation work or in many cases replacement by sand altogether. The total cost of small to large damages done to structure foundations and slabs including buildings, roads, houses, pools etc… by expanding clay soils is a large sum of money all over the world. The swell amount is affected by factors such as the presence of water, vertical stress on the soil, amount of clay in the soil volume, and the initial density.

Because of swell, walls and slabs may crack or loose their straightness, doors may stick, foundations can tilt, differential settlement may occur from one location to another location in the structure, which can cause large undue stresses in various locations of the structure which can even lead to failure.

Sands can be compacted better by vibration. Clay soils are not sensitive to vibration but they can be compacted by compression.

Other than entirely dry soils with 0% water content, which is rare, almost all soils contain at least some amount of water, even if very little. Therefore, a soil sample should be made of the solid soil materials plus the water in the pores plus the air. Of course for fully saturated soils, water (in theory) occupies 100 % of the pores of the soil and there is no air. Water has very important effects on soil properties and in turn how a foundation behaves.

Silty soils are susceptible to frost and heave cycles.

Well graded soil means, it contains reasonable amounts of different size particles, so that they fill each other’s gaps and therefore make the soil stronger. Gap graded soil means, a soil has large or small particles but very little in between, which means weaker soil than well graded soils. (For geologists for example, the definition of well and poor graded is reversed, but for civil engineers, strength of soil what matters, hence such definitions) 

There are also the organic soils, which we did not even mention above, because for engineering purposes these soils are considered useless… In all cases they are assumed to have zero strength and must be completely removed and replaced with stronger material for any construction project.

Soil classification symbols by soil group:

Gravel – G

Sand – S

Silt – M

Clay – C

Organic (silt, clay) – O

Organic (peat) – Pt

Soil classification symbols by soil characteristics:

Well Graded – W

Poorly Graded – P

Low Liquid Limit – L (we will talk about liquid limit later)

High Liquid Limit – H

So based on these, a gravel which is well graded would be labeled as GW, or a low liquid limit clay would be written as CL and so on.

In the next post of this series, we discuss “Soil Strength”

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