To understand what liquefaction is, we must first understand shear stress in soils, which in turn means, the strength of soil…

Load carrying capacity of any material arises from its shear strength. Soils, rocks, which carry our structures are no exception to this rule. The greater a foundation material has shear strength, the better it can support our structures with less settlement.

Shear strength is a result of how well the particles of a material stick to each other, and, the resistance they provide when they are made to slide over each other.

In other words,

Shear strength = The resistance provided particles sticking to each other + The resistance provided when particles resist sliding over each other

For soils, the main idea is the same as above, so let’s rewrite what we have just said above for soils. We can write shear strength of a soil material as:

This is the Coulomb’s equation for shear failure, where,

τ: Shear Strength

σ’: Effective strength

c’: cohesion

Ø’: Internal friction angle between soil particles

We will not explain how this equation is obtained here, which will be in another post. We only gave this equation to show that the shear strength for soil has indeed the two main components we mentioned above. And in this article our focus is the term σ’, effective strength, which means, the strength provided by the soil skeleton as a result of contact between soil particles. This is the main source of a soil’s strength. Pore water pressure contributes nothing to the strength because water practically has zero shear strength – this is why you cannot walk on water.

The apostrophe signs are for effective situation, which means, contribution from pore water is not included. If we include it, then we write another fundamental relationship in soil mechanics:

σ = σ’ + u

which was first introduced by Karl Terzaghi, father of modern soil mechanics, about 100 years ago. Here σ is total stress, σ’ is effective stress, and “u” is the pore water pressure. In other words, the total stress in soil is the sum of effective stress, which arises as a result of soil particles interacting, and pore pressure. This is why for example, we prefer foundations to remain above water table in the ground. Because in such locations pore pressure is zero and the soil skeleton can fully contribute to strength, which is all we have to obtain strength. But for depths below water table, for a given total stress, because of the equation above, effective component will be less than that, as there will also be pore pressure. This means the strength is weakened by that amount.

For example if we have saturated sand on which we apply new load, at first both the sand skeleton and water pressure resist the load, but in sand (which is highly permeable) water quickly dissipates and u drops to 0, and what remains is the effective strength that is countering the total stress as, σ = σ’. Given that we did not apply the load too suddenly, the sequence of events take place as described above. Note that in clays this happens too but at a far slower rate due to clay’s very low permeability, so u becomes zero only after a very long time, which could be years, as clay keeps settling (gets denser) very slowly (which means σ’ increases very slowly) which is the subject of consolidation, which we will cover in a future post.

After this fundamental background knowledge, we can now proceed with describing liquefaction.

Liquefaction:

Liquefaction of soil, misunderstood by many, is not simply settling of a structure due to soil deformation. Liquefaction means, soil practically becomes like a liquid and looses its ability to carry any load.

Imagine we have a structure on soil. The structure is able to stand there, because the soil material has the strength to carry it. Remember the equation, σ = σ’ + u. In the figure below, the soil is saturated, but the soil particles are in contact with each other, and therefore provide resistance.

If we disturb the soil however, such as by shaking it, such as during an earthquake, the soil can become like below. Remember a few paragraphs above, we said “given that we did not apply the load too suddenly”. Well, the situation below is when the load is applied suddenly and / or there is considerable shaking action on saturated sand.

Here, the soil particles lost contact. The entire soil mass practically becomes like a liquid. What happens in a saturated sand zone during an earthquake is that in the first instances of the shaking, water does not have enough time to escape, so its pressure increases, to such degree that the equation σ = σ’ + u, turns into σ = u as the effective strength, σ’ becomes 0. And since water cannot carry any load, the structure above starts to sink, as if sinking into a liquid. This is the process of liquefaction.

From what is described above, we can understand that, for liquefaction to occur:

-The soil must be made of mainly sand. Clays, and to a great extent silts, are not liquefiable. Their particles are simply too small for the shaking action to be able to liquefy the soil because water cannot move and penetrate inside with such ease as in the case of sand.

-The soil must be saturated.

-There must be a shaking action, such as earthquake, blast or vibration.

-The sand must be loose. The denser the sand, the less the effect of liquefaction will be.

So in areas where there is liquefaction potential, such as sandy soils with shallow water table below ground, liquefaction potential and its effects must be assessed, and proper measures must be taken beforehand, such as using deeper foundation systems, densifying the soil with various methods (which we intend to discuss in a post about ground improvement at a later date), if that location cannot be avoided altogether.

You can test all of the above by yourself. Pour some sand in a plate. Put an iron or wooden block on it. Then shake the plate. The block will not sink significantly. Now repeat the experiment, but this time make sand saturated with water first. When you start shaking, the block above will sink, because by shaking a saturated sand, you effectively can turn it into a liquid. This is the idea behind liquefaction.

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