Soil Liquefaction

One of the most dangerous forms of ground failure during an earthquake is the soil liquefaction. It can lean down buildings, cracks roads, breaking down bridges and demolishing lifelines that are underground. Have you ever wondered why you get dramatic ground movements when you shake in certain localities, it is usually the result of liquefaction. I shall attempt to define liquefaction in the most elementary terms; how, when, and under what circumstances liquefaction takes place; and how it can be prevented by engineers. You will get to know about the nature of liquefaction, when and how it takes place during an earthquake and the soil conditions leading to higher probabilities of liquefaction. You will also have real occurrences all over the world so that you can visualize how liquefaction affects prodigal buildings, roads and life. Besides that, we will also address basic tests that engineers apply in liquefaction risk and few common ways to reduce damage. The point is, merely, to offer an understandable, friendly overview of the subject that seems far more complex than it should be. As a student of civil engineering or someone who is simply interested in what happens with the ground when it shakes, this blog will enable you to find a general opinion of what the basics are without the technical jargon. 

 

Definition

The phenomenon in which saturated loose sand in undrained condition under rapid loading. It occurs due to high frequency of vibrations.

How Liquefaction Happens

1. A phenomenon in which soil behaves like a liquid is called liquefaction.
2. It usually occurs during seismic events. 
3. Seismic shaking creates stresses that increase pore water pressure between the soil particles.
4. When pore water pressure becomes greater than the soil bearing capacity, the particles separate, causing liquefaction.

                          

What Causes Soil Liquefaction?

1. Granular soils like sand and silt have larger void spaces and are therefore more prone to liquefaction.
2. Fully saturated soil cannot compact tightly, which increases the risk of liquefaction during earthquakes.
3. Earthquake vibrations rearrange soil particles, reducing the space between them  and increasing pore pressure.
4. When this pore pressure matches the effective stress of the soil, it loses shear strength that leads to liquefaction.
5. The consequences include sinking and tilting of buildings and other structures.

Conditions of liquefaction

      Loose, granular soil

Fully saturated soil

Strong shaking

High pore water pressure

Low effective stress

Young or recently deposited soils

 

Dense Soil NOT Liquefy Easily Why?

The fact that dense soil is resistant to liquefaction, is somehow related to how the soil particles act upon shaking. Suppose that you think in this way, what is the usual cause of liquefaction? The development of pore water pressure is when the loose soil tries to squeeze during an earthquake. Now than when you ask yourself the question, will a soil that already is packed together compress any more, the answer is no. Compaction of dense soil can hardly occur, and hardly any pressure may be accumulated. Quite to the contrary, when thick sands are violently shaken, they actually have a tendency of trying to spread out a little-a process known as dilation-and it takes away the pore pressure instead of adding to it. Let us see another: Take another soil, and suppose that the grains therein are but lightly touching, which one would you think was to lose strength first, and which where the particles are attached to each other? Why, the interlacing of the particles in dense soils is greatly more, and the shear strength is greater, and so it can resist deformation even on violent shaking. It is because of all these reasons that the dense soils are much more stable as compared to loose or newly deposited sands hence becoming much less likely to be liquefied.

Types of Liquefaction

There are mainly four types of liquefaction. Each of them is clearly explained with examples:

Flow Liquefaction

The aggressive type is flow liquefaction. A loose and damp sand layer may become weak at once and the entire mass may slide or sink like wet gravel. It will normally occur on slopes or riversides since the ground will already possess a direction to run to. 

Case in point: In the 1964 Niigata earthquake, certain river embankments just gave way and sections of the earth slid aside to the extent of knocking buildings over. 

Cyclic Liquefaction

Cyclic liquefaction is a very silent form of liquefaction. In case of an earthquake, the ground begins to weaken each time it shakes. It does not flow continuously but becomes soft enough to begin to adjust or lean the buildings. This is mostly observed in flat areas which have loose sands. 

Example: In the earthquake in Christchurch in 2011, the earthquake did not cause many houses to fall down but instead they sunk or tilted due to the inability to maintain stiffness in the sand layers.

Lateral Spreading

Lateral spreading refers to the side way movement due to a weaker layer below. It appears fine at the top of the crust, but the ground slides away, and roads are torn off and everything constructed in a course between a slope or the edge of a river. 

Observation: The port areas of the city moved sideways following the earthquake in Kobe in 1995, destroying the piers and interfering with alignment of various buildings.

Ground Settlement

The ground shaking appears after the shaking ceases. When the surplus pressure of water is eliminated, the soil compacted itself, and the ground sunk. At times it is minute; occasional enough to crack pavements or leave the floors in an uneven state. 

Example: Largely, the saturated low-lying areas subsidized after the 2005 Kashmir earthquake, and you could make out slight depressions in the pavements and slight lean towards small houses.

How engineers assess liquefaction

Standard penetration Test

This is the usual hammer test. You cut a hole in it, drop your sampler and simply count the number of hits by the hammer to get it to go down. The N-value is informing that the soil is loose or not. 

Fine sand = few = no good in earthquakes.

                                       

Cone Penetration Test

In the case of CPT, no first drilling. A cone is driven into the soil constantly at a fixed speed and resistance is recorded during this process. When it enters easily, then the soil is likely to be loose. If it pushes back, it's dense. It provides a good continuous reading, and therefore you can know where the weak areas are right away

Shear wave velocity

Here they measure the speed of the shearing waves in the ground. The wave is detected by sensors and a little impact is made. Rapid waves tend to be indicative of stiff and dense soil. Slow waves mean soft soil. When there are low Vs number, the soil may not stand up well when it is shaken. 

Cyclic triaxial

This examination is conducted in a laboratory. They put a sample of soil, saturate it, and put it in the chamber and fill it up and down in the chamber to approximate earthquake shaking. Then they observe the increase of pore pressure. The soil is heading to liquefaction in case the pressure continues to increase. Helps determine the number of times that it can shake a soil..

Cyclic simple shear

Similar idea to triaxial but the sample is moved sideways. That sideways motion is closer to what happens in an earthquake. If the sample quickly loses stiffness or builds pore pressure fast, that soil isn’t going to behave well in the field.

Groundwater level

Since liquefaction needs water, engineers always check where the water table sits. They put in standpipes or piezometers and take readings. If the water is near the surface, that soil layer is fully saturated and more likely to liquefy.

Liquefaction mitigation techniques

Liquefaction can be reduced by either making the soil stronger or helping the water escape during shaking. Engineers often start with the densification of loose sand since tighter soil doesn't liquefy that easily. In some cases, they install stone columns that not only add strength but also act as drains during an earthquake.

Another option is deep soil mixing where the soil is blended with cement to form solid columns which will be stable even under strong shaking. When the problem is mostly excess water pressure, drainage systems like gravel drains or vertical drains are added to provide the trapped water with a quick way out.

Compaction grouting is also used where thick grout is pumped into the ground to squeeze the soil into a denser state. If improving the soil isn't practical, pile foundations are the way to go, carrying the building load down to firmer layers so that the structure will stay safe even if the top layers liquefy.

Conclusion

Soil liquefaction is one of the major hazards when it comes to earthquakes, and with proper testing, planning, and engineering design, the impact can be hugely reduced. Understanding how soil behaves during shaking helps to build safer structures and reduce long-term damage within communities.

Frequently Asked Questions

Can liquefaction be predicted?

Not perfectly, but engineers can estimate the risk using tests like SPT or CPT and by checking groundwater levels.

Which soils are most at risk?

Mostly loose, saturated sands. Silty sand and recently deposited soil also act poorly during strong shaking.

Can buildings survive liquefaction?

Yes. If the foundation is designed properly, like using piles or improving the ground first, buildings can handle it.

How long does liquefaction last?

The actual “liquid-like” behavior only happens while the ground is shaking. Settlement and small movements can continue for a short time after.