In 1918, the upstream slope of the Calaveras Dam (California, USA) collapsed. Later, Allen Hazen studied the failure and defined soil liquefaction. In 1964, the Niigata earthquake (Japan) had a hugely destructive impact on structures. The primary cause of the destruction was the soil liquefaction triggered by the earthquake. This event brought soil liquefaction to the forefront, and extensive research was launched.
This article defines soil liquefaction as well as its causes, effects, and prevention methods. The following is the content list:
- What is Soil Liquefaction?
- What Causes Soil Liquefaction?
- Types of Soil Liquefaction
- What Are the Effects of Soil Liquefaction
- Soil Liquefaction Prevention Methods
Let’s have a detailed look –
What is Soil Liquefaction?
According to ‘Ch. Prahad and M. Chary et al.’ (Published in the International Journal of Engineering Research (IJER), soil liquefaction definition is: ‘Liquefaction is the phenomenon when there is a loss of strength in saturated and cohesionless soils because of increased pore water pressures and hence reduced effective stresses due to dynamic loading.’
To explain soil liquefaction meaning in simple terms, it occurs when soil loses its strength because of an external force. This external force is either the earthquake shock waves or mechanically triggered vibrations. In liquefaction, partially or fully saturated soil loses its firmness and strength and behaves like a liquid. Hence, the structures built on the liquefied soil collapse. Liquefaction of soil is a dangerous condition that causes massive losses.
What Causes Soil Liquefaction?
It should be noted that not all soils liquefy. So the question is: what causes soil liquefaction? In this section, we will discuss the soil’s properties that cause soil liquefaction.
Cohesion: Cohesion-less soil liquefies faster than cohesive soil. Cohesion is a property of soil that holds the soil particles together. For example, Sand has low cohesion, whereas clay has high cohesion.
Particle Size: The second property that influences the liquefaction of soil is particle size. As the soil particle size increases, the voids between the particles increase. As a result, soil with large particles is more susceptible to liquefaction than soil with small particles.
Compaction: The third factor that affects liquefaction is soil compaction. The susceptibility to liquefaction is inversely proportional to the density (compaction). As a result, adequate compaction may save the soil from liquefaction.
Saturation: The last factor affecting soil liquefaction is the saturation level of the soil. The liquefaction occurs only in saturated or partially-saturated soils.
The Factor of Safety to Liquefaction (FSL) is a metric used to assess the susceptibility of soil to liquefaction. It takes earthquake loading and soil liquefaction resistance into account. The soil is susceptible to liquefaction if the FSL is less than one.
FSL = CRR/CSR, where CRR means cyclic resistance ratio and CSR is cyclic stress ratio.
Types of Soil Liquefaction
There are two types of soil liquefaction: flow liquefaction and cyclic liquefaction. The major distinguishing factors between these two types of soil liquefaction are static equilibrium disturbance and cyclic loading.
1. Flow Liquefaction
When an external force (earthquake) disturbs the static equilibrium of the soil mass, the liquefaction is known as Flow Liquefaction. It is characterized by the relatively small initial trigger that starts the chain reaction. Then, the energy gets transferred from particle to particle and continues to increase until the entire soil mass is liquefied.
The flow liquefaction shows large and rapid movements. The ground can no longer support the structural load of the buildings, resulting in either sinking, tilting or collapsing of the structures.
2. Cyclic Liquefaction
The cyclic loading refers to a loading pattern that repeatedly occurs after an interval. It causes soil liquefaction in loose and saturated soil. Soils in loose conditions have less shear strength and are thus more susceptible to cyclic liquefaction. The deformations in cyclic liquefaction are in the lateral direction.
The deformations are larger in cyclic liquefaction than flow liquefaction. The severity of cyclic liquefaction depends upon the period of load application.
What Are the Effects of Soil Liquefaction?
Several countries around the world have faced the soil liquefaction problem. India also faced liquefaction in 1897 during the Shillong earthquake. The earthquake liquefied the Brahmaputra plain, causing destructive floods that devastated the plains and plateau.
Some of the common effects of soil liquefaction are as follows.
1. Building Failure
Building failure is one of the most common effects of soil liquefaction. The buildings rest on the soil. A building foundation is designed based on the soil-bearing capacity. In the liquefaction phenomenon, the soil-bearing capacity reduces to zero, and buildings begin to sink into the ground. The sinking can either be uniform or differential. As a result, the buildings sink, tilt or overturn.
2. Foundation Crack
Foundation cracks are another common effect of soil liquefaction. As mentioned above, sinking can either be uniform or differential. The foundation experiences non-uniform forces along its length and breadth during differential sinking. The uneven forces crack the foundation, rendering it incapable of safely transferring the building load to the ground.
Similarly, the pile foundation is also severely impacted by the liquefaction of the soil. The liquified soil exerts lateral stress on the pile and causes buckling. It reduces the pile foundation’s load-carrying capacity.
Here are some more reasons for your foundation’s cracks:
3. Damage to Utilities Due to Soil Liquefaction
All the utilities are underground nowadays. When the soil behaves like a liquid, it disturbs the alignment and connections of utilities. As a result, they malfunction.
4. Slope Erosion
Highways, railways, earthen dams, and landscaping projects extensively use earthen slopes. The soil flows to the ground and forms fissures in the slope during liquefaction. With this, the structure supported by the slope is jeopardised.
5. Retaining Wall Failure
A retaining wall is a structure that laterally supports the soil. Under the liquefaction, the soil behaves like a liquid; hence, the retaining wall may sink, slide, tilt or overturn.
Soil Liquefaction Prevention Methods
It is always advisable to avoid the liquefiable area before starting a project. If avoidance is not possible, use ground improvement techniques. The suitability of the method chosen depends upon the type of soil, its’ liquefaction potential, and the economy.
Many ground improvement techniques prevent soil liquefaction. The following are the four common soil liquefaction prevention methods –
- Vibro Compaction
- Dynamic Compaction
- Drainage Column
- Compaction Grouting
Any of the above-mentioned soil liquefaction prevention methods should not cost more than 5-10% of the total cost of the building.
1. Vibro Compaction
The vibro-compaction method is one of the most common soil liquefaction prevention methods. The principle of the vibro-compaction method is to compact the soil through vibrations that subsequently increase its density. The vibrations help in the compacting of cohesionless soil.
The technique employs vibrators that are lowered into the deep earth. As the deep strata are compacted, the hole is filled with soil and further compacted. The process is repeated until the borehole has been completely filled with soil.
2. Dynamic Compaction
The dynamic compaction technique is based on compaction by the sudden application of heavy loads. It uses a heavy-weight object that is repeatedly dropped from a height. The potential energy of the falling object converts to kinetic energy, which subsequently compacts the ground up to a certain depth. The depth of compaction depends upon the energy transferred, i.e., the weight of the pounder and the height of the fall.
If you want to know more about various types of compaction equipment, go through the below article:
3. Drainage Columns
The saturation of the soil is an important condition for soil liquefaction to occur. Therefore, to prevent the soil from liquefaction, it is necessary to allow the soil to release its pore water pressure. The drainage columns are boreholes made in the ground and filled with a permeable material. This material is generally graded gravel or stone that quickly drains the water from the soil.
The spacing of drainage columns depends upon the degree of saturation of the soil. Also, the column depth depends on the soil layer up to which the pressure is to be released.
4. Compaction Grouting
Grout is a mixture of cement, sand, and water. It is injected into the soil at a certain depth. Grout is a slow-flowing mixture; hence, it starts accumulating near the injection needle. This causes the nearby soil to compact. Grouting is a feasible technique for an existing structure. It also corrects the tilt in the structure, if any.
You are now familiar with soil liquefaction, its types, causes, effects and prevention methods. We have also written an article on the importance of topography and soil condition; click on the below link to read –
Importance of Topography & Soil Condition in Site Analysis
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Author Bio
Shazeb Ali –