Table of Contents
Liquefaction is a process in which the strength and stiffness of soil are decreased by earthquake shaking or other quick loadings.
Liquefaction and related problems have been behind massive amounts of damage in earthquakes around the whole world.
1. Soil Liquefaction
The liquefaction process generally takes place when loosely packed soil, and water-logged sediments at or near the ground surface decrease the strength of soil response to strong ground shaking (Earthquake).
Liquefaction that takes place below buildings and other structures can cause huge damage during earthquakes.
Liquefaction of soil makes structural instability in buildings, bridges, roads, etc. This phenomenon takes place due to different instances of structural failure.
The liquefied ground soil is unable to bear/support the stresses of its load from the foundations.
Due to this foundations will sink into the sand deposit and lead the building to lean and eventually collapse.
2. Process of Soil Liquefaction
a. The process of soil liquefaction is observed in the loose and saturated soil.
b. In this type of soil, the pores are filled with water, and the particles are not allowed to COM together even if they wish to become denser.
c. The action of forces like earthquake won’t help in consolidating this water instead the pressure from the water pores increases.
d. This will reduce the force between the soil particles resulting in softening and weakening of the soil mass.
3. Types of Soil Liquefaction
These processes can be classified into two main categories:
a. Flow liquefaction
b. Cyclic mobility.
Residual strength is the strength of liquefied soil.
a. Flow liquefaction:
Flow liquefaction is a process in which the static, equilibrium is damaged by static or dynamic loads in a soil deposit with low residual strength. It occurs when the static shear stresses in the soil exceed the shear strength of the liquefied soil.
Shaking destabilizes soil by increasing the space between grains and soil flow like a liquid.
b. Cyclic mobility:
Cyclic mobility is a liquefaction process, caused by cyclic loading, that takes place in soil deposits with static shear stresses lower than the soil strength. The distortion due to cyclic mobility produces incrementally because of static and dynamic stresses that exist during an earthquake.
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Table 1: Comparison of flow liquefaction and cyclic mobility.
|Flow liquefaction||Cyclic Softening|
|Cyclic liquefaction||Cyclic mobility|
|Loading Conditions||Static or Cyclic||Cyclic with stress reversal.||Cyclic without stress reversal.|
|Drainage Soil Response to Shear (Appendix I)||Strain Softening||Strain Softening and Strain Hardening||Strain Softening and Strain Hardening|
|Controlling Stresses||Static Shear Stress||Static and Cyclic Shear Stresses||Static and Cyclic Shear Stresses|
|Induced Stress State||In-situ shear stresses greater than the minimum undrained shear strength||Effective stress state reaches essentially zero||Zero Effective stress does not develop.|
|Failure or Deformation Potential||A sufficient volume of soil must strain to soften. Failure can result in a slide or flow depending upon internal geometry and stress state.||Strain-softened shear modulus can result in massive distortions during cyclic loading. Soils will try to stabilize upon the termination of cyclic loading.||Limited deformations, unless very loose soil results in flow liquefaction.|
|Soil Types||Any metastable saturated soil; very loose granular deposits, very sensitive clays, and loess deposits||Almost all saturated sands, with limited deformations in clayey soils.||Almost all saturated sands, with limited deformations in clayey soils.|
4. Effects of Soil Liquefaction
Here are some effects of Soil Liquefaction:-
a. Loss of bearing strength.
The ground can liquefy and lose its ability to support structure.
b. Lateral Spreading
The ground can slip down on very small slopes. It is mainly caused by cyclic mobility. Lateral spreading causes damage to the foundations of buildings, pipelines, and railway lines and causes shaking at pie due to increased lateral loads.
c. Sand Boil
Sand-laden water can be poured out from a buried liquefied layer and break out at the surface to result in sand volcanoes. The surrounding ground normally fractures and settles.
d. Flow Failures
Flow failures are the very disastrous ground failures takes placed by liquefaction. These failures generally flow large masses of soil laterally.
Flow Failures take place in loose saturated sands or silts on comparatively steep slopes.
The light structure that is buried in the ground (like pipeline sewers and nearly empty fuel tanks) can float to the surface when they are surrounded by liquefied soil.
Liquefied ground reconsolidates during an earthquake, the ground surface may settle and the underlying liquefied soil becomes denser.
g. Ground oscillation
Where the ground slope is too gentle or flat to permit lateral displacement, liquefaction at depth may repeat overlying soil layers from the beneath ground, permitting the upper soil to oscillate back and forth and up and down in the form of ground waves.
These oscillations are usually accompanied by the opening and closing of fissures fracture of rigid structures such as pavements and pipelines.
5. Factors Affecting Liquefaction
There is various factor that affects soil liquefaction:
a. Earthquake intensity and duration
b. Sol type
c. Soil relative density
d. Particle size distribution
e. Presence or absence of plastic fines
f. Groundwater table location (saturation)
g. Hydraulic conductivity
h. Placement conditions or depositional environment
I. Aging and cementation
j. Overburden pressure
k. Structure load
l. Historical liquefaction
6. Remedial Measures
1. Reduce foundation settlement.
2. Increase shear strength to improve slope stability.
3. Increase bearing capacity
4. Increase water permeability to accelerate drainage and thereby accelerate foundation settlement.
5. Prevent soil liquefaction during earthquakes.
6. Build a Liquefaction-resistant structure
8. Dynamic Compaction
9. Compaction Grouting
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|Read Also: Plastic limit test of Soil|
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