Factors affecting liquefaction
In this article, I discussed the factors affecting liquefaction in detail.
The factors affecting liquefaction are as follows
a) Soil Type:
Liquefaction occurs in cohesion-less soils as they lose their strength completely under vibration due to the development of pore pressures which in turn reduce the effective stress to zero. Liquefaction does not occur in the case of cohesive soils. Only highly sensitive clays may lose their strength substantially under vibration.
b) Grain Size and Its Distribution:
Fine and uniform sands are more prone to liquefaction than coarser ones. Since the permeability of coarse sand is greater than fine sand, the pore pressure developed during vibrations can dissipate faster.
c) Initial Relative Density:
It is one of the most important factors controlling liquefaction. Both pore pressures and settlement are considerably reduced during vibrations with an increase in initial relative density and hence chances of liquefaction and excessive settlement reduce with increased relative density.
d) Vibration Characteristics:
Out of the four parameters of dynamic load namely
(i) frequency
(ii) amplitude
(iii) acceleration
(iv) velocity
frequency and acceleration are more important. The frequency of the dynamic load plays a vital role, only if it is close to the natural frequency of the system. Further, the liquefaction depends on the type of the dynamic load i.e. whether it is a transient load or the load that causes steady vibration. For a given acceleration, liquefaction occurs only after a certain number of cycles imparted to the deposit. Further, horizontal vibrations have more severe effect than build-upvertical vibrations. Multi directional shaking is more severe than one directional loading
(Seed at al.1977), as the pore water pressure builds up is much faster and the stress ratio
required is about 10 percent less than that required for unidirectional shaking.
e) Location of Drainage and Dimension of Deposit
Sands are more pervious than fine grained soil. However, if a pervious deposit has large dimensions, the drainage path increases and the deposit may behave as un-drained, thereby, increasing the chances of liquefaction of such a deposit. The drainage path is reduced by the introduction of drains made out of highly pervious material.
f) Surcharge Load
If the surcharge load, i.e. the initial effective stress is large, then the transfer of stress from soil grains to pore water will require higher intensity vibrations or vibration for a longer duration. If the initial stress condition is not isotropic as in the field, then stress condition causing liquefaction depends upon K0 (coefficient of earth pressure at rest),characteristic and for K0> 5, the stress condition required to cause liquefaction increases by at least 50%.
g) Method of Soil Formation the
Sands unlike clays do not exhibit a characteristics structure. But recent investigations show that liquefaction characteristics of saturated sands under cyclic loading are significantly influenced by method of sample preparation and by soil structure.
h) Period under Sustained Load
The age of sand deposits may influence liquefaction characteristics. A 75% increase in liquefaction resistance has been reported on the liquefaction of undisturbed sand compared to its freshly prepared sample which may be due to some form of cementation or welding at contact points of sand particles and associated with secondary compression of soil.
i) Previous Strain History
Studies on liquefaction characteristics of freshly deposited sand and of similar deposit previously subjected to some strain history reveal, that although the prior strain history caused no significant change in the density of the sand, it increased the stress that causes liquefaction by a factor of 1.5.
j) Trapped Air
If air is trapped in saturated soil and pore pressure develops, a part of it is dissipated due to the compression of air, hence trapped air helps to reduce the possibility of liquefaction.
k) Groundwater Table
The most conducive condition to liquefaction is near the surface of the groundwater table. Unsaturated soil located above the groundwater table will not liquefy. At the location where the groundwater table significantly fluctuates, the liquefaction will also fluctuate.
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