liquefaction and liquefaction potential
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LIQUEFACTION AND LIQUEFACTION POTENTIAL
BY: Mahesh Raj Bhatt ME in Structural Engineering , Kathmandu University
1. LIQUEFACTION:What is liquefactionWhen does it occurLiquefaction of soilHow it worksShear strength of soilTypes of liquefaction2.LIQUEFACTION POTENTIALHow to identifyLiquefaction analysisPrediction Mitigations3. CONCLUSIONS:
LIQUEFACTION:In soil mechanics term liquefied first used byAllen Hazen (1918), In reference toFailure of Calaveras Dam in California.
Attention of engineer after -1964 Alaska Earthquake(Mw=9.2) good Friday earthquake - 1964 Niigata Earthquake( Ms=7.5) Japan -1989 loma-Prieta Earthquake and others. In soil mechanics the term "liquefied" was first used by Allen Hazen in reference to the 1918 failure of the Calaveras Dam in California.
attention of engineers after 1964 Niigata earthquake and 1964 Alaska earthquake. 1989 Loma Prieta earthquake, and in Port of Kobe during the 1995 Great Hanshin earthquak
What is Liquefaction? A phenomenon whereby a saturated or partially saturatedsoilsubstantially loses strength and stiffnessin response to an appliedstress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.
When does it occurs??
-when theeffective stressof soil is reduced to essentially zero, which corresponds to a complete loss ofshear strength
May be initiated byMonotonic Loading Cyclic loadingShock loadings(EQs)
Liquefaction of soil:Soils behave like a liquid. How and why?To understand the above phenomenon: some basics required regarding:
Total stress, (t)Pore water pressure (u) Effective stress (eff ) t= eff + u eff = t- u
How It Works??When the seismic waves pass through the soil, the vibrations cause the individual grains in the soil to move around and re-adjust their positions
This ultimately results in a decrease in volume of the soil mass as the grains pack more tightly together a reduction in porosity
Soil loose its strength because of loss of effective stressSaturated sand in ground vibration, -it tends to compact and decrease in volume ; -if no drainage, decrease in volume results -increase in pore water pressure IF Pore water pressure=overburden pressure THANeffective stress = zero, -sand looses strength completely and develops a liquefied state.
How pore pressure increase
Spring water analogy of soil layer
Shear strength of soil Shear strength, = c + t tanEffective stress gives more realistic behaviour of soil, Shear strength can be expressed as = c1 + (t u)tan1During the ground motion due to an earthquake, static pore pressure may by an amount udyn, then = c1 + (n u + udyn)tan1Let us consider a situation when u + udyn= n, then = c1 In cohesion less soil, c1= 0, hence = 0 (sand)
Influence of soil condition on liquefaction potential
Liquefaction damage 1964 Niigata japan
Alaska Earthquake (1964)
Types of liquefaction:1. Flow liquefaction 2. Cyclic Liquefaction: static> liquid state static< liquid state
-flow of soil mass -spreading of mass -slope >3 degree - slope