Liquefaction Case Study

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Liquefaction: Liquefaction is the phenomena of ground softening due to the build-up of excess pore water pressures in granular soils during an earthquake due to the application of cyclic shear stresses. The excess porewater pressure caused by seismic loading are unable to dissipate reducing the effective stress resulting in strength loss of the surrounding soil and the soil behaving like a liquid. (Marcuson 1978- Summary report Youd) Liquefaction is usually identified by observations of sand boils, ground fissures and lateral spreads at the ground surface. Liquefaction Susceptibility The susceptibility of soil deposits to liquefaction is determined by a combination of various factors to which they may be subjected and consist of, such as soil…show more content…
Several field tests such as the Standard Penetrometer Test(SPT), Cone Penetrometer Test(CPT), Shear Wave velocity(Vs ) and the Flat Dilatometer Test(DMT) are used to estimate the susceptibility to liquefaction. SPT’s and CPT’s are generally preferred because of the more extensive databases and past experiences. (Summary report Youd) Factor of Safety Against Liquefaction The cyclic resistance ratio (CRR) gives an estimate of the liquefaction resistance of soils in a deposit and is used along with the an estimation of the cyclic shear stress likely to be induced in the deposit in an earthquake (CSR) to give the margin of safety used in the prediction of liquefaction potential of a soil deposit. The factor of safety is defined as: F_L=CRR/CSR If the margin of safety(FL) is smaller than 1, the demand of the earthquake on the soils exceeds the estimated liquefaction capacity of the deposit and therefore liquefaction is predicted to…show more content…
O.D. and 1-3/8 in. I.D.) 12 in. into the ground in accordance with the New Zealand standard on CPT testing (NZS 4402.6.5.1:1988) . The Standard Penetration Test (SPT) is employed to determine the SPT N value, which gives an indication of the soil stiffness and can be empirically related to many engineering properties. The test is conducted inside a borehole. A split spoon sampler is attached to the bottom of a core barrel and lowered into position at the bottom of the borehole. The sampler is driven into the ground by a drop hammer weighing 68 kg falling through a height of 76 cm. The number of hammer blows is counted. The number required to drive the sampler three successive 150 mm increments is recorded. The first increment (0-150 mm) is not included in the N value as it is assumed that the top of the test area has been disturbed by the drilling process. The SPT N is the number of blows required to achieve penetration from 150-450 mm. The hammer weight, drop height, spoon diameter, rope diameter etc. are standard dimensions. After the test, the sample remaining inside the split spoon is preserved in an airtight container for inspection and

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