Laserfiche WebLink
• <br />4.0 LIQUEFACTION EVALUATION <br />Liquefaction refers to the sudden decrease in shearing resistance of a cohesion - <br />less soil, such as silt, sand, or gravel, caused by increased pore pressures <br />developed during earthquake shocks or similar dynamic loading. Saturated co- <br />hesionless soils can decrease in volume when subjected to ground vibrations or <br />shocks from earthquakes. If water from the soil pores cannot drain or drains too <br />slowly compared to the rate of loading, the tendency to decrease in volume may <br />result in an increase in pore water pressure and decrease in effective stress. If <br />the pore pressure increases to a point where it equals or exceeds the total stress, <br />the shear strength of the soil will decrease to a residual strength and may liquefy. <br />Generally, saturated loose granular soils less than 30 feet deep are most sus- <br />ceptible to liquefaction. Unsaturated and saturated granular soils more than <br />30 feet deep are less susceptible to liquefaction. <br />Because the ground -water table at the proposed site is deep, 125 to 150 feet <br />below ground surface, and the foundation soils mainly consist of medium dense <br />to dense granular soils, the potential for liquefaction is very low. An approximate <br />analysis was performed to evaluate liquefaction potential using the simplified <br />method (NAVFAC, 1982). In general, acceptable safety factors against lique- <br />faction range from 1.3 to 1.5 depending on the type of structure and conse- <br />quences of failure. EMCON's approximate analysis indicated a safety factor of <br />2.8 against liquefaction. <br />PJ9 9390217.00W 9 Rev. 0 May 23, 1989 <br />