Laserfiche WebLink
79 <br /> <br />5.3.6 LIQUEFACTION <br /> <br />Soil that is loose, sandy, silty, or saturated with water can result in soil liquefaction if <br />it is shaken intensely for an extended period. When ground liquefies in an earthquake, <br />it behaves like a liquid and may sink, spread, or erupt in sand boils. This can cause <br />pipes to break, roads and airport runways to buckle, and building foundations to be <br />damaged. Liquefaction can only occur under certain circumstances:4 <br />Loose Soils The soils must be loose, such as uncompacted or unconsolidated <br />sand and silt without much clay. This happens most often in the <br />Bay Area along the Bay shoreline, near creeks or other <br />waterways, on dry creek beds, and in areas of man-made fill, <br />such as the Marina District in San Francisco or parts of Alameda. <br />Soggy Soils The sand and silt must be soggy and saturated with water due <br />to a high water table. <br />Ground Shaking The ground must be shaken long and hard enough by the <br />earthquake to trigger liquefaction. <br />Liquefaction may not necessarily occur even if all three conditions are present. <br />Additionally, if liquefaction does occur, the ground may not move enough to have <br />significant impact on the built environment. As with ground shaking, several types of <br />maps depict liquefaction potential. Liquefaction susceptibility maps show areas with <br />soil types known to have the potential to liquefy with intense shaking. <br />PAST OCCURANCE AND POTENTIAL FOR OCCURANCE: <br />Figure 5-10 illustrates liquefaction susceptibility for San Leandro based on USGS soil <br />type maps in comparison to the entire bay area. However, site-specific investigations <br />are needed to confirm liquefactions susceptibility on any given site. Despite having <br />areas within the city that are susceptibility to liquefaction, San Leandro has no <br />historical occurrences of liquefaction. The risk of liquefaction is highest on former bay <br />lands which were filled in and built upon.17. Unless areas of liquefaction susceptibility <br />are subject to significant ground shaking, they are not likely to liquefy. Liquefaction <br />hazard maps express where the ground is both susceptible to liquefaction, and where <br />the ground is likely to be shaken long and intensely in an earthquake. In 2015, ABAG <br />produced maps that combine liquefaction susceptibility with USGS-generated <br />earthquake scenario maps to identify areas where there is a significant hazard of <br />liquefaction. Figure 5-11 represents an example which shows the liquefaction <br />potential in a M7.0 Hayward earthquake respectively. The maps combine the <br />liquefaction susceptibility and shaking information into a scenario-based liquefaction <br />potential map. <br /> <br />4 Perkins, J.B., (2001) <br />118