Glendale (PDF) - Hazard Mitigation Web Portal - State of California

Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 6 – Earthquakesdevelopment channels of the Arroyo Simi, where ground water is very shallow (Barrows et al.,1994).The types of ground failure typically associated with liquefaction are explained below.Lateral Spreading - Lateral displacement of surficial blocks of soil as the result of liquefaction in asubsurface layer is called lateral spreading. Even a very thin liquefied layer can act as a hazardousslip plane if it is continuous over a large enough area. Once liquefaction transforms the subsurfacelayer into a fluid-like mass, gravity plus inertial forces caused by the earthquake may move the massdownslope towards a cut slope or free face (such as a river channel or a canal). Lateral spreadingmost commonly occurs on gentle slopes that range between 0.3° and 3°, and can displace the groundsurface by several meters to tens of meters. Such movement damages pipelines, utilities, bridges,roads, and other structures. During the 1906 San Francisco earthquake, lateral spreads withdisplacements of only a few feet damaged every major pipeline. Thus, liquefaction compromised SanFrancisco’s ability to fight the fires that caused about 85 percent of the damage (Tinsley et al., 1985).Flow Failure - The most catastrophic mode of ground failure caused by liquefaction is flow failure.Flow failure usually occurs on slopes greater than 3°. Flows are principally liquefied soil or blocks ofintact material riding on a liquefied subsurface. Displacements are often in the tens of meters, but infavorable circumstances, soils can be displaced for tens of miles, at velocities of tens of miles perhour. For example, the extensive damage to Seward and Valdez, Alaska, during the 1964 GreatAlaskan earthquake was caused by submarine flow failures (Tinsley et al., 1985).Ground Oscillation - When liquefaction occurs at depth but the slope is too gentle to permit lateraldisplacement, the soil blocks that are not liquefied may separate from one another and oscillate onthe liquefied zone. The resulting ground oscillation may be accompanied by the opening and closingof fissures (cracks) and sand boils, potentially damaging structures and underground utilities(Tinsley et al., 1985).Loss of Bearing Strength - When a soil liquefies, loss of bearing strength may occur beneath astructure, possibly causing the building to settle and tip. If the structure is buoyant, it may floatupward. During the 1964 Niigata, Japan earthquake, buried septic tanks rose as much as 3 feet, andstructures in the Kwangishicho apartment complex tilted as much as 60° (Tinsley et al., 1985).Ground Lurching - Soft, saturated soils have been observed to move in a wave-like manner inresponse to intense seismic ground shaking, forming ridges or cracks on the ground surface. Atpresent, the potential for ground lurching to occur at a given site can be predicted only generally.Areas underlain by thick accumulation of colluvium and alluvium appear to be the most susceptibleto ground lurching. Under strong ground motion conditions, lurching can be expected in loose,cohesionless soils, or in clay-rich soils with high moisture content. In some cases, the deformationremains after the shaking stops (Barrows et al., 1994).Seismically Induced Slope FailureStrong ground motions can worsen existing unstable slope conditions, particularly if coupled withsaturated ground conditions. Seismically induced landslides can overrun structures, people orproperty, sever utility lines, and block roads, thereby hindering rescue operations after anearthquake. Over 11,000 landslides were mapped shortly after the Northridge earthquake, all withina 45-mile radius of the epicenter (Harp and Jibson, 1996). Although numerous types of earthquakeinducedlandslides have been identified, the most widespread type generally consists of shallowfailures involving surficial soils and the uppermost weathered bedrock in moderate to steep hillsideterrain (these are also called disrupted soil slides). Rock falls and rockslides on very steep slopes are2006 PAGE 6 - 30