Ordinance 2021-94 - Laserfiche WebLink

EVERETT COMPREHENSIVE PLAN LAND USE ELEMENT 11) with steep slopes, the soils are left in an post-glacial deposits such as alluvium (as in unstable condition. When these unstable river floodplains) or in areas which have been formations become saturated with water, they filled by human activities, particularly when become more likely to succumb to the forces these soils have a high moisture content. Such of gravity. When unconsolidated soils are soils are poorly compacted and when moist underlain by or are interbedded with a highly conditions are present, an earthquake will impermeable soil formation such as compacted cause the soil to liquefy. Certain engineering glacial till, they become saturated during and construction methods may reduce the heavy rains, because the water cannot rapidly likelihood of damage to structures resulting seep into the underlying non-porous material. from an earthquake. The Uniform Building Unconsolidated soils, steep slopes, saturation Code regulates the design and construction of of permeable soils above or beneath buildings located in seismic hazards areas. No impermeable formations combine with gra- additional zoning requirements are necessary vitational forces to cause landslides. Human to regulate structural design. Identification of induced factors can also increase the such areas is necessary in order to evaluate likelihood of landslides. These actions include development proposals in areas which may be diversion of water from rooftops and paved prone to liquefaction. Coastal areas which areas, improperly placed and compacted fills, have been filled, such as those at the Port of dumping of debris, road and utility cuts into Everett, are candidates for liquefaction during hillsides, excavation for building sites, and an earthquake. Upland areas where liquefac- failure of retaining walls. When such human tion may occur are sites which have been filled activities are combined with the other factors without proper engineering and compaction, mentioned, the potential for landslides sites affected by river deposited alluvium, and increases. wetland areas. Where the layer of alluvium or fill is thin enough to allow foundations to be Seismic Hazards. Seismic hazards in the placed on structurally sound soils, the Everett and Puget Sound area consist of two liquefaction hazard is minimal. kinds, ground shaking and ground failure. Rupture along fault lines is not a factor since Erosion Hazards Certain types of soils are the faults are located thousands of feet below more prone to erosion than others. As with glacial deposits and they are not discernible at landslides, erosion is more likely to occur on the surface. The consequences of seismic steep slopes, especially in erosion prone soils events are two types of hazards - landslides types, soils that have been disturbed by human and liquefaction. activities, and in the presence of rainfall and storm runoff. The most critical factor for Landslides which are likely to occur as a result control of erosion is construction management of a seismic event are the same areas and practices which limit clearing, require formations where landslides are likely to occur mitigative controls on development activity, under other non-seismic conditions. Seismic limitation of soil disturbance to dry seasons of 1 activity may trigger landslides in areas of the year, re-vegetation, and maintenance of landslide hazard. developed sites to prevent erosion after development. Liquefaction is a phenomenon where soil loses strength and its bearing capacity during an Ground water Aquifers Ground water earthquake. This phenomenon is most likely aquifers are a porous geological formation to occur on non-cohesive soils common to which hold surface water that has percolated LU-51