10200 25TH AVE SE Geotech Report 2018-01-02 MF Import

, j , Propased Madernization and Additioru to Subsurface Ezploration, Gealogic Hazards, Eisenhower.�liddle Schaof ond Geotechnica!Engineering Report Everett, Washrngton Geolo,qic Hazards and MitiRations II. GEOLOGIC HAZARDS AND MITIGATIONS The following discussion of potential geologic hazards is based on the geologic, slope, and ground and surface water conditions as observed and discussed herein. The discussion will be limited to seismic issues, landslides or mass-wasting, and erosion, including sed'nnent transport. 5.0 SEISMIC HAZARDS AND RECOMMENDED MITIGATION Earthquakes r :ur in the F'uget Sound Lowlar.:i with great regulariry. The vast major:ty of these events ..re small and are usually not felt by people. However, large earthquakes do occur as evidenced by the most recent 6.8-magnitude event on February 28, 2001 near Olympia Washington, the 1965, 6.5-magnitude event, and the 1949, 7.2-magnitude event. The 1949 earthquake appears to have been the largestin this area during recorded history. Evaluation of return rates indicates that an earthquake of the magnitude between 5.5 and 6.0 is likely within a given 20-year period. Generally, there are four types of potential geologic hazards associated with large seismic events: 1) surficial ground rupture; 2) seismically induced landslides; 3) liquefaction; and 4) ground motion. The potential for each of these hazards to adversely impact the proposed project is discussed below. , 5.1 Surficial Ground Rupmre Generally, the lac�est earthquakes which have occurred in the Puget Sound area are sub-crustal events with epicenters ranging from 50 to 70 kilometers in depth. For this reason, surficial faulting is thought to be relatively uncommon and knowledge ef fault locations is limited. In general, the risk of surface rupture at the site during a seismic event is thought ro be small. 5.2 Seismicallv Induced Landslicles The potential risk of damage to new structures by a seismically induced landslide is considered to be low due to relatively eentle slopes on the project and no sign of existin� slope instabiliry. 5.3 Liquefaction I.iquefaction is a process throu;h �vhich unconsolidated soil loses strer.gth as a result of vibra�ory shaking, such as that �vhich occurs during a seismic event. During normal conditions, the weieht of the soil is supported by both grain-to-grai❑ contacts and by the pressure within the pore spaces of the soil below the water table. Extreme vibratory shaking �Q1t�1O^•� �� '� ASSOCUTEDE.tRTNSC/E:VCES, lh'C. .t2(d-A'E0J7J9:IJ-Prn�ran]tx7JJJ9iA'E'�IIP-11^R FaP�e 7 �Q