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L <br /> SHANNON asWILSONs INC.L <br /> B 40 3 �� <br /> April 16, 2013 <br /> Port of Everett <br /> P.O. Box 538 <br /> Everett, WA 98206 <br /> Attn: Mr. Erik Gerking <br /> RE: RISK EVALUATION FOR BULKHEAD DAMAGE DURING 2,500-YEAR <br /> DESIGN EARTHQUAKE,PORT OF EVERETT EAST MARINA BULKHEAD <br /> REPLACEMENT PROJECT,EVERETT,WASHINGTON <br /> We understand that the Port of Everett(Port)has requested that the East Marina Bulkhead <br /> (14th/16th Street Bulkhead)be designed for a ground motion where soil liquefaction would be <br /> minimal. Based on our previous analyses (Shannon& Wilson letter dated January 29,2013), <br /> this condition ("minimal" liquefaction)would correspond to a ground motion with a return <br /> period of about 200 years. This ground motion is similar to the seismic design criteria that were <br /> used in 2002 for the 14th Street Bulkhead, connecting to the north end of the project. <br /> We understand that the structural design of the bulkhead was originally going to be performed <br /> using the International Building Code (IBC)to allow for future development behind the <br /> bulkhead. The 2009 IBC specifies that seismic design be performed using a ground motion with <br /> a return period of 2,475-years . For this seismic design level, we anticipate that the soil between <br /> the groundwater table and elevation -12 feet(approximate 20 feet)would liquefy. Liquefaction <br /> would result in a significant loss of soil strength against the deadman anchors, increased <br /> pressures on the bulkhead wall, ground settlement, and lateral movement towards the water <br /> (lateral spreading). This degree of liquefaction and lateral spreading would likely result in <br /> tilting, settlement, and damage of the bulkhead, structures,and utilities behind the wall. <br /> With the Port's selection of a lower level design ground motion, future development behind the <br /> bulkhead will need to be designed to accommodate liquefaction and its effects including ground <br /> settlement and lateral spreading. This letter provides information about the anticipated ground <br /> settlement and lateral spreading that could occur due to ground motions required by the IBC. <br /> DRAFT Seismic Risk Letter.docxr„Pnk. 21-1-21761-001 <br />