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January 28, 2005 <br /> Project:`'o. T-iG59 <br /> I <br /> i <br /> The Geolcgic :19up of the Liverer� 7.5- d/inute Quudrune!e, Snohomish Cowiry, IVasliington, by James P. Minard, <br /> I datcd 1935, shows soils in the vicinity of the site consisting of Vashon Till (Qv[). �1`e did not encoimter native <br /> soils in any of the test pits excavated o�-site. Howevcr, the silty sand with varying xmounts of gravel observed in <br /> the test pits were derived from tl�e VasUon Till unit. <br /> ITlie preceding discussion i� iniended as a ,cneral review of the soi] conditions observed. Pigw�es A-2 through A- <br /> 6 present more detailed descriptions of ehe subtiurf<ice conditions encountered in die test pits. The approximate <br /> � test pit locations are sho�vn on Figurc 2. <br /> � <br /> � 3 3 Ground�vater <br /> �Ve obscrved ground�vater,ecpaoe in sir m`the test pits excavated along the east end of the site. The seepage was <br /> � light and obscrved at depths ranging from �J.25 to 3.5 I;aet below the existing gruund surface. The seepage was <br /> perched �vithin thc upper ponions of the sifty �'v�d and sand with silt on the imderlying compact structural fill. <br /> � <br /> This is not a natural ground���ater flo�: regime, and we expect this shallow secpage will be eliminated �vith site <br /> j dec�'opment. Perched groundwate: levels and flow rates �vill fluctuate scasonally and typically reacri then <br /> ' !:i; .s�.levels during and shorely following the wet�einter months(October through May). <br /> � 3.4 Scismic <br /> ; 13ased on the compact condition of tlie fill soils and the underling local gcology, per Chapter l6 of tLe 2003 <br /> ' Intemational Building Code(IBC), site class"C"can be used in structural design. <br /> ILiquefaction is a phenomenon �t�hen tliere is a reduction or complete loss of soil strength due ro an ir.crease in <br /> ' water pressure induced by vibmtions. I.iquefaction mainly affccts geologically rccent deposits of fine-grained <br /> sands thm are below tl�e tround�vater table. Soils of this nature derive U�eir strength from intergranular friction. <br /> � The generated watcr pressure or pore pressure essentially separates die soil grains and eliminates this <br /> intergranular friction; th��s,eliminating the soiPs strength. <br /> � Due ro the well-graded nawro and mcdium dcnse to very dense conditimi of the underlying till fill soils observed <br /> at the site, it is our opinion Ihat there is nu risk ior liqucCac[ion to occur at this site during an earthquake. <br /> d.0 DISCUSSIO\ :�ND ]iECO�I\1L.NDATIONS <br /> i <br /> d.l Gencral <br /> � [3ased on our stud�-, it is our opinion the sitc is suitable Cor the proposcd development from a geoteclmical <br /> standpoint. 'Che structures can be supported on conventional spmad footin� foundations bearing on undisturbed <br /> i surfaces composed of the existing fill soils, or ne�e• strucmral fill placed above these competent existing site soils. <br /> Ploor slabs and paveme��ts can Uc similarly supported. <br /> � <br /> Page No. 3 <br /> �j� <br />