3702 TULALIP AVE 2016-01-01 MF Import

� SERS Public Sat 9-1-1 Communication Tower • PSI Report No. 704-05224-5 Decembar 20^, 1000 Page 8 alternative to select should be based on construction cost, avaiiabilitl� of local contractors, diKcuity to drill in the very dense silty sand, drilled pier casing requirements, and structurai mat footprint size requirements versus availab�e land. 5.3.1 Structural Mat Altemative Based on the results of our geotechnical investigation, it is our opinion that the proposed communication tower can be supported on a mat foundation designed for a maximum allowable soil bearing pressure of up to 3,000 psf, when founded on the undisturbed, stiff silty lean clay stratum first encountered in our boring (B-2)at a depth of about 5 feet below grade, or on an engineered, granular structural fill placed overlying the stiff silty lean clay stratum. The allowable bearing pressure includes a safety factor of 3 and is intended for dead loads and sustained live loads and can be increased by one-third for the total of all loads, including short-term wind or seismic loads. The mat foundation should extend to a minimum depth of 18 inches beneath the lowest adjacent exterior grade to provide irost protection. The foundation should be constructed at least 10 feet Iaterally from the top of any slopes, which should te permanentiy graded no steeper ihan 2 horizontal: 1 vertical. When sizing the mat, the structural engineer should evaluate the struc:ture for overtuming. Allowable lateral irictional resistance between the base of footings, including the guy anchors, and tne subgrade can be expressed as the applied vertical load multiplied by a coefficient of friction oi 0.30 for undisturbed, stiff silty lean clay. In addition, lateral loads may be resisted by an passive earth pressure based on an equivalent fluid density of 260 pounds per cubic foot (pc� on footings poured"neaP'against in-situ soils or properly backfilled with granular structural fill, respectively. These recommended values include a factor of safety of approximately 1.5, which is appropriate due to ihe amount of movement required to develop full passive resistance. We estimate that foundations designed and constructed in accordance with the above recommendations will experience total settlements generally less than 1-inch with differential settlements between columns generally Iess than Y:-inch. If foutings are constructed during wet weather, it may be necessary to protect the foundation excavation bottoms from disturbance during construction activities. In this regard, we recommend that a 3 to 4-inch thickness of crushed rock be placed at the bottom of the footing excavations immediately after the excavation is completed. If footings are constructed during the drier summer months, ihis crushed rock layer should not be required. 5.3.2 Drilled PierAlternative The !ower structure may be supported on straight-shaft, cast-in-place, reinforced concrete drilled piers extending into the stiff to hard silt� lean clay stratum. The piers should extend at least 15 (eet into this stratum and should have a minimum diameter of 24 inches lo (acilitate