222 W CASINO RD Geotech Report 2022-10-25

GeoTest Services, Inc. May 3,2019 Mukilteo School District 19-0194 N the planned portable location, we have presented options for foundation support in terms of the risk associated with that specific path for development. We recommend that the school district representatives and the project contractor weigh the cost of the options presented below against the increased potential risk of differential settlement associated with constructing new foundations on the variable fill soils that exist below the planned portable location. Low Risk Options Option#1—Limited/Full Overexcavation-Continuous and isolated spread footings could be founded on controlled density fill(CDF)that would extend vertically downward,below the proposed footing locations, to the native, Glacial Till contact. Glacial Till was observed within TP-2 at approximately 6.0 feet BGS, but was not encountered within the other three excavations, TP-2 through TP-4, that extended to between 8.5 to 9.0 feet BGS. We anticipate suitable native soil to be no more than 12 to 14 feet BGS. If this is the preferred path for development, CDF trenches need only to extend a nominal distance beyond the planned width of the subject footing. Alternatively, the variable, previously placed fill soils encountered below the planned portable location could be overexcavated down to the previously described native, Glacial Till contact and then replaced with suitable structural fill material to attain the proposed footing elevation as described in the Back fill and Compaction section of this report. Option #1 offers the least amount of settlement but at a greater cost than Option#2. Moderate Risk Option Option #2 — Limited Structural Fill Support If some level of anticipated differential settlement is acceptable to the project ownership, continuous or isolated spread footings could be founded on a minimum of 2 feet of properly compacted structural fill placed directly over properly prepared previously placed fill soils can provide foundation support for the proposed improvements. We recommend that qualified geotechnical personnel confirm that suitable bearing conditions have been reached prior to placement of structural fill or foundation formwork. A larger amount of settlement is anticipated with Option #2 but that may be somewhat mitigated if the portable building can be releveled after construction. Moreover, localized overexcavation, if necessary, can be backfilled to the design footing elevation with lean concrete, or on structural fill placed in accordance with the Back fill and Compaction section of this report. In areas requiring overexcavation to reach firm and unyielding soil, the limits of the overexcavation should extend laterally beyond the edge of each side of the footing a distance equal to the depth of the excavation below thebase of the footing. If lean concrete is used to backfill the overexcavation,the limits of the overexcavation need only extend a nominal distance beyond the width of the footing. In all cases, continuous and isolated spread footings should be founded 18 inches, minimum, below the lowest adjacent final grade for freeze/thaw protection. The footings should be sized in accordance with the structural engineer's prescribed design criteria and seismic considerations. Allowable Bearing Capacity Assuming the above foundation support criteria are satisfied, continuous or isolated spread footings founded directly on either a prism of CDF or a minimum of 2 feet of compacted structural fill placed 7