4027 4TH ST SE WWTP STAIR LANDING 2021-12-20

Geotechnical Engineering Letter NGA File No. 900014 Everett Water Pollution Control Facility July 30, 2014 Everett, Washington Page 3 CONCLUSIONS AND RECOMMENDATIONS General In our opinion, based on our understanding of the existing site conditions and the geotechnical report for the area, it is also our opinion that the staircases could be supported pin piles to reduce the potential for additional future settlement. The pin piles would transfer the staircase loads through the soft/loose fill and alluvial soils down to the underlying competent native soils. Pin Piles We recommend that the staircases be supported on pin piles to transfer structure loads down into the underlying competent materials. The structural engineer should design the new foundation supports and determine the location of the supports based on the recommendations provided in this letter. The preliminary pin pile layout and spacing are shown on the provided plans. It is our opinion that 2-inch diameter pin piles are suitable for this project. The piles should be driven to refusal using a hand operated 140-pound jackhammer. We recommend a design axial compression capacity of two tons for each pile. The refusal criterion for this pile and hammer size is defined as less than one inch of movement during 60 seconds of continuous driving. We recommend using galvanized extra strong (Schedule 80) steel pipe. There should be contingencies in the budget and design for additional/relocated piles to replace piles that may be obstructed by possible debris in the fill. Final pile depths should be expected to vary somewhat and will depend on the nature of the underlying soils. Depending on the actual makeup of the soil in the exact locations of the planned pin piles, we anticipate pile depths to be significant (80 plus feet). The pin piles could refuse at shallower depths with sand layers; however, for budgeting purposes the pin piles should be assumed to advance the full depth of the alluvium deposit. It may be prudent to install a test pile at each staircases location to better define anticipated pile depths: This should be verified in the field by the contractor under the supervision of NGA. Due to the relatively small slenderness ratio of pin piles, maintaining pin pile confinement and lateral support is essential to preventing pile buckling. Friction on the bottom of pin pile supported foundations should be neglected for the purpose of calculating lateral resistance; however, passive resistance on the below -grade portion of the foundation could be utilized. The passive resistance could be calculated based on a triangular pressure distribution NELSON GEOTECHNICAL ASSOCIATES, INC. C5/_1S)