10627 19TH AVE SE COOLEY SMILES DENTISTRY 2019-03-18

Geotechnical Engineering Evaluation NGA File No. 1008917 Cooley Smiles Commercial Development October 3,2017 Everett,Washington Page 5 CONCLUSIONS AND RECOMMENDATIONS General It is our opinion that the site is generally compatible with the planned development from a geotechnical standpoint. Our explorations indicated that the site is generally underlain by competent native glacial till soils at relatively shallow depths. The native glacial till soils should provide adequate support for foundation, slab, and pavement loads. We recommend that the planned structures be designed utilizing shallow foundations. Footings should be founded on the underlying medium dense or better native soil, or structural fill extending to these soils. The medium dense or better soil should typically be encountered approximately one to three feet below the existing surface, based on our explorations. However, deeper areas of undocumented fill or loose soils could be encountered in the unexplored areas of the site. This condition, if encountered, would require deeper excavations in foundation, slab, and pavement areas to remove the unsuitable soils. We also performed on-site infiltration testing based on the 2012 WSDOE Stormwater Management Manual for Western Washington, as amended in 2014. Our explorations generally encountered silty fine to medium sand with gravel that we interpreted as native glacial till soils at depth. Three small-scale pilot infiltration tests(small PITs)were conducted within each of the properties associated with the site. Based on our on-site testing, it is our opinion that the native glacial till soils encountered within the property are not conducive to traditional infiltration systems. However, it is our opinion that shallow low-impact design infiltration systems in the form of pervious pavements, bio swales, or rain gardens may be feasible at specific areas within the site depending on the final layout. This should be further discussed during final design with the civil engineer. Due to the medium dense to very dense native glacial soils encountered within our explorations,we recommend that any shallow infiltration systems installed on the site have an overflow component directed to an approved system. This is further discussed in the Stormwater Infiltration subsection of this report. The soils encountered on this site are considered moisture-sensitive, and will disturb easily when wet. We recommend that construction take place during the drier summer months, if possible. If construction is to take place during wet weather, the soils may disturb and additional expenses and delays may be expected due to the wet conditions. Additional expenses could include the need for placing a blanket of rock spalls to protect exposed subgrades and construction traffic areas. Some of the non-organic on-site soils could be used as structural fill, provided they could be compacted to specifications. This will depend greatly on the moisture content of the soils at the time of construction. NGA should be retained to determine if the on-site soils can be used as structural fill material during construction. NELSON GEOTECHNICAL ASSOCIATES, INC,