3800 RIVERFRONT BLVD Geotech Report 2023-05-04

I March 26, 2019 HWA Project No. 2015-061-21 Task 600-7 I Bearing Sand: Each of the borings was advanced into very dense sands, generally consisting of massive to stratified clean to slightly silty sand (Unified Soil Classification of SP and SP-SM). The upper sands were interpreted as advance outwash, consisting of glaciofluvial sediment Ideposited by glacial meltwater as the Puget Lobe advanced into the Puget Lowland. It was subsequently overridden by the advancing ice, causing over consolidation to a very dense I condition. Layers of hard silt with dark brown organics were encountered between sand layers. Due to the organic content, these are interpreted as pre-Vashon alluvium. Underlying and interbedded sand layers can be either alluvium of similar age, or older glacial outwash deposits. IThese could not be differentiated based on grain size and density. 3.4 GROUNDWATER IGroundwater was observed in all borings during drilling. Groundwater was typically encountered within the refuse, and sometimes within the alluvium. Depths to groundwater were I observed during drilling ranging from 16 to 30 feet and were generally deeper toward the river. These levels are shown on the logs and cross sections; however, it should be noted that these are not stabilized water levels as would be observed in a well. Actual groundwater levels are I typically higher than those observed on sampling rods during drilling. Based on our experience, groundwater elevations will vary seasonally and in response to significant precipitation events. 4.0 CONCLUSIONS AND RECOMMENDATIONS 4.1 GENERAL IThe Riverfront Boulevard alignment is underlain by a series of compressible materials consisting of landfill refuse and soft alluvial peats, silts and clays. These soils have been surcharged to I reduce the anticipated settlement across the site but are still expected to undergo significant settlement over the design life of the proposed roadway. Our analyses suggest that settlements will be differential in nature and magnitude. IDue to the anticipated settlements along the roadway, we recommend that Riverfront Boulevard be constructed with relatively flexible HWA pavement. We expect that anticipated settlements along the roadway will require an increased maintenance cycle and overlay program. The moisture sensitivity of the existing fill material will require the placement of an aggregate working surface along the entire roadway after the installation of the proposed utilities. IWe recommend that all subsurface utilities be constructed as fused HDPE piping to allow maximum flexibility. Flexible connections should be used at all locations where utilities I transition from grade supported to pile supported. We recommend that additional storm drain inlet structures should be positioned along the road alignment at locations where anticipated sags in the roadway are expected to develop. I I Geotechnical Report Revision 3 26 2019.doc 7 HWA GEOSCIENCES INC.