7505 SEAWAY BLVD Geotech Report 2022-10-25

November 11, 2016 HWA Project No. 2014-117-21 It consists of an unsorted mixture of clay, silt, sand and gravel, which is very dense, having been consolidated by the weight of greater than 2,000 feet of ice. It is also known to contain scattered cobbles and boulders. This unit is locally referred to as "hardpan", because of its induration and an appearance similar to concrete. Till is relatively impermeable, except where sandy zones are encountered. Generally, the till forms an impervious layer below which surface water cannot penetrate. 3.2 SUBSURFACE SOIL CONDITIONS Our interpretations of subsurface conditions are based on a review of available geologic and geotechnical information for the project site and our subsurface explorations. In general, the proposed Seaway Transit Center site is underlain by weathered and non-weathered glacial till with near-surface undocumented and roadway fill encountered along the eastern edge of the site. • Undocumented Fill—Undocumented fill material was encountered in machine-drilled borings BH-2, and BH-3, and in hand borings HH-1 and HH-2. Borings BH-2 and BH-3 were performed on the mounded eastern half of the project site. The undocumented fill noted at these locations ranged from a depth of about 5 feet at BH-2, to 12 feet at BH-3. The undocumented fill noted at BH-2 and BH-3 consisted of medium dense to dense, silty, gravelly, sand. A review of available aerial photos suggest that this undocumented fill was placed sometime between 1975 and 1990 and was most likely a result of the realignment of 75th Street SW. Other undocumented fill material was encountered in hand borings HH-1 and HH-2. This fill consisted of medium dense to dense,silty, gravelly, sand. A remnant HMA pavement structure was encountered at depths of 1.5 feet and 0.8 feet in hand-borings HH-1 and HH-2, respectively. This indicates the undocumented fill on the western portion of the property was possibly placed to support the remnant pavement structure. The purpose of this pavement structure is undocumented, and the pavement is not visible in any archived aerial photographs we have seen thus far; but it was possibly placed to facilitate delivery of the material comprising the mound on the eastern portion of the property. • Roadway Fill: Archived aerial photographs appear to indicate the roadway fill encountered at borings BH-5 and BH-6 was possibly placed when the southern portion of the wetland was filled, sometime between 1952 and 1968. The southern terminus of the present day wetland is at the north side of 75th Street SW. The previous terminus was 500 to 1,000 feet south of that location. Most of the road fill encounter in BH-5 and BH-6 consists of very loose to medium dense silty to very silty sand, with varying amount of gravel. The roadway fill soil at parts contains decomposing wood fragments. It should be noted that we encountered about 5 feet of well compacted, clean, well graded sand and 2014-117 Final Geotechnical Report 4 HWA GEOSCIENCES INC.