7000 HARDESON RD 2020-06-05 - Laserfiche WebLink

Geotechnical Engineering Report Community Transit Merrill Creek Operating Base 7100 Hardeson Road Everett, Washington February 28, 2018 RN File No. 3224-002A Page 11 a "Where there is a lack of usable space for rain garden/bioretention facilities at re- development sites, or where there is insufficient space within the existing public right- of-way on public road projects." o In our opinion, there is a lack of usable space in the narrow, landscaped islands at this re-development site. Sheet Flow Dispersion BMP T5.12: This BMP is discussed on page 5-6 of DOE Volume V. "This BMP can be used in any situation where concentrated flow can be avoided. The design guidelines state "Because flows are already dispersed as they leave the surface, they need only traverse a narrow band of adjacent vegetation for effective attenuation and treatment." Although concentrated flows are avoided because the parking and drive aisles have relatively flat slopes, the surface flow goes directly into a trench drain at the eastern edge of the paved area. Design guidelines call for a 2-foot wide transition zone and a 10-foot wide vegetated buffer for every 20-foot width of impervious surface. In our opinion, this BMP is not feasible because the edge of the asphalt abuts the trench drain leaving no room for a transitions zone or its associated buffer. Concentrated Flow Dispersion BMP T5.11: DOE page 5-3 of Volume V states that this BMP can be used "in any situation where concentrated flow can be dispersed through vegetation." The vegetation on the site is limited to the landscaped islands. The design guidelines discuss a vegetated flowpath of at least 50 feet. In our'opinion, based on the lack of a vegetated flowpath this BMP is infeasible. Utilities Our explorations indicate that deep dewatering will not be needed to install standard depth utilities. Anticipated groundwater is expected to be handled with pumps in the trenches. We also expect that some groundwater seepage may develop during and following the wetter times of the year. We expect this seepage to mostly occur in pockets. We do not expect significant volumes of water in these excavations. The soils likely to be exposed in utility trenches after site stripping are considered moisture sensitive. We recommend that they be considered for trench backfill during the drier portions of the year. Provided these soils are within 2 percent of their optimum moisture content, they should be suitable to meet compaction specifications. During the wet season, it may be difficult to achieve compaction specifications; therefore, soil amendment with kiln dust or cement may be needed to achieve proper compaction with the on-site materials. Pavement Subgrade The performance of roadway pavement is critically related to the conditions of the underlying subgrade. We recommend that the subgrade soils within the roadways be prepared as described in the Site Preparation and Grading subsection of this report. Prior to placing base material, the subgrade soils should be compacted to a non-yielding state with a vibratory roller compactor and then proof-rolled with a piece of heavy construction equipment, such as a fully- Robinson Noble, Inc.