1502 WALL ST IMAGINE CHILDRENS MUSEUM Geotech Report 2022-10-25

Imagine Children's Museum Building Addition Revised Geotechnical Engineering Report Everett, Washington Design Recommendations 6.3 Wet Weather Conditions If construction proceeds during an extended wet weather construction period and the moisture-sensitive, silty lodgement till soils become wet, they will become unstable. Therefore, the bids for site grading operations should be based upon the time of year that construction will proceed. It is expected that in wet conditions, additional soils may need to be removed and/or other stabilization methods used, such as a coarse, crushed rock working mat to develop a stable condition if silty subgrade soils are disturbed in the presence of excess moisture. The severity of construction problems will be dependent, in part, on the precautions that are taken by the contractor to protect the moisture- and disturbance-sensitive site soils. If overexcavation is necessary, it should be confirmed through continuous observation and testing by a representative of our firm. 6.4 Subgrade Protection The site soils that are expected below the building pad contain a high silt fraction and are considered to be moisture- and disturbance-sensitive. These soils will become unstable if disturbed by construction equipment while at elevated moisture contents, requiring additional soil removal at an increased cost. Therefore, in addition to the recommendations presented in the "Site Drainage and Surface Water Control" section of this report, site preparation and initial construction activities should be planned to minimize disturbance to the existing ground surface, particularly during extended wet weather periods and the wet season (typically October through May). Construction traffic should be restricted to specific drive areas to limit the area where disturbance of the subgrade will occur. If site stripping and grading activities are performed during extended dry weather periods, we anticipate that site stabilization requirements will be much less. However, it should be noted that most of the native soils were naturally wet at the time of our exploration and that intermittent wet weather periods during the summer months could delay earthwork if soil moisture conditions become elevated above the optimum moisture content, or if existing high moisture contents cannot be reduced by drying during favorable weather. If construction will proceed in the winter, we recommend the use of a working surface of sand and gravel, crushed rock, or quarry spalls to protect the silty soils, particularly in areas supporting concentrated equipment traffic. In winter construction staging areas, a minimum thickness of 12 inches of quarry spalls or 18 inches of pit run sand and gravel is recommended. If subgrade conditions are soft and silty, a geotextile separation fabric, such as Mirafi 500x or approved equal, should be used between the subgrade and the new fill. For building pads where floor slabs and foundation construction will be completed in the winter, a similar working surface should be used, composed of at least 12 inches of pit run sand and gravel or crushed rock. Construction of working surfaces from advancing fill pads could be used to avoid directly exposing the subgrade soils to vehicular traffic. If buildings will include exterior masonry, we recommend that a construction access area be prepared around the entire perimeter of the buildings, as recommended above for construction staging areas. Such March 28,2019;Revised April 16,2019 ASSOCIATED EARTH SCIENCES,IN .. 5GB/Id-190104E001-3-Projects\20190104\KE‘WP Page,8` //��