600 RIVERSIDE RD Geotech Report 2022-10-25

1 1 Landau Associates result, if sheet piles that penetrate the silt unit are used, we recommend that the tops of the sheeting be cut off below the ground surface and left in place. 3.6.3 Pipe Foundation Support We anticipate that the proposed utility pipes will be installed in either the fill or the upper sand unit. Consequently, the loose to medium dense sand with varying amounts of silt associated with these two units are expected at the invert elevations of the proposed pipes. In our opinion, this soil will generally provide suitable foundation support of the planned utilities, provided the trench excavations are properly dewatered and the foundation soil is maintained in an undisturbed condition. If the trench bottom becomes disturbed, or if soft silt, clay, organic soil, or otherwise unsuitable material is encountered in the base of the excavation, the trench bottom should be overexcavated to expose undisturbed foundation soil, to a maximum depth of 2 ft. Removal and replacement of unsuitable foundation material should be in accordance with Section 7-08.3(1)A of the 2016 WSDOT Standard Specifications. The overexcavation should be backfilled with suitable foundation material to provide a firm trench bottom. Foundation material should meet the requirements for Bank Run Gravel for Trench Backfill in Section 9-03.19 of the 2016 WSDOT Standard Specifications. Pipe foundation material should be placed and compacted in accordance with Section 2-03.3(14)C, Method C of the 2016 WSDOT Standard Specifications. Compaction and moisture control tests should be conducted in accordance with Section 2-03.3(14)D of the 2016 WSDOT Standard Specifications. The maximum dry density may also be determined by the ASTM D1557 test procedure. If suitable foundation soil is not encountered within a depth of 2 ft below the bottom of the trench, a woven geotextile, such as Mirafi 50OX or equivalent, should be placed over the bottom of the trench prior to the placement of backfill material. The recommended geotextile will provide separation between the trench bottom and pipe foundation material. 3.6.4 Pipe Bedding and Initial Backfill To provide uniform support of buried utility pipes, the pipe should be bedded in accordance with Section 7-08.3(1)C of the 2016 WSDOT Standard Specifications and WSDOT Standard Plan B-55.20-00 (WSDOT 2014). Bedding material for buried utility pipes, including flexible pipes, should consist of Gravel Backfill for Pipe Zone Bedding meeting the requirements Section 9-03.12(3) of the 2016 WSDOT Standard Specifications. For rigid pipes (concrete and ductile iron), the bedding material should be placed below and around the pipe in accordance with WSDOT Standard Plan B-55.20-00. Pipe zone backfill for rigid and metal pipes should meet the requirements of Section 7-08.3(3) of the 2016 WSDOT Standard Specifications. Excavated soil from the trench may be suitable for use as pipe zone backfill for rigid pipes provided that it is near its optimum moisture content and cobbles or boulders (i.e., materials with particle sizes greater than 3 inches) are removed from the soil. If additional material is needed for pipe zone backfill Geotechnical Engineering Report 0121030.010.011 Riverside Business Park March 22, 2018 Public Access Trails and Roadway Improvements 3-8