4920 EVERGREEN WAY STAPLES Geotech Report 2022-10-25

Geotechnical Engineering Report lierracon Staples Rockery Failure• Everett,Washington March 8, 2019 r Terracon Project No. 81175122 resistance of 20 ksf with an allowable side friction of 2 ksf for that portion of the pile embedded below ground surface in front of the wall. Factors of safety of 3 and 2 have been applied to the allowable end bearing and side friction, respectively. 4.5.2 Tiebacks For tiebacks, the bonded portion should be located sufficiently far behind the wall face to stabilize the wall. The "no-load" zone limit is the area behind the soldier pile equal to a lateral distance from the base of the excavation equal to the exposed wall height (H in feet) divided by four, or five feet, whichever is greater, and a line sloping up and back at 60 degrees from horizontal. The selection of tieback materials and installation methods should be the responsibility of the contractor. Because permanent tiebacks will be required, the tiebacks will need to be double corrosion protected. The actual adhesion values will depend on the materials and installation method and should be confirmed by testing. For non-pressure grouted tiebacks, the allowable design concrete/soil adhesion values of 1,000 and 2,000 psf can be used for soils above and below about 23 feet bgs, respectively(i.e. approximately elevation 305 feet). For pressure grouted tiebacks, an increase in adhesion value by a factor of two to three is to be achieved. We recommend all tiebacks be proof-tested to at least 130% of the design capacity prior to locking off at the specified post-tensioned design load. Prior to installation of production tiebacks, two verification tests to 200% of the design pullout capacity are recommended. The following are recommendations for tieback design: • minimum tieback spacing of four feet center to center • anchor holes drilled at an angle of 15 to 45 degrees down from horizontal • minimum bond length of 10 feet • minimum unbonded length of 15 feet The location and presence of existing features such as utilities should be checked during the design as these may affect the location and length of tieback anchors. 4.5.3 Lagging We recommend temporary timber lagging, or some other form of protection, be installed in all areas. Due to soil arching effects, lagging may be designed for 40 percent of static lateral earth pressure used for retaining wall design. Prompt and careful installation of lagging would reduce potential loss of ground. The requirements for lagging should be made the responsibility of the retaining wall subcontractor to prevent soil failure, sloughing, and loss of ground. Proper installation of lagging is critical to provide safe working conditions.We recommend that any voids between the lagging and soil be backfilled promptly. However, the backfill should not allow potential hydrostatic pressure to build-up behind the wall. Drainage behind the wall must be maintained. Responsive • Resourceful • Reliable 12