4920 EVERGREEN WAY STAPLES Geotech Report 2022-10-25

Geotechnical Engineering Report lierracon Staples Rockery Failure■ Everett,Washington March 8, 2019 • Terracon Project No. 81175122 4.5.4 Drainage and Permanent Facing To relieve hydrostatic pressure behind the wall, geocomposite drainage should be installed between the permanent facing and the temporary timber lagging. The geocomposite drain should span between soldier pile flanges. At the base of the wall, PVC plastic or metal pipes known as weepholes should project through the permanent facing to transmit water collected by the geocomposite drain to a drainline or gutter installed in front of the wall. The drain line or gutter should be sloped to provide positive gravity drainage or to a sump pit and pump. If installed below grade, the drain line should be surrounded by clean, free-draining granular material having less than 5 percent passing the No. 200 sieve. The free-draining aggregate should be encapsulated in a filter fabric. If controlling hydrostatic pressure behind the wall as described above is not possible, then combined hydrostatic and lateral earth pressures should be calculated for granular backfill, an equivalent fluid weighing of 82 pcf should be used for active earth pressure and (82)H psf for the maximum ordinate of the apparent earth pressure. These pressures do not include the influence of surcharge, equipment or pavement loading, which should be added. Heavy equipment should not operate within a distance closer than the exposed height of retaining walls to prevent lateral pressures more than those provided. A permanent facing constructed via shotcreting after installation of the drainage system is typical. The permanent facing should be designed to resist long-term static and seismic earth pressures. 4.5.5 Retaining Wall Construction The contractor should be required to prevent caving and loss of ground in all soldier pile drill holes. The retaining wall contractor will need to use methods to minimize caving and sloughing of the drill holes, such as the use of augercast methods or installation of casing. If more than one foot of water is present in the bottom of the hole, placement of concrete from the bottom of the hole will be required. When drillingboreholes for tieback installation, casingmaybe required toprevent cavingand loss q of ground. Grout should be pumped into the bond zone by tremie methods in order to remove water from the hole and continuously grout the anchor. Voids behind the lagging should be backfilled immediately with a permeable granular soil material or lean concrete. 4.5.6 Monitoring Monitoring of the retaining system during construction should include measurements of horizontal movements at the top of every soldier pile. The measuring system used for shoring monitoring should have an accuracy of at least 0.01-feet. All reference points on existing structures should be installed and readings taken prior to commencing construction. All reference points should be read prior to and during critical stages of construction. The frequency of readings will depend on Responsive• Resourceful • Reliable 13