Preliminary Geotechnical Engineering Report <br />Project No. 18-4986, 177 Day Island Road, Eugene, Oregon <br />NO " <br />again assuming level backfill against the wall. These values assume that the recommended <br />drainage provisions are incorporated, and hydrostatic pressures are not allowed to develop against <br />the wall. <br />During a seismic event, lateral earth pressures acting on below-grade structural walls will increase <br />by an incremental amount that corresponds to the earthquake loading. Based on the <br />Mononobe-Okabe equation and peak horizontal accelerations appropriate for the site location, <br />seismic loading should be modeled using the active or at-rest earth pressures recommended <br />above, plus an incremental rectangular-shaped seismic load of magnitude 6.5H, where H is the <br />total height of the wall. <br />We assume relatively level ground surface below the base of the walls. As such, we recommend <br />passive earth pressure of 320 pcf for use in design, assuming wall footings are cast against <br />competent native soils or engineered fill. If the ground surface slopes down and away from the <br />base of any of the walls, a lower passive earth pressure should be used and GeoPacific should be <br />contacted for additional recommendations. <br />A coefficient of friction of 0.42 may be assumed along the interface between the base of the wall <br />footing and subgrade soils. The recommended coefficient of friction and passive earth pressure <br />values do not include a safety factor, and an appropriate safety factor should be included in design. <br />The upper 12 inches of soil should be neglected in passive pressure computations unless it is <br />protected by pavement or slabs on grade. <br />The above recommendations for lateral earth pressures assume that the backfill behind the <br />subsurface walls will consist of properly compacted structural fill, and no adjacent surcharge <br />loading. If the walls will be subjected to the influence of surcharge loading within a horizontal <br />distance equal to or less than the height of the wall, the walls should be designed for the additional <br />horizontal pressure. For uniform surcharge pressures, a uniformly distributed lateral pressure of <br />0.3 times the surcharge pressure should be added. Traffic surcharges may be estimated using an <br />additional vertical load of 250 psf (2 feet of additional fill), in accordance with local practice. <br />The recommended equivalent fluid densities assume a free-draining condition behind the walls so <br />that hydrostatic pressures do not build-up. This can be accomplished by placing a 12 to 18-inch <br />wide zone of sand and gravel containing less than 5 percent passing the No. 200 sieve against the <br />walls. A 3-inch minimum diameter perforated, plastic drain pipe should be installed at the base of <br />the walls and connected to a suitable discharge point to remove water in this zone of sand and <br />gravel. The drain pipe should be wrapped in filter fabric (Mirafi 140N or other as approved by the <br />geotechnical engineer) to minimize clogging. <br />Wall drains are recommended to prevent detrimental effects of surface water runoff on foundations <br />- not to dewater groundwater. Drains should not be expected to eliminate all potential sources of <br />water entering a basement or beneath a slab-on-grade. An adequate grade to a low point outlet <br />drain in the crawlspace is required by code. Underslab drains are sometimes added beneath the <br />slab when placed over soils of low permeability and shallow, perched groundwater. <br />18-4986, 177 Day Island Road GRPT 11 GEOPACIFIC ENGINEERING, INC. <br />Version 1, August 27, 2018 <br />