10200 25TH AVE SE Geotech Report 2018-01-02 MF Import

. t , Propc.sed Modemization and Additians to SWisurface Exp[oration, Geofogic Ha;.ards, Eisenhower Middle School and Geotechnica!Engineering Repon Everett. WarhinRton DesiRn Recommerdations allowable bearing pressure of 3,000 psf including both dead and live loads. An increase of one-third may be used for short-term wind or seismic loading. 10.4 Alternate Foundation Support Usin� Rock-Filled Trenches Another alternative Cor foundations in existing fill areas would be to support foundations on rock-filled trenches that extend to suitable bearing soils beneath the existing fill. Where rock trenches are used, the building pad should be excavated as needed to provide a surface that is at least 2 feet lower than the building pad subgrade elevation. This will allow placement of a 2-foot-thick structural fill blanket for slab support. The building pad excavation should be proof-rolled and any soft or yieldiitg areas removed and replaced with structural fill. Rock trenches can then be installed. The trenches shou(d have a minimum width of 4 feet (or as designated by �'ze field engineer/engineering geologist) and be excavated down to suitable till, which was encountered at depths of up to 7 feet below the exist°ng ground surface at the time of exploration. All rock trenches must penetrate to the prescribed bearing stratum and no trenches should be founded in or above loose, organic, or existing fill soils. Because of the potential for caving, the actual trench width may be greater than specified. It wou13 be appropriate to backfill the trenches as the excavation proceeds to reduce caving. The use of.: larger, track-mounted backhoe will greatly speed trench excavation over the use of a conventional rubber-tired backhoe. In order to reduce dismrbance of the bearing soils exposed in the trench, we recommend that the teeth of the backhoe bucket be covered with a digging p�:te. To determine when suitable bearing has been achieved and to verify proper rock placement, the georechnical engineer/engineering geologist must be present an a full-time basis during footing trench excavation and backfill. A pump may be required to control seepage so that the bearing level can be visually determined. Seepage entering the excavation on an overnight basis must be removed prior to commencing trench excavation the following day. After the bearing stra;um has been reached, the trench should be immediately backfilled. We recommend the use of "railroad ballast" or 2- to 4-inch si2e crushed rock for backfill. The crushed rock must be tamped into place in maximum 2-foot lifts to achie�e a [ighdy packed mass; this may be done with either a "Hoepac" rype compactor mounte:i on the backhoe or more typically, with the bucket of the backhoe itself. Staging areas should be maintained so that the rock is not contaminated by mud prior ro placement in the trer.ch. Equipment access to trench locations should also be maintained. Once rock trench placement is complete, the building pad can be cleared a:�d brought to grade with structural fill. Spread footings may then be used for building support when placed and cenrered over properly constructed rock trenches that bear on suitable till. Footings which bear on approved rock trenches may be desiened for an allowable bearing pressure of 3,000 psf including both dead and live loads. An increase of one-third may be used for short-term wind or seismic loading. lmu�an ?3, 200d ASSOCIAT£D E.-IRTH SGE:VCES, 'VC. ��.\ .tlT4J-F'EOJ7J9dJ-Prn�ran7l,�JJ7i9iA'E�R'P-R=8 P3gC I7 �/1�) `.�