1901 MERRILL CREEK PKY 2018-01-01 MF Import

crushtd rock should be similar to WSDOT Standard Specification 9-03.9(3). In roadway aceas and pavement areas which will be subject to truck traftic, che thiclu�ases of asphalt concrete and � crushed rock should be increazed to 3 and 6 inches, rapectively. These design recommendations assume that pavement construction is accomplished during ex[ended periods of dry weather. � To promote positive drainage from beneath the pavement structure, we recommend a � minimum 6-inch-thick subbase be provided beneath the pavement section. The subbaze should consist of a free-draining sand and gravel with less than 5 percent fines by weight relative to the � � 3/4 fraction. In areaz where the ezisting fiil and native soils are soft or wet, an increased subbase � thickness may be required. r: The construction schedule should allow time for the pazking subgrade soils to "heal" prior . to placing crushed rock and asphalt concrete. The subbase surface will not serve as support for const�uction traffic and construction haul roads should be provided as discussed under the "Site � ; Preparation" section of this report. The crushed rock base course and subbase fill should both be compacted to at least 4 95 percent of the MDD determined in accordance with ASTM D-1557. Backfill in utiliry trenches � underlyu�g paved areu should also be compacted in accordance with the recommendations for strucmral fill pruented in this teport. I �� l.ii DRAINAGE CQNSIDERATIONS � Because of the depositional nature of the soils at the site where impermeable soils(typically glacial till and hazd silt) aze interlayered with granulaz soils (predominately sand and gravel), it � is likely that perched subsurface water or seeps ±nay be encountered during ezcavations. We �=, recommend that appropriate petmanent drainage facilities be incoiporated into the daign. Specific design of such facilities shouid be evaluated when final grading plans are available, and additional measures may be appropriate bazed on the observed stratigraphy of soils and seepage zones exposed in ezcavations. Control of surface water runoff is also critical. Collceted surface or subsurface water should not be discharged on slope faces. Roof drains should be connected to a tightline system � which is hydraulically independent from subgrade wall drains. The ground surface adjacent to building(s) should be sloped so that surface runoff flows away from the structure. We recommend that all surface water that may flow from highet azeaz of the site be intercepted by adequately designed swales, interceptor ditchu, or footing drains that tightline to a suitable discharge. An intercepto- ditch is constructed by ezcavating a trench, placing a nonwoven geotextile around the trench perimeter and a perfotated pipe near the bottom, and backfilling the trench with drain rock or sand and gravel with no fill. A schematic of a typical interceptor trench is provided as Figure 3. The upper portion of the site will ezperience seasonally petched ground water conditions because of the relatively impermeable giacial till that occurs at sNallow depths. Thereforo, we recommend perimeter footing drains (where basement wall drains or interceptor trenches are not G e o E n � i n e e r s 15 Fiie No. I�QO-006-73-11�0l111297