EXH0381021690

._,0_1108th Ave.. Suite 400, Bellevue, WA 9_04 _: {42_) 455-3555 *. Fax (425} 453-9179 ,, Architectural Fax (425) 40u-25 ." To Michael Cheytae, PMP Date October 26, 2000 Third Runway Program Leader From Jim Thomson, P.E., I-INTB Mike Barley, P.E., Hart Crowser Michael Kem-ick, P.E., H.art Crowser Subject "Third Runway Embankment Infiltration Summary Sometimeago, theThird Runwry DesignTeam was asked to considertakingstepstoward providing enhancedinfiltrationwithinthe embankment. Because of thestormwaterdetention benefitinfiltrationprovides,the team had atreadyreviewedthe possibilitiesof enhanced infiltrationand believedthe projectcoutdprovideopportunityin thefollowingways: • Use of flattercrossslopeson the airfieldprovidinggreater stormwaterresidencetime on pervioussurfaces, • Longerwater coursesoverpervioussudaces prior to stormsewercollectionalso increasingresidencetime. • Use of a varietyof naturallyoccuringfillmaterialsallowing forsomeinfi[trationwhile continuingto providestability. Recently, the designteam wasasked to provideadditionalinfiltrationbeyondwhatwould occurfrom the design approachoutlinedabove. We haveevaluatedalternatives,which would meet both designgoals of enhanced infiltrationandembankmentstability. The design team believesthat artificiallyincreasingthe amountof infiltrationbeyondstandarddesignand constructionpracticefor an embankmentof thisdepthwouldincreasethe riskof Instability, increase constructioncomplexityand clbsts,and may resultin adverseimpactsbeyondthe embankmentlimits. We believethe benefit,whichmightbe providedthroughenhanced infiltration,is out-weighedbythe increasedrisksto ttie embankmenL Embankmentsthat are designedand constructedfor stormwater retention,reservoirs, etc. are typicallybuiltas zoned embankments,withdifferentsoilmaterial zones usedto control seepage and providenecessarydrainageor strengthto accommodatethe resultantpore pressures The proposedThirdRunwayembankmentis a type of zoned embankment,but its zones were designedto optimizesupportof the airfield pavementsectionswith minimumlong term risk. The increasedrisksresultingfrom enhanced infiltrationare: • Increasedriskof surficialslopestabilityproblemsrelatedto seepage • Increaserisk of subgradeliquefactionbase stability • Increasedconstructioncomplexity • Longtermmaintenanceissues A more detaileddiscussionof the increasedrisksfollows in the nextsectionof this memorandum. AR 021690 Ftlen_m¢ V_NTP6911klZT.._'mbankmcnNnfiltr-mcmo, doc Printed IO/2(YO0 DOE9117/0100 5 J " , Detaiis oflncreas,ed Risk I. ,- 1- Increased Risk of Surficial Slope Stability Problems Rela:'..dto Seepage , . Side slopesfor the proposedembankment are…

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Add to CalendarMWednesday Mar 11, 9:00am - 11:35am
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Call to order – Roll Call – Quorum Established – Land Acknowledgement We would like to acknowledge that our Agency operates in four present-day counties, which are the traditional lands of the Coast Salish people, past and present. This acknowledgement does not take the place of authentic relationships with indigenous communities in our jurisdiction and [...]