July 2009 5

Ice Jams and the Bridge TwoicejamsontheMissouriRivernearBismarck,onetothesouthandtheothertothenorth,dominatedlocalandnationalnewsinMarchof2009(fig.1).Thesouthernicejamcausedfloodwatersfrom theMissouri River to rise to 1,634 feet (above sea level).Anelevationof1,634.28feet(a16foot-readingintheBismarckstreamgauge)isconsideredfloodstage.ThiswasthehighestriverlevelseenatBismarcksince1952,oneyearpriortocompletionofGarrisonDam.ThedamislocatedontheMissouriRiver75milesupstreamfromBismarck.Thenorthernicejam,tenmilesnorthofBismarck,heldwaterbackand,inalllikelihood,preventedthefloodwatersatBismarckfromrisingtohigherelevations.

ThedevelopmentofspringicejamsontheMissouriRiverwouldnot have come as a surprise to George ShattuckMorison, thebridge engineer that designed and built the Northern PacificRailway bridge on the Missouri River between Bismarck andMandanintheearly1880s.Morison,aresidentofNewYorkCity,came to Bismarck in the spring of 1880 to scout for a suitablebridgelocationandtoobtainfirsthandknowledgeoftheMissouriRiver.Afterall,thiswastobethefirstbridgetospantheUpperMissouri River. While in Bismarck he witnessed amassive icejamandnotedthatblocksof icehadpileduptoa thicknessof20feetinsomeplaces.AnicejamthefollowingspringbetweenBismarckandFortAbrahamLincolncausedfloodwaterstospilloutoverthebottomlandtoaheightof1,644.4feet(fig.2).TheseicejamsgreatlyconcernedMorisonandplayedasignificantroleinhisbridgedesign.Tominimizethechancethathisbridgewould

besweptawaybyice,MorisonreducedthewidthoftheMissouriRiveratthebridgesitewithadike,permanentlyalteringtheriverintheBismarckarea,andspacedthepiers400feetapart.Asaresult,heonlyhadtoplacetwoofhisfourmassivegranitebridgepiers in thewater where theywould be vulnerable to ice. Toprotectthesetwomiddlepiers,hedesignedthemwith30-foot-high icebreaker sectionson theupstreamportion so that theycouldliterallyplowthroughtheice.Inaddition,hestrengthenedtheicebreakersectionsbydowelingallofthegranitestonesintotheunderlyinglayerandplatingthenoseoftheice-breakerswith¾-inch steel. Healsohad the stonemasons smooth the stonesaroundtheicecutterstopromotedownstreammovementoftheice(fig.3).

TheNorthernPacificRailwayBridgeatBismarckwaswitnesstomanyspringicejamsandfloodsbetweentheyears1882and1953.Ninemajorfloodevents(abovethe1,637.28foot elevation)were

Ice Jams, Landslides, and the Northern Pacific Railway Bridge at Bismarck

by Ed Murphy

Figure 1. Demolition crews set off a series of charges to weaken thesouthernicejamontheMissouriRiveratBismarckonMarch25,2009.Asecondseriesofchargesandsaltwereusedtofurtherweakentheice.PhotocourtesyoftheBismarckTribune.

Figure 2. Shaded relief map of the Bismarck-Mandan area. GeorgeMorison’s dike (green line) narrowed the Missouri River channel andconstrained it against its east bank for more than one mile near thebridgesite.ThelocationofFortAbrahamLincolnnotedbyFL.

6 DMR Newsletter

recorded inBismarckduring thattimeperiod (fig. 4). Morisondidnothavetowaitlongtoseehowhisbridgewouldstandupagainsthighwater.TheMissouriRiverreachedarecordhighof1649.88 feet in 1883 followed closelyby anear-record level of1,649.38in1887(fig.5).

Slope Stability and the BridgeAstimewas tobearout,Morisonproperlydesignedhisbridgetowithstandtheforceoficejamsandhighwater.Whathedidnot foreseewas that slope instability would create decades of

problemsandalmostleadtoabandonmentofhisbridge.Shortlyafter the railroad bridge was completed, the east pier beganslidingtowardstheriver.Forthenext68years,NorthernPacificengineersappliedavarietyofslopestabilizationtechniquesinanattempttostopthepierfrommoving.Asaresultoftheirefforts,thebridgeoffersaninterestinghistoricalreviewofgeotechnicaltheoriesandapplications.Thesetechniquesincludedexcavatinga pit above the bridge site to isolate the pier from the slidingmaterial,thekeyingoftherockaboveandbelowtheslidingzonewithblocksofconcreteinanefforttolockthematerialtogether

Figure 3. Morisondesigned the twomiddlepiers thatstandintheriverwithasteeledgeto“plow”throughiceto reduce the chanceof ice jams formingbeneath thebridge.Thegraniteblocksalongeithersideofthemetaledgeweresmoothedtoencourageicemovementawayfromthepier.Thephotographsweretakeninthefallof2003whentheMissouriRiverwasverylow.

Figure 4. AgraphofannualpeakheightsoftheUSGeologicalSurveygaugingstationontheMissouriRiveratBismarck.Peakheightsfrom1929-2009arefromtheUSGeologicalSurveygaugingstationatBismarck.GarrisonDamwascompletedin1953.

July 2009 7

andthusstabilizetheslide,extendingtheeastpier’sfoundationtoagreaterdepthtogetbelowtheslidingzone,erectingacofferdamaroundthepiertoisolateitfromthelandslide,andconstructionofagroundwaterintercepttunneltostabilizetheareabyreducingporewaterpressure.Ultimately,alloftheseattemptstostabilizetheareafailed. It isappropriatethatafternearlysevendecadesofconfoundingand frustrating engineers this slope stability problem wouldfinallybesolvedbyoneofthemostfamouscivilengineersthateverlived,RalphB.Peck.Bythelate1940s,theeastapproachtothebridgehadtobestraightenedbecausethelargelocomotivesofthe1930sand1940shaddifficultynegotiatingit.Atthattime,the Association of American Railroads had a contract with theEngineeringDepartmentattheUniversityofIllinoistoinvestigateslope stability problems. Ralph Peck, an engineering professorat the University of Illinois and co-author of two of the mostinfluentialtextbooksingeotechnicalengineering,Soil Mechanics in Engineering Practice and Foundation Engineering, tookovertheproject.Peck’sfirstresponsewastoinitiateasubsurfaceboringinvestigationtoidentifythesedimentsintheproblemareaandtodeterminetheirsoilproperties(fig.6).Heusedtheresultsofthatinvestigationtodeterminethefactorofsafety(theangleofaslopebelowwhichitshouldnotfail)fortheslopeabovethepier.Peckseizedtheopportunitythatwasaffordedduringthestraighteningof the railroad track to remove a considerable amount of thehillside above the east pier and reduce the remaining slope towithinhisdesiredfactorofsafety.Finally,afterallofthoseyears,hiseffortsresultedinrelativestabilityoftheslopeabovetheeastbridgepier(fig.7).

Ralph Peck donated his papers regarding the Northern PacificRailwayBridgetotheNorthDakotaGeologicalSurveyin1993(fig.8).Hementionedthatthisprojectheldspecialmeaningtohimbecauseoneofhisfather’sfirstjobshadbeentodesigntheeasttail span for this bridge. Duringhis long and illustrious career,Dr.Peckwasinvolvedinmorethan1,000consultingprojectsandwas the recipient of numerous accolades and awards includingtheprestigiousNationalMedalofSciencewhichwasawardedbyPresidentFordin1974.RalphPeckdiedin2008attheageof95.

Figure 5.TheMissouriRiverfloodinginthespringof1884.WaterextendsallthewayfromtherailroadbridgewesttothepresentlocationofNDHighway1806southofMandan.Theicebreakersonthemiddlepiersareunderwaterindicatingthatthewaterlevelexceeded1,644.4feet.Theseoriginalbridgespanswerereplacedin1905becausetheycouldnolongersafelysupporttheever-increasingweightoflocomotives.PhotocourtesyoftheStateHistoricalSocietyofNorthDakota.

Figure 6.SeverallandslidescarpsarevisiblealongtheeastslopesoftheMissouriRiverValleyabovetheBismarckRailroadBridge.ThetopofaCityofBismarckwaterreservoir isvisiblejustbelowthelargesttreeatthetopoftheeastslope.PhotographtakenbyRalphPeckin1951.

8 DMR Newsletter

History and the BridgeIdiscoveredthelongandcolorfulhistoryoftheBismarckRailroadBridgelandslidein the early 1990s while investigatinglandslides in marine and nonmarine rocksinwesternandsouth-centralNorthDakota. I found duringmy research ofthislandslidethattheearlyhistoryoftheBismarckRailroadBridgewasinterwovenwith theBismarckWaterCompany. Theleaky reservoirs of the Bismarck WaterCompanywerelocatedonahilltopabovethebridgesiteandwereblamedbymanyoftheNorthernPacificengineersfortheinstability of the east pier. The watercompany reservoirs reportedly leakedbetween 6,000 and 18,000 gallons ofwater per day. The 1,000 foot-longtunnelthattheNorthernPacificRailwayengineers constructedabove thebridgesitein1903wasmeanttointerceptwaterleakingfromthereservoirsbeforeitcoulddestabilizetheslopeabovetheeastpier.Several residentsofBismarckwhowerechildrenduringthe1920sand1930shavetoldmetheyplayedandcollectedgartersnakesattheentrancetothetunnel,neverdaringtoventuretoofarintowhattheyhadassumedwasanoldcoalmine.

Asthisnewsletterwasgoingtopress,asmalllandslidedevelopednorthofthebridgesitethatforcedtheCityofBismarcktocloseasectionofRiverRoaduntilitcouldberepaired.

Figure 7. AphotographlookingwesttotheNorthernPacificRailwayBridgeontheMissouriRiverbetweenBismarckandMandan,NorthDakota.ThephotowastakeninthefallwhenwaterlevelsintheMissouriRiverwererelativelylow.Foramorecompletearticleonthebridgesee:Murphy,E.C.,1995,TheNorthernPacificRailwayBridgeatBismarck:NorthDakotaHistory,JournaloftheNorthernPlains,StateHistoricalSocietyofNorthDakota,Vol62,No.2,p.2-19.

Figure 8.SomeofthepapersdonatedtotheStateofNorthDakotabythelateRalphB.Peck.