timber shoring

TrenchingandExcavationOperations

HMTRI2000 Page58 TrenchingREV1

TIMBERSHORINGSYSTEMS

OBJECTIVES

Uponcompletionofthissection,theparticipantshouldbeableto:

1. Depictthedifferentpartsofashoringsystemandtheirfunction.

2. Clarifytheroleofplywoodintrenchingoperations.

3. Demonstrate the correct use of the timber shoring tables indesigningatimbershoringsystem.

4. Explainthelimitationsonthetimbershoringtables.



Introduction

The safety concept of behind timber and aluminum shoring isfundamentallydifferentfromshieldsystems.Recallthatshieldingsystemsweremobile and not in contact with the trench wall, and that shield protected theworkersfromthecavein.Timberandaluminumshoringsystemsaredesignedtoprotectworkersbypressingagainstthefaceofthetrenchandpreventingthecavein fromoccurring in the firstplace. Todo this,OSHAhasdevised tablesthatdetermine themosteffectiveshoringconfigurations. Wewill lookat thesetablesshortly.

ShoringSystem astructuresuchasametalhydraulic,mechanicalortimbershoringsystemthatsupportsthesidesofanexcavationandwhichisdesignedtopreventcaveins.

Thecompetentpersonhasfourshoringoptionsfromwhichtochoose:

OPTION1:Usethetables,whichOSHAprovides.Thisrequiressoiltesting.

OPTION 2: Use the manufacturer's tabulated data that is supplied with theshoringsystem.

OPTION3:UsetablesdesignedbyRegisteredProfessionalEngineer.

OPTION 4: Use a shoring system designed by a Registered ProfessionalEngineer.

PARTSOFASHORINGSYSTEM

CROSSBRACE

These are horizontal members, which extend perpendicularly to eachtrenchwall.Bytighteningthecrossbraces,pressureisappliedtothesoilinthetrenchwalls.Crossbracesabewooden,hydraulic,pneumaticorascrewbrace(trench jack). Mechanical crossbraces usually has a limited span. Widetrenches will often have timber crossbraces which using wedges tighten. Ifmechanical crossbraces are used, they must be used within their strengthrange,andthemanufacturer'stabulateddataforthisdevicemustbepresentatthe job site. It is also important not to overextend mechanical crossbraces.Thiscommonlyoccurswithscrewbraces.

Kickout theaccidentalreleaseofacrossbrace.



UPRIGHT

These members are placed vertically against the trench wall. Uprightsdistribute thecompression forcegeneratedby thecrossbraces toawiderareaonthetrenchwallandarecommonlymadeofwoodormetal.Theterm"upright"implies that there are gaps between individual planks. Sheeting is an uprightsystemwheretherearenogapsbetweenplanks.

Sheeting themembers of a shoring system that retain the earth in positionandinturnaresupportedbyothermembersoftheshoringsystem.



Closesheetingisformedwhenindividualuprightsarebuttedupagainsteachothertoformasolidplankwall.It'scommonlyusedtoguardagainstlocalSpallingandraveling.Tightsheeting,ontheotherhand,isusedinsaturatedorsubmergedsoilconditions.Theedgesoftheuprightsareaminimumof3inchesthickandconstructedina"tongueandgroove"fashion.Whenassembled,theyformawatertightbarrier. Steeluprightscalled sheetpiling interlock to formawatertightwall.Commonly,thesheetpileisapermanentsoilbarrierthatwillbeleftbehindaftertheconstructionisover.

Canplywoodbeusedassheeting? No.Cananyplywoodbeusedasaprimaryloadcarryingmember?No.Plywoodcanonlybeusedtocontrol localraveling,andregular3/4"plywoodisnotapprovedforthistaskunlessithasbeensodesignatedbyaRegisteredProfessionalEngineer.Plywoodthatcanbeusedtocontrollocalravelingis14plyarcticwhitebirch(FinlandFormor"inform"),11/8" softwoodplywood,oranyotherplywoodconfiguration,whichhasequalorbetterstrengthandstiffnesscharacteristics.Forexample,OSHAhasapprovedthe use of two sheets of 3/4" plywood screwed together with the face grainparallel to the trenchline(edgeof the trench)asasubstitution for3/4" informs.HORIZONTAL AND VERTICAL CROSSBRACE DISTANCES AREUNAFFECTED BY THE PRESENCE OR ABSENCE OF PLYWOOD. Pleaserefer to thearticle "OSHAGives Interpretation forPlywood" in theappendix formore clarification. NOTE: The article only addresses the use of plywood foraluminumshoringconfigurations.

WALE

Wales(orstringers)arehorizontalmembersthatpressagainsttheuprightsanddistributethecompressiveforceofacrossbracetothoseuprightswhichdonothave their own crossbrace. This allows a compressive span between crossbraces.Thespacebetweenthecrossbracespermitspipesorothermaterialstobeloweredintothetrench.Insomecases,walesmaynotberequired.

Mudsill Awalewhichisplacedatthebottomofthetrenchandisimbeddedin the soil. Mudsills prevent the unsupported bottoms of uprights from beingdisplacedintothetrench.



TIMBERSHORING

Timbershoringsystemsareveryversatilesincetheyarecustombuilttofitthetrench.Deepandlongtrenchesareprobablybettersuitedtotimbershoringthan any other form of protective system. Obviously, it is suited well forexcavations where significant time is to be spent in one area. For thoseapplicationswheretheworkingareaofanexcavationisconstantlymoving,asinlaying a conduit, shield systems or trench boxes are the protective devices ofchoice.

Specificwoodisrequiredforuseinatimbershoringsystem.Oakwithabendingstrengthof850psiandDouglasfirwithabendingstrengthof1500psiarespecificallymentionedin theStandard.Manufacturedcomponentsarealsoallowediftheypossesequivalentstrengthandtabulateddataforthedeviceisatthejobsite.NOTE:Thetimbershoringtablesarespecificallydesignedforeitheroak orDouglas fir, or equivalent, and careshould beexercisednot tomix thewoodtypesinashoringsystem.Thesematerialsmustbefreefromdamageordefects, maintained in good condition, inspected, and removed from service ifdamaged.



SHORINGTABLES

There are six tables, which provide construction and wood dimensiondirection for timber shoring systems. Each soil type has two tables, one foractualsizedlumberandone for"nominal"size lumber. Theactualsizelumberuses oak with a minimum bending strength of 850 psi or its equivalent as itsselectionfortimber.NominalsizetablesuseDouglasfirwithabendingstrengthof1500psioritsequivalentasitswoodofchoice. Since thebendingstrengthrequirements for Douglas fir are higher, the wood dimension requirements forDouglasfirareslightlysmallerthanforoakinsomeconfigurations.

Thetablesgivevariousoptionsforshoringconstruction.Ifheavierlumberis available, the table allows for longer spans and greater special distancesbetweenloadbearingmembers.Ifsmallerdimensionlumberispresent,optionstoproduceatightersystemarepresentaswell.

The wood dimensions and construction distances are based upon theanticipatedloadplusa2footsurcharge.ThedesignsputforthbythetablesDONOTapplywhen:

Asurchargeloadinexcessof2feetisplacedonthesoiladjacenttothetrench

Verticalloadsinexcessof240LBaredistributedonaonefootsectionofthecenterofthecrossbrace

Surchargeloadsarepresent fromequipmentweighing20,000poundsormore

Timbershoringismixedwithslopingunlesstheslopeisflatterthan3H:1Vorthedimensionsoftheshoringmembersarebasedonthetotaldepthofthetrench.



TABLEINTERPRETATION

The tables offer member spacing information as well as timber size. Alldistances are measured center to center. Spacing interpretations for crossbraces,uprights,andWalescanbesummarizedinthefollowingdiagram:

Whenplacingaconduitonthefloorofatrench, it is helpful to have the bottomcrossbrace as high as possible. TheStandard addresses three possiblescenarios:

If the uprights arenot imbeddedat thebase of the trench, the lowest crossbracecanbenohigherthan30inches.



If theuprightsareimbedded, thelowestcrossbracemaybenohigher than36inches.

If the uprights are imbedded and amudsill is used, the lowest crossbracemaybenohigherthan42inches.



Itisimportanttoknowhowtospacethecrossbracesatthetopofthetrenchaswell,especiallyifthetrenchdepthdoesnotallowthespacingtocomeouteven.TheStandardindicates that thespanfromthe topcrossbrace to thetopof thetrenchshallbenomorethan1/2oftheverticalcrossbracespacing.

SHORINGSYSTEMCONSTRUCTION

Manydifferent techniquesareusedinconstructinga timbershoringsystem.Usually,itisconstructedasthetrenchisdug.Uprightsareinstalledfirstandareusually supported by hydraulic shores until the timber crossbraces or trenchjackscanbesecured.Then,thehydraulicshoresareremoved.Afewcautionsonshoringconstructionmustbevoiced:

DO NOT ALLOW WORKERS TO WORK IN AN UNPROTECTEDTRENCHWHILECONSTRUCTINGTHESHORINGSYSTEM

Membersmustbeplacedunderloadduringconstruction.Donotwaitforthetrenchfacetomovebeforethesystemisloaded.Tightenthesystemwithwedges,trenchjacks,orotherdevices

Donotoverloadmembers.

Watchsoilforsignsoffailure

Shoring members must be secured together by nailing or some othereffectivemeans.



TIMBERSHORINGINSPECTION

It is critical that the competent person inspects the timber shoring before thebeginning of work each day and after each hazardincreasing event. Whileinspecting, the competent person is allowed to climb on the shoring system.Thingstowatchforinclude:

Tightnessofmembers

Membersfittightagainstsoil(gapswheresoilhaseroded)

Signsofsoilfailure

Crossbracesperpendiculartouprights(inbothaxis).

TIMBERSHORINGREMOVAL

Greatcaremustbeexercisedwhenremovingtheshoringsystem.Allpersonnelmustbeprotectedfromacaveinduringsupportremoval.Thesystemmustberemoved by disconnecting thebottom first andworking toward the top. Often,hydraulic systems are used to support the uprights while the timbers crossbracesare removed. Once the shoring system has been removed, backfillingshouldproceedassoonaspossible.

timber shoring

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