Construction Methods - Chapter06.pdf - po ang - Januszek66

RockExcavation

6–1INTRODUCTION

RockCharacteristics

Rockmaybeclassifiedasigneous,sedimentary,ormetamorphic,accordingtoitsorigin.

Igneousrock formedwhentheearth’smoltenmaterialcooled.Becauseofitsorigin,itis

quitehomogeneousandisthereforethemostdifficulttypeofrocktoexcavate.Examples

ofigneousrockaregraniteandbasalt. Sedimentaryrock wasformedbytheprecipitationof

materialfromwaterorair.Asaresult,itishighlystratifiedandhasmanyplanesofweak-

ness.Thusitisthemosteasilyexcavatedtypeofrock.Examplesincludesandstone,shale,

andlimestone. Metamorphicrock originatedasigneousorsedimentaryrockbuthasbeen

changedbyheat,pressure,orchemicalactionintoadifferenttypeofrock.Metamorphic

rockisintermediatebetweenigneousrockandsedimentaryrockinitsdifficultyofexca-

vation.Examplesofmetamorphicrockincludeslate,marble,andschist.

Thedifficultyinvolvedinexcavatingrockdependsonanumberoffactorsinaddi-

tiontotherocktype.Someofthesefactorsincludetheextentoffracturesandotherplanes

ofweakness,theamountofweatheringthathasoccurred,thepredominantgrainsize,

whethertherockhasacrystallinestructure,rockbrittleness,androckhardness.

RockInvestigation

Relativehardnessismeasuredon Moh’sscale from1(talc)to10(diamond).Asarule,any

rockthatcanbescratchedbyaknifeblade(hardnessabout5)canbeeasilyexcavatedby

rippingorothermechanicalmethods.Forharderrock,additionalinvestigationisrequired

toevaluatetherockcharacteristicsdescribedabove.Theprincipalmethodsforinvestigat-

ingsubsurfaceconditionsincludedrilling,excavatingtestpits,andmakingseismicmeas-

urements.Drillingmaybeusedtoremovecoresamplesfromtherockortopermitvisual

observationofrockconditions.Coresamplesmaybevisuallyinspectedaswellastestedin

thelaboratory.ObservationinatestpitorinspectionbyTVcamerasplacedintodrilled

holeswillreveallayerthickness,theextentoffracturingandweathering,andthepresence

155

156

CHAPTER6

Figure6–1 Schematicrepresentationofseismicrefractiontest.

Figure6–2 Graphofrefractiontestdata.

ofwater.Useoftherefractionseismographpermitsarapiddeterminationofrocksound-

nessbymeasuringthevelocityatwhichsoundtravelsthroughtherock.

Inperformingaseismicrefractiontest,asoundsourceandanumberofreceivers(geo-

phones)aresetup,asillustratedinFigure6–1.Thetimerequiredforasoundwavetotravel

fromthesoundsourcetoeachreceiverismeasuredandplottedagainstthedistancefromthe

soundsource,asillustratedinFigure6–2.Inthisplottheslopeofeachsegmentofthecurve

representsthesoundvelocityinthecorrespondingsubsurfacelayer.Thisvelocityhasbeen

foundtorangefromabout1000ft/s(305m/s)inloosesoiltoabout20,000ft/s(6100m/s)

insoundrock.Sincetherelationshipbetweentheangleofincidenceandtheangleofre-

fractionofasoundwavecrossingtheinterfaceoftworocklayersisafunctionoftheirre-

spectivesoundvelocities,theseismicrefractiontestmethodmayalsobeusedtodetermine

157

ROCKEXCAVATION

thethicknessofrocklayers.Equation6–1maybeusedtodeterminethethicknessofthe

upperlayerwhenthesoundvelocityincreaseswithlayerdepth,thatis,whenthevelocity

inthetoplayerislessthanthevelocityinthesecondlayer,whichistheusualcaseinthe

field.

(6–1)

where H 1 % thicknessofupperlayer(ftorm)

D 1 % distancefromsoundsourcetofirstintersectionoflinesontime-distance

graph(ftorm)(pointA,Figure6–2)

V 1 % velocityinupperlayer(ft/sorm/s)

V 2 % velocityinsecondlayer(ft/sorm/s)

EXAMPLE6–1

Findtheseismicwavevelocityanddepthoftheuppersoillayerbasedonthefollowingre-

fractionseismographdata:

DistancefromSound

SourcetoGeophone

T ime(ms)

10 3.05 5

20 6.10 10

30 9.15 15

40 12.20 20

50 15.25 22

60 18.30 24

70 21.35 26

80 24.40 28

SOLUTION

PlottimeoftravelagainstdistancefromsoundsourcetogeophoneasshowninFigure6–3.

% 40 #

% 2000ft >

0.020 #

B % 12.2 #

% 610m

0.020 #

% 80 # 40

0.028 # 0.020 % 5000ft >

B % 24.4 # 12.2

0.028 # 0.020 % 1525m

158

CHAPTER6

Figure6–3 Graphofrefractiontestdata,Example6–1.

1 % 40

¢ 5000 # 2000

5000 " 2000 ≤

% 13.1ft

B % 12.2

¢ 1525 # 610

1525 " 610 ≤

% 4.0m

Rock-HandlingSystems

Theprocessofrockmovingmaybeconsideredinfourphases:loosening,loading,hauling,

andcompacting.ThemethodsemployedforrockcompactingarediscussedinChapter5.

Therefore,thisdiscussionofrock-handlingsystemswillbelimitedtothephasesofloos-

ening,loading,andhauling.Theprincipalmethodsandequipmentavailableforaccom-

plishingeachofthesephasesarelistedinTable6–1.

Thetraditionalmethodforexcavatingrockinvolvesdrillingblastholesintherock,

loadingtheholeswithexplosives,detonatingtheexplosives,loadingthefracturedrock

intohaulunitswithpowershovels,andhaulingtherockawayintrucksorwagons.Newer

alternativesincludetheuseoftractor-mountedripperstoloosenrock,theuseofwheel

Table6–1 Principalrock-handlingsystems

Operation EquipmentandProcess

Loosen Drillandblast

Rip

Load Shovel

Wheelloader

Haul Truck

Wagon

Loadandhaul Reinforcedscraper

159

ROCKEXCAVATION

Figure6–4 Largetunnelingmachine.(CourtesyofTheRobbinsCompany)

loaderstoloadfracturedrockintohaulunits,andtheuseofreinforced(or“specialappli-

cation”)scraperstobothloadandhaulfracturedrock.Theequipmentandproceduresuti-

lizedareexplainedinSections6–2to6–4.Theselectionoftherock-handlingsystemtobe

employedinaparticularsituationshouldbebasedonmaximizingthecontractor’sprofit

fromtheoperation.

Tunneling

Tunnelinginrockisaspecializedformofrockexcavationthathastraditionallybeenac-

complishedbydrillingandblasting.Recently,however, tunnelingmachines ormechanical

moles equippedwithmultiplecutterheadsandcapableofexcavatingtofulltunneldiameter

havecomeintoincreasinguse.ThetunnelingmachineshowninFigure6–4weighs285tons

(258t),producesathrustof1,850,000lb(8229kN),anddrillsa19-ft(5.8-m)–diameterhole.

Someofthespecializedtermsusedintunnelingincludejumbos,hydraulicjumbos,and

muckingmachines. A jumbo isalargemobileframeequippedwithplatformsatseveral

elevationstoenabledrillsandworkerstoworkonthefulltunnelfaceatonetime.The

hydraulicjumbo illustratedinFigure6–5isaself-propelledmachineequippedwithanum-

berofhydraulicdrills,eachmountedonitsownhydraulicboom.Suchamachineiscapable

ofdrillingblastholesacrossthefulltunnelfaceatonetime.A muckingmachine isaformof

shovelespeciallydesignedforloadingfracturedrockintohaulunitsduringtunnelexcavation.

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