IS 14448: Code of practice for reinforcement of rock slopes with plane wedge failure

16 Pages·1997·1.3 MB·English

by  Bureau of Indian Standards

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IS 14448 (1997): Code of practice for reinforcement of rock
slopes with plane wedge failure [CED 48: Rock Mechanics]
“!ान $ एक न’ भारत का +नम-ण”
Satyanarayan Gangaram Pitroda
““IInnvveenntt aa NNeeww IInnddiiaa UUssiinngg KKnnoowwlleeddggee””
“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता हहहहै””ै”
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“Knowledge is such a treasure which cannot be stolen”

IS 14448: 1997
(Reaffirmed - 2012)

Indian Standard
CODE OF PRACTICE FOR
REINFORCEMENT OF ROCK SLOPES
WITH PLANE WEDGE FAILURE
ICS 93.020
1997
BUREAU OF INDIAN STANDARDS
MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI 110002

October 1997 Price Group 5
AMENDMENT NO.1 DECEMBER 2006
TO
IS 14448: 1997CODE OF PRACTICE FOR
REINFORCEMENT OFROCK SLOPES
WITH PLANE WEDGE FAILURE
(Page 1, clause 3.5.2, heading and line 1) - Substitute 'Tendon' for
'Tandon'.
(Page2, clause4.1, line S) - Substitute 'parameters affecting the slope
stability'lor'slopeparameters'.
(Page2,clause4.2.4, line8)- Insert'that.is' after'parallel'.
(Page3,clause6.1, line3)- Substitute'occurs'lor'occures'.
(Page 3, clause 6.3.6, line 7) - Substitute 'hydrofracturing' for
'hydrofacturing'.
(J)age 4,clause6.4, line1)- Delete '(F. A.L.)'.
(Page4,clause6.4.2.3, line2)- Substitute '6'for '5'.
(Page4, clause7.1, lines2 and3) - Delete 'that is' and insert'Itshould
beensuredthat.'after'peovided'.
(Page 4,clause7.1,line4)- Substitute'vegetation'lor'vegitation'.
(Page 5,clause8.1, line1)- Substitute'offor 'or'.
(Page 5,clause8.4, line 3)- Substitute 'obtained'for 'obtianed'.
(Page 5,clause9.2.1.1, line4)- Substitute '9'for ';'.
[J)age6, Fig. 2(a)]- Substitute ''tip'for ''Vb'.
(CED48)
•Reprolf&PbyUnit,8IS,NewDelhi,India
RockMechanicsSectionalCommittee,CED48
FOREWORD
This Indian Standard wasadoptedbytheBureauofIndianStandards,afterthedraftfinalizedbyRock
MechanicsSectionalCommitteehadbeenapprovedbytheOvilEngineeringDivisionCouncil.
This IndianStandardisanattempt tobringmoreIndianStandardscoveringtypesofreinforcementand
its design invarious rock engineeringapplications.
Landslides, slips, rockfalls are someof the termsUsed to describe the movement of rocks under the
influenceofthegravity. Thesemovementssometimesbecomeseriouslydamagingorevendisastrous. In
highways,foundationsandhydraulicengineeringstructuresinrock,thereisaneedforassessingthedegree
ofstabilityofman-madeaswell asnaturalslopes.
Stability can usually be improvedby addinglimproving the shear resistance of the rock-mass or by
increasingthesystemsresistingforceagainstslidingbytyingoranchoringlboltingtheslidingrockwedge
orslabtotheparentrock-mass.
Rock-reinforcementcanbeeffectivelyusedtostabilizetheunstablenaturalorartificialslopes,which are
isolatedbyjointsandfaults. Therock-reinforcementconsistingofrock-bolts,rockanchors/cableanchors,.
canbeusedmorefrequentlywithorwithoutsteelstraps,welded meshor chainlink, mesh/fabric-link,
etc.Thefrequentuseofrock-reinforcementtostabilizetherock-slopesisduetoitsfollowingadvantages:
a) Versatility,
b) Simpletoapplyinvariousrockconditions,
c) Relativelyinexpensive,
d) Flexibleperformance,and
e) Easilycombinedwithothercontrolmeasures.
TechnicalCommitteeresponsiblefortheformulationofthisstandardisgiveninAnnexA
Inreportingtheresultofatestoranalysisinaccordancewiththisstandard,ifthefinalvalue,observedor
calculated,is tobe roundedoff,it shallbedoneinaccordancewithIS2: 1960'Rulesforroundingoff
numericalvalues(revised)'. Thenumberofsignificantplacesretainedintheroundedoffvalueshouldbe
thesameasthatoftheSpecifiedvaluein thisstandard.
IS 14448:1997
Indian Standard
CODE OF PRACTICE FOR
REINFORCEMENT OF ROCK SLOPES
WITH PLANE WEDGE FAILURE
1 SCOPE anchoring/reinforcing. This term also includes
subterranean cavities and artificial as well as
Thisstandardcoversthedesignaspectsor'remtor
naturalslopes.
cementof rock-slopes with planewedge failuret
•
Thisstandarddoesnotcoverthetypeofrockbolts 3.4 Rock-Medium
and their installation. This will becovered by Rocksareunderstoodin theirnaturalpositionand
Indian Standardon 'TypeofRockbolts andtheir
infills,anditincludessolidrock-masses.
installation'(draftunderpreparation).
3.5 Anchor
2 REFERENCES
Itisadevicetotransferforces inagivendirection
The Indian Standards given below contain fromthestructuretotherock-medium. Ananchor
provisions which through reference in this text, iscomposedofthreeparts.
constituteprovisionofthisstandard. Atthetime
3.5.1 TheAnchorHead
of publication, the editionsindicated werevalid.
Allstandardsaresubjecttorevision,andpartiesto Itis situatedat theexternal (movable) endofthe
agreementsbasedonthisstandardareencouraged anchor,itisused forconnectingtheface/baseplate
to investigate the possibility ofapplying the most andtransferring ofanchoring forces to the rock
recenteditionsofthestandardsindicatedbelow: massandalsousedforprestressingoftheanchors.
ISNo. Title
3.5.2 TheTandon
456 :1978 Code of practice for plain and Tandonisthemiddlepartoftheanchorconnect
reinforced concrete (third ingthe head and the root. The tandon usually
revision) allows,byits elasticdeformation,establishingof
permanent prestress of the anchor during the
4031 Methods of physical test for
anchoringprocess.
(Part5) :1988 hydraulic cement:Part 5Deter
mination of initial and final 3.5.3 TheAnchorRoot
settingtimes(firs:revision)
It is situated at the internal (distant)end of the
11309 :1985 Method for conducting pull-out anchorandis usedforfixing the anchorinto the
testoranchorbarsandrockbolts rock-mass.
3.5.4 AnchorRope/Cable
3 TERMINOLOGY Itconsistsofstrandsofsteelwireshavinghighyield
3.1 Forthepurposeof thisdraftcode,thefollow stress. Generally thesteelwires havesize2-7mm
ingdefinitionsshallapply. diameter.
3.6 Anchoraae
3.2 Anchoring
This ensures the co-operation of the rock
By anchoring, a compressive force is introduced
medium with the structure, consisting of steel
into the rock-mass directly towards the critical
bars/bolts and bundles/strands of steel wires/
joint. Theanchoringcreatesacompressivestress
ropes/cables.Theeffectofanchoringisincreased
whichincreases theinherentstrengthoftherock
byprestressing theanchorageinitsfreesection,
mass and help the rock-mass to support itself
that is betweentheendfixedin therockandthe
againstsliding.
externalanchorflxedinthestructure.
3.3 Structure 3.7 FixedAnchorLength (F.A.1.,.)
Itdenotesanyconstructionworkonor belowthe It is the length of that part of the rock-anchor
surfaceofslopewhosestabilityistobe securedby farthestawayfromthestructuresoverwhichtensile
IS 14448:1997
forces are transmitted to the surroundingrock- control measures depending upon the riskin
mass. F.A. L. shallDotbe lessthan60and100d. volvedinit,inshorttermaswellas inlongterm.
fordeformedandplainbarsrespectively,
4.2 Model ofFailures
where
4.2.1 There are mainly three type of modesof
d. == diaofbar/bolt/cable.
fallures intherockslopes namely planarslide, 3
The extensive field studyhas shown that F. A. L. dimensionalwedgefailureandtopplingfailure. In
shallbeasperTable1: thiscodeonlyonemodethatisplanewedgefailure
isdescribed(ste Fig.1).
Table1 FIxedAnchorLenathforDifferentRock
4.2.2 Theplanewedgefailureoccursundergravity
Conditions
alone when a rock-mass rests on an inclined
8) 2m,forveryaoodrockcondiliona (RMR- 81to1(0) geologicaldiscontinuity,suchasbeddinglweakness
b) 3m,foraoodrockOODditionl (RNa• 61to 80) planethat daylightsintothefreeslopefacethatis
c) 4m,forfairlpoorroe.*condilioDi (RMR- 21to 60) 'l'f>'l'p [su Fig. 1(b»). Incaseof3dimensional
d) 6m,forvaypoorrockconditions (RMR. 0to 20) wedgefailure, plane maybe assumed atapparent
dipofintersectionofjoints('Pi).
4 REQUIREMENTOFSTABIU1Y
4.2.3 MovementshowninFig. l(c),supposesthat
4.1 The stability of natural or artifidal slopes
therestraintofslidinghasbeenovercomenotonly
generallydepend upon the geometry, frequency along the surface ofsliding/failure but along the
andorientationofjoint sets,dipofslopeandits
lateralsidesoftheslideaswell.
planeof weaknessand condition of the slopes.
Other slope parameters are climate, hydrology, 4.2.4 Sliding shall occurwhenthe inclinationof
tectonic movements, presenceof brecciaand theplaneofSlip shallbe greater than thefriction
humanactivitiesin immediateand/or adjacent angleofthat planethat is'lip >tPJ [s~t Fig.1(b)].
area, underground openings. Blasting bring However,inthecaseofunreinforcedsteepslopes
about,years later. changesaffectingthestability ('I'r > goo - 'lip+tPj - A), failure may occurby
ofslopes. over-topplingandthesameisnotconsideredhere.
Intoppingfailure.thestrikesofslopeandjointsare
4.1.1 When any slope (naturaVartificial) shows
nearlyparallelwithin± ISOandthedipdirection
the signof instability, then it becomesessential
to stabilize the slope by adopting the effective ofbeddingplanes/continuousjointsisoppositeto
(0)
(b)
ee) (d)
FIo.l P1...ANBWEDGBFAILUREGEOMETRY

2
IS14448:1997
that oftheslope.. The horizontal component of geological, climatic, hydrological, tectonic move
earthquake acceleration Is accounted for by A ments and human activities in the immediate
which is equal to tan-aa. Safecutslopeangleto andloradjacentareaofthestructures.
check over-toppling is therefore equal to
6.2 ForstabilizJngtherockslopes,cementgrouted
(cxf - 'l'p +tPj - A).safecutslopeangletocheck
rock-anchors/bolts/cablesshouldbe used.
3dimensionalwedgefanureisequaltotheangleof
intersectionofcriticaljointplanes. 6.3 CementGrout
4.2.5 The most likely plane of weakness/critical Thedelaybetweendrillingtheholeanditsgrouting
plane,alongwhichslidingwilloccurinrelationto shouldalwaysbekeptminimumandasapolicy,one
theorien18tionoftheslopefaceshouldbeassesssed should always drill and grout the anchor on the
from the kinematicmodel analysis/stereographic sameday. The possibilityofsignificant shrinkage
plots, in the groutdiminishing tbe intimatecontactbe
tweenthe groutandrockmightprovideaweaker
S PRINCIPLEOFROCKREINFORCEMENT surfacethan that adjacentto thesteel. Hence,it
5.1 Therockreinforcementreinforcesandmobil shouldbeensuredthat thegrouteitherincludes a
zes the inherent strength of rock-medium which smallamount ofa suitableexpanding elementor
helpstheslopetosupportitself. that the mixis such that the shrinkagewouldbe
insignificant.
5.2 The rock shall be reinforced by the steel
bolts/cables to take over the tensilestressesand Thefollowingspecificationsforacementgroutare
alsoapartoftheshearstresseswhichincreasesthe recommendedtoprovidetlowability,expansionon
stabilizingforceordecreasesthedisturbinglsliding hardeningand higher shear strength to transfer
force. loadfromtheanchorthroughthegrouttotherock.
5.3 Therock-medium shallbelockedbyinduced 6.3.1 Thereshallbenoindicationsofthefalseset
prestressing in the boltswhichactivates/increases [see IS4031 (Part5»).
thefrictionalforcesalongthesurfaceofseparation
6.3.% Aflow timeof2Sto30seconds throughthe
in the anchorage zone and transfer the load of
standard flow cone (volume 1 725 ml, orifice
unstablerock wedgeto the deeper parent rock
diameter12.5mm).
mass.
6.3.3 Minimumcompressivestrength(ona7.5em
-S.4 The densityof rock-reinforcement caneasily
diametercylinder and 15emlong)shouldbe 285
be modified bychanging itsdiameter,lengthand
kwcm2,after28days.Thewatercementratioofthe
spacingwhichisfrequently requiredbylocalrock groutshouldbe0.4to0.5.
massconditions. Anotheradvantageisthatitcan
be easily combined with additional control 6.3.4 Theamountofaluminiumpowdernecessary
measuressuchasshotcretinganddrainagesystem, toobtainexpansion(toavoldshrinkage)shouldnot
etc. bemorethan0.005percentbyweightofcement,for
a grout of 0.4 to 0.5watercement ratio. Other
!.! Tests tobeConducted admixturesmayalsobeusedtoreduceshrinkage.
!.5.1 Kinematic Test
Excessi'lequantityofaluminiumpowdershouldbe
Thistestshallbeagraphicalrepresentationofrock avoidedbecauseitproducesfoaming inthegrout,
faults/joints, etc, on the spherical stereographic especiallywhenthewatercementratioisalsohigh.
planefromwhichthe mostlikely unstablewedge/ Hence,bothshouldbestrictlycontrolled,
criticalplaneofweaknessmaybeassessed.
'.3.5 FQrcement mortar grout, equal part of
!.5.2 Pull-outTest cement and sand are first mixedand sufficient
wateristhen addedto bringthe mortar toastiff
Thepull-outtestshouldbeconductedatsiteasper consistency. The maximumparticle sizeofsand
IS 11309 for assessing the anchor load bearing shouldlotexceed2mm.
capacity and for the checking of the designed/as
sumedF.A. L.ofaparticularanchor. 6.3.6 The situation where pressure grout is
required, the pressure should be limited to
, DESIGNCONSIDERATIONSOF 50-70 percent of overburden pressure,although
ROCK-REINFORCEMENT 15percentoverburdenpressureshouldbeapplied
whenever need arises. The use of excessive
6.1 The rock-boltor cable reinforcementsystem
pressure loading gives rise to possibility of
shouldbe designedtostabilizetheslippageofthe
slope(naturaVartificial),whichoccuresduetothe hydrofacturingand surface heaving.
3
IS 14448:1997
6.3.7 Thediameterofdrill-holeshouldbe30mm 6.4.2.2 The rIXed anchor lengthshall not beless
morethandiameteroftheanchorsothatsufficient tban 60 and 100 times the diameterofdeformed
grout-cover preventscorrosion.Suitable preven anchorbarsandplainanchorbarsrespectively.
tivemeasuresshallbeadoptedtopreventcorrosion
6.4.2.3 The fixedanchor lengthshall not be less
dependinguponaggressivenatureofrockmass.
than2.3,4,andSmforverygood,good,fair/poor/
6.4 Therock-anchor(F. A 1.) mayfailinoneor and very poorrockconditionsrespectively.
moreofthefollowing.
The anchoragelengthshall beobtained from the
6.4.1 FailureofRock/GroutBond
followingespresston;
Thefollowingexpressionforthelengthofanchorage
. P.F .....(2)
based on an equivalent uniform distribution for L-~
bondstressas skinfriction alongthefixedanchor xuT.
maybeused:
where
L a P.F. /1'C.D.T .•..(1) L =Effective/fIXedanchorlength,in mm;
where F = Factorotsafety;
L =Effective/rIjXedanchorlength(F.A. L.) in D :: (d. + 30 mm); d. =- Diameterof anchor
bar,in mm;and
mm;
P =Pull-outforceperanchor,inN; fa =Permissibleshearstressin N/mm2•
F == Factorofsafety;
6.4.3 Failurec[th«Anchoror TopAnchorage
T = Bondstressatfailure, inN/mm2;and
D =Diameterofthe borehole (d.+ 30mm), Thediameteroftheanchorbarshallbe calculated
inmm. fromthefolloWing expression usingyieldstressof
steel(cry);whichcanbe takenfromIS456
Theactualdistributionofbondstressisexponen-
tial anddependsupon the elasticmodulusof the P= A". Uy/F .....(3)
rock, grout material, diameter and length of the
anchorage. However, theassumption ofuniform where
distribution holds good for rocks of low elastic AI l=IS Areaofsteelbar,inmm2;
modulus. Hence, a factor of safetyof 3 to 5 is Of Yieldstress of steel used for anchor, in
recommended. mm2; .
=
F Factor of safety whichwillbe takenas
Thesafebondstrength(TrlF)tobeusedforprelimi
1.30;and
narydesign ofrockanchors,isgiven inTable2,if P =Pullforce(bothstaticanddynamic),inN.
testdataisnotavailable.
6.4.4 BearingFailure ofRock-Mass atthe Face of
Anchorage
Table2 SafeBondStrengthfor Different
In order to minimise bearing failure (which
RockConditions
depends upon the load carrying capacityof the
Verypoortopoorrock (RMR- 0to40) 0.35-0.70NJmm2
rock-mass),alargebearingplatefoundedonathin
condition
F.ur,lO&oodrock (RMR- 41to 80) 0.70-1.05Nlmm2 grout overa largecontactarea shouldbe recom
cpndltion mended.
Veryloodrock (RMR- 81to1(0) 1.0S-1.40Nlmm2
7 PROCEDUREFORINSTALlATIONOF
condition
ROCK ANCIIORS
However,theabovesafebondstrengthshouldnot
bemorethanonethirtiethoftheminimumuniaxial 7.1 Thesite shallbe madeclearand cleanwhere
compressivestrengthofrockmaterialorthegrout. therockanchors/rockboltsaretobeprovidedthat
6.4.2 Failurea/theGrout/AnchorBond isclayoverburdenorloosepiecesofrockordebris
orvegitationshouldbe removeduptoaclearrock
Ifthe uniaxial compressive strength of the grout
formation.
(fc)is knownthe permissible/allowable stress in
shear (fa) may be obtained from the following 7.2 The drill holes should be driven slightly of
expression: largerdiameter(about 30mm)than the diameter
T. - fellS < 1.2N/mm2 (forplainbars) ofsteelbarswhichistobeprovidedasanchorbars.
T. - jJI0 < 2.5N/mm2 (fordeformedbars) 7.3 Drillholeshouldbewashedwith watertillthe
6.4.2.1 .Someumes anchor bars are pulled out clearwaterflows out fromit. Then the drillhole
because ofinsufficient fixedanchorlength that is shouldbefilledwithcementgrout. Plainorribbed
lengthofanchorbarsbeyondcriticaldiscontinuity. steel shallbedriveninto the drillhole filled with
4