Part 11 - Stabilisation

8/10/2019 Part 11 - Stabilisation

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PART 11: STABILISATION OF ENGINEERING STRUCTURE INROCK MASS PRINCIPLE & METHOD Construction cost of structures (tunnel & slope) will increase if they need some

methods of stabilisation. The cost will increase further if the stabilisation is of higherdegree & to be maintained for a long period of time. Appropriate design &construction method help to reduce the amount & cost of stabilisation.

Design & construction method that are not according to correct procedures willinduce signicant disturbance to rock mass conse!uently more problems onpro"iding stabilisation methods and the associated cost#

As soon as a tunnel is excavated,surrounding rock mass will be disturbed formation of yield zone. Design & methodof construction must be carefullyconsidered so that disturbance tosurrounding rock is reduced (thinner yieldzone, less aected volume!.

As soon as a slo"e is excavated, surrounding rockmass will be disturbed formation of yield zone.Design & method of construction must be carefullyconsidered so that disturbance to surrounding rock isreduced (thinner yield zone, less aected volume!.

$sually stabilisation of structure is an aspect thatmust be implemented. %tabilisation method is basically to increase '% byincreasing resisting forces.

FOS = [ resisi!" #$r%es ' [ (is)r*i!" #$r%es

Disturbing forces are factors that sometimes beyond our control (uncertainties)

rainfall weathering rate groundwater rock mass earth!uakes etc.

esisting forces are methods that can be implemented to increase '% slopeproles a"oiding problem areas & installing stabilisation method.

+,- is I./$r-! -*$) M$(e $# I!s-*i0i i! R$%2 -!( Me,$( $#S-*i0is-i$!3

Di*erent mode of instability in rock re!uires di*erent methods of stabilisation

(support or reinforcement method). Therefore it is important to understand+, -hat is the pre"ailing instability in the eca"ated rock mass/ (is it plane wedge

toppling or rock fall)., 0ow specic method of stabilisation works to produce e*ecti"e stability to the

unstable rock mass/

1ost of stabilisation methods for rocks are di*erent from those used in soils. This ismainly due to the mode of instability in rock (the less weathered ones) are mainlyinduces by weakness plane rather than material strength.

S-*i0is-i$! -!( Rei!#$r%e.e! $# R$%2:


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The basic principle of rock mass stabilisation method is to help rock mass to stabiliseitself. Correct installation of method can help to mobilise the inherent2internalstrength a"ailable in the rock (e.g. frictional or residual strength of fractured rockshear strength of discontinuities).

1ethods of stabilisation for rock are di"ided into 3+

(4)%upport system (sistem penyokong)(3)einforcement system (sistem pengukuhan)

S)//$r Sse.:The use of support elements (e.g. steel $,section pre,cast slabs shotcrete & wire,

mesh) that are installed on rock surface.

This system increased the stability of rock body by imposing a conning pressureinto the rock.

The conning pressure increased the contact between fracture surfaces (5oints) andconse!uently ele"ated the eisting frictional strength in the fractured rock.

Rei!#$r%e.e! Sse.:

The use of reinforcement elements (e.g. dowel rockbolt rock anchor & grout) that

are installed inside the rock body.This system stabilised the rock by increasing its internal inherent strength.

The element must be installed across a weakness plane.

6n principal it is similar to the action of rebars in reinforced concrete or pre,tensionedrebar in pre,stressed concrete.

I./$r-! C$!si(er-i$!s i! Se0e%i!" S)i-*0e S-*i0is-i$! Me,$(:4)Types si7e and geometrical orientation of weakness planes presence in the rock

(5oint fault and bedding plane).3)1ode2type of instability in rock mass (plane wedge 8eural & rock fall). The mode

of unstability is dictated by the type orientation and sets of weakness plane (ig

44.4).9)$nderstanding of the reinforcement mechanisms (reinforcing mode) of each method

i.e. how each element reacts with the eisting instability so to increase '% (a5ah44.3).

:)Type of rock strengths weathering grade (0- & C-). -eaker rocks like schist &highly weathered granite are not suitable to be reinforced with rock,bolt & dowel.


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#ig $$.$% ode of instability in rock is controlled by ty"e, number of set andorientation of weakness "lanes.


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'lane failure is normally associated withone set and almost "arallel inclineddiscontinuities (inclined bedding "lanein fold, faults. sually the unstableblocks are relatively larger in sizeore than ) sets of *oint intersectingwith each other may result in rock fall(smaller loose rock blocks! whicheventually "roducing or larger sizefailure (ravelling!


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A single larger size *oint (in granite! can lead to"lane failure, and two intersect *oints may leadto wedge failure

'lane failure + rock fall arger unstable blocksresting on an inclined

*oint "lane or bedding "lane canlead to

"lane failure


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-hree or more intersecting *oint sets that are closely s"aced may "roduce smallerlooseunstable rock blocks and eventually lead to rock falls

/ock fall


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0ombination of several small intersecting*oints & larger *oints may "roduce rock

falls and wedge failure

1ne or two *oint sets that are nearvertical "osition can "roduce 2exural or

to""ling failure

3sectionsteel arch su""ort methods installed on tunnel surface, for eectiveness

the system must be active (it must yields along with the movement of rock in thetunnel walls


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4tabilisation of large tunnel usingshotcrete + concrete slab + rock bolt +wire mesh

4teel 3section as su""ort with concreteslabs as lining in between the 3sections

5asic design of rock bolt & dowel


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ain com"onents of rock bolt and dowel

A rock bolt is an active (tensioned! dowel usedfor stabilising rock *oints. 4imilar to the use oftensioned rebar in "re3stressed concrete

6nstalled rock bolt with s"hericalbearing "late & tensioning nut.4"herical seat ensure evendistribution of bolt tension


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7arious ty"es and surface 8nished ofrebar used as for rock bolt, materialcan steel, 8bre reinforced concrete(#/0!

/ock bolt & dowel can be used to stabiliseunstable blocks. 5olt must be of su9cientlength so that it extends well beyond the

weakness "lanes thus anchoring theunstable blocks to a stronger rock

se of rock bolt as tunnel reinforcement se of rock bolt as stabilisingmechanism for rock slo"e

R$%2 D$4e05 R$%2 B$0 -!( R$%2 A!%,$r:

6n principle these three reinforcement elements are similar in design. The only

di*erences are their stabilisation capacity purpose and installation.They consist essentially of tendon2bar anchorage (bonding agent) bearing plate &

tensioning de"ice.


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ock bolt is more epensi"e to install than rock dowel they are e*ecti"e for long,term stabilisation (ci"il engineering structure).

ock dowel is for short,term reinforcement (for mining & temporary support in

tunnel).

They are used to increase the inherent strength of weakness planes in rock e.g.shear strength of unstable 5oint.

or larger unstable blocks (e.g. unstable blocks of se"eral tonnes in plane failure)

rock anchor is recommended. 6nstead of tendon rock anchor consists of high capacity multiple strands steel cables

(cables used in suspension bridge).

T,e Di6ere!%e Be4ee! D$4e0 -!( R$%2 B$0:

Dowel is passi"e bolt installed without any tension.

Tension in dowel de"elop e"entually after there is mo"ement in 5oints it intersect.

ock bolt is acti"e dowel installed with pretension (usually about (;< =< > Tb).

Due to this pre,tension rock bolt ehibits se"eral positi"e characteristics aspermanent reinforcement.

:esan /ockbolt yangdi"asang condong terhada"satah kegagalan, "eningkatandaya "enahan

4liding of rough *oint surfaces may lead todilation (o"ening of *oint!. Dilation induces

tension into rock dowel and additionaltension to rock bolt


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/ock bolt intersecting rough clean *oint%dilation leads to an increase in bolttension ("ositive eect!

/ock bolt intersecting 8lled *oint% nodilation due to com"ressibility of in8ll,

this leads to eventual loss (cree"! inbolt tension (negative eect!

/oof section is the most critical "art of a tunnel. /ock bolt of dierent lengths are

used to reinforced highly *ointedfractured roof section (reduce roof failure rock fall &wedge failure!


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;xcavation of rectangular tunnel inhorizontally bedded sedimentary rock mayinduce a "henomenon called hen the lower & u""erbeds are bolted together they form

a com"osite beam (beam setara!,the resulting bed is thicker (thus

smaller ! and higher shear strengthat the bedding "lane.


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4teel wire3mesh (chain3link orwelded mesh! is used to restrainsmaller size unstable rock blocksfrom falling down es"ecially the


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4teel welded mesh to restrainsmaller size, unstable rock blocksfrom falling down, es"ecially the


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ixing of shotcrete 4"raying of shotcrete on rock surface whichhas been 8xed with welded mesh


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4hotcrete + wire mesh + wee" holes

-he dierence in"ro"erties betweensoil and rock must beunderstood, methodsa""licable to rockmay not be suitable

forsoil (or weak rocke.g. schist! shotcrete is onlya""licable for strongrock for goodbonding betweenshotcrete & slo"e

face

4ince shotcrete isim"ermeable towater it must beinstalled withwee"3holes. -his todrain water out ofthe slo"e orotherwise therewill be built3u" of"ore water"ressure in theslo"e.


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>eathered strong rock likegranite (?one @ ! are notsuitable to be reinforced usingrockboltdowel. 6n weak rock thebonding between bolt andsurrounding rock are weak.>eak bonding will not "roduce

su9cient tension in the bolt

inerals arrangement (slatey &schistosity! in metamor"hic rocksinduce weakening eect into theserocks (almost similar to grannular soil!./einforcement method using rock bolt,dowel & rock anchor are notrecommended in these rocks mainlydue to weak bonding

4tability & drainage are the main"roblems with slo"e excavated in schist& slate (metamor"hic rocks!. ;ven withgentle and low angle "ro8le (B m height

with benching! the slo"e remainsunstable an im"ortant note for slo"e

in metamor"hic rockC

ock slopes can fail in "arious modes & each method of stabilisation is onlysuitable2e*ecti"e for specic mode of unstability.

$sually more than one methods are re!uired to stabilise a rock slope.


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Commonly used method are+ rock bolt ? wire,mesh ? shotcrete.

@ong,term e*ect due to weathering is reduced by minimising the presence ofweathering agents through drainage system+ surface drainage & weep,holes.

%prayed concrete2shotcrete (special cement mi of 3


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>edge failure is associated withunstable medium to large size blockthat tends to slide along intersecting

weakness "lanes (*oints or faults!.

4imilar to "lane failure, reinforcing theunstable block to a more stable rock mass isrecommended using rock bolts (for medium

block! or rock anchor (for large blocks!.

-o""ling or rocks fall is associated withsmaller size blocks. #ailure is not alongdistinctive sliding "lanes. -he unstablerocks are blocks that have beense"arated from the main rock body byintersecting *oints (F ) sets!. >ater can"enetrate the slo"e easily due to many*oint sets. /einforcement alone is notsuitable for this ty"e of unstability.4u""ort system like wire3mesh &

shotcrete together with rock bolts arerecommended.

#lexural to""ling is unstability due torock blocks that are se"arated by at leastone set of near vertical *oints or bedding"lanes. #ailure is associated with sliding

"lane. -he slender (thin! vertical rockblocks can be made stronger by bondingthem together (com"osite beams! using

rock bolts (smaller blocks! or rock anchor(larger blocks!


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Drainage system (method of stabilisation! isan essential measure for rock slo"esexcavated in tro"ical countries (high annualrainfall!. -he drain also acts as buer zonefor any fallen rock

Drainage system (method ofstabilisation! at the crest of

"rotected slo"


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#ig $$.)% ethods of stabilisation for rock slo"e

0om"rehensive drainage system (surface & internal drainageG for large size slo"esand where water is a critical factor.


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Part 11 - Stabilisation

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