Observational Methods and NATM

7/27/2019 Observational Methods and NATM

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Observational Methods and

NATM


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Predict io n of geotechnic al behaviour is often dif f icu l t , therefore i tis sometimes appropriate to adopt the observational methodapproach, in which the design is reviewed dur ing cons truct ion.

Ac cord ing to Peckobservational methodhas the fo l lowingprocedura l steps:

Exploration sufficient to establish at least the general nature, patternand properties of the deposits, but not necessarily in detail

The assessment of the most probable conditions and the mostunfavourable conceivable deviations from these conditions, in thisassessment geology often play a major role

The establishment of the design based on a working hypothesis ofbehaviour anticipated under the most probably conditions

The selection of quantities to be observed as construction proceedsand the calculation of their anticipated values on the basis of theworking hypothesis

The calculation of values of the same quantities under the mostunfavourable conditions compatible with the available data concerning

the subsurface conditions The selection in advance of a course of action or modification of

design for every foreseeable significant deviation of the observationalfindings from those predicted on the basis of the working hypothesis

The measurement of quantities to be observed and the evaluation ofactual conditions

The modification of design to suit actual conditions


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The method is inapplicable where there is nopossibility to alter the design duringconstruction. The ability to modify the design is

appropriate if the method is to be applied onlyduring construction and the focus is on thetemporary conditions.

However, there are situations where the method

could be applied after construction, e.g long-term monitoring of dams and buildings.

Peck emphasises the importance of asking thecritical questions. These must ensure that the

observations are appropriate and meaningful.The key is to combine comprehens iveness

wi threl iabi l i ty , repeatabi l i ty and s impl ic i ty.Observations are often far more elaborate and

costly than necessary.


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The Base Design developed in (c) will typically be based onanalysis, such as finite element.

Possible modes of failure particularly those of a sudden or

brittle nature, or those who could lead to progressivecollapse must be assessed carefully.

It is a fundamental element of the Observational Method toovercome the limitations of analysis by addressing actualconditions.

The design in (c) may therefore present difficultiesassociated with the term most probably, and in practice (c)has been interpreted as unlikely to be exceeded.

Some margin of conservatism is always necessary; it maytherefore be more appropriate base the design on amoderately conservativeapproach.A moderatelyconservative design would be less conservative than aconventional design, but more conservative than one basedon Pecks most probable.


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Feedback from Observations

Feedback and assessment from observations must betimelyin order to confirm predictions or to provide adequatewarning of any undue trends in ground movements or loadings.

There must be sufficient time to enable planned contingencymeasures to be implemented effectively. This emphasises afurther aspect of the Observational Method.

Measurements of quantities must occur at the required timesduring a construction sequence. It may be necessary to interrupt

construction progress and may even influence the wayconstruction is sequenced.

Other Observational Approaches

As set out by Peck, the procedures (a) (h) for theObservational Method may be unnecessarily cumbersome andoften impossible to achieve.

Further, the most probable condition in (c) is very difficult tofind in a statistically reliable manner. Simpler versions of anobservational approach have been suggested, as e.g. by MuirWood.

Management of observational approaches are often described

in flowcharts, often including risk levels and responses.


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System for Observational approach to tunnel design


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Eurocode 7 (EC7) includes the following remarks concerning anobservational method.

Four requirements shall all be made before construction is started:

The l imits of behaviou r, which are acceptable, shall be established. The range of behaviourshall be assessed and it shall be shown that

there is an acceptable probability that the actual behaviou r wi l l bewith in the acceptable l imits.

A plan of moni tor ingshall be devised which will reveal whether theactual behaviour lies within the acceptable limits. The monitoring shall

make this clear at a suff ic ient ear ly stage; and with s uff ic ient lyshor t in terva ls to allow c ont ingency act ions to be under takensuccessfu l ly. The response time on the instruments and theprocedures for analysing the results shall be sufficiently rapid in relationto the possible evolution of the system.

A plan of cont ing ency act ions shal l be devisedwhich may beadopted if the monitoring reveals behaviour outside acceptable limits.

Dur ing con struct ion th e mon i tor ing shal l be carr ied ou t asplanned and add it ional or replacement monito r ing shall beund ertaken if this becomes necessary. The results of themonito r ing sh all be assessed at app ropr iate stages and theplanned cont ingency act ions shal l be put in op eration i f th isbecomes necessary.


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NATM New Austrian Tunnelling Method

One of the most well known methods using

some elements of an observational approach isthe New Austrian Tunnelling Method, or NATM.

The method, has often been mentioned as a

value engineered version of tunnelling due to

its use of light, informal support. It has long been

understood that the ground, if allowed to deform

slightly, is capable of contributing to its own

support. NATM, with its use of modern means ofmonitoring and surface stabilisation, such as

shotcrete and rock bolts, utilizes this effect

systematically.


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Tradit ional tunnel l ingused first timber supports andlater on steel arch supports in order to stabilise a tunneltemporarily until the final support was installed. The finalsupport was masonry or a concrete arch. Rock loadsdeveloped due to disintegration and detrimentalloosening of the surrounding rock and loosened rockexerted loads onto the support due to the weight of aloosened rock bulb (described by Komerell, Terzaghiand others). Detrimental loosening was caused by theavailable excavation techniques, the support means andthe long period required to complete a tunnel sectionwith many sequential intermediate construction stages.

The result was very irregular heavy loading resulting inthick lining arches occupying a considerable percentageof the tunnel cross-section (in the early trans-Alpinetunnels the permanent structure may occupy as much as40% of the excavated profile)


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NATM:With a f lex ib le pr imary suppo rt a new equi l ibr iumshall be reached. This shal l be contro l led by in -si tudeformation measurements. After this new equi l ibr ium isreached an inner arch shall be bu i l t . In specif ic cases the

inner arch can be om it ted.

The New Austrian Tunnelling Method constitutes a design where thesurrounding rock- or soil formations of a tunnel are integrated intoan overall ring like support structure. Thus the formations willthemselves be part of this support structure.

With the excavation of a tunnel the primary stress field in the rockmass is changed into a more unfavourable secondary stress field.Under the rock archwe understand those zones around a tunnelwhere most of the time dependent stress rearrangement processestakes place. This includes the plastic as well as the elastic behaving

zone.

Under the act ivat ion of a rock archwe understand ouractivities tomaintain or to improve the carrying capacity of the rock mass, toutilise this carrying capacity and to influence a favourabledevelopment of the secondary stress field.


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The main pr inc iples of NATM are:

The main load-bearing component of the tunnel is the surrounding rockmass. Support is informal i.e. it consists of earth/rock-anchors andshotcrete, but support and final lining have confining function only.

Maintain strength of the rock m assand avoid detrimental looseningby careful excavation and by immediate application of support andstrengthening means. Shotcrete and rock bolts applied close to theexcavation face help to maintain the integrity of the rock mass.

Rounded tunnel shape: avoid stress concentrations in corners whereprogressive failure mechanisms start.

Flexible thin l in ing :The primary support shall be thin-walled in order tominimise bending moments and to facilitate the stress rearrangementprocess without exposing the lining to unfavourable sectional forces.

Additional support requirement shall not be added by increasing liningthickness but by bolting. The lin ing shal l be in ful l con tact with theexposed rock . Shotc rete ful f i ls this requirement.

Statically the tunnel is considered as a thick-walled tube consisting of

the rock and lining. The closing of the ring is therefore important, i.e. thetotal per iphery includ ing the invert mus t be app l ied with shotc rete.

In si tu measurements:Observation of tunnel behaviour duringconstruction is an integral part of NATM. With the monitoring andinterpretation of deformations, strains and stresses it is possible tooptimise working procedures and support requirements.


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The concept of NATM is to controldeformations and stress rearrangement

process in order to obtain a required safetylevel. Requirements differ depending on thetype of project in a subway project in builtup areas stability and settlements may bedecisive, in other tunnels stability only maybe observed. The NATM method isuniversal, but particularlysuitable for

irregular shapes. It can therefore be appliedfor underground transitions where a TBMtunnel must have another shape ordiameter.


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Observations of tunnel behaviour

One of the most important factors in the successful

application of observational methods like NATM isthe observation of tunnel behaviour during

construction. Monitoring and interpretation of

deformations, strains and stresses are important to

optimise working procedures and supportrequirements, which vary from one project to the

other. In-situ observation is therefore essential, in

order to keep the possible failures under control.

Considerable information related to the use of

instruments in monitoring soils and rocks are

available from instrument manufacturers.


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Example measurement instrumentation in a tunnel

lined with shotcrete.

1.Deformation of theexcavated tunnel surface/Convergence tapeSurveying marks

2.Deformation of the ground

surrounding the tunnel/Extensometer

3.Monitoring of groundsupport element anchor/Total anchor force

4.Monitoring of groundsupport elementshotcrete shell/

Pressure cellsEmbedments gauge


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NATM Process on site

Cutting a length of tunnel here with a roadheader


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Applying layer of shotcrete on reinforcement mesh


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Primary lining applied to whole cavity, which remains

under observation.


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Final lining applied. Running tunnels continued.


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Completed underground transition


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Sketch of mechanical process and sequence of failure

around a cavity by stress rearrangement pressure

Main Pressure

Stage 1 Stage 2 Stage 3


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Schematic representat ion o f stresses around a circular

cavi ty wi th hydro static pressure


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The Fenner-Pachercurveshows the relationshipbetween the deformation

R/R and required supportresistance Pi.Simplistically, the moredeformation is allowed, theless resistance is needed.In practice, the support

resistance reaches aminimum at a certain radialdeformation, and supportrequirements increase ifdeformations becomeexcessive.

Fenner-Pacher-typediagrams can begenerated to help evaluatethe support methods bestsuited to the conditions.


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Skin resistance which counteracts the radial stresses forming around the cavity, becomes smaller intime, and the radius of the cavity decreases simultaneously. These relations are given by theequations of Fenner-Talobre and Kastner.

Pi = -c Cotg + [c Cotg + P0 (1 - Sin ) ]( r / R)

where;

Pi = skin resistance

C = cohesion

= angle of internal frictionR = radius of the protective zone

r = radius of the cavity

P0 = H; overburdenFollowing the main principle of NATM, the protective ring around the cavity (R-r), is a load carryingpart of the structure. The carrying capacity of the rock arch is formulated as;

PiR =

where;

Pi = resistance of rock arch (t/m2)

S = length of shear plane (m)

R = shear strength of rock (t/m2) = angle of internal friction (0)

b = height of shear zone (m)

nR = normal stress on shear plane (t/m2)

Sin

Sin

1

2

2/2/

S

b

SinS

b

Cos RnR


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, R, nR, can be measured in laboratories, where as S

can be measured in meters , on a drawing made to scale.


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Generally two separate supports are carried

out. The first is a flexible outer arch orprotective support designed to stabilize thestructure accordingly. It consists of a

systematically anchored rock arch withsurface protection, possibly reinforced by ribsand closed by an invert.

The behaviour of the protective support and

the surrounding rock during the readjustmentprocess can be monitored by a measuringsystem.


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The second means of support is an inner concrete arch, generally not

carried out before the outer arch has reached equilibrium. In addition to

acting as a final, functional lining (for installation of tunnel equipment

etc.) its aim is to establish or increase the safety factors as necessary.


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The resistance of the lining material (shotcrete) is:

An additional reinforcement (steel ribs, etc.) gives

a resistance of:

where;

)2/(sin b

d

P s

s

s

t

)2/(sin b

FP

s

stst

st

i

s

s

stsst

E

E

15


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The lining resistance is:

PiL = Pi

s + Pist

The anchors are acting with a radialpressure:

With the lateral pressure given by:

3 = pis + pi

st + piA

and with Mohrs envelope, the shearresistance of the rock mass R and theshear angle is determined, assuming thatthe principal stresses are parallel and atright angles to the excavation line.

et

fP

st

p

st

A

i


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The carrying capacity of the rock arch is

given by:

The resistance of the anchors against the

movement of the shear body towards the

cavity is:

2/

sin

2/

cos

b

S

b

SP

R

n

RR

i

)2/(

cos

bet

afP

st

p

st

A

i


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The total carrying capacity of the outer arch is then:

mini

A

i

R

i

L

i

w

i PPPPP


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Numerical example for NATM

Tunnel size: 12.10 x 12.00 m(fig.10)

H = 15.0 m overburdenaccording to tests on samplesfound: = 27, c = 100 t/m(three axial tests). When weopen a cavity the stressequilibrium spoiled and forestablishing new equilibrium

condition achieved bysupporting as follows.

Use the supporting ring whichdevelops around the cavity afterexcavation as a self-supportingdevice and select a type of

supporting which can bear thedeveloped rock loads anddeformable when necessary.

Design the inner lining underfinal loads


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The (1) supporting system is capable of carrying safely the

loads, the (2) lining is for safety and to bear the additional

loads which are probable to develop after the supports are

installed.Supporting will consists in this example:

a: Shotcrete (15+10) cm in layers by two shots

b: Bolts spaced 2.00 x 2.00 m in rings with diameter 26 mm.

c: Rib steel channel supports (2 x 14)

d: by ground supporting ring

To find the radius of disturbed zone R:

Talobre formula:

sin1

sin2

0 )()sin1(cotcot

R

rPgcgcPi


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Values entered into the formula:

= 27

= 2.5 t/m

H = 15.0 m

P0 = H = 2.5 x 15.0 = 37.5 t/m

C = 100 t/m

R = 6.45 m

27sin127sin2

)05.6

()27sin1(5.3727cot10027cot100

R

gPi

450.01450.02

)05.6

()27sin1(5.3796.110096.1100 x

iR

P

005.6

08.2173.196

66.1

R66.1

05.6

08.217

3.196

R

R = 6.45 m


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Shotcrete:

d = 25 cm

c28 = 160 kg/cm compressive strength shear in

concrete (assume 20% ofc28)the capacity carrying loads

h = 0.20 160 = 32 kg/cm = 320 t/m

d = thickness of shotcrete in (cm)

= /4 - /2 angle of shear plane with verticalb = shear failure height of the cavity (see Fig.10)

sin 31.5 = 0.520;

b/2= r cos = 6.05xcos31.5=5.15

2/9.2915.552.0

32025.0 mtpci


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bol ts

Bolt 26 mm

St III, sh = 4000 kg/cm

Spacing=2x2 m

f = 5.3cm

2/53.0200200

40003.5cmkgpbi

2/3.5 mtpbi


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steel r ibst = spacing 2.00 m

F = 2 20.4 cm = 40.8 cm =0.00408 m

st = c 15 = 320 15 = 4800 t/m

/56.3600.215.552.0

480000408.0

2

sin

mt

tb

FP ststi

N = normal stress = 142 t/m

read on XZ = minor stress = 37.2 t/m

R= shear stress = 170 t/m read on Y

Connect AB and find the centre (W) draw the

circle. Tangent at the point B (BB) so

OB = Cohesion = 100 t/m = internal friction angle (27)R calculated (Talobre formula) as 6.45 m. Width of the

protective ring 6.45-6.05 = 0.4m drawn through A, B

and the intersection bisecting with the middle ring (C);

ABC shear failure line drawn and thus (S) measured.

Bolt length l = 4.00 m is taken and inclination

measured

Pi = pic + pib + pist = 29.9 + 5.3 + 36.56 = 70.26 t/m


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bearing capacity of thesupporting ring

Sin = sin 27 = 0.450Cos = cos 27 = 0.891thus

S = 4.54 m (from figure 17)

= 27b/2 = 5.15 m

R = 170 t/mN = 142 t/menter the formula

15.5

450.01426.4

15.5

891.01706.4

RCp

2/22.78 mtpRC


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The resistance of the bolts (anchors) against the movement of theshear body towards the cavity is:

(b/2, a = 4.20 m, = 35.5 from Fig 17)

< 5.3 t/m

shear= 4000 kg/cm ; e.t1 = bolts arrangement = 2.00 2.00 m

So the total bearing capacity of supporting will bePi = Pi

c + Pib + Pi

st + PiR = 29.9 + 3.43 + 36.56 + 78.22 = 160.1 t/m

2/

cos

bte

fap

shb

i

2

/43.315.500.200.2

81.0400020.520.4mtp

b

i

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Observational Methods and NATM

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