NATM BASICS

Chapter 8

Observational Methods and NATM

Because prediction of geotechnical behaviour is often difficult, it is sometimes appropriate to adopt the approach known as the observational method, in which the design is reviewed during construction.

The principles of the Observational Method were first described in detail by R.B. Peck (1969). Pecks method provides a basis for understanding of the requirements, although when reviewing the method later Peck felt that his efforts to formalize it were too contrived and rigid. Observational methods have in fact always been used to a degree in applied soil mechanics.

The Observational Method has a specific meaning. Peck set forward the following procedural steps:

(a) Exploration sufficient to establish at least the general nature, pattern and properties of the deposits, but not necessarily in detail(b) The assessment of the most probable conditions and the most unfavourable conceivable deviations from these conditions, in this assessment geology often play a major role

(c) The establishment of the design based on a working hypothesis of behaviour anticipated under the most probably conditions

(d) The selection of quantities to be observed as construction proceeds and the calculation of their anticipated values on the basis of the working hypothesis

(e) The calculation of values of the same quantities under the most unfavourable conditions compatible with the available data concerning the subsurface conditions

(f) The selection in advance of a course of action or modification of design for every foreseeable significant deviation of the observational findings from those predicted on the basis of the working hypothesis

(g) The measurement of quantities to be observed and the evaluation of actual conditions

(h) The modification of design to suit actual conditions

The method is inapplicable where there is no possibility to alter the design during construction. The ability to modify the design is appropriate if the method is to be applied only during construction and the focus is on the temporary 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 the critical questions. These must ensure that the observations are appropriate and meaningful. The key is to combine comprehensiveness with reliability, repeatability and simplicity. Observations are often far more elaborate and costly than necessary.

The Base Design

The base design developed in (c) will typically be based on analysis, such as finite element. However, analysis cannot replace judgement. Possible modes of failure particularly those of a sudden or brittle nature, or those who could lead to progressive collapse must be assessed carefully. It is a fundamental element of the Observational Method to overcome the limitations of analysis by addressing actual conditions. The design in (c) may therefore present difficulties associated 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 may therefore be more appropriate base the design on a moderately conservative approach. A moderately conservative design would be less conservative than a conventional design, but more conservative than one based on Pecks most probable, so that modifications to the original design become exceptional, not the rule.

Feedback from Observations

Feedback and assessment from observations must be timely in order to confirm predictions or to provide adequate warning of any undue trends in ground movements or loadings. There must be sufficient time to enable planned contingency measures to be implemented effectively. This emphasises a further aspect of the Observational Method. Measurements of quantities must occur at the required times during a construction sequence. It may be necessary to interrupt construction progress and may even influence the way construction is sequenced.

Other Observational ApproachesAs set out by Peck, the procedures (a) (h) for the Observational Method may be unnecessarily cumbersome and often impossible to achieve. Further, the most probable condition in (c) is very difficult to find in a statistically reliable manner. Simpler versions of an observational approach have been suggested, as e.g. by Muir Wood.

Management of observational approaches are often described in flowcharts, often including risk levels and responses.

Figure 8.1: System for Observational approach to tunnel design

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

Four requirements shall all be made before construction is started:

1. The limits of behaviour, which are acceptable, shall be established.

2. The range of behaviour shall be assessed and it shall be shown that there is an acceptable probability that the actual behaviour will be within the acceptable limits.

3. A plan of monitoring shall be devised which will reveal whether the actual behaviour lies within the acceptable limits. The monitoring shall make this clear at a sufficient early stage; and with sufficiently short intervals to allow contingency actions to be undertaken successfully. The response time on the instruments and the procedures for analysing the results shall be sufficiently rapid in relation to the possible evolution of the system.

4. A plan of contingency actions shall be devised which may be adopted if the monitoring reveals behaviour outside acceptable limits.

During construction the monitoring shall be carried out as planned and additional or replacement monitoring shall be undertaken if this becomes necessary. The results of the monitoring shall be assessed at appropriate stages and the planned contingency actions shall be put in operation if this becomes necessary.

Figure 8.2: Management review process for in-tunnel monitoring (CIRIA 1997)

The organisational procedure for a project using an observational approach is illustrated in Figure 8.2. There must always be emphasis on the time element to enable supplementary measures (contingency actions) to be put in place, including administrative actions.

Although primarily a tool for the geotechnical area, the Observational Method is not limited to tunnel construction. For example, during construction of the Copenhagen Metro, the method was applied for a yes/no decision for constructing a temporary waler beam at a metro station box. The base design was not to build the waler, and the observed parameters were deflection of the station walls, settlement of above buildings, etc.

NATM New Austrian Tunnelling MethodOne of the most well known methods using some elements of an observational approach it the New Austrian Tunnelling Method, or NATM. The method, which is in fact a broader concept of geotechnical engineering than a single 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 of monitoring and surface stabilisation, such as shotcrete and rock bolts, utilizes this effect systematically.

Historical background

Traditional tunnelling used first timber supports and later on steel arch supports in order to stabilise a tunnel temporarily until the final support was installed. The final support was masonry or a concrete arch. Rock loads developed due to disintegration and detrimental loosening of the surrounding rock and loosened rock exerted loads onto the support due to the weight of a loosened rock bulb (described by Komerell, Terzaghi and others). Detrimental loosening was caused by the available excavation techniques, the support means and the long period required to complete a tunnel section with many sequential intermediate construction stages. The result was very irregular heavy loading resulting in thick lining arches occupying a considerable percentage of the tunnel cross-section (in the early trans-Alpine tunnels the permanent structure may occupy as much as 40% of the excavated profile)

In the first part of the 20th century tunnellers and scientists at that time understood the necessity to reduce deformations in order to utilise the carrying capacity of the rock mass, and the reciprocal relationship between support resistance and deformations.

The New Austrian Tunneling Method (NATM) grew out of experience with the old methods. In his book "Gebirgsdruck und Tunnelbau" 1944 Prof. L.v. Rabcewicz published a systematic of rock pressure phenomena and their interpretation. In his Patent of 1948 the basic principles of the concept were formulated. The essence was as follows:

With a flexible primary support a new equilibrium shall be reached. This shall be controlled by in-situ deformation measurements. After this new equilibrium is reached an inner arch shall be built. In specific cases the inner arch can be omitted.

From 1956 to 1958 Rabcewicz built the first large size tunnels in Venezuela according to these principles. In Austria the first attempts were made in the fifties with smaller hydro tunnels. In 1963 the "New Austrian Tunneling Method" was introduced at the Geomechanics Colloquy in Salzburg. The method has been further developed with regard to sequences of excavation works and supports. Support means and auxiliary measures have been improved, instrumentation, interpretations techniques and skills have extended the applicability of NATM to heavily squeezing ground with extreme deformations, to soft ground in built up areas and to large sections or complicated geometrical tunnel configurations.

NATM concept

The New Austrian Tunnelling Method constitutes a design where the surrounding rock- or soil formations of a tunnel are integrated into an overall ring like support structure. Thus the formations will themselves be part of this support structure. The definition together with the main principles was published in 1980.

With the excavation of a tunnel the primary stress field in the rock mass is changed into a more unfavourable secondary stress field. Under the rock arch we understand those zones around a tunnel where most of the time dependent stress rearrangement processes takes place. This includes the plastic as well as the elastic behaving zone.

Under the activation of a rock arch we understand our activities to maintain or to improve the carrying capacity of the rock mass, to utilise this carrying capacity and to influence a favourable development of the secondary stress field.

The main principles of NATM are:

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

Maintain strength of the rock mass and avoid detrimental loosening by careful excavation and by immediate application of support and strengthening means. Shotcrete and rock bolts applied close to the excavation face help to maintain the integrity of the rock mass.

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

Flexible thin lining: The primary support shall be thin-walled in order to minimise bending moments and to facilitate the stress rearrangement process without exposing the lining to unfavourable sectional forces. Additional support requirement shall not be added by increasing lining thickness but by bolting. The lining shall be in full contact with the exposed rock. Shotcrete fulfils 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. the total periphery including the invert must be applied with shotcrete. In situ measurements: Observation of tunnel behaviour during construction is an integral part of NATM. With the monitoring and interpretation of deformations, strains and stresses it is possible to optimise working procedures and support requirements.

The concept of NATM is to control deformations and stress rearrangement process in order to obtain a required safety level. Requirements differ depending on the type of project in a subway project in built up areas stability and settlements may be decisive, in other tunnels stability only may be observed. The NATM method is universal, but particularly suitable for irregular shapes. It can therefore be applied for underground transitions where a TBM tunnel must have another shape or diameter.

Observations of tunnel behaviour

One of the most important factors in the successful application of observational methods like NATM is the observation of tunnel behaviour during construction. Monitoring and interpretation of deformations, strains and stresses are important to optimise working procedures and support requirements, 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. Figure 8.3 shows an example instrumentation in a tunnel lined with shotcrete.

LegendMeasuring objectiveInstrument

1Deformation of the excavated tunnel surfaceConvergence tape

Surveying marks

2Deformation of the ground surrounding the tunnelExtensometer

3Monitoring of ground support element anchorTotal anchor force

4Monitoring of ground support element shotcrete shellPressure cells

Embedments gauge

Figure 8.3: Examples of NATM tunnel measurement equipment

Measurements of tunnel behaviour can be automatic, but it is outside the scope of these notes to go into detail. Reference is made to instrumentation manufacturers catalogues.

NMT

A variant of NATM using steel-fiber reinforced shotcrete instead of mesh-reinforced concrete is referred to as the Norwegian Method of Tunnelling (NMT). The method is most suitable in jointed rock which tends to overreach. By 1984 NMT had replaced NATM in such ground conditions in Sweden and Norway. NMT is often used with drill and blast tunnelling, but can be used in conjunction with TBMs in clay zones.

NATM Process on site

The simplified steps of an underground transition created with NATM are shown below.

1

Cutting a length of tunnel,

here with a roadheader

2

Applying layer of shotcrete on reinforcement mesh

3

Primary lining applied to whole cavity, which remains under observation.

4

Final lining applied. Running tunnels continued.

5

Completed underground transition

Tunnel stresses and failure mechanisms

The following main failure mechanisms are observed:

Chimney failure

Dome failure

These mechanisms develop from the roof in case of loosening of the rock mass or lack of horizontal stress in the roof.

Split tensile and buckling failure - develops near the excavation line in straight side walls or inverts

Shear failure - develops in high pressure exerting rock if lateral confinement is insufficient.

These failures are generally of progressive nature and can be kept under control with the information of careful in situ observation. Other problem areas may be rock burst in brittle behaving rock under high uniaxial stress or swelling rock like clays or anhydrite. Rock sample testing should be carried out under laboratory conditions as well as in situ examinations. The gained values of the rock-mechanical properties, their variability in particular long-term changes and also the effects of water inflow must be taken into account.

Figure 8.4: Sketch of mechanical process and sequence of failure around a cavity by stress rearrangement pressure.

Figure 8.5: Schematic representation of stresses around a circular cavity with hydrostatic pressure

The Fenner-Pacher curve

The Fenner-Pacher curve shows the relationship between the deformation R/R and required support resistance Pi. Simplistically, the more deformation is allowed, the less resistance is needed. In practice, the support resistance reaches a minimum at a certain radial deformation, and support requirements increase if deformations become excessive.

Fenner-Pacher-type diagrams can be generated to help evaluate the support methods best suited to the conditions. In terms of analysis, it is convenient to carry out quick and simple convergence-confinement calculations with and without support. Resistances of shotcrete, steel reinforcement, and anchors/bolts can be calculated. Functions of support elements, radial stresses, support resistances of inner and outer arches, deformations, and failures can be analysed with respect to time.

Figure 8.6: The Fenner-Pacher Curve.

A classical approach is that of a hole in a homogenous uniformly stressed solid which behaves linearly elastically up to a certain stage of stress and perfectly plastically thereafter. The radial and tangential stresses assumptions around the opened cavity, and the plastic zone can be seen in figure 8.7.

Key

r=radius of cavity

R=radius of plastic zone

(r=radial deformation

(0=primary stress condition

(s=tectonic stresses

(r0 and (t0=radial and tangential stresses respectively with (r=0

(r1 and (t1=radial and tangential stresses respectively for r-(r.

Figure 8.7: Schematic representation of stresses around a cavity.

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

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

where;

Pi = skin resistance

C = cohesion

( = angle of internal friction

R = radius of the protective zone

r = radius of the cavity

P0 = ( H; overburden

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

PiR =

where;

PiR = 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)

(, (R, (nR, can be measured in laboratories, where as S can be measured in meters , on a drawing made to scale.

n

Figure 8.8: Skin resistance Pi required to establish equilibrium of a cavity as a function of angle of internal friction and P0 = (H.

Principles of dimensioning the supporting system

Generally two separate supports are carried out. The first is a flexible outer arch or protective support designed to stabilize the structure accordingly. It consists of a systematically anchored rock arch with surface protection, possibly reinforced by ribs and closed by an invert.

The behaviour of the protective support and the surrounding rock during the readjustment process can be monitored by a measuring system.

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.

In order to plan a project and design standard sections for the documents it is necessary to establish the required carrying capacity of the support for different types of rock.

The carrying capacity of the outer arch can be decided by the (r/(r curve, which is characteristic for any type of rock and primary stress condition.

Figure 8.9: Schematic representation of the(r, (r1, r/R1 and T1 for supports of different yield 1 and 2, and time of application. Key: (r0=radial stress for r/R=1; (r=required radial stress as a function of (r; pia and pi1=support resistances of outer and inner arches respectively; s=safety factor; and C and C=loaded and unloaded condition of the inner arches respectively.

The required radial stress pia to obtain equilibrium decreases if the border zone is allowed to yield and a plastic zone develops simultaneously. The rate of the decrease being mainly a function of the primary stress condition (0 and the angle of the internal friction ( of the rock as a rule diminishes rapidly. At any intersection between pi and the (r curve, equilibrium is reached for the respective support resistance.

It is a particular feature of NATM that the intersections always take place at the descending branch of the curve. For instance, should the support partially fail for any reason, a new equilibrium comes into being without any additional strengthening at a lower point of intersection, as long as this lower point does not fall below the minimum of the (r curve (marked B on figure 9) where the detrimental loosening starts.

With conventional methods on the other hand, the intersection point is usually situated at the ascending branch of the (r curve. With any failure, the intersection point moves to the right and the supporting structure has to be strengthened above its former carrying capacity.

Loosening is considered detrimental, with open cracks and fissures appear in such as way that the rock is no longer capable of conveying shear and compressive stresses. The weight of the loosened masses is added to the lining, actually causing the free area of the cavity to increase.

To be able to plot the (r/(r curve, the following parameters have to be established: the primary stress condition (0 with the direction of principal stresses, the angle of internal friction (, the uniaxial compressive strength (gd parallel and normal to stratification, and the corresponding modulus of deformation and elasticity.

These parameters can be determined by measuring and the course of the curve computed by the finite-element method, taking into consideration the method of excavation (full-face driving or subdivision of the section).

While, after Kastner, the (r0 for (r = 0 is theoretically given by the equations:

(here c = cohesion and = angle of internal friction)

Establishing of (rmin is influenced by the magnitude of (r0 on one hand and geological conditions on the other. This can be explained by the following example.

With a road tunnel situated in fairly compact rock with a small overburden, the tangential border stresses only slightly exceeding its uniaxial compressive strength, Pi min will be very small, particularly if the rock has in addition a high standing capacity.

The same type of rock under a large overburden is bound to develop a fairly large plastic zone causing significant deformations. The rock in this case becoming fractures to a depth of several meters, requires a far greater Pi min, the more so it should it be crossed by a system of even joints instead of being interlocked, due to e.g. well-defined interlocking of the layers.

The value of the required carrying capacity of the outer arch Pia must be chosen as to combine maximum economy with and acceptable degree of safety, and Pia should therefore be as close as practically possible to Pi min in order to obtain a sufficient factor of safety from the additional lining resistance Pi1 of the inner arch.

Should a stiffer type of support be chosen for the outer arch (as marked 2 in figure 8.9 for example), the intersection with the (r curve is bound to rise, while the safety factor simultaneously decreases.

The minimum carrying capacity of the inner arch is decided by the smallest lining thickness that will allow suitable compaction of the concrete. Should a greater Pi1 be required, the thickness can be chosen according to Pia and the required safety factor s.

Once the carrying capacity of the outer arch has been established for certain standard sections, the means of strengthening can be chosen and computed accordingly. The computation has been slightly altered in the mean time and it is therefore shown again in figure 8.11. The resistance of the lining material (shotcrete) is:

An additional reinforcement (steel ribs, etc.) gives a resistance of:

where;

(for concrete)

The lining resistance is:

PiL = Pis + Pist

The anchors are acting with a radial pressure:

With the lateral pressure given by:

(3 = pis + pist + piA

and with Mohrs envelope, the shear resistance of the rock mass (R and the shear angle ( is determined, assuming that the principal stresses are parallel and at right angles to the excavation line.

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:

The total carrying capacity of the outer arch is then:

Figure 8.10: Shear forces

Figure 8.11: Design scheme of arch for a given carrying capacity

Design nomenclature

SymbolUnitDescription

PisMp/mResistance of shotcrete

PistMp/mResistance of steel

PiRMp/mResistance of rock arch

PiAMp/mResistance of anchors

PiWMp/mTotal support capacity

bMHeight of shear zone

dMThickness of lining

e,tMDistance between rock bolts

sMLength of shear plane

wMWidth of carrying ring

(gsMp/mUniaxial compressive strength of rock

(Angle of internal friction

cMp/mCohesion of rock

(sMp/mShear resistance of lining material

(stMp/mProportaion of shear resistance of reinforcement

Est, EsMp/mModulus of elasticity of reinforcement and lining material, respectively

(sShear angle of lining material

FstcmArea of rock bolts

(pstMp/mProportional limit of anchor steel

(RMp/mShear strength of rock

(nRMp/mNormal strength of rock

(Shear angle of rock

(Average inclination of shear plane

(Inclination of anchors

As to the reciprocal mode of action of the basic supporting members of the NATM, shotcrete and the anchored rock-arch, experience shows the following:

1. With the same type of rock and overburden the relationship between the size of the joint-bodies and the excavation area is decisive for the mobility of the material;

2. With small sections (ie -10-16 cm) and joint bodies of a few dm, a simple shotcrete sealing with d = 3 cm = 0.017 R usually stabilizes the tunnel;

3. With an underground power station of 400-600 m2 on the other hand, a rock with joint-bodies of this size behaves like a cohesionless mass, and a simple shotcrete lining of 0.017 R = 19-24 cm would never do. A systematically anchored rock arch is imperative in this case.

The surrounding ground acts as the main carrying member, the shotcrete lining merely having the function of stabilizing the surface between the anchorage points.

The greater is the (r( and the section of the cavity and, the smaller is (, the more important is the system anchoring in comparison with the shotcrete.

With conventional wedge or expansion bolts the plate exerts the supporting action, and the anchor is always stressed equally over its whole length. Grouted anchors have their main carrying effect from the bond between the grout and rock. The bond consists mostly of friction caused by the tangential stresses in the surrounding rock (besides a minor share of adhesion).

The tensile stress of the anchor increases from zero at the end to a maximum at the plate, and any radial border stresses possibly remaining are additionally conveyed to the anchor by the plate Fig. 8.12

The movement of the rock towards the cavity is inhibited in this way, and an arch effect is creates between neighbouring anchors, as is shown in the drawing.

The carrying capacity piB can be described analytically in simplified form by the equation:

piB = ld((a+tan( (mt) +F(r(fe(e

The term F(r can possibly rise to (r.

Although the carrying capacity of both the expansion and the grouted type anchors is the same (limited by the tensile strength of the steel), the stabilizing effect of grouted anchors is much greater.

As a further reinforcing measure in NATM, light steel ribs of the channel-section type are used, connected by overlapping joints and fastened to the rock by the anchors.

The ribs serve primarily as a protection for the tunnelling crew against rock fall and as local reinforcement to bridge across zones of geological weakness. The static share of the ribs in the lining resistance is relatively low.

The stiffness of the ribs contrasts with the relatively high yielding capacity of the shotcrete, and with large sections and deformations minor cracks in the shotcrete along the ribs must be reckoned with.

Final dimensioning is based on measurement.

Inseparably connected with the NATM, and a basic feature of the method, is a sophisticated measuring programme. Deformations and stresses are controlled systematically, allowing determination of whether the chosen support-resistance corresponds with the type of rock in question, and what kind of additional reinforcing measures are needed, if any.

In a case of the lining being over-dimensioned, the reinforcing measures can straight away be reduced accordingly when the same or similar mechanical conditions of the rock are encountered during further tunnel driving.

Figure 8.12: Schematic showing the mode of action of the grouted anchors.Key: (ri = radial border stress, (ti= tangential border stress, (t = tangential stress, (r = radial stress, (0 = (H, (tm = average tangential stress on l, a = adhesion of grout/rock, PiB= lining resistance of anchor, fe = area of anchor steel, (e = tensional strength of steel, F = area of plate, d = grouted diameter hole.

An empirical dimensioning is carried out in this way, based on the principles explained here.

During the execution of a series of NATM tunnelling works during the last many years satisfactory measuring systems have been developed.

In order to control the behaviour of the outer arch and surrounding the different construction stages in practice, main measuring sections are chosen at distances determined by the significant geological points.

These will be equipped with double extensometers and convergence measuring devices to measure deformations and pressure pads to measure radial and tangential stresses. In addition, roof and floor points can be monitored geodetically.

In-between the main measuring sections, secondary points are selected at suitable distances where only convergence are made. Readings are made every other day at the beginning, decreasing to once a month according to the velocity of deformation and change of stresses. The measurement results are plotted in graphs as a function of time, which enables the changes in the rock caused by mechanical

R1-R8 : Radial pressure pads

T1-T8 : Tangential pressure pads

H1, H2, H3: Convergence measuring lines

E1-E6 : Long extensometers

Ea1-Ea6: Short extensometers

VF, Vs: Geodesic control points

Figure 8.13: Standard main measuring section

This method of establishing stress-time graphs gives a high degree of safety, allowing any situation to be recognized long before it becomes critical.

Since the readjustment process takes a very long time, being possibly influenced locally by subsequent alterations of the geological conditions (e.g. increase in the water content of the surrounding rock), it is essential from both the practical and the theoretical point of view to measure also the stresses and deformations of the inner lining

This is done by placing a series of tangential pressure pads or strain gauges, both in pairs outside and inside the lining, and also by using convergence measuring devices.

Figure 8.14: A drawing showing some of the results of measurement made at the Austrian Tauern Tunnel North, showing the conspicuous decrease of deformation with respect to time and the excavation and outer lining section. Key: H1-H3 = convergence measurement readings, E4 and E5 = long extensometer readings.

Numerical example for NATM

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

H = 15.0 m overburden according to tests on samples found: (= 27, c = 100 t/m (three axial tests). For establishing a new equilibrium condition in the opened cavity, following supporting is going to be used:1. Use the supporting ring which develops around the cavity after excavation as a self-supporting device and select a type of supporting which can bear the developed rock loads and deformable when necessary.

2. Design the inner lining under final loads

Figure 8.15: Numerical Example Tunnel Section

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 of:

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 the disturbed zone R:

Talobre formula:

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

a) Shotcrete:

d = 25 cm

(c28 = 160 kg/cm compressive strength shear in concrete (assume 20% of (c28)

the capacity carrying load

(sh = 0.20 160 = 32 kg/cm = 320 t/m

d = thickness of shotcrete in (cm)

( = (/4 - (/2 angle of shear plane with vertical

b = shear failure height of the cavity (see Fig.10)

;sin 31.5 = 0.520; b/2= r cos (= 6.05xcos31.5=5.15

b) bolts

Bolt 26 mm

St III, (sh = 4000 kg/cm

Spacing=2x2 m

f = 5.3cm

c) steel ribs

t = spacing 2.00 m

F = 2 20.4 cm = 40.8 cm = 0.00408 m

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

(N = normal stress = 142 t/mread on X

(Z = minor stress = 37.2 t/m

(R = shear stress = 170 t/mread on Y

Figure 8.16: Finding stresses geometrically

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

d) bearing capacity of the supporting ring

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

thusS = 4.54 m (from figure 17)

( = 27

b/2 = 5.15 m

(R = 170 t/m

(N = 142 t/m

enter the formula

Figure 8.17

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

(b/2, a = 4.20 m, ( = 35.5from 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 be

Pi = Pic + Pib + Pist + PiR = 29.9 + 3.43 + 36.56 + 78.22 = 160.1 t/m

t/m

Failed implementation and criticism of NATM

It may be useful for the student to be aware of some of the controversy that has surrounded NATM.

The name itself has been used to cover a variety of different approaches to tunnelling, at times surrounded with too much philosophical baggage. The method is quite practical, however. It is a universal principle of good tunnelling to adapt the support to the expected behaviour of the ground. As regards the ring-like support structures, any opening in the ground causes at least a degree of ground support to be conferred by circumferential stress on the ground.

More importantly, since the beginning of NATM engineers and geologists doubted the existence of sufficient bearing capacity of the ground. In principle at least, the criticism was unfounded, but there were indeed a dozen accidents within the first thirty years use of the method. Also in more recent years, a number of NATM tunnels collapsed, and debates on the method are still continuing. A well-known NATM tunnel collapse was the Heathrow Express Tunnel in October 1994, causing extensive media coverage. There were allegations that the method was unsuitable for tunnelling in the London Clay; allegations that was deemed groundless by NATM proponents the success of the method in many other clay conditions all over the world. Two collapses in Germany, the Munich Metro and Krieberg Tunnels received similar media attention, and in Turkey the Bolu tunnel experienced massive problems.

The collapse of the Heathrow Express tunnel in 1994 caused a symposium by the Health and Safety Executive (HSE) in the UK. The HSE was at that time aware of totally 116 NATM related collapses.

When analysing the background of these collapses, the most important fact is that the large majority of NATM tunnel collapses have occurred during construction. Moreover, the principle collapse in NATM is the face failure, i.e. collapses have occurred only at the face where the lining is still weak and cantilevered. Completed, correctly constructed NATM linings have almost never failed.

In literature, and in the NATM debate in the tunnelling society, one particular item is addressed as very important: skill and experience of the site-engineering team. This view is defended by the fact, that the majority of incidents have been associated with night shifts. For instance, the 1994 Heathrow Express Tunnel collapse took place at 01.00h.

A contractor, however, need not be an expert in NATM tunnelling provided the correct business relations are established to obtain expertise elsewhere. The design of NATM tunnels relies heavily on the available data of the soil conditions. During the discussions it has been claimed, that designers of NATM tunnels have often had insufficient data to work with, and in fact most NATM collapses are connected with 'unexpected ground conditions'.

NATM is a safe method if properly applied. Proper application requires:

Theory and calculations

Experience of engineers

Good labour skills

Excellent monitoring/instrumentation.

Eurocode 7 BSI 1995

PAGE 185Chapter 8

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Main Pressure

Stage 1

Stage 2

Stage 3

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