8. risk analyses in tunneling 8.1 fundamentals · - 431 - wbi-print 6 wbi gmbh, henricistr. 50,...

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8. Risk analyses in tunneling

8.1 Fundamentals

In recent years the necessity for systematic identification andevaluation of risks has become more and more important. The termrisk or risk analysis and management respectively, in subsurfaceconstruction is connected with applications or objectives, whichare partly very different from each other (see e. g. Vigl et al.,2002; Kovari and Bosshard, 2003; Maidl and Handke, 2003; Jäckleand Wittke, 2003; Ziegler and Zwick, 2003; Heimer et al., 2004).

In the following, a procedure will be presented, which allows toquantify and estimate respectively, the risk of an increase ofconstruction costs and time, resulting from the geotechnical con-ditions of a tunneling project.

In addition to the geotechnical and technical risks, a number offurther risks for example due to approval and planning may lead toincreases of costs and construction time. Also the selection ofexperts which are involved in the planning process on the basis ofa price competition, is already a risk on its own. The latter as-pects, however, will not be dealt with here.

According to the opinion of the authors, simple, universally ap-plicable methods for identification of geotechnical risks is notavailable. The potential risks, resulting from the regional geo-logical and hydrogeological conditions, in each individual casemust be identified by ground experts and planners in an earlystage of design. The identified risks can then be reduced by meansof the selection of suitable construction methods and, if neces-sary, additional measures on the basis of targeted exploration andinvestigation programs, as well as corresponding stability analy-ses. Thus, construction methods which are combined with high riskscan be avoided.

In a further step the remaining risks may be assessed with regardsto costs or construction time, respectively. For this purpose arisk simulation analysis can be carried out.

Whether all decisive potential damages are identified determinethe value of a risk analysis to a great extent. If essentialrisks, as for example squeezing rock zones or karstic in cavities

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are not identified in the course of planning of a tunneling proj-ect, a risk analysis is more or less worthless. A risk analysissimulation also makes only little sense if the input data are notreliable.

In spite of these difficulties, potential increases of costsshould not be covered by an overall surcharge upon the total con-struction costs, which is solely derived from experience gainedfrom other projects. Such a procedure postulates that the geotech-nical conditions and also the planning carried out are comparable,which is not justified in most cases.

Definitions

"Risk" as well as related terms as "risk management", "risk analy-sis", "residual risk" etd. in common speech and also in the tech-nical language are often used with different meaning. Therefore,the terms which are applied subsequently, are defined in the fol-lowing.

Damage patternA potential critical situation or an unwanted event are nameddamage pattern. The term damage pattern can also be appliedfor the description of a deviation from predicted conditions,e. g. less favorable characteristic geotechnical parameters.

Damage/extent of damage (D)The negative consequence of an event, which results from adamage pattern is referred to as damage. The extent of damageD is quantified in the term of increases of costs or construc-tion time.

Probability of occurrence (p)The probability of occurrence p normally is estimated by anexpert for tunneling or geotechnics. It can vary between 0 and1.0.

Risk (R)The risk R related to a damage pattern, is defined as productof extent of damage D and probability of occurrence p. Exem-plarily, the risk R for a damage pattern is represented in adiagram in Fig. 8.1, in which the probability of occurrence pis plotted over the extent of damage D.

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Residual riskThe risk, which remains after consideration of risk reducingconstruction measures, is referred to as residual risk R'(Fig. 8.1). The acceptable residual risk usually is assessedin different ways. If the residual risk predominantly leads toincreases of costs and construction time, the client can spec-ify the accepted risk by himself. Measures to limit the risks,which can lead to a hazard of life and limb, must be excludedfrom such a judgement. Such risks must be detected and reducedby means of an escape and rescue concept.

Fig 8.1: Reduction of a risk R for a damage pattern Pi to theresidual risk R'

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8.2 Procedure

The risk analysis always has to be carried out project-oriented.In many cases the risk analysis in five steps as represented inFig. 8.2 has been applied successfully:

Step 1: Selection of the construction method and the type of TBMrespectively

The risk analysis starts with this step, because in tunneling theselected method of heading and for mechanized tunneling also thetype of TBM, have an important influence on the risks. Certainkinds of risk can only occur, when certain construction methods ortypes of TBM are applied.

Step 2: Identification of potential damage pattern

All potential and reasonable damage patterns must be identifiedand evaluated for the selected construction method and type of TBMrespectively. In section 4.1.1 some potential risks for the dif-ferent types of TBMs are pointed out. When a TBM-S for instance isused in rock, the following damage patterns regarding the stabil-ity of the unsupported, excavated opening are possible:

- rock collapses, clogging of the cutterhead,

- tearing out of the temporary face,

- breakouts above the cutterhead,

- rock mechanical parameters less favorable, or zones withsmall rock strength (e. g. fault zones) longer than ex-pected,

- intense inflow of water.

In connection with the annular gap grouting also the followingdamage patterns may lead to hindrances for a mechanized tunnelheading (see section 5.7):

- Penetration of the mortar into the steering gap and to-wards the area of the cutterhead,

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- plugging of the injection lines,

- damaging of the brush sealings,

- damaging or tearing of the external steel sheets of thesealing.

Fig. 8.2: Procedure for a risk analysis

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The following damage patterns may lead to a considerable reductionof the operability of the excavation tools and the conveyor capa-bility:

- Overloading of single discs in case of mixed-face-conditions,

- clogging of discs in weak rock,

- high wear of the cutterhead and the excavation toolscaused by a high abrasivity of the rock or the forming ofa grinding paste, composed of water and quartz,

- drifting of the TBM in case of wedging rock layers,

- adhesion of the excavation tools,

- loosening of rock wedges from the temporary face and theroof,

- accumulation of excavated material at the temporary face.

Further damage patterns, which can for example be assigned to thebearing capacity of the lining, the serviceability and durabilityof the tunnel, logistics, environmental aspects as well as plan-ning and awarding of the contract, possibly must be considered inaddition.

Besides the risks, also the chances which may lead to savings ofcosts and construction time for a certain construction method,should be evaluated in a risk analysis. Examples for this are:

- higher rates of advance,

- smaller thickness of segments,

- no sealing in dry rock areas (e. g. umbrella sealing insteadof circumferential sealing, or watertight concrete instead ofplastic liner) as well as installation of an unreinforced orfiber-reinforced internal lining, in case of the double seg-mental lining method.

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If the potential savings are based on variants of construction,which deviate from the applicable regulations and safety conceptsin the respective country, the feasibility of these variants needsto be checked.

Step 3: Evaluation of the damage pattern

In order to evaluate the damage pattern, the extent of damage, e.g. the costs C of the corresponding damage pattern, as well as itsprobability of occurrence p need to be estimated.

Whereas the extent of damage can be estimated comparatively sim-ple, by means of costs and time evaluations, the estimation of theprobability of occurrence obviously is more difficult. The accu-rate evaluation of the probabilities of occurrence of certain dam-age patterns, requires the knowledge of the statistical distribu-tions of all influencing input parameters, which usually are notavailable. Therefore, the assessment of the probabilities of oc-currence is based on experience values, as it is also the casewhen the characteristic parameters of the ground are specified.These, for example, can be gained from other projects with compa-rable ground conditions.

If sufficiently extensive and representative data are availablefor individual influencing variables, e. g. the characteristic pa-rameters of the ground, statistical distributions may be derivedfor these variables, which then can be used to estimate the prob-abilities of occurrence (Ziegler, 2002). Also in such cases, how-ever, the estimation of the probabilities of occurrence should bemade by an expert (Ziegler and Zwick, 2003). Also a detailed ex-ploration of the ground conditions with the aid of an explorationadit, or a test gallery, combined with monitoring and back analy-sis of the corresponding results can lead to an improved estimateof probability of occurrence of certain damage patterns.

A risk analysis, which is completely based on the opinion of ex-perts (expert's survey) is proposed by Ziegler (2003). Here, bymeans of the example of the assessment of different conveying sys-tems for the transportation of soil excavated by a TBM, 11 crite-ria are selected, which are assigned to the topics technical pa-rameters, economical parameters and the sensibility for failure.These criteria then are evaluated by selected experts, with marksranging from 1 to 10. This procedure, however, is controversial

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discussed, because the evaluation is based exclusively on the sub-jective assessment and experience of the involved experts.

Step 4: Measures to reduce the identified risks

If the identified risks are regarded as acceptable by the clientand the specialists, planners and authorities involved in the pro-ject, the procedure can be directly continued with step 5, therisk simulation analysis (Fig. 8.2).

If the risks connected with one or more damages patterns are how-ever not acceptable, it has to be investigated, whether measuresto reduce the risks to an acceptable level are available. The re-duction of a risk to an acceptable residual risk can be obtainedby both, measures which cause a drop of the probability of occur-rence, and measures which lead to a decrease of the extent of dam-age.

If the risks, even by additional measures, cannot be adequatelydealt with or if they cannot be reduced economically, the origi-nally selected construction method, or in case of mechanized tun-neling, the type of the TBM, respectively, should be ruled out asnot acceptable and thus should be replaced by a different con-struction method or type of TBM respectively. Thus, all majorrisks which are not acceptable, can be eliminated already in theplanning stage (Fig. 8.2).

Step 5: Risk simulation analysis

After completion of steps 1 to 4 a risk simulation analysis can becarried out for the selected construction method under considera-tion of the acceptable risks (Fig. 8.2). The single risks, con-sisting of the extent of damage and the probability of occurrence,for this are combined in order to evaluate the overall risk quan-titatively. The procedure of a risk simulation analysis is de-scribed in section 8.3.

If different methods of construction or TBM's are planned to becompared, the risk analysis needs to be carried out separately foreach construction method, including the measures to avoid and toeliminate damages.

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8.3 Risk simulation analysis

The risks which are associated with tunneling, are often evaluatedonly qualitatively or in a simplified manner. As an example, oftenonly chances and risks are taken into account in cost evaluations,which are likely to occur. If the probability of occurrence of arisk or chance is expected to be comparatively small, the corre-sponding costs and savings respectively, are often not consideredat all.

This procedure leads to an amount, which is expected to reflectthe costs for the project. Information on the potential range ofthe construction costs and on the corresponding probabilities ofoccurrence cannot be derived from this figure.

One possibility to achieve the corresponding information is tocarry out a risk simulation analysis. It allows to combine therisks resulting from varying input parameters with each other, andthus to quantify the overall risk.

The procedure for the combination of scattering variables, whichare represented in the form of a frequency distribution, will beexplained in the following, by means of a simple example, whichhas no technical relevance.

Three balls identified by numbers 1 to 3 are located in a box. Theprobability that the ball indicated with number 1 is drawn,amounts to 1/3. The probability that the balls indicated with thenumbers 2 and 3 are drawn, in each case is also 1/3. In a secondbox another three balls are located, which are identified by thenumbers 4 to 6. The probability that one of these balls is drawn,here also amounts to 1/3. Now one ball is to be drawn from eachbox. The question is, how high is the probability that the sum ofthe numbers, both balls are indicated with, is higher or equal to7.

Since in both boxes three balls are located, 3 x 3 = 9 combina-tions for the drawing of the two balls from boxes 1 and 2 areavailable (Fig. 8.3). The probability that one of the 9 possiblecombinations is drawn, can be determined by the product of theprobabilities resulting for draw of each ball. The probabilitythat the combination of the balls identified with the numbers 1and 4 is drawn, for instance is 1/3 x 1/3 = 1/9 (Fig. 8.3). Since

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the probability of drawing is equal for all balls, this value re-sults for all combinations. The sum of the probabilities of all 9combinations results to 9 x 1/9 = 1, because the probability thatone of the 9 possible combinations is drawn, is 1 or 100 %.

Fig. 8.3: Simple example for a risk simulation analysis, possi-ble combinations

In Fig. 8.4a the possible combinations for this example arelisted. The sum of the numbers on the balls from boxes 1 and 2, aswell as the corresponding probabilities of occurrence are given.

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Fig. 8.4: Example for a risk simulation analysis:a) Possible combinations; b) frequency distribution;c) cumulative frequency

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The sum of the numbers on the balls from boxes 1 and 2 now can beordered according to size, and then be represented in the form ofa frequency distribution (Fig. 8.4b). The cumulative frequency isevaluated from the frequency distribution, by adding all prob-abilities for combinations reaching or increasing a certain sum ofnumbers. The probability that the sum of numbers on the balls fromboxes 1 and 2 is greater or equal 7, which is called probabilityof excess, can be gained from the representation of the cumulativefrequency as 6/9. The individual probabilities of excess can beconnected by a curve, which represents the so-called risk profile(Fig. 8.4c).

The scattering variables, which in the above explained example arerepresented by the numbers on the balls, in a risk simulationanalysis are also referred to as risk variables, because they nor-mally lead to increases of costs and construction time. In theconsidered example the increases of costs and construction timeare represented by the sum of numbers on the balls from boxes 1and 2.

In the following the application of the risk analysis and the risksimulation analysis will be demonstrated for an example of a roadtunnel carried out according to the NATM. The potential risks andthe associated increases of costs are investigated for two plan-ning variants. The example indicates that the comparison of costscan also include an evaluation of risks due to misinterpretationor unexpected occurrences of damage.

8.4 Tunnel Leutenbach, assessment of two planning variantswith the aid of a risk analysis

8.4.1 Description of project

The federal road B 14 is one of the most important traffic routesin the region of the river Neckar (Germany). Due to narrow cross-town links, the capability of this road is limited and cannot meetthe requirements resulting from an increased volume of trafficanymore. Therefore, a four-lane extension of the federal road withseveral local bypasses is planned.

The extended B 14 coming from Stuttgart, currently ends at thejunction Winnenden Süd, and in continuation is planned to under-cross the area between Leutenbach and Winnenden in a 1080 m long

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tunnel. The tunnel starts west of Leutenbach, in an abandoned claypit of a former brick yard, and undercrosses the country roadK 1847, the Buchenbach (creek) and the railroad line Stuttgart –Nuremberg (Fig. 8.5). The eastern portal of the tunnel is locatednorth of Winnenden, immediately adjacent to the railtracks. Subse-quently, the new B 14 runs parallel to the railtracks and jointsthe existing B 14 close to Nellmersbach.

Fig. 8.5: Tunnel Leutenbach, site plan (Frenzl et al., 2005)

8.4.2 Planning variants

In the course of the planning for the tunnel amongst others twovariants were investigated, which differ with respect to the loca-tion of the gradient and the shape of the cross-section. The low-est point of the gradient of both variants is approx. located inthe middle of the tunnel (Fig. 8.6). The gradient of both variantscoming from the west dips with approx. 4.1 % and ascends towardsthe eastern portal with approx. 3.9 %. At the lowest point, how-ever, the gradient of variant 1 is less curved than that of vari-ant 2. In variant 1 the tunnel's roof cuts the Quaternary layersin the area of the Buchenbach and the railroad embankment which

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consist of loam. Furthermore, the spacing between the bottom ofthe Buchenbach and the tunnel's roof only amounts to approx.1.5 m. After heavy rainfall, the discharge of the Buchenbach canrise up to more than 40 m³/s. As a result of the lowering of thegradient for variant 2, the tunnel cross-section according to theresults of exploration in this area is located completely withinthe shell limestone. The spacing between the bottom of the Buchen-bach and the tunnel's roof is increased to approx. 5 m.

Fig. 8.6: Longitudinal section with ground profile and variants1 and 2 (Frenzl et al., 2005)

The tunnel is planned to be constructed according to the cut-and-cover method over a length of 371 m in the western part, and over50 m in the eastern part (Fig. 8.6). The 659 m long section in be-tween, is planned to be driven by the mining method.

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For this section, variant 1 foresees a double tube cross-section,with a central buttress of reinforced concrete, which is to beconstructed in advance (Fig. 8.7). Variant 2 in this section com-prises two single tubes with a rock mass column in between (Fig.8.8). The tunnel tubes, which are excavated according to the NATM,must be provided with a sealing between the internal and the ex-ternal lining, which is made of plaster liners.

Fig. 8.7: Double tube with central buttress, variant 1 (Frenzlet al., 2005)

Fig. 8.8: Two separate tubes with lowered gradient, variant 2(Frenzl et al., 2005)

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8.4.3 Risk analysis

General

The decision, which variant should be the basis for the tender wasmade with the aid of a risk analysis.

Only the tunnel section, which has to be constructed by NATM isconsidered. The tunnel sections which are to be constructed bymeans of the cut-and-cover method are almost equal for both vari-ants, and thus have no significant influence on the decision to bemade.

Damage patterns

The selected most favorable construction method for the tunneltubes to be driven by underground means, is the NATM. The decisivedamage patterns, elaborated for the Tunnel Leutenbach, are com-piled in tables 8.1 to 8.4. The damage patterns are distinguishedwith regard to

- the geological and hydrogeological conditions (Table 8.1),

- excavation and support of the tunnel (Table 8.2),

- the sealing and the internal lining (Table 8.3) and

- miscellaneous causes (Table 8.4).

The tables contain a description and explanation of each damagepattern. Furthermore, for each damage pattern the maximum extentof damage is predicted.

In order to evaluate the damage patterns the probabilities of oc-currence are estimated under consideration of the available expe-rience as well as the geotechnical and operational boundary condi-tions.

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The evaluation of the damage patterns subsequently is demonstratedby means of the example "extension of the sections which need tobe supported with pipe umbrellas" given in Table 8.1. In sectionswith too small rock overburden, or where soil exists at the tun-nels roof, the heading is planned to be supported by pipe umbrel-las. If the transition between rock and soil is located lower thanexpected, an extension of the corresponding sections, which needsupport by pipe umbrellas, is necessary. Thus, the extension ofthe section which needs to be supported by pipe umbrellas, is in-troduced as a damage pattern. Except for the basic variant (no ex-tension required) two scenarios were considered.

Scenario A leads to an extension of 3 x 40 m = 120 m for variant 1and of 2 x 100 m = 200 m for variant 2. Assuming that the costsfor the pipe umbrella amount to 5000 € per meter, the extent ofdamage for scenario A result to 0.6 x 106 € (variant 1) and of 1.0x 106 € (variant 2) respectively. The corresponding probabilitiesof occurrence are estimated on the basis of the results of thesite investigation. Based on the elevation of the transition be-tween rock and soil encountered in the various exploration bore-holes, the probabilities of occurrence for this scenario wereevaluated to 0.005 (variant 1) and 0.2 (variant 2) (Fig. 8.9).

Fig. 8.9: Damage pattern "extension of the tunnel section,which needs to be supported with pipe umbrellas":a) variant 1; b) variant 2

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For scenario B the prolongation of the tunnel section supportedwith pipe umbrellas is assumed to be 3 x 80 m = 240 m for vari-ant 1, and 2 x 150 m = 300 m for variant 2. Thus, the correspond-ing extent of damage amounts to 1.2 x 106 € (variant 1) and to1.5 x 106 € (variant 2) respectively. The probabilities of occur-rence again are estimated on the basis of the results of the siteinvestigation, resulting to 0.02 for variant 1 and 0.1 for variant2 (Fig. 8.9).

Since the sum of the probabilities of occurrence for each damagepattern must be equal to 1.0, the probability of occurrence forthe case that no prolongation of the section supported with pipeumbrellas is required, is 0.93 for variant 1 and 0.7 for variant 2(Fig. 8.9).

Risk simulation analysis

For both variants the extent of damage and the probabilities ofoccurrence for the various damage patterns were combined. The num-ber N of the combinations which have to be investigated, resultsfrom the product of the numbers of individual cases ni, for eachdamage pattern i (i = 1, 2, ...., m):

m321 n..........nnnN �� . (8.1)

The number of cases for the damage pattern "extension of the tun-nel section, which needs to be supported with pipe umbrellas" foreach variant amounts to ni = 3.

The total number of combinations for both investigated variantsamounts to approx. 8.5 x 106 (variant 1) and approx. 25.5 x 106

(variant 2).

The probability of occurrence for one of the approx. 8.5 x 106 or25.5 x 106 combinations respectively, is to be computed by theproduct of the probabilities of occurrence of all cases, which arecombined with each other:

jm,m2j,21j,1k p...ppp �� (8.2)

where p1,ji: Probability occurrence for the case ji of damagepattern i,

k = 1, ..., N.

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The sum of the probabilities of all combinations is equal to 1.0,thus, the following formula is true:

0.1pN

1kk ��

�

. (8.3)

For the risk simulation analysis for the considered example, forsimplification it is assumed that the overall damage of a combina-tion k, corresponds to the sum of the extent of damages of allcases, which are combined with each other:

Sk = S1,j1 + S2,j2 + ... + Sm,jm

where Si,ji: extent of damage for case ji of damage pattern i.

This procedure however requires, that there are no causally deter-mined interdependencies between the damage patterns. This meansthat the damage patterns are statistically independent. In thegeneral case of interdependency between the damage patterns, theextent of damages of combined cases can be correlated with eachother, by arbitrary functions. As a consequence, the number ofcombinations is reduced in comparison to the case of statisticalindependence of the damage patterns.

For both variants, the obtained extents of damages of all combina-tions were ordered according to size, and were represented as fre-quency distribution with their probability of occurrence (see Fig.8.4b). From this frequency distribution, the cumulative frequen-cies were evaluated (see Fig. 8.4c).

Fig. 8.10 shows the evaluated risk profiles for both investigatedvariants. From this representation, the probability that the over-all extent of damage exceeds a certain value can be determined.For variant 1 for example, the extent of damage for a probabilityof excess of 50 % results to 0.81 x 106 €. The corresponding extentof damage for variant 2 results to 1.4 x 106 € and thus is0.65 x 106 € higher than for variant 1. For a probability of excessof 10 % the extents of damage amount to 1.8 x 106 € (variant 1) and3.1 x 106 € (variant 2) respectively.

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Fig. 8.10: Comparison of variants, risk profiles obtained fromrisk simulation analyses (Frenzl et al., 2005)

Evaluation of risks and selection of variant

Fig. 8.11 shows bar diagrams in which the calculated costs of47.44 x 106 € (variant 1) and 43.16 x 106 € (variant 2) as well asthe increases of costs with the corresponding probabilities of ex-cess of 50 %, 10 % and 0 %, which were evaluated by means of therisk simulation analyses, are represented.

Fig. 8.11: Comparison of variants, construction costs under con-sideration of the risks (Frenzl et al., 2005)

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The risk analysis leads to the result that variant 2 is cheaperthan variant 1, also when all risks are taken into account in theassessment. According to the cost evaluation without considerationof risks, a cost benefit of approx. 4.28 x 106 € results. If aprobability of excess of 50 % is assumed, the construction costsof both variants increase by approx. 2 % to 3 %, and the differ-ence of costs reduces to 48.25 x 106 € - 44.62 x 106 € = 3.63 x 106

€. When a probability of excess of 10 % is assumed, the cost ad-vantage for variant 2 still results to 49.24 x 106 € - 46.26 x 106

€ = 2.98 x 106 €. Even if all considered damages would occur, vari-ant 2 is cheaper than variant 1, by approx. 1.5 x 106 € (Fig.8.11).

Because of the described evaluations, the tender for the TunnelLeutenbach was based on variant 2.

8.5 Cost overruns, which occured in connection with machine-driven tunnels

Exemplarily, the cost differences between billing and originalcontract, which resulted for machine-driven tunnels in Switzer-land, Germany and the Netherlands are summarized in tables 8.5 and8.6.

For the tunnels in Switzerland, which were constructed accordingto the double segmental lining method, descreases of costs (TunnelGubrist and Tunnel Zürichberg) and additional costs of up to ap-prox. 40 % (Tunnel Zürich-Brunau) occurred. The tunnels Bösberg,Mont Russelin and Gorgier caused additional costs of 21 % up to32.6 %. These could be traced back to a large extent, to unex-pected geological conditions and change orders of the owner. Inthe case of the Tunnel Zürich-Brunau (construction lot 2.01), thebilling amount was 40.7 % higher than the contract amount. Thistunnel required particular support measures in urban areas (lowoverburden). Furthermore, the conditioning and disposal of mud,caused extra costs. The cost overruns of the other six tunnelslisted in table 8.5, amounted to approx. 1 % up to 7.4 % of thetotal order value.

The billing amounts of the tunneling projects in Germany and theNetherlands, which are listed in Table 8.6, except for the WeserTunnel and the Westerschelde Tunnel, are practically within thescope of the contract amounts. Both mentioned tunnels were driven

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in soil, by means of the single segmental lining method. In thecase of the Weser Tunnel, the additional costs are predominantlycaused by modifications of planning, an extension and a loweringof the tunnel. The extra costs which arised for the WesterscheldeTunnel, can be traced back to additionally required cross-cuts,and to a large extent to the buckling of the tail-skin, which hap-pened in both tunnel tubes in the area with the highest waterpressures.

In connection with the given examples, substantial increases ofcosts and construction time occurred, in particular due to thegeological conditions and due to change of orders of the clients.If these risks could be eliminated, the deviations of costs be-tween billing and contract amounts, for the tunnels listed in ta-bles 8.5 and 8.6 vary between – 5 % and + 7 %. The risk analyses,which are to be carried out in future in connection with largetunneling projects, therefore, should predominantly aim at theidentification of the risks due to the ground conditions and theconstruction method, in order to consider and evaluate these risksadequately already during planning.

8. risk analyses in tunneling 8.1 fundamentals · - 431 - wbi-print 6 wbi gmbh, henricistr. 50,...

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