The 2013 ISRM Franklin lecture

Rock Mechanics for Resources, Energy and Environment – Kwasniewski & Łydzba (eds)© 2013 Taylor & Francis Group, London, ISBN 978-1-138-00080-3

Engineering aspects of geotechnical tunnel design

A. Goricki3G Gruppe Geotechnik Graz ZT GmbH, Graz, Austria

ABSTRACT: The geotechnical design of underground structures deals with the interaction between the groundand the structure. The key element of this behavior is the potential failure mode of the ground, which mainlydepends on the ground conditions including ground parameter, water and primary stress condition as well asthe excavation geometry. Typical ground behaviors for underground structures are discussed and engineeringdesign methods are presented. To deal with the uncertainties of the ground and the complexity of the subsurfacebuildings a behavior based design methodology is discussed in combination with the risk management processas described in ISO 31000. The integration of the processes leads to a sound design process, which allows theapplication of individual and problem-oriented engineering design tools as well as the adaptation of the designduring the construction phase to minimize the geological and geotechnical risks if required.

1 INTRODUCTION

The “geotechnical design of underground structures”can be described as the design dealing with the inter-action between the ground and the structure. Withfocus on an economic construction according to itspre-defined specifications the geotechnical designmainly covers the design of the excavation and theprimary support but also additional measures such aslowering of the ground water level or injections tochange the ground properties. Generally the geotech-nical design includes all aspects, which deal with theground interactions.

Underground structures are often complex struc-tures with geometrically different elements such asshafts, tunnels or caverns. These structures are exca-vated in various ground conditions concerning geo-logical units, overburden, primary stresses or groundwater with different levels of uncertainties of thepredictions. Due to this the geotechnical design ofunderground structures is often very complex andrequires comprehensive understanding of the geo-logical, geotechnical and structural designs and itsinteractions.

At the moment a comprehensive method for thegeotechnical design of underground structures, espe-cially in rock mass, is neither defined in standards norinternationally accepted as state of the art. Various dif-ferent approaches are applied based on regional expe-rience or specifications of local clients and authorities.The typical methods applied for geotechnical designare discussed by various authors (e.g. Hudson 2001,Palmstrom & Stille 2007 or Feng & Hudson 2011) andcan be summarized as

– Empirical methods based on experience fromcomparable projects;

– Closed form solutions;

– Numerical analyses; and– Combinations of the methods mentioned above.

Independent of the applied method, the most impor-tant aspect of the geotechnical design of undergroundstructures is the interaction between the ground and thestructure.This “behavior” can be described as the reac-tion of the ground to any change of the natural in situconditions due to construction works such as excava-tion, support installation or ground improvement. It isobvious that this behavior is dominated by the groundconditions such as rock mass parameters, ground wateror primary stress condition as well as by the exca-vation process, the type of support measures and theapplied installation process. It is the result of the com-plex interaction within the system ground, excavationand support and consequently a key element of anygeotechnical design.

In the following a systematic structure for thegeotechnical design of underground structures ispresented, including general design principles, rockmechanical aspects concerning failure mechanismsand ground behavior as well as aspects of risk man-agement. Additionally focus is set on the applicabilityof the geotechnical design during all design stagesincluding the construction phase of the undergroundstructure.

2 GROUND BEHAVIOR – BASIS FORGEOTECHNICAL DESIGN

The failure mechanism, or more general, the behavioris often the key element in engineering design pro-cedures. In structural engineering, for example, it istypical that different design approaches are appliedfor different loading conditions, which lead to dif-ferent modes of failure. In the design of reinforced

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concrete structures (CEN 2011) it is a matter ofcourse that different design processes and analysesare used depending on the loading, which causes dif-ferent failure mechanisms of the structural element.Consequently the design analyses are different for ten-sion, pressure or torsion due to the different modes offailure.

Also in soil and rock mechanics the potentialfailure mechanisms as well as the behavior of theground are used as the key factors for the develop-ment of geotechnical designs. In rock slope design,for example, it is state of the art to use differentdesign approaches depending on the expected slopefailure mode such as plane failure, wedge failure,circular failure or toppling. The applied models andanalyses are adapted to the expected slope behav-ior and different and independent design approachesare used.

Also for the design of underground structures thelogical engineering design approach is to predict thepotential behavior without support and then developmeasures to change or modify it to the requiredbehavior of the compound system ground-excavation-support.

The behavior of the ground varies significantlydepending on the ground properties such as rock,discontinuity or rock mass parameters but also pri-mary stress condition and ground water condition.It is important to recognize that different combina-tions of the parameters can lead to absolutely differentbehavior of the rock mass. For example, with theincrease of primary stresses the behavior of hard mas-sive rock could change from stable to spalling or rockburst. Fractured rock mass might show progressivegravitational failure in case of very low stresses andchange its behavior with increasing stresses to limitedgravitational failure (due to increasing confinementpressure), local shear failure and a kind of “plastic”behavior in case of very high stresses compared tothe rock mass strength. Additionally ground watermay change the behavior depending on pressure andquantity of inflow.

Besides ground properties also the geometry ofthe underground structure has a significant impactonto the behavior. The span of a tunnel or cav-ern or the geometry of the excavation has a majorinfluence on the development of failure modes. A rect-angular excavation, for example, shows more stressinduced failure than an excavation with a round pro-file and gravitational block failure is more probableand has larger volume if the excavation becomesbigger.

Such simple examples show that the behavior of anunderground excavation can change due to variationsof the ground and boundary conditions. This under-lines the importance of the geological and geotechni-cal investigation and prediction as another importantaspect of the geotechnical design by defining theprecision of the relevant design parameters. For theconstruction of a second tunnel tube of an infrastruc-ture tunnel for example, very detailed information is

available from the investigation and especially fromthe construction of the first tunnel. Besides the detailedgeological data from tunnel documentation also obser-vations of the actual behavior and the effectiveness ofsupport concepts are existing (for example MetsovoTunnel see Goricki & Rachaniotis 2011). On the otherhand only very limited data with high uncertaintiesmight be available in case of a deep tunnel in remotemountainous area, where field mapping or drilling isnot possible due to topographical reasons (for exampleRohtang tunnel see Reichenspurner 2013). This mightlead to basic differences in the structure and method-ology of a geotechnical design only due to the qualityand precision of the available input data.

For the sound development of a geotechnical designit is necessary to separate the underground structureinto sections with homogeneous or comparable geo-metrical and geotechnical conditions. Due to this splitinto smaller tasks it becomes possible to develop theground behavior of any underground structure, inde-pendent of its geometrical complexity or extension andindependent of the heterogeneity of the ground. Basedon the ground behavior, developed for each sectionwith comparable conditions, the geotechnical designcan be developed and excavation and support mea-sures can be designed. Considering the big varietyof possible parameter combinations and conditionsfor underground structures in general, it is obviousthat different design approaches in terms of modelsand analyses must be used to capture all the differ-ent possible behaviors and modes of failure. Thereforea geotechnical design procedure based on groundbehavior is required to prepare a framework for alldifferent tasks, which have to be performed duringa geotechnical design of an underground structure ingeneral.

3 GEOTECHNICAL DESIGN PROCEDURE –BASED ON GROUND BEHAVIOR

In the past various design procedures for undergroundstructures were developed (e.g. Hoek & Brown 1980,ITA 1988 or Bieniawski 1992), more detailed discus-sion and further developments of these methods aregiven for example in Goricki (2003) or Palmstrom &Stille (2007). In the following the “Guideline for thegeotechnical design of underground structures withconventional excavation” published by Austrian Soci-ety for Geomechanics (2010) is introduced briefly. Itconsists of two consistent design procedures, one forthe design phase and another for the adaptation of thedesign during the construction phase.

3.1 Geotechnical design during design phase

The design procedure consists of mainly 5 stepsstarting with the description of the basic geologicarchitecture and proceeds by defining geotechnicalrelevant key parameters for each ground type. The

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key parameter values and distributions are deter-mined from available information and/or estimatedwith engineering and geological judgment. The valuesare constantly updated as pertinent information isobtained. Ground Types are then defined accordingto their key parameters.

The second step involves the evaluation of thepotential ground behaviors considering each GroundType and local influencing factors, including the rel-ative orientation of relevant discontinuities to theexcavation, ground water conditions, stress situation,etc.This process results in the definition of project spe-cific Ground BehaviorTypes.The ground behavior hasto be evaluated for the full cross sectional area withoutconsidering any modifications including the excava-tion method or sequence and support or other auxiliarymeasures. Eleven general categories are listed in theguideline.

– Stable;– Potential of discontinuity controlled block fall;– Shallow failure;– Voluminous stress induced failure;– Rock burst;– Buckling;– Crown failure;– Raveling ground;– Flowing ground;– Swelling ground; and– Ground with frequently changing deformation

characteristics.

In case more than one Ground Behavior Type isidentified in one of the general categories, sub typescan be assigned. The Ground Behavior Types formthe basis for determining the excavation and supportmethods as well as assist in evaluating monitoring dataduring the excavation.

In the third to fifth step, different excavation andsupport measures are evaluated and acceptable meth-ods are determined based on the Ground BehaviorTypes. The System Behavior is a result of the inter-action between the ground behavior and the selectedexcavation and support schemes. The evaluated Sys-tem Behavior has to be compared to the definedrequirements. If the System Behavior does not complywith the requirements, the excavation and/or sup-port scheme has to be modified until compliance isobtained. It is emphasized, that different boundaryconditions or different requirements may lead to dif-ferent support and excavation methods for the sameGround Behavior Type even within one project.

In the sixth step, based on steps 1 through 5the alignment is divided into “homogeneous” regionswith similar excavation and support requirements. Aframework plan indicates the excavation and supportmethods available for each region, and contains lim-its and criteria for possible variations or modificationson site.

In the final step of the design process the geotech-nical design must be transformed into a cost and timeestimate for the tender process.

3.2 Geotechnical design adaptation duringconstruction phase

Due to the fact that in many cases the ground condi-tions cannot be defined with the required accuracyprior to construction, a continuous updating of thegeotechnical model and an adjustment of excavationand support to the actual ground conditions duringconstruction is required. The final determination ofexcavation methods, as well as support type and quan-tity is often only possible on site. In order to guaranteethe required safety, a safety management plan needsto be followed.

Step 1: To be able to determine the encounteredGround Type, the geological documentation dur-ing construction has to be targeted to collect andrecord the relevant parameters that have the greatestinfluence on the ground behavior. The geologi-cal and geotechnical data collected and evaluatedon site are the basis for the extrapolation andprediction of the ground conditions into a rep-resentative ground volume, which determines thebehavior.

Step 2: Based on the predicted ground conditions thesystem behavior in the section ahead has to beassessed and compared with the framework plan.Particular attention has to be paid on potentialfailure modes.

Step 3: To determine the appropriate excavation andsupport the criteria laid out in the frameworkplan have to be followed. Consequently, the actualground conditions continuously have to be com-pared to the prediction for compliance. Based onthe additional data obtained during constructionthe excavation and support methods are deter-mined to achieve economic and safe tunnel con-struction. The System Behavior is predicted forthe next excavation sections, considering groundconditions and the chosen construction measures.Both, excavation and support, to a major extent,have to be determined prior to the excavation.After the initial excavation only minor modifica-tions, like additional bolts, are possible. This factstresses the importance of a continuous short-termprediction.

Step 4: By monitoring the system behavior thecompliance with the requirements and criteriadefined in the geotechnical safety managementplan is checked. In case of differences betweenthe observed and predicted behavior occur, theparameters and criteria have to be reviewed. Whenthe displacements or support utilization are higherthan predicted, a detailed investigation into thereasons for the different System Behavior has tobe conducted, and if required improvement mea-sures (like increase of support) ordered. In case theSystem Behavior is more favorable than expected,the reasons have to be analyzed as well, andthe findings shall be used to modify the designaccordingly.

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Figure 1. Interaction between risk, measures and costs.

4 GEOTECHNCIAL DESIGN AND RISKMANAGEMENT

4.1 General

The main goal of a geotechnical design is the defi-nition of measures to construct a safe and economicunderground structure. Due to the uncertainties of theground, the risks related to construction methods andthe usual demand for cost reduction also the results ofthe geotechnical design are affected by uncertaintiesand risks. To deal with these risks it is proposed toapply the principles of risk management and combineit with the typical procedures of geotechnical design aspresented above (Goricki et al. 2002, Schubert 2011).For such approach it is important to clearly differen-tiate between basic condition, measures and effect ofmeasures.

In the design of underground structures the behav-ior of the ground is evaluated and, if the predictedground behavior is not acceptable, e.g. in case of pre-dicted ground instabilities, measures such as bolting orshotcreting are designed to achieve an acceptable andstable behavior. In terms of risk management, a notacceptable risk was reduced due to measures to fulfillthe risk criteria and became acceptable. Additionallya pre-defined factor of safety is usually consideredin engineering designs, which also influences the risklevel.

The interaction between risk, measures and costs isshown quantitatively in Figure 1. Without any mea-sures the probability for occurrence of damages ishighest and might lead to catastrophic conditions. Ifintensive measures are implemented, the risk decreasesto a minimum but the costs for the measures willincrease significantly and might lead to unreasonablehigh costs. With a decrease of the risk due to mea-sures the costs for possible damages decrease, whilethe costs for measures develop opposite to the costs ofrisk. At a certain point the total costs, calculated as thesum of costs from possible damages and costs for mit-igation measures, show a minimum. Independent tothese minimum total costs an “acceptable mitigationthreshold” is introduced, which defines the balance

between acceptable residual risk (1) and the necessarycosts (2) to align this residual risk. In a technical designthis threshold can also be described as the minimumdesign criteria. Additionally any kind of uncertaintiesor a factor of safety can be considered by movingthe threshold, which influences the risk as well as themeasures and their costs. Besides this defined min-imum safety or maximum risk level also definitionsconcerning the costs can set the boundary conditions.

By applying the principles of risk managementto geotechnical designs it can easily be seen thatthe design of excavation and support to control theground behavior follows these correlations in general.If a ground, for example, has a potential for gravi-tational discontinuity controlled over break of rockblocks, the design of an underground structure can bedeveloped with various options. Without any rock sup-port progressive failure with systematic voluminousover break will occur, which requires intensive andcostly repair works (unacceptable risk without costsfor measures). In case of forepoling with spiles theapplied support measures are generally cheap and onlyminor over break occurs, which leads to a well bal-ance between risk reduction and costs for measures(acceptable risk with some costs for measures).

In a third scenario the excavation is supportedwith heavy pipe umbrella forepoling with high costsand slow progress, which results in cero potentialfor overbreak (acceptable risk with high costs). Thissimple example shows that the application of risk man-agement to geotechnical design does fully complywith the design approach based on ground behav-ior and additionally supports the engineering designdecisions.

4.2 Risk management according to ISO 31000

The ISO 31000 (International Organization for Stan-dardization 2009) provides a generic approach in termsof principles and guidelines for managing any form ofrisk in a systematic and logical process. Risk is definedas the uncertainty on objects, which is often expressedin term of a combination of consequences of an eventand the associated likelihood of occurrence. Risk man-agement is defined as coordinated activities to directand control something with regard to risk. The pro-cess of risk management as an integrative part of themanagement consists mainly of systematic applica-tion of management policies, procedures and practicesto activities of communicating, consulting, establish-ing the context, and identifying, analyzing, evaluating,treating, monitoring and reviewing risk.

Figure 2 shows the process risk management withits main elements and their interaction. In the begin-ning the overall context including objective, internaland external influencing parameters, boundary condi-tions, strategies and risk criteria shall be defined. Thenthe risks are assessed by using tools of risk identifi-cation, risk analysis and risk evaluation. Based on thisunderstanding the risk treatment is developed, whichinvolves the selecting and implementing of one or

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Figure 2. General risk management process; from ISO31000.

more option for modifying the risks. The risk treat-ment is a cyclic process of developing a treatment,assessing a treatment, deciding about acceptable resid-ual risk levels and checking the effectiveness of thattreatment by comparing with risk level. The risk treat-ment options can include avoiding a risk, taking orincreasing a risk, removing a risk source, changingthe likelihood, changing the consequences, sharing arisk or retaining a risk. A risk treatment plan describesthe implementation of the chosen treatment options.

Finally, both monitoring and review should beplanned as part of the risk management process andshall ensure

– control in design and operation;– obtain further information to improve risk assess-

ment;– analyzing and learning lessons from events,

changes, trends, successes and failures;– detection of changes in the context including risk

criteria or risk itself; and– identifying emerging risks.

4.3 Geotechnical design as risk managementprocedure

By comparing the geotechnical design procedure asdescribed in chapter 3 with the risk managementprocedure from ISO 31000 it can be seen, that thegeotechnical design procedure does generally containall relevant elements of a risk management process.

Figure 3 shows the procedure of geotechnicaldesign in the center and the main elements of riskmanagement at the sides in dark beams. In the designstage the description of the ground conditions and theevaluation of the ground behavior covers the elementsof risk identification, risk analysis and risk evaluationwhich can be summarized as risk assessment. Basedon the results of this risk assessment the constructionconcept, the excavation and the support is designed ina way that the behavior fulfills the previously definedrequirement.

Figure 3. Integration of the geotechnical design procedure(as defined in chapter 3) in the risk management process asdefined in ISO 31000.

These requirements are risk criteria and the pro-cess of design and check of the modified behaviorequals the process of risk treatment. During construc-tion many input data but also the behavior of theground with measures can be observed and interactiveprocess of monitoring and reviewing must be executed.

4.4 Monitoring and review

A key element in the geotechnical design as wellas in the risk management is the monitoring andreview process. It allows to verify the success ofthe applied measures and if necessary to modify themeasures to gain the expected result. In geotechnicaldesign there are two main sources for uncertainties, thecharacterization of the ground and the geotechnicaldesign models for behavior without and with mea-sures. The monitoring of the ground conditions duringconstruction (geological and geotechnical documen-tation) allows the observation of the actual conditionand usually leads to an increase of the ground datain quantity and quality. Due to this a verification ofthe predicted ground conditions can be done. Addi-tionally the behavior of the ground influenced by theapplied measures can be observed and compared withthe predicted behavior of the system ground withmeasures.

The difficulties to characterize ground materialsand the complexity of modeling underground exca-vations and support measures might lead to an actualbehavior deviating from the predicted. In such a caseit is essential to modify the measures once definedaccording to the actual ground conditions, behaviorand construction specific conditions. This must bedone on site and requires a clear structure and full

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implementation of a design representative on site asdescribed in general in the risk management process.

The step of monitoring and review is not limitedto the construction phase. Also during the differentdesign phases the design decisions have to be reviewedin case of additional data or more detailed informationis available for example from additional investigation.

During monitoring and reviewing it is again essen-tial to focus on the basic failure modes, the behaviorof the ground. If the variations in ground conditionschange the behavior of the underground excavation,for example due to higher primary stresses or due towater inflows, the measures have to be adapted to thenew potential failure modes. By, for example chang-ing the bolting according to the stress orientation orby locally lowering the water pressure, stable condi-tions can be developed more successful than by simplyadding unspecific support.

The elementary demand for a geotechnical designis to adapt the relevant measures to the actual groundcondition and behavior and hence control the groundrelated risks continuously during all project phases.Consequently the geotechnical design is the contin-uous evaluation of ground behavior (check of risks),the design of a proper excavation and support system(definition of measures and check of risk reduction)and the comparison with design criteria and boundaryconditions to reach a balance between required safetylevel and costs.

5 TYPICAL GROUND BEHAVIOR ANDENGINEERING METHODS

5.1 General

In the chapters above the importance of the groundbehavior as basis for the geotechnical design of under-ground structures is pointed out. In the following themost important types of ground behavior are describedand engineering methods for evaluation, analysis anddesign of measures are discussed briefly.As the groundbehavior is highly depending on the project spe-cific ground and boundary conditions, the descriptionsbelow shall be used as a general guidance for a designengineer.

The classification into different behavior types mustbe done project specific depending on the detailing ofthe design due to the heterogeneity of and the knowl-edge about the underground conditions as well as dueto the project phase, the importance of the project andthe costs of the underground structure. Most impor-tant for the grouping into ground behavior types is theconsideration of the potential soil or rock mechani-cal failure mechanisms. Consequently, basic behaviortypes can be defined, which create the basis for theproject specific work. In the past various authors havedeveloped grouping or classification of behavior typesfor underground structures (e.g. Hoek et al. 1995,Goricki 2003, Palmstrom & Stille 2007).

In the following a classification of the main behav-ior types by considering the potential failure modes

due to gravity, stress, water and swelling is given. Eachtype of ground behavior is discussed briefly includingthe basic mode of failure, the important parameters andthe typical methods for design and analysis with andwithout measures. The excavation and support mea-sures itself are not discussed in this context. Due to thisit is important to understand that the modes of failuresare theoretical and will be avoided by the design andinstallation of the proper excavation and support sys-tem. Consequently the theoretical modes of failure –the plain ground behavior – cannot be observed duringconstruction.

The models used for analyses, evaluation of thebehavior and design of measures are not limited toany method but must be applicable for the problem,the behavior and the mode of failure respectively.Therefore also empirical methods can be used if theexperience is comparable with the geometrical, groundand boundary conditions. Additionally it is requiredthat the data are collected from technically successfuland economically traceable projects during the designand construction phases.

5.2 Gravity induced behavior – discontinuitycontrolled blocks

Mechanism: gravity induced falling, sliding or rotat-ing of blocks into the excavation, along discontinuitieswith potential for local shear failure. The kinematicfreedom and the exceeding of the tension and shearstrength along the discontinuities are the basic prereq-uisite for this behavior (Goodman & Shi 1985, Pötsch2011). The blocks can fail locally or systematicallywith various depths and volumes.

Important parameters: number, orientation and dis-tance of discontinuities or degree of fracturing; wavi-ness, roughness persistence, aperture and fillings ofdiscontinuities or tension and shear strength in gen-eral; strength and deformability of the rock material,water pressure, primary stress conditions. Addition-ally the excavation geometry and the secondary stresscondition around the excavation do also have signif-icant importance especially for the estimation of theover break volume.

Methods for design and analysis: for the evaluationof this behavior kinematic and mechanical models areused. Depending on the project specific problem onlykinematic models, kinematic models in combinationwith mechanical methods, or 2D and 3D numericaldistinct element methods are used. Depending on theavailable ground data and the geometry of the under-ground structure a systematic (e.g. for a tunnel) ora discrete (e.g. for a cavern) excavation and supportsystem can be developed.

Comment: the behavior of gravity induced blockfailure is generally very sensitive to the discontinu-ity properties and the stress condition. Due to this theknowledge about the ground conditions has a signifi-cant impact onto the predictability of the behavior. Incase of shallow underground structures it might be pos-sible to use a distinct model while for deep tunnels only

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Figure 4. Gravity induced, discontinuity controlled blockfailure in a tunnel; photo by W. Schubert.

Figure 5. Raveling of highly fractured rock mass(Peridotite/Serpentinite) in the beginning stage of a pro-gressive failure after local instabilities during works at thebench/side wall; photo by Goricki.

a random prediction of this behavior might be used andfinal support decisions depend on the observations ofthe ground conditions during construction.

5.3 Gravity induced behavior – raveling ground

Mechanism: gravity induced raveling and falling ofgenerally dry soil material or highly fractured andpoorly interlocked rock mass (e.g. fault material) intothe excavation; the ground has no or low cohesion andthe failure occurs usually progressively.

Important parameters: degree of fracturing, inter-locking, grain size distribution, discontinuity fillings,

cohesion or tension and shear strength in general;primary stress condition.

Methods for design and analysis: the mechanismmainly depends on the tension between the particlesand consequently on the parameter cohesion, in caseof applying Mohr-Coulomb failure criterion. Espe-cially in soil mechanics many analytical models havebeen developed, from simple comparison of cohesionwith free span or height of excavation surfaces tosophisticated numerical DEM models.

Comment: The exact prediction of the limit stateof raveling (detaching of a particle) is generally dif-ficult but tendencies and scenarios can be developedfrom the models as well as from observations duringsite investigation such as collapse of drill holes or col-lapse of exploratory pit side walls. In case of fracturedrock mass the limit between the behaviors of grav-ity induced falling of discontinuity controlled blocksand gravity induced raveling of particles is difficult todefine. The main difference could be described by theinfluence of bolting, if distinct bolting concepts canavoid failure or not.

5.4 Stress induced behavior

Mechanism: the loading of the rock mass due to sec-ondary stresses around the underground excavationexceeds the rock mass strength.This leads to the devel-opment of fractures along discontinuities and throughintact rock. Depending on the stress condition and therock mass properties the stress induced failures canoccur differently from plastic to brittle.

Important parameters: deformation and strengthparameters of intact rock, discontinuities and rockmass; the significance of the parameters highlydepends on the detailed mode of stress induced failure.

Methods for design and analysis: closed formsolutions and numerical models, which describe thestrain/stress condition, the utilization of the groundsurrounding the excavation or more detailed fracturepropagation are the typical methods for modeling.Therefore it is important that the relevant groundparameters are considered and that the applied modelcan reproduce or represent the potential failure mech-anism. Anisotropic ground properties for example ordistinct zones of weakness must be considered in theapplied model if the behavior of the ground is triggeredby these influences (Goricki et al. 2005).

As mentioned above, different modes of stressinduced failure may propagate due to different com-binations of ground parameters, stress conditions andexcavation geometry. In the following typical types ofstress induced failure are discussed briefly.

5.4.1 Stress induced behavior – shear failure in lowstress environment

Even with generally low stress magnitudes the load-ing of the rock mass exceeds the rock mass strength.Especially due to low confinement stresses (e.g. incase of shallow overburden) progressive shear failureslead to voluminous over break in the crown area and

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Figure 6. Day-lighting shear failure of a tunnel in rock mass(weathered gneiss) with shallow overburden, upper photofrom surface, lower photo from tunnel; photos by Goricki.

chimney like roof failures. Figure 6 shows the result ofa progressive shear failure, which developed up to theground surface. The failure occurred during the benchexcavation in weathered gneiss material with an over-burden of approx. 5 m plus a 1.5 m thick upper soillayer.

This mode of failure can usually be observed in soilmaterial with shallow overburden but can also prop-agate in highly fracture or weathered rock mass or inrock mass with steeply dipping discontinuities and lowshear strength.

5.4.2 Stress induced behavior – shear failure inhigh stress environment

The loading of the rock mass exceeds the rock massstrength and the rock mass is significantly utilized.Due to the high stress level and the triaxial loading con-dition around the excavation shear failures develop.Figure 7 shows the development of a discrete shearfailure at the side wall of a tunnel top heading. Withincreasing confinement pressure the shear fracturesbecome more distributed in the rock mass, which leadsto “plastic behavior” (Jaeger et al. 2007) in combi-nation with large and uniform displacements of theexcavation surfaces. The anisotropic properties of therock mass have a significant influence on the develop-ment of the behavior and must be considered properlyin the model.

Modeling of this type of behavior was published byvarious authors in the past (e.g. Schubert 1996, Gorickiet al. 2006, Anagnostou & Cantanieni 2007, Radoncicet al. 2009, Hoek & Marinos 2009, Barla et al. 2010).

Figure 7. Shear failure at the right top heading side wall,temporary stabilization with tree trunks; photo by G. Feder.

Figure 8. Rock burst in deep tunnel in massive rock mass;from (Ortlepp 2000).

5.4.3 Stress induced behavior – brittle failure inhigh stress environment

The loading of hard, massive and brittle rock massexceeds the high rock or rock mass strength. The brit-tle failure develops close to the excavation surfacedue to the generally uniaxial loading condition in thisarea. The failure, which mainly depends on the rockmass properties, the stress level and the orientationof the primary stresses can propagate within a widevariety from local spalling to violent rock burst. Thefractures develop parallel to the principle stress andcreate thin rock plates. Figure 8 shows the result ofa rock burst of a small tunnel in massive rock mass.The modeling of this behavior can either be done byevaluation of relevant rock mass parameter (such asuniaxial compression strength, post failure behavioror elastic parameters) in combination with the primaryand secondary stress condition or by applying numer-ical models with appropriate failure criteria. Differentmodels and approaches are published concerning aproper and realistic analysis of this type of behavior(e.g. Hoek et al. 1995, Kaiser et al. 1996, Martin &Christiansson 2009, Kaiser 2010).

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Figure 9. shows a typical failure of a tunnel floor due toswelling.

5.5 Swelling ground

Mechanism: Swelling is the volumetric increase ofrock or rock mass due to chemical processes in com-bination with water. Swelling pressure can developif the volumetric strains are kinematically limited.The behavior is time dependent and dominated bythe interaction between water, rock mass mineralogyand the stress environment. Considering equilibriumthe swelling process can start by adding water and/orreducing the stress level.

Important parameters: mineralogy of the rock androck mass, water, permeability, primary and secondarystress condition.

Methods for design and analysis: One method is todetermine the swelling pressure based on ground con-dition, mineralogical condition and specific laboratorytests (ISRM 1999) in combination with the potentialinfiltration of water. These loads can then be used fortypical design methods such as closed form solutionsor numerical analysis. Another method is the directimplementation of the swelling behavior in the anal-ysis by using specifically developed constitutive lawsfor example in combination with numerical models.The mechanism and models are discussed in litera-ture such as Einstein (1996), Wittke-Gattermann &Wittke (2004), Rauh (2009), Anagnostou et al. (2010)or Steiner et al. (2010).

Comment: Swelling, especially anhydrite swellingcan lead to large deformations of the ground in com-bination with high stresses on underground structures.The timely development can continue through yearsand even small variations of the environment canagain change the behavior. To handle such behav-ior two basically different design approaches, stiffand ductile, were developed. The decisions aboutthe support system as well as the applied designmodels must be done project specific based onengineering understanding of the potential swellingbehavior, especially considering the significant impactof measures onto the development of the behavioritself.

Figure 10. Flowing ground in a tunnel due to intensive waterinflow; reference unknown.

5.6 Flowing ground

Mechanism: flowing of intensively fractured rockmass or soil material into the excavation due to highwater content. The initial water pressure is higher thanthe interlocking, cohesion or bonding strength of thesoil and rock mass particles. The failure occurs usuallyprogressively.

Important parameters: water pressure and quan-tity, permeability, degree of fracturing, interlocking,grain size distribution, cohesion or tension and shearstrength.

Methods for design and analysis: A simple methodis to compare the tensile strength or cohesion of therock mass with the predicted ground water condi-tions. Such simple evaluation is often sufficient inrock tunneling to decide about measures to reduce theground water inflow or pressure or to increase the rockmass strength. More detailed models are developed insoil mechanics especially in combination with shallowurban tunneling.

Comment: In hard and fractured rock mass waterinflows, independent to quantities and quantities dooften only affect the construction progress but notcreate significant ground failure. Such water inflowswithout influence on the excavation stability are notdiscussed in this context as the ground can be classifiedas stable with mainly contractual impact.

6 IMPLEMENTATION OF BEHAVIOR BASEDDESIGN IN CONSTRUCTION PROCESS

In the following it is assumed that an undergroundstructure in natural ground is under excavation at anyrandom position within the project.

In the design phase construction measures includ-ing construction method, process and materials weredefined. Additionally a prediction of the behavior ofthe structure and the surrounding environment wasprepared. This includes the stability as well as the ser-viceability and covers for example loading, utilizationand strain of structural elements as well as under-ground and surface displacement, maximum waterinflows and more. Additionally the monitoring system

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was defined in the design and expected as well as alertand maximum allowable values were quantified.

Before the excavation of the next cycle, a decisionabout the final layout of excavation and support sys-tem need to be done. Therefore some fundamentalworks concerning the update of the design to the siteconditions are necessary.

In a first step a geological and geotechnical progno-sis for the next few excavation meters needs to be done.This prognosis is an update of the existing design dueto the information gained from the already preformedexcavation and/or from additional investigations e.g.ahead of the tunnel face. It has to include geologi-cal conditions, ground parameters, stress and waterconditions as well as a prediction of the theoreticalground behavior due to excavation without any support(potential failure mechanism).This short term progno-sis can be similar, slightly or significantly different tothe descriptions in the original design.

In a second step the applicability of the design forthe ground as described in the short term predictionmust be verified. Therefore the basic design assump-tions including ground conditions, tunnel geometryand boundary conditions such as maximum surfacedisplacements or lowering of ground water table mustbe checked.

In a third step observations of the ground behaviorfrom already constructed tunnel section in comparableconditions are evaluated, if available.The data from theobservations can be used to re-evaluate the potentialfailure modes of the ground and the predicted behav-ior as the effects of the interaction between ground,excavation and support.

In the fourth step, the data as described in the previ-ous steps, the updated short term prediction, the designassumptions for this particular section and the expe-rience gained from previous observations, are used toevaluate the original design. In case of deviations thegeotechnical design for the next excavation must beadapted, modified or maybe entirely re-designed.

Any change of the geotechnical design must fulfillthe same criteria as applied during the design phaseincluding analysis in case of significant changes.Addi-tionally a detailed description of the expected behaviordue to the changes must be given. The design changesmust be done by qualified engineers who are aware ofthe design as well as in the construction. Small adap-tations and modifications must be part of the routinework on site while sever design changes might be doneby an additional design team ideally supervised by anonsite design engineer.

The above described decision making process mustbe fully integrated part of the project risk manage-ment process. This guarantees that the design and itsadaptation to the actual ground conditions is alwaysintegrative part of the on-site decisions and that thebasic focus of the design always remains the mitiga-tion of unacceptable risks to acceptable conditions byapplication of measures.

During excavation the actual ground conditionsare compared with the prediction and, in case of

deviations, modifications can immediately be devel-oped by following the procedure as described abovestarting with step 1 for the actual excavation round.

After construction the observation and evaluationof the behavior of the ground and the structural ele-ments continues. The experience shall be used for theoptimization of future construction works as describedin step three. In case of deviations from the predictedbehavior an evaluation must be done and, if requiredmeasures must be developed based on the principlesof project risk management.

7 CONCLUSIONS

The geotechnical design of underground structuresmainly deals with the interaction between the groundand the structure in the different construction stages. Itoften has to deal with difficult and complex conditionsdue to differences and uncertainties within the ground,the structures and the boundary conditions. In order todevelop an economic and safe underground structureit is essential to develop a sound geotechnical designduring all project phases.

The behavior of the ground is the key elementfor the development of the geotechnical design. Withthe understanding of the interaction between groundand construction works including mechanisms andpotential failure modes, a proper design and modelingtechniques can be applied. Depending on the specificconditions any engineering method representing thestate of the art in soil and rock mechanics as wellas in structural mechanics can be used to investigatethe mechanisms and develop the required geotechnicalmeasures concerning excavation and support.

For a systematic development of the geotechnicaldesign during all project phases, including construc-tion, it is necessary to follow a general procedure,which allows systematic adaptation of the design tothe different levels of information. It is proposed touse the process of risk management as described inISO 31000, which provides a comprehensive frame-work for the development of the geotechnical design.By considering the potential modes of ground failureas risks and the designed ground support as mitigationmeasures, the actual risks of underground structure canbe evaluated and controlled in any project phase. Espe-cially the adaptation of the geotechnical design duringthe construction phase due to possible changes ofthe ground conditions, collection of additional grounddata and the observation of the actual behavior canbe developed very well within the risk managementprocess.

The proposed methodology provides a consistentdesign approach for the development, adaptation andapplication of the geotechnical design in any projectphase. Based on the predicted ground behavior theproper engineering design method and tool can beselected and within the framework of the risk manage-ment process the geotechnical design can be developed

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independent of the difficulty of the ground conditionsor the complexity of the underground structure. Thedesign modifications, due to additionally gained infor-mation, become an integrative part of the decisionmaking process on-site independent to the design andconstruction method.

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