recommendations on how gbr can be prepared for rock tunnel projects

8/10/2019 Recommendations on How GBR Can Be Prepared for Rock Tunnel Projects

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343

RECOMMENDATIONS ON HOW GEOTECHNICALBASELINE REPORTS CAN BE PREPARED FOR ROCK

TUNNEL PROJECTS

Paul Heslop Jacobs Engineering

Christopher Caruso Jacobs Associates

ABSTRACTThe issues associated with using Geotechnical Baseline Reports on tunneling projectshave been the subject of extensive debate for several years. This paper attempts tosummarize these issues and it presents research to show that despite the ongoingdebate, there appears to have been limited changes or improvements made to addressthese problems over the last 5 to 10 years. The paper then provides recommendationsto help mitigate some of the issues identi ed, which include recommending changes towhat baselines should be used and how they should be presented. In particular it rec-ommends only developing baselines that directly address speci c aspects of groundbehavior and/or other possible claims, as opposed to simply providing baselines forindividual rock properties, which are often inconsistent, open to misinterpretation orhave no direct relevance to a potential claim.

INTRODUCTIONGeotechnical Baseline Reports are commonly used on large tunneling projects to helpmitigate the risks associated with unforeseen ground conditions. However, despitebeing commonly used there is still an extensive and ongoing debate regarding theireffectiveness and there are still many perceived problems associated with using them.

The intent of this paper is to provide recommendation on how GBRs for rock tun-nel projects can be improved. In the rst part of the paper we identify and discuss theperceived problems with using GBRs. We then investigate and identify the causes ofthese problems before providing recommendations on how these problems and issuescan be mitigated. The paper focuses speci cally on hard rock tunneling projects, how -ever many of the issues identi ed and recommendations provided are considered tobe equally applicable to other ground conditions. It is hoped that this research will sub-sequently help to improve the effectiveness of future Geotechnical Baseline Reports.

BACKGROUND AND RESEARCHIn preparing this paper we wanted to understand why GBRs were not being moreeffectively in helping to reduce claims and mitigate risks on many of the recent highpro le tunnel projects within the US. As a basic concept GBRs should work well, so wewanted to understand how they could be improved and what are the type and extentof the problems being encountered when using GBRs. We started our research by

undertaking an extensive literature review of technical papers and magazine articles,all published over the last 5 years, which discussed the various problems with usingGBRs. This allowed us to compile a list of the perceived problems associated withGBRs as shown in Table 1. In the industry, a wide range of problems have developedand we discuss the reasons for and extent of these problems in more detail below.


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344 Geotechnical Considerations

Table 1. Summary of problems associated with geotechnical baseline reports for rocktunnel projects

Problem Discussion

C o n c e p

t u a

l I s s u e s

1 Is a GBR a risk transfer or risk shar-

ing tool?

Many believe GBRs are not used to help

share risk as intended, but are used to transferownership of the ground risk from the owner tothe contractor.

2 GBRs only baseline individual rockproperties they do not baselineground behavior or constructionissues .

GBRs often only provide baselines for individ-ual rock properties and they do not speci callyaddress construction or design issues.

3 The GBR is often not consistent withthe rest of the Contract documents.

GBRs should be consistent with speci -cations, drawings and any other contractdocumentation.

B a s e

l i n e

D e

f n

i t i o n

I s s u e s

1 It is often unclear what statementsin a GBR can be relied upon as abaseline.

Extensive interpretation is often provided (i.e.,geological sections and/or factual tables) as

Appendices and it is unclear if these form partof the baseline.

2 GBRs do not provide baselinesthat are relevant and they do notadequately describe the condi-tions to be expected. Many GBRsalso baseline an extensive array ofgeotechnical parameters, includingmany baselines that are not relevantto tunneling.

While this appears to represent a comprehen-sive GBR which should protect the Owners,experience shows it often provides more prob-lems and chance of errors, contradiction ormisinterpretation by the Contractor (see Table2 for more information).

3 Baselines often con ict with otherbaselines or with other informationprovided in the report.

This can occur when multiple baselines areprovided that address the same or similarissues, such as providing Q, RQD and RMRvalues to describe the same rock type.

4 GBRs often include overly conser-vative baselines and/or baselinesthat are not consistent with the siteinvestigation data.

GBRs that present conditions that are moreadverse, arbitrary and/or unrealistic oftenare perceived as an attempt by the Owner tounfairly transfer risk to the Contractor. Lists ofrelevant rock tunnel baselines are shown inTable 5 and Table 6.

5 Baseline statements are often inde -nite, ambiguous or qualitative .

GBR statements often use the terms like may,could and possible, which make it dif cult forthe Contractor to rely upon the statements.

6 The assumptions used in develop-ing many baselines are often notprovided, leading to uncertainty andmisinterpretation.

For example, many ground classi cationschemes such as the Q System (Barton, 1974)require certain assumptions to be made,. i.e.,the selection of an appropriate SRF value.However, this is often not provided with thebaseline, leaving it open to debate later.

7 How should a baseline be quanti edor presented ? Consultants tend toprovide a full range of possibilities.i.e., maximum, minimum, averages,ranges and graphical techniquessuch as contouring.

Providing multiple baselines for the sameproperty leads to confusion and uncertaintyregarding what constitutes a change in abaseline. For example, if a single UCS value atone location is outside of the speci ed rangebut the average of all samples is close to theaverage, is this different?

(table continues)


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How Geotechnical Baseline Reports Can Be Prepared 345

In addition to this literature review we then independently reviewed twenty- ve (25)recent GBRs that were prepared for large rock tunnel projects within the US over thelast 10 year. We also reviewed a number of GBRs from international tunneling projectsin order to compare and identify any differences. In reviewing these GBRs we wantedto see how effective they were in describing the anticipated ground conditions. We alsowanted to understand what baselines are typically being provided, how relevant thesebaselines are, and how they are typically being presented. This review work allowed usto verify the extent of the problems which were identi ed in our literature review. It wasnecessary to employ a certain degree of judgment in attempting to quantify how effec-tive these baselines, as we were not involved in preparing these reports ourselves. Inselecting GBRs to review, we sought reports prepared by a wide range of consultantsand clients from various geographical locations across the US, to avoid any bias andto get a representative picture of how GBRs for rock tunnels are currently being pre-pared. Based on our research, we have reduced the problems with GBRs into threebasic types.

DISCUSSIONProblems Associated with How the Baselines Are Defined and PresentedOur research shows that a common problem with GBRs is that many of the baselinesprovided are poorly de ned. The baselines are not clearly presented; they can often

be ambiguous, overly conservative, and irrelevant, they can also be inconsistent orcon ict with other contract documentation such as drawings or speci cations. Figure 1was prepared following our review of existing GBRs and helps to show the signi canceof these problems.

Figure 1 shows a list of baselines that were provided in the Rock Tunnel GBRswe reviewed; it also shows the frequency with which these baselines occurred. It alsoshows for several of the key baselines (i.e., UCS and Joint Orientation) how realisticor useful the baselines were. We can con rm that baselines are often presented withvery wide ranges, which make them of limited use in practice. Our research showsthat many of the key rock properties for design and construction are also not always

provided; in fact we found that very few GBRs reviewed provided a comprehensive listof relevant baselines. For example Q values, which are a key design and constructionparameter for describing rock quality, were only provided in fewer than 25% of GBRsreviewed. In Table 2 we show the relevance and importance of these baselines in termsof design and construction issues.

Our research also highlighted that the way in which baselines are presented isextremely important in helping to reduce ambiguity or contradiction. Baseline valuescan be presented in a variety of different ways including maximum, minimum and

Problem Discussion

P r a c

t i c a

l I m p

l e m e n

t a t i o n 1 How should the baselines be

evaluated ?If baselines are exceeded, there is often noguidance given in the GBR as to what shouldbe done and what the Contractor can be com-pensated for, i.e., direct or in-direct costs.

2 How should the baselines bemeasured and veri ed duringconstruction?

The baselines should be presented in a waythat are easily measurable during construction,considering the proposed means and methods.

Table 1. Summary of problems associated with geotechnical baseline reports for rocktunnel projects (continued)


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Figure 1. Distribution of baseline properties in geotechnical baseline reports reviewed

average values; they can also be de ned using ranges and/or even graphically usinggraphs, geological sections, or contours. The problem with using multiple approachesis that several baselines are provided for the same property, often leading to confu-sion. Our research also highlighted the importance in a GBR to clearly identify whatstatements are baselines and what statements are not. This is especially important ifsections including interpretation or discussion based on previous tunnel experienceare provided. We found only 36% of the GBRs reviewed clearly identi ed and de nedwhat were contract baselines. We also found that only 48% of reviewed GBRs pro-vided any type of glossary to help de ne the terms used in the report and only 29% of


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Table 2. Relevance of geotechnical properties for design and construction


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GBRs provided any information on what assumptions or classi cations had been usedin developing the baselines.

Problems Associated with the Practical Implementation of the Geotechnical

Baselines During Construction A commonly overlooked yet vitally important part of any GBR should be a discussionon how baselines should be measured and evaluated during construction. This is acommon problem encountered when using GBRs in practice. Only 16% of the GBRsreviewed discussed how baselines should be measured during construction and only20% discussed any allowable tolerances to the baselines.

For example, consider two baselines commonly provided for rock tunnels, the UCSof the rock and the amount of rock cover above the tunnel. In practice, how are UCSvalues to be measured in the event that a differing site condition exists related to rockstrength? In order to justify the rock strength, are additional borehole and core samples

required to be taken along the alignment and, if so, when, where, and how many testsare needed? If point load testing can be used on collected representative rock samples,what correlations should be used to determine equivalent UCS values? In order to ver -ify or demonstrate changes in rock cover, are additional boreholes required? If probehole data can be used, then when, where and how many probes should be used?

Problems Associated with the Concept or Intent of the GeotechnicalBaseline ReportThe principal purpose of a GBR as de ned in the UTRC Geotechnical Baseline Reportsfor Underground Construction (1997), is to set baselines for geotechnical conditionsanticipated to be encountered during underground and subsurface construction, inorder to provide clear indications in the contract for resolution of disputes concerningsubsurface conditions.

GBRs are needed because there needs to be a fair way to manage the groundrisks, especially for design build projects. Traditionally, on these projects the Ownersessentially pass on the ground risks to the Contractor, who relies on a contingency tohelp mitigate these risks. However, due to the competitive nature of these contractsContractors often nd themselves with insuf cient contingencies and are unable tocomplete these projects if they incur signi cant cost increases resulting from anychanges in the ground conditions.

A commonly reported problem is that GBRs are often used as a risk transfer toolas opposed to a risk management tool. This typically manifests itself through the useof overly conservative and/or unrealistic baselines. Based on our research, it is clear tosee that this is still true. As illustrated in Figure 1 many of the most common baselinesprovided, such as the UCS, Cerchar Abrasivity, RQD, hydraulic conductivity, and jointorientation, have often used, in our opinion, conservative and/or unrealistic ranges forthe baselines. We believe this is the single biggest problem with using GBRs by far.There needs to be a greater effort in getting all parties to understand the importance ofapproaching GBRs in a fair and reasonable way. It takes all parties to understand theirroles and responsibilities, otherwise the concept and approach is destined for failure. If

GBRs are not prepared properly, it is our opinion that a bad GBR is worse than havingno GBR at all. Another conceptual issue raised is the lack of focus on design and construction

issues when it comes to developing baselines. Our research shows that there is still atendency to focus on describing and providing baselines only for individual rock prop-erties. For example GBRs typically provide baselines for rock permeability and notgroundwater in ow, or rock quality and not initial support requirements, or rock strengthand not cutter wear. Figure 1 shows that this is still true; there is still a tendency to focus


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on rock properties that are obtained from the site investigation as opposed to speci cconstruction or design issues.

RECOMMENDATIONS FOR FUTURE GBRsIn 1997 the UTRC report stated the following:

Improvements are needed to overcome the following shortcomings in contractual geo-technical interpretative reports:

Baselines may not adequately describe the conditions to be expected. Baseline statements are often inde nite, too broad, ambiguous or qualitative,

resulting in disputes over what was indicated in the contract. Baselines may present conditions that are more adverse than indicated by the

data, or just plain arbitrary and unrealistic, without discussion or explanation

for such apparent discrepancies. Discussion often repeats material on drawings or speci cations Baseline statements are sometimes in con ict with the drawings or

speci cations The effects of means and methods of construction on ground behavior are not

well described

Based on our research these recommendations from 16 years ago are still truetoday and this demonstrates that as an industry there is still scope for improvement in

how we prepare and use GBRs. The problems with GBRs highlighted in the rst partof this paper are associated with a variety of different reasons, including the generalapproach and intended use (or misuse) of these reports, the way in which the data ispresented and the way in which the baselines are measured and used in practice. Wehave provided recommendations on how these problems can be addressed; these aresummarized in Table 3 and discussed in more detail below.

In terms of solving the conceptual problems there is no simple x, this simplyrequires a change in mindset and approach and an acceptance that all parties involvedneed to play their part in making GBRs work. If the Owner, Contractor or Consultantsdo not work openly with a spirit of fairness, GBRs will continue to be of limited use.Consultants should work with Owners to develop meaningful baselines and Ownersshould understand that by providing realistic baselines they are in fact reducing thecost and eliminating the need for contingencies. If different ground conditions areencountered later this should not be seen as error but as an adjustment to what is thetrue cost of the project. Consultants should present baselines that re ect their ownunderstanding of the expected ground conditions; they should avoid the approach ofproviding conservative baselines in the belief that they are helping the Owner to elimi-nate ground related claims.

The recently revised GBR guideline (Geotechnical Baseline Report for Construction,2007, ASCE) speci cally addresses this issue and the need to provide more realisticbaselines. Although beyond the scope of this paper international experience particu-larly in Hong Kong, Singapore and Australia show how the use of two stage GBRs orthe use of balanced baselines could be useful tools in helping to ensure that groundrisks are shared fairly between the Owners and Contractors and help to ensure thatGBRs are used more effectively. For example the use of two stage GBRs are nowstandard practice on all current MTRC tunnel project in Hong Kong. Two stage GBRsallow the Contractor as part of his bid to provide his own interpretation of where hebelieves the baselines should be set and to identify were he thinks are the areas of


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Table 3. Recommendations for preparing geotechnical baseline reports for rock tunnelprojects

Recommendations Discussion

C o n c e p

t u a

l I s s u e s

Be clear about what is a baseline.

We recommend allowing the Contractto rely on all information within thereport, including any geological plansand sections. It is recommended tolimit interpretation or make this consis-tent with the baseline statements.

This should eliminate confusion and help avoid

contradiction and con iction. Often the exten -sive interpretation provided in the GBR con ictswith the actual baselines provided.

Prepare baselines that relate directlyto a construction or design issue, asopposed to simply providing a list ofindividual rock properties. Care shouldhowever be taken to incorporate the

impact of the Contractors means andmethods.

We are never going to be able to accuratelydescribe all ground encountered but it should bepossible to provide a minimum criterion for keyaspects of the tunnel support and construction.For example provide baselines for groundwater

in ow instead of rock permeability, or baselinea minimum initial support or rock quality (Q val-ues) instead of simply providing individual jointor rock properties.

Provide reasonable baselines basedon the understanding of the interpreta-tion and expected ground behavior.

Avoid playing contractual games inan attempt to minimize claims.

Avoid unnecessary or overly conservativebaselines. Contractors should have the right toexpect that the baselines presented are reason-able. Unrealistic or ultraconservative baselinesthat shift unreasonable risk to a contractorshould be discouraged and are contrary to theoverall intent of the GBR.

Consider the use of alternativecontractual approaches when usingGBRs, including the use of a 2 stageGBR or by using Balanced Baselines(Doyle, 2006).

The use of a 2 step GBR allows the Contractoras part of his tender to show how he hasinterpreted the risk and where he believes fairbaselines should be set. This allows the impactof the proposed means and methods to also beconsidered.In the spirit of partnering Balanced Baselinescould be used as a way to share the risk and/or reward and to encourage a fair and openapproach to determining baseline.

B a

s e

l i n e

D e

f n

i t i o n s

See Table 4 for discussion and recommendations on how speci c baselines can bepresented.Only provide one baseline for any rockproperty or design/construction issue.

This will help to reduce contradicting and dupli-cate baselines.

Use Rock Mass Types or Classes tocharacterize the rock along the align-ment, as opposed to lithology.

The use of Rock mass classes allow you togroup different rock types with similar propertiesand behavior together. This allows variations inrock properties to be better de ned and helps toeliminate variations.

Do not baseline properties that can beheavily in uenced by the quality of the

contractors means and methods.

For example it is not recommended to providebaselines for rock over-break.

Be careful in providing baselines thatrequire interpretation or assumptions.

It is not recommended to provide baseline ofproperties where there is the need for inter-pretation or where there is the need to makeassumptions. For example if Q baseline valuesare speci ed then the SRF values to assumeshould be provided.

(table continues)


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concern. In reviewing the resulting tender submissions the Owner can assess moreclearly how the Contractors have used the baselines as part of their proposal and canchose to adopt the revised version.

Balanced baselines are an approach proposed by (Doyle 2006). If balanced base-lines are used in addition to Contractors receiving extra payment if the conditions aremore adverse than those in the baseline, it is suggested that the Owner should alsoreceive a reduction in the contract price, for any less adverse site conditions that areencountered. In this situation both the Contractor and the Owner would have balanced

risks in regards to subsurface site conditions. In addition to these ideas the use ofGeotechnical Contingency Funds can also help to ensure the partnering and effec-tiveness of GBR reports, a good example of the use of this approach is on the Port ofMiami Tunnel Project.

In terms of addressing the structural problems associated with GBRs, such as pro-viding more relevant baselines and clarifying how we present baselines to help elimi-nate ambiguity and contradiction we believe that this can be more easily addressed.Speci c recommendations on how baselines can be presented and used during con -struction of rock tunnels are shown in Table 4.

We also recommend when developing future baselines for rock tunnel that wefocus more on the behavior of the ground and speci c design and construction issues,as opposed to simply providing a list of rock properties. Consulting Engineers andGeologists have understandably dif culty trying to develop speci c numerical base -lines for a wide range of geotechnical properties. This often results in the develop-ment of wide ranges for the various baselines. It is unlikely that we can ever expectto accurately describe miles of varying rock conditions, so it is recommended to focuson design and construction requirements which could be easier to quantify. This rec-ommendation was in fact made in the UTRC (1997) report; a checklist was provided

Table 3. Recommendations for preparing geotechnical baseline reports for rock tunnelprojects (continued)

Recommendations DiscussionShow GDR test data where possible

to show that you are being consistentand transparent.

It is recommended to be transparent and show

the data that has been used to help develop thebaselines. If the baseline provided is differentfrom the testing data then these differencesshould be clearly explained.

P r a c

t i c a

l I m p

l e m e n

t a t i o n

Clearly specify how during construc-tion the baseline rock properties willbe measured. This testing should startfrom day one and baselines shouldbe continually assessed by the Owneror CM during construction even whenground conditions are as expected.

It is important to clearly speci c the type andfrequency of any testing that is required toevaluate the various baselines. This will help toeliminate any uncertainty or misinterpretation inthe event of a change condition.

Clearly specify how during construc-tion the baseline rock properties willbe evaluated and clearly identify whatcosts or delays will the Contractor becompensated for.

The baselines should clarify the what in thequestion conditions materially different towhat? Just because the ground is different thisdoes not necessarily mean that this will impactthe Contractor.

Require the Contractor to developcontingency measures to addresswhat would happened if there was adiffering site condition at the start ofthe project.

This will help everyone to understand the costand schedule impacts involved and help toreduce delays caused by differing site condi-tions .This should ultimately help save time andeliminate debate and uncertainty.


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outlining what should be provided in a GBR, where in addition to providing baselines

for ground characterization, it is also recommended addressing the following designand construction issues. Based on the results of our research we have also added anestimate in Table 5 and Table 6 of how frequently these considerations were providedin the GBRs we have reviewed.

Contractors are often frustrated because they feel that they are not always pro-vided with the baselines they need, the results of our analysis clearly show this to betrue (see Figure 1 and Table 4). Owners also feel taken advantage of when individualbaselines are used to justify claims in a manner not intended or the baselines arenot respected in the dispute resolution process. These concerns can be helped byConsultants providing more relevant baselines and baselines that address the designand construction considerations highlighted in Table 4, Table 5, and Table 6. We shouldonly baseline rock properties that are necessary for a contractor to evaluate means andmethods, estimate ground behavior for his initial and/or permanent support require-ments and develop a construction schedule.

Finally it is recommended that we pay more attention to how baselines should bemeasured and assessed during construction. In the GBRs we reviewed, measurementand payment was discussed in less than 20% of the GBRs. It is important to clearlystate how baselines are to measured using the expected means and methods, it is alsoimportant to explain how any changes in baselines will be evaluated. If a baseline isexceeded it is important to understand how this has impacted the work, and what costor delays may occur.

CONCLUSIONIn summary our research has highlighted a wide variety of problems can be encoun-tered with using GBRs on rock tunnel projects and we have shown that many of theseissues are continuing to occur on recent projects. To help mitigate these issues thefollowing recommendations and conclusions have been made.

Table 4. Design and construction considerations for GBRs (taken from UTRC Report 1997)Design Considerations Construction Considerations

Description of ground classi cationschemes used.

25% Required sequence ofconstruction

10%

Criteria and methodologies used for thedesign of ground support and ground stabi-lization, including ground loadings.

5% Anticipated ground behaviorin response to constructionoperations.

45%

Criteria and basis for nal design. 0% Rational behind groundimprovement.

0%

Environmental performance considerationssuch as limitations on settlement and lower-ing of groundwater levels.

0% Identi cation of speci c construc -tion dif culties.

75%

The manner in which different supportrequirements have been developed for

different ground types, and the protocols tobe followed in the eld for determination ofground support types for payment, refer-ence to speci cations for detailed descrip -tions of methods/sequences

0% Rational behind baselines forgroundwater in ow to be encoun -

tered during construction.

20%

The need and rational for ground perfor-mance instrumentation included in thedrawings and speci cations.

50% Identi cation of sources of delay,faults, gas, obstructions etc.

75%


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Table 5. Recommendations and checklist for how rock tunnel baselines can bepresented (1 of 2)

GeotechnicalBaselines Priority Recommendations

Testing andSampling

G e o l o g

i c a

l I n t e r p r e

t a t i o n

Rock Type(Lithology)

High Provide clear descriptions of the rockunits, using a recognized rock classi ca -tion system.

Rock Mapping

Top of Rock/Rock Cover

High Tabulate the minimum rock coverexpected along the alignment, not rec-ommended to provide contour plots, espe-cially if they are computer generated.

Probing and Additional SiteInvestigation

Rock MassTypes

High Different rock types should be groupedinto Rock Mass Classes if they havesimilar characteristics and behavior. RockMass Classi cation schemes such asRMR, Q and/or GSI should be used tode ne each Rock Class.

Rock Mapping & Additional SiteInvestigation

WeatheringGrade

High A standard weathering classi cationscheme should be used (i.e., ISRM) andeach grade should be clearly de ned forexample by using SPT(N), RQD or TCRtesting results.


I n t a c

t R o c

k P r o p e r t

i e s

RockMineralogy(i.e., HardMineralContent)

High Provide a range of values; this will typi -cally include an assessment of the QuartContent.

Field Sampling(Testing)

Unit Weight Low Provide a realistic range of unit weights. Field Sampling(Testing)

Uncon nedCompressiveStrength

High Provide a clear range of realistic UCSstrength values (It is not recommended toallow correlations with Point load testing).


TensileStrength(BrazilianTest)

Low Provide a clear range of realistic tensilestrength values.


YoungsModulus (E)

Low Provide a clear range of E values, beclear about how E vales were measuredi.e., Secant modulus E 50


Cerchar AbrasivityIndex (CAI)

High Provide a clear range of CAI values. Field Sampling(Testing)

R o c

k M a s s

P r o p e r t

i e s

&

C l a s s

i f c a

t i o n s

StrengthProperties(Angle ofFriction &Cohesion)

High Provide a clear range of strengthproperties for the rock mass; be clear tospecify how these were derived and whatassumptions have been made.


Rock MassStiffness(YoungsModulus &PoissonsRatio)

High Provide a clear range of E values, beclear about how the E values were mea-sured or derived.


(table continues)


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Table 5. Recommendations and checklist for how rock tunnel baselines can bepresented (1 of 2) (continued)


Testing andSampling

R o c

k M a s s

P r o p e r t

i e s

& C l a s s

i f c a

t i o n s

( c o n

t i n u e

d )

SeismicVelocity

Low Provide a clear range of seismic velocityvalues, be clear about how seismic veloc-ity values were measured or derived.

Additional SiteInvestigation

Rock MassPermeability

High Provide a clear range of rock masspermeability for each rock mass class,although it may be advisable to provide

joint aperture and/or in ow estimates.


GSI andHoek &BrownConstants

High Provide a clear range of GSI values foreach rock mass class, state clearly anyassumptions made.

Rock Mapping

Q System High Provide a clear range of Q values (with-out SRF and J w component) for each rockmass class, state clearly any assumptionsmade.

Interpretation dur-ing mapping

Rock MassRating (RMR)

High Provide a clear range of RMR values foreach rock mass class, state clearly anyassumptions made.

Rock Mapping

Rock MassIndex (RMi)

Low Provide a clear range of RMi values foreach rock mass class, state clearly anyassumptions made.

Rock Mapping

RQD High Provide a clear range of RQD values foreach rock mass class, state clearly anyassumptions made.


R o c k

J o

i n t i n g

Number &Orientation ofJoint Sets

High Provide a clear number and range oforientations for the discontinuities ineach rock mass class, state clearly anyassumptions made. Recommended for

joint orientation to only specify generaldip directions i.e., NE and avoid usingstereonets to display results as they areopen to interpretation.


Joint ShearStrength

High Provide a clear range of c and phi valuesfor each rock mass class, state clearlyany assumptions made.


Joint Spacing& Persistence

High Provide a clear range of joint spacing andpersistence values for each rock massclass, state clearly any assumptionsmade.



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Table 6. Recommendations and checklist for how rock tunnel baselines can bepresented (2 of 2)


Testing andSampling

R o c

k J o

i n t i n g Joint Conditions(including water)

High Provide a clear range of joint conditiondescriptions; where possible provide jointaperture, alteration, roughness and wavi-ness data for each rock class. This datamust be consistent with the data used todevelop any rock mass classi cation sys -tems such as Q, RMR or GSI estimates.

RockMapping &


F a u

l t i n g

Number, Location& Orientation

High Provide the number and location of knowfaults on the geological sections andclearly de ne their orientation.

RockMapping &


Fault Thickness &Properties

High Provide a clear range for a fault thicknessand strength properties, state clearly anyassumptions made i.e., if true or apparentthickness has been used.

RockMapping &


S t r e s s Insitu Stress

(including Ko)High Provide a clear value for assumed insitu

vertical stress and a range of values forhorizontal stress (including K o ranges),state clearly any assumptions made.

Additional SiteInvestigation &Field Sampling(Testing)

G r o u n

d w a

t e r

GroundwaterLevel

High Provide a clear range of groundwaterlevels along the tunnel alignment, stateclearly any assumptions made. Thisshould include any allowance for oodlevels and seasonal variations should beconsidered.


GroundwaterIn ows

High Provide a clear range of groundwa-ter in ow values for each rock massclass, state clearly any assumptionsmade. Values for immediate ush owsand steady state conditions should beprovided.


S p e c

i f c

G r o u n

d R i s k s

Slake Durability As

Required

If appropriate provide a range of values to

address the potential for slaking for eachrock mass class.

Field Sampling

(Testing)

Swelling Potential AsRequired

If appropriate provide a range of values toaddress the potential for swelling for eachrock mass class.


Solution Featuresand Voids

AsRequired

Recommend to identify the length oftunnel that may be impacted by the pres-ence of solution features and voids. Avoidtrying to identify speci c void volumes asthis tends to result in the development ofconservative baselines.


(table continues)


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Table 6. Recommendations and checklist for how rock tunnel baselines can bepresented (2 of 2) (continued)


Testing andSampling

G r o u n

d B e

h a v

i o r

Ground FailureTypes

High Provide a clear range of expected groundbehaviors for each rock mass class,state clearly any assumptions made.Recommended to use an acceptableground behavior classi cation schemesuch as that proposed by Terzaghi 1977.

Rock Mapping

Rock Loading High Provide a clear range of expectedrock loading for each rock mass class,state clearly any assumptions made.Recommended to use an acceptableclassi cation such as that proposed by

Barton 1974 (Q System).

Rock Mapping

C o n s

t r u c t

i o n

C o n s

i d e r a

t i o n s

Overbreak& Volume ofExcavatedMaterial

Not Recommended to provide baselines for these properties arethey are strongly related to the quality of the Contractors means andmethods.

ExcavationTechniques

High Provide a clear discussion of anticipatedexcavation techniques, including any limi-tations or potential problems for speci cmeans and methods.

SiteObservations

ConstructionSequence

High Provide a clear discussion of antici-pated construction sequences, includ-ing expected maximum unsupportedexcavation lengths, standup time andthe need to use of split heading/benchconstruction.

SiteObservations

Initial SupportRequirements

High Provide a clear discussion of anticipatedinitial support requirements, including theneed for pre-support, face, crown andwall support and nal lining support.

Field Mapping& SiteInvestigation

ContaminatedGround or

Groundwater

AsRequired

If appropriate provide discussion toaddress the potential for contaminated

ground and/or groundwater.

SiteInvestigation &

Field Sampling(Testing)Gas High Provide a clear statement on the classi -

cation (i.e., OSHA) of the tunnel in termsof gassy or non-gassy.

Field Testing

Obstructions(natural orman-made)

High If appropriate provide discussion toaddress the potential for encounteringeither natural (i.e., boulders) or mad-made (i.e., foundations) obstructions.The location of these should be clearlyidenti ed and a description of the obstruc -

tion provided. In the case of bouldersavoid specifying the size and number ofactual boulders as this tends to result inoverly conservative baselines, it is recom-mended to identify a length of tunnel thatmay be impacted by this.

SiteObservationsand addi-tional SiteInvestigation


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GBRs should in addition to characterizing the expected ground condition alsoprovide baselines for speci c design and construction issues.

Realistic and relevant baselines should only be provided (see Table 5 andTable 6).

Baselines should be clearly presented and repetition and con iction shouldbe avoided.

Assumptions or terminology used should be clearly provided; this shouldinclude providing a glossary of terms and any other references or testing usedin developing the baselines.

Discussion should be provided on how baselines are to be measured consid-ering the expected means and methods to be used.

Discussion should also be provided on how the baselines will be evaluated inthe event of a change condition.

Finally GBRs are intended to be a risk sharing not a risk transfer tool, it is thereforeimportant that all parties involved understand their role. GBRs are intended to be atrue measure of ground behavior based on a reasonable interpretation of the availabledata not simply a conservative description of the site investigation data. Based on ourresearch we found the best GBRs were those that provided a realistic interpretation ofthe expected ground conditions that included an assessment of ground behavior andconstruction implications.

REFERENCES ASCE. 1997. Geotechnical Baseline Reports for Underground Construction

Guidelines and Practices, The Technical Committee on Geotechnical Reports ofthe Underground Technology Research Council (Yellow Book).

ASCE 2007. Geotechnical Baseline Report for Construction: Suggested Guidelines,The Technical Committee on Geotechnical Reports of the Underground TechnologyResearch Council (Gold Book).

Black, R.J. 2009. The New Economic Reality: Implications for the Construction Industryin Hong Kong, Hong Kong Construction Association.

Doyle. J. 2006. Balanced baselines a fairer allocation of uncertain risk.Freeman, T., Klein, S., Korbin, G., and Quick, W. 2003. Geotechnical Baseline

ReportsA Review. 2003 RETC Proceedings .Geotechnical Baseline Reports: A Bene cial Tool, or a Scourge on the Industry? TBM:

Tunnel Business Magazine, August 2009.

recommendations on how gbr can be prepared for rock tunnel projects

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