Intelligent Tunnel Design

Robbins TBM

Every successful tunneling project results from the successful implementation of a three-step process involving planning, design, and construction. Tunnel planning is concerned primarily with what the Owner needs in order to obtain the satisfactory, long-term utilization of the finished facility. Tunnel design is itself a two-part process whereby the tunnel Designer assists the Owner in developing all of the design criteria required for the finished facility and all of the constructability criteria needed to create the underground space inside of which the finished facility will be built. Tunnel construction is also a two-part process whereby the Contractor accomplishes all of the work required both to create the underground space and, then, to build the finished facility.

Intelligent Tunnel Design is also a cooperative effort whereby the tunnel Owner, the tunnel Designer, and the tunnel Contractor work together in order to provide a finished facility that has long-term value for the project Owner. Upfront cost is a concern and should be minimized through the use of effective, efficient design, but long-term value is the key to success for tunneling projects. In essence, long after the tunnel Designer and the tunnel Contractor have disappeared from the scene the tunnel Owner still benefits for many decades from the completed project facility.

Intelligent Tunnel Design

Based on the above Introduction, what, then, are the basic elements of Intelligent Tunnel Design and given below is a listing of those nine elements: Intelligent Tunnel Design diagram

  1. Project Layout
  2. Subsurface Investigation
  3. Ground Behavior vs. Ground Control
  4. Project Design
  5. Construction Methods
  6. Third Party Impacts
  7. The Contract Document
  8. Project Procurement
  9. Construction Monitoring

Project Layout – Tunnel planning is an extremely complicated process that is highly dependent on the purpose of the finished facility. For instance, the design criteria for any form of water tunnel will be radically different as compared to a transportation facility. Underground space can also be used for storage, libraries, museums, pedestrian access and a variety of other purposes; each of which is dependent upon the establishment of a wide variety of specific operational criteria. Also included in project planning are considerations for community and environmental impact, land acquisition, rights-of-way, the cost of operations and maintenance, project financing, and contracting practices.=

One of the primary outputs from the project planning effort is the project layout. In essence, the project planning effort establishes the size, shape, depth and alignment of the proposed tunnel structure that is required in order to fulfill the Owner’s needs. Hopefully, and in many cases, these sizes and shapes of openings, together with the tunnel portals and/or shafts can be used to actually construct those facilities. If not, then the tunnel Designer and Owner must provide the Contractor with additional surface sites, supplemental shafts, and/or additional rights-of-way in order to minimize the cost of construction. The logistics associated with constructing a tunnel are extremely complex and can add substantially to the project cost if the Contractor is forced to work from remote locations and/or from within restricted sites. In the final analysis, the ultimate cost of an underground project is established by what is required to be accomplished in the contract document, but the Owner and its Designer must perform their work based on the realization that substantial amounts of money can be saved by making the project as easy as possible to construct.

Unfortunately, it is beyond the scope of this paper to discuss all of the intricacies of project planning, but what will be discussed herein is what the tunnel Designer needs to do in order to create the underground space needed for the finished facility. In general, most of the cost and most of the risk associated with underground construction is related to how well and how effectively the Contractor can create the underground space needed to construct the finished facility in the ground conditions that exist at the project site. Intelligent Tunnel Design is, therefore, a process whereby the Owner and its Designer does that which is required to assist the Contractor in accomplishing the above objective.

Subsurface Investigation – Without doubt, the single most important aspect of Intelligent Tunnel Design is the implementation of a comprehensive subsurface investigation. Almost every important decision relating to tunnel planning, design and construction is dependent upon one’s knowledge of the existing ground condition. If major elements of the subsurface condition such as faults, stratigraphy, weathering, groundwater issues and/or hazardous substances are left undiscovered by the subsurface investigation, then major problems with significant cost and scheduling impacts are in the offing. In the final analysis, you either know what exists underground or you do not, and trying to design an underground opening without adequate subsurface information is a fool’s errand.

Literally hundreds of papers, books and articles have been written about what is required to produce a satisfactory subsurface investigation for a tunneling project and it is beyond the scope of this paper to discuss all of that information. In general, the authors of this paper prefer to perform the subsurface investigation in three phases related to tunnel planning, tunnel design, and tunnel construction culminating in the preparation of two extremely important contract documents; the Geotechnical Data Report and the Geotechnical Baseline Report.

The Geotechnical Data Report (GDR) is exactly what it says; i.e. a collection of all of the field and laboratory data developed for and related to the project. The GDR is, therefore, a collection of facts that includes an introductory text describing how the work was performed followed by a collection of the results of that work. It is important to note, however, that all of the subsurface investigative efforts must be performed to exacting, accepted standards so that the data are reliable and that they accurately portray the engineering properties of the various underground deposits. It is also important to note that the GDR should not include any form of geotechnical interpretation such as a subsurface profile connecting the test borings. The interpretation of subsurface data is a separate part of tunnel design as discussed below.

Beginning approximately thirty years ago, and formalized in an ASCE publication entitled Geotechnical Baseline Reports for Underground Construction that was originally published in 1997 and updated in 2007, it was decided that the project Owner and its tunnel Designer should provide “interpretations” and “baselines” for the project data contained in the GDR. As provided in the 2007 edition of the ASCE GBR document given below is the stated purpose of a GBR:

1.3 Purpose of the GBR

The principal purpose of the GBR is to set clear realistic baselines for conditions anticipated to be encountered during subsurface construction, and thereby provide all bidders with a single contractual interpretation that can be relied upon in preparing their bids. Other key objectives of the GBR include:

  • presentation of the geotechnical and construction considerations that formed the basis of design for the subsurface components and for specific requirements that may be included in the specifications;
  • enhancement of the Contractor’s understanding of the key project constraints, and important requirements in the contract plans and specifications that need to be identified and addressed during bid preparation and construction;
  • assistance to the Contractor or DB team in evaluating the requirements for excavating and supporting the ground; and
  • guidance to the Owner in administering the contract and monitoring performance during construction.

The ASCE GBR publication also provides an extensive collection of guidelines about how to prepare a GBR that have established, for all intents and purposes, the professional standards for the preparation of this important contract document.

Therefore, Intelligent Tunnel Design includes the implementation of an accurate and comprehensive program of subsurface explorations for every underground project and the preparation of both a GDR and a GBR for inclusion in the contract documents. In addition, and in accordance with accepted professional practice, no attempt by the Owner to disclaim any of this data and/or the related interpretations and baselines should take place.

excavator in a tunnel

In closing this section of the paper, it is also necessary to make reference to the topic of hazardous materials. Many geotechnical engineering companies do not specialize in the practice of evaluating the possibility of encountering underground hazardous materials but the unanticipated encountering of contaminated ground or groundwater on a tunneling project is a very serious concern. If such a possibility does, in fact, exist, then it is imperative that the project Owner authorize a hazardous substance investigation, and similarly to the subsurface investigation, make that information available to prospective Contractors prior to bidding.

Ground Behavior vs. Ground Control – Intelligent Tunnel Design involves an enormous amount of “brainstorming” associated with the best methods for constructing the proposed project in the ground conditions as described by the subsurface investigation. In order to build a tunnel opening, it is necessary to provide satisfactory answers to each of the following three questions:

  1. What is the best method for excavating the ground as revealed by the test borings and to the dimensions required for the project?
  2. What is required in order to control adverse ground behaviors from negatively impacting either the tunneling operation itself and/or any overlying or adjacent, existing third party structures such as utility lines and building foundations?
  3. What is the best method for supporting the ground after the required openings are made in a manner that is safe for the workers and stable for the third parties?

Each of the three topics listed above involves a mind-boggling evaluation of innumerable construction techniques that might be appropriate for any given underground project. Depending on the size and shape of the various tunnels and shafts and the various ground conditions in which those structures will be built, the tunnel Designer can call upon a vast array of construction methods to excavate, to control and to support that ground. In addition to the actual tunneling methods themselves the Designer can also call upon various ground improvement methods such as dewatering, freezing, and/or grouting in order to make the ground easier to control and support.

In order to accomplish the ground behavior vs. ground control brainstorming effort, it is necessary to assemble a team of highly experienced individuals from different aspects of the industry such as engineers, engineering geologists, estimators, suppliers, and construction personnel. One of the most important concerns associated with Intelligent Tunnel Design is the fact that the best way to know how to design an underground opening is to know how to build it. In no other area of civil engineering expertise are the design and construction processes so intimately intertwined; i.e. the design will guide construction, but the proposed construction methods will also have a significant impact on how the project should be designed.

Project Design – As stated above, the primary focus of this report is on the design procedures required to produce the openings inside of which the finished facility will be constructed. With that goal in mind, it is important to note that all of the temporary structural elements required for tunnel support come into direct contact with and interact with the ground. Steel ribs, rock bolts, and shotcrete are used most often to support the rock, and steel ribs and boards, steel liner plates, and jacked pipe can be used to support the soil.

Sometimes, the temporary support can be augmented such as by providing additional layers of shotcrete in order to provide a final lining. And, finally, tunnel linings such as bolted and gasketed concrete segments can be used in conjunction with TBMs both to support the ground and to provide the final lining.

However, and this is a big however, project designers must be careful when specifying various construction procedures that they believe are “necessary” in order to create the underground openings because this is where the interface between design and construction can become highly problematic. Having been involved with the design and construction of literally hundreds of tunneling projects the authors of this paper have become well aware of two of the most critical aspects of the downside of tunnel design as listed below:

  1. Lots of tunnel designers are simply not aware of all of the construction techniques and procedures available to highly experienced contractors for both controlling and supporting the ground, and
  2. Lots of tunnel contractors do not seem to fully appreciate the almost unlimited ability for adverse ground reactions to cause trouble for their proposed construction procedures.

Without doubt, the appropriate and successful combination of the two considerations listed above is the key to Intelligent Tunnel Design. Tunnel Designers should not specify more than is required to build the proposed openings and tunnel Contractors should not do less than is required to build those openings in a safe and stable manner. In the final analysis, an appropriate balancing of tunnel design requirements with tunnel construction procedures is the primary goal of Intelligent Tunnel Design©.

Construction Methods – Tunnel construction is a complicated combination of the work that needs to be performed in order to advance the tunnel heading as quickly as possible and the logistical issues associated with accomplishing that task. Working from inside the space created for the tunnel and actually accomplishing something in that restricted space at the tunnel heading is not a simple task. Over time, and based on actual on-site experiences, successful tunneling contractors have become very good at performing these procedures, but if the ground introduces unanticipated problems for those procedures then the cost of completing the tunnel can quickly spiral out of control. In addition, the contract documents can require work to be performed in the tunnel that might not be necessary and might add considerably to the cost of building the project.

For instance, many tunnel designers like to specify the requirement for probe hole drilling and grouting from the face of a TBM tunnel excavated in rock. This approach might provide some comfort for the tunnel designer that he is covering all of his bases and reducing project risk, but is it necessary? Probe hole drilling and grouting means that the entire tunneling operation must be halted and all of the costs associated with the entire project must be dedicated to this one activity. For some ground conditions that might be required, but more often than not that is not the case. Hence, and as discussed above, an appropriate balancing of design risk and construction methods is what is required for “successful” tunneling.

Third Party Impacts – Overlying all of the above design and construction considerations is the issue of third party impacts. Sometimes design and construction procedures must be specifically tailored to the protection of existing, overlying and adjacent third parties. For instance, if you are tunneling directly below an existing 6-foot-diameter sanitary sewer or adjacent to an historic church, it is necessary to avoid doing anything that will cause those structures to be damaged. For the sewer, it might be possible to grout the ground surrounding the sewer in order to make certain that that ground will not be destabilized during tunneling. For the church, it might be necessary to underpin the church foundations prior to tunneling so that the tunnel will not cause detrimental settlements and cracking of the church. As above, this interface between design requirements and construction methods can be difficult to accomplish relative to existing third parties, but, when done is a proper manner, it is a critical component of successful tunneling.

The Contract Document – The contract document for a tunneling project is radically different as compared to a vertical building project for three important reasons: the contact with the ground, the amount of temporary construction, and the complexity of the finished facility. For a vertical building, maybe 10% of the structure comes into direct contact with the ground as compared to a tunnel where 100% of every portion of the structure has a ground/structure interface. For a tunnel, anywhere from 2/3’s to 3/4’s of the cost and the risk of the project is related to all of the temporary construction needed to create the space inside of which the finished facility will be constructed as compared to a vertical building where the space needed for the building is simply open air. And finally, a vertical building requires a large number of design and construction specialties such as for the architectural, structural, ventilation, lighting, mechanical, and finish work as compared to a tunnel where the finished facility is, most often, simply the final lining.

As a result of the above, the contract document for a vertical building project consists of three parts: the general conditions, the specifications, and the drawings. A single drawing showing the results of test borings may be included and a geotechnical report summarizing the subsurface information and providing design criteria may be made available for information, but most building contractors perform most of their work in the local area and are frequently more knowledgeable about the local ground conditions than are the designers.

For a tunneling project the contract document consists of five parts: the general conditions, the specifications, and the drawings, as listed above, and both the GDR and the GBR that become part of the contract. For a structure that is 100% in contact with the ground and also contains a differing site condition clause it is imperative for the project Owner to provide project specific contract indications for both the ground conditions and, to some degree, for the ground behaviors that the Contractor can expect to encounter during construction. Although this commitment on the part of the Owner establishes some “risk” for the Owner, this risk does not simply disappear if the Owner attempts to disclaim the subsurface information. The majority of legal and DRB decisions associated with this topic clearly state that the Contractor has a right to rely on whatever subsurface information was obtained for the tunnel despite any attempt by the Owner to disclaim that information. Hence, for tunneling projects, there is a huge emphasis on both the amount and the accuracy of subsurface information that must be provided in the contract document for bidding and construction purposes.

It is hard to emphasis enough the importance of preparing a contract document that is specifically tailored to tunneling if the Owner has any hope of completing the project without claims for extra costs and schedule extensions. Once the contract is prepared and released for bidding purposes, the Contractor has a right to rely upon the basic assumption that if he does everything as described in that contract then the Owner will be pleased with the outcome and the Contractor will be paid for his work as outlined in the contract. If, however, the ground conditions themselves or the specified construction procedures intended to deal with those ground conditions are inaccurate or inappropriate for some reason, then the Contractor will seek additional compensation associated with those discrepancies. Hence, when an Owner and its Designer put a contract out to bid for a tunneling project they must be satisfied, to the best of their ability, that that contract paints a straightforward and consistent picture of what the Contractor needs to do in order for the project to be declared successful. Such is the primary objective and the challenge when preparing a contract document for a tunneling project.

Project Procurement – Successful tunneling Contractors are, as a result of sheer survival, highly sensitive to what is presented in the contract documents and highly perceptive about what it will take to be the low bidder for a particular project. In general, the tunneling industry can be described as a highly competitive “fraternity” where all of the major players know each other and all are quick to either joint venture and/or to compete depending on an enormous collection of contractual requirements and market conditions for each particular project. Tunneling contractors also have strong opinions about which Owners and which Designers are “fair” with respect to resolving the inevitable discrepancies and controversies that develop during construction and are not the least bit shy about resorting to all forms of “dispute resolution,” including litigation, if they feel that the contract documents were materially inadequate for any reason.

That being said, given below are some of the important topics that a tunneling contractor will look for in the contract document when deciding whether or not to bid a particular project:

  • Does the contract contain a valid Differing Site Conditions clause?
  • Does the contract contain a comprehensive Geotechnical Data Report without any disclaimers?
  • Is the Owner utilizing reasonable and appropriate procedures for assembling a group of “qualified” Contractors to bid the project?
  • Are the third party requirements established in the contract reasonable and well defined?
  • Is there a provision for partnering?
  • Is there a reasonable amount of time provided to prepare a responsive bid?
  • Has the Owner surrounded itself with well experienced tunneling professionals that will work with the Contractor during construction?
  • Does the contract contain provisions for alternative dispute resolution such as for a Dispute Review Board?

One topic that is not listed above and for good reason is the Geotechnical Baseline Report. Provided earlier in this paper is the ASCE stated purpose of a GBR, which, to summarize, should provide the Contractor with “realistic” descriptions of what he needs to know and needs to do in order to bid the work in a proper manner; and, then, to build the work as described by the contract document. In addition, the GBR should represent a “reasonable interpretation” of the subsurface information assembled for the project that is compatible with and supportive of the plans and specifications included in the contract. Such is the overriding goal of a “good” GBR.

However, and this is a big however, that is not always the case as sometimes the contract contains a “bad” GBR! By definition a bad GBR represents one last, mistaken attempt by the Owner and it Designers to shed liability onto the Contractor by establishing extremely conservative and unreasonable “interpretations” and “baselines” for the work. Sometimes an Owner believes that by grossly exaggerating the information uncovered during the subsurface investigation he can create a wall of liability protection that will somehow “insulate” the Owner from a DSC claim. Unfortunately, and more often than not, this plan will backfire. For instance, it is highly likely that experienced tunneling contractors will realize what is happening and 1) come to the realization that the Owner is not serious about working together, and 2) begin to carry large contingencies in its bid to cover the liability implications of a bad GBR. Even worse, if things do not work out in the field as the Owner anticipated, experienced tunneling Contractors can be both highly imaginative and highly aggressive in pursuing various claims against the Owner almost no matter what is written in the contract. In general, and based on experience, a majority of claim resolutions by DRBs and by the courts have been somewhat sympathetic to the Contractor based on a realization of how truly difficult it is to bid a tunneling project and to accomplish all of the hard work and complex logistical requirements needed to build a tunnel in a proper manner.

Hence, a “good” GBR is a thing of beauty and will legitimately contribute to the successful outcome of a tunneling project. A “bad” GBR does not. In fact, a bad GBR is less than worthless because it actually contributes to the creation of an adversarial relationship that is so highly detrimental to a successful project. In conclusion, the concept of a good GBR, as defined by the ASCE Guidelines, is a positive part of a tunneling contract while a bad GBR is not.

Construction Monitoring – The last topic that needs to be discussed as part of Intelligent Tunnel Design is construction monitoring. It is absolutely imperative that the tunnel Owner assign experienced tunneling professionals to observe and document every aspect of construction activity taking place in the field. In general, these activities consist of observing ground conditions and ground behaviors, documenting construction means and methods, and monitoring the behavior of overlying and adjacent third parties.

Good construction monitoring is one more important part of Intelligent Tunnel Design. Accurate project records are essential to making sure that the work is being performed in accordance with contract requirements and that the ground conditions and ground behaviors are compatible with the geotechnical documents. If trouble does develop, then both the Owner and the Contractor must have in their possession accurate and complete project records that can be used to help resolve alleged claims either in the field, as a result of alternative dispute resolution (i.e., a Dispute Review Board), or, if all else fails, as a result of litigation. It is the height of false economy for the Owner of a tunnel project to believe that a tunnel can be constructed without adequate professional observation and assistance in the field.

Summary and Conclusion

Given below is a flow chart that summarizes all of the topics discussed above and serves as the basis for Intelligent Tunnel Design.

Using the above flow chart in a manner as described in this paper is the best way to pave the way for a successful tunneling project. In summary, and as shown in the chart Intelligent Tunnel Design© consists of the following:

Project Layout – Create the space that is needed both for construction and for the final facility.
Subsurface Investigation – Perform a comprehensive subsurface investigation and prepare an accurate GDR and a “Good” GBR.

Ground Behavior vs. Ground Control – Do a good job of brainstorming all of the design requirements and construction procedures needed to build the subsurface space in the existing ground condition.

Project Design – Establish all of the design criteria needed both to create a safe and stable underground opening and to provide a highly satisfactory finished facility.

Construction Methods – Specify only those construction methods that are necessary to construct the temporary facilities in a safe and stable manner.

Third Party Impacts – Make certain that proposed design criteria and construction methods are sufficient to safeguard existing overlying and adjacent third parties.

Contract Document – Assemble a contract document that is specifically tailored to tunnel construction and which adequately describes everything that needs to be performed by the Contractor in order to accomplish the above design and construction requirements.

Project Procurement – Create a positive and cooperative environment for the successful completion of your tunneling project.

Construction Monitoring – Assign highly experienced tunneling professionals to monitor and record all aspects of construction activities.

If you actually do all of the activities listed above as part of your plan for Intelligent Tunnel Design©, then there is every reason to believe that you can and will achieve a successful tunneling project.

Dr. Gary S. Brierley is President of Doctor Mole, Inc. Dr. Arthur J. McGinn is President, Brierley Associates Corporation.


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