Geotechnical Risk Assessment for Tunneling Projects



The flow chart shown below represents what the author believes is the nine-step process that must be implemented if you want to produce successful tunneling projects. A detailed discussion of this topic was presented in a paper titled Intelligent Tunnel Design in the August 2017 issue of TBM: Tunnel Business Magazine and the primary purpose of this paper is to emphasize the four steps as listed below where geotechnical professionals have the most input and the most responsibility for the successful outcome of a tunneling project:

  • Performance of the Subsurface Investigation,
  • Assessment of Ground Behaviors,
  • Preparation of the Contract Document, and
  • Provision of Construction Monitoring

Tunnels are radically different as compared to above-ground buildings primarily because they come into contact with the ground for 100 percent of all construction activities. Above-ground buildings come into contact with the ground for maybe 10 percent of construction and the contractor working on an above-ground project is always happy when he finishes with the foundation and basement portions of the project and is able to start work on the superstructure.

Tunneling contractors do not have that luxury. In addition, the project schedule for a tunnel is largely dependent on how fast the face of excavation can be advanced. If adverse ground behaviors interfere with face advancement, then the entire project can be subjected to significant delay. Finally, most tunnels are built in densely populated urban areas and come into contact with a huge number of third-party facilities such as existing utilities, highway and railroad rights-of-way, and building foundations that can be negatively impacted by ground movements caused by tunneling. All in all, it is probably safe to say that you need to be on your toes if you intend to design and/or construct tunneling projects – and that includes having the assistance of experienced and knowledgeable geotechnical professionals.

An evaluation of the risks associated with a tunneling project can also be divided into four parts as listed below; each of which is associated with the input provided by geotechnical professionals.

  • Risk Identification – is closely related to the subsurface investigation,
  • Risk Minimization – is largely associated with the evaluation of ground behaviors,
  • Risk Allocation – is heavily related to the geotechnical portions of the contract document, and, finally,
  • Risk Management – is associated with the observation and monitoring of ground conditions and ground behaviors during construction.

Risk Identification/Subsurface Investigation

Intelligent Tunnel DesignSuccessful tunneling can be defined as producing a satisfactory finished facility for no more money and in no more time than is required to deal with the existing ground condition. Hence, the question becomes, “How well do you know the existing ground condition?”

Successful tunneling is also a four-step process that includes excavating the ground, controlling the ground during the process of excavation, supporting the ground in a safe and stable manner as the tunnel advances, and, finally, constructing the finished facility – and all of the activities associated with excavating, controlling and supporting the ground are related to input provided by geotechnical professionals. It is also during the accomplishment of these three activities that the tunnel comes into contact with the ground and encounters most of the risks associated with tunneling.

RELATED: Intelligent Tunnel Design

The subsurface investigation is the only process whereby tunnel designers can obtain an accurate description of the existing ground condition. It is beyond the scope of this paper to explain all of the techniques and procedures available to geotechnical engineers for investigating the ground, but this is one of the most important aspects of successful tunneling. Some tunnel owners are primarily concerned with the cost of a subsurface investigation and seek to find the lowest cost provider of those services. This is a very bad idea. The subsurface investigation for a tunneling project must be thought of as an investment in successful project completion and the owner’s primary objective in obtaining the services of geotechnical professionals must be focused on obtaining the most knowledgeable and experienced people for that role.

Geotechnical engineers working on a tunneling project are almost always asked to produce two reports: a Geotechnical Data Report (GDR) and a Geotechnical Baseline Report (GBR). The GDR is defined as an accurate and thorough factual description of all the information collected as a result of the subsurface investigation. As the police officer on the old television show Dragnet used to say to the eyewitness; “Just give me the facts.” Similarly, the GDR should not contain any interpretation of the data. The GBR is the report that is intended to provide all parties working on the project with the geological and geotechnical interpretations required both to design and to construct the project and a brief description of how one goes about preparing a GBR will be provided in the next section of this paper.

Very often, persons associated with tunneling projects will tell you that the GBR is the most important geotechnical report associated with a tunneling project, but that is not true. The GDR is the most important report. Without good data about the existing ground condition, it is difficult to identify the major risks associated with a tunneling project and, therefore, difficult to accomplish an appropriate interpretive effort. For instance, let us assume for the moment that the existing ground contains a major fault, but that fault is not discovered as a result of the subsurface investigation. Obviously, when the tunnel encounters this fault during construction the project will be subjected to potentially massive delays and cost increases that will be difficult to manage and which could very well result in litigation; i.e. not a good outcome.

When it comes to the geotechnical scope of work established for a tunneling project, one should not be thinking about cutting corners and/or reducing costs. The subsurface investigation is primarily responsible for identifying most of the risks associated with a tunneling project and a significant level of effort must be expended to accomplish that task. In the final analysis, and as stated above, almost all of the major decisions about how one goes about designing and constructing a tunneling project are based upon how well one understands the ground conditions that are actually present along the proposed tunnel alignment.

Risk Minimization/Ground Behavior

Having obtained the subsurface data, it is necessary to decide what that data means with respect to tunnel design, tunnel construction, and the potential for impact to third parties located along the proposed tunnel alignment. Risk minimization involves decisions about what is required to excavate the ground, to control the ground during the process of excavation, and to support that ground as the tunnel advances; each of these decisions are closely related to one’s knowledge of ground conditions and one’s expectations about how that ground will behave and/or react to the process of tunneling. When brainstorming those decisions, it is also necessary to evaluate various methods for modifying ground behavior (i.e. improving the ground) by dewatering, grouting, or freezing.

Almost without exception the subsurface interpretive effort begins by plotting the subsurface profile along the proposed tunnel alignment as revealed by the subsurface investigation. Each test boring is plotted on the profile and an evaluation is then made about the types of ground that will be encountered by the tunnel including different deposits of soil and rock, the soil/rock interface, and, most importantly, the groundwater regime. Once the borings are plotted it is then necessary to decide how the various layers can be connected to show a profile and this depends largely on one’s knowledge of the geological processes that produced the soil and rock deposits dating back many thousands or even millions of years and one of the most important contributors to the subsurface profile process is a well-trained and highly experienced engineering geologist.

Engineering geologists are trained not only to identify various depositional environments but also to apply that knowledge to the preparation of an accurate description of the subsurface profile. The engineering geologist must also be aware of how the project will be designed and constructed in order to provide useful information about anticipated ground behaviors. Sometimes, a good engineering geologist is able to anticipate subsurface features that were not even indicated by the subsurface explorations such as faults or weathering features based on geologic maps, aerial photographs, or information obtained from similar geological environments.

Having developed the subsurface profile, it is now necessary to plot the results of all of the laboratory tests on the profile in order to obtain a complete picture of possible ground behaviors. It is all well and good to know that your tunnel will be excavated in a deposit of clay but is that clay soft or stiff, will it squeeze or swell, or is it highly plastic and able to “stick” to the tunneling equipment. Sandy soils can be loose or dense, highly abrasive, and have highly variable values of permeability; all of which must be taken into account during your brainstorming effort.

RELATED: Incorporating Risk Management into the Contact Document

Rock deposits are equally complex and are described by a suite of laboratory test results that have been developed almost exclusively for tunneling projects. Laboratory testing for tunnels in rock is based upon exacting American Society for Testing and Materials (ASTM) standards that must be applied with great precision to the testing procedures. Inaccurate rock test results can have highly detrimental impacts on how anticipated ground conditions are described in the contract document.

Lastly, it is necessary to describe how the groundwater regime will interact with the tunneling operation. Groundwater has three potentially important and independent impacts on a tunneling project; i.e. how much water is expected to enter the tunnel, at what pressure, and is that water “contaminated” in some manner. Large water inflows into a tunnel can disrupt tunneling procedures and greatly slow tunnel progress. High groundwater pressures can cause the ground to flow or collapse, resulting in extra measures to control and/or to support the ground. Groundwater contamination can create unsafe conditions for the workers and even naturally occurring elements in the groundwater can make it difficult to dispose of at the ground surface without expensive treatment facilities. Tunnels can also encounter manmade contaminates such as gasoline or cleaning fluid and other problems such as combustible gases.
As a result of the above, the geotechnical engineer must create some model, some description, and/or some characterization of the ground that can be carried forward into design. Reasonable and appropriate assumptions must be made about soil stratigraphy, soil and rock properties, and groundwater characteristics such that a good representation of the existing ground is possible. At its best, this work is the output of a process involving judgment, knowledge, experience and maybe a little luck, which results in a good working model for, but probably not an absolutely perfect description of, the ground.

Based on the above interpretive effort and on one’s knowledge of tunneling methods and ground improvement techniques, it is possible to develop a plan for your tunneling project that reduces risk exposure to what is referred to in tunneling terminology to “as low as reasonably possible” (ALARP). It is truly not possible to eliminate all risk for a tunneling project as Mother Nature has a perverse ability to make things difficult for tunneling contractors. However, the bottom line on all of the above is to assemble a contract package that you sincerely believe is appropriate for the existing ground condition and that will allow the contractor to price the work required to create the space required for construction of the finished facility.

The actual contract document that is used to summarize all of the above thoughts, evaluations, and decisions is the GBR. A complete description about how one goes about preparing a GBR is described in the Geotechnical Baseline Reports for Construction, Suggested Guidelines by the American Society of Civil Engineers, 2007, and given below are quotes from that document about the purpose and scope of a GBR:

1.2 The Geotechnical Baseline Report

Projects involving subsurface excavation present many risks, all of which must be assumed by either owner or the contractor. The greatest risks are associated with the materials encountered and their behavior during excavation and installation of support.

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.
  • Enhancement of the contractor’s understanding of the key project constraints.
  • Assistance to the contractor in evaluating the requirements for excavating and supporting the ground; and
  • Guidance to the owner in administering the contract and monitoring performance during construction.

From the above it can be ascertained that the primary objective of a GBR is to help the contractor understand important ground characteristics and anticipated ground behaviors in order to improve the probability for project success and that is the essence of a good GBR. Good tunneling must also be thought of as a team effort whereby the owner, the designer, and the contractor work together to produce a successful project. The contractor is not the enemy and one of the most important roles of the geotechnical engineer is to make a sincere effort to provide the contractor with the subsurface information necessary to evaluate the best methods for excavating, controlling and supporting the ground.

Risk Allocation/Contract Document

The contract document for a tunneling project consists of five parts:

  • General Conditions,
  • Plans,
  • Specifications,
  • GDR, and
  • GBR

One of the most significant risks that must be “allocated” in the contract documents is the risk of a differing site condition. As defined by numerous contract forms, a differing site condition is a ground condition discovered during tunneling that differs materially from those ground conditions indicated by the contract or which differs materially from those ground conditions that would normally be anticipated by the contractor under similar circumstances. For instance, if the contractor encountered rock where none was indicated by the test borings or encountered archeological remains or abandoned utility lines when there was no provision in the contract for that possibility, then the contractor would be eligible to file a claim for a differing site condition. The other possibility for a differing site condition associated with tunneling projects is related to claims by the contractor that the ground is not “behaving” as he was led to believe by contract indications, including the project specifications.

Virtually all claims for differing site conditions are filed during construction when the contractor is attempting to excavate, control, and/or support the ground and revolve around the answers to two questions:

  1. What did the contract indicate about ground conditions?
  2. What ground conditions did the contractor actually encounter during construction?

Clearly, the vast majority of contract indications related to ground conditions have to do with work performed by the geotechnical engineer. As indicated above, all of the data about ground conditions is contained within the GDR and all of the interpretations derived from that data will be presented and explained in the GBR. Hence, and as a result, the geotechnical engineer will be front and center during the evaluation of a differing site condition. Another big problem associated with a differing site condition is the possibility of causing significant project delay. As indicated above, tunneling projects have a serial construction schedule which means that if the face of excavation is not advancing then pretty much all activities on the project are delayed and the costs associated with that delay can be substantial.

Clearly, it is not possible to discuss all of the possible ramifications of a claim of differing site condition in this short paper (i.e. numerous entire volumes of legal proceedings have been published about this topic) but it is important to note that all three steps for risk assessment as discussed above involve services provided by the geotechnical engineer. Subsurface risk identification, subsurface risk minimization, and subsurface risk allocation are all central to successful tunneling and are part of the package of services provided by geotechnical engineers. As a result, it is right to say that this activity is not for the faint of heart and should only be performed by geotechnical engineers who are well versed in the intricacies and nuances associated with tunneling projects. Performing subsurface investigations and preparing the geotechnical reports for inclusion in the contract document for a tunnel is not a great opportunity for on-the-job training. It is also imperative for tunnel owners to realize and to accept the importance of good quality geotechnical services when it comes to tunneling and to be prepared to make the investments that are required in order to make certain that this work is being performed in a proper manner.

Risk Management/Construction Monitoring

As mentioned above, a claim of differing site condition involves providing answers to two questions:

  1. What did the contract indicate about ground conditions?
  2. What ground conditions did the contractor actually encounter during construction?

The process of tunneling is a highly dynamic and transient activity and you either know what was happening on a day-to-day basis or you do not. Most specifications for tunneling projects require the contractor to monitor his activities on a daily basis and to prepare reports associated with those activities, but it is also absolutely essential that the owner have an independent set of reports describing those same activities from the owner’s perspective. Accurate project records with copious photographs, information from data loggers on the tunnel boring machine (TBM), and observations of ground reactions are all required when evaluating the validity of a claim of differing site condition.

It is also necessary for the owner to make certain that the field work is being performed in accordance with contract requirements for the means and methods of construction but even more so for the adequacy of the finished facility. Tunnels are designed to last a long time and are very difficult and expensive to repair if they are not constructed in a proper manner. Hence, all portions of the finished facility that could be impacted by adverse ground reactions must be observed and constructed in accordance with contract requirements.

It is more or less inevitable that some form of claim or dispute will develop during the construction of a tunneling project and the owner must be prepared to react in a positive and proactive manner to those situations. One of the most important aspects of successful tunneling as stated above is the ability of all parties associated with the project including owners, contractors, and designers (with the help of geotechnical engineers) to work together as a team. Conversely, it is not a good thing when this team devolves into a series of shouting matches. An excellent set of good quality project records from both the contractor and the owner contributes both to the team effort and to the timely and, hopefully, satisfactory resolution of contractor claims.


The entire process of risk assessment for tunneling projects is intimately associated with the services provided by geotechnical engineers, geologists, and engineering geologists. Tunneling risks, to a large degree, are related to how well one understands the existing ground condition, and, more importantly, the ground behaviors and/or reactions that will occur during tunnel construction. Hence, it is fair to say that successful tunneling projects go hand-in-hand with the successful provision of comprehensive contributions from knowledgeable and experienced geotechnical professionals.


Brierley, G.S. (Oct. 2017) “Incorporating Risk Management into the Contact Document.” Tunnel Business Magazine.
Brierley, G.S. (Sept. 2017) “Intelligent Tunnel Design.” Tunnel Business Magazine.
Brierley, G.S. (April 2017) “Differing Site Conditions OR Why I Believe Mother Nature is a Bitch” Tunnel Business Magazine.
Brierley, G.S. (September/October 2015) “Geotechnical Risk Management.” Geostrata Magazine.
Brierley, G.S. (March/April 2014) “So, Why Do You Want to Write a GBR?” Geostrata Magazine.
Brierley, G.S. (Feb. 2014) “To GBR or Not to GBR; Is that the Question?” Geo-Congress.
Essex, R. (2007) “Geotechnical Baseline Reports for Construction, Suggested Guidelines.” American Society of Civil Engineers.
Hatem, D. (1998) “Subsurface Conditions: Subsurface Investigations and Geotechnical Report Preparation.” Chapter 3
Hatem, D. (1997) “Geotechnical Baselines – Professional Liability Implications.” The CA/T Professional Liability Reporter, Vol.3, No. 1
Edgerton, W. (2008) “Recommended Contract Practices for Underground Construction.” Society for Mining, Metallurgy, and Exploration, Inc.

Gary S. Brierley is president of Dr. Mole Inc. He is a chairman of the TBM editorial advisory board and a frequent contributor to the magazine.


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