[EDITOR’S NOTE: This is the sixth in a series of articles from Dr. Gary S. Brierley reflecting on the history of tunneling. This first article appeared in the August 2014 issue of TBM: Tunnel Business Magazine and examined tunneling from its ancient roots to the present day. Subsequent articles examine specific elements within the tunneling market, with particular attention paid to the U.S. market. This installment discusses the period of tunneling from 1970 to 1990, which saw wholesale changes in virtually all aspects of the industry.]
What can you say about the tunneling industry from 1970 to 1990? Well, the word “revolutionary” comes to mind. Revolutionary is defined as complete or fundamental change, and that is exactly what happened to the tunneling industry from 1970 to 1990.
If you entered a soil tunnel prior to 1970, you actually saw the ground at the face of excavation. By 1990, all you saw was concrete and steel. And the same was the case for rock tunnels. Prior to 1970, most rock tunnels were excavated by drilling and blasting. After 1990, almost all civil rock tunnels were excavated by TBMs.
In addition, all other aspects of tunneling – including subsurface investigations, design procedures, planning activities, and contracting practices – experienced similar, fundamental changes that converted tunneling projects from an expensive, last resort option for water and transportation infrastructure projects to what is now considered the preferred alternative. Nothing provides a longer term value and/or a less intrusive community and environmental impact, especially for densely populated urban areas, than does an underground project. Given below is a quote from Ellis Armstrong from his paper titled “The Challenge of Tunneling,” which was published in the proceedings for the very first Rapid Excavation and Tunneling Conference (RETC) held in Chicago in 1972:
From this great reservoir of knowledge and ability, we have assembled here North America’s greatest collection of expertise in excavation technology.
We are here because we are aware of the critical need to accelerate the development of underground rapid excavation and tunneling technology in order to meet two great worldwide as well as continental concerns: (1) urbanization of the population, and (2) development and conservation of natural resources with due concern for the needs of mankind and the necessity of protecting and enhancing the environment of this fragile biosphere.
At the 1974 RETC conference Dr. Ralph Peck presented a paper titled “The Technology of Underground Construction, Present and Future” with the following concluding paragraph:
In this discussion, possibly you have detected one strong personal conviction: that underground construction is advancing technologically on many fronts, and will continue to do so. I do not look for the earth shattering breakthrough that will utterly revolutionize underground construction. Rather, I foresee overall progress, rapid in some areas, and slower in others.
The gains in technology will be fruitful only if applied, and application will depend mostly on three factors: a better working relationship among owners, designers, and constructors; a transfer of attention by designers from the analysis of the lining to consideration of the behavior of the surrounding medium; and the encouragement and use of geotechnical specialists to develop exploratory and observational programs that will furnish, for a given expenditure, the maximum pertinent information for design and construction.
As usual, Peck was on the right track except for his observation that the changes to tunnel design and construction would not be revolutionary. In fact, tunneling was revolutionized largely along the lines of what Peck foresaw, i.e. contracting practices, ground behavior, and subsurface explorations.
During the early 1970s, the federal government was heavily involved in funding numerous tunneling projects for highways, subways and water tunnels and was actively involved in funding the U.S. National Committee on Tunneling Technology (USNCTT). Most of the leading practitioners of that era were members of USNCTT and took an active role in furthering tunnel design and contracting practices. During that time frame numerous federally funded research projects were undertaken, the results of which led to publication of the following:
Better Contracting for Underground Construction (published in 1974)
Proceedings on the Workshop on Tunnel Lining Design (1979)
Geotechnical Site Investigations for Underground Projects (1984)
It was also during this period of time that RETC, the Underground Technology Research Council (UTRC) and the America Underground-Space Association (AUA) became active in promoting underground construction and in recommending contracting practices for underground projects.
Better Contracting for Underground Construction was a well researched and well written document that has had an enduring positive impact on contacting practices for tunneling projects. In particular, this publication led directly to improved subsurface investigations, to the use of alternative dispute resolution, to the widespread acceptance of the differing site conditions clause, and to the use of geotechnical interpretive reports with an emphasis on anticipated ground behavior – more or less as Dr. Peck had predicted. Given below are summary discussions of each of these topics.
Prior to 1970, it was common practice for tunnel owners to provide prospective bidders with the results of subsurface investigations without accepting responsibility for their accuracy and with no explanation of the design and/or construction significance of that information. In the terminology of the day, prospective contractors were expected to make “reasonable and prudent” assumptions about this information and to provide adequate “contingencies” in the bid for their anticipated construction procedures. This method of approach was clearly lacking as a contractual mandate and resulted in numerous, large claims related to both inappropriate construction procedures and damage to adjacent third parties. As a result of those difficulties, and based on a recommendation from its Board of Engineering Consultants, the Washington Metropolitan Area Transit Authority (WMATA) implemented the use of Geotechnical Design Summary Reports (GDSR) for its projects. At about that same time, the Colorado Department of Transportation (CDOT) re-introduced the use of a Dispute Review Board (DRB) for construction of the second bore of the Eisenhower Tunnel largely upon the recommendation of Al Mathews. Both of these concepts, which were shown to be successful, led directly to two additional important publications by the UTRC:
Avoiding and Resolving Disputes in Underground Construction (1989)
Geotechnical Baseline Reports for Underground Construction (1997)
Most recently (2008), the Society of Mining, Metallurgy & Exploration (SME) published a comprehensive update of the 1974 report on better contracting titled Recommended Contract Practices for Underground Construction. It is no exaggeration to state that this book is required reading for every owner, designer and contractor involved with underground construction.
Again, as stated by Peck, it is not enough to know what type of ground will be encountered by a proposed tunnel, it is also necessary to anticipate how that ground will respond and/or react to the tunneling process. Understanding ground behavior and making a conscious decision about the best way to control that behavior within acceptable limits are central to all further decisions related to tunnel design and construction, about how to protect third parties, and about how to avoid expensive disputes. As a result of this realization, the period from 1970 to 1990 saw great strides in the art and science of soil mechanics, rock mechanics, engineering geology, and the observational approach.
With respect to soil tunnels, new models were developed about how pressurized face equipment could be used to control adverse ground reactions associated with flowing and squeezing soils. Great strides were also made in anticipating the amount of water inflow to be encountered during open face tunneling. For rock tunnels, new models needed to be developed with respect to the speed and cost of excavating rock with a TBM as compared to drilling and blasting. Design criteria for supporting rock masses with rock bolts and shotcrete needed to be established and verified as a result of field observations. Substantial advances in engineering geology allowed greatly improved methods for anticipating the types and quantities of soil and rock deposits to be encountered during tunneling. Accurate predictions of ground behavior led to the implementation of comprehensive observational programs to be used during construction for comparison with those predictions. All in all, an emphasis on ground behavior rather than ground type has improved all aspects of tunnel design and construction.
It was also during the period 1970 to 1990 that advances were made with respect to computer models for anticipating ground behavior, which is a topic that the author of this article feels has had dubious impacts on tunnel design and construction. Computer modeling creates the “impression” of reliability, but the accuracy of computer “output” is totally dependent upon the accuracy and completeness of computer “input,” which is highly problematic for large, long structures in highly variable ground conditions. Tunnel design and construction is and always will be dependent upon practical experience; i.e. experience derived by actually being in the field observing and dealing with Mother Nature’s unlimited ability to “cause trouble” for tunneling projects. Without that experience, computer modeling is approximately equivalent to “back of the envelope” estimates of what might happen underground based upon input parameters that can only be described as “guestimates” of various soil and rock properties. Computer modeling can be beneficial, but it is hardly the be-all and end-all of an accurate prediction of ground behavior.
Probably the area of tunnel design and construction that benefitted most dramatically from advances made during the period 1970 to 1990 was subsurface investigations and geotechnical report preparation. Although it is difficult to summarize all of the literature devoted to this topic in the last 40 or so years, the following is now considered to be an appropriate approach to this topic by responsible tunnel owners:
1. Conduct a comprehensive and well documented subsurface investigation and present the results of that investigation to prospective bidders in a Geotechnical Data Report that is made part of the contract document without disclaimers. Prior to 1970, the emphasis for subsurface investigations was on their cost. After 1990, the emphasis for subsurface investigations was on their value and on the need to provide a reliable package of subsurface information for all aspects of tunnel design and construction.
2. Prior to 1970, any interpretation of the subsurface data performed as part of design was either withheld from prospective bidders altogether or was provided for information only and completely disclaimed. After 1990, it became standard practice to prepare some form of geotechnical interpretative report, preferably in the form of a Geotechnical Baseline Report, and to make it part of the contract document. Considerable confusion still exists with respect to the purpose and scope of a GBR for each different type of tunnel, but the consensus is that GBRs have improved tunnel design and construction and have greatly mitigated contractual disputes by providing prospective bidders with geotechnical “baselines” for input into their bids. Those baselines have been shown to greatly limit the need for large bid contingencies associated with ground conditions and have saved large sums of money for many projects.
The above two developments with respect to subsurface investigations, in and of themselves, have greatly improved tunnel design and construction and have facilitated the use of underground construction to solve myriad infrastructure needs with minimal environmental impact and community disruption.
Previous installments in this series of articles have concentrated on the technical developments that occurred in the tunneling industry during the period 1970 to 1990 with respect to tunnel boring machines, drilling and blasting, small-diameter tunnels, and pressurized face equipment, but the non-technical developments with respect to contracting practices were equally significant. All in all, the tunneling industry has set the stage for a very bright future that will provide infrastructure projects with lasting value to the world’s metropolitan areas.
[AUTHOR’S NOTE: I would like to thank Dr. Ron Heuer for his contributions to this paper. Not only did Ron help with the paper, but he was actually involved in many of the positive developments discussed above during the period 1970 to 1990.]