By Dr. Gary Brierley
Whenever one wishes to investigate a technical topic such as tunneling, it is always interesting to begin with a discussion of the historical perspective. For tunneling, this historical discussion can be divided into three principle eras:
The Ancient Era — From Biblical times to 1880
The Industrial Era — From 1880 to 1970
The Modern Era — From 1970 to Present
Editor’s Note: This is the first in a series of articles from Dr. Gary S. Brierley reflecting on the history of tunneling. This first article examines tunneling from its ancient roots to the present day. Subsequent articles will examine specific elements within the tunneling market, with particular attention paid to the U.S. market.
Without a doubt, the best description of ancient tunneling practices was provided in Henry S. Drinker’s seminal treatise on this topic titled Tunneling, Explosive Compounds, and Rock Drills that was published by John Wiley & Sons in 1878. Drinker claimed that his book discussed all tunneling activities from the time of Ramses II until 1878. Drinker was a professor at Lehigh University and his research including nearly 2,000 tunnel case histories with particular emphasis on tunneling practices in Germany, Italy, Great Britain and the United States; although tunneling practices in more than 15 other countries were also documented.
As one might expect, the majority of early tunnels were constructed for use as tombs and/or religious practices where the builders were free to choose their sites so as to avoid the need for ground support. With time, however, tunneling began to be used for drainage, water supply, sewers and transportation projects (including canals) where tunnel location became paramount and various, sometimes difficult, ground conditions needed to be managed. In his discussion of tunnel projects, Drinker is liberal in his use of the word “astonishing” when describing the size, shape, depth and length of these ancient openings such as two-lane “highways” extending for thousands of feet. For one project, Drinker discussed the use of 30,000 workers toiling for 11 years.
In his treatise, Mr. Drinker refers to his “Modern Era” of tunneling beginning in the 1700s with the regular use of black powder and various methods of timbering. Final linings were provided using bricks, dressing stone and cement, but only when absolutely required by bad ground. As recorded, the vast majority of ancient tunnels were constructed in rock until the late 1700s when consideration was given to tunneling under the Thames River in London. Apparently, the French had built a soft ground tunnel for a canal in 1803 that required continuous arching and this set the stage for numerous similar projects in the following 50 years. In Great Britain alone, it was estimated that a total of more than 200,000 feet of canal tunnels had been built by 1850. It was also during this period of time that tunnels began to be built for railroads that vastly increased both tunnel size and the need for tunneling through difficult ground. Foremost among these railroad projects was the Mont Cenis Tunnel in Europe and the Hoosac Tunnel in the United States.
Whenever bad ground was encountered (whether faulted material in rock or any type of soft ground), ancient tunnelers invariably resorted to some form of timbering to support the ground both at the face and along the length of the tunnel. Sometimes, timber was used for the final lining, but, more often than not a final lining of brick, dressing stone or cement was installed inside the timber support. One of the great costs associated with using timber for tunnel support was the large size of opening required in order to allow for construction of the inner, final lining. During this process, literally scores of carpenters were employed and entire forests were denuded in order to provide the raw materials. A few quotes from Drinker are particularly apt:
- There probably is no branch of engineering in which the situations to be met and the difficulties to be overcome are so diverse and so varied as in tunneling. It, therefore, is well to take advantage of all experiences to shape one to good judgment, when occasion may arise for its exercise in new work.
- Above all things, we should remember that correct timbering seeks to avoid rather than draw pressure, to meet it at once, and check the tendency to fall or to swell, as any movement of the kind, if not promptly checked, grows in an increasing ratio; and in excavating and timbering, we must always bear in mind the immense advantage, in soft ground, of early draining out the water by a bottom-heading.
We all like to think about how smart we are, but the ancients also knew a thing or two about tunneling in bad ground.
In the final analysis, however, prior to 1880, only the judgment and experience of persons in the tunnel were available to decide on the best method for initial support and on the thickness and strength of the final lining including the use of “inverted” arches in especially heavy ground conditions. No analytical methods and/or engineering precepts were available to predict ground loads that needed to be supported for any given ground condition. It is truly amazing what ancient tunnelers were able to accomplish based on the tools and procedures available to them at that time.
One other extremely important development that occurred near the end of the Ancient Era of tunneling was the introduction of tunneling shields. The “shield” was patented by Marc Isambard Brunel in 1828. His original device was highly compartmentalized and totally mechanical in that hundreds of screw jacks were attached to individual breasting boards; each of which needed to be reset before the shield could move forward even a few feet. Brunel’s shield was only used for the construction of one tunnel in the 1820s, the first Thames River Tunnel, but that effort was so time consuming and so expensive that no shield tunneling was attempted again until the late 1860s. Brunel can be said to have invented the concept of a shield, but he was not able to perfect a practical application.
Shield technology took a great step forward in 1878 based on a provisional patent developed by a Mr. Barlow. In general, this patent described four main features for his proposed shield:
- A cylindrical overlapping skin,
- A cast iron final lining,
- The placement of grout outside the lining, and
- The use of screw jacks to move the entire shield forward as a single unit.
It was this type of machine that was designed by James Greathead to build the Tower Subway under the Thames River in London. At about the same time, a Mr. Beach was using a shield to build a subway under Broadway in New York City. Beach’s shield utilized a series of horizontal shelves to control the face and the first application of hydraulic rams to propel the shield. (As an interesting side note, you can observe photographic reproductions of the Broadway subway in the wallpaper at some Subway sandwich shops.) After 1880, it was the Barlow/Beach/Greathead method of tunneling that set the stage for all shield tunneling in the Industrial Era. It was also during the Industrial Era that compressed air began to be used for tunneling in water-bearing strata.
To summarize, tunneling practices during the Ancient Era had been totally upended by 1880 based on scientific and engineering developments associated with the beginning of the Industrial Age. Dynamite, compressed air rock drills, lining construction and various subsidiary developments such as ventilation and survey control had totally revolutionized rock tunneling. In addition, shield technology with compressed air was showing great promise for the excavation and support of all types of soil conditions that had hitherto been extremely costly, time consuming and dangerous to penetrate.
The most interesting aspect of the Industrial Era of tunneling is that there were no big ideas associated with tunneling. During this entire period it appears that tunnel constructors were content to make incremental improvements to those ideas that had been established by 1880. In rock tunneling there were continuous improvements in drilling equipment and blasting procedures as well as surveying, ventilation, mucking and consolidation grouting. Richardson and Mayo’s book titled Practical Tunnel Drilling in 1941 would have been easily recognizable to tunnel engineers in 1891 or 1921. For instance, one entire chapter of Mayo’s book is devoted to soft-ground timbering together with photos and diagrams that were essentially identical to those published by Drinker in 1878. For shield-driven tunnels, cast-iron segments had become common, although steel and precast concrete segments were also being used. Shields were stronger and more mechanized, but their design was little changed from 1900 and a good deal of face control for the shield was still provided by timber. Compressed air tunneling was used for difficult ground but construction dewatering was now available as a viable alternative.
With respect to ground support, the biggest changes were with respect to steel ribs and steel liner plates. Beginning in the 1920s, steel was used more often because it was easier to install, it was stronger than wood, it greatly reduced the tunnel cross-section, and it could be encased in the final cast-in-place concrete lining. Steel ribs with timber backpacking and wood lagging, when required, became very common and well established in the 1930s and 1940s. Steel liner plates were also used, when required, for various tunneling applications in unstable soil or faulted rock. Cast-in-place concrete, with or without reinforcement, was almost universally used for the final lining with various types of pipe and/or grouted steel linings used in tunnels subjected to internal pressure.
Beginning in the 1930s and accelerating through the 40s, 50s, and 60s, several new, big ideas began to impact the art and science of tunnel design and construction. Behind the scenes, but important to all of these developments, was the emergence of geotechnical engineering, engineering geology, and subsurface exploration based largely on the work of Dr. Karl Terzaghi. Although numerous individuals participated in this process, Terzaghi was the guiding light for these engineering specialties, especially the influence of pore pressure on the behavior of soil and rock and the overriding significance of field observations. For design, Terzaghi recommended intense geological mapping exercises and subsurface exploration programs and, during construction, field observations in order to make certain that the ground was behaving as anticipated.
With respect to tunneling, Terzaghi’s treatise on Rock Defects and Loads on Tunnel Support published by the Commercial Shearing & Stamping Company in 1946 is, to this day, one of the best and most informative discussions of this topic in the tunneling literature. It is truly amazing to read this treatise and to realize not only how insightful and well informed was Terzaghi, but also to realize how well developed was the expertise later identified as “Engineering Geology.” Largely as a result of this publication, Terzaghi became the first person to relate soil and rock properties and behavioral characteristics to the ground loads being imposed on a tunnel lining. For anyone interested in tunneling this paper is still a must read.
Following hard on the heels of the above design developments were numerous construction innovations as listed below:
- Rock Bolts
- Tunnel Boring Machines
- Pressurized Face Shields
- Ground Improvement Technologies
Beginning in the mid-1950s and continuing through the 60s, each of the above construction technologies had profound impact on the costs, risks, schedules and applications of underground construction for both civil engineering and mining projects. In fact, beginning in the 1960s and continuing through the 90s, the cost of tunneling, especially as compared to other types of civil engineering projects, was actually reduced, making tunnels the preferred alternative for many water resource and transportation projects. Tunneling was also shown to be highly compatible with many of the environmental and community impact concerns that were developing during that time period.
Rock Bolts – For the first time ever it was possible to reinforce a rock mass. Drilling holes and inserting steel dowels into the rock produced three profound effects on the rock:
- Rock dowels were used to support loose blocks of rock around the crown and sidewalls of the tunnel, creating a safe environment, and promoting arch action.
- Rock reinforcement also caused the loading scenarios to become more uniform by preventing large, concentrated loads from impacting the lining. Uniform loading conditions promote lining thrust and result in smaller moments.
- In bad ground, longer and more frequent rock bolts were used to create a stable arch of rock around the opening that supported a majority of the load being imposed by the ground.
Hence, in bad ground, rock bolts were used to facilitate and promote the formation of a rock arch that is highly effective at resisting heavy ground pressure.
Shotcrete – Although gunite had been used in tunnels for some time, primarily to prevent raveling in shafts and to control faulted material, it wasn’t until the early 1960s that shotcrete began to be used as a tunnel lining. Shotcrete provides a high capacity and highly versatile support mechanism especially in blocky, drilled and blasted rock masses. Shotcrete can be installed close to the face, it is highly compatible with staged excavations, and it can be increased in thickness as the heading advances and/or the opening is made larger. In some applications it can also be used as the final lining. Shocrete and rock bolts, used together, can be made to support almost any ground condition.
Tunnel Boring Machines – TBMs essentially allowed shields to be used in rock. TBMs cause much less disturbance to the rock, create a uniform circular opening, and provide a chamber in which the initial support can be erected. Initial support methods compatible with TBMs include rock bolts, steel ribs, liner plates, temporary segments, and one-pass bolted and gasketed segments. Although shotcrete can be used with TBMs, shotcrete rebound can be very damaging to the trailing gear.
Pressurized Face Shields – Earth pressure balance and slurry shields greatly facilitated the types of soils that could be excavated and the rate of advance in those soils, but actually limited the types of linings that could be used with those shields. Because of the pressure exerted at the face, it is also necessary to withstand that pressure along the body of the tunnel. Hence, liner plates, jacked pipe, and/or bolted and gasketed segments must be used with a pressurized shield. These linings must also be designed to withstand the handling stresses and jacking forces imposed by the shield. Open face shields, however, are still appropriate for many soil conditions and can be used with ribs and boards or temporary segments.
Ground Improvement Techniques – Dewatering, grouting, freezing and other forms of ground improvement can have huge, beneficial effects both on the type of lining and the required capacity of that lining. Tunnel linings, and especially shaft linings, can benefit from the increase in ground strength and decrease in ground compressibility resulting from ground improvement procedures.
In summary, the Modern Era of tunneling began in 1970 and resulted in significant advancements in the in the art and science of tunneling which will be discussed in the following installments of this retrospective on tunneling. For now, it is interesting to note two of the most interesting quotes from Drinker’s book in 1878!:
It is worthy of note that this art of tunneling has gone in past ages hand in hand with the higher civilization of each era. As a people become more civilized, its civilization can be gauged by its progress in tunnel construction, and whatever be the particular motive, the result is always the same.
Finally, with modern civilization, we have tunneling in its last and greatest development, and we see that in proportion to the civilization of a people will be found their development in this art. This is most natural, for, of all branches of construction, it is one of the most difficult. A barbarous people may, perhaps, develop a high degree of perfection in the mere art of open-air building, where stone can be piled on stone, and rafter fitted to rafter, in the light of day; but it takes the energy, knowledge, experience, and skill of an educated and trained class of men to cope with the unknown dangers of the dark depths that are to be invaded by the tunnel-man.
I don’t know about you the reader, but the author of this paper would like to think of himself as part of this higher form of civilization.