A Historical Perspective
[EDITOR’S NOTE: This is the second 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 will examine specific elements within the tunneling market, with particular attention paid to the U.S. market. This installment discusses hard-rock tunnel boring machines.]
There have been numerous excellent articles written about the history of tunnel boring machines and the primary objective of this paper is to provide an overview of those articles for the casual reader. The “invention” of a machine capable of excavating and supporting large-diameter tunnels in hard rock was one of the greatest achievements in the history of tunneling, and to say that this method of rock excavation revolutionized the design and construction of tunneling projects would be an understatement. Even more interesting is the realization that most of the important developments with respect to the art and science of TBMs have taken place within the lifespan of old-timers in the business, such as myself.
The first TBM intended to cut hard rock was the Wilson Patent Stone Cutting Machine. It was invented in 1851 and was mobilized to the east portal of the famous Hoosac Tunnel in North Adams, Massachusetts. This machine was designed to cut a 13-in. wide circumferential ring, 26 ft in diameter in solid granite. After advancing several feet, it was intended to withdraw the machine and blast the rock from inside the ring. This machine was built from cast iron, powered by steam, and employed roller cutters that were amazingly similar to what is being used today. That machine had actually advanced in the rock for 10 ft when it became obvious that it was neither strong enough nor powerful enough to cut such hard rock. (As an aside, it is interesting to note that my hometown happens to be North Adams, Massachusetts, and as a result of that coincidence, I have actually visited the east portal of the Hoosac Tunnel and have observed, with my own two eyes, the grooves cut in the rock by the Wilson TBM.)
A second TBM was also mobilized at the west portal of the Hoosac Tunnel. This machine was 10 ft in diameter with a conical head and was intended to cut a circular top heading in the badly faulted ground. Initial experiments with this machine were promising, but the contractor who developed this equipment was forced into bankruptcy before it could be fully utilized and the replacement contractor abandoned the TBM in favor of using staged excavation. Thus ended some early experiments with TBMs, and for the next 100 years nearly every rock tunnel located anywhere in the world was excavated by drilling and blasting.
Fast forwarding to the 1950s, numerous successful mechanical devices were being used for coal mining when, in 1952, a fellow named James Robbins was asked to utilize these concepts for the construction of tunnels at South Dakota’s Oahe Dam. The cutterhead for his TBM utilized rows of drag bits and disc cutters to excavate weak shale. In essence, the drag bits cut grooves in the rock into which the roller cutters broke the rock. Like many inventors, James Robbins was obsessed with the success of his concept and was able to convince several project owners and contractors to support his experiments and to deal with various problems that developed during construction. Unfortunately, James Robbins was killed in a plane crash in 1958 but his determination, his foresight, and his son, Dick Robbins, combined to set the stage for many of the subsequent developments related to tunnel boring machines. In general, these developments centered on the following:
- What is the best way to cut the rock?
- What is the best way to stabilize, propel and steer the TBM?
- What is the best way to provide backup equipment and maintenance procedures?
- What are the best methods for handling materials both into and out of the tunnel?
- What is the best way to provide a reliable cost estimate for TBM tunneling?
James Robbins’ TBM at Oahe Dam was little more than a soil shield with a rotating cutter head. Although machine simplistic compared to modern day TBMs, it introduced two of the most basic concepts upon which all subsequent TBMs are based:
- A rotating, cutting head at the face that is capable of cutting the rock and allowing the advance of a shield, and
- A circular shield in rock that is available to protect the workers and equipment and inside of which tunnel support can be erected.
Based on the continuous improvement of these two concepts, all modern day TBMs are derived.
With respect to cutting the rock, early concepts were based on either button cutters, such as used in oil field drilling or continuous coal mining machines. It wasn’t long, however, before technical innovation and basic research began to concentrate on roller cutters as the best method for excavating hard rock. Today, we take this concept for granted, but it took many years of both field and laboratory investigations in order to perfect the concept of using roller cutters to fracture hard rock.
During this time frame an interesting project took place in Toronto that had important implications for TBM design. Initially, this machine was equipped with both fixed cutters and roller cutters, but, based on field observations, when the fixed cutters were removed, it was observed that the rate of advance for the machine did not decrease. Hence, it was discovered that roller cutters could simultaneously cut the grooves and fracture the rock into those grooves.
Another significant breakthrough project for TBMs was a hydroelectric project in Tasmania in 1961. This TBM, as designed and built by Robbins, featured roller cutters and floating grippers.
Prior to this project, machine propulsion and steering were provided by a rigid steel frame but using this method made it difficult to steer the TBM. At Tasmania, Robbins utilized a floating gripper assembly that greatly improved tunneling efficiency. Also utilized for the Tasmania project were high-capacity, metal-to-metal cutter bearings. Hence, this project clearly confirmed the concept of using roller cutters to both cut a groove in the rock and then fracture the rock by tension between those grooves. As a result of those innovations, the Tasmania TBM produced a rate of advance of 229 ft per week, which was twice the world record advance rate for drill-and-blast. Clearly, the tunneling industry had found a new way to economically excavate tunnels in hard rock.
With this concept firmly in mind, laboratory experiments at the Colorado School of Mines (CSM) began to codify important TBM machine characteristics and the rock mass variables necessary to maximize the rates of advance for a TBM. Since a great deal of industry practice was considered confidential at that time, it took an independent laboratory program to make these concepts generally available. It was at this time, during the mid to late 1970s, that a young graduate student named Levent Ozdemir was asked to participate in a National Science Foundation research project on rock excavation; truly a fortuitous occurrence both for Ozdemir, who would go on to teach at CSM for 30 years, and for the tunneling industry at large. As a result, full-scale laboratory tests with different types of cutters and bearings, different thrusts and torques, and different cutter spacings were used to both maximize performance and to standardize the methods for cost estimating. Thus, the concept of predictive modeling that is now universally applied to TBM tunneling projects was developed.
Over time and as a result of many difficult case histories, TBM technology has been advanced to deal with unstable ground conditions at the face, swelling, squeezing and overstressed rock, difficult ground such as faults and wet, fractured rock, and many other schedule-wrecking problems. Integral to many of those developments was the use of “double-shielded” TBMs. In 1972, the Robbins Company developed the first double-shielded machine for use on a hydroelectric project in southern Italy. Ground conditions for this project consisted of both competent and fractured granitic rock – hence the need for a two-in-one machine design. In competent rock, the rear shield gripped the rock and provided the thrust reaction necessary for the front shield to move forward. In fractured rock, the rear shield was retracted and the machine was thrust off of the temporary lining. Once again, this represented a significant technological advance based on field experimentation. In addition to shielding, field experimentation also led to developments in recessed cutters, back-loading cutters, and the erection of high-capacity, bolted and gasketed tunnel linings for propelling TBMs through unstable ground.
Clearly the above write-up provides only an outline of important TBM developments, but the fact remains that roller cutters with lots of thrust and lots of torque were shown to be the preferred method for cost-effectively excavating civil engineering tunnels in hard rock. However, as will be discussed in the next installment of this series, the drill-and-blast industry did not sit idly by while this was happening.
[AUTHOR’S NOTE: For more information about TBM history, the reader is referred to an excellent and extremely comprehensive book authored by Barbara Stack and published by John Wiley & Sons in 1982 entitled, “Handbook of Mining and Tunnelling Machinery.” It is not an exaggeration to say that Stack did for the history of tunneling from 1878 to 1982 what Henry S. Drinker accomplished with his book published in 1878.]
Dr. Gary S. Brierley is president of Dr. Mole Inc. He began his career in 1968 with a Bachelor’s Degree in Civil Engineering from Tufts University and Masters and Doctoral Degrees from the University of Illinois in 1970 and 1975, respectively. During that time Dr. Brierley was fortunate to work on the instrumentation program for DuPont Circle Subway Station in Washington, D.C., a project that formed the basis for his doctoral dissertation. Since that time, Dr. Brierley has devoted his entire professional career to the design and construction management of underground openings.