“Sweet Spot” Engineering
By Don Del Nero

The underground environment is the most challenging environment to work in – even more challenging than outer space.

If anyone has ever played baseball you will quickly recognize what it means to hit the ball on the “sweet spot” of the bat. A simple definition for sweet spot is the place on the bat barrel where the contact between bat and ball results in the best hit – the ball leaves the bat with the greatest velocity and therefore the greatest chance for the player to get on base.

With experience, even from a distance, you can hear (and often see from the trajectory) the distinct sound that comes from a batter hitting the ball on the bat’s sweet spot. Every time an owner, engineer or contractor takes on a project, in very similar fashion, we also need to stay in the technology’s “sweet spot” so we get the “best hit” possible.

Allow me to elaborate on how we continue to get the best hit possible? Just like the sweet spot is unique to a given bat, each underground technology also has a unique sweet spot. The sweet spot of an underground technology is where the ground conditions, diameter and length are within the proven capabilities of the method. The underground industry can, and must, pay attention to and take advantage of the sweet spot or performance envelope unique to each technology.

We are in an age of great fiscal consternation and downward pressure to take unnecessary risks and extend the performance envelope beyond what is prudent. In very general terms, the more rudimentary the technology, the less capital cost compared with a more sophisticated technology. This sometimes means a difficult underground project is attempted with a technology that is not appropriate or proven for the ground conditions, diameter or length anticipated.

Ostensibly, this trend is rooted in the need to reduce capital costs, which can be a noble objective or simply a way to be competitive and win work. I’d like to make the case that we should stay within the performance envelope or sweet spot of the technology even under fiscal pressures.

Why should an owner or engineer adopt this philosophy? Consider this perspective - Contractors don’t take risk, they price risk. So, the more risk, the higher the price and the greater the chance for a “poor hit.” It is simply a matter of dollars and sense.

Also, the underground environment is the most challenging environment to work in – even more challenging than outer space. In outer space the conditions are well known and certain. Underground construction has to deal with mining in some of the most severe conditions including mixed face ground, perched water, running and flowing sands, rock formations having UCS of more than 40,000 psi, highly abrasive rock containing quartz and garnet, squeezing rock, Karstic rock, slaking rock, rock over-break, gas-laden rock, fault-laden ground, fracture-laden ground, groundwater heads greater than 15 bar, cobble- and boulder-laden ground, tills that are as hard as weak rock, sticky clays and claystones, expansive clays, and unpredictable groundwater flow regimes.

As a basic case in point, we sometimes face the issue of an engineer, owner or contractor trying to use traditional auger boring techniques for under-crossings (roads, waterways, etc.), which have been used for years, when the ground conditions clearly point toward the more expensive pressurized face technology. This can also happen when a well meaning project manager does not consult with an underground specialist and turns directly to “what has worked in the past.” No matter the “marketplace pressures,” our response should be to remain disciplined and avoid risky and/or unproven designs for our valued clients and prosperous industry. To address this generally undesirable trend, a good philosophy to follow is succinctly captured in the following motto: “Design for Success….not for the Newspaper.”

There are many ways to maintain acceptable risk levels in our designs, but this is one of the most important principles to “keep sacred.” In essence, relative to underground technologies, as stated, we need to stay within the reasonable and proven performance envelope, i.e. sweet spot, for the technology being considered.

Choosing the Right Underground Technology

Ultimately, the goal of good design practice is to select the right underground technology for the anticipated ground conditions. This is the single greatest thing we can do to reduce project risk. To do that end, the following axioms reduce risk and enhance the chances the right underground technology will be implemented.

Axiom No. 1: Follow industry best practice – There are numerous industry guides published by various underground organizations in the marketplace. Take advantage of them and save yourself a lot of headaches (and protracted litigation).

Axiom No. 2: Engage an underground specialist. This business is not conducive to on-the-job training. Means and methods are where most of the money is and therefore every design team should include an underground specialist with extensive field experience to know what will work and what will not work.

Axiom No. 3: Plan ahead and apprise the owner of the risk profile as it relates to each underground technology. Most owners do not have underground experts on staff and need this advice.

Axiom No. 4: Use established standards/specifications. There are more and more published standards available in the marketplace. Get involved in UCA of SME, BTS, TAC and NASTT. The return on investment is well-worth it.

Axiom No. 5: Conduct a subsurface investigation (SI) appropriate for the underground methods under consideration and one that has a site-specific focus.

Axiom No. 6: Understand the ground conditions, ground behavior and groundwater behavior and use ground reports tailored to the underground industry.

Axiom No. 7: Conduct risk management from project inception through the end of construction and incorporate as many risk management tools as possible in the contract documents.

Axiom No. 8: Implement formal underground alternatives evaluations. Methods selection, if under the auspices of the design team, should not be a “back of the envelope” exercise.

Axiom No. 9: Take full advantage of industry forums and magazines. Relearning mistakes is a painful price to pay and there are numerous industry experts who are more than willing to share their lessons learned. We have a great “family” and need to lean on each other.

Axiom No. 10: Use the “Design for Success” philosophy.

Axiom No. 5 includes the following concepts: a) an SI program for aboveground or near-surface infrastructure is typically far different than for an underground project, b) a licensed geotechnical engineer, engineering geologist or geologist should be in the field during the SI program, c) caution should be exercised in using a commodity geotechnical firm to conduct the SI program (more often than not their pedigree in the underground industry is light), d) a desktop study should be incorporated into the SI program, e) the SI program should be done in multiple phases, f) a strategic SI program is not about blanketing the tunnel horizon with traditional exploration tools but with those best equipped to diagnose the anticipated features, g) borings should not be routinely spaced but strategically located based on discontinuities identified in desk-top reconnaissance and mapping, h) reliance solely on simple split spoon sampling and NX rock coring is often insufficient for the needs of an underground project, and i) the results of an effective SI program will enhance the ability of a tunnel contractor providing equipment specifically designed to deal with project-specific geologic conditions and associated ground behavior.

Axiom No. 6 includes the following concepts: a) a geotechnical baseline report (GBR) is the most prominent ground report for an underground project, b) a GBR fits hand-in-hand with the payment provisions and financial arrangements in the contract documents, c) front-end provisions and technical specifications are often not in a “mature” state until the 60 percent design, so a GBR is best started at that design stage, d) a GBR is the framework to compare actual ground conditions, e) the GBR must be integrated with payment provisions and pricing strategies, f) a GBR is as much a commercial document as much as it is a technical document, g) a GBR could be considered specification for the ground, and h) a GBR limits a tunnel contractor’s subject interpretation.

Axiom No. 7 includes the following concepts: a) good risk management practice takes a deliberate (and sometimes monumental) effort, b) good risk management uses preventive measures, contingency measures and control measures, c) tunnel contractors will perceive the level of owner sophistication by how risk is addressed in the contract documents, d) it is not unusual for tunnel contractors to walk if a robust risk management strategy has not been incorporated into the contract documents, e) trying to shed all risk on the contractor is a false economy (ignorance is bliss, but it won’t stand up in court!), f) not all risks will be identified through the risk management process … not every lightning bolt can be predicted, g) all DSC clauses were not created equal, h) don’t forget about highlighting residual risks to the owner in the haste of developing contract documents, and i) establish risk allocation that is commensurate with owner’s risk tolerance level.

Axiom No. 10 includes the following concepts: a) use proven technologies, b) stay within the proven performance envelope of the underground technology, c) use uniform diameters as appropriate d) use uniform drive lengths, e) minimize shafts, f) locate shafts around permanent structures if possible, g) evaluate alternate alignments, h) use a pilot study or conduct the shortest drive first (do the easiest first to get ground truth), and i) incorporate operations and maintenance considerations from design inception.

Summary

If our valued clients come to us, sometimes from genuine inspiration, or from contractor inquiry, and would like to go beyond proven applications to reduce capital costs or expand experience base, we are professionally obligated to apprise the team of the technical, financial, schedule and maybe even operational risks that they would be taking on. This is a transcendent principle that should be considered across whatever enterprise engaged in. This does not mean the industry should not look for opportunities to apply innovative solutions to the challenges facing the world, but that we only take calculated risks and apprise all parties involved of the risks in applying a technology outside of its sweet spot. This approach does not squelch innovation, but helps to avoid unwanted publicity and fosters confidence in the market sector. In other terms, mavericks are good for the industry, cowboys are not. All of us desire continued growth and innovation, which are hallmarks of our industry. So the recommendation is clear, use seasoned underground specialists and good design practice to stay in the sweet spot!

Don Del Nero, P.E., C.D.T., is a Global Tunnel Specialist and Vice President for CH2M HILL and resides in Atlanta. For additional information on underground technologies, you may contact him at don.delnero@ch2m.com.