Tunneling vs. Open Cuts: How to make the tough calls

As the use of underground space for construction and design becomes more commonplace, engineers around the world often debate the benefits and disadvantage of tunneling vs. open cut systems.  As cities continue to expand their transportation systems, utilities infrastructure, and the interconnectivity of neighboring buildings, choosing the most cost-effective solution is a key concern.

The selection of the route may also depend on precise alignment constraints and environmental issues.  The final determination may ultimately rely on the nature of the related soil, rock, and water table, as well.  Other factors to consider include the diameter, depth, shape, and length of the passageway along with all related utility requirements.  To determine which option is most efficient and economical, engineers must first conduct a detailed and thorough site inspection.

Advantages and disadvantages of tunneling vs. open cuts

Tunneling has become so pervasive that The Federal Highway Administration (FHWA) of the U.S. Department of Transportation (DOT) offers an open source handbook which provides engineers with specific construction guidelines.  The FHWA Tunnel Manual provides valuable insights into the planning, design, construction, maintenance, and rehabilitation of both bored and mined tunneling systems, including the following highlights.

• Tunneling is generally considered more economical compared to open cut trenches, especially regarding structures of perhaps greater than 60 feet in depth.
• Tunneling systems tend to limit occurrences of above-ground disruptions and occurrences of noise pollution during the construction process, in many cases.
• Tunneling can also reduce or eliminate construction delays, especially in construction areas of high altitudes where snow and ice hazards are more likely.
• Tunneling can also be a viable alternative to bridge systems, especially regarding the construction of utilities infrastructure.
• Because of the increased safety issues and more restricted surface and air interferences, tunnel design requires a more highly skilled and experienced engineering team.
• Open cut trench systems, on the other hand, usually require far less sophisticated equipment and staffing to perform the construction operations.
• Open cut trenches also usually require a substantially reduced initial cost investment.  
• Open cut systems are often the more cost-effective choice in areas where the underground material is predominantly rock material that is more difficult to excavate.
• Open cut solutions can sometimes prove more costly, particularly if the intended route consists of varying degrees of topography grades.

Many engineers are surprised to learn that even tunnels usually have a slight slope in their design. An average of 0.2% gradient ensures proper drainage requirements.  In cases of lengthier tunneling systems, the entrances and exits at either end will often have alternating slopes to enhance air ventilation before, during and after the construction process is complete.

Meanwhile, federal inspection standards for tunnel design are far more complex as compared to open cut trenches.  Before deciding between tunneling vs. open cut systems, each of these factors must be thoroughly investigated to ensure the maximum safety of the structure and the reduced negative consequences to the above-ground environment.