What is the current state of the tunneling field?
Tunneling has become an even more popular means of conveyance as cities’ populations grow and their infrastructure ages. Plus, owners are very concerned about preventing disruptions to the public. This all makes tunneling an attractive option for projects.
Tunneling is becoming a very viable means of going from point A to point B, whether it’s a wet or dry conveyance, utilities, or traffic. The interesting aspect of tunneling is that there are many different ways to get from point A to point B, depending on the size, length, and purpose of the tunnel and subsurface conditions.
Why is tunneling an attractive option in some projects?
A tunnel can serve multiple roles. For example, maybe you are building a conveyance system for a wastewater treatment plant with an average capacity of 15 million gallons per day (mgd). However, if that plant needs to accommodate a storm surge of 30 mgd and you have only a small amount of space, your options are limited. When you start looking for some place to store 30 million gallons of water, you can increase the diameter of the conveyance pipe a few feet so that it doubles as a stormwater storage tank. No one sees it, no one is aware of it, and no one is bothered by it.
Can you give an example of CDM’s tunneling work?
The Brightwater conveyance system. The project, in King County, Washington, includes four separate large diameter tunnels—each ranging from 13 to 19 feet in diameter, from 2- to 4-miles long, in extremely challenging soil conditions. The groundwater pressure in one alignment is more than 7 bars. Anytime the pressure goes above 3.5 bars it becomes a challenge. And the soil is also abrasive.
The magnitude of these conditions is uncommon, especially when you combine a soft ground tunnel with high-abrasion conditions, a 400-foot depth, and a 4-mile length. It becomes a challenge. In these types of conditions installing a rescue shaft becomes a very expensive option, so you need to think of options. We did our homework and determined the best alternatives. You need to be aware of the risks, and that comes from experience and working with the owner to develop a plan to manage those risks.
What does the future hold for the tunneling industry?
Based on the information gathered during the Brightwater design, CDM has approved funding for research and development of the effects of soil abrasion on tunnel boring machine cutter wear. This wear affects the rate of progress and cost of the project. Currently there is no standard to assess the magnitude of this problem.
Also, ground freezing will become more common in the United States as tunnel shafts get deeper and land become less available for the more common method of slurry wall support. Ground freezing is also a common method of ground support for cross passages between parallel tunnels.
Michael B. Gilbert, P.E., has more than 30 years of experience in geotechnical engineering, with much of his experience specifically focused on tunneling projects.