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He’s Got Tunnel Vision

Published: October 15, 2015

There are a lot of things that can go wrong when you’re building a tunnel—especially in soft ground.

No one knows that better than Luis Arboleda-Monsalve, a member of the Civil Engineering and Construction Engineering Management Department since 2014. The new faculty member, who earned his doctorate from Northwestern University in 2014, offers a first-ever class on tunnel-building at CSULB.

The course focuses on the analysis, design and construction considerations of tunnels and access shafts.

“There are two things I want our students to get,” he explained. “The first is what are the challenges, from the soil mechanics point of view, that are related to the problems of tunneling in soft ground? In tunnels dug in soft ground, such as in the San Francisco Bay mud, Boston Blue Clay or Chicago Clays, tunnels deal with soil with very little strength. Second, tunneling through rock is a different story. Our students will need to know how stable the rock and the pre-existent bedding planes to avoid falling rocks on the site that would lead to the tunnel collapse.”

Arboleda-Monsalve is interested in the high-speed rail planned by the California High Speed Rail Authority that will connect Los Angeles to San Francisco in two to three hours at speed capable of more than 220 mph.

“When it comes to Southern California drilling, engineers will encounter slippery, granular alluvium deposits and formations ranging from recent Paleozoic formations to Mesozoic intrusive rocks,” he said.

Then there are the San Gabriel Mountains, which present their own unique challenges.

“There are several options in the L.A. area,” he explained. “The most feasible from a technical point of view is a tunnel that would pass under the protected San Gabriel Mountains. That fact you are drilling under mountains suggests there will have to be portions of the tunnel in rocks. The combination of rocks with non-cohesive alluvium deposits complicates the construction. Then you must consider water. Water is the critical part of most tunnel constructions. What I tell my students is, as a forensic engineer if you are trying to explain a geotechnical failure or collapse, most likely by saying ‘because of water’, you would be very close to the actual explanation. The presence of very high water tables would trigger massive floods of granular material and water inside of the tunnel opening if knowledge of the subsurface conditions and adequate construction techniques are not used in the tunneling operation.”

One of the biggest challenges to building tunnels from L.A. to San Francisco is how earthquakes would affect their performance. According to Arboleda-Monsalve, there is a consensus that underground structures are less vulnerable to earthquakes than most surface structures.

“However,” he said, “the presence of major active faults crossing the tunnel alignment, during a seismic event displacements are induced which may impact the operation of these type of underground structures. It is a different challenge when you can control the materials. When you’re building a bridge or a high-rise structure, you have some control over the strength and quality of the construction materials. But what about the soils? It is nature. Even after the most detailed subsurface investigation, you won’t know how variable soil are until you start digging. As you need excavate, you start discovering the actual subsurface conditions of the site. There are always things you can’t foresee in the design stages of these type of projects.”

Luis Arboleda-Monsalve
Luis Arboleda-Monsalve

There have been significant technological advances in tunnel construction.

“Look at the recent tunnel built right under the city of London: The so-called Fifteen Billion Pound Railway,” he said about the project that had a team of more than 10,000 engineers and construction workers. “I want to teach our students how to analyze and design those access shafts to put these massive Tunnel-Boring Machines (TBM). Digging tunnels right under a city is not an easy task. Pipes, cables, sewers and other utilities crowd the ground. There are platforms to build and escalators. The presence of deep foundations of the existing structures. All these things make the underground of major cities extremely crowded. The only option is to drive these tunneling boring machines through the tightest of gaps. There can be as little as 30 centimeters to spare when you’re driving one of these big TBMs. That means passing those giant machines only 12 inches away from an obstacle.”

Arboleda-Monsalve brings to the new class plenty of experience in the real world of engineering. He worked as a bridge engineer from 2007-10 for the Indianapolis-based Janssen and Spaans Engineering Inc. where he specialized in the design, construction engineering, inspection and rehabilitation of infrastructure projects including long-span bridges in the United States.

Modern laboratory equipment, such as Cyclic Triaxial Testing devices, are coming to CSULB thanks to the recent effort made by the College of Engineering.

“That device will basically crush soil samples,” he explained. “A sample from the field is brought to our laboratory at CSULB to measure the stress and strain response of soils to monotonic and cyclic loading. Not too many universities and commercial laboratories in the U.S. are able to perform that kind of testing. Cal State Long Beach will be on the cutting edge. With the new technology, CSULB will be able to analyze soil behavior better than most commercial laboratories. The construction industry will see that CSULB has this level of equipment and will soon knock on our door to use our equipment for advanced testing needed in their projects.”

His tunnel “sophistication” has changed the way Arboleda-Monsalve sees California.

“As a soil engineer, I would like to have Superman’s X-ray vision,” he laughed. “I would like to see what is under the San Gabriel Mountains. I am convinced that there is lots of potential to build underground in Southern California. Southern California is on top of competent soils. These days, now that we are running out of space above ground, we should go underground.”