Modyfy: January 2022

Digging tunnels might not seem like the most cutting-edge industry, but the need for rapid and precise boring has become a recent hotbed for innovation. Most of the momentum in the space can be credited to the Boring Company, one of Elon's many "side projects." After joining the CU Hyperloop club at my school, the University of Colorado Boulder, I learned about what goes into creating a tunnel boring machine (TBM). As part of the Boring Company's Not-A-Boring competition, universities and hobbyist groups from around the world design and build a TBM with the goal of digging a tunnel the fastest. For the last competition, the tunnel had to be 30 meters long and half a meter in diameter.

There are many aspects that go into building the machine that builds the tunnel. Our team was split into subteams such as Excavation, Soil Removal, Tunnel Support, Propulsion, and many others. I joined the team as a freshman and quickly was thrust into the design process. Our club had roughly a year to design, build, and test our machine before the competition. I took interest in the Soil Removal and Tunnel Support areas, becoming the lead for both subteams after one semester.

When dirt is excavated by the cutting head, it has to be processed and sent to the tunnel entrance. This encompasses the Soil Removal subteam. For this competition cycle, our team decided to take the slurry approach. Water is introduced to a soil chamber directly behind the cutting head and then pumped by a sewage pump to the tunnel entrance. There was a lot of design work that I did in terms of figuring out how to integrate soil mixing and processing mechanisms. One example of this was the agitation rods attached to the back of the cutting head. Although a lot of time and effort went into the soil removal system, most of it was changed for this year's new machine due to clogging issues with the pump and large water requirements for the slurry.

I believe my most significant contribution to the team came with my idea of implementing a continuous and flexible tunnel support system. In conventional TBMs, heavy concrete pipe sections are slowly placed into the bored dirt to support the weight of the soil. This process not only increases dig time but also is very expensive. Since our club did not have a large budget, I knew the solution had to be cost-effective. After a lot of discussion and brainstorming, I had the idea of using a "dog-tunnel" mechanism. As the TBM progresses, the tunnel support structure unfolds behind it. This allows for continuous and rapid excavation. For the to work, the support structure is held in place at the tunnel entrance by stakes.

To keep costs low, this support structure was constructed out of heavy-duty poly-tarp and 1/2" steel rings. We made these material choices after conducting a quarter-scale centrifuge test. The test allows for a scale model to be subjected to the appropriate pressures by submerging it in a soil box and then spinning that box in a centrifuge. All the materials are kept consistent in this model, but all dimensions are scaled down by a factor of four. By doing some rough calculations, we estimated the support structure would need to handle 25 kPa of vertical pressure and 12.5 kPa of lateral pressure. To our surprise, the centrifuge test revealed an overall factor of safety of 3 (the model didn't reach failure but the centrifuge couldn't spin any faster with the given load).

At this point, we also realized that this data was for a fully tensioned support section. Therefore, the tarp and ring segments needed to be stretched before making contact with the soil. This realization lead to the development of a release mechanism that incorporated several linear actuators along with a stepper motor-driven carriage. Essentially, the actuators would hold two rings in place while the carriage stretched them apart by traveling on a leadscrew.

Unfortunately, financial issues caused our club not to reach an adequate budget. We didn't have funding to fully complete our TBM. But, we decided that it would still be good to conduct some tests at the competition. However, we were proud that our design proposals and technical documents submitted to the Boring Company put us into the "Digging Dozen" - 12 teams out of about 500 submissions that were invited to Las Vegas for the competition. Most teams also ran into funding issues and only one team ended up digging.

The main test we wanted to run was with the tunnel support structure. Essentially replicating digging conditions and putting a portion of the structure into the ground at an appropriate depth to stress-test it. Over the summer of 2021, the manufacturing of the support structure began. It was a back-breaking task. For our 10 meter long test segment, 1/2" steel rods needed to be bent into rings with a 0.5m internal diameter and then sewed into pockets on the tarp sections. With our funding problems, everything was done in-house. However, we were able to finish the segment in time for the competition.

While in Vegas, I lead the efforts for conducting this stress test. First, a long sloping channel was dug by site staff to place the segment into. I designed a staking mechanism that used rebar, a plywood sheet, and several steel pieces to ensure that the support structure could be fully-tensioned while under soil load. The next issue after situating the segment was data collection. After some team brainstorming, we decided to build a sensor carriage that would be inserted into the support section and slowly pulled out with a pulley system. This system would collect one data set before soil loading and then another after loading. LIDAR and ultrasonic sensors on this carriage would record the distance to the top of the tunnel so that a deformation value could be computed.

Although very time-consuming, the data from these tests almost perfectly aligned with our earlier centrifuge test. My support structure held up very well against the immense soil pressures and proved to be a good alternative to the conventional rigid segments. However, for this year's competition, there are some vital improvements to be made. For one, the release mechanism needs to be simplified and tested. Additionally, the problem of settlement, soil shifting after subsurface excavation, must be explored. All in all, I think that this experience was extremely valuable. As a team lead, I was honestly very surprised to see my novel, cost-effective design solution work on the first try. There is still a significant amount of work to do before the system is implemented in an actual TBM, but I think it's a pretty good start. See pictures taken throughout the competition cycle below. Click here for a cool centrifuge video.