Remembering Dr. Evert Hoek’s Thoughts on the Art of Tunnelling in Rock

Rock tunnelling is both a science and an art, the late Dr. Evert Hoek believed. In his lecture, "The Art of Tunnelling in Rock," he notes that theoretical knowledge, experience-based judgment, adaptability, and a deep understanding of the site’s geology are all key for successful tunnelling.??

Here, we’ll cover the key insights from Dr. Hoek's lecture and specific methods he thought could address complex tunnelling challenges, including maintaining tunnel profiles, managing unstable rock, and mitigating high stress environments.??

Missed the first article in this series? Read it here.??

1. What is the Art in Rock Tunnelling??

The science of tunnelling focuses on calculations and models, and the art lies in the judgment and practical strategies that guide the construction process. In unpredictable conditions, tunnelling is often a balance between technical knowledge and hands-on decision-making. Dr. Hoek also highlighted the importance of recognizing the most probable failure modes and selecting the right excavation sequences and support methods.?

"The art means the things that we have to do in conjunction with science of tunnel design to ensure that the tunnels actually are constructed safely and work." (0:27-0:40)?

2. Maintaining Tunnel Profiles??

Any deviation from the intended tunnel profile can lead to inefficiencies, increased costs, or delays, and the tunnel’s profile is often fixed by its function – such as a highway tunnel or an underground powerhouse. So, to maintain the intended profile, optimize the tunnel's efficiency, and avoid costly setbacks, careful planning and execution are needed.?

Dr. Hoek also explained that critical components, like ventilation and drainage systems, can only be properly installed and function as intended when the tunnel dimensions are correct.??

"Our job is to devise ways of excavating that tunnel to that profile and maintaining the profile because any loss of the profile is a loss of efficiency, a loss of money, a loss of time." (1:37-1:50)?

3. Pilot Tunnelling??

Pilot tunnelling is a technique that helps manage unstable rock conditions. It involves creating a smaller initial tunnel so that engineers can install support systems more safely before fully expanding it. This gives greater control over the excavation process and reduces the risk of catastrophic rock failures, which is especially useful in geological conditions with large uncertainties. Additionally, starting with a pilot tunnel means engineers can better assess the behavior of the rock under stress.?

"If you take your 8-meter span and you drive a tunnel 6 meters wide in the middle of it... it's pretty much guaranteed that that wedge would be stable, and you can get in there and put the bolts in." (8:10-8:45)?

4. Dealing with High Stress Environments?

High stress environments can lead to rock spalling, which means splinters or slivers can break off when tunnel boundary stresses exceed the rock’s tensile strength. Dr. Hoek explains that understanding the stress distribution around the tunnel can help you manage it, and flexible support systems like cable lacing can contain the rock and control deformation without compromising safety.??

An important feature of the spalling process, Dr. Hoek noted, is that if the rock is prevented from falling away initially, it becomes self-stabilizing. But without proper retention, the spalling will continue as new surfaces are exposed.?

"The stresses when they're high enough on the boundary that you've created by excavating the tunnel, exceed the tensile strength of the rock, and you get splinters or plates or slivers of rock forming." (11:48-12:02)?

5. Techniques for Poor Rock Conditions?

Advanced techniques are often required to create a stable tunnel environment in extremely poor rock conditions. One approach involves drilling steel pipes (forepoles) ahead of the tunnel face and injecting grout to form a protective arch so that tunnelling can safely continue. This technique is particularly valuable in environments where the rock mass is highly fractured or behaves more like soil than solid rock.??

Dr. Hoek also gave an example of how forepoles – closely spaced pipes that are drilled and filled with concrete – can form an umbrella arch above the excavation tunnel to stabilize the rock and prevent collapse. Additionally, fiberglass rods can be grouted into the tunnel face as supplementary reinforcement, offering support that can be easily cut during excavation, thus not impeding progress.?

"[The forepoles] are close together and form more or less a continuous arch above the face to protect you. And they successfully worked their way through a very large fault using this technique." (28:06-28:20)?

Conclusion?

Dr. Evert Hoek's insights show us that effective tunnelling requires adaptability, experience, and well-considered support systems, and that it is important to understand the most probable failure modes and proper support methods to manage evolving rock conditions. Techniques such as maintaining tunnel profiles, pilot tunnelling, and drilling steel pipes show how blending the art and the science of tunnelling can help you navigate unpredictable geology and manage successful projects.?