‘Man at work: the magnificence of construction’. These few words form the foundation of this themed issue dedicated to geo-environmental loads acting on tunnels. As individuals, we have the opportunity to contribute positively to our surroundings by developing infrastructure, including tunnels. This perspective stands in stark contrast to the prevalent destruction evident in war-torn areas today. In contrast, all the articles in this issue show the dedicated efforts of many individuals and engineers to conscientiously design and maintain their constructions (tunnels) over time. A positive claim we can make about our job as civil engineers is that it represents ‘the greatness of building against the tragedy of destroying’.
Constructing tunnels is a fantastic but quite intricate job. We don't get to choose the conditions we work with, and here's why: the materials we want to excavate are already there. Our task is to understand how they behave through on-site, laboratory, theoretical and numerical studies. Only after that can we start excavating, essentially disturbing something that has been untouched for thousands of years.
The aim of this themed issue is to equip geotechnical engineers to handle various challenges, referred to here as geo-environmental ‘loads’. Each paper serves as a piece of a puzzle, and when combined, they form a clearer picture.
Geo-environmental ‘loads’ encompass a broad range of different actions on tunnels within a specific context. To assist the reader, the first group of papers focuses on natural loads, including geological aspects (from paper #1 to #5), water-related considerations (papers #6 and #7) and temperature effects (paper #8).
The second group deals with what we term anthropic ‘loads’ (from paper #9 to #12). This section explores how tunnels interact with human-made environments, adding another layer of complexity to the understanding of tunnel engineering.
Before entering into the specifics of each paper, let's revisit the three Vitruvian principles (from the first century BCE): firmitas, utilitas, venustas, which translate to solidity, utility and beauty. The absence of any of these elements indicates a deficiency in our work. While this themed issue primarily focuses on the concept of firmitas (solidity), you may also discern traces of utilitas and venustas in each article. The holistic consideration of these principles ensures a well-rounded exploration of the topics covered in this themed issue.
The geological ‘loads’ begin with an active landslide triggered by tunnel excavation, as discussed by Zhang et al. (2023). It is fascinating to see how the authors applied the firmitas concept through on-site evaluations, numerical predictions, countermeasures, and validation using monitored data post-construction trying to stop the landslide.
Newman et al. (2023) focus on the changing geology on tunnel boring machine (TBM) earth pressure balance (EPB) performance. They successfully ‘controlled’ potential ground hazards (geological ‘load’) by conducting detailed ground investigations and creating an accurate pre-construction geological model.
Addressing voids in karst areas represents another geological challenge, as highlighted in the study by Yan et al. (2023). The findings offer valuable insights for similar projects, covering key technologies for tunnel construction, karst-disposal measures and quasi real-time design and activation of countermeasures.
Squeezing in weak rock masses presents another geological challenge, addressed by Kadkhodaei et al. (2023) using a machine learning method called gene expression programming (GEP). This innovative approach, requiring a suitable data set during tunnel design and excavation, helps predict squeezing phenomena.
Du et al. (2023) propose an analytical method to handle high stress in deep tunnels, distinct from the machine learning approach. Their method quickly calculates internal forces along the entire tunnel lining, considering rock mass stress release for deep-buried tunnels.
Addressing the water-related challenges, Su and Bloodworth (2023) tackle groundwater pressure issues leading to failures in sprayed waterproof membranes. They establish a conceptual relationship between potential groundwater pressure locations and stress states. The study uses numerical analyses to understand potential failure mechanisms in composite sprayed concrete lining (SCL) tunnels.
Unconventional water-related ‘loads’ are related to immersed tunnels. Back-silting, tide effects, and short and long displacements during tunnel immersion in construction phases are comprehensively discussed by Wang et al. (2023a). It is fascinating to discover this unique tunnel scenario where there's no soil excavation involved. Instead, a precast tunnel is immersed at the bottom of the sea.
Considering temperature challenges, Chhun et al. (2023) conduct long-term temperature monitoring within tunnels in cold regions. This paper stands out as it offers a robust data set of climatic data inside and outside a tunnel, coupled with traffic loads. This data set proves valuable for numerical analyses of temporal heat transfer.
The papers addressing anthropic ‘loads’ start with the consideration of the effects of deep bridge foundations (piles) located in close proximity to the tunnel axis. Wang et al. (2023b) address this through an analytical solution, providing valuable insights into tunnel–pile interaction. This solution proves beneficial for designers working on similar projects.
Jones et al. (2023) present an almost 20-year stress history in the primary lining of a concourse tunnel, measured using radial and tangential pressure cells on and in the sprayed concrete. This remarkable case study demonstrates the feasibility of long-term monitoring, significantly reducing uncertainty in tunnel design.
Nematollahi and Dias (2023) show the suitability of a three-dimensional (3D) numerical model in handling a non-symmetric load caused by a retaining wall interacting with shallow twin tunnels. The combination of on-site monitoring and calculations using various constitutive models makes this paper a reference case study, validating the predictions of 3D numerical models.
Rattia et al. (2023) propose a simple yet effective model for estimating ground surface settlements due to a TBM in drained and undrained conditions. Analogous to the hyperbolic behaviour of stress–strain curves in soils, this model estimates immediate surface settlement with validity ensured through relevant case studies.
In conclusion, given the diverse range of specific geo-environmental loads on tunnels, geotechnical engineers now have an additional set of information and tools to design and maintain both new and existing tunnels over the years.
