Interaction Mechanism of Tunnel Excavation and Overlying Building Loads in Shallow-Buried Coastal Urban Tunnels
This study investigates the interaction between shallow-buried tunnel excavation and overlying building loads in coastal urban areas through model tests, numerical simulations, and in-situ monitoring, revealing that excavation sequence, groundwater seepage, and building surcharge collectively govern deformation patterns, with the upper bench excavation under buildings identified as the most critical stage causing significant settlement and structural strain.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Beneath the bustling streets of coastal cities, a quiet but critical engineering challenge unfolds where tunnels meet the ground above them. In these areas, the earth is often a complex mix of loose soil, weathered rock, and water, sitting just below the surface. When engineers dig a massive tunnel through this ground, they remove the material that has been holding the weight of the city for centuries. This removal causes the ground to shift and settle, a process that can threaten the stability of buildings standing directly overhead. The situation becomes even more complicated when the ground is saturated with water. In such environments, digging a hole allows water to flow into the tunnel, changing the pressure that holds the soil particles together. This interaction between the solid earth, the flowing water, and the heavy weight of buildings creates a delicate balance that, if disturbed, can lead to significant damage or even collapse. Understanding exactly how these forces work together is essential for keeping cities safe as they continue to expand their underground transportation networks.
Researchers at the China University of Geosciences (Beijing) set out to unravel this complex relationship by studying a specific section of the Second Qingdao Jiaozhou Bay Subsea Tunnel. This tunnel is a vital piece of infrastructure, but its onshore section presents a unique set of difficulties: it is buried very shallowly, spans an enormous width, and passes directly beneath existing buildings in a water-rich coastal environment. To understand the mechanics of this interaction, the team did not rely on a single method. Instead, they combined three approaches: they built a scaled-down physical model of the tunnel and buildings in a laboratory, they ran detailed computer simulations that accounted for the flow of water through the soil, and they analyzed real-time data collected from the actual tunnel construction site. By weaving these three threads together, they were able to watch how the ground and the buildings reacted step-by-step as the tunnel was dug.
The physical model was a precise miniature of the real world, constructed inside a large box filled with specially designed materials that mimicked the strength and water-flow properties of the actual granite and soil. The researchers placed tiny buildings on top of this model, including a small brick structure and a taller frame building, and applied loads to them to simulate their real weight. They then dug the tunnel in the model using a specific method that involved removing the soil in stages, first from the left side and then the right, moving from the top down. As they dug, they watched how the ground settled and how the water pressure changed. They found that the most dangerous moment did not happen when the tunnel was first opened, but rather during a specific phase of the excavation. The most significant movement and stress release occurred when the upper section of the right-hand side of the tunnel was being dug directly beneath the buildings. It was at this precise moment that the ground above the tunnel sank the most, and the water pressure dropped the fastest.
The presence of water proved to be a major amplifier of these effects. The study showed that when water is allowed to drain out of the soil as the tunnel is dug, the ground settles significantly more than it would in dry conditions. In their computer simulations, the researchers found that accounting for this water flow increased the maximum sinking of the tunnel roof by 27.3 percent compared to a scenario where water was ignored. This happens because the water pressure that helps hold the soil grains together is lost as the water escapes into the tunnel, causing the soil to compress and the ground above to drop. The researchers observed that this drainage effect was most intense near the top of the tunnel, which is why the upper stage of excavation was so critical. The water did not just add weight; it fundamentally changed how the soil behaved, making it weaker and more prone to sudden movement.
The position of the buildings relative to the tunnel also played a decisive role in how the damage was distributed. When a building sat directly above the tunnel, the ground settled in a relatively symmetrical pattern, but the area of settlement became narrower and deeper, concentrating the stress on the building's foundation. However, when the tunnel passed slightly to the side of a building, or at an angle, the ground did not settle evenly. Instead, one side of the building would sink more than the other, causing the structure to tilt. This uneven sinking created twisting forces and extra strain on the building's frame, particularly at the corners and where the beams met the columns. The study highlighted that simply measuring how much the ground sank in the center was not enough; engineers also needed to watch for these tilting and twisting effects, which could be just as dangerous for the structural integrity of the building.
The findings from the laboratory model and the computer simulations matched closely with the data collected from the actual tunnel construction in Qingdao. In the real world, as the tunnel face moved beneath the buildings, the ground settled rapidly during the upper bench excavation, confirming the patterns seen in the experiments. This agreement gives engineers confidence in the results. The research suggests that for similar projects in coastal cities, the most critical time to monitor safety is when the upper part of the tunnel is being excavated directly under a building. During this window, engineers should pay close attention not just to how much the ground is sinking, but also to how fast the water pressure is dropping and whether the buildings are starting to tilt. By focusing their monitoring efforts on these specific moments and factors, construction teams can better protect the structures above and ensure the safety of the workers below.
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