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Analysis of pressure behavior and pressure gradient in the excavation chamber of the Hampton Roads Bridge Tunnel variable density tunnel boring machine
Garin Paredes, Alexa
Garin Paredes, Alexa
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2025
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Monitoring tunnel boring machine (TBM) face pressure using excavation chamber (EC) pressure sensors is key to maintaining face stability and preventing issues like sinkholes or losses in face pressure. However, understanding what happens inside the EC of a tunnel boring machine (TBM) is difficult without interventions. The limited research done on variable density (VD) TBMs only discusses their use in difficult ground conditions or the use of high-density slurry. This research investigates the pressure behavior in the EC in a VD TBM and evaluates if the pressure gradient can be used to estimate density in the EC. The incorporation of five levels of EC sensors (left and right sides) in the TBM enables a detailed investigation of slurry pressure distribution.
First, the factors influencing EC pressure behavior, such as advance speed and feed pumping into the EC, among others, were identified. Then, an analysis was performed to investigate the behavior of EC pressure sensors during different operational modes (excavation, bypass, stop, and pipe building). A linear EC pressure increase with depth is often assumed in hydrostatic conditions. To examine this assumption, a linear regression analysis was employed to assess the pressure gradient within the EC during excavation and standstill. R2 (a statistical measure indicating the goodness of fit of a model) and the uncertainty related to each regression were analyzed. A predominantly linear relationship between pressure and depth was observed on both the left and right sides of the EC. Therefore, the slope of the linear regression (pressure gradient) was used as a density estimate.
The pressure gradient was initially analyzed across the entire alignment, followed by a more focused examination of individual excavation cycles through different advances. This analysis revealed an inconsistency where gradient values fall below feed density, especially during the initial alignment phase (until approximately advance 220). Since this is not physically possible, it suggested that the gradient, as calculated from pressure measurements, may not be a reliable indicator of the actual density distribution within the chamber. However, the gradient effectively reflects density decreases in situations where no soil is excavated but feed flow is pumped into the EC.
The possibility of dividing the EC into three sections and obtaining a gradient for each section was also analyzed. The results of this analysis were ambiguous, with some sections showing no change in density during excavation and others indicating densities below 1 t/m³. Furthermore, the gradients calculated for each section did not accurately reflect the density conditions within the EC.
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