Assessing the timing and magnitude of precipitation-induced seepage into tunnels bored through fractured rock

Seepage into tunnels bored through fractured rock is a common occurrence that can cause significant problems for the construction process, tunnel longevity, and regional hydrogeology. Predictions of seepage using analytical solutions and numerical models are often inaccurate due to the inherent assumptions, volumetric averaging of fractures, and lack of important hydrogeological features. This study seeks to better understand tunnel infiltration processes through the application of a high-resolution, integrated hydrologic model. First, a conceptual model is developed for this research using the factors shown by previous studies to control net infiltration and seepage. A stochastic fracture continuum is generated for bedrock using FRACK, which maps discrete fracture networks to a finite difference grid with heterogeneous, anisotropic permeability fields. An integrated hydrologic model, ParFlow is then used to investigate the control exhibited by factors such as climatic forcing; vegetation; soil type and depth; bedrock type; fracture spacing; and tunnel depth on the timing and magnitude of seepage into tunnels. Simulations are run using hourly meteorological forcing. Surface and subsurface properties are adjusted individually to investigate the change in seepage response for varying hydrogeology and land cover. Results show that fracture spacing and connectivity, bedrock type, and overburden are particularly important pieces in obtaining reliable seepage estimates. Higher fracture spacing causes higher total seepage at a more constant rate than a lesser spacing, which exhibits a much larger range of fluctuation in seepage volumes. More permeable and porous bedrock increases lag times while reducing seepage volumes that remain relatively constant over time. Thicker and less conductive soils both increase lag times and reduce seepage magnitude. Tunnels, precipitation, infiltration, seepage, fractured rock.