Seepage Analysis and Piping Control for the Soft-Ground Stretch of a Hydropower Headrace Tunnel Adjacent to a Valley Slope

Short reaches of pressure tunnel driven through erodible soil close to an open valley slope present a hazard that neither pressure-tunnel practice nor hydraulic-structures practice addresses in full; leakage through the cracked concrete lining emerges on the free face and, where the hydraulic exit gradient approaches the critical value, can initiate backward-erosion piping that endangers the slope and ultimately the tunnel itself. This paper proposes a six-step framework for assessing and controlling that hazard, coupling ground and boundary characterization, groundwater-state determination, head-dependent lining characterization, sectional finite-element seepage modelling, piping evaluation against a safe gradient anchored to the erodible fraction of the soil, and model-based ground-treatment design bounded by constructability. Its distinguishing feature is the propagation of information between steps; the lining is represented not by a fixed permeability but as a semi-pervious element whose equivalent conductivity follows from lining–ground load sharing, and this head-dependent source drives the seepage model whose exit gradients in turn set the quantitative target that the ground treatment is dimensioned to meet. The framework is demonstrated on the roughly 136-m soft-ground stretch of the headrace tunnel of the Neelum–Jhelum Hydroelectric Project, Pakistan, where the tunnel passes through coarse river-terrace alluvium along a valley slope under internal pressure heads of 18–24 m. Conductivities of the alluvium (truncated mean 7.25 × 10−6 m/s) and bedrock (3.89 × 10−7 m/s) were characterized from 67 borehole tests. Because the compressible alluvium accepts only 17% of the net internal pressure, the computed lining conductivity during first filling approaches that of the surrounding ground, making first filling on drained ground, rather than long-term operation, the condition that governs the leakage, with a cumulative leakage of 9.1 L/s against 2.7 L/s; the maximum exit gradients of the two cases are, however, similar. Steady-state analyses of twenty-six cross-sections give exit gradients at the slope face reaching 1.06 at the critical gradient, against a site-specific safe gradient of 0.20 derived from the erodible sub-19-mm fraction using a factor of safety of 5. A parametric study shows that a permeation-grouted annulus 5 m thick at a target conductivity of 6.5 × 10−8 m/s reduces the first-filling leakage by about 90%, lowers the phreatic intercept on the slope face by about 15 m, and reduces the exit gradient at the reference section to 0.215. Evaluated at all sections, however, the safe gradient is still exceeded at most of them in first filling and at all of them in operation, when the flow to the slope is driven by leakage from the reservoir rim, so a loaded filter on the seepage face is added to the grouted annulus. Grout materials, injection rates, confinement-limited injection pressures and a verification programme are specified for implementation. The case study demonstrates the application of the framework from characterization to a treatment design; the computed treatment performance remains to be confirmed by the verification testing and first-filling observations specified, and the workflow is transferable to shallow pressure tunnels in erodible soils generally.