Long-term pore water pressure changes around subsurface structures

Abstract

Geotechnical engineering guidelines mandate the use of the most onerous hydraulic criteria for the design of earth retaining structures below the water table. Consequently, favourable local conditions, including the geometry of the structure, are not usually exploited. This means that retaining walls in particular are typically designed to resist hydrostatic pressures below the water table. Investigations have shown however that pore water pressures, axial stresses and bending moments reduce when groundwater seepage is allowed through the segmented linings of shallow tunnels. Contiguous pile retaining walls, by their nature, are also permeable. Allowing for groundwater seepage through the gaps in a retaining wall formed from contiguous piles could result in the pore water pressures on the active side of the wall being less than behind conventional impermeable retaining walls such as diaphragm walls.
Numerical simulations, laboratory flow tank experiments and long-term field monitoring were conducted to determine the impact of pile gaps on the hydraulic conditions around contiguous piles. A relationship between the resulting bulk permeability of the equivalent structure and the pile gap was derived from 2D numerical analyses and verified by flow tank experiments. This expression can be used to calculate bulk permeability values for uniform retaining walls representing circular piles in 2D numerical simulations. The permeability relationship was used to calculate and assign equivalent bulk permeability values for a continuous retaining wall of uniform cross-section during the back analysis of the hydraulic conditions around the contiguous pile retaining wall at CTRL, Ashford. Pore water pressures and horizontal total stresses from the back analyses were consistent with those from the field measurements but were much lower than behind retaining walls formed from secant piles in similar conditions. Dimensionless charts were presented to estimate the groundwater level and the increased settlement observed behind contiguous pile retaining walls. The results demonstrated that the economic advantages of allowing through-wall seepage are greater than the perceived disadvantages.

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