The Impact of Groundwater on Excavation Displacement
The groundwater level relative to the excavation depth, as well as pore water pressure and the resulting seepage force, can reduce the stability of excavations stabilized by nailing methods (such as nailing, soldier pile walls, and top-down systems).
In designing and constructing nailed walls, particular attention must be paid to the groundwater conditions behind the excavation wall.
Key issues related to groundwater level behind the excavation wall include: the presence of pore water pressure, reduced excavation stability, decreased soil strength, increased corrosion potential of nails, decreased stability of boreholes for nail installation, and other design and construction considerations.
US Department of Transportation Guidelines
Volume 7 of the U.S. Department of Transportation’s guidelines on nailed walls provides recommendations for designing and constructing nailed walls under high groundwater conditions and in saturated soils.
According to these guidelines, geocomposite drainage strips are used behind the shotcrete facing to facilitate adequate drainage behind the wall, reducing water’s impact on excavation displacement.
These strips drain water behind the wall and route it to a drainage pipe at the toe of the excavation, directing it outwards. Given the groundwater level behind the wall, seepage flow after excavation creates inward hydraulic forces that, along with water pressure, lower the excavation’s factor of safety and establish a fixed phreatic surface.
In Figure 1, wall displacement normalized by wall height is shown for various groundwater levels. The greatest displacement occurs when the groundwater level is at the ground surface, while the least occurs when it is below the final excavation depth.
As shown in the figure, increasing groundwater level can increase wall deformation by up to 57%. Figure 2 illustrates normalized axial forces in nails at various wall heights for different groundwater levels.
The figure indicates that in nailed retaining walls, changes in axial forces due to groundwater level variations are more significant in the upper and lower nail rows compared to middle ones.
Hence, it can be concluded that groundwater level changes have a greater effect on edge nails (top and bottom), highlighting the need for special attention to these nails in saturated soils.
Since upper nails cross unsaturated zones, they tend to bear higher loads. Groundwater fluctuations significantly affect the deformation and resistance behavior of nailed walls.
Proper drainage behind the wall creates a stable phreatic surface, and the resulting saturated and unsaturated zones impact wall performance differently.
Figure 3 shows the variations in maximum axial nail force and maximum wall displacement at different groundwater levels. While axial force changes by about 7%, wall displacement varies by approximately 57%.
If the groundwater level rises to half the excavation height, the rate of wall displacement increases mildly. However, if it rises to the ground surface, wall displacement increases significantly—by as much as 71%.
