Load Transfer Mechanism in Stabilized Walls by Nailing Method
Introduction
Before discussing the analysis of stabilized walls constructed by the nailing method, the fundamental load transfer mechanisms and wall reactions during various construction stages are briefly explained.
Reaction and Load Transfer During Nailing Construction
During the installation of nails (or reinforcing elements in nailing), the reaction of the stabilized wall and load transfer mechanisms can be summarized as follows:
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Initial Excavation Stage:
Excavation begins at the ground surface, and the first excavation stage is completed. Due to soil stability without facing protection, the upper soil behind the excavation face (before installing the first row of nails) remains stable or nearly stable. Soil resistance is mobilized along the most probable critical failure surface, allowing temporary soil stability without support. -
Excavation-Induced Deformations:
During excavation, the stabilized wall may deform. At this point, another probable slip surface develops at the bottom of the excavation lift. The critical failure surface at this stage differs from that of the previous excavation level. -
Nail Installation and Load Transfer:
Nails are then installed. Subsequent soil displacements above the excavation lift generate additional loads transferred to the nails, which in turn develop tensile forces within themselves. -
Requirement for Nail Embedment:
For overall stability, nails must extend beyond the probable failure surface. As excavation depth increases, the volume of retained soil mass grows. -
Increasing Interface Stresses and Nail Forces:
With an increasing retained soil zone, soil-nail interface stresses and axial forces in nails increase. Tensile forces generated are transferred through the nail anchorage to stabilize the potential sliding mass.
Distribution of Slip Surfaces and Nail Tensile Forces
While tensile forces in middle and lower nails increase due to deeper excavation, tensile forces in some upper nails may decrease because of load redistribution.
As the critical failure surface deepens and enlarges, the contribution of upper nails to the stability of the sliding mass diminishes. In some cases, upper nails may have negligible effect on deep critical failure surfaces. However, upper nails are not redundant as they contribute to stabilizing early excavation stages and help reduce lateral displacements.
Analysis of Nailing Wall During Construction and After Completion
To identify the most critical nail forces at each excavation level, wall analysis under loading conditions “during construction” and “after construction” must be considered.
The most critical situation might occur after wall completion, due to combined long-term design loads (e.g., dead loads, live loads, traffic) and ultimate loads (e.g., earthquakes). Alternatively, the most critical condition may arise during construction when the lowest excavation level remains unsupported temporarily, and nails and shotcrete facing are not yet installed.
Safety factors for temporary excavation conditions are typically lower than those for long-term conditions because the risk period of failure is shorter during construction.
