Investigation of the Ultimate Excavation Depth for Pits Supported by a “Diaphragm Wall”

A diaphragm wall is one of the most widely used types of retaining structures for vertical excavation support in dense urban environments, where deformation control and the stability of the soil mass are of critical importance. Despite the extensive application of diaphragm wall technology, the problem of selecting an optimal wall thickness—considering excavation depth, temporary support configuration during construction, and site-specific geotechnical conditions—remains highly relevant for both designers and researchers. The aim of this study is to determine the relationship between diaphragm wall thickness and the maximum achievable excavation depth, as well as to assess the influence of different support schemes (cantilever and temporarily braced) on the stress–strain behavior of the structure. The investigation considers wall thicknesses ranging from 300 mm to 1000 mm, which correspond to typical parameters for medium- and large‑depth urban excavations. Numerical modeling was performed using the finite element method, accounting for nonlinear soil behavior, staged construction, and soil–structure interaction. The analysis shows that increasing the diaphragm wall thickness has a limited effect on reducing bending moments, particularly in the cantilever configuration. In contrast, the use of a temporary strut significantly alters the deformation pattern and enables an increase in excavation depth at each construction stage. For the examined range of wall thicknesses, the maximum depth of a single excavation stage is approximately 1.0 m for the cantilever scheme, whereas the introduction of a temporary strut allows this value to increase to as much as 2.4 m, depending on wall thickness and the stiffness of the bracing element. Thus, the key factor governing the achievable excavation depth is not the wall thickness itself, but rather the structural support scheme employed during construction.The obtained results have practical significance for optimizing excavation technologies in complex urban environments. Increasing the excavation depth at a single stage reduces the number of construction cycles, shortens the overall construction duration, and lowers the risks associated with deformations of retaining structures and adjacent buildings. The findings demonstrate that a rational combination of diaphragm wall thickness and temporary bracing elements can enhance the efficiency and safety of the construction process without substantially increasing the material consumption of the retaining structure.