To assess the feasibility of employing experimental studies and observations in the monitoring process to estimate the uncertain parameters of excavation retaining wall elements, specifically focusing on sheet pile lengths. Methods: The methods proposed in this paper are centered on determining the actual rigidity of the excavation retaining wall elements (steel sheet piles) under real working conditions within the context of existing structures. Essentially, the methods outlined herein represent a full-scale experimental study conducted at an active construction site. Results: This paper presents the findings related to the identification of the missing sheet pile parameters, specifically sheet lengths, through numerical modelling and field investigations that included deformation measurements, which were then compared to the calculated values. Practical significance: The practical relevance of this research is largely attributed to the distinctive challenges faced in the construction and renovation of buildings and structures in densely populated urban environments, where the effective use of underground space is vital. Furthermore, the significance is heightened by the specific ground conditions of St. Petersburg, which are marked by weak clay soils that are topped by relatively thin layers of fine and dusty sands. Geotechnical engineers often face the challenge of incomplete construction projects that necessitate completion due to the instability of the technical condition at the point where construction was interrupted. It is crucial to finalize the ongoing construction and installation works; however, the existing underground structures, such as piles, excavation retaining elements, foundations, or other installations, cannot be removed, as their dismantling could jeopardize adjacent buildings. Consequently, it is imperative to maintain and make use of the existing structures and their components while collecting accurate information about these structures, particularly, about their strength and deformation characteristics.