Comprehensive assessment of the impact of deep excavations and underground structures on settlements and tilts of buildings in conditions of dense urban development taking into account construction staging

Abstract

The article is devoted to a comprehensive assessment of settlements and tilts of existing buildings during the development of deep excavations and construction of underground structures in conditions of dense urban development, where stress redistribution, changes in filtration regime, and staged technological impacts form a non-uniform settlement trough and increase the risk of exceeding permissible deformations, especially for historical brick buildings on weak water-saturated soils and man-made embankments. An approach is presented that combines numerical modeling in Plaxis 2D/3D with the Hardening Soil Small Strain elasto-plastic model, statistical processing of geotechnical monitoring data from 14 facilities with excavation depths of 8–22 m, and comparison with regulated deformability criteria. It is quantitatively shown that during excavation stages, about 70% of the total settlements are formed; for an excavation 16 m deep, the maximum single contribution is from excavation to elevation −10.2 m (28.8%), while preliminary tensioning of struts causes limited reverse heave (up to 2% in modulus), and during transfer of loads from temporary struts to permanent slabs and their dismantling, about 14.1% of settlements are added. A relationship between vertical settlements and horizontal displacements of the retaining structure is established: for rigid systems (secant bored piles, diaphragm wall), relative displacements are grouped at 0.15–0.16% of depth, whereas for sheet piles they reach 0.29%. For the near zone, a high correlation of settlement with the maximum deflection of the retaining structure (r=0.93) is established, and for the far zone — enhancement of the role of dewatering; with an increase in pumping duration to 14 months, the share of the consolidation component increases to 40.9%. A regression model with R²=0.87 highlights the determining significance of the depth-to-distance ratio to the building (β=0,41), retaining structure stiffness (β=−0.33), and dewatering duration (β=0,22), while accounting for staging improves the accuracy of settlement and tilt predictions by 34% and 47% on average, respectively; for buildings closer than 0.5 excavation depth, adjustment of calculated tilts by a factor of 1.15 is justified, along with differentiated monitoring prioritizing inclinometry in the near zone and piezometry in the far zone, as well as organizational measures to reduce dewatering periods and application of technologies minimizing open drainage.