Abstract: Basal heave analyses for circular foundation pits are commonly derived from two-dimensional upper-bound mechanisms that neglect three-dimensional spatial effects, resulting in failure surfaces that deviate from actual pit behavior. Building on the variable-mechanism upper-limit method and informed by numerical simulation, this study establishes a three-dimensional deformation mechanism for circular foundation pits subjected to backfill loading. A kinematically admissible velocity field is constructed to incorporate spatial effects, from which an upper-bound solution for the basal-heave safety factor is derived. Parametric analyses are conducted to evaluate the influence of pit radius, soil cohesion, and embedded-wall ratio on the basal-heave safety factor. Results indicate that incorporating three-dimensional effects significantly improves the accuracy of upper-bound predictions. The safety factor increases as pit radius decreases, reflecting the strengthening influence of spatial confinement; soil cohesion exerts a dominant, approximately linear effect; and excessive increases in embedded ratio yield diminishing returns in basal-heave resistance. A case study verifies the validity of the proposed formulation. The findings provide a more realistic theoretical framework for the design and construction of circular foundation pits, particularly where deformation control and basal stability are critical.

Key words: Upper Limit Method, Circular Foundation Pit, Safety Factor, Petroleum Exploration