Analytical design of RC–micropile connections in building underpinning
Abstract
The construction sector plays a key role in achieving global sustainability targets, making extending the service life of existing buildings a priority. In this context, micropile underpinning is widely used to restore the structural performance of foundations affected by severe geotechnical conditions. However, current structural and geotechnical standards provide limited and fragmented guidance for the analytical verification of load transfer mechanisms between micropiles and existing foundations. This study proposes a practical analytical framework to evaluate these connection interfaces through a two-interface decomposition of the load-transfer mechanism, by considering two governing mechanisms: (i) bond behavior between the steel casing and grout, and (ii) shear transfer between the grout and the existing concrete footing. The methodology is analytically verified through a full-scale underpinning project involving 158 micropiles, combining normative formulations, empirical approaches, and standardized pull-out test data. Results from the analyzed case study indicate that relying solely on the natural bond between smooth steel casings and conventional concrete joints results in a significant reduction in load transfer capacity, limiting it to approximately 15–20% of the theoretical structural capacity. In contrast, the use of welded corrugated connectors and high-performance expansive mortars significantly enhances interface capacity, allowing safe load transfer within practical geometric constraints. The study also provides a mechanical interpretation of how these solutions modify the internal strut-and-tie mechanism, improving stress distribution and reducing the risk of premature failure. These findings also highlight the potential of integrating the proposed analytical framework into digital design environments (e.g., BIM-based workflows), where interface-level verification could be incorporated as parametric checks to support decision-making in foundation retrofitting projects. The proposed framework offers a rational and practical basis for the design of micropile underpinning connections, contributing to more efficient and sustainable foundation retrofit solutions.