Thermomechanical Performance and Bearing Characteristics of a Large-Diameter, Extra-Long Energy Pile: An Intercalibrated DFOS Analysis

Abstract

This study conducted a comprehensive field trial test on a large-diameter, extra-long, rock-embedded energy pile subjected to mechanical, thermal, and coupled thermomechanical loads. This paper introduces a novel approach for evaluating the thermomechanical behavior and bearing capacity of the test pile using an intercalibrated distributed fiber optic sensing (DFOS) method based on distributed temperature sensing (DTS) and Brillouin optical frequency domain analysis (BOFDA). An innovative temperature self-compensation method for BOFDA has been developed, representing the first application to energy pile monitoring. The results show that integrating the BOFDA and DTS temperature measurements achieves high-resolution distributed sensing along the extra-long pile. The novel temperature self-compensation method for BOFDA shows enhanced reliability to existing instruments in optimizing DFOS strain measurements. The localized asymmetry of axial forces along both sides of the large-diameter pile shaft during the static load test was effectively captured by high-resolution DFOS strain sensing. The study proposed thermal and mechanical responses for a large-diameter, extra-long, rock-embedded energy pile under various loads and compared with the existing studies based on the experimental results. Under mechanical load, the axial force distribution along the pile showed inconsistency, with varying degrees of shaft resistance at different depths. Thermal loading identified a neutral point near the pile’s base, from which additional shaft resistance linearly increased toward both ends. The pile was reloaded while maintaining the thermal load, resulting in the conversion of negative axial resistance to a positive state at the pile base. The intercalibrated DFOS method has demonstrated reliable high-resolution distributed sensing on a large-diameter, extra-long, rock-embedded energy pile, indicating its great potential for wide application in large-scale geotechnical engineering projects.