In this paper, a finite element numerical model of thermal-hydro-mechanical of energy piles under multi-layer geological conditions was established, and field tests of ultra-long energy pile (1000-mm-diameter, 44-m-long) were carried out to reveal the temperature distribution and mechanical properties of energy pile under typical working conditions. Based on the analytical results, a softening shear model of the energy–soil interface under the condition of large shear displacement was proposed with the load transfer method, and the reliability of the model was verified. The model can simulate the shear–displacement relationship of the pile–soil interface under different geological conditions.

Phase change materials are promising for medium-temperature thermal energy storage due to their high latent heat storage capacity. However, their applications are limited by low thermal conductivity and the trade-off between thermal conductivity enhancement and latent heat retention when conductive fillers are introduced. In this study, polydimethylsiloxane-modified graphene oxide (MGO) was incorporated into a sebacic acid–succinic acid (SeA–SuA) eutectic PCM to improve filler dispersion and filler–matrix contact while maintaining thermal storage performance. At an MGO loading of 0.25 wt%, the composite exhibited a melting peak temperature of 128.9 °C, a melting enthalpy of 234.8 J g⁻¹, and a room-temperature thermal conductivity of 0.499 W m⁻¹ K⁻¹. Compared with the composite containing unmodified graphene oxide (GO) at the same loading, the MGO-based PCM exhibited better latent heat retention and higher thermal conductivity, which may be associated with reduced filler aggregation and improved filler–matrix contact. After 800 accelerated thermal cycles, the composite retained 95.2% of its initial melting enthalpy and 87.8% of its initial thermal conductivity. These results demonstrate that combining eutectic composition regulation with GO surface modification is a feasible strategy for achieving balanced thermal storage, transport, and cycling performance in medium-temperature composite PCMs.