Numerical Investigations of Energy Micropiled Raft in Hypoplastic Clay Subjected to Long-Term Cyclic Thermal Loading

This paper presents the results of a series of fully coupled thermohydromechanical finite element simulations of the behavior of large energy micropiled raft foundations in soft to medium stiff clays. A parametric analysis was conducted by varying the spacing between the energy micropiles and considering the cases of lightly overconsolidated clay and more heavily overconsolidated clay. A recent advanced thermohypoplastic model was adopted to describe soil behavior. The simulations were performed in two stages: in Stage 1, the raft was subjected to a constant external load under isothermal drained conditions; in Stage 2, with the external load kept constant, the energy micropiles were subjected to a large number of thermal loading cycles. The results show that micropiles helped in reducing settlements during isothermal loading, with effectiveness decreasing as the spacing increased. In Stage 2, all the considered rafts suffered from a progressive accumulation of permanent settlements associated with a thermally induced ratcheting phenomenon. The accumulation of inelastic strains, caused by the progressive buildup of excess pore water pressure, eventually resulted in settlements that were greater than those of the raft alone after some years of continuous operation of the energy system. The heat flux exchanged between the micropiles and the soil increased as the spacing increased, with the best performance obtained for a spacing of 7.6 m, for which the micropiles behaved as single isolated heat exchangers. However, under such conditions, the micropiles were almost ineffective as settlement reducers. On the contrary, for the smallest spacing considered (1.9 m), the micropiles performed well in controlling the settlements of the raft but very poorly from the energetic standpoint. The optimum compromise between mechanical and thermal performance was achieved with an intermediate spacing of 3.8 m.