Experimental and numerical study on thermo-mechanical behaviour of energy pile under different constraints

Thermo -mechanical (TM) behaviour of energy pile (EP) is strongly affected by the pile constraints. The influence mechanism of different constraints on TM behaviour of EP should be revealed. In this paper, the model experiments in laboratory are carried out to research effects of pile end constraints and sand compactness on TM properties of EP. The research results display that the strain of end bearing pile increases gradually along pile depth due to the larger constraint at the pile tip, while the strain distribution of friction pile is large at both ends and small in the middle. Compared with friction pile, the end bearing pile has larger pile tip soil pressure and smaller side friction of pile. The displacement change of EP decreases during the operation in summer mode after applying 0.5 kN load on the pile top. The displacement of end bearing pile can restore to 0 mm, while the friction pile has a settlement of -0.028 mm. In winter mode, both the end bearing pile and friction pile produce unrecoverable settlement, and applying load on pile top will further aggravate the settlement amplitude. As for influence of sand compactness, compared with loose sand, the dense sand has greater thermal conductivity which benefits to the thermal efficiency of EP. This results in smaller pile excess temperature and recovery rate of surrounding soi1 temperature. Accordingly, the pile top displacement and strain also decrease due to small temperature variation, but the pile side friction and side soil pressure increase. The mechanical properties of EP under the condition of different pile -soil parameters are simulated by a 3-D numerical model, which is validated by experiments. It is shown that when the Poisson's ratio of the soil increases from 0.2 to 0.4, the top displacement of the pile increases from -7.35 mm to -8.76 mm, which attributes to the increase in the lateral deformation of the soil, resulting in a decrease in the soil constraint on the pile. Although the distributions of pile side friction along the pile depth are basically the same for different poisson's ratios, there are differences at the pile tip. Also, increasing the pile -soil interface friction coefficient will result in the enhancement of constraint and pile side friction and the decrease of pile body displacement and thus improve the mechanical properties of EP. The study helps to reveal the influence mechanism of various constraints on the TM properties of EP and can provide reference for the safe and efficient operation of EP.