Structural design and performance of a steel joint for driven precast concrete energy piles

Abstract

Geothermal energy piles are deep foundations primarily used to transfer building loads down to competent ground layers, while they can be used as a heating and cooling source/sink for buildings. Most energy pile foundations are cast-in-place piles, while precast concrete driven energy piles (DEPs) have not been utilized due to the lack of a suitable joint to connect pile segments. This study presents a detailed structural design and performance of a novel steel joint for precast concrete DEPs. Firstly, the two most common joint sizes are designed based on the finite element method, and the dimensions and steel grades of the joint are presented. Secondly, the casting procedure of energy piles using the newly designed joint is explained in detail. Thirdly, the impact and bending tests performed according to the standards are presented, the standard procedures are critically evaluated, and recommendations are provided. The impact test results show that both joint sizes remained undamaged after being subjected to 1000 blows, imposing a minimum of 28 MPa of stress. The small and large sizes of joints have a flexural stiffness (EI) of 3500 kN.m2 and 7720 kN.m2, respectively, according to the standard flexural tests. Lastly, the flexural stiffness distribution of the pile and joints is measured using digital image correlation (DIC) and compared with the current method recommended by the standard. The current standard requirements measure flexural stiffness based on point sensor measurements, and the distribution of flexural stiffness along the pile and joint is not evaluated. Standard stiffness is affected by the concrete and reinforcements in the pile; however, there is no standard requirement for the type/grade of the concrete and reinforcements used for manufacturing the test specimens.