A load transfer approach for studying the cyclic behavior of thermo-active piles M.E. Suryatriyastuti a , H. Mroueh a,⇑ , S. Burlon b a Laboratoire Génie Civil et géo-Environnement (LGCgE) – Polytech’Lille, Université Lille1 Sciences et Technologies, Villeneuve d’Ascq 59655, France b Institut Français des Sciences et Technologies des Transports, de l’Aménagement, et des Réseaux (IFSTTAR), Marne la Vallée 77447, France article info Article history: Received 15 April 2013 Received in revised form 22 July 2013 Accepted 29 September 2013 Available online 19 October 2013 Keywords: Thermo-active piles Cyclic thermal loading Soil–pile interface t–z Function Modjoin Finite difference method Three-dimensional modeling abstract Unsatisfactory understanding of thermally induced axial stress and mobilized shaft friction in the thermo-active piles has led to a cautious and conservative design of such piles. Despite the fact that the number of construction works using this type of piles has been rapidly increasing since the last 20 years and none of them witnessed any structural damage, the question that still remains is how to overcome the cyclic thermal effects in such piles to optimize the design method. This paper presents a soil–pile interaction design method of an axially loaded thermo-active pile based on a load transfer approach by introducing a proposed t–z cyclic function. The proposed t–z function comprises a cyclic hardening/softening mechanism which is able to count the degradation of the soil–pile capacity during two-way cyclic thermal loading in the thermo-active pile. The proposed t–z function is then compared to a constitutive law of soil–pile interface behavior under cyclic loading, the Modjoin law. Afterwards, numerical analyses of a thermo-active pile located in cohesionless soil are conducted using the two cyclic laws in order to comprehend the response of such pile under combined mechanical and cyclic thermal loads. The behaviors of the pile resulting from the two laws show a good agreement in rendering the cyc- lic degradation effects. At last, the results permit us to estimate the change in axial stress and shaft fric- tion induced by temperature variations that should be taken into account in the geotechnical design of the thermo-active pile. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The thermo-active piles have successfully incorporated the heat exchanger elements to the structural pile foundations. Other than supporting the static weight of structure, the thermo-active piles are used to provide thermal energy to the overlying building by cir- culating the heat carrier fluid inside the piles [1,2]. In consequence, the thermo-active piles are subjected not only to the mechanical loading of the overlying structure but also to a two-way cyclic thermal loading (i.e. seasonal thermal loading) according to the thermal needs of building. Installation of the thermo-active piles in European countries [3–6] showed that the usage of these piles is advantageous in increasing the energy performance and in min- imizing the annual cost [6,7]. But at the same time, this latter pre- sents high risk on the mechanical resistance of both foundations and upper structure [8–10] because the circulating warm fluid dur- ing summer produces a pile expansion and the circulating cool fluid during winter produces a pile contraction [11,12]. Besides, no design code that takes into account the thermal interaction on the geotechnical capacity of pile foundations is available yet [13]. For years, contractors have done constructions with thermo-active piles based on empirical considerations or on a conservative design method by increasing the safety factor [13,14]. As a result, a bigger dimension of pile and a higher piling cost are required. Due to the limited knowledge concerning the impact of thermal operation on the geotechnical performance, the response of ther- mo-active piles under combined mechanical and cyclic thermal loads becomes a major interest for establishing a more effective geotechnical design criterion. Since the ratio of the pile diameter to the pile length is very small, the temperature variations injected in the pile affects mainly the pile axial response [15]. In situ expe- riences of the new building at Swiss Federal Institute of Technology in Lausanne [10] and the Lambeth College in London [8] have re- marked an important change in mobilized shaft friction and axial load distribution at the soil–structure interface by the change of temperature [16]. These changes are stated to be dependent on the degree of axial fixity at the head and the toe of pile foundation [8,16]. While most reliable method to determine the response of the piles is based on the results obtained from pile load tests, this method can be expensive and time-consuming [17]. Other alterna- tive means to study the axially loaded piles is by conducting 0266-352X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compgeo.2013.09.021 ⇑ Corresponding author. E-mail addresses: msuryatr@polytech-lille.fr (M.E. Suryatriyastuti), hussein. mroueh@polytech-lille.fr (H. Mroueh). Computers and Geotechnics 55 (2014) 378–391 Contents lists available at ScienceDirect Computers and Geotechnics journal homepage: www.elsevier.com/locate/compgeo