The influence of freezing-thawing cycles on the geotechnical performance of an end-bearing energy pile

Abstract

Energy piles are getting more attention for their sustainable and economic potential. The fluid pipes are integrated in the foundation piles, which makes it cost-attractive, but also relatively new to the Dutch geotechnical industry. By lowering the temperature limit of the anti-freeze fluid in the pipes the heat extraction and thereby also the necessary pile length can be optimized. However, the resulting thermal loads might influence the structural function and are therefore a main obstruction for the industry. Within this thesis three aspects of freezing-thawing of an axially compressed end-bearing energy pile are evaluated: (i) frost resistance of concrete, (ii) thermal analysis and (iii) geotechnical analysis. The objective is to work towards a conclusion on the degradation effects of freezing-thawing processes along the edge of an end-bearing energy pile and come up with practical advice for the design. The behaviour of the concrete is evaluated by literature, wherein the concrete thermal conductivity and mixture are key elements. For the thermal analysis a pile group simulation and a detailed single pile analysis is performed in Comsol Multiphysics. The soil stratigraphy is simplified and based on the Western part of the Netherlands. The input originates from the heat-cool demand of a single household related to the monthly gas usage in the Netherlands. Typical heat extraction for this specific case, i.e. long-term average amount of 7 W/m, for 7 years did not result in freezing temperatures at the edge of the most critical piles. Freezing occurred after an increased heat extraction until 10 W/m. Furthermore, the influence of daily cycles in the single pile analysis damped by the thermal resistance of the concrete pile. The geotechnical analysis is assessed in an axisymmetric model in Plaxis with and without a fully-coupled thermo-hydro-mechanical (THM) module. The first set of analyses with THM examines the influence of expansion and contraction of materials including phase changes, but without changes in soil mechanical and physical properties. Two situations are considered that are different in terms of the position of their neutral plane and negative shaft resistance. The resulting settlements can be distinguished as initial structural load and settlements due to thermal seasonal cycles, which fluctuate between 1 mm. A ratcheting behaviour was observed and resulted in a settlement of 12 and 3 mm over 5 years. However, the volumetric strain accumulation around the pile indicates unrealistic results in the THM module and conclude that the simulation of freezing-thawing cycles has to be done in a more advanced constitutive model which can capture freezing-thawing behaviour of soils. Within the second set of analyses the impact of the soil strength and stiffness due to a freezing-thawing cycle was investigated for the two situations to determine their effect on the pile behaviour in terms of load-displacement and shear distribution. The results indicate that effective cohesion is most influencing the pile behaviour and it indicates that no need exist to change the design approach of end-bearing piles as long as the potential negative shaft resistance along the pile over all layers above the bearing layer is included in the design calculation and considered fully mobilized.