An analytical method for developing a theoretical load-settlement curve for axially loaded piles in clay is presented. The method is based on the correlation of the ratio of load transfer to soil shear strength as a function of pile movement. The results of studies of field tests of instrumented piles and laboratory tests of small piles in clay are used to obtain the desired correlation. The correlation is presented in the form of a family of curves that are obtained when the ratios of load transfer to soil shear strength versus pile movement are plotted as a function of depth. The validity of the family of correlation curves is checked by comparing computed and actual load-settlement curves for some typical field tests. Results obtained by this method indicate that the method may be used effectively to determine load-carrying capacity for axially loaded piles in clay.

Closure to “Load Transfer for Axially Loaded Piles in Clay”

Energy piles are an effective and economic means of using geothermal energy resources for heating and cooling buildings, contributing to legislative requirements for renewable energy in new construction. While such piles have been used for around 25 years with no apparent detrimental effect, there is limited understanding of their thermo-mechanical behaviour. This paper synthesises the results from three published field studies and illustrates some of the engineering behaviour of such piles during heating and cooling. Simplified load transfer mechanisms for a single pile subjected to pure thermal loadings (i.e. without mechanical load) and combined thermo-mechanical loadings have been developed and are used to interpret the field data with regard to change in axial stress and shaft friction during heating and cooling. The effect of end restraint and ground conditions on the thermo-mechanical response of energy piles is discussed. Values of change in axial stress and mobilised shaft friction due to thermal...

Thermo-mechanical Behaviour of Energy Piles

Review of analytical models for heat transfer by vertical ground heat exchangers (GHEs): A perspective of time and space scales

There is currently a lack of established calculation method for the geotechnical design of heat exchanger piles, although the technology is experiencing a fast expansion. Instead of quantifying the effects of temperature changes on the static behavior of heat exchanger piles, the common geotechnical practice is to apply a large overall security factor. This is done in order to be on the side of safety with respect to thermal effects. The few existing in situ experiments show that applying a thermal load induces a significant change in the stress-strain state of a pile. This paper presents a geotechnical numerical analysis method, based on the load transfer approach, which assesses the main effects of temperature changes on pile behavior. The method is validated on the basis of two in situ measurements of the loads and deformations experienced by heat exchanger test piles. The occurrence of critical design situations is further discussed. Some conclusions are formulated on concrete failure and the full mobilization of the pile shaft friction and base resistance during the operation of the heat exchange system.

Geotechnical Analysis of Heat Exchanger Piles

Eight full-scale energy foundations were constructed for a new building at the US Air Force Academy. The foundations are being used to demonstrate this technology to the United States Department of Defense and have several experimental features in order to study their thermal–mechanical behavior. Three of the foundations are instrumented with strain gages and thermistors, and their thermo-mechanical response during a heating and cooling test was evaluated. For a temperature increase of 18 °C, the maximum thermal axial stress ranged from 4.0 to 5.1 MPa, which is approximately 25 % of the compressive strength of concrete (estimated at 21 MPa), and the maximum upward displacement ranged from 1.4 to 1.7 mm, which should not cause angular distortions sufficient enough to cause structural or aesthetic damage of the building. The end restraint provided by the building was observed to change depending on the location of the foundation. The heat flux per meter was measured by evaluating the temperatures and flow rates of a heat exchanger fluid entering and exiting the foundations. The heat flux values were consistent with those in the literature, and the foundation with the three continuous heat exchanger loops was found to have the greatest heat flux per meter. The transient thermal conductivity of the subsurface measured using the temperatures of the subsurface surrounding the foundation ranged from 2.0 to 2.3 W/mK, which is consistent with results from thermal response tests on energy foundations reported in the literature.

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Evaluation of thermo-mechanical and thermal behavior of full-scale energy foundations

Very limited information is available regarding the impact of heating and cooling processes on the geotechnical performance of piled foundations incorporating pipe loops for ground-source heat-pump systems (so-called energy piles). A pile-loading test that incorporated temperature cycles while under an extended period of maintained loading was undertaken to investigate the behaviour of an energy pile installed in London Clay. Testing was carried out over a period of about seven weeks, with conventional loading tests carried out either side of an extended loading test with thermal cycles. Using an optical fibre sensor system, and other more conventional instrumentation, temperature and strain profiles were observed in the test pile, an adjacent bore-hole, two of the anchor piles, and the heat sink pile. Details of load and movement at the pile head, of ambient air temperature and of the input/output temperature of fluid within the heating system were also recorded. Thermodynamic behaviour observed during t...

Energy pile test at Lambeth College, London: geotechnical and thermodynamic aspects of pile response to heat cycles

A descriptive framework for explaining the contribution of the foundation material and various levels of end-restraint, to the response of piles that are loaded thermomechanically, is presented. Th...

A framework for understanding energy pile behaviour

Based on discussions at the international workshop on “Thermoactive geotechnical systems for near-surface geothermal energy”, hosted at Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland (http://www.olgun.cee.vt.edu/workshop/), this article attempts to provide a broad overview of the analysis methods used for evaluation of systems that use either boreholes or geo-structures for heat exchange. It identifies commonalities where knowledge transfer from the former to the latter can be made, and highlights where there are significant differences that may limit this cross-fertilisation. The article then focusses on recent developments and current understanding pertaining to the analysis of the thermo-mechanical interaction between a geostructure and the ground, and how this may be incorporated into the geotechnical design of energy geostructures.

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P.J. Bourne-Webb

Papers

Energy pile test at Lambeth College, London: geotechnical and thermodynamic aspects of pile response to heat cycles

Thermo-mechanical Behaviour of Energy Piles

A framework for understanding energy pile behaviour

Analysis and design methods for energy geostructures

Thermal and mechanical aspects of the response of embedded retaining walls used as shallow geothermal heat exchangers