Thermally-induced ratcheting behaviour of laterally-loaded reinforced concrete energy piles in sand

Abstract

An energy pile is an energy geostructure that has been increasingly used in practice for supporting and reducing carbon emissions from buildings or other infrastructure. It is subjected to vertical loading from the self-weight of the superstructure above, possibly horizontal loading due to pile eccentricity and cyclic thermal loading due to daily and seasonal temperature change. Thermally-induced pile ratcheting – the accumulated and irreversible pile head displacement towards the loading direction upon repeated pile heating-cooling occurs during the operation of energy piles. However, there is no investigation on any thermally-induced ratcheting for the laterally-loaded reinforced concrete pile. Because of the lack of data concerning the thermomechanical behaviour of the laterally-loaded reinforced concrete (RC) piles, any numerical modelling method for this kind of problem cannot be validated.

In this thesis, firstly, a type of model RC consisting of mortar (plaster, sand and water) and copper powder, along with a steel reinforcing cage and silicone pipes was presented. It was designed to (1) correctly capture quasi-brittle structural response; (2) match thermal properties of field RC, without changing the mechanical properties. In 1- soil-structure interaction tests, a model RC pile could serve as an effective heat exchanger for transferring heat from a heat-carrier fluid pipe embedded within the mortar to the surrounding sand. It exhibited accumulated and irreversible pile head settlement upon heating and cooling cycles.

Secondly, this small-scale model RC pile was employed for testing soil-structure interaction upon horizontal pile-head loading within a centrifuge. Nonlinear finite-element modelling was also presented to back-analyse centrifuge observations and explore the influence of the constitutive models used. The physical model RC pile could (1) reproduce the pile failure mechanism by forming realistic tension crack patterns, and plastic hinges and (2) give hardening responses upon horizontal loading. The effectiveness of three different soil constitutive models, from simple elastic-perfectly plastic, to strain hardening, and strain softening was evaluated. The soil nonlinearity concerning an equivalent mobilised shear modulus was essential to capture the pre-peak load-displacement response of the laterally loaded pile. It could be incorporated into an iterative and a sublayer approach. It found that the limiting lateral soil pressure distributions exhibited a peak above the plastic hinge with the strain-softening model for both the dry and saturated tests. Comparisons of measured and predicted results demonstrated that for the laterally-loaded pile issue, the load-displacement response could be well approximated by models which did not incorporate strain softening, even though the soil behaviour itself exhibited a strong softening response.

Finally, a series of centrifuge and FE modelling of the thermomechanical behaviour of the RC energy pile subjected to different horizontal working loads in the sand was presented. The numerical modelling was to back-analyse centrifuge tests on thermomechanical behaviour of soil-pile interaction. By adopting the validated FE model, sensitivity analyses were conducted to identify influential factors that govern the thermally-induced pile ratcheting behaviour. A loading- and temperature- controlled system for use in the centrifuge was built to apply the constant horizontal load and heating-cooling cycles upon the RC pile. Thermally-induced pile ratcheting for the laterally-loaded RC pile was observed under the high- condition. The back-analyses demonstrated that (1) isotropic sand thermal properties (i.e. ignoring the influence of mechanically- and/or thermally induced volume change on sand thermal properties) had no/minimal impact on the sand temperature prediction; (2) the post-peak soil softening behaviour at large-strain played an essential role in predicting the thermally-induced pile head horizontal displacement at greater (two) numbers of thermal cycles; (3) the accumulation of the plastic shear strain due to the cyclic mechanical loading upon pile heating and cooling was proven to be a reason for the pile head ratcheting. Sensitivity analyses revealed that (1) thermal conductivity (over 0.54 to 3.56 W/(mK)) had no/negligible effect on the pile ratcheting performance and the thermomechanical sand-pile interaction; (2) thermally-induced pile bending moments were not sufficient to cause structural yield or to reach the ultimate limit state.

Date of Award	2020
Original language	English
Sponsors	China Scholarship Council
Supervisor	Jonathan Knappett (Supervisor), Anthony Leung (Supervisor) & Ioannis Anastasopoulos (Supervisor)