Resumo em Inglês:

Abstract The multi-physical phenomena, particularly water content and temperature variations, governing the behaviour of soils should be considered in the design and analysis of the energy geostructures. Soil temperature and water content variations impose a significant risk on the stability and serviceability of existing and future geostructures. Although potential failure modes, impacts at a system scale, and the response of saturated soils to thermal loads are previously discussed, interpretation of the thermo-hydro-mechanical behaviour of partially saturated soils in the context of energy geostructures is not thoroughly investigated. In this regard, this paper brings together the experimental data from several laboratory investigations to attain a comprehensive understanding of the partially saturated fine-grained soils response under thermo-hydro-mechanical loading, which plays a vital role in the analysis of the soil behaviour and energy geostructures in contact with them. In this paper, the effect of thermal loading in different matric suctions and hydraulic loading at different temperatures on soil preconsolidation stress, water content variation, thermal and hydraulic conductivities, and compression indexes are studied. Furthermore, soil thermal deformation is studied in detail for different overconsolidation ratios and matric suctions.

Resumo em Inglês:

Abstract The energy crisis scenario currently going on in Brazil, along with the need to reduce greenhouse emissions, lead to an urgent need to design buildings with greater energy efficiency. Shallow geothermal energy surges as a sustainable alternative to reduce the electricity consumption in buildings related to air conditioning and water heating systems. In Brazil, the use of this technique is still incipient due to the lack of studies that demonstrate its viability in the country's climatic conditions, as well as underground temperature and demand for acclimatization of buildings. The aim of the current work is to present a site characterization for the first geothermal energy investigation carried out in the Brazilian South region. This study is being conducted at the Geotechnical Experimental Site of the State University of Ponta Grossa, and this paper describes the physical, mineralogical, thermal and mechanical characteristics of this site, which comprises a thick layer of a lateritic sandy clay soil over a silty sand layer. The preliminary results of the ground temperatures are consistent with the common trends reported in the literature, showing more expressive oscillations near the ground surface, and becoming approximately constant at higher depths.

Resumo em Inglês:

Abstract Energy piles are often closely spaced such that the thermal response of one pile might affect the response of neighbouring piles through heat transfer in the soil. This paper examines the changes in extracted energy and ground temperature of two cast-in-place bored energy piles installed below a six-storey building, with a diameter of 0.6 m, length of 10 m and 3.5 m centre-to-centre distance. Field experiments were conducted on singular and dual piles for cooling and cyclic temperatures, continuous daily operation of the ground source heat pump (GSHP) and intermittent cyclic operations with stoppage-to-operating ratios of 2:1 and 1:2. Compared to single pile operations, greater energy was extracted from dual piles for all operating modes despite thermal interaction through the soil volume between the dual piles. The larger stoppage-to-operating ratio of the GSHP induced lower pile and ground temperature changes and higher energy extraction than continuous operation for single and dual pile operations. The larger stoppage-to-operating ratio of the GSHP reduced the thermal interaction between the dual piles by imposing lower ground temperature changes compared to continuous operation. The results demonstrate the practical significance of managing the GSPHs operating modes for optimal thermal performance of multiple closely spaced energy piles.

Resumo em Inglês:

Abstract The use of geothermal energy for heating and cooling purposes is an environmentally-friendly and cost-effective alternative with the potential to replace fossil fuels and help to mitigate global warming as well. The paper illustrates the geothermal potential evaluation of a portion of the Lyon-Turin base tunnel considering the thermal activation of the tunnel concrete segmental lining. The international infrastructure will connect Italy to France, crossing the Alps and meeting uncommon climate conditions, reaching a peak temperature of 47 °C, due to its significant rock overburden (up to 2500 m) under the Ambin massif. A thermo-hydraulic numerical model was used to simulate the heat exchange of the system and quantify the power achievable by thermally activating a 10 km-long section of the base tunnel. Sensitivity analyses were performed to investigate the influence of the heat carrier fluid and the air flow velocities as well as the inlet temperature on the heat exchange. Moreover, four different operational conditions were compared to allow for assessing the overall thermal performance of the energy lining in terms of heat exploited and of the capacity of cooling the tunnel.

Resumo em Inglês:

Abstract Knowledge of the soil-temperature relation and its peculiarities have been widely discussed in Geotechnics. Previous studies show that heated fine-grained soil induces pore pressure, and as a result, the soil exhibits a consolidation effect similar to that of conventional consolidation when pore pressure is dissipated. The outcome of this process depends on soil characteristics such as the overconsolidation ratio and plasticity. Consolidated soil typically induces positive pore pressure while overconsolidated one induces negative pore pressure when heated. The phenomenon described was explored in this study aimed at the thermal consolidation product regarding the decrease in void ratio and consequent increase in undrained shear strength (Su) of a soil submitted to heating. These aspects were investigated using physical models at 1g gravity field, built on a laboratory scale. The heat source was placed inside a driven torpedo pile model in a medium consisting of a kaolin and metakaolin mixture. The undrained shear strength profile was defined by T-Bar tests involving different effective stresses and consolidation temperatures. The results show that for both mediums (normally consolidated and overconsolidated), undrained shear strength tends to grow proportionally to the temperature variation and permanent volume change is reached after a heating-cooling cycle. On the other hand, undrained shear strength indicates less significant variations for higher overconsolidation ratios.

Resumo em Inglês:

Abstract The worldwide consumption of electric energy destined for air conditioners, expected to triple by 2050, can be lessened by geothermal piles, which transfer heat from the internal environment of buildings to the subsoil. This paper shows the influence of pile geometry and properties of soil, pile, and pipe materials on the heat transfer of a geothermal pile to the surrounding soil, to support design from the viewpoint of thermal performance optimization. A numerical model was developed with ANSYS CFX 19.2, a high-performance Computational Fluid Dynamics tool, and calibrated using data from a thermal response test performed in a saturated sandy soil in São Paulo, Brazil. A parametric analysis was carried out varying pile length, diameter, and slenderness; soil and pile material conductivities; degree of saturation; fluid inlet temperature; fluid flow rate; and pipe thermal resistance. Results show that the fluid inlet temperature is the most influential parameter on the thermal performance of the pile. Heat transfer grows when geometrical parameters (diameter and length) are increased mainly due to an increase in heat exchange surface area, whereas the normalized heat transfer rate per unit of surface area of the pile is practically unaltered. Higher soil, pipe and pile thermal conductivities improve thermal performance. The degree of saturation increases the thermal conductivity of the soil; however, the effect is not remarkable on the system’s thermal performance for saturation degrees higher than 20%. The fluid flow must be turbulent but increases above a certain flow rate do not improve the thermal performance.

Resumo em Inglês:

Abstract This study presents the results from centrifuge modeling experiments performed to understand the effects of temperature changes on the vertical pullout capacity of scale-model torpedo piles embedded in soft clay layers. The model torpedo pile is a pointed stainless-steel cylinder with fins at the top, installed by self-weight to the base of a clay layer using a stepper motor. An internal electrical resistance heater was used to control the pile temperature. The torpedo pile was first heated until the temperature and pore water pressure of the surrounding clay layer stabilized (drained conditions), after which the torpedo pile was cooled. Pullout tests performed on torpedo piles indicate that allowing drainage of excess pore water pressures induced by heating to different temperatures followed by cooling leads to an increase in axial pullout capacity with maximum temperature but does not affect the pullout stiffness. Push-pull T-bar penetration tests performed before and after pile heating indicate that an increase in undrained shear strength of the clay occurs near the torpedo pile, and post-test gravimetric water content measurements indicate a greater decrease in void ratio occurred in the soil layers heated to higher temperatures. The pullout capacity of the torpedo pile was found to follow a linear trend with maximum pile temperature change, but with a smaller slope than that observed for end-bearing energy piles tested in previous studies in the same clay.

Resumo em Inglês:

Abstract The anchoring of floating platforms is one of many processes in the oil industry that requires innovative strategies. In this respect, there is interest in developing techniques that improve the shear strength of soft soils in order to increase the bearing capacity of offshore foundations anchored in these soils. Normally consolidated clay soil is known to undergo thermal consolidation when submitted to temperature cycles. The present study aimed to assess the impact of a temperature cycle on soft kaolin clay using a reduced-scale physical model submitted to heating at maximum temperatures of 85 °C and 65 °C, followed by cooling. Variables such as pore pressure, temperature at different soil depths and displacement were monitored during the thermal cycle to better understand the phenomenon. The strength profile before and after heating was determined via T-bar tests conducted in different positions in relation to the heat source. The temperature variation increased the undrained shear strength of the soil directly proportional to the temperature applied and inversely proportional to the radial distance from the heat source, reaching improvements of 123%. In this respect, it is believed that applying a temperature cycle to normally consolidated soft clayey soil can improve the pullout capacity of offshore foundations.

Resumo em Inglês:

Abstract This paper provides preliminary results on geothermal energy piles (GEPs) for the thermal climatization of structures founded on the typical tropical unsaturated soil of the central region of Brazil. The research employed a series of prototype simulations of a thermal pile embedded into a calibration chamber with compacted unsaturated soil. It closely simulates the behavior of a (prototype) section from real scale GEPs founded in the geotechnical media of the region, in terms of compactness, mineralogy, water content and thermal variables. One of the on-going thermal tests was numerically simulated with a Multiphysics commercial software to calibrate a model and expand the results to possible scenarios of distinct (laboratory) GEP performance. The analyses will base future simulations of typical foundation layouts for large-scale structures founded on tropical soils of the region to verify the thermal energy efficiency under average operational conditions. Besides the known limitations of this research, still at early stage, an initial assessment was achieved to design shallow geothermal systems for local conditions.

Resumo em Inglês:

Abstract The use of shallow geothermal energy through energy piles for the air-conditioning of buildings is increasing worldwide. This type of renewable energy technology is still not utilized in Brazil, where the hot dominating weather regions and the air cooling demand predominate. In this case of unbalanced heat transfer to the ground, the efficiency of the system may decrease with time due to the excessive heat injection into the soil. In order to investigate the possibility of an efficient application of this technology in São Paulo city, a balanced use of the ground for a ground-source heat pump (GSHP) system utilizing energy piles is evaluated in the present paper. Energy foundations were designed to meet the balanced heating and cooling loads (air conditioning and water heating) of a hypothetical business hotel building located in a site at the campus of the University of São Paulo, where thermal response tests (TRTs) were conducted on different types of energy pile. The number of energy piles required to supply the building thermal loads were estimated using the pile heat exchanger modelling software PILESIM 2.1 and compared with an analytical model prediction. The evaluations were done for three different types of pile tested at the site chosen for this study: micropiles, steel pipe, and continuous flight auger (CFA) piles. The results indicate that the ground heat extraction should be considered for the use of GSHP systems with energy piles in air cooling-dominated scenarios similar to the case studied here.

Resumo em Inglês:

Abstract This paper addresses the topic of site characterization for the design of Shallow Geothermal Energy (SGE) systems, namely of thermoactive geostructures, which are geotechnical structures, such as piles, retaining walls and tunnel linings, also used as heat exchangers as part of closed-loop SGE systems. Such solutions, being increasingly adopted for buildings’ and infrastructures’ heating and/or cooling, are considered sustainable and cost effective. For the design of the primary circuit of the SGE system, which is embedded within the superficial soil layers, a comprehensive knowledge of the ground condition at the site is mandatory. This includes the evaluation of the energy features and whether the system can provide the required energy needs during the operational period, as well as the verification of the structural and geotechnical safety and functionality requirements. The site characterization for SGE systems involves different stages, from desk studies to detailed characterization, including in-situ trials, laboratory testing of undisturbed soil samples and the study of possible interferences. The specific aspects that will be addressed are: (i) the assessment of the site undisturbed ground temperature and its hydrogeological features; (ii) the thermal and thermomechanical characterization of the different soil layers; (iii) the investigation of the ground-heat exchanger thermal resistance; (iv) the collection of information related to the environmental constraints and to potential interferences among multiple users, which are related to the service life of the structure. The overall aim is to ensure a proper design of the SGE system for guaranteeing its sustainability in the long term.