Thermo-active Diaphragm Walls Research Articles

Investigation on heat transfer performance of energy diaphragm walls under groundwater flow condition

The flow of groundwater affects the transport of heat, subsequently impacting the heat transfer performance of energy diaphragm walls. To investigate the effect of groundwater flow on the thermal performance of energy diaphragm walls in the actual project, a three-dimensional finite element model of the thermo-hydro coupling was established. This model's accuracy was confirmed using in-situ experimental data. Subsequent analyses explored the influence of groundwater flow velocity on the long-term heat transfer efficiency of energy diaphragm walls and the temperature field distribution in both the wall and adjacent soil during winter/summer cyclical operations. Studies have shown that the increase of groundwater flow velocity can promote the migration of heat accumulated in the wall and in the soil around the wall to a position farther away from the wall along the flow direction, thereby promoting the long-term heat exchange efficiency of the thermo-active diaphragm walls. Moreover, as winter and summer operating conditions transition, the previously accumulated heat in the wall and surrounding soil aids in boosting the current mode's heat exchange efficiency. In addition, groundwater flow also facilitates the movement of heat accumulated at the bottom of the excavation face, allowing the ground's temperature field to stabilize faster. This indirectly bolsters the heat exchange efficiency of the energy diaphragm walls. The research results may enrich the understanding of the heat transfer behavior of energy diaphragm walls under actual working conditions.

Effect of climate change on the behaviour of thermo-active diaphragm walls

Energy geo-structures are becoming more common as a renewable energy solution which utilises shallow geothermal energy to provide heating and cooling to buildings and civil infrastructure projects. There has been an upward trend in the use of thermo-active geo-structures in the United Kingdom, from 150 in 2005 to over 5000 in 2012 [2]. Studies so far have shown that diaphragm walls subjected to combined thermo-mechanical loading show overall increases in lateral displacements, bending moments, shear forces, axial forces, and settlements on the retained side with thermal cycles [6, 7, 8].
 This study uses a variation of a validated numerical model [4] to predict the behaviour of thermo-active diaphragm walls in the longer-term including accounting for the influence of climate change. This numerical model also assesses the impact of different modelling assumptions on the model output by comparing a simplified model with a more complex model where atmospheric temperatures affecting ground surface temperatures are included. Research from the IPCC shows increases in ambient temperatures and in maximum extreme temperatures as a result of climate change. Two models are compared in the IPCC report [5], Representative Concentration Pathway (RCP) 2.6 where governments across the world work together to meet the recommendations in the report to reduce overall warming, and RCP8.5 where the current trends of heating continue without any governmental interventions. According to climate model RCP2.6, global mean surface air temperatures will increase by around 1.5°C and extreme daily temperatures (i.e. days where temperatures exceed expected temperatures for the time of year) will increase by around 2°C over the 50-year period chosen for this study, whilst using RCP8.5 global mean surface air temperature increases by approximately 3°C and extreme daily temperatures increase by approximately 5°C.
 This study uses the same model geometry as a model validated through centrifuge testing [4] which is a 5m deep cantilever diaphragm wall, like those that would be used in deep excavations for open-topped tunnels in large civil infrastructure projects. The numerical model was developed using the semi-coupled thermo-hydro-mechanical (THM) thermal module available with PLAXIS 2D and uses the HS-Small soil model [3] to allow for variation in shear modulus at small strains (i.e. those associated with retaining walls) to be captured more accurately. Thermal loads were applied by applying heat flows through a plate element with the same mechanical properties of the model diaphragm wall used in centrifuge testing. This model attempts to capture the influence of atmospheric temperatures by applying thermal boundary conditions to the ground surface as well as to the thermo-active diaphragm wall. Studies have demonstrated that a more complex model capturing atmospheric temperatures increases the impact of thermal loading [4], whilst also capturing a more realistic temperature profile throughout the entire depth of the soil body both adjacent to the wall and in the free field condition. In this study, atmospheric temperatures were captured in RCP2.6 by cycling between 30°C and -5°C over the course of the cycle to represent a typical ‘hottest’ and ‘coldest’ day of the year in cycle 1, increasing to 32°C and -5°C in line with the temperature increases described in the IPCC report [5]. Ambient mean temperatures between heating and cooling cycles increased from 20°C to 21.5°C over the 50-year period. In the same model, the temperature within the diaphragm
 wall cycled between 35°C and 10°C in cycle 1, representing the typical operation of a ground source heat pump (GSHP) in heating and cooling. These temperature inputs increased to 37°C and 10°C over the 50-year period. The RCP8.5 atmospheric temperatures cycled between 30°C and -5°C to represent summer and winter in cycle 1 and increase to 35°C and -5°C over the 50-year period. Thermal flows in the diaphragm wall cycled between 35°C and 10°C in cycle 1 and increased to 40°C and 10°C over the 50-year period. The results from this study show increases in lateral displacement, maximum bending moments, positive and negative shear forces and axial forces (compressive (-) and tensile (+)). These results are shown in Figure 1.
 Significantly, the RCP2.6 model shows that these increases begin to stabilise over the 50-year period, with the increases in the final cycles becoming negligible. With the RCP8.5 model these increases are increasing linearly at the end of the modelling period which can potentially have significant implications not only in terms of serviceability of the structure, but potentially on ultimate limit state as linear increases suggest that design limits will eventually be reached. The findings from this study generally agree with other similar studies, such as a study that analysed the geotechnical performance of energy piles with additional environmental loading as a result of climate change which found that additional compressive and tensile stresses are generated during heating and cooling phases, respectively [1].

On the performance of energy walls by monitoring assessment and numerical modelling: a case in Italy

Thermoactive diaphragm walls have proved their efficiency for near-surface geothermal energy use. To get insights into the heat transfer process occurring between the heat exchanger pipes and the surrounding boundaries, an instrumented real case located in northern Italy was taken as reference. Combining on-site monitoring data with computational simulations, the role of the basement space in governing the heat fluxes in the different seasons, and the energy performance of the diaphragm wall and of the ground source heat pump system, is highlighted. The numerical analysis represents an effective predictive tool, but is also highly sensitive with respect to parameters of uncertain or complex definition, such as the boundary thermal conditions and the thermal input at the pipe inlet. Intermittent operation and idle periods require a refined simulation, discretising the time dependency of the input variables in appropriate short time steps.

Finite element modeling of thermo-active diaphragm walls

There are two major challenges faced by modern society: energy security, and lowering carbon dioxide gas emissions. Thermo-active diaphragm walls have a large potential to remedy one of these problems, since they are a renewable energy technology that uses underground infrastructure as a heat exchange medium. However, extensive research is required to determine the effects of cyclic heating and cooling on their geotechnical and structural performance. In this paper, a series of detailed finite element analyses are carried out to capture the fully coupled thermo-hydro-mechanical response of the ground and diaphragm wall. It is demonstrated that the thermal operation of the diaphragm wall causes changes in soil temperature, thermal expansion/shrinkage of pore water, and total stress applied on the diaphragm wall. These, in turn, cause displacements of the diaphragm wall and variations of the bending moments. However, these effects on the performance of diaphragm wall are not significant. The thermally induced bending strain is mainly governed by the temperature differential and uneven thermal expansion/shrinkage across the wall.

Investigation of long-term behaviour of thermal wall by finite element analysis

Energy foundations utilise the natural thermal energy stored underground for the space heating and/or cooling of buildings. This technology can be used for lowering carbon dioxide gas emissions. However, there has been very limited research on the effects of cyclic heating and cooling on the structural performance of thermo-active diaphragm walls (thermal walls). An investigation of the long-term behaviour of a thermal wall is conducted in this study by a finite element analysis. The complex thermo-hydro-mechanical (THM) responses due to the operation of the thermal wall are analysed. With no operation of the thermal wall, the earth pressure and the wall movement change due to the dissipation of the excess pore pressure developing from the construction. However, there is only a small change in the bending moment of the wall. With the operation of the thermal wall, the thermal differential across the diaphragm wall induces thermal strain, and therefore, an increase in curvatures, resulting in an increase in the bending moment compared with no operation of the thermal wall. This study shows the necessity of examining the thermally induced effects of a thermal wall in the design, including variations in the bending moment of the wall, the cyclic changes in the earth pressure acting on the diaphragm wall, and the thermally induced soil shrinkage/expansion.

Fundamentals of the coupled thermo-hydro-mechanical behaviour of thermo-active retaining walls

Geotechnical structures can be employed to provide renewable and cost-effective thermal energy to buildings. To date, limited field studies regarding thermo-active retaining walls exist and therefore their mechanical response under non-isothermal conditions requires further research to comprehend their behaviour. This paper investigates the response of a hypothetical thermo-active diaphragm wall by performing finite element analysis to characterise in detail its short and long term response. The soil-structure interaction mechanisms arising from the coupled thermo-hydro-mechanical nature of soil behaviour are for the first time identified and shown to be complex and highly non-linear. Subsequently, simpler modelling approaches are used to isolate and quantify the impact of the various identified mechanisms on the design of thermo-active retaining walls. It is concluded that simpler approaches tend to overestimate structural forces developing due to temperature changes in the retaining wall, while severely underestimating the associated ground movements, which are highly influenced by the development of thermally-induced excess pore water pressures. Furthermore, the results suggest that the behaviour of thermo-active retaining walls is highly transient in nature, as a result of the high rates of heat transfer and pore water pressure dissipation under plane strain assumptions.

Assessment of thermal behaviour of thermo-active diaphragm walls based on monitoring data

Thermo-active diaphragm walls have proved their effectiveness in the thermal conditioning of buildings and infrastructures. However, some aspects still need to be investigated in order to tailor methods and tools for an accurate prediction of their energy and structural performance. In this perspective, some issues are addressed that concern the definition of models for the numerical analysis, in particular issues about the modelling of geometry and thermal boundary conditions. Taking advantage of a monitoring programme on a real full-scale structure, this research focuses on the assessment of heat transfer process and thermal response of diaphragm wall and soil mass on the basis of field data. Understanding of the heat transfer process contributes to the definition of the time-dependent thermal boundary conditions at the excavation side. From the analysis of thermal gradients in the wall, the condition at the excavation side is recognised as a major factor that influences the heat transfer process, governing the direction of the heat flux in different seasons of operation of the geothermal system.

Thermo-hydro-mechanical coupling analysis of a thermo-active diaphragm wall

Thermo-active diaphragm walls that combine load bearing ability with a ground source heat pump (GSHP) are considered to be one of the new technologies in geotechnical engineering. Despite the vast range of potential applications, current thermo-active diaphragm wall designs have very limited use from a geotechnical aspect. This paper investigates the wall–soil interaction behaviour of a thermo-active diaphragm wall by conducting a thermo-hydro-mechanical finite element analysis. The GSHP operates by circulating cold coolant into the thermo-active diaphragm wall during winter. Soil contraction and small changes in the earth pressures acting on the wall are observed. The strain reversal effect makes the soil stiffness increase when the wall moves in the unexcavated side direction, and hence gives different trends for long-term wall movements compared to the linear elastic model. The GSHP operation makes the wall move in a cyclic manner, and the seasonal variation is approximately 0.5–1 mm, caused by two factors: the thermal effects on the deformation of the diaphragm wall itself and the thermally induced volume change of the soil and pore water. In addition, it is found that the change in bending moment of the wall due to the seasonal GSHP cycle is caused mainly by the thermal differential across the wall during the winter, because the seasonal changes in earth pressures acting on the diaphragm wall are very limited.

Investigation on the behaviour of a thermo-active diaphragm wall by thermo-mechanical analyses

The thermo-active diaphragm walls are traditional retaining structures that embed heat exchangers for the exploitation of the near surface geothermal energy, used in the thermal conditioning of buildings and infrastructures. The coupled energetic and structural function of these so called energy walls requires some investigation in order to optimize the embedded circuit and assess the possible occurrence of significant consequences, in terms of temperature variations within the soils mass and thermal effects on the stress/strain state of the structure. In this contribution, the behaviour of an energy wall is assessed by finite element thermal analyses, that allow to investigate the energy performance and the short and long term influence on the soil temperatures, and by finite element thermo-mechanical analyses, to highlight the wall geotechnical and structural response. A one year cycle of heating/cooling operating mode of the geothermal system has been considered and the effects have been discussed in terms of soil–structure interaction and structural internal actions. The results show that the thermally induced mechanical effects are not negligible, especially as variations of the internal axial forces and bending moments. Although they seem to be not detrimental to the geotechnical and structural safety, they require a careful evaluation in order to predict possible situations of unexpected overstress conditions.