# Long-Term Thermo-Hydraulic Numerical Assessment of Thermo-Active Piles--A Case of Tropical Soils
## 2 Numerical, Modeling of the Long-Term Performance of Thermo-Active Piles
### Ground Investigations  
#### 2.3.1 Tropical Soil Thermal Properties--In Situ Measurements (TRT)  
As tropical weathering involves decomposition and chemical-mineralogical and structural transformation, tropical soils have different properties and behaviors from the soils in temperate/cold regions [31]. In Brazil, lateritic soils are abundant. They are the product of an intensive weathering called laterization. The recurrence of wet and dry seasons is an important feature of laterite formation. The laterization process is defined by the migration of particles through water infiltration, creating a highly porous layer with almost exclusively the most stable minerals (e.g., quartz and kaolinite) and enriching the soil with iron and aluminum and their associated oxides such as sesquioxide [32]. According to [PERSON] (1975), the lixiviation and particle cementation are responsible for the formation of aggregates and a porous structure, which generally results in a soil with an open structure, low specific gravity, high void ratio, high permeability, and high resistance to erosion [33]. The macro-porous structure makes these soils highly compressible, aside from usually exhibiting collapsible behavior. To choose the most appropriate sites for the implantation of thermo-active geostructures, the various thermal, hydraulic, and mechanical properties of sites should be taken into consideration. In terms of porosity, water content, hydraulic conductivity, density, deformability and strength, standard test methods are used for the quantification of these properties of soils. For their thermal properties, the \"geothe \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Parameters**} & \multicolumn{1}{c}{**T1—Brasilia**} & \multicolumn{1}{c}{**T2—Porto Alegre**} & \multicolumn{1}{c}{**T3—Manaus**} \\ \hline \multirow{2}{*}{Heating} & Time [day] & 133 & 187 & - \\  & \(P_{max}\) [W/m] & 30 & 45 & - \\ \hline \multirow{2}{*}{Cooling} & Time [day] & 227 & 173 & 360 \\  & \(P_{max}\) [W/m] & 45 & 45 & 45 \\ \hline Imbalance & \% & 37–63 & 52–48 & 0–100 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The assessment of heating and cooling demand over 1 year for the three types of thermal solicitations. ommended. Thermal conductivity and volumetric heat capacity are the most significant thermal properties of soils which are crucial to an engineering design [34]. Figure 4: Annual ambient temperature and finite heat source power for three cities (1 year/1 cyclic). Figure 3: Flow chart for energy output for pile heat exchangers. Note: for piles greater than 0.4 m in diameter, the spacing is usually larger and the above performance can be improved upon, possibly by up to 50% for large (>1 m) diameters. A Thermal Response Test (TRT) was performed by [PERSON] and [PERSON] in Sao Paulo city [35]. It was carried out by injecting a constant heating power via the heat circulating fluid within a thermo-active micropile. The pile with a diameter of 0.35 m and a length of 15 m was equipped with a U-shaped pipe. During approximately 10 days of operation, the inlet fluid was fed at a constant flow rate of 3.52e-4 m\({}^{3}\)/s and the inlet temperature was monitored along with the outlet temperature. The site represents a saturated clayey sand condition of the Brazilian southeast region. The groundwater table varies seasonally from 2-3 m below the ground surface and was 1.9 m when the test was performed. The TRT results were interpreted based on [PERSON]'s linear heat source theory. Parameters were derived from this test:  
* The undisturbed ground temperature: 297.85 K,
* The effective thermal conductivity \(\lambda_{\mathit{off}}\). 2.82 W/m/K,
* The average pile thermal resistance \(R_{\mathit{b}}\): 0.13 m-K/W. It is important to know that such thermal response tests are not enough to determine the properties of soils.