The distribution of temperature in sand soils was measured through laboratory tests, and the temperature influence on friction resistances at the concrete-soil interface was analyzed. Based on the results of laboratory tests, the finite element model was established using the sequential thermal coupling method. The influences of temperature on the bearing characteristics of energy pile were analyzed. The analysis results show that the cyclic temperature will cause additional displacement along pile depth. It is pointed out that if applied vertical loads at energy pile head exceed the value from which nonlinear settlements would be initiated, irrecoverable additional settlement will occur at pile head. Based on the analysis results, a simplified approach was proposed to estimate the zero point of additional displacement along pile shaft and the additional axial pile force. The comparison between the calculated results obtained by the proposed method and that of ABAQUS on single energy pile was given to verify the accuracy of the proposed method. It is shown that reasonable predictions can be obtained without expensive and time-consuming analyses by the proposed method in this paper.
Over the past decades, many experimental and theoretical researches have been carried out to analyze the bearing characteristics of pile foundations, because of their extensive application in many infrastructures and structures [
As carrier of heat exchanger, piles will expand and contract while enduring heating and cooling, resulting in thermomechanical phenomena. In the past years, some efforts had been made to study the mechanisms of thermomechanical soil-structure interaction. In situ tests were carried out to investigate the changes of pile characteristics due to temperature, and the results shown that additional thermal stresses would be mobilized in the pile during the heating and cooling. The change of energy pile bearing characteristic was subjected to the restraint conditions of the tested piles, and it was also found that the use of energy piles causes significant thermally induced additional deformation in the pile itself [
To obtain the characteristics of energy pile foundations during heating-cooling cycles, numerical method was also used to analyze the variations of pile stresses and settlements caused by temperature. Scholars also have carried out many researches to analyze the characteristics of piles during the heating and cooling processes [
As discussed above, it is conclude that both the thermal-induced displacements and stresses must be taken into account in the geotechnical design of energy piles despite being acceptable under normal working conditions. The additional axial force and pile shaft friction resistance change are complex, being closely related to the site engineering geological conditions. In this paper, the conduction characteristics of temperature on concrete-soil interface were tested through laboratory tests, and a finite element model was established based on the results from the above laboratory tests. The influences of temperature load on bearing characteristics of a single energy pile were analyzed, and a simplified method was proposed to estimate the zero point of additional displacement and the additional pile axial force was calculated by the proposed method. The comparisons between the calculated results obtained by the proposed method and that of ABAQUS were given to verify the accuracy of the proposed method. It is shown that reasonable predictions can be obtained without expensive and time-consuming analyses by the proposed method in this paper.
To study the changes of bearing behavior caused by temperature, it is necessary to study the change of temperature with time at the concrete-soil interface and its distribution around the concrete and surrounding soil. A self-developed friction resistance testing device, which can be considering the influence of temperature as shown in Figure
Parameters of model tests.
Sand parameters | Water content |
|
|
Size (mm) |
---|---|---|---|---|
5% | 3.1 | 0.9 | 300x300x75 | |
Concrete parameters | Size (mm) | |||
500x350x100 |
Temperature conduction and friction testing device.
The device consists of upper and lower shear boxes with unequal sizes. The upper shear box is fixed on the reaction force rack, and the lower shear box is connected to the reaction force rack through the guide rails, sliding along the guide rails during tests. The upper shear box has a vertical compression plate that can apply normal stress to the soil sample. The loading device is fixed at one side of the lower shear box, and horizontal loads can be applied, and a displacement sensor is provided testing horizontal displacement. The above device was also described by the WANG et al
The constant temperature heating system is mainly composed of a heating pool, an electric heater, delivery tubes, a thermal fluid, a delivery pump, a temperature probe, and an intelligent thermostat. The temperature probe measures the thermal fluid temperature in the heating pool, and the signal output of the temperature probe is connected with the signal receiving end of the intelligent controller. When the temperature of the thermal fluid in the heating pool reaches the set value, the intelligent temperature controller controls the electric heater to stop working. The delivery pump is connected with the catheter, and the catheter is embedded in the preparation process of the concrete. After the temperature of the thermal fluid reaches a set value, the constant-temperature thermal fluid passes through the inside of the concrete by the delivery tubes. By heat conduction, the temperature of test block can be to a constant value, and the soil is heated through the block. The change of temperature in the soil is measured through the embedded temperature sensor.
11 flexible tubes are embedded in the test concrete block at equal distance, the distance between the hoses and top of the test concrete block is about 10mm, and the length of the hoses is 3 m. When the concrete reaches the certain strength, the wood models were removed and maintained. The surface of the test concrete block is polished, as shown in Figure
Molding process of the test concrete block.
When the temperature of heating fluid is stable, the temperature sensor values were begun recording. The tested results were shown in Figure
Experimented results of temperature conduction and friction.
Temperature changes at pile-soil interface with time
Curves of interface friction resistance-displacement
According to the above laboratory test results, it can be seen that the temperature at the concrete-soil interface increases with time, and tends to be constant with time. The temperature has no significant effect on the contact friction between concrete and soils. Based on the results mentioned above, a finite element model was established, as shown in Figure
The simplified diagram for calculation model.
As for the related parameters, Alessandro et al. [
Parameters of finite element model.
Parameter | Value | Parameter | Value |
---|---|---|---|
Elastic modulus of pile GPa | 30 | Pile unit weight kN/m3 | 25 |
Soil elastic modulus MPa | 20 | Soil unit weight kN/m3 | 20 |
Coefficient of linear thermal expansion for pile °C−1 | 1x10−5 | Friction angle of soil ° | 30 |
Coefficient of linear thermal expansion for soil °C−1 | 2x10−6 | Soil Poisson’s ratio | 0.35 |
Pile conductivity W/m/K | 1.74 | Soil cohesion kPa | 5 |
Specific heat capacity of soil J/kg/K | 1305 | Pile Poisson's ratio | 0.15 |
Specific heat capacity of pile J/kg/K | 1706 | Soil conductivity W/m/K | 1.16 |
The axial symmetry finite element model was adopted to analyze the influence of temperature on the pile. The soil adopts More-Coulomb model, and the pile was deemed to be elastic. The effect of the liquid flow on the temperature distribution in the heating pipe was neglected. Based on the measurements in laboratory tests, the influence of temperature change on the mechanical properties of the contact interface was not considered. The finite element analysis adopted sequential thermomechanical coupling method. The implication of this method is that stress does not affect temperature distribution, but temperature causes stress changes. In the process of heat conduction analysis, the heat conduction elements were adopted and the three-dimensional stress elements were adopted in the process of coupled thermomechanical analysis.
According to relevant experimental test data [
The load-settlement curves from the calculated model were shown in Figure
Curves of load and displacements of pile.
Displacements of pile top and end caused by temperature.
It could be seen from Figure
The displacement differences between pile top and end caused by temperature were shown in Figure
As discussed above, the conclusion that heating and cooling would lead to additional displacement of energy pile with pile head load of
Even though many existing methods about the settlement of pile foundation and load transfer mechanism analysis of single pile, these methods for the energy pile are not very applicable due to the temperature load.
The behavior between the pile shaft and the surrounding soils in this paper is described by a simple linear model. For the linear relationship, the pile side friction is increasing linearly with gradually increasing relative displacement between pile shaft and surrounding soils. The relationship between the pile end loads and the settlements also can be expressed by the linear model based on the existing research results. The relationships can be expressed in the following:
As discussed above, it can be seen that there was a zero point of displacement change at which the additional displacement caused by temperature was zero, while the pile top load was not heavier than the value,
Temperature-induced displacements along pile depth.
Variations of pile axial force caused by temperature.
Settlement difference caused by heating 30d and cooling 60d.
Zero point of displacement change.
The zero point of displacement change is considered as origin of coordinate, and the length from zero point to the pile head has the value of
The additional axial pile shaft force from the pile head to the zero point can be obtained by
The analytic solutions to (
The additional axial pile shaft force from the zero point to the pile end can be obtained by
The analytic solutions to (
In multilayered soils, assume the zero point of displacement change is in the
Calculation model for zero point in multilayered soils.
All the parameters in the above equation are shown in Figure
The additional axial pile shaft force from the pile top to the zero point can be obtained by
In order to determine the specific position of layers in (
(1) Confirm the values of
(2) Assume that the zero point is in the lowest layer and calculate to obtain the value of
(3) If the solution of
(4) Calculate the additional axial pile shaft force using (
The proposed method is an approximate calculation method, and the existing tests mainly focused on heat transfer analysis of pile to study the responses of heat conduction, and the bearing characteristics of field tested pile were also carried out by many scholars. But the bearing characteristics were influenced by many factors such as the stratums and groundwater flow. Therefore, the finite element method is adopted to compare with the results obtained by the above proposed method. The parameters in the analysis adopted the values from the Table
Parameters of case study.
Layer of soils | Thickness of soil layer (m) | Parameters | Values(kPa/mm) |
---|---|---|---|
Layer 1 | 5 |
|
1.094 |
Layer 2 | 5 |
|
1.276 |
Layer 3 | 5 |
|
2.161 |
Layer 4 | 5 |
|
2.719 |
Layer 5 | 5 |
|
2.771 |
Layer 6 | 5 |
|
2.875 |
Layer 7 | 5 |
|
3.066 |
Layer 8 | 5 |
|
3.241 |
|
|||
|
|||
|
|||
|
|||
|
|||
|
Comparisons between the results form ABAQUS and those computed by the proposed approach are shown in Figure
Result comparisons between the proposed method and finite method (ABAQUS).
As discussed above, the following conclusions can be obtained: The frictional resistance at the concrete-soil interface has no obvious relationship to the variation of temperature. The cycle temperature would lead to unrecoverable additional displacement at energy pile heat with heavy loads from which nonlinear settlement occurred on the load-settlement curve. The position of the zero point of displacement change has no obvious relationship to temperature change of pile body. The main factors of influence on the point are the initial soil stiffness at the pile base and shear stiffness of soils surrounding pile and the coefficients of linear expansion for pile. The proposed method can be used to estimate the zero point of additional displacement and additional pile axial force along pile length.
All the data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research presented in this paper was supported by the National Natural Science Foundation of China (Research Grant nos. 51708496 and 51579217) and the Zhejiang Provincial Natural Science Foundation (Research Grant nos. LY16E080010, LY19E080013, and Q19E080021). These financial supports are gratefully acknowledged.