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| A novel visualization apparatus for freezing soils and its application in freezing-thawing test#br# |
| ZHOU Yongyi，ZHANG Jianjing#br# |
(School of Civil Engineering，Southwest Jiaotong University，Chengdu，Sichuan 610031，China)
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Abstract In order to improve the temperature control accuracy and to monitor the growth process of ice lens in real-time，a novel visualization apparatus for freezing soil test，mainly including a solid-state refrigeration module and a CCD visualization module，was designed based on solid-state refrigeration and linear array CCD(Charge Coupled Device) scanning technology. The neural network was adopted in the solid-state refrigeration module to optimize the parameters of control algorithm so that the test apparatus has the self-adaption capability to temperature conditions of the controlled object. In the visualization module，a linear array scanning structure with the maximum resolution of 4 800 dpi×4 800 dpi was assembled，which has the advantages of simple operation and high imaging accuracy. Performance test results show that the developed apparatus has a constant temperature control accuracy of ±0.002 ℃. In the variable temperature control process with an amplitude of 0.05 ℃，the temperature overshoot is 0.009℃. When the amplitude increases to 0.5 ℃，the temperature overshoot is only 0.089 ℃. The deviation does not exceed ±0.01 ℃ even in the linear cooling process with a cooling rate of 1.2 ℃/h. The application test with sinusoidal temperature boundary was carried out. The test results show that the control system is robust with high accuracy and that the ice lens as small as 0.5 mm width can be observed by the scanning imaging module. It is also indicated from the test data that both the supercooling of pore water and the freezing-thawing history have a significant influence on the temperature profile and the growth of ice lens. Hence，the prefreezing of soil samples，aiming at eliminating the supercooling process，will change the initial state of soil samples and affect the test results. Furthermore，the single freezing process cannot simulate the influence of freezing-thawing history on frost heave characteristics，especially for remoulded samples. The novel apparatus can provide test conditions for further researching the influence of these factors on test results.
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2020, Vol. 39 
