Methods and exploration for mechanical testing and microscopic testing of loess under freeze-thaw cycles

: The stability of loess subgrade is affected by many factors. Freeze-thaw cycles are a common factor that causes the strength of loess subgrades to be damaged. In order to study the influence of freeze-thaw cycles on the stability of loess subgrades, reveal the mechanism of freeze-thaw cycles on the strength of loess subgrades. It is necessary to sample the loess soil samples in this area, and perform compaction test, freeze-thaw cycle test, unconfined compression test and electron microscope scanning test on the soil sample to analyze the influence of freeze-thaw cycle on the strength and microstructure of loess soil. . In this way, the potential relationship between the microstructure and the strength of the soil can be obtained. The results found that the freeze-thaw cycle of loess led to a trend of first increase and then decrease in soil strength. The microstructure showed a trend of smaller soil particles and increased soil voids. The potential relationship between the microstructure and the strength of the soil was analyzed. It explains the conclusion that the strength of the soil first increases and then decreases.


Introduction
The loess is in the cyclical change of climate and seasons, and the soil is also in the cyclical state of freezing and thawing of cooling and warming at this time, and this kind of freezing and thawing has a great influence on the physical and mechanical properties of loess. Previous studies by scholars have shown that for loess that has undergone multiple freeze-thaw cycles, the freeze-thaw cycles will weaken the shear strength of the loess, the ability to resist deformation is weakened, and the structure of the loess will be destroyed. Therefore, the performance of the loess under the freeze-thaw cycle The research has an important impact on the construction of the loess area. Yang Gengshe et al. [1] used nuclear magnetic resonance technology to study the distribution of pores in undisturbed loess under the action of freeze-thaw, and believed that the content of micro and small pores increased, while the content of medium and large pores decreased under alternate freeze-thaw cycles. Liu Lei [2] conducted indoor triaxial tests on Taiyuan loess under different initial conditions, and believed that the shear strength of Taiyuan loess showed a decreasing trend with the increase of water content. When the water content was large, its deformation characteristics showed strain hardening. Shear shrinkage, when the water content is small, its deformation characteristics are shown as strain softening and dilatancy. Yuan Kangfeng [3] used conventional compression apparatus and triaxial apparatus to study the law between the shear strength and structural parameters of loess on a certain slope in southern Shanxi. When the moisture content is low, the structure is stronger. When the pressure is constant, the shear strength of loess decreases with the increase of water content, and when the water content is constant, the shear strength of loess increases with the increase of pressure. When the moisture content is constant, the shear strength increases with the increase of structural parameters. The greater the structure, the stronger the macroporous structure and bonding strength, and the greater the shear strength. Aoyama et al. [4] found through experiments that the cohesion of the soil body decreased under the action of freezing and thawing, but the internal friction angle hardly changed. Liu Leqing [5] found that with the decrease of freezing and thawing temperature, the unconfined compressive strength of unconfined loess and remolded loess decreased, but the unconfined compressive strength of remolded loess changed unevenly, indicating that temperature has an effect on remolded loess. The intensity of the impact is serious. CHAMBERLAIN EJ [6] and other studies have found that the freeze-thaw cycle will change the structure of the soil. When water freezes under the action of freezing, the frost heave force will be generated. Due to the action of ice, larger pores or cracks will be formed, and the nature of the soil It also matters a lot. Ye Wanjun et al. [7] analyzed that during the freezing and thawing process of soil, the significant increase of moisture in the frozen zone is an important reason for the attenuation of soil strength and the increase of pores, and the relationship between soil strength and pore area is inversely proportional. Edwin J et al. [8] observed the changes in the permeability and structure of four types of finegrained soils with the freeze-thaw process, and believed that the freeze-thaw cycle can cause a decrease in porosity and an increase in vertical permeability. Although no exact relationship has been established, it can be It is determined that the soil with the highest plasticity index has the largest change in permeability. Van Bochove et al. [9] believed that the freeze-thaw cycle changed the physical and biological structure of silty clay. Maria HohmannPorebska [10] believes that the cohesion of geological parameters is greatly affected by freezing and thawing. Chamberlain EJ, Blouin SE et al. [11] performed three freeze-thaw cycles on four cohesive soils with a plasticity index ranging from 0 to 20 under a confining pressure of 1.7 to 140 kPa. analyze. Under the conditions of vertical unidirectional freezing and thawing and pressure and sealing, the consolidated sample will be strengthened due to the freezing and thawing cycle, and the void ratio will decrease, but the vertical permeability will increase; for fine-grained soils, vertical permeability will increase. The increase in performance is due to the formation of vertical polygonal cracks in the sample during the freeze-thaw cycle; for coarse-grained soil, it is mainly due to the flocculation of fine particles in the soil, which leads to a decrease in the volume of the coarsegrained soil. small. And it is proposed that the occurrence of vertical cracks is mainly due to the formation of negative pore water pressure and water migration during the freezing process. .Boonsinsuk P [12] and others through repeated freezing and thawing tests, found that as the number of freezing and thawing increases, the shear strength index damage degree of composite soil materials increases. Wang Jing [13] found that although the number of freeze-thaw cycles is different, the lower the negative temperature during freezing, the more significant the strength of the soil after melting. However, when the negative temperature is lower than -10°C during freezing, the attenuation of the strength will reach a steady state. . Ndèye Fatou Ngom [14] uses a generalized cylinder to model the soil microstructure on the basis of computer high-resolution three-dimensional tomography (CT) soil samples, and the pores in the soil can be extracted by a set threshold. Qin Hui [15] carried out the wave velocity test of compacted loess, analyzed the change law of shear wave velocity and compression wave velocity of compacted loess under different freeze-thaw cycle conditions, and found that with the increase of the number of freeze-thaw cycles and water content , The form of soil sample failure presents a trend of transition from brittle failure to plastic failure.

Experiment method
It is required to conduct compaction test, freeze-thaw cycle test, unconfined compression test and scanning electron microscope test on soil samples to analyze the influence of freeze-thaw cycle on the strength and microstructure of loess soil. Compaction test According to the "Code for Design of Highway Subgrade" [20], a heavy-duty compactor is used to carry out compaction tests on the modified expansive soil with different lime content. The preparation method of dry compaction test specimens for lime-improved expansive soil: (1) After the soil sample is air-dried to a moisture content of less than 10%, it is passed through a 5 mm sieve, and the soil sample under the sieve is taken for later use. (2) Determine the moisture content of the airdried soil sample and divide the soil sample into several parts (each soil sample is 2.5-3.0 kg). (3) After adding water to each soil sample to the set moisture content, seal it in a plastic bag for 24 hours so that the soil sample is fully infiltrated and the moisture content is uniform. (4) Perform compaction after mixing, and determine the maximum dry density and optimal moisture content of different samples according to the test method.

Preparation method of improved soil
The selected soil samples are dried, crushed, and sieved. Mix the admixture with the treated loess in proportion.
Calculate the water quality according to the water content of the fly ash soil, add the weighed water to the mixture, stir it evenly, and let it stand in a humidifier for 24 hours. The moisturized mixture is placed in a mold for uniform compaction, and the mixture in the mold is compacted from both ends by three times of compaction. Use a demolding instrument to eject the test piece from the mold. In order to prevent moisture loss, seal the prepared sample with a plastic film and place it in a curing box (the temperature of the curing box is (20±1)°C, and the relative humidity is (96±2)%), in the humidity box Keep in good health for 7 days. Make the test piece with a degree of compaction greater than 95%, and then perform multiple freeze-thaw cycles on the soil sample. In the experiment, try to use the same batch of test pieces and perform different freeze-thaw cycles.

Freeze-thaw cycle test method
It is necessary to simulate or measure the main concentration range of the water content of loess within this depth based on the test results of the soil temperature at the borrow site and taking into account the depth of the borrow. In order to facilitate the direct shear test directly after the freeze-thaw cycle, the direct shear ring knife can be directly used for sample preparation, and the soil samples with different moisture contents are prepared by the drip method according to the test requirements, and the soil samples are wrapped with plastic film to keep the moisture content. Change. Since the moisture content of the soil sample will migrate to the surface during the freezing process, in order to make the moisture content of the soil sample even after melting, it is melted outdoors and allowed to stand for 24 hours, and then a direct shear test is performed. [16] The freeze-thaw cycle adopts a temperature-controlled freezer. The temperature setting of the temperature cycle process should be as close as possible to the ambient temperature of the test site, and the setting temperature should be as low as -10 degrees Celsius. Place the specimen in the freezer for 24 hours to simulate a frost heave During the process, the temperature of the freezer is adjusted to positive degrees Celsius, and the test piece is placed in the freezer for 24 hours to simulate a melting process, which is a complete freeze-thaw cycle process. If necessary, temperature sensing tools can be set in the soil to observe the soil temperature in real time.

Microstructure research methods
In the past, optical microscope, CT and other testing technologies, due to their low resolution, could not meet the requirements for studying soil images. The current high-resolution image testing technology available is mainly scanning electron microscopy. Try to use the same material and the same batch of specimens with the same manufacturing process to prepare soil samples through different freeze-thaw cycles. After the soil sample is successfully prepared, the soil sample is carefully broken apart to expose a fresh surface, and trimmed with a knife into small pieces that meet the irradiation requirements, and the microstructure changes are observed. Dehydration of soil samples: The samples used in the scanning electron microscope need to be in a completely dry state, so the processed soil samples need to be dehydrated. The dehydration technology of soil samples is divided into three types: drying method, air-drying method and freeze-drying method. The air-drying method is to make the soil sample reach a testable standard in a natural state in a cool, dark and ventilated environment; the drying method is generally to place the soil in a drying box at 105°C and keep it for 12 hours to reach the soil. Completely dry; freeze-drying method uses liquid nitrogen freezing technology to instantaneously freeze the soil sample, and then depressurize it in a freeze dryer, so that the liquid phase of the soil is sublimated. Combining the three dehydration techniques, the drying method can dry the soil sample in a shorter time and more economically. It should be noted that the disturbance to the soil sample is minimized during the drying process. Gold spraying of soil samples: As the two-ash modified loess is not a good conductor, it is necessary to spray gold technology to enhance the conductivity of the soil samples, so as to obtain a good microstructure image of the soil. Use ion sputtering instrument to spray gold on the soil sample, and then paste the soil sample on the disc with conductive tape for use. When scanning, a typical position should be selected for scanning. The selected scanning point should be at a position where the particles of the lime-ash soil are evenly distributed and the pores are well distributed, avoiding large particles or large pores, so that the microstructure image of the improved loess is typical and representative. The magnification of the microscopic image determines whether the image can truly reflect the microstructure of the soil. Excessive magnification will cause the image to be unrepresentative, and too small a magnification will result in blurred imaging and large errors. It should be selected according to the actual situation.

Experimental results
In the freeze-thaw cycle test of loess, the strength of the soil is affected by water content, confining pressure, compaction, as well as the loess itself and the improved soil. When the water content is low, the changes in water content are very sensitive to changes in the structural properties of loess. As the confining pressure gradually increases, the change range of structural parameters becomes smaller. The increase in confining pressure makes the structural parameters of the soil show a trend of first becoming larger and then smaller. The larger the confining pressure, the more stable the structural parameters and the smaller the strain value. When the degree of compaction is different, the strain when it becomes stable is also different [17]. Multiple freeze-thaw cycles did not significantly affect the overall moisture content of the loess samples.

The influence of freeze-thaw cycles on the compressive strength of loess
After the freeze-thaw cycle of the soil sample with high moisture content, the surface of the soil sample appeared pitted, the content of large pores increased significantly, the surface became looser, and there were tiny cracks, and there were also small cracks in the contact with the ring knife. The surface changes of soil samples with low water content are not obvious. When the number of freeze-thaw cycles is greater than a certain number of times, the compressive strength tends to be stable; the compressive strength of the modified loess with 2% cement dosage has the largest decrease rate, and the modified loess specimens break and fail after 9 freeze-thaw cycles. The 3% cement dosage improves the resistance of loess. The second rate of compressive strength decline is caused by the low cement dosage and the poor resistance to freezing and thawing of the structure formed by cement hydration products and soil particles. When the cement dosage or compaction coefficient is small, the strength of the modified loess will be greatly reduced under the effect of freezing and thawing, and the freezing resistance will be poor [18]. Under the same freezing and thawing temperature, the relationship between the unconfined compressive strength of remolded loess and the number of freezing and thawing cycles is basically the same as that of undisturbed loess, and both show strain softening. The first few freeze-thaw cycles will increase the strength of the reshaped loess. The freeze-thaw cycles destroy the skeleton of the reshaped loess particles. The large pores between the solid particles are filled with small particles, and the overall connection of the reshaped loess is improved. The results of repeated freezing and thawing cycles on the strength are consistent with the undisturbed loess, and all decrease with the decrease of freezing and thawing temperature. The overall trend is to increase first, then decrease and then increase [19].

The influence of freeze-thaw cycles on cohesion and internal friction angle
The freeze-thaw cycle has a significant influence on the cohesive force, and the cohesive force decreases exponentially with the increase of the number of freezethaw cycles under the same moisture content. For different moisture content, the greater the moisture content, the greater the reduction in cohesion after freezing and thawing cycles. After the same freeze-thaw cycle, as the freezing temperature decreases, the cohesive force does not change much after freezing and thawing. The internal friction angle increases slightly with the increase of the number of freeze-thaw cycles, and the final increase is not large. The main reason is that the proportion of large pores in the soil sample decreases after the freeze-thaw cycle, which leads to an increase in the contact points between soil particles. Causes a slight increase in the angle of internal friction.

The effect of freeze-thaw cycles on microstructure
With the increase of the number of freeze-thaw cycles, the number of small particles of remodeled loess gradually increases, the large pores are transformed into small pores, the arrangement of the particles is loose, and the cementing capacity between the particles decreases. As the number of freeze-thaw cycles increases, the primary structure of loess is damaged more severely. Most of the large particles are decomposed into small particles and filled into the gaps between the particles, so that the particles are arranged tightly and the connection capacity is enhanced. Freezing and thawing will cause changes in the soil particle system. With the increase in the number of freeze-thaw cycles, the pore area ratio of the soil shows a trend of first increasing and then becoming stable, and the small pores in the soil continue to transform into medium and large pores. , The strength of the soil is negatively correlated with the pore area [7].

Conclusions
(1) Summarize the precautions in the freeze-thaw cycle experiment of loess, consider the actual situation of the project, and set the temperature in the freeze-thaw cycle of the test loess to simulate the actual situation. The addition of admixtures may enhance the water retention and heat preservation capacity of the loess.
(2) Sort out some loess strength tests under dry-wet cycle conditions, introduce the methods and procedures of compaction test, as well as the humidification and drying methods in dry-wet cycle experiments, and the precautions in electron microscope scanning experiments.
(3) Summarize the influence of freeze-thaw cycles on the strength of loess. With the increase of the number of freeze-thaw cycles, the strength of loess first increases and then decreases. Summarize the influence of freezethaw cycles on the microstructure of loess. With the increase in the number of freeze-thaw cycles, the experimental loess showed a trend of smaller soil particles and larger soil voids, and the original structure of the loess was seriously damaged.