# The Influence of Regional Freeze–Thaw Cycles on Loess Landslides: Analysis of Strength Deterioration of Loess with Changes in Pore Structure

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## Abstract

**:**

## 1. Introduction

_{2}) distribution and free induction decay (FID) measurements.

## 2. Test Scheme

#### 2.1. Materials and Methods

_{3}undisturbed loess in Gaoling District, Xi’an, with a depth of 5–8 m. The particle gradation curve is shown in Figure 2. Prepare a standard cylinder with a size of 39.1 mm × 80 mm (diameter × height), dry and saturate the prepared sample, and seal it for later test use.

#### 2.2. Freeze–Thaw Cycle Tests

#### 2.3. SEM

#### 2.4. Nuclear Magnetic Resonance Tests

#### 2.5. Triaxial Compression Tests

## 3. Test Results

#### 3.1. SEM Results

#### 3.2. NMR Test Results

_{2}spectrum distribution curve of the sample, as shown in Figure 5 (T

_{2}is the relaxation time of the pore fluid). The T

_{2}spectral distribution showed a significant right shift after the freeze–thaw cycle. As the number of freeze–thaw cycles increases, the two peaks move upwards as a whole, and the trough between the two peaks becomes less obvious as the curve moves up. It shows that the expansion of the internal pores of the loess is in a process of dynamic change under the conditions of freeze–thaw cycles. Therefore, the micropores in the soil are gradually transitioning to larger pores as the number of freeze–thaw cycles increases.

#### 3.3. Triaxial Compression Test Results

- (1)
- The freezing and thawing times of loess are divided into 6 gradients. The peak intensity decreases with increasing freeze–thaw times, under different confining pressure conditions.
- (2)
- The greater the number of freeze–thaw cycles, the less the peak intensity degradation of the gradient compared to the previous one.
- (3)
- As the confining pressure increases, the phenomenon of peak intensity degradation becomes less obvious.

## 4. Discussion

#### 4.1. Strength Damage

#### 4.2. SEM Test Result Analysis

- (1)
- Probability entropy

_{m}is the probability entropy, and the value range is [0,1]. The larger the value, the lower the order of the particle arrangement and the more chaotic the arrangement. Divide 0~180° into n zones, and the angle range of each zone. ${m}_{i}$ is the number of particles whose particle long axis is in the i-th interval. M is the total number of particles.

- (2)
- Area probability distribution index

#### 4.3. NMR Test Result Analysis

_{2}spectrum distribution curve in Figure 5. The T

_{2}value and pore structure in the soil satisfy the following equation:

_{2}spectral value is proportional to the pore radius.

_{2}spectrum curve obtained by the NMR experiment was subjected to the inversion of the Equation (5) to obtain a pore distribution curve of saturated undisturbed loess, as shown in Figure 10. It can be seen from Figure 8 that the pore distribution of the loess mainly has two peaks, and the larger the peak value, the larger the proportion of the pore volume corresponding to the pore diameter. When NMR is used to test the water in the soil, according to the relaxation time (T

_{2}) of the water in different occurrence states, after inversion by Equation (5), it can be divided into 4 types of pores according to the pore size [28].

- (1)
- The freeze–thaw cycle is 0 to 10 times, and the pore volume of each type varies greatly, which indicates that the soil mesostructure has a higher degree of freeze–thaw damage and a faster damage rate.
- (2)
- When the number of freeze–thaw cycles is 10~30, the change of pore volume of each type is small, which shows that the degree of freeze–thaw damage of the mesostructure of the soil body is reduced and the damage rate is reduced. This is because the soil forms new pore structure characteristics, which changes the degree of influence of the freeze–thaw cycle on the microstructure of the soil. After the number of freeze–thaw cycles reached 50 times, there was almost no change in the pore volume of each type. This was because the soil was basically destroyed and the pore structure was stabilized to a certain extent.

#### 4.4. Correlation Between Strength and Pore Structure

#### 4.4.1. Changes in Porosity

#### 4.4.2. Verification of Test Results

## 5. Conclusions

- (1)
- The freeze–thaw damage evolution of saturated undisturbed loess under freeze–thaw cycles was observed by scanning electron microscopy. It is found that the micro and small pores in the soil increase first and then decrease, and the medium and large pores reduce first and then increase. This is closely related to changes in the internal microstructure of the soil. The T2 spectrum distribution curve of saturated undisturbed loess under freeze–thaw cycles was obtained by using nuclear magnetic resonance technology. The pore distribution curve of loess under freeze–thaw cycles is obtained by equation inversion, which shows that the micro and small pores in the loess are gradually transitioning to the larger medium and large pores with the increase of the number of freeze–thaw cycles. It is reported that the freeze–thaw cycle is a dynamic process for the internal pore expansion of loess. The results obtained by NMR and SEM experiments confirm each other, and the pore distribution has similar changes.
- (2)
- For saturated undisturbed loess, the freeze–thaw cycle breaks the original balance of the sample itself. During the freezing process, the soil particles are squeezed by the growth of ice crystals, the volume expands, and pores and fissures develop, forming a new soil skeleton structure. During the melting process, the melting of solid ice inside the rock sample cannot cause the complete restoration of the deformation of the soil skeleton particles. Therefore, during the freeze–thaw cycle, due to the effect of the frost-heaving force, the loess sample shows a decrease in strength and an increase in porosity.
- (3)
- The strength of saturated undisturbed loess under freeze–thaw cycles is related to the change of porosity. The measurement of porosity is relatively simple. Therefore, consider establishing a functional relationship with the shear strength based on the change in porosity. According to the concept of freeze–thaw damage, an exponential function distribution between the two is derived. Fitting the variation of the porosity and the shear strength under different cycle times, the correlation between the two is good. Therefore, this research provides a new method for the non-destructive analysis of the strength of saturated loess by freezing and thawing, and a new idea for analyzing the loess landslide caused by the freezing and thawing cycle.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Spatial distribution of landslides in the Loess Plateau of the Yellow River Basin [8].

**Figure 5.**T

_{2}spectral curve distribution under different freeze–thaw cycles (N is the number of freeze–thaw cycles).

**Figure 6.**Stress–strain curves under different confining pressures under freeze–thaw cycles. (

**a**) σ

_{3}= 200 kPa, (

**b**) σ

_{3}= 300 kPa, (

**b**) σ

_{3}= 300 kPa, (

**c**) σ

_{3}= 400 kPa.

**Figure 7.**Strength degradation. (

**a**) Shear strength loss rate under freeze–thaw cycles. (

**b**) Mechanical parameters.

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**MDPI and ACS Style**

Li, Z.; Yang, G.; Liu, H.
The Influence of Regional Freeze–Thaw Cycles on Loess Landslides: Analysis of Strength Deterioration of Loess with Changes in Pore Structure. *Water* **2020**, *12*, 3047.
https://doi.org/10.3390/w12113047

**AMA Style**

Li Z, Yang G, Liu H.
The Influence of Regional Freeze–Thaw Cycles on Loess Landslides: Analysis of Strength Deterioration of Loess with Changes in Pore Structure. *Water*. 2020; 12(11):3047.
https://doi.org/10.3390/w12113047

**Chicago/Turabian Style**

Li, Zuyong, Gengshe Yang, and Hui Liu.
2020. "The Influence of Regional Freeze–Thaw Cycles on Loess Landslides: Analysis of Strength Deterioration of Loess with Changes in Pore Structure" *Water* 12, no. 11: 3047.
https://doi.org/10.3390/w12113047