Skip to main content
PLOS ONE logoLink to PLOS ONE
. 2024 Apr 25;19(4):e0302409. doi: 10.1371/journal.pone.0302409

Effect of freeze‒thaw cycles on root–Soil composite mechanical properties and slope stability

Ruihong Wang 1,2, Zexin Jing 2, Hao Luo 2,*, Shun Bao 2, Jingru Jia 2, Xiaoyu Zhan 2
Editor: Shaker Qaidi3
PMCID: PMC11045164  PMID: 38662726

Abstract

Natural disasters such as landslides often occur on soil slopes in seasonally frozen areas that undergo freeze‒thaw cycling. Ecological slope protection is an effective way to prevent such disasters. To explore the change in the mechanical properties of soil under the influence of both root reinforcement and freeze‒thaw cycles and its influence on slope stability, the Baijiabao landslide in the Three Gorges Reservoir area was taken as an example. The mechanical properties of soil under different confining pressures, vegetation coverages (VCs) and numbers of freeze‒thaw cycles were studied via mechanical tests, such as triaxial compression tests, wave velocity tests and FLAC3D simulations. The results show that the shear strength of a root–soil composite increases with increasing confining pressure and VC and decreases with increasing number of freeze‒thaw cycles. Bermuda grass roots and confining pressure jointly improve the durability of soil under freeze‒thaw conditions. However, with an increase in the number of freeze‒thaw cycles, the resistance of root reinforcement to freeze‒thaw action gradually decreases. The observed effect of freeze‒thaw cycles on soil degradation was divided into three stages: a significant decrease in strength, a slight decrease in strength and strength stability. Freeze‒thaw cycles and VC mainly affect the cohesion of the soil and have little effect on the internal friction angle. Compared with that of a bare soil slope, the safety factor of a slope covered with plants is larger, the maximum displacement of a landslide is smaller, and it is less affected by freezing and thawing. These findings can provide a reference for research on ecological slope protection technology.

Introduction

A considerable number of small-scale shallow soil landslides have occurred in the Three Gorges Reservoir area in recent years [14], posing a significant danger to the safety and stability of reservoir slopes, as well as people’s lives and property. A large temperature difference is a major contributor to these landslides [5]. Colder temperatures cause the water in the soil to continuously undergo the liquid–solid phase transition, destroying the bonds between soil particles and reducing the strength and stability of the soil as a whole. In particular, in the process of frost heave, the formation of ice crystals in the soil increases the distance between the soil particles, resulting in the volume expansion of the soil, which causes damage to and deformation of the soil. Compared with that of ordinary soil, the water content of soil in water level fluctuation areas is much greater, and the performance degradation caused by freeze‒thaw cycles is more severe [1, 5]. As a result, steps must be taken to mitigate the impacts of freeze‒thaw cycles on slopes, and plant roots play a significant role in maintaining shallow slope stability [6, 7]. Vegetation has been widely utilized in ecological projects in recent years to efficiently reduce soil erosion and stabilize slopes [8, 9].

Freeze‒thaw cycles have a great influence on the structural and physical and mechanical properties of soils, and many scholars have conducted series of experimental studies on the effects of freeze‒thaw cycles on various soils. The results of Chamberlain et al. [1012] showed that freezing of the water in soil led to crack formation and increased porosity. The change in the basic physical properties of soil is also related to this phenomenon. Yang et al. [1317] reported that the shear strength of soil decreased after freeze‒thaw cycling. Some scholars have further explored the influence of freeze‒thaw cycles on the shear strength parameters of rock and soil [18]. The test results of Aoyama et al. [19] showed the freeze‒thaw cycling will lead to the weakening of soil cohesion, although the internal friction angle will not change much. The research results of Ogata et al. [20] showed that the internal friction angle of soil increased as the number of freeze‒thaw cycles increased, although the cohesion decreased. Wang et al. [21] measured the shear strength and elastic modulus of soil before and after freezing and thawing and reported that the internal friction angle increased while the elastic modulus and cohesion decreased.

Appropriate reinforcement measures should be adopted to resist the deterioration effect of freeze‒thaw cycles on soil. Lime reinforcement, fiber-reinforced concrete reinforcement and other technologies are widely used in the field of soil reinforcement [22]. Numerous studies have shown that while plants improve the quality of the ecological environment, their roots can also be seen as a natural fiber that plays a significant role in reinforcing the shallow soil of the slope, significantly increasing soil strength [23, 24]. Ding et al. [25] used plant roots of three vegetation types to reinforce the soil of high and steep slopes. The results showed that the soil strength of the three vegetation types improved to varying degrees. Noorasyikin et al. [26] used bermudagrass to reinforce sand and clay. Bermudagrass had a greater effect on the clay than on the sand and enhanced the cohesion of the clay. Through pull-out tests, Su et al. [27] quantitatively characterized the anchorage effect of a root system on soil.

Although there has been significant research on the reinforcement effect of roots on soil and the deterioration effect of freeze‒thaw cycles on soil, there is limited research on the mechanical properties of soil under the combined action of both processes. Additionally, there are few indicators available for the quantitative evaluation of the interaction between the two. This study explores the influence of root content, confining pressure, and freeze‒thaw cycles on the mechanical properties of soil and the resistance mechanism of roots to freeze‒thaw cycles. The resistance effects of roots and freeze‒thaw cycles are quantitatively characterized, and the stability of the Baijiabao landslide under different working conditions is analyzed via simulation model calculations. This study is based on unconsolidated–undrained (UU) tests, wave velocity tests, and optical microscopic observations.

Materials and methods

Test materials and research areas

The plant roots and soil used in this study were bermudagrass roots and undisturbed soil from the Baijiabao landslide in the Three Gorges Reservoir area. The Baijiabao landslide is located in Hubei Province, China. Its altitude is 110°45′33.4′′, and its latitude is 30°58′59.9′. Overall, the landslide is higher in the west and lower in the east. Homologous gullies split the northern and southern sides of the landslide body. The boundary of the landslide along both sides and the trailing edge is a contact surface between rock and soil. The trailing edge’s terrain is shaped like a circular chair, with the front and center portions being gentle and the back portion being steep. The landslide body has a slope of 10° to 30°. The planar form of the landslide body is tongue-shaped, and it is stepped in profile. The landslide body is 550 m long and 400 m wide. The thickest part of the landslide body is approximately 80 m thick, and the average thickness is 45 m. Its volume is approximately 990×104 m3. Since the landslide formed, its subsequent movement has exhibited seasonal variations. Since monitoring began in November 2006, the landslide has experienced periodic deformation and slip in the winter of each year. To decrease the displacement rate, a large amount of bermudagrass was planted on the slope surface to reinforce the soil. The maximum freezing depth of the soil is approximately 30 cm, and the reinforcement depth of bermudagrass roots is 10~30 cm. The morphology of the landslide is shown in Fig 1.

Fig 1. The morphology of the landslide.

Fig 1

The collected soil samples were subjected to numerous soil tests according to the "Standard for Soil Test Method" (GB/T50123-1999) specification [28]. The physical parameters obtained are shown in Table 1. The soil samples were classified as a clay of high plasticity (CH) according to the Unified Soil Classification System (USCS). The gradation curve of the soil sample is shown in Fig 2.

Table 1. Physical parameters of the soil samples.

density ρ (g·cm-3) water content w (%) maximum dry density ρdmax (g·cm-3) optimum moisture content wop (%) soil particle proportion Gs plasticity index IP
1.977 21.54 1.9082 18 2.67 18.87

Fig 2. The gradation curve of the soil sample.

Fig 2

Sample preparation

The sample preparation method was similar to that in References [6, 22, 24]. The soil samples were dried by a dryer, and the larger stones and impurities were removed. The prepared Bermuda grass roots were washed and allowed to dry naturally. The target dry density was 1.63 g·cm3 to ensure that the compactness of the sample was approximately 85%. The total mass of pure water required to reach the target moisture content was weighed according to the dry soil mass. Pure water was sprayed on the surface of the dry soil with a sprayer and uniformly mixed in with a stirrer until the sample reached the target moisture content of 18%.

The root-containing remolded soil samples were prepared by the compression method [26]. The preparation of root-containing samples by the this method was completed by an automatic pressing prototype with a static constant pressure. The sample preparation process is shown in Fig 3. The mass of each sample was 721.2 g. The configured root–soil mixtures were loaded into containers and mixed evenly. The weighed soil samples were divided into 5 parts for sample preparation. Cylindrical samples with dimensions of 61.8 mm × 125 mm were created in accordance with the specifications of the testing device. Each pressed sample was sealed with cling film and placed in a sealed container to prevent water loss. Five groups of samples with different root contents were prepared. Five different numbers of freeze‒thaw cycles were applied to the five samples in each group with a different root content; 4 samples were tested for each condition, for a total of 100 samples.

Fig 3. The Sample preparation process.

Fig 3

Test scheme

The test instrument used in this paper is a saturated-unsaturated soil stress path triaxial test system jointly developed by Nanjing Soil Instrument Factory and the authors’ research group, as shown in Fig 5.

Fig 5. The test steps.

Fig 5

Notably, previous research using traditional methods has often demonstrated a relationship between root content and soil strength [2931]. However, because the roots of naturally growing plants are buried underground, it is impossible to intuitively judge the reinforcement of soil through the growth state of vegetation, making some research less instructive for actual slope reinforcement projects. To address this issue, this work used the point-frequency (PF) method [32, 33] to evaluate plant cover within a fixed range of slope surfaces. The soil in this range was measured to calculate the root content (root mass/dry soil mass), and a relationship was established between the vegetation coverage (VC) and root content (Fig 4). The root contents were 0%, 0.0856%, 0.1262%, 0.1786%, and 0.2149%, corresponding to VC contents of 0%, 30%, 50%, 70%, and 90%, respectively, so that the reinforcement of slope surface soil can be judged according to the growth state of the vegetation.

Fig 4. The vegetation coverage (VC) and root content.

Fig 4

D0, D2, D4, D6, and D8 represent root–soil composite samples with 0, 2, 4, 6, and 8 freeze‒thaw cycles, respectively, and G0, G3, G5, G7, and G9 represent root–soil composite samples with 0%, 30%, 50%, 70%, and 90% VC, respectively. The test scheme is shown in Table 2.

Table 2. Test scheme.

group water content w (%) dry density ρd (g·cm-3) VC (%) freeze‒thaw cycles confining pressure σ2 = σ3 (kPa)
1 18.0 1.63 0 0, 2, 4, 6, and 8 50
100
200
2 30 0, 2, 4, 6, and 8 50
100
200
3 50 0, 2, 4, 6, and 8 50
100
200
4 70 0, 2, 4, 6, and 8 50
100
200
5 90 0, 2, 4, 6, and 8 50
100
200

The Three Gorges Reservoir region is a seasonally frozen soil area, according to temperature data for the Yichang sector of the Three Gorges Reservoir area from 1971 to 2022 published by the China Meteorological Center’s data sharing platform. The data suggest that the temperature difference between day and night was significant in the winter, with the minimum temperature at night reaching -9.8°C and the highest temperature during the day reaching 23°C. As a result, in this experiment, the melting temperature was set to 25°C, and the freezing temperature was set to -10°C: a freeze‒thaw cycle of 12 hours of freezing and 12 hours of thawing was employed. The uniform distribution of root aggregates in the sample is obviously different from the actual situation of root growth from the surface downward into the soil in the field; therefore, all surfaces of the samples were regarded as freeze‒thaw surfaces, and circumferential freeze‒thaw was adopted instead of unidirectional freeze‒-thaw, which is more reasonable and is consistent with the freeze‒thaw method in References [1214]. According to our preliminary experimental results, the soil strength does not decrease after more than 8 freeze‒thaw cycles. Therefore, the numbers of freeze‒thaw cycles investigated here are 0, 2, 4, 6, and 8. The Test steps is shown in Fig 5.

After freeze‒thaw treatment, a sample was subjected to a wave velocity test with a YL-SWT shear wave velocity tester. During the test, a sensor and transmitter were installed on the soil sample to ensure that the position was accurate and stable. Then, the data acquisition device and the recorder were connected, the sound wave signal was transmitted, the sound wave propagation time was recorded, and the wave velocity was calculated. Subsequently, UU tests [34] were conducted under confining pressures of 50 kPa, 100 kPa, and 200 kPa. The axial strain rate range for the UU tests, as per the geotechnical test standard, was set to 1%~3%, which is equivalent to a loading rate of 1.25~3.75 mm·min-1. During the tests, the loading rate was set to 1.25 mm·min-1 based on the sample size. A test was considered complete when the stress‒strain curve reached its peak shear strength value and the axial strain continued to increase by approximately 3% to 5% beyond the peak value. If there was no peak, the test ended when the axial strain reached 15%. Fig 5 illustrates the detailed test steps.

Results and dissection

Stress–strain curve characteristics

The stress‒strain curves of the samples tested with different VCs and confining pressures and after different numbers of freeze‒thaw cycles are shown in Fig 6. There was no noticeable peak stress in either the bare soil or root-containing soil, and there was no softening phenomenon in the corresponding stress‒strain curves. The deviatoric stress increased as the axial strain increased, and the rate of increase in stress increased more quickly in the early stage of testing and then slowed and eventually stabilized in the late stage. The continuous strain hardening phenomenon becomes increasingly clear as the confining pressure and VC increase, as shown in Fig 6A. Fig 6B indicates that under constant confining pressure and VC conditions, the deviatoric stress decreases as the number of freeze‒thaw cycles increases, showing that the freeze‒thaw cycling weakens the soil.

Fig 6.

Fig 6

Sample stress‒strain curves: (a) The influence of VC and confining pressure; (b) The influence of VC and number of freeze‒thaw cycles.

Images of the soil samples after 8 freeze‒thaw cycles are shown in Fig 7 for morphological comparison. Many folds and holes formed on the surface of the three samples, and few soil particles detached, suggesting that the freeze‒thaw activity disrupted the internal structure of the soil. The sample morphology demonstrated a typical end effect (wide in the middle, narrow at both ends), and the severity of the end effect decreased as VC increased, indicating that the root system could effectively inhibit the compaction and expansion caused by loading and frost heave.

Fig 7. Shear failure image after 8 freeze‒thaw cycles.

Fig 7

Shear strength characteristics

Fig 8A shows the effect of VC on the shear strength of the root–soil composites under a 200 kPa confining pressure and after various numbers of freeze‒thaw cycles. In the absence of freeze‒thaw action, the roots increased the soil strength by 18.10%, and the reinforcement effect was remarkable. The slope of the fitting line gradually decreased as the number of freeze‒thaw cycles increased; the slope decreased from 0.661 to 0.338, showing that the freeze‒thaw treatment diminished the reinforcing effect of the roots on the soil. These findings are in good agreement with those of previous studies [14, 17]. When the number of freeze‒thaw cycles was 8, the roots only increased the soil strength by 13.69%.

Fig 8. Shear strength variation curve.

Fig 8

(a) Relationship between shear strength and VC (σ3 = 200 kPa); (b) Relationship between shear strength and number of freeze‒thaw cycles (σ3 = 200 kPa).

The effect of the number of freeze‒thaw cycles on the shear strength of the root–soil composites with various VC contents under a confining pressure of 200 kPa is reflected in the data shown in Fig 8B. It is evident that the correlation between the shear strength and the number of freeze‒thaw cycles is exponentially negative. The freeze‒thaw treatment reduces the soil shear strength more significantly in the early stage of testing than in the late stage, which was also reported in previous studies [15, 19].

To quantitatively reveal the resistance of roots to freeze‒thaw deterioration, the freeze‒thaw–root combined action factor λij was defined.

λij=τi0τ00τi0τij (1)

where i is the VC and j is the number of freeze‒thaw cycles.

According to λij, the extent to which the VC reinforcement effect of the soil can offset the freeze‒thaw deterioration effect of the soil can be intuitively determined. When λij is greater than 1, the reinforcement effect is greater than the deterioration effect. When λij is less than 1, λij is the proportion of the reinforcement effect offsetting the freeze‒thaw effect. According to the results of λij calculated from Fig 8, the data in Table 3 were obtained.

Table 3. The λi−j values for different VC and freeze‒thaw cycle test conditions.

λ i−j i = 3 i = 5 i = 7 i = 9
j = 2 0.57 1.14 1.71 1.69
j = 4 0.26 0.45 0.69 0.87
j = 6 0.20 0.29 0.46 0.61
j = 8 0.15 0.22 0.34 0.49

According to Table 3, λ is greater than 1 only when j = 2 and i = 5, 7, and 9, which means that only the reinforcement of roots in these three cases can completely offset the degradation of freeze‒thaw cycles. When i ≥ 4, no matter the value of j, the degradation of soil strength caused by freeze‒thaw cycles cannot be completely offset. When i = 8, the maximum offset that the reinforcement effect of the root system can provide is less than half the deterioration effect (0.49). In addition, under the condition that i is constant, although λ decreases as j increases, the rate of decrease gradually slows until it tends to stabilize. For example, the difference between λ3−2 and λ3−4 is 0.31, the difference between λ3−4 and λ3−6 is 0.06, and the difference between λ3−6 and λ3−8 is 0.05. This shows that the rate of reduction in soil strength will gradually decrease with increasing number of freeze‒thaw cycles until it tends to stabilize, i.e., the degradation of soil by freeze‒thaw cycles will inevitably progress through three stages: a large strength deterioration stage, a small strength reduction stage and a stable strength stage.

Shear strength parameters

By studying the variation in the shear strength parameters (c, φ) of soil samples with VC after freeze‒thaw action, the influence of the two on the basic physical and mechanical properties of soil can be analyzed. Therefore, we draw the Mohr stress circle and calculate the c and φ values of the soil samples according to the triaxial tests under three confining pressures of 50 kPa, 100 kPa and 200 kPa. A plane rectangular coordinate system is constructed, and the changes in c and φ of the different soil samples are shown in Fig 9A and 9B. In Fig 9A, with increasing VC, the cohesion shows different degrees of increase. When the number of freeze‒thaw cycles is 0, 2, 4, 6 and 8, the cohesion of the root–soil composites with VC contents of 30%, 50%, 70% and 90% increase by 2.89%~13.04%, 9.39%~22.56%, 14.39%~20.53%, 9.55%~30.6% and 20.06%~73.25%, respectively, compared with that of the rootless soil. In contrast, the internal friction angle does not change significantly, and the overall change range is small.

Fig 9. The relationship of the shear strength parameter and strength damage coefficient with the number of freeze‒thaw cycles.

Fig 9

(a) cohesion; (b) internal friction angle; (c) Kc; and (d) Kφ.

To quantitatively reveal the influence of the two on c and φ, the cohesive damage residual coefficient Kc and the internal friction angle damage residual coefficient Kφ are defined.

Kc=cnc0 (2)
Kφ=φnφ0 (3)

where cn and c0 are the cohesion values after n freeze‒thaw cycles and zero freeze‒thaw cycles, respectively, and φn and φ0 are the internal friction angle values after n freeze‒thaw cycles and zero freeze‒thaw cycles, respectively.

The variations in KC and Kφ are shown in Fig 9C and 9D, respectively.

Shear wave velocity

The wave velocity test results are shown in Fig 10. A decrease in the wave velocity is a sign of damage and destruction of the internal structure of the soil by freeze‒thaw action. The lower the wave velocity, the more internal pores there are in the soil. The figure shows that the decrease in the wave velocity for all the samples is greatest after the first two freeze‒thaw cycles; thereafter, it steadily decreases and begins to stabilize, which is consistent with the change in the freeze‒thaw–root combined action factor λi−j. Additionally, as VC increases due to an increase in the fraction of root material in the sample, the initial wave velocity of the sample decreases.

Fig 10. Shear wave velocity test results.

Fig 10

Changes in the soil microstructure

Fig 11 shows the microstructure of the bare soil and root–soil composites before and after freezing and thawing under a microscope. Prior to freezing and thawing, the soil particles in both the root–soil composite and bare soil samples were intact. However, the freezing and thawing process altered the pore distribution properties, resulting in an increased porosity and pore size of the samples. After eight cycles of freezing and thawing, both types of samples exhibited numerous holes, some of which developed into microfissures. Before freezing and thawing, the roots inside the root–soil composite were intertwined to form a complex supporting system, and the roots and soil were in close contact at root–soil interfaces. However, after the freezing and thawing process, holes appeared around the root system. This was due to the presence of water within the roots. During frost heave, water froze around the root, causing it to separate from the soil interface. Upon thawing, microholes formed around the root system.

Fig 11. Microstructure of the bare soil and root–soil composite samples before and after freezing and thawing.

Fig 11

Slope stability simulation

FLAC3D was used to simulate the deformation of the shallow soil of the Baijiabao landslide body, and the safety factor was calculated to explore the influence of VC and freeze‒thaw cycles on slope stability. The calculation model selects the I-I’ section of the Baijiabao landslide as the calculation section and establishes a two-dimensional model, as shown in Fig 12. The length, width and height of the model are 800 m, 100 m and 320 m, respectively, which are the same as the actual landslide size. Fixed constraints are set on the bottom, front and rear boundaries of the model, normal constraints are set on the side boundaries, and the top surface is a free boundary. The mesh of the Baijiabao landslide model is divided into triangular node elements. A total of 29945 elements were generated, covering a total of 6484 nodes. The model follows the Mohr–Coulomb yield criterion.

Fig 12. Baijiabao landslide simulation model.

Fig 12

(a) I-I’ profile (Fig 1); (b) Numerical simulation effect diagram; (c) Grids and nodes.

Many geological survey reports on the Baijiabao landslide were consulted, as were the physical and mechanical parameter data of shallow rock and soil masses of the same type of landslide in the Three Gorges Reservoir area. After comprehensive consideration, the simulation calculation parameters of the landslide deformation displacement were determined, as shown in Table 4.

Table 4. Deformation calculation parameters of the Baijiabao landslide.

Parameter name Density Ρ (kg/m3) Shear modulus G (Pa) Bulk modulus K (Pa) Poisson ’s ratio u Bonding force C (Pa) Internal friction angle Φ (°)
Bedrock 2800 1.08e10 1.56e10 0.22 2.45e6 42
Sliding mass 2190 5.25e7 7.45e7 0.25 1.41e4 26
Sliding belt 2090 3.5e7 4e6 0.44 6e3 22

In this work, the strength reduction method is used to reduce the c and φ values of the bare soil and root–soil composite, and the shear strength parameter of the reduced soil is substituted into the model for calculation. Finally, the displacement and safety factor of the bare soil slope and vegetation-covered slope are obtained.

Slope stability without freeze‒thaw cycles

Table 5 shows that as the VC increases, the maximum displacement of the slope decreases, while the safety factor increases. The soil displacement of the simulated vegetation-covered slope is 2.4%~4.1% less than that of the bare soil slope. When VC is 90%, the slope safety factor reaches 2.43, which is 6.11% greater than that of the bare soil slope.

Table 5. Slope displacement and safety factor statistics of different VCs.

VC (%) Slope maximum displacement (mm) Safety factor of slope Stability improvement ratio
0 11.98 2.29 0
30 11.69 2.38 3.93%
50 11.56 2.39 4.37%
70 11.56 2.41 5.24%
90 11.49 2.43 6.11%

Slope stability after freeze‒thaw cycles

To explore the influence of freeze‒thaw cycling on the stability of the shallow soil of the Baijiabao landslide, the maximum displacement and safety factor of the bare soil slope and VC 90% slope after freeze‒thaw cycling were compared. The results are shown in Table 6.

Table 6. Analysis table of the displacement and safety factor of bare soil and vegetation-covered slopes after different numbers of freeze‒thaw cycles.

Freeze‒thaw cycles Maximum displacement of slope (mm) Safety factor of slope The stability reduction rate
Bare soil slope Vegetation slope Bare soil slope Vegetation slope Bare soil slope Vegetation slope
0 11.98 11.56 2.29 2.41 0 0
2 12.52 11.94 2.13 2.30 6.98% 4.56%
4 13.04 12.55 2.01 2.13 12.23% 11.60%
6 13.78 13.24 1.86 1.96 18.78% 18.67%
8 14.99 13.60 1.68 1.89 26.64% 21.58%

The change trends of the safety factor and displacement of the bare soil slope and vegetation-covered slope are roughly the same. In the process of enhancing the freeze‒thaw effect of the shallow surface soil of the slope, the deformation of the shallow surface soil of the slope increases, and the safety factor decreases gradually, which fully shows that the freeze‒thaw cycling will have an adverse effect on the stability of the shallow surface soil of the slope. When the bare soil slope experienced 8 freeze‒thaw cycles, the displacement of the soil was the largest, increasing by 8.78% compared with that without freeze‒thaw cycles, while the displacement of the vegetation-covered slope increased by only 5.21%. Under the same number of freeze‒thaw cycles, the safety factor of the vegetation-covered slope was 2.42%~5.06% smaller than that of the bare soil slope, indicating that the presence of plant roots could effectively inhibit the slip deformation of the shallow soil of the slope. The degradation of the vegetation-covered slope after freeze‒thaw action was generally weaker than that of the bare soil slope.

The influence mechanism of freeze‒thaw cycles and roots on the mechanical properties of soil

According to the above test results, combined with the research of other scholars, the attenuation mechanism of the root–soil composite strength due to freeze‒thaw action is briefly summarized below (Fig 13):

Fig 13. Strength attenuation mechanism of the root–soil composite under freeze‒thaw action.

Fig 13

  1. Root reinforcement without freeze‒thaw action: Without freeze‒thaw action, a root–soil composite, in which the roots are intertwined, forms a uniform and effective “network” support system of roots [3537]. At the same time, there is strong cohesion between soil particles and between roots and soil particles. At this point, if the root–soil composite experiences shear failure, the aforementioned two bonding pressures must be resisted simultaneously. The greater the number of roots is, the more complicated the support system, the greater the cohesion between the roots and soil particles, and the greater the shear strength of the whole root–soil composite.

  2. The decrease in cohesion during freezing and the resistance of roots and high confining pressure: The free water and weakly bound water in soil freeze and expand throughout the freezing process at low temperatures. When the water content of the root system increases, an ice crystal ring will form at water surface. The root and soil interfaces with water change into root system-ice crystal bonds and ice crystal-soil bonds. Moreover, the expansion and extrusion of ice crystals breakdown certain soil particles, increasing the porosity of the soil. The interfaces of many soil particles change into ice crystal-soil bonds, resulting in a considerable loss of cohesion [30, 31]. However, the support system established by complex root intertwining remains, limiting the freezing expansion effect. Moreover, a greater external confining pressure suppresses the freezing effect to some extent, preventing the bonding effect from decreasing during freezing.

  3. Particles lose contact during melting: As melting begins, ice crystals melt into water, and the root system-ice crystal and ice crystal-soil bonds vanish, resulting in the reduction of particle contact.

  4. Repeated freezing and thawing leads to a further reduction in soil particle bonding until the soil becomes stable: When the soil undergoes freeze‒thaw action again, the free water and some weakly bound water in the soil will migrate and accumulate again, and its volume will expand again as it freezes. However, the increase in soil pores caused by the phase change expansion of water during the previous freezing effect provides additional space. The general structure of the soil is less damaged by the second freezing than it is by the first freezing. The rate of attenuation slows, but the strength of the soil continues to decrease. The strength of the soil constantly declines as the number of freeze‒thaw cycles increases. However, after a given number of freeze‒thaw cycles, the impact of the water phase change on the soil structure diminishes, and the strength of the soil tends to stabilize. Due to the strengthening of soil due to the root system, the root–soil composite maintains its strength more than the bare soil does.

Conclusions

In this paper, freeze‒thaw cycle tests and UU tests were carried out on root–soil composites in the Three Gorges Reservoir area, and the effects of freeze‒thaw cycles, VC and confining pressure on the mechanical properties of the resulting root–soil composites were analyzed. The freeze‒thaw–root combined action factor λi−j was defined. According to λi−j, the reinforcement effect of different root contents on soil can be used to quantitatively express how many freeze‒thaw cycles can be resisted. According to the c and φ values obtained from the UU tests, a numerical simulation analysis was carried out on the stability of the Baijiabao landslide, and the following conclusions were obtained:

  1. With increasing confining pressure and VC, the strain hardening phenomenon of the root–soil composite became more obvious, the end effect was weakened, and the maximum increase in shear strength reached 18.1%, indicating that the Bermuda grass roots and confining pressure together enhance the freeze‒thaw durability of the soil.

  2. As the number of freeze‒thaw cycles increased, the shear strength of the bare soil and root–soil composite decreased in stages. The freeze‒thaw deterioration was divided into three stages, namely, the large strength reduction stage, the small strength reduction stage and the stable strength stage.

  3. When the number of freeze‒thaw cycles was at least 4, λi−j was always less than 1; the degradation of the soil strength caused by the freeze‒thaw action could not be completely offset by the root system, regardless of the root content. When the number of freeze‒thaw cycles reached 8, the root system could offset less than half the degradation of the soil strength due to freezing and thawing (λi−j = 0.49).

  4. The cohesive damage residual coefficient Kc increased with increasing VC and decreased with increasing number of freeze‒thaw cycles, which is basically consistent with the trend of the change in shear strength. The internal friction angle damage residual coefficient Kφ always fluctuated around 0.1 and had no obvious correlation with VC or the number of freeze‒thaw cycles. This shows that both VC and freeze‒thaw action change the strength of soil by affecting the cohesion of the soil rather than the internal friction angle.

  5. When there is no freezing or thawing, the maximum displacement of the shallow surface soil of the slope will decrease with increasing VC, which reflects that the plant root system effectively constrains the sliding displacement of the shallow surface of the slope. When experiencing freeze‒thaw cycles, the displacement of the shallow surface soil of the landslide increases, and the safety factor decreases. The freeze‒thaw effect has a negative effect on the stability of the shallow surface soil of the slope. Under the same number of freeze‒thaw cycles, the safety factor of the vegetation-covered slope is 2.42%~5.06% smaller than that of the bare soil slope, indicating that the existence of roots resists the adverse effects of freeze‒thaw action on slope stability.

Notably, in theory, the reinforcement effect of root content on soil becomes limited after a threshold root content. When it exceeds this threshold root content, too many roots will agglomerate in the soil to form a weak surface and destroy the integrity of the soil. However, it is difficult for the coverage of naturally growing Bermuda grass on slopes to exceed 90%; that is, it is difficult for the root content to exceed the maximum root content of 0.22% tested in this paper. Therefore, combined with the actual natural background of the Three Gorges Reservoir area, when using bermudagrass for slope protection, the planting density should be increased as much as possible to increase its reinforcement effect on the soil.

Supporting information

S1 File. Original data.

(ZIP)

pone.0302409.s001.zip (6.3MB, zip)

Acknowledgments

The authors gratefully acknowledge the Yangtze River Scientific Research Institute for its help in the field sampling process.

Data Availability

All relevant data are within the manuscript and its Supporting Information files.

Funding Statement

This research was funded by the National Natural Science Foundation of China (grant number 51979151); the Natural Science Foundation of Hubei Province Outstanding Youth Project(Grant number 2021CFA090); and the Three Gorges Key Laboratory of Geological Hazards of the Ministry of Education (China Three Gorges University) (Grant number 2020KDZ07). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

References

  • 1.Zheng Y, Li S, Ullah K. Increased Occurrence and Intensity of Consecutive Rainfall Events in the China’s Three Gorges Reservoir Area Under Global Warming. Earth and Space Science. 2020; 7.8. doi: 10.1029/2020EA001188 [DOI] [Google Scholar]
  • 2.Tang H, Wasowski J, Juang C. Geohazards in the three Gorges Reservoir Area, China-Lessons learned from decades of research. Engineering Geology. 2019; 261. doi: 10.1016/j.enggeo.2019 105267. [DOI] [Google Scholar]
  • 3.Shang M, Xu X, Zhang G, et al. Back Analysis of a Recent Progressive Failure in China Three Gorges Reservoir Area: Shanshucao Landslide. Journal of Testing and Evaluation. 2019; 47(3). doi: 10.1520/JTE20170717 [DOI] [Google Scholar]
  • 4.Luo S, Huang D, Peng J, Tomás R. Influence of permeability on the stability of dual-structure landslide with different deposit-bedding interface morphology: The case of the three Gorges Reservoir area, China. Engineering Geology. 2022; 296. doi: 10.1016/j.enggeo.2021.106480 [DOI] [Google Scholar]
  • 5.Cui H, Jiang S, Ren L, Xiao W, Yuan F, Wang M, Wei L. Dynamics and potential synchronization of regional precipitation concentration and drought-flood abrupt alternation under the influence of reservoir climate. Journal of Hydrology: Regional Studies. 2022; 42. 10.1016/j.ejrh. 2022.101147. [DOI] [Google Scholar]
  • 6.Mickovski S, Bengough A, Bransby M, Davies M, Hallett P, Sonnenberg R. The effect of roots on soil reinforcement. Journal of Biomechanics. 2006; 39(S1): S353–S353. doi: 10.1016/S0021-9290(06)84407-6 [DOI] [Google Scholar]
  • 7.WALDRON L. J., and SUREN DAKESSIAN."SOIL REINFORCEMENT BY ROOTS." Soil Science 132.6(1981). [Google Scholar]
  • 8.Ma R, Hu F, Xu C, Liu J, Yu Z, Liu G, Zhao S, Zheng F. Vegetation restoration enhances soil erosion resistance through decreasing the net repulsive force between soil particles. Catena. 2023; 226. doi: 10.1016/j.catena.2023.107085 [DOI] [Google Scholar]
  • 9.Tian P, Tian X, Geng R, Zhao G, Yang L, Mu X, et al. Response of soil erosion to vegetation restoration and terracing on the Loess Plateau. Catena. 2023; 227. https://doi.org/10.1016/ j.catena.2023.107103. [Google Scholar]
  • 10.Chamberlain E, Gow A. Effect of freezing and thawing on the permeability and structure of soils. Engineering Geology. 1979; 13(1):73–92. doi: 10.1016/B978-0-444-41782-4.50012-9 [DOI] [Google Scholar]
  • 11.Hou C, Cui Z, Yuan L. Accumulated deformation and microstructure of deep silty clay subjected to two freezing-thawing cycles under cyclic loading. Arabian Journal of Geosciences. 2020; 13(12):1–13. doi: 10.1007/s12517-020-05427-2 [DOI] [Google Scholar]
  • 12.Miao W, Meng S, Sun Y, Fu H. Shear strength of frozen clay under freezing-thawing cycles using triaxial tests. Earthquake Engineering and Engineering Vibration. 2018; 17(4):761–769. https://doi.org/ 10.1007/s11803-018-0474-5. [Google Scholar]
  • 13.Lu Y, Lu S, Alonso E, Wang L, Xu L, Li Z. Volume changes and mechanical degradation of a compacted expansive soil under freeze-thaw cycles. Cold Regions Science and Technology. 2019; 157206–214. doi: 10.1016/j.coldregions.2018.10.008 [DOI] [Google Scholar]
  • 14.Li W, Chai S, Xue M, Wang P, Li F. Structural damage and shear performance degradation of fiber-lime-soil under freeze-thaw cycling. Geotextiles and Geomembranes. 2022; 50(5):845–857. doi: 10.1016/J.GEOTEXMEM.2022.04.005 [DOI] [Google Scholar]
  • 15.Qin Z, Lai Y, Tian Y, Zhang M. Effect of freeze-thaw cycles on soil engineering properties of reservoir bank slopes at the northern foot of Tianshan Mountain. Journal of Mountain Science. 2021; 18(2):1–17. doi: 10.1007/s11629-020-6215-z33456447 [DOI] [Google Scholar]
  • 16.Yuan G, Che A, Tang H. Evaluation of soil damage degree under freeze–thaw cycles through electrical measurements. Engineering Geology. 2021; 106297–. doi: 10.1016/j.enggeo.2021.106297 [DOI] [Google Scholar]
  • 17.Broms Bengt B, Yao Leslie Y.C. Shear Strength of a Soil After Freezing and Thawing. Journal of the Soil Mechanics and Foundations Division. 1964; 90. (4). doi: 10.1061/JSFEAQ.0000629 [DOI] [Google Scholar]
  • 18.Guo C, Zhang Y, et al. Freeze-thaw cycle effects on granite and the formation mechanism of long-runout landslides: insights from the Luanshibao case study in the Tibetan Plateau, China. Bulletin of Engineering Geology and the Environment 82.10 (2023): 394. doi: 10.1007/s10064-023-03427-6 [DOI] [Google Scholar]
  • 19.Aoyama K, Ogawa S, Fukuda M. Temperature Dependencies of Mechanical Properties of Soils Subjected to Freezing and Thawing. Kinosita S, Fukuda M. Proceedings of the 4th International Symposium on Ground Freezing Sapporo, Japan. Rotterdam: A.A Balkema, 1985; 217–222.
  • 20.Ogata N, Kataoka T, Komiya A. Effect of Freezing-Thawing on The Mechanical Properties of Soil. Kinosita S, Fukuda M. Proceedings of the 4th International Symposium on Ground Freezing Sapporo, Japan. Rotterdam: A.A Balkema, 1985; 201–207.
  • 21.Wang D, Ma W, Niu Y H, Chang X, Wen Z. Effect of cyclic freezing and thawing on mechanical properties of Qinghai-Tibet clay. Clod regions science & Technology Engineering Geology. 2007; (13): 34–43. doi: 10.1016/j.coldregions.2006.09.008 [DOI] [Google Scholar]
  • 22.Hadi Sahlabadi Seyed, et al. Freeze–thaw durability of cement-stabilized soil reinforced with polypropylene/basalt fibers. Journal of Materials in Civil Engineering 33.9 (2021): 04021232. doi: 10.1061/(ASCE)MT.1943-5533.0003905 [DOI] [Google Scholar]
  • 23.Ding H, Xue L, Liu H, Li L, Wang H, Zhai M.et al. Influence of Root Volume, Plant Spacing, and Planting Pattern of Tap-like Tree Root System on Slope Protection Effect. Forests. 2022; 13(11):1925–1925. doi: 10.3390/f13111925 [DOI] [Google Scholar]
  • 24.Fang Shuai, et al. Effects of Herbaceous Plant Roots on the Soil Shear Strength of the Collapsing Walls of Benggang in Southeast China. Forests. 2022; 13.11. [Google Scholar]
  • 25.Ding H, Zhang H, Liu B, Huang H. Study on Mechanical Properties of Soil Stabilization by Different Vegetation Roots on High Steep Slope. Sustainability. 2023; 15(3):2569–2569. https://doi.org/10.3390/ su15032569. [Google Scholar]
  • 26.Noorasyikin M, Zainab M, Derahman A, Dan M, Madun A, Yusof Z, et al. Mechanical properties of Bermuda grass roots towards sandy and clay soil for slope reinforcement. Physics and Chemistry of the Earth. 2022; 128. doi: 10.1016/j.pce.2022.103261 [DOI] [Google Scholar]
  • 27.Su L, Hu B, Xie Q, Yu F, Zhang C. Experimental and theoretical study of mechanical properties of root-soil interface for slope protection. Journal of Mountain Science. 2020; v.17(11):197–208. doi: 10.1007/s11629-020-6077-4 [DOI] [Google Scholar]
  • 28.“Standard for Soil Test Method”(GB / T50123-1999). Ministry of Construction, P.R. China.(in Chinese)
  • 29.Zhang C, Chen L, Liu Y, Ji X, Liu X. Triaxial compression test of soil–root composites to evaluate influence of roots on soil shear strength. Ecological Engineering. 2010. doi: 10.1016/j.ecoleng 2009.09.005. [DOI] [Google Scholar]
  • 30.Orakoglu M, Liu J. Effect of freeze-thaw cycles on triaxial strength properties of fiber-reinforced clayey soil. KSCE Journal of Civil Engineering. 2017; 21 (6):2128–2140. doi: 10.1007/s12205-017-0960-8 [DOI] [Google Scholar]
  • 31.Orakoglu M, Liu J, Lin R, Tian Y. Performance of Clay Soil Reinforced with Fly Ash and Lignin Fiber Subjected to Freeze-Thaw Cycles. Journal of Cold Regions Engineering. 2017; 31 (4). https://doi.org/ 10.1061/(ASCE)CR.1943-5495.0000139. [Google Scholar]
  • 32.Vanha‐Majamaa I., Salemaa M., Tuominen S., Mikkola K. Digitized photographs in vegetation analysis‐a comparison of cover estimates. Applied Vegetation Science. 2000; 3(1):89–94. https://doi.org/10.2307/ 1478922. [Google Scholar]
  • 33.Bråkenhielm S, Liu Q. Comparison of field methods in vegetation monitoring. Water, Air, and Soil Pollution. 1995; (79): 75–87. doi: 10.1007/BF01100431 [DOI] [Google Scholar]
  • 34.Jiang Y, Mehtab A, Su J, Muhammad U, Shamsher S, Li J, et al. Effect of root orientation on the strength characteristics of loess in drained and undrained triaxial tests. Engineering Geology. 2021; (prepublish):106459-. doi: 10.1016/j.enggeo.2021.106459 [DOI] [Google Scholar]
  • 35.Wang B, Liu J, Li Z, Morreale S, Schneider R, Xu D, et al. The contributions of root morphological characteristics and soil property to soil infiltration in a reseeded desert steppe. Catena. 2023; 225. doi: 10.1016/j.catena.2023.107020 [DOI] [Google Scholar]
  • 36.Meijer G. A generic form of fibre bundle models for root reinforcement of soil. Plant and Soil. 2021; 468 (1–2):1–21. doi: 10.1007/s11104-021-05039-z [DOI] [Google Scholar]
  • 37.Su X, Zhou Z, Liu J, Cao L, Liu J, Wang P. Estimating slope stability by the root reinforcement mechanism of Artemisia sacrorum on the Loess Plateau of China. Ecological Modelling. 2021; 444. doi: 10.1016/j.ecolmodel.2021.109473 [DOI] [Google Scholar]

Decision Letter 0

Shaker Qaidi

13 Feb 2024

PONE-D-23-42173Influence of freeze-thaw and root combined action on soil mechanical characteristics and stability in the water-level-fluctuating zone of Baijiabao landslide in the Three Gorges Reservoir Area​PLOS ONE

Dear Dr. Luo,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

==============================

Dear Authors,

The evaluations from the peer reviewers regarding your submitted work have been duly received. Upon reviewing their feedback, it is evident that they recommend that you revise your manuscript. Therefore, the authors should consider each comment and decide on the best course of action for their research.

==============================

Please submit your revised manuscript by Mar 29 2024 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

  • A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

  • A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

  • An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Shaker Qaidi

Academic Editor

PLOS ONE

Journal requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. In your Methods section, please provide additional information regarding the permits you obtained for the work. Please ensure you have included the full name of the authority that approved the field site access and, if no permits were required, a brief statement explaining why.

3. Please note that PLOS ONE has specific guidelines on code sharing for submissions in which author-generated code underpins the findings in the manuscript. In these cases, all author-generated code must be made available without restrictions upon publication of the work. Please review our guidelines at https://journals.plos.org/plosone/s/materials-and-software-sharing#loc-sharing-code and ensure that your code is shared in a way that follows best practice and facilitates reproducibility and reuse.

4. Thank you for stating the following financial disclosure:

“This research was funded by the National Natural Science Foundation of China (grant number 51979151); the Natural Science Foundation of Hubei Province Outstanding Youth Project(Grant number 2021CFA090); and the Three Gorges Key Laboratory of Geological Hazards of the Ministry of Education (China Three Gorges University) (Grant number 2020KDZ07).”

Please state what role the funders took in the study.  If the funders had no role, please state: "The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript."

If this statement is not correct you must amend it as needed.

Please include this amended Role of Funder statement in your cover letter; we will change the online submission form on your behalf.

5. We note that Figure 1 in your submission contain [map/satellite] images which may be copyrighted. All PLOS content is published under the Creative Commons Attribution License (CC BY 4.0), which means that the manuscript, images, and Supporting Information files will be freely available online, and any third party is permitted to access, download, copy, distribute, and use these materials in any way, even commercially, with proper attribution. For these reasons, we cannot publish previously copyrighted maps or satellite images created using proprietary data, such as Google software (Google Maps, Street View, and Earth). For more information, see our copyright guidelines: http://journals.plos.org/plosone/s/licenses-and-copyright.

We require you to either (1) present written permission from the copyright holder to publish these figures specifically under the CC BY 4.0 license, or (2) remove the figures from your submission:

1. You may seek permission from the original copyright holder of Figure 1 to publish the content specifically under the CC BY 4.0 license. 

We recommend that you contact the original copyright holder with the Content Permission Form (http://journals.plos.org/plosone/s/file?id=7c09/content-permission-form.pdf) and the following text:

“I request permission for the open-access journal PLOS ONE to publish XXX under the Creative Commons Attribution License (CCAL) CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). Please be aware that this license allows unrestricted use and distribution, even commercially, by third parties. Please reply and provide explicit written permission to publish XXX under a CC BY license and complete the attached form.”

Please upload the completed Content Permission Form or other proof of granted permissions as an "Other" file with your submission.

In the figure caption of the copyrighted figure, please include the following text: “Reprinted from [ref] under a CC BY license, with permission from [name of publisher], original copyright [original copyright year].”

2. If you are unable to obtain permission from the original copyright holder to publish these figures under the CC BY 4.0 license or if the copyright holder’s requirements are incompatible with the CC BY 4.0 license, please either i) remove the figure or ii) supply a replacement figure that complies with the CC BY 4.0 license. Please check copyright information on all replacement figures and update the figure caption with source information. If applicable, please specify in the figure caption text when a figure is similar but not identical to the original image and is therefore for illustrative purposes only.

The following resources for replacing copyrighted map figures may be helpful:

USGS National Map Viewer (public domain): http://viewer.nationalmap.gov/viewer/

The Gateway to Astronaut Photography of Earth (public domain): http://eol.jsc.nasa.gov/sseop/clickmap/

Maps at the CIA (public domain): https://www.cia.gov/library/publications/the-world-factbook/index.html and https://www.cia.gov/library/publications/cia-maps-publications/index.html

NASA Earth Observatory (public domain): http://earthobservatory.nasa.gov/

Landsat: http://landsat.visibleearth.nasa.gov/

USGS EROS (Earth Resources Observatory and Science (EROS) Center) (public domain): http://eros.usgs.gov/#

Natural Earth (public domain): http://www.naturalearthdata.com/

6. We note that Figures 3, 5 and 13 in your submission contain copyrighted images. All PLOS content is published under the Creative Commons Attribution License (CC BY 4.0), which means that the manuscript, images, and Supporting Information files will be freely available online, and any third party is permitted to access, download, copy, distribute, and use these materials in any way, even commercially, with proper attribution. For more information, see our copyright guidelines: http://journals.plos.org/plosone/s/licenses-and-copyright.

We require you to either (1) present written permission from the copyright holder to publish these figures specifically under the CC BY 4.0 license, or (2) remove the figures from your submission:

1. You may seek permission from the original copyright holder of Figures 3, 5 and 13 to publish the content specifically under the CC BY 4.0 license.

We recommend that you contact the original copyright holder with the Content Permission Form (http://journals.plos.org/plosone/s/file?id=7c09/content-permission-form.pdf) and the following text:

“I request permission for the open-access journal PLOS ONE to publish XXX under the Creative Commons Attribution License (CCAL) CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). Please be aware that this license allows unrestricted use and distribution, even commercially, by third parties. Please reply and provide explicit written permission to publish XXX under a CC BY license and complete the attached form.”

Please upload the completed Content Permission Form or other proof of granted permissions as an "Other" file with your submission.

In the figure caption of the copyrighted figure, please include the following text: “Reprinted from [ref] under a CC BY license, with permission from [name of publisher], original copyright [original copyright year].”

2. If you are unable to obtain permission from the original copyright holder to publish these figures under the CC BY 4.0 license or if the copyright holder’s requirements are incompatible with the CC BY 4.0 license, please either i) remove the figure or ii) supply a replacement figure that complies with the CC BY 4.0 license. Please check copyright information on all replacement figures and update the figure caption with source information. If applicable, please specify in the figure caption text when a figure is similar but not identical to the original image and is therefore for illustrative purposes only.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: This paper entitled “Influence of freeze-thaw and root combined action on soil mechanical characteristics and stability in the water-level-fluctuating zone of Baijiabao landslide in the Three Gorges Reservoir Area” discusses the strength variation of soil under the combined action of freeze-thaw and root system, and the resistance of root reinforcement to freeze-thaw deterioration is quantitatively expressed. The paper may be valuable for the civil engineering community. However, it needs some significant improvements before further processing.

Title: Title is very long. Please edit and revise the title and write more suitable title.

Abstract: The abstract should ideally include the main contributions and implications of the study. The abstract section should be improved considering the following structure: Introduction, problem statement, methodology, results, and conclusion.

Introduction: The introduction provides a clear and concise overview of the research topic, setting the stage for the study.

Please add a paragraph at the beginning of introduction about effect of natural effect such as freeze-thaw and dry-wet cycles on geotechnical parameters of geomaterials. Consider providing more context or background information to help readers understand the significance of the research problem.

The literature review needs to be completed, and it is not apparent what the novelty of this paper is. Your literature review should be updated. There are many new papers related to your research: https://doi.org/10.1061/(ASCE)MT.1943-5533.0003905, https://doi.org/10.1007/s10064-023-03427-6.

At the end of the Introduction, where the author introduces the central claim of his research or reasserts this claim, the paper should be including a paragraph to explain the importance of the subject, novelty, and originality of the paper.

materials and methods: Please write soil type based on USCS classification.

Please write in more details about the details of wave velocity test.

Test scheme: Please present a Table with more details instead of Table 2 for test program for showing details of tests.

On what basis have the number and temperatures of freeze-thaw cycles been chosen?

Results: in my opinion “results and dissection” is more suitable than “results and analysis” for section 3.

Ensure that all figures and tables are properly labeled and referred to in the text.

After summarizing your key findings, compare your results with previous studies.

The Manuscript should be polished by native speakers or a designated editing company to improve readability.

Some quantitative findings should support the conclusion. Please write the conclusion in more detail.

Reviewer #2: Your manuscript "Influence of freeze-thaw and root combined action on soil mechanical characteristics and stability in the water-level-fluctuating zone of Baijiabao landslide in the Three Gorges Reservoir Area"(PONE-D-23-42173) requires amendment. Although it is of interest, several details need to be clarified or corrected.

1. In line 37. “Compared to ordinary soil, the soil in the water-level-fluctuating zone has a substantially greater water content and the degradation in the soil of the water-level-fluctuating zone is more severe.” Is there a direct basis for this conclusion to be introduced as background? Citing relevant literature to strengthen the testimony is a good way to improve.

2. In line 78. “lt is a typical soil landslide induced by the failure of shallow soil strength caused by freeze-thaw action.” However, in line 82, the maximum freezing depth of soil is about 30 cm. Is the actual depth of the sliding belt only 30cm? It is recommended that this be explained in the text or in a diagram.

3. The gradation curve of soil sample in Fig.2 is incomplete. According to the standard of geotechnical test method, when the mass of the specimen with particle size less than 0.075mm is more than 10% of the total mass, the composition of the particles with particle size less than 0.075mm should be determined according to the densitometer method or pipette method.

4. In line 94. Previous pull-out test results are mentioned. However, no relevant literature citations or experimental results are given or presented.

5. According to Figure 4, the highest measured root content in the field was over 20%, while the experimental design was as high as 90%?

6. The layout of Table 2 is not aesthetically pleasing, and the parentheses try not to be misplaced.

7. Figure 5 is too redundant, flowcharts and diagrams of experimental procedures should be concise and clear, and detailed descriptions already in the text are not recommended to be repeated in flowcharts.

8. About Shear Strength Parameters. In general, fully saturated specimens of triaxial UU tests have only one effective stress Mohr's circle at the time of damage, and the envelope of the damage Mohr's circle for all the different enclosing pressures is a nearly horizontal straight line. It is therefore not possible to determine an effective shear strength parameters by measuring pore water pressure. It is usually only used to determine the undrained shear strength Cu. Therefore the angle of internal friction in Fig. 9(b) taken to 17° is needed to discussion. A detailed description of the triaxial test and shear strength parameters can be found in the soil mechanics textbook.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Meysam Bayat

Reviewer #2: No

**********

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

PLoS One. 2024 Apr 25;19(4):e0302409. doi: 10.1371/journal.pone.0302409.r002

Author response to Decision Letter 0


19 Mar 2024

Response to Reviewer Comments

Response to Academic Editor

Point 1: Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming.

Response 1: We have now re-typed and checked the format in strict accordance with the manuscript template you gave. If there is still a problem of incorrect formatting, please give us instant feedback, and we will hand over to a professional editing and polishing company for typesetting after the manuscript content is determined to be unchanged.

Point 2: In your Methods section, please provide additional information regarding the permits you obtained for the work. Please ensure you have included the full name of the authority that approved the field site access and, if no permits were required, a brief statement explaining why.

Response 2: The research area ( Baijiabao landslide ) in this study is an open area, so no additional work permit is required.

Point 3: Please note that PLOS ONE has specific guidelines on code sharing for submissions in which author-generated code underpins the findings in the manuscript. In these cases, all author-generated code must be made available without restrictions upon publication of the work.

Response 3: We are able to provide unlimited data and code of this manuscript, we have uploaded all the relevant data of our chart, if there are other data need to contact us at any time.

Point 4: Please state what role the funders took in the study. If the funders had no role, please state: "The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript."

Response 4: The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Point 5: We note that Figure 1 in your submission contain [map/satellite] images which may be copyrighted……

Response 5: According to your request, we have now deleted the satellite map in Figure 1 and only retained the engineering geological map of the landslide.

Point 6: We note that Figures 3, 5 and 13 in your submission contain copyrighted images……

Response 6: We have now replaced similar images in Figure 3, Figure 5 and Figure 13 and annotated them in the revised manuscript.

Response to Reviewer 1 Comments

Point 1: Title is very long. Please edit and revise the title and write more suitable title.

Response 1: Thank you for your suggestion , we realized that the title is really too specific, now we refer to similar articles, and the title changes more streamlined.

Point 2: The abstract should ideally include the main contributions and implications of the study. The abstract section should be improved considering the following structure: Introduction, problem statement, methodology, results, and conclusion.

Response 2: Thank you for your suggestion , We have now improved and supplemented the abstract according to the structure you suggested.

Point 3: Please add a paragraph at the beginning of introduction about effect of natural effect such as freeze-thaw and dry-wet cycles on geotechnical parameters of geomaterials. Consider providing more context or background information to help readers understand the significance of the research problem.

Response 3: Thank you for your suggestion , The background information we have given before may be too short. Now we supplement the description of the failure process of soil by freeze-thaw action and the significance of studying the soil in the water-level-fluctuating zone in the first paragraph of the introduction. The detailed revisions are shown in lines 41-53 of the revised manuscript.

Point 4: The literature review needs to be completed, and it is not apparent what the novelty of this paper is. Your literature review should be updated.

Response 4: Thank you very much for your comments and kindly provided us with the literature closely related to our research. We have cited the literature you provided and supplemented more related literature.

Point 5: At the end of the Introduction, where the author introduces the central claim of his research or reasserts this claim, the paper should be including a paragraph to explain the importance of the subject, novelty, and originality of the paper.

Response 5: We have added a paragraph at the end of the introduction to emphasize the innovation and importance of our research according to your suggestions.

Point 6: Please write soil type based on USCS classification.

Response 6: Thank you for your suggestion, we have now given the soil type according to the USCS classification, as shown in the 106-107 lines of the revised manuscript.

Point 7: Please write in more details about the details of wave velocity test.

Response 7: Thank you for your suggestion, We have now given the instrument model used in the wave velocity test and the specific steps of the wave velocity test, as shown in lines 166-170 of the revised manuscript. We also supplemented the photos of the soil samples during the wave velocity test in Fig.5.

Point 8: Please present a Table with more details instead of Table 2 for test program for showing details of tests.

Response 8: We now modify and optimize Table 2.

Point 9: On what basis have the number and temperatures of freeze-thaw cycles been chosen?

Response 9: Thank you for your suggestion, The choice of temperature is based on the original text we have given, in the revised manuscript; the selection of the number of freeze-thaw cycles is based on our pre-test. When the number of freeze-thaw cycles exceeds 8 times, the strength of the sample basically does not decrease. We have now supplemented and explained it, as shown in the 152-165 lines of the revised manuscript.

Point 10: In my opinion “results and dissection” is more suitable than “results and analysis” for section 3.

Response 10: Thank you for your suggestion, We have revised the title of section 3 according to your suggestion.

Point 11: After summarizing your key findings, compare your results with previous studies.

Response 11: Thank you for your suggestion, We have added a comparison with other research results in the text in accordance with your recommendations.

Point 12: The Manuscript should be polished by native speakers or a designated editing company to improve readability.

Response 12: Thank you for your suggestion, We have now submitted the manuscript to the AJE official editing company for a comprehensive polishing.

Point 13: Some quantitative findings should support the conclusion. Please write the conclusion in more detail.

Response 13: Thank you for your suggestion, We have now added more quantitative data to the conclusion.

Response to Reviewer 2 Comments

Point 1: In line 37. “Compared to ordinary soil, the soil in the water-level-fluctuating zone has a substantially greater water content and the degradation in the soil of the water-level-fluctuating zone is more severe.” Is there a direct basis for this conclusion to be introduced as background? Citing relevant literature to strengthen the testimony is a good way to improve.

Response 1: Thank you for your suggestion , The soil in the water-level-fluctuating zone is formed in the water body and is in the underwater environment. It is immersed in water for a long time and has stronger water absorption. Therefore, compared with the soil on land, its water content is usually higher. We have now added the corresponding literature to prove this in accordance with your recommendations.

Point 2: In line 78. “lt is a typical soil landslide induced by the failure of shallow soil strength caused by freeze-thaw action.” However, in line 82, the maximum freezing depth of soil is about 30 cm. Is the actual depth of the sliding belt only 30cm? It is recommended that this be explained in the text or in a diagram.

Response 2: Thank you very much for pointing out our mistakes in language expression. The landslide is not entirely caused by freeze-thaw. What we want to express here is that the subsequent slip of the landslide after formation is mainly caused by freeze-thaw. Freezing and thawing and the root system mainly affect the changes in the shallow surface soil above the slide zone, thus indirectly leading to the subsequent sliding of the slope. The evidence is that the slip of the landslide in winter is much larger than that in other seasons. Obviously, our misrepresentation is misleading, and we have now made corresponding modifications. Thank you again for pointing out our mistakes, which is very important to us.

Point 3: The gradation curve of soil sample in Fig.2 is incomplete. According to the standard of geotechnical test method, when the mass of the specimen with particle size less than 0.075mm is more than 10% of the total mass, the composition of the particles with particle size less than 0.075mm should be determined according to the densitometer method or pipette method.

Response 4: Thank you for your suggestion ,We have now completed the gradation curve.

Point 4: In line 94. Previous pull-out test results are mentioned. However, no relevant literature citations or experimental results are given or presented.

Response 5: Thank you for your suggestion , We have added literature to confirm this.

Point 5: According to Figure 4, the highest measured root content in the field was over 20%, while the experimental design was as high as 90%?

Response 6: Fig.4 shows the relationship between underground root content and surface vegetation coverage. The maximum root content is 21.49 %, and the corresponding vegetation coverage is 90 %. Because the roots are buried underground and cannot be observed, we use vegetation coverage instead of root content, which can facilitate technicians to directly judge the reinforcement effect according to the degree of vegetation coverage on the surface. The relevant explanations are given in lines 137-146 of the revised manuscript.

Point 6: The layout of Table 2 is not aesthetically pleasing, and the parentheses try not to be misplaced.

Response 7: Thank you for your suggestion, We now modify and optimize Table 2.

Point 7: Figure 5 is too redundant, flowcharts and diagrams of experimental procedures should be concise and clear, and detailed descriptions already in the text are not recommended to be repeated in flowcharts.

Response 7: Thank you for your suggestion, We have now redrawn Figure 5 to make it more streamlined.

Point 8: About Shear Strength Parameters. In general, fully saturated specimens of triaxial UU tests have only one effective stress Mohr's circle at the time of damage, and the envelope of the damage Mohr's circle for all the different enclosing pressures is a nearly horizontal straight line. It is therefore not possible to determine an effective shear strength parameters by measuring pore water pressure. It is usually only used to determine the undrained shear strength Cu. Therefore the angle of internal friction in Fig. 9(b) taken to 17° is needed to discussion. A detailed description of the triaxial test and shear strength parameters can be found in the soil mechanics textbook.

Response 8: Thank you very much for your opinion. As you said, for saturated clay, the Mohr stress circle envelope obtained by UU test will approach the horizontal line, so the φ value cannot be obtained. For the unsaturated clay used in this paper, the UU test can not fully and accurately reflect the real mechanical behavior of unsaturated soil, and the obtained φ value is not accurate, so we think that your opinion is undoubtedly correct. In fact, the values of c and φ measured by different test methods ( UU, CU, CD ) are different. The UU test is carried out under undrained conditions. The volume of the sample is constant during the test, and the water content is constant. Changing the surrounding pressure increment will not change the effective stress in the sample, but only cause the change of pore water pressure. If the pre-shear consolidation pressure of the sample is large, the UU test will obtain a larger cohesion value and a smaller φ value. Under the action of consolidation pressure and pore water discharge, the spacing of soil particles in CU and CD tests is gradually shortened, the interaction between particles is strengthened, and the relative movement of particles is more difficult. Therefore, the measured φ value is usually larger than that of UU test. However, a large number of literatures have proved that the differences in the specific values of the parameters caused by different test methods usually do not affect the overall trend of the parameters when the same factor changes, so we believe that our conclusions are still reasonable. Thank you again for your suggestion, we think this clarification is very necessary.

Attachment

Submitted filename: Response to Reviewers.docx

pone.0302409.s002.docx (98.6KB, docx)

Decision Letter 1

Shaker Qaidi

3 Apr 2024

​Effect of freeze‒thaw cycles on root–soil composite mechanical properties and slope stability

PONE-D-23-42173R1

Dear Dr. Luo,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice will be generated when your article is formally accepted. Please note, if your institution has a publishing partnership with PLOS and your article meets the relevant criteria, all or part of your publication costs will be covered. Please make sure your user information is up-to-date by logging into Editorial Manager at Editorial Manager® and clicking the ‘Update My Information' link at the top of the page. If you have any questions relating to publication charges, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Shaker Qaidi

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Dear Authors,

I am pleased to inform you that your manuscript has been accepted for publication in our journal.

The reviewers acknowledged the importance of your work and found that it makes a significant contribution to the field. Your research methods were sound, the data supports the conclusions, and the paper is well-written overall.

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The new version of the paper is wholly modified compared to the original version, and the article is acceptable for publication.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Meysam Bayat

**********

Associated Data

    This section collects any data citations, data availability statements, or supplementary materials included in this article.

    Supplementary Materials

    S1 File. Original data.

    (ZIP)

    pone.0302409.s001.zip (6.3MB, zip)
    Attachment

    Submitted filename: Response to Reviewers.docx

    pone.0302409.s002.docx (98.6KB, docx)

    Data Availability Statement

    All relevant data are within the manuscript and its Supporting Information files.


    Articles from PLOS ONE are provided here courtesy of PLOS

    RESOURCES