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^{#}These authors contributed to this work equally and should be regarded as co-first authors

In the seasonal permafrost region with loess distribution, the influence of freeze-thaw cycles on the engineering performance of reinforced loess must be paid attention to. Many studies have shown that the use of fiber materials can improve the engineering performance of soil and its ability to resist freeze-thaw cycles. At the same time, as eco-environmental protection has become the focus, which has been paid more and more attention to, it has become a trend to find new environmentally friendly improved materials that can replace traditional chemical additives. The purpose of this paper uses new environmental-friendly improved materials to reinforce the engineering performance of loess, improve the ability of loess to resist freeze-thaw cycles, and reduce the negative impact on the ecological environment. To reinforce the engineering performance of loess and improve its ability to resist freeze-thaw cycles, lignin fiber is used as a reinforcing material. Through a series of laboratory tests, the unconfined compressive strength (UCS) of lignin fiber-reinforced loess under different freeze-thaw cycles was studied. The effects of lignin fiber content and freeze-thaw cycles on the strength and deformation modulus of loess were analyzed. Combined with the microstructure features, the change mechanism of lignin fiber-reinforced loess strength under freeze-thaw cycles was discussed. The results show that lignin fiber can improve the UCS of loess under freeze-thaw cycles, but the strengthening effect no longer increases with the increase of fiber content. When the fiber content is less than 1%, the UCS growth rate of loess is the fastest under freeze-thaw cycles. And the UCS of loess with 1% fiber content is the most stable under freeze-thaw cycles. The freeze-thaw cycles increase the deformation modulus of loess with 1% fiber content, and its ability to resist deformation is obviously better than loess with 1.5%, 2% and 3% fiber content. The fiber content over 1% will weaken the strengthening effect of lignin fiber-reinforced loess, and the optimum fiber content of lignin fiber-reinforced loess under freeze-thaw cycles is 1%.

Loess is a kind of silt-dominated Quaternary sediment, which is mostly distributed in arid and semi-arid areas in the middle latitudes of the earth [

In recent years, there have been continuous reports on the mechanical properties of reinforced soil by new improved materials, especially fiber-reinforced soil under freeze-thaw cycles. Zaimoglu [

Lignin fiber has huge storage, which not only has the advantages of fiber materials, but also has the characteristics of strong renewability, low cost and low environmental impact [

The study analyzes the effect of freeze-thaw cycles on the strength of lignin fiber-reinforced loess through a series of unconfined compressive strength tests, and summarize the law of different lignin fiber content to enhance the strength of loess under the condition of freeze-thaw cycles. And combined with the microstructure features, the reason for the difference of this intensity change law is discussed. Finally, the optimal lignin fiber content to improve the freeze-thaw resistance of loess is given. The results can fill the gap in the study of the effect of freeze-thaw cycles on the strength of lignin fiber-reinforced loess, and provide a reference for the follow-up study of lignin fiber-reinforced loess under the condition of freeze-thaw cycles. It is also helpful to the application of environmental protection material lignin fiber in the loess area.

The loess used in the tests is typical loess in northwest China, which comes from Xiji County, Ningxia Hui Autonomous Region. The loess texture is uniform and the color is yellowish (

Property | Value |
---|---|

Specific gravity | 2.72 |

Liquid limit (%) | 24.0 |

Plastic limit (%) | 14.5 |

Plasticity index | 9.5 |

Maximum dry density (g/cm^{3}) |
1.79 |

Optimum moisture content (%) | 14.5 |

Cohesion (kPa) | 35.4 |

frictional angle (°) | 30.3 |

modulus of compression (MPa) | 14.6 |

Property | Value |
---|---|

Length (mm) | <1 |

Average diameter (μm) | 40 |

Bulk density (g/L) | 27 |

Moisture content (%) | <5 |

Ash content (%) | 18 |

Heat resisting ability (°C) | 230 |

pH | 7.0 |

The lignin fiber content of the specimen used in the study was m = 0%, 1%, 1.5%, 2% and 3%, respectively. The content is the ratio mass of fiber to dry loess [^{3} and the compaction coefficient 94% [

After curing, the specimens were subjected to freeze-thaw cycles in batches in a high and low temperature test instrument (

Fiber content (%) | Number of freeze-thaw cycles | Temperature (°C) | Time (hours) | ||
---|---|---|---|---|---|

freeze | thaw | Freeze | thaw | ||

0 | 0, 1, 5, 10, 15 | −20 | 20 | 12 | 12 |

1 | |||||

1.5 | |||||

2 | |||||

3 |

The YYW- 2 strain controlled unconfined compression testing machine was used in the UCS test, and the loading rate was 2.4 mm/min (

Representative microstructure pictures of lignin fiber-reinforced loess were got by KYKY-2800B scanning electron microscope. The specimen of electron microscope needs to be prepared before scanning. The preparation process is: First, the vacuum freeze drying equipment was used to dry the loess specimen. Then the suitable part was selected in the loess specimen and the flake electron microscope specimen was made through manual grinding. Finally, the flake specimen was glued to the sample bracket with conductive adhesive, and the gold was sprayed by an ion sputtering instrument. The magnification of the microstructure image of the specimen got by scanning was 400 times, which is helpful to analyze the reason for the change of macroscopic mechanical properties of reinforced loess caused by freeze-thaw cycles.

According to the unconfined compressive strength test data, the stress-strain curves of different fiber content specimens under different freeze-thaw cycles are shown in

As shown in

The influence of fiber on the stress-strain relationship of loess under freeze-thaw cycles was further explained. The peak stress and final stress (i.e., the stress at the end of the test) of the specimens after 10, 15 and 20 freeze-thaw cycles were statistically analyzed. The results are shown in

Fiber content (m) (%) | Peak stress | Final stress | ||||
---|---|---|---|---|---|---|

Geometric mean (kPa) | Standard deviation (%) | Standard error (%) | Geometric mean (kPa) | Standard deviation (%) | Standard error (%) | |

0 | 119 | 1.15 | 0.67 | 24 | 2.65 | 1.53 |

1 | 180 | 0.58 | 0.33 | 49 | 12.34 | 7.13 |

1.5 | 160 | 2.89 | 1.67 | 46 | 14.49 | 8.35 |

2 | 163 | 4.73 | 2.73 | 52 | 17.56 | 10.14 |

3 | 213 | 1.73 | 1.00 | 53 | 16.17 | 9.3 |

To more intuitively analyze the influence of freeze-thaw cycles and fiber content on the unconfined compressive strength of reinforced loess, the dimensionless quantitative treatment was carried out. The UCS (_{m}/S_{m = 0}_{m}_{m = 0}

As shown in

After 5 freeze-thaw cycles, the UCS ratio of 1% fiber content specimen still does not change significantly. The UCS ratios of 1.5%, 2%, and 3% fiber content specimens decrease significantly, and they are again smaller than the specimens without freeze-thaw cycles with the same fiber content. The UCS ratio of 1.5% fiber content specimen has been less than 1% fiber content specimen. At this time, with the increase of fiber content, the strengthening effect of fiber on loess begins to show the law of first strengthening and then weakening, and then second strengthening.

After 10 freeze-thaw cycles, the UCS ratios of 1% and 3% fiber content specimen remain basically stable. Like 1.5% fiber content specimens, the UCS ratio of 2% fiber content specimens begins to be less than 1% fiber content specimens. The change rule of UCS ratio is basically the same as that after 5 freeze-thaw cycles. The UCS ratios of the specimens after 15 and 20 freeze-thaw cycles are not significantly different from those after 10 freeze-thaw cycles, and the change rules of UCS ratio do not change. After 10, 15, and 20 freeze-thaw cycles, the UCS ratios of 1%, 1.5%, 2%, and 3% fiber content specimens ranges from 1.51 to 1.53, 1.32 to 1.38, 1.33 to 1.41, and 1.76 to 1.81, respectively. Although the UCS ratio of fiber-reinforced loess no longer increases with the increase of fiber content, the UCS ratios of 1%, 1.5%, 2%, and 3% fiber content specimens are still all greater than 1. This means that although the fiber can enhance the strength of loess under the condition of freeze-thaw cycles. However, when the strength of fiber-reinforced loess begins to stabilize under the action of freeze-thaw cycles, there is no positive correlation between reinforcement effect and fiber content.

Therefore, in order to further illustrate the strengthening effect of fiber content on loess UCS under the influence of freeze-thaw cycles, the change rate of UCS ratio of each fiber content range after different freeze-thaw cycles is listed in

Fiber content range (%) | Rate of change of different freeze-thaw cycles (%) | |||||
---|---|---|---|---|---|---|

0 | 1 | 5 | 10 | 15 | 20 | |

0∼1 | 47 | 49 | 47 | 52 | 53 | 51 |

1∼1.5 | 10 | 26 | −23 | −30 | −37 | −37 |

1.5∼2 | 21 | 26 | 26 | 7 | 5 | 2 |

2∼3 | 37 | 36 | 36 | 40 | 45 | 43 |

As we can see from

Based on the above analysis, it can be found that the freeze-thaw cycle will weaken the strengthening effect of fiber on UCS of loess, but the weakening extent of loess with different fiber content is different. Therefore, it is necessary to analyze the change of UCS loss rate (Δ_{n}_{n=0}) of the same fiber content specimen during the freeze-thaw cycle. _{n}_{n = 0}) without freeze-thaw cycles and UCS (_{n}) after a given number (n) of freeze-thaw cycles.

According to

However, after 5 freeze-thaw cycles, the UCS loss rates of 0%, 1%, 1.5%, 2%, and 3% fiber content specimens are 0.09, 0.09, 0.19, 0.17, and 0.16, respectively. The UCS loss rates of 0% and 1% fiber content specimens are less than 1.5%, 2%, and 3% fiber content specimens. After 5 freeze-thaw cycles, 1.5%, 2%, and 3% fiber-reinforced loess are more affected than 0% and 1% fiber-reinforced loess. The UCS loss rates of 0% and 1% fiber content specimens after 10, 15, and 20 freeze-thaw cycles are between 0.12 and 0.14, 0.10 and 0.11, respectively, which change little compared with those of 1 and 5 freeze-thaw cycles. It can be considered that the response of 0% and 1% fiber-reinforced loess to freeze-thaw cycles begins to decrease after 1 freeze-thaw cycle, and their strength changes begin to stabilize. And the UCS loss rate of 1% fiber-reinforced loess is slightly lower than that of unreinforced loess under the condition of freeze-thaw cycles.

The UCS loss rates of 1.5%, 2%, and 3% fiber content specimens have little difference after 1 freeze-thaw cycle. After 5 freeze-thaw cycles, the UCS loss rates of 1.5%, 2% and 3% fiber content specimens are 0.19, 0.17, and 0.16, respectively, and the UCS loss rates of 1.5% fiber content specimens are the highest among the three. After 10 freeze-thaw cycles, the UCS loss rates of 1.5%, 2%, and 3% fiber content specimens are 0.21, 0.24, and 0.21, respectively. The UCS loss rate of 2% fiber content specimen is higher than that of 1.5% fiber content specimen, which is the highest among the three. After 5, 10, 15, and 20 freeze-thaw cycles, the UCS loss rate of the specimens with 3% fiber is the lowest among the three. The UCS loss rates of 1.5%, 2%, and 3% fiber content specimens after 10, 15, and 20 freeze-thaw cycles are between 0.21 and 0.23, 0.24 and 0.28, 0.21 and 0.22, respectively. It can be considered that 1.5%, 2%, and 3% fiber-reinforced loess begin to enter a stable stage after 10 freeze-thaw cycles, and 2% fiber-reinforced loess after stabilization is most affected by the freeze-thaw cycles. By comparing the UCS loss rate of all fiber content specimens, it can be found that 1% fiber-reinforced loess has the best stability under freeze-thaw cycles, the strongest ability to resist freeze-thaw cycles, and 2% fiber-reinforced loess has the worst ability to resist freeze-thaw cycles.

_{50} was used as the evaluation index to evaluate the deformation properties of lignin fiber-reinforced loess. _{50} is the secant slope from the origin to the point of 50% axial failure strain. The calculation formula is as follows [_{1/2} is the stress value when the axial failure strain reaches half; _{f} is the axial failure strain corresponding to the peak stress. _{50} is also quantified dimensionless to eliminate the influence of different factors on deformation modulus. The modulus ratio (_{50m}_{50m=0}) of specimens with the same freeze-thaw cycles is defined as the ratio of the deformation modulus (_{50m}) of reinforced loess with given fiber content (m) to deformation modulus (_{50m=0}) of unreinforced loess in the same freeze-thaw cycle, which can reflect the effect of fiber content on the deformation modulus of loess under the same freeze-thaw cycle. The variation of the modulus ratio with fiber content of the specimen with the same number of freeze-thaw cycles is shown in

As shown in

The effects of freeze-thaw cycles on specimens with different fiber content are different. _{50n}_{50n = 0}) of the specimen with the same fiber content is the deformation modulus (_{50n}) of the specimen with the same fiber content after a given number of freeze-thaw cycles divided by the deformation modulus (_{50n = 0}) of the specimen without freeze-thaw cycles.

According to

The modulus ratio of 1.5%, 2%, and 3% fiber content specimens decrease at first and then tend to be stable. The modulus ratio of 1.5%, 2%, and 3% fiber content specimens decrease most significantly after 1 freeze-thaw cycle, and remain stable after 10 freeze-thaw cycles. The modulus ratios of 1.5%, 2%, and 3% fiber content specimens after 10, 15, and 20 freeze-thaw cycles are in the range from 0.57 to 0.67, 0.52 to 0.58, and 0.65 to 0.66, respectively. The freeze-thaw cycle greatly reduces the deformation modulus of 1.5%, 2%, and 3% fiber-reinforced loess, weakening their ability to resist deformation.

Based on the results of UCS test, it can be found that the relationship between macroscopic mechanical properties and fiber content was different between the specimens without freeze-thaw cycles and those after freeze-thaw cycles. In order to analyze the reasons for this difference, the representative microstructure pictures of fiber-reinforced loess after 0 and 10 freeze-thaw cycles were compared and analyzed. The results of the analysis are shown in

Without freeze-thaw cycles, the unreinforced loess (m = 0%) had obvious overhead pores, the contact between soil particles was mainly point contact, the surface of particles had less cohesive substances, and the pore filling is insufficient. With the increase of fiber content, the pores of the reinforced loess become smaller, the pore connectivity became worse, the pore distribution became more uniform, the cohesive polymer on the particle surface increased, and the soil became denser. This phenomenon is related to the filling of loess pores by tiny particles (detritus) in lignin fiber. The strength of reinforced loess without freeze-thaw cycles increases with the increase of fiber content, which should be closely related to the filling effect and the interaction between fiber and soil interface [

By comparing the microstructure characteristics of reinforced loess before and after freeze-thaw cycles, it could be found that the pores in 0%, 1%, and 3% fiber-reinforced loess all slightly increase after 10 freeze-thaw cycles. This phenomenon is obviously caused by the change of water volume in the soil during freeze-thaw cycles. Except for water, the strength of loess is obviously related to fiber content and soil properties. Therefore, the coupling effect of fiber, loess and water may be the reason for the different degrees of response of reinforced loess with different fiber content to freeze-thaw cycles. There are differences in the properties of lignin fiber and loess materials. After the addition of lignin fiber to the loess, the hydrophilic groups in the lignin fiber will adsorb the water in the loess. The lubrication formed by the accumulation of water in loess on the surface of fiber and detritus will weaken the strengthening effect of fiber on loess [

As shown in

In this paper, the effects of freeze-thaw cycles and fiber content on lignin fiber-reinforced loess have been discussed, and the following conclusions can be drawn:

Lignin fiber can reduce the softening degree of frozen-thawed loess under unconfined conditions, and strengthen the UCS of loess. Without freeze-thaw cycles, the strengthening effect of lignin fiber on loess increases with the increase in fiber content, but the growth rate is different. After 5 freeze-thaw cycles, the strengthening effect of lignin fiber on loess increases at first and then decreases, and then enhances again with the increase in fiber content. Under different freeze-thaw cycles, the enhancement rate of UCS of lignin fiber-reinforced loess is the fastest when the fiber content increases from 0% to 1%.

Under the condition of freeze-thaw cycles, the stable cycle times and UCS loss rate of reinforced loess with different fiber content are different. The stable cycle times and strength loss rate of high fiber content reinforced loess are significantly higher than those of low fiber content reinforced loess. Under the condition of freeze-thaw cycles, the reinforced loess with 1% fiber content enters the stable state fastest, the strength loss rate is the smallest, and the stability is the best.

Under the condition of freeze-thaw cycles, the deformation modulus of reinforced loess will decrease with the increase of fiber content, until the fiber content increases to a certain extent (over 1.5%). The effect of freeze-thaw cycles on the deformation modulus of reinforced loess with different fiber content is different. The deformation modulus of reinforced loess with 0% and 1% fiber content increase under freeze-thaw cycles, and its ability to resist deformation becomes stronger.

When the fiber content stay in a certain range, the strengthening effect of lignin fiber on loess will be weakened due to the coupling effect of fiber, loess and water. The freeze-thaw effect will further amplify this weakening effect, and when the fiber content increases to a certain extent (1%), the strengthening effect on loess begins to weaken. Excessive fiber content is not conducive to the improvement of loess strength, and the best fiber content of lignin fiber-reinforced loess under freeze-thaw cycles is 1%.

Authors are grateful for the support provided by Master Liu Fuqiang who participated in the test.