# Effect of Soil Moisture Content on the Shear Strength of Dicranopteris Linearis-Rooted Soil in Different Soil Layers of Collapsing Wall

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

^{2}, which is more than 90 times higher than the allowable soil loss in the south [5]. This is also called an “ecological ulcer” in tropical and subtropical China [7], which seriously endangers human production and life (Zhang et al., 2020). A collapsing wall is a steep cliff formed by undercutting or collapse of hillside soil [5], and its stability is the key to the occurrence and expansion of Benggang. Shear properties are an important index to quantify the stability of landslides. Under the action of external force, the shear resistance of soil changes greatly. Research on the mechanical properties of collapsing wall soil is of great significance for revealing the process of collapse and the development of Benggang.

## 2. Materials and Methods

#### 2.1. Study Area

#### 2.2. Root Sample Collection and Pretreatment

^{−3}, respectively. After the survey, the roots of well-growing Dicranopteris linearis were collected by the excavation method [36]. To prevent the water loss of roots during transportation, the collected root samples were covered with fresh soil, wrapped in a foam box, and quickly transported to the laboratory. After cleaning (Figure 2a), half of the samples was trimmed into 2 cm root segments for rooted soil preparation according to the size of the shear ring. Meanwhile, roots that were straight, uniform and free of diseases were selected for the tensile test (Figure 2b). The root tensile test was carried out by an electronic fabric tension machine (the instrument test force range was 0~2500 N). The results showed that the average root diameter and tensile strength of Dicranopteris linearis were 0.49 ± 0.18 mm and 18.42 ± 9.36 MPa, respectively.

#### 2.3. Soil Collection and Basic Properties

#### 2.4. Experimental Design

^{−3}and the root content was 0.75 g 100 cm

^{−3}. The design of SMC referred to the variation range of SMC in the field. Four gradients of 15%, 20%, 25% and 30% were set (Table 1), and three replicates were prepared for each treatment.

_{1}is the SMC of wet soil (%), ρ is the constant, 1.35 g cm

^{−3}in this experiment, and v is the volume of the shearing ring (cm

^{3}).

#### 2.5. Remodeling Soil Preparation and Shear Test

^{−3}or 0 g 100 cm

^{−}

^{3}was randomly and uniformly mixed with the soil with target SMC to simulate its distribution in the field soil. The operations were as follows: first, the soil was mixed with roots. Second, the mixed sample was stirred many times until the root was evenly distributed in the soil. Then, the root–soil mixture was filled into the shear ring and compacted. Finally, the mixture was sealed and placed in an incubator for 24 h. Before the shear test, a scanning electron microscope (Phenom ProX, Amsterdam, The Netherlands) was used to explore the microstructure of the root–soil interface.

^{−}

^{1}and 8 mm, respectively. Because the root distribution depth of Dicranopteris linearis is shallow and the collapse rate of soil is fast, the normal stress and shear rate parameters were set to 25 to 100 kPa and 0.8 mm min

^{−}

^{1}, respectively. When the displacement reached 6 mm, the shear test ended and the shear index was calculated:

#### 2.6. The WWM

_{r}= k × T

_{r}× RAR

_{r}represents the cohesion increment (kPa), k is the constant (1.2), T

_{r}is the root tensile strength (MPa) and RAR is the root area ratio (%). According to the calculation, when the root content is 0.75 g 100 cm

^{−3}, the average RAR of the soil with roots is 0.048%.

#### 2.7. Statistical Analysis

^{2}.

^{2}is the determining coefficient. M

_{i}and P

_{i}represent the true and predicted shear strengths of i, respectively. $\overline{M}$ and $\overline{P}$ are the mean values of the true and the predicted shear strengths, respectively. The closer the NSE is to 1, the better the fitting effect of the model.

## 3. Results

#### 3.1. Relationship between SMC and Shear Strength of Collapsing Wall Rooted Soil

#### 3.2. Relationship between SMC and Cohesion of Collapsing Wall Rooted Soil

^{2}values greater than 0.7 and passed the significance regression equation test.

#### 3.3. Relationship between SMC and Internal Friction Angle of Collapsing Wall Rooted Soil

#### 3.4. Relationship between SMC and Cohesion Increment Predicted by the Wu–Waldron Model

_{R}, k, T

_{R}and RAR are given in Formula (4).

## 4. Discussion

#### 4.1. Effect of SMC on the Shear Strength of the Rooted Soil in the Collapsing Wall

#### 4.2. Effect of SMC on the Cohesion of the Rooted Soil in the Collapsing Wall

#### 4.3. Effect of SMC on the Internal Friction Angle of the Rooted Soil in the Collapsing Wall

#### 4.4. WWM Correction and Shear Strength Model Building

_{rs}, C

_{r}, C

_{s}, σ and φ are given in Formulas (3) and (4).

_{r}and combined with the relation between the SMC of plain soil and the internal friction angle, a shear strength model of the Dicranopteris linearis-rooted soil in three soil layers was built.

- (1)
- In the LL:τ
_{rs}= (−13.61w^{2}+ 4.07w + 0.53) × T_{R}× RAR + C_{s}+ σtan(−95.51w + 53.68)

R^{2}= 0.93, p < 0.01, NSE = 0.92 - (2)
- In the SL:τ
_{rs}= (−6.63w^{2}+ 1.71w + 0.14) × T_{R}× RAR + C_{s}+ σtan(−6.58lnw + 28.22)

R^{2}= 0.98, p < 0.01, NSE = 0.96 - (3)
- In the DL:τ
_{rs}= (−6.65w^{2}+ 2.84w − 0.22) × T_{R}× RAR + C_{s}+ σtan(−6.04lnw + 29.83)

R^{2}= 0.97, p < 0.01, NSE = 0.97

_{rs}, C

_{r}, C

_{s}, σ and φ are given in Formulas (3) and (4).

^{2}and NSE index of the new model are above 0.90. Comparing the true values and predicted values of the rooted soils, it can be seen that they approach the line of 1:1 (Figure 11). This indicates that the model can accurately predict the effect of ferns on the shear strength of collapsing walls.

## 5. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 4.**Properties of sampled soil in collapsing wall. Different lowercase letters represent significant difference in the soil properties among three soil layers (p < 0.05).

**Figure 5.**Relationship between shear strength of collapsing wall soil and SMC. (

**a**,

**c**,

**e**) LL, SL and DL without roots, respectively. (

**b**,

**d**,

**f**) LL, SL and DL with roots, respectively.

**Figure 6.**Relationship between cohesion and SMC in three soil layers of collapsing wall. (

**a**) Unrooted soil. (

**b**) Rooted soil.

**Figure 7.**The relationship between soil internal friction angle and SMC. (

**a**) Unrooted soil. (

**b**) Rooted soil.

**Figure 8.**The relationship between simulated and measured cohesion increment and SMC in the LL (

**a**), SL (

**b**) and DL (

**c**).

**Figure 10.**Scanning characteristics of the LL (

**a**), SL (

**c**) and DL (

**e**) with roots at 20% SMC; and the LL (

**b**), SL (

**d**) and DL (

**f**) rooted soils at 25% SMC.

**Figure 11.**Measured shear strength versus predicted shear strength in three layers of collapsing wall.

Soil Layer | Rooted Soil | Plain Soil | ||||
---|---|---|---|---|---|---|

Root Content /g 100 cm ^{−3} | Bulk Density /g cm ^{−3} | SMC /% | Root Content /g 100 cm ^{−3} | Bulk Density /g cm ^{−3} | SMC /% | |

LL | 0.75 | 1.35 | 15, 20, 25, 30 | 0.00 | 1.35 | 15, 20, 25, 30 |

SL | 0.75 | 1.35 | 15, 20, 25, 30 | 0.00 | 1.35 | 15, 20, 25, 30 |

DL | 0.75 | 1.35 | 15, 20, 25, 30 | 0.00 | 1.35 | 15, 20, 25, 30 |

Soil Layer | Root Content /g 100 cm ^{−3} | Fitting Equations | Optimal SMC/% | Optimal Cohesion/kPa | R^{2} | p | n |
---|---|---|---|---|---|---|---|

LL | 0.75 | C = −0.24 w^{2} + 10.92 w − 63.71 | 22.78 | 60.62 | 0.98 | <0.01 | 12 |

SL | 0.75 | C = −0.12 w^{2} + 4.64 w − 16.47 | 19.67 | 29.14 | 0.97 | <0.01 | 12 |

DL | 0.75 | C = −0.08 w^{2} + 2.96 w − 2.26 | 18.39 | 24.92 | 0.98 | <0.01 | 12 |

Soil Layer | SMC/% | Cohesion Increment/kPa | Rates of Increment/% | Average Rates of Increment/% |
---|---|---|---|---|

LL | 15 | 7.14 | 18.52 | 13.72 |

20 | 7.75 | 14.88 | ||

25 | 5.46 | 10.29 | ||

30 | 4.87 | 11.18 | ||

SL | 15 | 2.06 | 8.08 | 6.22 |

20 | 2.32 | 8.97 | ||

25 | 0.96 | 3.92 | ||

30 | 0.64 | 3.90 | ||

DL | 15 | 0.50 | 2.16 | 2.66 |

20 | 0.75 | 3.15 | ||

25 | 0.65 | 3.00 | ||

30 | 0.31 | 2.28 |

**Table 4.**Fitting relationship between internal friction angle and SMC of rooted soil in collapsing wall.

Soil Layer | Root Content /g 100 cm ^{−3} | Fitting Equations | R^{2} | p | n |
---|---|---|---|---|---|

LL | 0.75 | φ = −0.89w + 52.36 | 0.99 | <0.01 | 12 |

SL | 0.75 | φ = −0.33w + 46.27 | 0.99 | <0.01 | 12 |

DL | 0.75 | φ = −0.36w + 47.42 | 0.99 | <0.01 | 12 |

**Table 5.**Variation in internal friction angle increment with SMC of root–soil complex in three soil layers of collapsing wall.

Soil Layer | SMC/% | Internal Friction Angle Increment/° | Rates of Increment /% | Average Rates of Increment/% |
---|---|---|---|---|

LL | 15 | 0.18 | 0.46 | 1.07 |

20 | −0.44 | −1.28 | ||

25 | 0.14 | 0.48 | ||

30 | 1.16 | 4.64 | ||

SL | 15 | 0.54 | 1.33 | 2.50 |

20 | 1.14 | 2.92 | ||

25 | 0.89 | 2.41 | ||

30 | 1.19 | 3.35 | ||

DL | 15 | −0.81 | −1.93 | −1.83 |

20 | −0.71 | −1.75 | ||

25 | −0.42 | −1.10 | ||

30 | −0.94 | −2.55 |

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## Share and Cite

**MDPI and ACS Style**

Zhou, M.; Zhu, Q.; Wang, H.; Wang, X.; Zhan, Y.; Lin, J.; Zhang, Y.; Huang, Y.; Jiang, F.
Effect of Soil Moisture Content on the Shear Strength of Dicranopteris Linearis-Rooted Soil in Different Soil Layers of Collapsing Wall. *Forests* **2024**, *15*, 460.
https://doi.org/10.3390/f15030460

**AMA Style**

Zhou M, Zhu Q, Wang H, Wang X, Zhan Y, Lin J, Zhang Y, Huang Y, Jiang F.
Effect of Soil Moisture Content on the Shear Strength of Dicranopteris Linearis-Rooted Soil in Different Soil Layers of Collapsing Wall. *Forests*. 2024; 15(3):460.
https://doi.org/10.3390/f15030460

**Chicago/Turabian Style**

Zhou, Man, Qin Zhu, He Wang, Xiaopeng Wang, Yuanyuan Zhan, Jinshi Lin, Yue Zhang, Yanhe Huang, and Fangshi Jiang.
2024. "Effect of Soil Moisture Content on the Shear Strength of Dicranopteris Linearis-Rooted Soil in Different Soil Layers of Collapsing Wall" *Forests* 15, no. 3: 460.
https://doi.org/10.3390/f15030460