# The Influence of Drying-Wetting Cycles on the Suction Stress of Compacted Loess and the Associated Microscopic Mechanism

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

^{2}and accounting for about 6% of China’s total land area [3,4,5,6]. At present, with the implementation and advancement of China’s “Western Development” strategy, more and more infrastructure projects are being constructed in the Loess Plateau [4]. Local materials are usually used for construction, and the loess is compacted as the filling material in roadbed slopes [7]. However, variable rainfall intensity, changes in groundwater, and surface evaporation in the Loess Plateau, especially in the Yan ‘an area, create a regular dry-moist cycle in local soils. These climate changes cause the drying-wetting cycles to occur under natural conditions, which affect soil hydraulic properties [8] and shear strength [9], important factors which influence the fill and loess slope stability.

_{0}consolidation test on granite residual soil, further proving the effectiveness of SSCC in describing the consolidation and shear strength characteristics of unsaturated soils. Jiang et al. [25] found that SSCC varies significantly with different dry densities and soil water content based on loess SWCC data. Song and Hong [26] compared the modified SSCC of granite and mudstone soils and concluded that the mineral composition and particle size distribution of unsaturated soil has a significant impact on its characteristics.

## 2. Materials and Methods

#### 2.1. Materials

_{3}loess at a depth of 3 m below the surface. The specific sampling process is as follows: after the sampling point is determined, the surface soil is removed, the sample is manually obtained from a depth of 3 m to reduce disturbance, and the sample is carefully cut into a soil column with a diameter of 10 cm and a height of 20 cm, which is immediately put into the sampling cylinder, stored in bubble film, and transported back to the laboratory on the same day. The basic physical properties of the loess samples were measured according to ASTM 2006 Standard Test Methods [27], as shown in Table 1. The particle group analysis was performed with the Bettersize 2000 laser particle size tester. The results showed that the loess sample was mainly composed of silt and sand (namely 90.04%) and clay particles (namely 9.96%) (Table 1 and Figure 2).

#### 2.2. Sample Preparation

^{3}, 1.55 g/cm

^{3}, and 1.65 g/cm

^{3}), put into a mold with a diameter of 61.8 mm and a height of 20 mm, and then statically compacted at a constant displacement rate of 0.4 mm/min. After compaction, the samples were taken out of the mold. The loess samples used in this study were all prepared using this method.

#### 2.3. Drying-Wetting Cycles

#### 2.4. Microstructural Investigation

#### 2.4.1. NMR Test

_{2LM}is the geometric mean of the T

_{2}distribution.

^{3}as an example, ${k}_{s}$ is $1.18\times {10}^{-13}$ m

^{2}; $\varnothing $ is 0.49; T

_{2LM}is 0.52989 ms, so ${\rho}_{2}$ is 2.69 $\mathsf{\mu}\mathrm{m}$/ms. Substituting the obtained ρ

_{2}into Equation (1), the T

_{2}curve can be converted into a pore size distribution curve. The pore size distribution curves in this study are all converted from the T

_{2}curve using this method.

#### 2.4.2. SEM Test

#### 2.5. SWCC Test

## 3. Results and Discussion

#### 3.1. SWCC

^{3}had the smallest change in SWCC morphology before and after the drying-wetting cycles, indicating that the drying-wetting cycles weakly impacted the porosity of the sample at this density. As dry density increased, the SWCC morphology changes before and after the drying-wetting cycles became increasingly obvious, indicating that the influence of drying-wetting cycles on SWCC increased with an increase in dry density.

#### 3.2. SSCC

^{3}) sample almost overlap, indicating that, under the same matric suction, the reduction in suction stress was very weak. As the dry density increased from 1.45 g/cm

^{3}to 1.65 g/cm

^{3}, the downward shift of the SSCC after the drying-wetting cycles also increased, meaning that the attenuation of the suction stress increased. In other words, the influence of drying-wetting cycles on SSCC increased with dry density. This is similar to the change in SWCC, from which we can infer that the drying-wetting cycles have a more significant impact on the pore structure of high-pressure compacted loess samples than that of low-pressure compacted samples.

#### 3.3. Microscopic Analysis of Changes in the Suction Stress of Compacted Loess Samples

#### 3.3.1. SEM Observations

^{3}and 1.65 g/cm

^{3}were used to analyze the influence of drying-wetting cycles on loess microstructure. The figure illustrates that there are inter- and intra-aggregate pores in the compacted loess sample. This double pore structure is caused by compaction under a lower than optimal moisture content [45]. After three drying-wetting cycles, the aggregate and the contact types of the compacted loess changed to varying degrees. The number of face contacts between aggregates decreased, with more point contacts appearing, and the numbers of pores between aggregates increased. Among them, when the dry density was 1.65 g/cm

^{3}, the change in sample microstructure was the most significant, and when the dry density was 1.45 g/cm

^{3}, the change in microstructure was relatively small.

^{3}dry density samples did not change significantly after the drying-wetting cycles, but the proportion of inter-aggregate pores in the overall microstructure increased, indicating that the drying-wetting cycles would destroy the structural integrity of the compacted loess, while other indicators of microstructure changed little. That is to say, the change in the overall microstructure of this density is the least among the three dry density samples, which was consistent with the finding that the suction stress of the 1.45 g/cm

^{3}dry density loess sample decreased after drying-wetting cycles, but the amplitude was small (Figure 4b).

^{3}, the samples had a large size differential, and consisted of large and small aggregates before the drying-wetting cycles. The aggregates were mainly bonded together in the form of face contact with poor pore connectivity. However, after the drying-wetting cycles, the aggregate size of the sample decreased and was more uniform. The contact form between aggregates changed to point contact with more apparent aggregate contours and greater apparent porosity. The change in aggregate contact form indicated that water dissolved cement during the drying-wetting cycles, causing some small and medium-sized pores to develop into macropores [46], increasing the soil porosity ratio and the volume of macropores. At the same time, due to irreversible van der Waals forces, the size of soil aggregates increased after the wetting-drying cycle, which made clay closer to the aggregates, and created larger aggregates [47]. The increase in aggregates means that the specific surface area decreased.

#### 3.3.2. NMR Analysis

_{2}) distribution curve of the compacted loess sample was obtained through the nuclear magnetic resonance experiment, and the pore size distribution curve of the loess sample before and after different degrees of compaction and drying-wetting cycles was obtained by using Equation (1) (Figure 6).

^{3}were 24, 9.4, and 8.7 μm, respectively. The diameter of the dominant pores inside the soil decreased as dry density increased. In addition, the dominant pore diameter of the loess samples with the same dry density increased to varying degrees after the drying-wetting cycles. The dominant pore diameters of samples with dry densities of 1.45, 1.55, and 1.65 g/cm

^{3}increased to 29.7, 24.3, and 23.5 μm, respectively, after wetting and drying. Compared with the other two dry densities, the increase in the dominant pore diameter of the 1.45 g/cm

^{3}sample was least affected by the drying-wetting cycles, which may be one reason that this sample also showed the smallest change in suction stress before and after the drying-wetting cycles.

^{3}loess sample as an example, the dominant pore diameter before the drying-wetting cycles was 9.4μm, and the corresponding matric suction was 15.2 kPa, which is almost the same as the AEV (that is, near the inflection point of SSCC). This fact means that the equivalent matric suction, or air entry value, can be used as a threshold value to divide the SSCC into two sections: In the first section (0–15.2 kPa), the matric suction corresponds to the large pores in the compacted loess sample (that is, the inter-aggregate pores), in this pore diameter range or matric suction range, the suction stress of sample increases sharply with the increase in the matric suction. In the second section (>15.2 kPa), the matric suction corresponds to the small pores in the compacted loess sample (that is, the intra-aggregate pores); in this pore diameter range or within the range of matric suction, SSCC changes from steep to slow, and tends to be constant. This rule is still consistent for the sample after the drying-wetting cycles, or when the dominant pore diameter was 24.3 μm and the equivalent matric suction was 5.87 kPa. This finding shows that, for small pores, there was a subtle change in suction stress with matric suction, while for large pores, the change in suction stress with matric suction is strong. In other words, the SSCC of the compacted loess was closely related to the change in the inter-aggregate pores or the dominant pore diameter, while the influence of the intra-aggregate pores was relatively weak. The more obvious the influence of the drying-wetting cycles on the dominant pore diameter, the stronger the corresponding change in suction stress. This also explains the significant change in the dominant pore diameter of the 1.65 g/cm

^{3}dry density loess sample after the drying-wetting cycles, followed by smaller changes in the 1.55 and 1.45 g/cm

^{3}samples, while the corresponding change range of the suction stress characteristic curve also decreased sequentially.

#### 3.4. Suction Stress Calculation Model Based on PSD Parameters

^{3}/g); V

_{s}is total pore volume (mm

^{3}/g); V

_{r}is residual pore volume (mm

^{3}/g); and$l,m$ and $n$ are fitting parameters.

^{2}were all greater than 0.99, indicating that the fitting effect was excellent, and also that formula (7) is suitable for the cumulative PSD curve.

^{3}were prepared according to the same method. Some of these samples were subjected to three dry and wet cycles. After testing their SWCC, based on the SWCC parameters, the suction stress before and after the drying-wetting cycles was calculated, and then the matric suction and suction stress data were fitted using Equation (11). The fitted results are shown in Figure 9 with a correlation coefficient of 0.976. This shows that the suction stress calculation model based on the PSD index proposed in this study is a reasonable and feasible estimation method for the loess in the Yan ‘an area.

## 4. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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Maximum Dry Density (g/cm^{3}) | Optimal Water Content (%) | Specific Gravity | Liquid Limit (%) | Plastic Limit (%) | Grain Size Fractions (%) | ||
---|---|---|---|---|---|---|---|

<0.005 mm | 0.005–0.075 mm | >0.075 mm | |||||

1.69 | 14.11 | 2.72 | 28.9 | 16.1 | 9.96 | 82.22 | 7.82 |

The Sample | a | b | c | AEV (kPa) | R^{2} |
---|---|---|---|---|---|

1.45 Cycle 0 | 17.996 | 1.911 | 1.105 | 6.59 | 0.998 |

1.45 Cycle 3 | 13.578 | 1.843 | 1.214 | 5.66 | 0.998 |

1.55 Cycle 0 | 18.937 | 1.601 | 0.888 | 14.8 | 0.998 |

1.55 Cycle 3 | 8.724 | 1.016 | 1.404 | 5.89 | 0.997 |

1.65 Cycle 0 | 26.08 | 1.684 | 0.715 | 17.01 | 0.999 |

1.65 Cycle 3 | 7.406 | 1.202 | 0.957 | 6.04 | 0.998 |

The Sample | a | b | c | R^{2} |
---|---|---|---|---|

1.45 Cycle 0 | 19.564 | 1.425 | 1.532 | 0.991 |

1.45 Cycle 3 | 5.771 | 1.337 | 1.585 | 0.991 |

1.55 Cycle 0 | 35.971 | 1.859 | 0.752 | 0.989 |

1.55 Cycle 3 | 11.007 | 0.651 | 1.333 | 0.996 |

1.65 Cycle 0 | 47.719 | 0.889 | 0.45 | 0.998 |

1.65 Cycle 3 | 6.36 | 0.432 | 1.284 | 0.995 |

The Sample | l | m | n | R^{2} |
---|---|---|---|---|

1.45 Cycle 0 | 20.499 | 1.238 | 2.126 | 0.999 |

1.45 Cycle 3 | 29.667 | 1.233 | 1.981 | 0.998 |

1.55 Cycle 0 | 11.007 | 1.252 | 3.534 | 0.999 |

1.55 Cycle 3 | 25.541 | 1.206 | 2.392 | 0.998 |

1.65 Cycle 0 | 4.561 | 1.214 | 4.334 | 0.999 |

1.65 Cycle 3 | 28.31 | 1.201 | 2.545 | 0.997 |

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**MDPI and ACS Style**

Nie, Y.; Ni, W.; Li, X.; Wang, H.; Yuan, K.; Guo, Y.; Tuo, W.
The Influence of Drying-Wetting Cycles on the Suction Stress of Compacted Loess and the Associated Microscopic Mechanism. *Water* **2021**, *13*, 1809.
https://doi.org/10.3390/w13131809

**AMA Style**

Nie Y, Ni W, Li X, Wang H, Yuan K, Guo Y, Tuo W.
The Influence of Drying-Wetting Cycles on the Suction Stress of Compacted Loess and the Associated Microscopic Mechanism. *Water*. 2021; 13(13):1809.
https://doi.org/10.3390/w13131809

**Chicago/Turabian Style**

Nie, Yongpeng, Wankui Ni, Xiangning Li, Haiman Wang, Kangze Yuan, Yexia Guo, and Wenxin Tuo.
2021. "The Influence of Drying-Wetting Cycles on the Suction Stress of Compacted Loess and the Associated Microscopic Mechanism" *Water* 13, no. 13: 1809.
https://doi.org/10.3390/w13131809