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The distribution of chlorine saline soils is extensive in Haixi region of Qinghai Province in Northwest China. Its natural and geographical conditions are unique, and the external environment varies greatly. To study the effects of variable external environment on the mechanical characteristics of chlorine saline soils, a number of unconsolidated undrained (UU) dynamic triaxial tests under different confining pressure, moisture content, and loading frequency were carried out. The dynamic stress–dynamic strain, failure strength, dynamic elastic modulus, and parameter of shear strength were analyzed. The triaxial test results demonstrated that the stress–strain curves of the soil were strain-hardening. The failure strength and dynamic elastic modulus increased with the increasing of confining pressure; the law with moisture content and loading frequency were inconsistent. The dynamic cohesion and dynamic friction angle increased with the increasing of loading frequency, but decreased with the increasing of moisture content. Besides, the significance analysis theory was used to analyze the effect degree of different factors. It found that the effects of confining pressure, loading frequency, and the interaction between confining pressure and frequency on mechanical characteristics were significant, but the moisture content had less effect.

Saline soil is a special type of soil that distributes to various degrees in many countries and regions. Similarly, saline soil is also widely distributed in China which covers approximately a total area of 36.9 million ha, accounting for 4.88% of Chinese available land area [

In recent years, many studies have been performed to investigate the engineering characteristics of saline soil. The shear strength of saline soil increases with increasing salt content due to the presence of salt crystals in soil [

The research results on the mechanical characteristics of saline soil are mostly concentrated on static conditions. But the saline soil of the roadbed not only bears the influence of changes in external factors such as temperature, but also sustains tens of thousands of traffic loading. Therefore, UU triaxial dynamic tests were developed on chlorine saline soil with different factors, and SPSS software was used for the significance test. The effect of single factor and interaction between factors on the mechanical characteristics of chlorine saline soil was analyzed. The results provided a useful reference for the engineering construction in the area of over-chlorine saline soil.

The chlorine saline soil for the test was taken from a highway in the Qarhan Salt Lake area of Qinghai Province, China. The area has a typical plateau continental climate. There is little precipitation, the rain and heat are in the same season, and the precipitation varies greatly with the spatial distribution in this area. The basic physical test was measured according to Test Methods of Soils for Highway Engineering [

Particle-size curve of chlorine saline soil.

Basic physical parameters of chlorine saline soil.

Liquid limit _{L} (%) | Plastic limit _{P} (%) | Plastic index IP | Liquid index _{L} | Maximum dry density _{dmax} (g·cm^{−3}) | Optimum moisture content _{opt} (%) |
---|---|---|---|---|---|

20.47 | 7.44 | 13.03 | <0 | 1.81 | 5.20 |

Chemical composition analysis of chlorine saline soil.

Anion ( | Cation ( | ||||||
---|---|---|---|---|---|---|---|

Cl^{−} | SO_{4}^{2−} | CO_{3}^{2−} | HCO_{3}^{−} | Na^{+} | K^{+} | Ca^{2+} | Mg^{2+} |

17.950 | 0.320 | 0.010 | 0.049 | 11.160 | 0.061 | 0.22 | 0.20 |

The test was completed by a dynamic triaxial test system of Global Digital Systems, shown in Figure

GDS dynamic triaxial test system.

The saline soil sample needed to be dried and passed through a 2 mm sieve. The soil sample with the target moisture content was prepared according to the test requirements and stuffed for more than 12 h. The reshaped triaxial samples with a compacting degree of 95%, a diameter of 39.1 mm, and a height of 80 mm were prepared by 5-layer compaction with a three-part mold. The sinusoidal wave was applied to simulate traffic cyclic load through the stress control single-stage loading method, shown in Figure

The curve of axial single-stage loading.

Test plan design.

Confining pressure (kPa) | Moisture content (%) | Frequency (Hz) |
---|---|---|

200 | 3.2 | 0.5 |

300 | 5.2 | 1.0 |

400 | 7.2 | 2.0 |

For saline soils with high salinity, the analysis of mechanical characteristics should not only take into account the particularity of the soil itself, but also the effect of moisture content, frequency, confining pressure, and other factors. In addition, the interaction between multiple factors should not be completely ignored. If there is no interaction between the influencing factors, the effect of a single factor can be studied separately and then superimposed. But if the interaction between multiple factors is obvious, the comprehensive effect of the interaction should be considered. The significance test can be used to study the influence of different factors on the mechanical characteristics of high-salinity soil. In other words, the

For the three factors _{i}, _{j}, and _{k}); the observed value of the test index can be obtained as _{ijk}. The computational method of symbols is expressed in the following equations:

The sum of squares of the total variance is obtained by the following equation:

The sum of squares of the variance of factors

The estimated variances _{A}, _{B}, and _{C} of factors _{A}, _{B}, _{C}, and _{E} are, respectively, the degree of freedom of factors

As for the significant study of three factors, the interaction between

Further, the sum of squares of the variance of interaction effect of factors

_{α} (_{1}, _{2}) was the critical value of

_{α} (_{1}, _{2}).

Significant level | 0.1 | 0.05 | 0.025 | 0.01 | 0.005 | 0.001 |
---|---|---|---|---|---|---|

_{1} = 2, _{2} = 4 | 4.32 | 6.94 | 10.65 | 18.00 | 26.28 | 61.25 |

_{1} = 2, _{2} = 8 | 3.11 | 4.46 | 6.06 | 8.65 | 11.04 | 18.49 |

_{1} = 4, _{2} = 8 | 2.81 | 3.84 | 5.05 | 7.01 | 8.81 | 14.39 |

The maximum dynamic stress and maximum dynamic strain (the vertices of each stress–strain hysteresis loop, Figure

Hysteresis curve and backbone curve.

Dynamic stress–dynamic strain relationship curves. (a) _{3} = 300 kPa, and (c) _{3} = 300 kPa.

The effects of different confining pressures, moisture content, and frequency on stress–strain curves were compared and analyzed. As shown in Figure

The hyperbolic model of equation (_{d} and _{d} are dynamic stress and dynamic strain and

Furthermore, equation (_{d}⟶ +∞ in equation (

The results of the related fitting parameters in the experiment were _{dmax}, as shown in Table

Chlorine saline soil test conditions and the fitting values of backbone curve parameters.

Test group | _{3} (MPa) | _{dmax} (MPa) | ^{2} | |||||
---|---|---|---|---|---|---|---|---|

BYZ1 | 0.2 | 1.0 | 5.2 | 20786.65 | 1.65 | 54.07 | 0.38 | 0.997 |

0.3 | 18695.38 | 2.97 | 25.78 | 0.73 | 0.997 | |||

0.4 | 34231.06 | 3.75 | 35.95 | 0.95 | 0.992 | |||

BYZ2 | 0.3 | 0.5 | 5.2 | 30981.62 | 3.38 | 143.98 | 0.22 | 0.977 |

1.0 | 18695.38 | 2.97 | 25.78 | 0.73 | 0.998 | |||

2.0 | 28604.87 | 2.57 | 39.48 | 0.72 | 0.994 | |||

BYZ3 | 0.3 | 1.0 | 3.2 | 1441.33 | 4.57 | 2.41 | 0.60 | 0.993 |

5.2 | 18695.38 | 2.97 | 25.78 | 0.73 | 0.998 | |||

7.2 | 12695.06 | 4.21 | 19.13 | 0.67 | 0.997 |

According to the different types of dynamic stress–strain curves, the principles for determining the failure strength of soil are also different. From the achieved stress–strain curves, the average value of the dynamic stress after the stress–strain curve stabilizes is taken as the failure strength.

The change curves of failure strength under different influencing factors obtained from the test results were shown in Figure

Variety regularity of failure strength with moisture content and frequency. (a)

As shown in Figure

Figure

Figure

Taking confining pressure, moisture content, and frequency, respectively, as factors

Significance test of failure strength considering interactions.

Source of variance | Sum of squares | Degree of freedom | Significance | |
---|---|---|---|---|

Confining pressure | 682595.48 | 2 | 133.48 | I |

Moisture content | 367.00 | 2 | 0.07 | IV |

Frequency | 1328069.81 | 2 | 259.71 | I |

19176.49 | 4 | 1.88 | IV | |

165960.64 | 4 | 16.23 | I | |

21128.29 | 4 | 2.07 | IV | |

Errors | 20454.99 | 8 | ||

Sums | 2237752.70 | 26 |

According to the significant levels given in Table

The elastic modulus is a key parameter to describe the properties of soil, and it also plays an important role in deformation and stability analysis in geotechnical engineering; then, it is usually acquired in smaller strain. The computational methods of static elastic modulus and dynamic elastic modulus are different. Lee et al. [_{d} is dynamic elastic modulus, _{d} and _{d} are, respectively, axial dynamic stress and axial dynamic strain; the computational method is as shown in equations (

Figure

Variety regularity of dynamic elastic modulus with moisture content and frequency. (a) (

Table

Significance test of dynamic elastic modulus.

Source of variance | Sum of squares | Degree of freedom | Significance | |
---|---|---|---|---|

Confining pressure | 1852.77 | 2 | 36.38 | I |

Moisture content | 371.96 | 2 | 10.95 | II |

Frequency | 1123.09 | 2 | 33.08 | I |

201.41 | 4 | 2.97 | III | |

302.18 | 4 | 4.45 | III | |

657.07 | 4 | 9.68 | II | |

Errors | 118.85 | 8 | ||

Sums | 3855.02 | 26 |

The properties of soil are extremely complex, so the strength of soil should not be simply considered as the strength of mineral particles, but the interaction effect between particles must be considered. Similarly, the shear strength of soil also depends on many factors, which are generally divided into two categories. One is the properties of the soil itself, such as physical properties; the other is the external conditions of the soil, such as stress and strain conditions. In the practical application, the most common theory of shear strength is the Mohr–Coulomb strength criterion with only two parameters: cohesion

The cohesion is mainly provided by physical and chemical forces such as electrostatic force and Van der Waals’ force and is affected by ion concentration, ion valence, and the mineral composition of particles. The friction angle can reflect the mutual movement and bite between particles, such as friction caused by sliding between particles.

According to the results of the unconsolidated undrained triaxial test, take _{d} and dynamic friction angle _{d}.

Figure

Variety regularity of shear strength parameter with water content and frequency. (a) Dynamic cohesion. (b) Dynamic friction angle.

With the increase of moisture content, the pore water filled between soil particles would increase, the electrostatic attraction, Van der Waals’ force and the ion concentration in the soil would decrease, and the electrokinetic potential of the colloid in the soil would increase, resulting in the enhancement of the mutual interaction between the colloids. And the thickness of hydration film on the surface of soil particles increased with the increasing of moisture content. Besides, the salt solution acted as a lubricant in the soil and weakened the bonding force between soil particles. Therefore, the dynamic friction angle and dynamic cohesion of chlorine saline soil decrease with the increase of moisture content, and shear strength also decreased. If loading frequency was too high, the effect of cyclic loading would be inadequate on the soil, and it would have little effect on the biting force and bonding force between soil particles. The soil could resist external deformation. Therefore, the shear strength increased with the increase of loading frequency.

The significance analysis results of the effect of moisture content and frequency on the dynamic cohesion and dynamic friction angle of chlorine saline soil are indicated in Tables

Significant test of cohesion of influencing factors.

Source of variance | Sum of squares | Degree of freedom | Significance | |
---|---|---|---|---|

Moisture content | 887.44 | 2 | 4.10 | IV |

Frequency | 701.97 | 2 | 3.24 | I |

Errors | 432.83 | 4 | ||

Sums | 2022.23 | 8 |

Significant test of the friction angle of influencing factors.

Source of variance | Sum of squares | Degree of freedom | Significance | |
---|---|---|---|---|

Moisture content | 87.85 | 2 | 2.00 | IV |

Frequency | 623.97 | 2 | 14.21 | I |

Errors | 87.83 | 4 | ||

Sums | 799.65 | 8 |

According to the significance test level given in Table

Based on the above analysis, it could be seen that not only single factors such as confining pressure, moisture content, and frequency had a certain effect on the mechanical characteristics of chlorine saline soil, but also the effect of the interaction of multiple factors might be significant. Therefore, it was necessary to consider the effect of a single factor and the interaction between multiple factors on the mechanical characteristics of soil comprehensively, instead of analyzing the independent influence of each single factor only.

The mechanical characteristics of chlorine saline soil affected by a number of factors were studied, and the following conclusions have been drawn:

The dynamic stress–dynamic strain curve of chlorine saline soil was strain-hardening. Although the hyperbolic model was not well fitted, the power function model achieved a quite good fit of over 97%.

The confining pressure, frequency, and the interaction between confining pressure and frequency could significantly affect the failure strength and dynamic elastic modulus. Some discrepancies in the changes of the failure strength and dynamic elastic modulus with factors were observed.

The effect of frequency on dynamic cohesion and dynamic friction angle was more significant, which increased with the increase of frequency. While moisture content had a weaker effect, dynamic cohesion and dynamic friction angle decreased as moisture content increased.

Based on the significance test theory, the effect of confining pressure and frequency on the mechanical properties of chlorine saline soil was significant, but the effect of moisture content was weaker. It could be obtained that the effect of interaction between confining pressure and frequency was significant simultaneously. Therefore, when studying the mechanical characteristics of soil, the interaction between multiple factors needs to be taken into consideration.

The data used in the paper have been uploaded on the Baidu Netdisk (

The authors declare that they have no conflicts of interest regarding the publication of this paper.

This research was supported by the National Natural Science Foundation (NSFC) under Grant no. 41471052 and the Science and Technology Department Project of Qinghai Province under Grant no. 2021-ZJ-908.