^{1}

^{2}

^{1}

^{2}

^{3}

^{4}

^{1}

^{2}

^{1}

^{2}

^{3}

^{4}

Small-strain shear modulus,

Soft clay is found in coastal plains, estuary deltas, lake basins around, mountain valleys, and other areas, most of which belong to the middle or late Fourth Age. Figure

Clay content of China [

For soft clay, soil destruction is a complex and cumulative process defined by strain as a standard of destruction (in general, it is 5 or 10%) [

The undrained triaxial tests were performed by Shirley and Hampton [

Many indoor tests have been carried out and widely used to study the stress-strain properties of soil at small shear strain levels/amplitudes (

In spite of many studies dealing with the characterization of soft clay, the change law of

To address the above problems, we first prepared the soft clay samples and measured the physical indexes (Section

Soft clay in the test was derived from the Chashan Higher Education Park, Wenzhou University. Soil distribution maps within China and in Zhejiang Province are depicted as Figure

Soil distribution maps within China and in Zhejiang Province [

The stratigraphic map of Wenzhou, Zhejiang Province [

The soil depth is 5-7 m, which is typical for the marine soft clay on the southeast coast of China [^{3}/h as the standard. The mechanical parameters of soft clay are given in Table

Mechanical parameters and values [

Soil parameter | ^{3}) | ||||||
---|---|---|---|---|---|---|---|

Value ranges | 1.7~1.75 | 42~50 | 26 | 57 | 31 | 54.3 | 94.6 |

The tests are carried out using a combination of cyclic triaxial test system and piezoelectric ceramic bender element system developed by GDS, UK [

GDS dynamic triaxial test system (a) and bending element test system (b).

The test items and accuracy of the dynamic triaxial test system.

No. | Test items | Range value | Accuracy |
---|---|---|---|

1 | Axial displacement | 100 mm | Measurement resolution reaches 0.08 |

2 | Axial stress | 2 kPa | Accuracy can reach 0.1%, that minimum resolution is 0.2 kPa |

3 | Confining stress range | 3 MPa | Accuracy reaches 1 kPa |

4 | Back pressure range | 2 MPa | Accuracy reaches 1 kPa |

The test control and data recording of the dynamic triaxial test system are recorded in real time by the GDSLAB software, which can achieve the conventional triaxial compression test, the stress-controlled dynamic triaxial test, and the strain-controlled dynamic triaxial test. The method of bending element test is a simple and intuitive method with a clear principle. It is noted that the combination device used in this study can improve the accuracy of the wave velocity. Soil is a particle material and has a variety of contact forces-deformation behavior. When a point in the soil is stressed by the release of energy, a small strain will spread in the condition of elastic waves. The velocity of elastic wave depends on the elastic modulus of the soil. Therefore, if it can be measured for the small-strain elastic wave and velocity, the elastic parameters of the soil can be determined.

Figure

Working principle of shear wave velocity tested by bending element [

Dynamic tests are performed using two methods: (1) Stress control: a certain number of constant amplitude cyclic load stress are imposed by the bending element system to test

The test scheme design.

No. | CSR | Cyclic strain (%) | ||
---|---|---|---|---|

1 | 50 | 0.02 | 0.476 | — |

2 | 50 | 0.02 | 0.551 | — |

3 | 50 | 0.02 | 0.625 | — |

4 | 100 | 0.02 | 0.489 | — |

5 | 100 | 0.02 | 0.516 | — |

6 | 100 | 0.02 | 0.571 | — |

7 | 100 | 0.02 | 0.625 | — |

8 | 200 | 0.02 | 0.362 | — |

9 | 200 | 0.02 | 0.392 | — |

10 | 200 | 0.02 | 0.447 | — |

11 | 200 | 0.02 | 0.517 | — |

12 | 300 | 0.02 | 0.432 | — |

13 | 300 | 0.02 | 0.472 | — |

14 | 300 | 0.02 | 0.569 | — |

15 | 100 | 0.02 | — | 0.5 |

16 | 100 | 0.02 | — | 1 |

17 | 100 | 0.02 | — | 2 |

18 | 200 | 0.02 | — | 0.75 |

19 | 200 | 0.02 | — | 2 |

20 | 200 | 0.02 | — | 3 |

Because of its structure, expectedly, intact soft clay has a nonlinear feature between its strength and confining stress. Moreover, dynamic characteristic of soft clay varied significantly under the same confining stress. To analyze the dynamic stress levels and characteristics under different confining stresses, it was expressed by the CSR in the test, i.e.,

Curves of biaxial cycling obtained from the triaxial test results: strain development curve with cycle time (a) and a typical hysteresis (b).

Figure

Strain and pore pressure curve under 200 kPa confining stress: strain curve (a) and pore pressure (b).

For the confining stress of 100 kPa and 200 kPa, the curves of dynamic strain and different CSRs are illustrated in Figure

Curves of dynamic strain and different cyclic stress ratios: 100 kPa (a); 200 kPa (b).

Figure

Pore pressure development curves when confining stress is 100 kPa and 200 kPa.

When the confining stress is 200 kPa, stress-strain hysteresis loops of four different cyclic stress ratios (0.362, 0.392, 0.447, and 0.512) are shown in Figure

Stress-strain hysteresis loops at different CSRs when confining stress is 200 kPa.

One can observe three notable features in Figure

The attenuation of

The change of

As can be seen in Figure

At 100 kPa, 200 kPa, and 300 kPa, the law of the effect of cyclic loading history on

Effect of cyclic loading history on

As can be seen,

The normalized

Determined failure criterion area of intact soft clay based on

Failure criteria on the basis of double-amplitude dynamic strain.

By applying the cyclic loading, we found that the turning point of the double-amplitude strain of soft clay structure destruction was not constant, but it was closely related to the load amplitude. The double-amplitude strain value will increase with the increase of dynamic stress amplitude. The greater the dynamic stress amplitude, the earlier the corresponding turning point occurs. Moreover, these turning points are basically distributed on a straight line. The test results were in agreement with References [

From Figure

Therefore, we concluded that making use of

In this paper, the dynamic triaxial joint bending element test and the resonance column test were conducted for saturated soft clay in the southeast coast of China to investigate the interaction between

The larger the CSR, the smaller the effective stress reduction, and the earlier the occurrence of exponential growth. The increase in pore pressure is consistent with the trend of strain development. The failure points are not the same for different CSRs. The larger the CSRs, the earlier the occurrence of the failure point

The rectangular area of failure points was drawn in this paper, which it defined by normalized effective stress

A short pause causes a small increase of 1-2% in

Small-strain shear modulus

Initial small-strain shear modulus

Cyclic stress ratio

Overconsolidation ratio

Variable confining stress

Density of soft clay

Water content

Void ratio

Plastic limit

Liquid limit

Plasticity index;

Clay content

Fine content

Axial strain

Deviatoric stress

Peak deviatoric stress

Effective major principal stress

Effective minor principal stress

Effective confining stress

Effective mean confining stress

Initial effective mean confining stress

Pore pressure coefficient

Initial confining pressure

The numbers of the loop loaded

Vibration corresponding to the turning point

Imposed dynamic stress

Cumulative strain

Total axial strain

Rebound strain

Double amplitude of cyclic axial strain

Strain value that corresponds to the turning point of the strain

Pore water pressure

Excess pore water pressure

Excess pore water pressure

Elapsed time (s)

Any two time nodes in the secondary consolidation phase after the completion of the main solidification

The modulus changes from

Shear modulus measured when the confining stress is applied 1000 min

Expression of long-term time effects defined by modulus changes to time ratios

Expression of long-term time effects defined by the relationship between normalized modulus and time

Normalized effective stress

Normalized shear modulus.

All data supporting this study are included within the article.

The authors declare no conflict of interest.

This research was financially supported by the National Basic Research Program of China (973 Program) (Grant No. 2015CB057903), Fundamental Research Funds for the Central Universities, and Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant Nos. 2019B74214 and SJKY19_04533). The authors express their sincere thanks for the above financial supports.