Shear Strength Deterioration of Compacted Residual Soils under a Wind Turbine due to Drying-Wetting Cycles and Vibrations

The soil beneath a wind turbine withstands not only environmental impacts but also continuous vibrations transmitted from the superstructure. This paper presents an experimental study of the deterioration characteristics of shear strengths of residual soils affected by drying-wetting cycles and continuous vibrations. A series of triaxial tests were performed on compacted residual soil specimens after various drying-wetting cycles and vibrations. The influences of drying-wetting cycles and vibrations on the shear strengths of residual soils with different compaction degrees were analyzed. The results demonstrate that the shear strength and cohesion of compacted residual soils decreased as the number of drying-wetting cycles increased, and they tended to be stable after three drying-wetting cycles. The angle of internal friction decreased linearly with the reduction of compaction degree but was generally not affected by drying-wetting cycles. The shear strength of compacted residual soils also decreased because of continuous vibrations. After 10000 vibrations, the strength was stabilized gradually. Both the cohesion and angle of internal friction showed dynamic attenuation phenomenon. Finally, a modified Mohr–Coulomb strength equation considering the effects of drying-wetting cycles and vibrations was established. This equation could be used to predict the shear strength of compacted residual soils and further estimate the embedded depth of wind turbine foundations.

Strength Deterioration Mechanism of Bentonite Modified Loess After Wetting- Drying Cycles

The employment of bentonite modified loess (BML) is a common method of constructing the anti-seepage lining of landfills in the loess region of China, and its long-term secure performance is threatened by wetting-drying (W-D) cycles. Taking the remolded loess (RL) and BML with 15% in mass of bentonite as research objects, the W-D cycles test, scanning electron microscope (SEM) test and direct shear test were carried out to analyze the effects of W-D cycles on the microstructure and shear strength of samples. The regression equations between strength and micro-pore structure parameters were established by the multivariate linear stepwise regression method. The damage mechanism of BML after W-D cycles was studied by establishing damage degree models based on pore area ratio and cohesion. Results indicate that the water absorption and expansion of bentonite effectively block the intergranular pores, resulting in more medium and small pores and more pronounced surface contact of particles. After W-D cycles, the particle arrangement of samples before and after bentonite modification tends to be loose. Both the pore area ratio and fractal dimension increase and tend to stabilize after five cycles. The BML exhibits lower pore area ratio and greater fractal dimension while its cohesion and internal friction angle show more significant decrease after W-D cycles than those of RL. The damage variables based on pore area ratio and cohesion well describe the W-D induced damage of loess before and after modification from macro- and micro-scale perspectives. The damage degree of samples increases with W-D cycles, but the increment decreases.

Simplified calculation of flexural strength deterioration of reinforced concrete T-beams exposed to ISO 834 standard fire

Reinforced concrete (RC) T-shaped cross-section beam (so-called T-beam) is a common structural member in buildings where beams and slabs are monolithically cast together. In this paper, a simplified calculation method based on Russian design standard SP 468.1325800.2019 is introduced to determine the flexural strength of RC T-beams when exposed to ISO 834 standard fire. The idea of 500oC isotherm method, which is stipulated in both Eurocodes (EC2-1.2) and SP 468, is applied associated with specifications of temperature distribution on T-beams’ cross sections and the temperature-dependent mechanical properties of concrete and reinforcing steel. A case study is conducted to explicitly calculate the flexural strength deterioration (FSD) of T-beams compared to that at ambient temperature. A calculation sheet is established for parametric studies, from which the results show that the FSD factor of RC T-beams is adversely proportional to the dimensions of the beam’s web and flange. However, the effect of these components of T-beams is not significant.

Strength Deterioration Mechanism of Bentonite Modified Loess After Wetting-Drying Cycles

The employment of bentonite modified loess (BML) is a common method of constructing the anti-seepage lining of landfills in the loess region of China, and its long-term secure performance is threatened by wetting-drying (W-D) cycles. Taking the remolded loess (RL) and BML with 15% in mass of bentonite as research objects, the W-D cycles test, scanning electron microscope (SEM) test and direct shear test were carried out to analyze the effects of W-D cycles on the microstructure and shear strength of samples. The regression equations between strength and micro-pore structure parameters were established by multivariate linear stepwise regression method. The damage mechanism of BML after W-D cycles was studied by establishing damage degree models based on porosity and cohesion. Results indicate that clay minerals such as montmorillonite in BML absorb water and expand to fill the macropores, resulting in more medium and small pores and more pronounced surface contact of particles. After W-D cycles, the particle arrangement of samples before and after bentonite modification tends to be loose. Both the porosity and fractal dimension increase and tend to stabilize after five cycles. The BML exhibits lower porosity and greater fractal dimension while its cohesion and internal friction angle show more significant decrease after W-D cycles than those of RL. The damage variables based on porosity and cohesion well describe the W-D induced damage of loess before and after modification from macro- and micro-scale perspectives. The damage degree of samples increases with W-D cycles, but the increment decreases.

Influencing Factors and Prediction Model for the Antierosion Performance of Cement-Improved Loess Compacted Using Different Compaction Methods

To analyze the antierosion performance of cement-improved loess (CIL), several influencing factors have been investigated based on two different compaction methods, which include the quasi-static compaction method (QSCM) and the vertical vibration compaction method (VVCM). Then, a prediction model for the cumulative erosion mass loss (CEML) has been established. The effects of erosion on the strength deterioration of CIL were also studied. The results show that, compared with QSCM, specimens compacted using the VVCM have better antierosion performance. As the cement content and the compaction coefficient are increased by 1%, the antierosion performance is increased by 16% and 6.2%, respectively. The eroding time has a significant effect on the antierosion performance of CIL, and the CEML increases linearly with an increase in the eroding time. The compressive strength of CIL decreases significantly due to erosion, and based on the average deterioration degree of the specimens, the design criteria for strength of CIL are proposed, which can provide reference for the design of CIL.