Dynamic Nonlinear and Residual Deformation Behaviors of the Fly Ash-Modified Loess

Metastable loess soils can deform, inducing geological and engineering disasters. Therefore, the behavior of the loess under dynamic load is gaining massive attention from researchers to improve the strength of the soils. Fly ash mixed with loess can improve strength and reduce construction costs and environmental pollution. Moreover, it has strong economic and social benefits. This paper investigates the influence of fly ash on the dynamic properties of the modified loess through a series of dynamic triaxial tests of the fly ash modified loess with different fly ash contents. The treated soil samples were prepared using a static compaction method in both ends and cured for 28 days. The dynamic shear modulus ratio, the damping ratio, and the dynamic residual strain of the modified loess were analyzed. The variation characteristics of the dynamic shear modulus ratio and damping ratio with the dynamic shear strain of the fly ash modified loess were obtained. The effect of fly ash content on the dynamic nonlinear parameters of the modified loess was also investigated. In addition, the relationship between the dynamic residual strain and the fly ash content was discussed. The results show that the dynamic shear modulus ratio of fly ash modified loess decreases nonlinearly with the increase in the dynamic shear strain. However, the attenuation rate difference of the curves is small. The damping ratio increases gradually with increasing dynamic shear strain. Under a certain dynamic shear strain level, the damping ratio decreases with the increase in the fly ash content. The dynamic residual strain increases with the increase in the dynamic stress. However, it decreases with the increase in the fly ash content. When the fly ash content is between 10% and 20%, the dynamic residual strain of fly ash modified loess is reduced rapidly. However, when the fly ash content exceeds 20%, the dynamic residual strain decreases slowly. The fly ash content of 20% could be suggested as an optimal content for seismic resistance of the loess foundation.

Development of a machine vision system for recording experimental data in the study of torsional stiffness by dynamic mechanical analysis

A machine vision system has been developed for recording data obtained on an experimental installation for studying relaxation transitions of polymers, as well as studying the dynamic shear modulus during phase transformations in alloys based on titanium nickelide. The computer vision system is based on simple algorithms implemented in the Java language in the IDE “Processing”. The conditions for the experiment and the procedure for processing the experimental data obtained on the basis of the machine vision system are determined.

Empirical Formulas of Shear Modulus and Damping Ratio for Geopolymer-Stabilized Coarse-Grained Soils

The contribution of gravel fraction on the maximum shear modulus (Gmax), dynamic shear modulus ratio (G/Gmax), and damping ratio (λ) of cementitious coarse-grained soils has not been fully understood yet. Large-scale triaxial cyclic tests for geopolymer-stabilized coarse-grained soils (GSCGSs) were conducted with different volumetric block proportions (VBPs) under various confining pressures (CPs) for investigating their dynamic behaviors and energy dissipation mechanisms. Results indicate that the Gmax of GSCGS increases linearly with VBPs but nonlinearly with CP. High VBPs will probably result in a gentle decrease in G/Gmax and a rapid increase in normalized λ (λnor), while the opposite is the case for a high CP. With the shear strain amplitude being normalized, the G/Gmax and λnor are distributed in a narrow band with low dispersion and thus can be well-described by empirical functions of the normalized shear strain amplitude.

Experimental Study of the Engineering Mechanical Properties of the Foundation Soil for Offshore Wind Power Platforms

Most of the coastal beach zone in the world is rich in wind energy reserves and has great potential for offshore wind power development. However, the sedimentary environment in the coastal area is complex and changeable, and the nature of the foundation soil of offshore wind power platforms is weak and complex, which is quite different from that in the land areas. In order to systematically study the mechanical properties of marine foundation soils, a series of geotechnical tests are carried out on representative undisturbed seabed soils, such as basic laboratory geotechnical tests, bender element tests, undrained triaxial shear tests, and resonance column tests. The test results show that shear wave velocity (Vs) of marine silt and silty clay increases linearly with the buried depth; the stress-strain relationship curves of silty clay and silt present two different modes of development: strain hardening and strain softening, the undrained shear strength (Sd) of the two types of marine soils decreases with the increase of the void ratio (e), and both present a good single correlation. Based on the relationship between Sd and Vs from the laboratory test of disturbed seabed soils, an undrained strength evaluation method of undisturbed seabed soils under the current stratum conditions incorporating in situ shear wave velocity is established. The dynamic shear modulus (G) in the various strain ranges of undisturbed silty clay and silt increases regularly with the buried depth (H). Meanwhile, the maximum dynamic shear modulus (Gmax) linearly increases with the increase of H, whereas the attenuation relationship of G decreases with the increase of H. The prediction method of G based on buried depth is established with high accuracy.

Experimental Study on the Dynamic Modulus and Damping Ratio of Compacted Loess under Circular Dynamic Stress Paths

Dynamic loads such as earthquakes and traffic will simultaneously generate vertical dynamic stress and horizontal shear stress in the foundation soil. When the vertical dynamic stress amplitude is twice the horizontal shear dynamic stress amplitude, and the phase difference between them is 90°, a circular dynamic stress path is formed in the τ z θ d ∼ σ zd − σ θ d / 2 stress coordinate system. To simulate the stress state of soil in the area of the circular dynamic stress path caused by bidirectional dynamic stress coupling, a series of tests of compacted loess under the action of a circular dynamic stress path were carried out using a hollow cylindrical torsion shear apparatus. The effects of the mean principal stress, dry density, and deviatoric stress ratio (the ratio of deviator stress to average principal stress) on the dynamic modulus and damping ratio of compacted loess were mainly studied. The test results show that, under the action of the circular dynamic stress path, the larger the mean principal stress is, the larger the dynamic compression modulus and dynamic shear modulus are. The dynamic compression modulus increases obviously with increasing dry density, but the dynamic shear modulus increases only slightly. When the deviator stress ratio increases from 0 to 0.4, the dynamic compression modulus and dynamic shear modulus increase to a certain extent. In addition, the greater the dry density and deviatoric stress ratio are, the greater the initial dynamic compression modulus and initial dynamic shear modulus of the compacted loess. The dynamic compression damping ratio of compacted loess increases with increasing mean principal stress, but the dynamic shear damping ratio decreases with increasing mean principal stress. Dry density basically has no effect on the dynamic compression damping ratio and dynamic shear damping ratio of compacted loess. When the dynamic strain exceeds 1%, the greater the deviatoric stress ratio is, the smaller the dynamic compression damping ratio and the dynamic shear damping ratio are. The research results can provide reference for the study of dynamic modulus and damping ratio of loess under special stress paths.

Experimental Study of the Dynamic Shear Modulus Ratio and Damping Ratio of the Quaternary Sedimentary Soils in the Offshore Areas of the Yellow Sea

The effects of marine and continental sedimentary environments and geological ages on the dynamic shear modulus ratio and damping ratio of the Quaternary sedimentary soils in the offshore areas of the Yellow Sea were analyzed by using a resonant column device (GCTS, USA). The results show the following: (1) The G max of various marine soils increases with the depth and shows a typical linear relationship. (2) The marine transgression has significantly different effects on the dynamic shear modulus ratio versus the shear strain amplitude curves (i.e., G / G max ~ γ a curves) and the damping ratio versus the shear strain amplitude curves (i.e., λ ~ γ a curves) of the different soil types in the offshore areas of the Yellow Sea. The effects of marine transgression were strong on clays, moderate on silty clays, and minor on silts. (3) The geological ages have noticeable effects on the G / G max ~ γ a curves of the tested marine silty clays, marine silts, and continental silty clays, but the effects of geological ages on the λ ~ γ a curves are minimal. The fitting parameters and recommended empirical equations of the G / G max ~ γ a and λ ~ γ a curves for each type of the tested soils (silty clay, clay, and silt) were obtained mirroring the effects of sedimentary environments and geological ages.

EXPERIMENTAL STUDY ON THE DYNAMIC SHEAR MODULUS AND DAMPING RATIO OF SATURATED SAND UNDER CYCLIC LOADING

Initial shear stress is inevitable in actual engineering slopes, subgrades and foundations, and soils exhibit different dynamic characteristics under the influence of initial shear stress. Using a dynamic triaxial test system, this study explores the dynamic shear modulus and damping ratio of saturated sand from Wenchuan, investigates the effects of the initial shear stress and vibration frequencies on the dynamic shear modulus and damping ratio of saturated sand and proposes a normalised dynamic shear modulus formula that considers the initial shear stress and vibration frequency. Results show a threshold dynamic shear strain of the saturated sand. When the dynamic shear strain is below this threshold, the dynamic shear modulus significantly increases with the initial shear stress and vibration frequency. Otherwise, the influence of the initial shear stress and vibration frequency gradually decreases and eventually stabilises. The initial shear stress significantly affects the normalised dynamic shear modulus/strain curves where a larger initial shear stress corresponds to a higher curve. Meanwhile, the vibration frequency only exerts a slight influence. The curves under different frequencies are generally within the same band and fall near the Seed upper envelope. The initial shear stress also has a significant influence on the damping ratio where a larger initial shear stress corresponds to a smaller damping ratio. On the basis of the experimental results, a normalised dynamic shear modulus/shear strain formula that considers the influence of the initial shear stress and vibration frequency is established. Fitting results indicate that this formula shows good agreement with the test data.

Normalized Shear Modulus and Damping Ratio of Soil–Rock Mixtures With Different Volumetric Block Proportions

The volume fraction of rock blocks plays a particularly significant role in static/dynamic shear behaviors of soil–rock mixtures (SRM). Large-scale cyclic triaxial tests for SRM with different volumetric block proportions (VBPs) were performed at different confining pressures to investigate the reduction of dynamic shear modulus (G) and the increase of damping ratio (λ). Results indicate that VBP has a significant effect on the dynamic behaviors of SRM. The higher VBP is more likely to result in a gentler reduction of G and a faster increase of λ. The variations of dynamic shear modulus ratio (G/G0) and normalized damping ratio (λnor) fall within relatively narrow bands but are very different with gravelly soils and sands due to VBP with particle size larger than 2 mm. The G/G0 and λnor can be characterized by empirical functions about normalized shear strain amplitude (γnor).

Structural features of composite materials made of hydrolyzed birch wood

Structural features and physical and mechanical characteristics of plate composite materials are investigated. The materials are obtained from hydrolyzed birch wood by hot pressing without the addition of binding components. Wood processing is carried out by the method of explosive autohydrolysis without chemical reagents. The influence of pre-moistening of wood on the structure and properties of the composite material is studied. The structural features of the amorphous and crystalline components of the composite material are studied. It was found that the composite material obtained from pre-dried and pre-moistened wood retains the crystalline phase that is present in the original wood. Changes in the structure of wood when obtaining composite materials based on it occur in the amorphous component. Based on the temperature dependences of the dynamic shear modulus and the tangent of the angle of mechanical losses, information on the glass transition temperature of a complex of amorphous components of a composite material is obtained. It was found that the region of transition of lignin and hemicellulose macrochains from a glassy to a highly elastic state in the composite material is shifted towards low temperatures in comparison with the original wood. The offset is more than 70K. It is assumed that structural plasticization is the main cause of the detected effect. Pre-moistening of wood does not affect the position of the temperature transition in the amorphous component of the composite material. The study of the diffusion and sorption of water vapor in the samples of the material shows the presence of large structural inhomogeneities. Diffusion processes obey Fick’s second law and correlate with the density of samples. Data on density, static bending strength, water absorption and swelling characteristics of composite material samples were obtained. It is shown that the use of pre-moistening of wood before barothermal treatment significantly improves the structural uniformity of the resulting material. The value of the dynamic shear modulus at room temperature in comparison with the same indicator for the material obtained on the basis of dry wood increases three times. Mechanical losses are reduced, mechanical strength increases.