Predicting Vp and constrained modulus reduction curve based on Vs and shear modulus reduction curve in accordance with poroelastic theory

The compression wave velocity (Vp) of sediments plays a key role in seismic wave amplification of vertical motion and is required in site response analysis. However, such information is usually lacking during field exploration (e.g., surface wave method) because only shear wave velocity (Vs) is obtained. This study aims to predict Vp based on Vs empirically and theoretically, especially focusing on saturated conditions. The empirical approach is to establish the Vp correlation dependency on Poisson's ratio and Vs, and the theoretical approach is based on poroelastic theory that accounts for the interaction between fluid and soil skeleton. The Engineering Geological Database for the Taiwan Strong Motion Instrumentation Program and the Kiban Kyoshin Network database in Japan are adopted to establish an empirical model and validate poroelastic theory. The validated poroelastic approach is used to develop a constrained modulus reduction curve dependency on the porosity, Vs, Poisson's ratio, and degree of saturation with a shear modulus reduction curve. The proposed approach can be used to develop generic Vp profiles and constrained modulus reduction curves for the site response to vertical motion given a site specific Vs profile.

Normalized Modulus Reduction and Damping Ratio Curves for Bay of Campeche Calcareous Clay to Carbonate Mud

Equations to calculate the modulus reduction curve (G/Gmax-γ) and material damping ratio curve (D-γ) of calcareous clay and clayey carbonate mud of the Bay of Campeche and Tabasco Coastline are developed. This was achieved using a database of 156 resonant column tests and 468 strain-controlled cyclic direct simple shear tests performed in clays with 10 % ≤ CaCO3 ≤90 %. The effects of carbonate content (CaCO3), mean effective confining pressure (σ′m), plasticity index (PI), and overconsolidation ratio (OCR) on the shape of the modulus reduction and material damping ratio curves are shown based on the available laboratory data and the equations developed to calculate these curves. It is shown that as CaCO3 increases, the normalized shear modulus (G/Gmax) curve tends to shift downward and the damping ratio (D) curve tends to shift upward; as σ′m and PI increase, the G/Gmax curve tends to shift upward and the damping ratio curve tends to shift downward; and the value of OCR has practically no effect on the position of the curves. The validation of the calculated values of G/Gmax and D shows the best predictions are found at low shear strains for G/Gmax and at large shear strains for D, falling within ± 25 % of the measured values, and shows that due to limitations in the model at large strains (γ > 1 %) for G/Gmax and at low strains (γ < 0.05 %) for D, the calculated values fall within ± 50 % of the measured values. The equations developed to calculate the curves of G/Gmax-γ and D-γ of calcareous clay and clayey carbonate mud are recommended for preliminary or perhaps even final seismic site response evaluations. However, considering the scatter of the data points around the curves, the equations should be used with caution, and parametric and sensitivity studies are strongly recommended to assess the importance of this scatter. In large critical projects, direct experimental determinations of G/Gmax and D for the soils of interest are suggested to be more appropriate.

Synergy between Phenoxy and CSR Tougheners on the Fracture Toughness of Highly Cross-Linked Epoxy-Based Composites

A remarkable synergistic increase in fracture toughness by 130% is demonstrated for a CFRP high performance epoxy composite when adding an equal weight combination of phenoxy thermoplastic and core-shell rubber (CSR) toughening agents, as compared to a single toughener at a comparable total concentration of around 10 wt%. The dual-toughened matrix exhibits an unusual morphological arrangement of the two toughener agents. The interlaminar shear strength of the composites is also synergistically improved by about 75% as compared to the reference while the compression modulus reduction and viscosity increase are significantly smaller than for the single phenoxy toughened system. A partial filtering of the CSR particles by the dense CF fabric during pre-pregging leads to a less than optimum CSR dispersion in the composites, showing that the synergy can be further optimized, possibly to the same level as the unreinforced systems.

Stress-independent parameters for stress-strain relationship and damping in nonlinear one-dimensional seismic site response analysis

The modulus reduction and damping curves represent the nonlinear behavior of soil under cyclic load. In the literature, those curves were produced from lab tests of soil at particular confining stresses. This study developed a set of parameters that can be used to normalize the modulus reduction and damping curves to be stress-independent. The proposed formulations for the stress-independent parameters were implemented in the finite element code SRAP and validated through producing shear modulus reduction and damping curves that match the existed ones. Nonlinear 1D seismic site response analyses were conducted for centrifuge experiments to verify the developed computer code. Comparisons of the analysis results between SRAP and another computer code were presented in terms of maximum and minimum displacement, peak ground acceleration, maximum shear strain profiles, and response spectra. Keywords: backbone curve; hysteretic damping; dynamic soil model; stress-independent parameters; finite element method; nonlinear 1D seismic site response analysis.

A Simplified Approach to Estimating the Collapsible Behavior of Loess

Due to the particularity and complexity of loess, it is challenging to estimate its collapsible behavior numerically at present. This paper aims to propose a simplified approach, which is named as the modulus reduction method, to estimate the collapsible behavior of loess. For loess upon wetting, the modulus reduction method assumes that loess collapses as a result of strength reduction due to the additional stress induced by increasing bulk density. Thus, special attention is given to the confirmation and determination approaches of bulk density and deformation modulus of loess upon wetting. Subsequently, a comparative numerical analysis based on the modulus reduction method and the force-water equivalent method, which is commonly used for the analysis of negative skin friction on piles in collapsible soil, is investigated. It turns out that the result obtained by the modulus reduction method is more consistent with the collapse mechanism of loess compared with that derived by the force-water equivalent method. Finally, a case history concerning a published field test of loess upon wetting is studied, and the result shows that the simulated deformation characteristics by adopting the modulus reduction method agree excellently well with the measured data. The case study validates that the modulus reduction method is feasible to analyze the collapse of loess and suitable for the numerical simulation involving collapsible loess.

Effect of Nonlinear Soil Model on Seismic Response of Slopes Composed of Granular Soil

A series of two-dimensional finite element analyses are performed to simulate the seismic response of slope composed of granular soil. Four sets of input parameters for the nonlinear soil model are used to fit the reference the shear modulus reduction and damping curves, thereby to evaluate the influence of the nonlinear soil model. The first set is fitted to the shear modulus reduction curve. The second and third sets are fitted simultaneously to both shear modulus reduction and damping curves. The final set applied the shear strength adjustment to adequately capture the nonlinear soil response at large strains. The accuracy of each set of parameters are evaluated through comparison with centrifuge model test measurements. It is observed that the nonlinear soil model has a marginal influence on the acceleration response. On the contrary, the vertical settlement is highly influenced by the nonlinear soil model. The discrepancy is shown to increase with an increase in the intensity of the input ground motion. It is demonstrated that the adjustment for the shear strength is important in performing seismic analyses of slopes, which is most often ignored in practice. Based on the results, practical guidelines on how to select the parameters for the nonlinear soil model are provided.