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Liquefaction can significantly alter the ground response. However, no existing design spectrum accounts for the severity of soil liquefaction. This work aims to develop correction factors that can be used to adjust code-based design spectra to reflect the specific liquefaction susceptibility of a site. The correction factor is derived as the ratio of response spectra calculated by two types of 1D nonlinear site response analyses: effective stress analysis, which can model porewater pressure (PWP) generation, and total stress analysis. We considered seven real profiles and 200 motions in our analysis. Four combinations of soil nonlinear models and PWP generation models are also utilized to account for epistemic uncertainties. Results show that the response spectral ratio for liquefied sites typically falls below one for periods less than 1–2 s and rises above one for longer periods. Meanwhile, the response spectral ratio reflects the overall liquefaction susceptibility influenced by PWP, factor of safety, and liquefiable layer depth, while the liquefaction potential index (LPI) captures their complex interplay. Accordingly, we propose four LPI-dependent factors: three correction factors for peak ground acceleration, 0.2 s spectral acceleration (Sa), and 1.0 s Sa, and a long-period adjustment factor applicable for periods exceeding 1 s. The correction factors linearly decrease with increasing LPI, while the adjustment factor exhibits the opposite trend. A design spectrum for a liquefiable site can be readily constructed by adjusting the code-based design spectrum using the proposed correction factor, as illustrated in the example. This approach is applicable as long as LPI is available from a simplified liquefaction analysis or a liquefaction hazard map.

After the occurrence of a large earthquake, engineering seismologists are often requested to estimate strong ground motions at a site where strong motion data were not obtained. The goal of this study was to test the ability of a class of methods that uses Fourier phase characteristics for the post-earthquake ground motion estimation, making use of the precious opportunity provide by the ESG6 Blind Prediction Steps 2&3. It was also part of the goal of this study to test the performance of the effective stress analyses to account for soil nonlinearity. In addition to the dataset provided by the organizer of the blind prediction, the author used additional accelerometric data from a nearby JMA site. To simulate ground motions for an M5.9 earthquake at the target site “KUMA”, the Fourier amplitude spectrum was estimated from the spectral ratio between KUMA and the nearby JMA site. The Fourier phase spectrum was approximated by the spectrum of another event at KUMA. Comparison between the estimated and recorded ground motions after the blind prediction revealed that the estimated ground motions were fairly consistent with the observed ground motions, indicating the effectiveness of the method when the rupture process of the target event is simple and the soil nonlinearity at the target site is not significant. To simulate ground motions at KUMA for the M6.5 foreshock and the M7.3 mainshock of the 2016 Kumamoto earthquake sequence, the author conducted effective stress analyses using a program called “FLIP” to account for soil nonlinearity. Comparison between the estimated and recorded ground motions after the blind prediction indicated that the low-frequency components were overestimated and the high-frequency components were underestimated. The strong soil nonlinearity considered in the effective stress analyses was the main cause of the discrepancy. One explanation for this result could be that the nonlinear soil behavior at KUMA during the foreshock and the mainshock was not a strong one. Another explanation could be that the effect of soil nonlinearity was already included in the records at JMA and the effect of soil nonlinearity was double counted in the results submitted by the author.

This article presents a comprehensive study of dynamic soil properties [namely, initial shear modulus-Gmax; normalized shear modulus reduction (G/Gmax); and damping ratio (D) variation curves] and pore water pressure parameters of a river bed sand (Brahmaputra sand), sampled from a highly active seismic region (northeast India). Two independent high quality apparatus (resonant column-RC and cyclic triaxial-CTX) are adopted in the study. Resonant column apparatus was used to obtain the small strain properties (up to 0.1%) while CTX equipment was adopted to obtain the high strain properties along with the pore water pressure parameters. The results obtained from both the equipment are combined to provide a comprehensive data of dynamic soil properties over wide range of strains. A modified hyperbolic formulation was suggested for efficient simulation of G/Gmax and D variations with shear strain. Based on the CTX results, a pore water pressure generation model is presented. Furthermore, a nonlinear effective stress ground response study incorporating the pore water pressure generation, is performed using the recorded earthquake motions of varying peak bed rock acceleration (PBRA) in the region, to demonstrate the applicability of proposed dynamic soil properties and pore pressure parameters. High amplification for low PBRA ground motions (< 0.10 g) was observed and attenuation of seismic waves was witnessed beyond a PBRA of 0.10 g near the surficial stratum due to the induced high strains and the resulting high hysteretic damping of the soil. Also, increased excess pore pressure generation with increased PBRA of the input motion was observed and the considered soil stratum is expected to liquefy beyond a PBRA of 0.1 g. The established properties can be handy to the design engineers during seismic design of structures in the northeast Indian region.

Economic gas production from unconventional shale-gas reservoirs requires hydraulic fracturing to reconnect and open existing fractures which are lower than the actual fracture gradient. Estimating sensitive parameters of fractured sweet spots such as effective stress parameter and fracture properties from azimuthal seismic amplitude data can be useful to optimize the development of shale-gas fractured reservoirs. Based on the anisotropic Gassmann’s equation and the relationship between porosity and effective stress, we propose a simplified expression of the saturated stiffness tensor for a horizontal transverse isotropic (HTI) model formed by a single rotationally symmetric set of vertical fractures. Using the perturbation in the stiffness matrix, we derive a linearized reflection coefficient as a function of effective stress parameter and fracture parameters and corresponding Fourier series expression. The sensitivity analysis of Fourier coefficients (FCs) shows that the zeroth FC is sensitive to effective stress parameter, and the second FC is sensitive to fracture parameters. Thus, we propose to estimate the effective stress parameter and fracture weaknesses using a three-step azimuthal FCs inversion involving 1) estimating the FCs using azimuthal seismic data, 2) estimating the effective stress parameter using the zeroth FC, and 3) estimating the normal and tangential fracture weaknesses using the second FC. A synthetic seismic data example reveals that the effective stress parameter and fracture parameters can be reliably estimated even with moderate noise. Test on a real data set implies that the proposed inversion method can generate meaningful results that are useful for identifying abnormal pressure and fractures in reservoirs.Article HighlightsPP-wave reflection coefficient and the corresponding Fourier series expression are derived in terms of effective stress parameter and fracture weaknesses for an HTI modelA three-step azimuthal FCs inversion for estimating the effective stress parameter and fracture weaknesses from azimuthal seismic data is establishedTests on synthetic and real datasets imply the proposed inversion approach has potential in generating reliable results of the effective stress parameter and fracture weaknesses for seismic characterization of abnormal pressure and fractured reservoir identification

Investigation of the effects of salinity concentration on shear strength of compressed bentonite and its sand mixture is desideratum to evaluate the safety of engineered barriers for disposal of highly radioactive nuclear waste. In this research study, the compacted bentonite–sand mixtures were first saturated using distilled water and NaCl solutions under a specified stress in oedometers. Then the specimens were subjected to shearing in a direct shear apparatus. The experimental results revealed that the shear strength increases at higher NaCl concentration and high sand content of mixture. By employing the modified effective stress (pe) containing the osmotic suction effect of porewater, the highest shear strength (τf), referred to as the peak shear strength (PSS) from hereon, of mixture in different NaCl concentrations can be represented by a unique line in the τf–pe plane. As per Mohr–Coulomb principle, the mixtures with a given bentonite content possess the same parameters of shear strength (effective angle of internal friction φ′ and effective cohesion c′), when they are placed and allowed to saturate in NaCl solution at various concentrations. On the contrary, the appearance of sand particles on the shearing surface offers more resilience to the movement of montmorillonite particles which increases the internal friction angle φ′ but poses negligible effects on the cohesion c′. When the effective stress approaches a threshold value, the skeleton of sand particles is formed in the mixture with high sand content bringing about a significant change in the shear strength of bentonite.

Fractures are common in rock masses, and the examination of the electrical properties of fractured rocks under the combined scenario of thermal-hydro-mechanical is of great significance for the application of electrical methods for deep underground engineering investigation. The resistivity of limestone specimens containing prefabricated fractures was measured under different combinations of confining pressure, pore pressure and temperature. The formation factor was then calculated. The results showed that the formation factor increased with the increase in confining pressure and decline in pore pressure and temperature. The coupling effect between the three factors in changing resistivity is very significant. After comparing the specimen strain, the fracture specimen formation factor was found to be closely related to the fracture deformation. The difference between confining pressure and pore pressure in changing the rock formation factor was analyzed, and the concept of effective stress for the fractured rock formation factor was proposed. A formation factor model for single-fractured rocks under the thermal-hydro-mechanical coupling was derived, and its accuracy was verified by comparison with experimental results. Finally, the consistency of the effective stress coefficient of fractured rock deformation and the effective stress coefficient of formation factor was analyzed.

The concept of effective stress is key for understanding the dependence of rock elastic and compaction behaviors on stress and pore-fluid pressure. Previous studies on the concept largely used data acquired on siliciclastic rocks. Carbonate rocks, however, display elastic and compaction behaviors that can be very different than those of siliciclastic rocks. For example, applying most velocity-to-pore-pressure transforms in the context of carbonate reservoirs can be quite challenging. Our study used an experimental approach (a disequilibrium compaction scenario) to assess effective stress coefficient (n) for velocities in three carbonate samples displaying comparable porosities but different dominant pore types (in terms of shape and compliance). Different saturating fluids (nitrogen and distilled water) were used, one at a time, which allowed us to compare both pore and fluid type effects on the coefficient n between these rocks. We found that n is generally bounded by unity. The exception is with nVs (n derived from shear wave velocities) obtained under nitrogen-saturated conditions; nVs is higher than 1 on the three studied samples. Under nitrogen-saturated conditions, the less compliant the main pore types in a given rock are, the higher the value of nVs is. Higher-than-unity values of nVs indicate a deviation from the behavior predicted by existing theories. This could stem from (i) the fact that theoretical analyses assume a pore fluid whose properties are not comparable to those of nitrogen and/or (ii) the way the bulk volumetric strain (a main factor in elastic wave propagation) is incorporated into those theories.

Several sites located between Road No.28 and Akitsu River in downtown Mashiki were liquefied during the mainshock of the 2016 Kumamoto earthquake. According to the building damage survey results, only a few buildings were damaged in areas proximate to the Akitsu River, where liquefaction occurred, however, serious building damage occurred in neighboring regions. Therefore, the effect of soil liquefaction on strong ground motions in Mashiki should be ascertained. Moreover, the distribution of visible and invisible liquefaction is required to be estimated as well. In this study, the distribution of depth of groundwater level in Mashiki was studied, which decreased from 14 to 0 m from northeast to southwest. Thereafter, the nonlinearities of the shallow layers at four borehole drilling sites were identified from the experimental data using the Ramberg–Osgood relationship. Subsequently, the dynamic nonlinear effective stress analysis of the one-dimensional soil column was performed to 592 sites in Mashiki between the seismological bedrock and ground surface to estimate the distribution of strong ground motions during the mainshock. First, the ground motions estimated by the nonlinear analysis corresponded to the ground motions observed at the Kik-net KMMH16. Second, the soil nonlinearity of shallow layers was considerably strong in the entire target area especially in the southern Mashiki, and the PGV distribution was similar to the building damage distribution after the mainshock. Furthermore, the estimated distribution of the soil liquefaction site was similar to the observed results, whereas certain invisible-liquefaction sites were estimated in the north and middle of the target area.

The response of subsoil strata subjected to seismic excitations plays an important role in governing the response of the overlying superstructures at any site. Ground response analysis (GRA) helps to assess the influence of soil characteristics on the propagating seismic stress waves from the bedrock level to the ground surface during an earthquake. For the northeastern region of India, located in the highest seismic zone in the country, conducting an extensive GRA study is of prime importance. Conventionally, most of the GRA studies are carried out using the equivalent linear method, which, being a simplistic approach, cannot capture the nonlinear behavior of soil during seismic shaking. This paper presents the outcomes of a one-dimensional effective stress based nonlinear GRA conducted for Guwahati city (located in northeast India) incorporating the non-Masing load/unload/reload characteristics. The various ground response parameters evaluated from this study help in assessing the ground shaking, soil amplification, and site responses expected in this region. 2D contour maps, which are representative of the distribution of some of these parameters throughout Guwahati city, are also developed. The results presented herein can serve as guidelines for the design of foundations and superstructures in this region.

Soils and rocks are commonly characterized as a porous or fractured medium, with liquid and gaseous fluids occupying and moving in the void space. The presence of water in the void space remarkably influences the deformation behaviors, mechanical properties and stress states of soils and rocks. It has been well recognized that the induced volume change, deformation and shear strength decrease of soils and rocks do not depend on the total stress applied, but on the effective stress dened at the saturated state due to the difference between the total stress and the uid pressure in the pore space. The deformation of soils and rocks further alters the pore or fracture network and induces a nonnegligible variation in hydraulic properties (Kirby 1991; Chen et al. 2007; Li et al. 2014a). Therefore, the concept of effective stress plays a dominant role in understanding the coupled hydromechanical behaviors of soils and fractured rocks.