Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal.
In-flight measurement of shear-wave velocity (Vs) in centrifuge model test is important for real-time characterization and has multiple engineering applications. Accurate measurement of Vs by bender elements (BE) in centrifuge models is still challenging, in part because of the curved raypath of shear-wave propagation. By focusing on the variable g-fields inside the models, this paper provides new equations with improved Vs-depth function to raypaths of shear waves for two typical centrifuge model setups. Parametric analyses including the centrifuge specifications, testing layout, and soil characteristics were carried out to study their effects on raypaths and Vs accuracy. The results show that testing layout has significant effect on Vs accuracy whereas soil characteristics have considerable effect. Variable g-fields will cause a further reduction of Vs accuracy, which is dominated by centrifuge radius. To secure an accurate Vs measurement in centrifuges, it is recommended that Di/L should be larger than 0.5 for cross hole measurement, and curved raypath inversion is necessary for tomography at shallow depth (Di/L<0.5). Then two centrifuge model tests with BE testing corresponding to two cases of variable g-fields were performed to validate the proposed recommendations, where the apparent velocity is proved accurate enough to be used as the actual velocity when testing layouts meet the requirement.
This paper focuses on elastic stiffness parameters for axial, horizontal, and vertical motions of a pipeline relative to the seabed, with the aim of expressing these parameters in terms of fundamental elastic properties of the soil. Limited information exists in the literature on the axial elastic response of on-bottom pipelines, particularly for nonhomogeneous soil. Therefore, an approximate analytical approach was developed for axial stiffness, focusing on the case of shear modulus proportional to depth. The solution was then verified through numerical analysis. Further numerical analysis was carried out to obtain relationships for horizontal and vertical elastic stiffnesses of on-bottom pipelines. Finally, relationships among elastic stiffnesses were developed. Here recommendations are made for the selection of proper elastic stiffnesses in all three directions of motion. These recommendations allow consistent and rigorous modeling of elastic pipe–seabed interactions with application to the analysis of pipeline laying, buckling, walking, and on-bottom stability.
The stability of geotechnical earth structures is often affected by associated uncertainties present in geotechnical parameters, if they are not properly accounted for. The present paper aims at quantifying these uncertainties and proposes a modification factor, namely probabilistic risk factor (Rf) for each geotechnical random variable. A gravity retaining wall is analyzed by a pseudostatic method of analysis against four modes of failure namely, sliding, overturning, eccentricity, and bearing. The effect of variation of properties of backfill and foundation soil on stability of the wall for various earthquake conditions is analyzed. Rf simultaneously identifies the effects of Pf of a gravity retaining wall subjected to earthquake loading and also the sensitivity of geotechnical random variables on different modes of failure. The geotechnical random variables are modified by Rf and applied in design. It is observed that, apart from the seismic horizontal and vertical pseudostatic acceleration coefficients kh and kv, friction angle of backfill soil (φ1), and cohesion of foundation soil (c2) are the major guiding geotechnical parameters in stability analysis of the gravity retaining wall. Parametric studies are carried out for different combinations of kh, kv, and φ, and risk factors-based on the formulated approach are proposed for each case. Finally, design guidelines are proposed for different variations of random variables and earthquake conditions. A case study is also presented, which deals with the application of proposed risk factors to a series of 54 retaining walls in Hodogaya Ward and Naka Ward of the Yokohama municipality area in Japan.
The results of fully coupled, three-dimensional (3D), nonlinear finite-element analyses of structures founded on liquefiable soils are compared with centrifuge experiments. The goal is to provide insight into the numerical model’s capabilities in predicting the key engineering demand parameters that control building performance on softened ground for a range of structures, soil profiles, and ground motions. Experimental and numerical observations will also guide future analyses and mitigation decisions. The numerical model captured excess pore pressures and accelerations, the dominant displacement mechanisms under the foundation, and therefore building’s settlement, tilt, and interstory drift. Both experimental and numerical results showed that increasing the structure’s contact pressure and height/width (H/B) ratio generally reduces net excess pore pressure ratios in soil but amplifies the structure’s tilting tendencies and total drift. The settlement response of a structure with a greater pressure and H/B ratio was also more sensitive to soil-structure-interaction induced forces, which could at times amplify on a denser soil with less softening. A denser soil profile also increased building’s flexural drift in all cases by reducing excess pore pressures and rocking drift, while amplifying foundation accelerations and total drift. Numerical simulations captured these trends well. These experimental and numerical results point to the importance of taking into account a building’s dynamic properties and overall performance in mitigation design.
Field observations suggest that preshaken natural sands in some seismic regions have high liquefaction resistance as a result of geologic aging and/or preshaking. This paper focuses on the young silty sand deposits located in the Imperial Valley of California. Recent deposition and intense seismic activity in the Valley suggest that preshaking is the main cause of their increased liquefaction resistance. The first part of the paper examines the liquefiable layer at the Wildlife site, which may have been deposited by flooding approximately between 1905–1907. The site was instrumented with accelerometers and piezometers in 2005, providing data over the last 10 years. The following conclusions are reached from this and from the catalog information on earthquakes before 2005: (1) Since 1907, the Wildlife layer has been subjected to approximately 60–70 earthquakes having amax≥0.1 g at the site, which caused pore pressure buildup in the layer; (2) most of these earthquakes generated excess pore pressures but generally did not liquefy the soil (Events A); and (3) approximately 10 or 20% of all earthquakes were capable of liquefying the layer immediately after deposition (Events B). This information was used to plan a centrifuge experiment that crudely simulated the history of the Wildlife site. In this test, 66 base shakings were applied to the base of a 6-m prototype homogeneous deposit of loose saturated silty sand, with a ratio of one Event B for every 10 Events A. Events B liquefied the deposit at the beginning but not at the end of the experiment. Events A liquefied the deposit at very shallow depths at the beginning but stopped liquefying it very soon into the experiment. Finally, an Event B caused the next Event A to generate more excess pore pressures, with this effect being canceled rapidly by a couple of subsequent Events A. The lack of liquefaction by Events B after heavy preshaking in the experiment is consistent with the Wildlife layer response to the 2010, Mw=7.2, El Mayor-Cucupah earthquake, an Event B that generated only a 19% pore pressure ratio at the site.
A nonlinear elastic method to analyze the reinforcement loads at the potential failure surface of a reinforced soil composite without facing restriction is developed in this study. The method makes use of the compatible deformations of soil and reinforcement at the potential failure surface and employs the hyperbolic stress-strain relationship to determine the tangential modulus of soil. Nonconstant Poisson’s ratio, nonlinear reinforcement load-strain behavior, and nonuniform reinforcement spacing can be taken into account. Effect of soil compaction is conveniently modeled by an equivalent compaction pressure and a pressure-dependent unloading-reloading Young’s modulus. The method can be implemented by a simple computer code. Four numerical model walls and four large-scale tests of reinforced soil composites were employed to validate the proposed method. It was shown that the method can be used to analyze the reinforcement loads of reinforced soil composites under working stress conditions. The method excludes the necessity to carry out sophisticated numerical analyses to determine the reinforcement loads.
Sandstone is encountered in many hydropower projects in China, and its permeability evolution characteristics during the deformation process are one of the most important issues in engineering design. In this study, microstructure observation and mineral content analysis of a sandstone are performed, and triaxial flow experiments under different fluid pressures are carried out. The experiment results show that the permeability evolution with volumetric strain is associated with microcrack development in the process of rock deformation and failure. The volumetric strain is closely related to the permeability and fluid pressure, and it plays a role of linking the flow field and the stress field. A conceptual model is suggested to describe the permeability evolution of the rock. Scanning electron microscope analysis results on some failed specimen pieces illustrate that the fluid pressure plays a role in the development of microcracks. A high fluid pressure can accelerate crack propagation and rock failure, decrease the crack damage stress, and increase the flow in the sandstone.
Seismic compression is the accrual of volumetric strains in unsaturated soils caused by cyclic loading and has caused significant damages to buildings and other structures during earthquakes. To date, the available methods for predicting the severity of seismic compression have mainly been simplified procedures, in which a number of equivalent cycles are used to represent the duration of earthquake loading. Often, however, the number of equivalent cycles is computed inconsistently with the underlying mechanics of seismic compression. This paper proposes a non-simplified procedure for predicting the severity of seismic compression. The procedure is based on a modified version of the Richart-Newmark cumulative damage hypothesis, wherein volumetric strain is used as the damage metric. The proposed model was calibrated using data from 425 constant-amplitude sinusoidal strain-controlled cyclic simple shear tests performed on clean sand and validated using test data from samples subjected to variable-amplitude sinusoidal and earthquake loadings. In addition to predicting the severity of seismic compression, the proposed model can be used to compute number of equivalent shear-strain cycles for use in simplified models, consistent with the seismic compression phenomenon. In comparison with other proposed nonsimplified models for computing seismic compression, the proposed model gives good agreement with the measured seismic compression.
Linear interpolation between isotropically consolidated undrained strength envelopes and fully drained (effective stress) strength envelopes is commonly used to calculate undrained strengths for slope stability analyses of rapid drawdown. However, the current linear interpolation method presents numerical problems at low effective normal stresses with soils that possess an effective stress cohesion intercept. The resulting undrained shear strength envelopes are unrealistic at low normal stresses. An improved interpolation method has been developed that interpolates a single undrained strength envelope for the entire slip surface. The improved interpolation method produces identical results to the current method for cases where c′ equals zero.
Soil compressibility models with physically correct asymptotic void ratios are required to analyze situations that involve a wide stress range. Previously suggested models and other functions are adapted to satisfy asymptotic void ratios at low and high stress levels; all updated models involve four parameters. Compiled consolidation data for remolded and natural clays are used to test the models and to develop correlations between model parameters and index properties. Models can adequately fit soil compression data for a wide range of stresses and soil types; in particular, models that involve the power of the stress σ′β display higher flexibility to capture the brittle response of some natural soils. The use of a single continuous function avoids numerical discontinuities or the need for ad hoc procedures to determine the yield stress. The tangent stiffness—readily computed for all models—should not be mistaken for the small-strain constant-fabric stiffness.
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