Recent Studies on Seismic Centrifuge Modeling of Liquefaction and Its Effects on Deep Foundations

The effects of liquefaction on deep foundations are very damaging and costly, and they keep recurring in many earthquakes. The first part of the paper reviews the field experience of deep foundations affected by liquefaction during earthquakes in the last few decades, as well as the main lessons learned. The second part of the paper presents results of physical modeling of deep foundations in the presence of liquefaction conducted mostly in the U.S. and Japan in the 1990’s, with emphasis on the work done by the authors and others at the 100 g-ton RPI centrifuge. Centrifuge models of instrumented single piles and pile groups embedded in both level and sloping liquefiable soil deposits have been excited in-flight by a suitable base acceleration. End-bearing and floating piles with and without a pile cap, with or without a mass above ground, free at the top or connected to a lateral or rotational spring to simulate the superstructure\u27s stiffness, with the foundation embedded in two- or three-layer soil profiles, have been tested. Tests with a mass above ground have allowed backfiguring the degradation of the lateral resistance of the loose saturated sand against the pile as the soil liquefies, while tests in sloping ground without a mass have allowed studying the effect of lateral spreading. Interpretations of these centrifuge experiments and their relation to field observations, soil properties, theory and analytical procedures are also discussed

Recent Studies on Seismic Centrifuge Modeling of Liquefaction and Its Effects on Deep Foundations

The effects of liquefaction on deep foundations are very damaging and costly, and they keep recurring in many earthquakes. The first part of the paper reviews the field experience of deep foundations affected by liquefaction during earthquakes in the last few decades, as well as the main lessons learned. The second part of the paper presents results of physical modeling of deep foundations in the presence of liquefaction conducted mostly in the U.S. and Japan in the 1990’s, with emphasis on the work done by the authors and others at the 100 g-ton RPI centrifuge. Centrifuge models of instrumented single piles and pile groups embedded in both level and sloping liquefiable soil deposits have been excited in-flight by a suitable base acceleration. End-bearing and floating piles with and without a pile cap, with or without a mass above ground, free at the top or connected to a lateral or rotational spring to simulate the superstructure\u27s stiffness, with the foundation embedded in two- or three-layer soil profiles, have been tested. Tests with a mass above ground have allowed backfiguring the degradation of the lateral resistance of the loose saturated sand against the pile as the soil liquefies, while tests in sloping ground without a mass have allowed studying the effect of lateral spreading. Interpretations of these centrifuge experiments and their relation to field observations, soil properties, theory and analytical procedures are also discussed

A Numerical Study of Lateral Spreading Behind a Caisson Type Quay Wall

A series of centrifuge model tests were conducted at Rensselaer Polytechnic Institute to study the seismic response of a caisson-type waterfront quay wall system, and the liquefaction and deformation characteristics of the saturated cohesionless backfill. Using a nonlinear two-phase (solid-fluid) finite element program, a numerical study of the above centrifuge tests is performed. In this paper, the centrifuge tests and formulation of the employed finite element program are briefly described, and the numerical simulation results are compared to the experimental records. It is shown that the extent of liquefaction, the deformation pattern of the soil-wall system, and the magnitude of lateral spreading obtained from the computational code are similar to actual observations in the centrifuge tests. Computational parametric studies are then conducted by varying soil relative density and soil permeability to investigate the spatial extent of liquefaction in backfill material and its effect on the magnitude of ground lateral spreading. It is concluded that the dynamic properties and permeability of backfill material are among the most influential factors in dictating seismic performance of a quay wall system

Case Histories of Liquefaction in Loose Sand Fills During the 1989 Loma Prieta Earthquake: Comparison With Large Scale and Centrifuge Shaking Tests

The paper focuses on loose clean and silty sand fills that liquefied during the 1989 Loma Prieta, California earthquake. Available field case histories of liquefaction that include measured shear wave velocity from the Andrus et al. (2003) database are used. The liquefaction behavior observed in these field case histories is compared with the results of two large scale and six centrifuge shaking tests conducted by the authors. System identification and site response analyses are used to obtain the corresponding cyclic shear stress ratios in the tests. Due consideration is given to the shaking duration and 1D versus 2D shaking in the laboratory and field. The comparison between field and shaking tests is very good, with both case histories and shaking tests validating well the Andrus and Stokoe (2000) liquefaction chart for Mw = 7.0. This agreement also serves to validate the large scale and centrifuge testing techniques presented, as tools that can be used toward improved methods for liquefaction evaluation and mitigation of sandy fills

DEM Simulation of Liquefaction-Induced Lateral Spreading

This paper reports the results of a model-based simulation of 1g shake table tests of sloping saturated granular deposits subjected to seismic excitations. The simulation technique utilizes a transient fully-coupled continuum fluid discrete particle model of the watersaturated soil. The fluid (water) phase is idealized at a macroscale using an Eulerian averaged form of Navier-Stokes equations. The solid particles are modeled at the microscale as an assemblage of discrete spheres using the discrete element method. The interphase momentum transfer is accounted for using an established relationship. Numerical simulations were conducted to investigate the liquefaction induced lateral spreading of a mild-sloped semi-infinite deposit subjected to a dynamic base excitation. The employed model reproduced a number of response patterns observed in the 1g experiment. In addition, the simulation results captured the initiation of sliding at failure planes, the propagation of liquefaction front and associated large strain localization, and the redistribution of void space during shaking

Stabilization of liquefiable soils using colloidal silica grout

Journal of Materials in Civil Engineering, 19(1): pp. 33-40.Passive site stabilization is a new technology proposed for nondisruptive mitigation of liquefaction risk at developed sites susceptible to liquefaction. It is based on the concept of slowly injecting colloidal silica at the edge of a site with subsequent delivery to the target location using natural or augmented groundwater flow. Colloidal silica is an aqueous dispersion of silica nanoparticles that can be made to gel by adjusting the pH or salt concentration of the dispersion. It stabilizes liquefiable soils by cementing individual grains together in addition to reducing the hydraulic conductivity of the formation. Centrifuge modeling was used to investigate the effect of colloidal silica treatment on the liquefaction and deformation resistance of loose, liquefiable sands during centrifuge in-flight shaking. Loose sand was successfully saturated with colloidal silica grout and subsequently subjected to two shaking events to evaluate the response of the treated sand layer. The treated soil did not liquefy during either shaking event. In addition, a box model was used to investigate the ability to uniformly deliver colloidal silica to loose sands using low-head injection wells. Five injection and two extraction wells were used to deliver stabilizer in a fairly uniform pattern to the loose sand formation. The results of the box model testing will be used to design future centrifuge model tests modeling other delivery methods of the grout

Mortality of emergency abdominal surgery in high-, middle- and low-income countries

Background: Surgical mortality data are collected routinely in high-income countries, yet virtually no low- or middle-income countries have outcome surveillance in place. The aim was prospectively to collect worldwide mortality data following emergency abdominal surgery, comparing findings across countries with a low, middle or high Human Development Index (HDI). Methods: This was a prospective, multicentre, cohort study. Self-selected hospitals performing emergency surgery submitted prespecified data for consecutive patients from at least one 2-week interval during July to December 2014. Postoperative mortality was analysed by hierarchical multivariable logistic regression. Results: Data were obtained for 10 745 patients from 357 centres in 58 countries; 6538 were from high-, 2889 from middle- and 1318 from low-HDI settings. The overall mortality rate was 1⋅6 per cent at 24 h (high 1⋅1 per cent, middle 1⋅9 per cent, low 3⋅4 per cent; P < 0⋅001), increasing to 5⋅4 per cent by 30 days (high 4⋅5 per cent, middle 6⋅0 per cent, low 8⋅6 per cent; P < 0⋅001). Of the 578 patients who died, 404 (69⋅9 per cent) did so between 24 h and 30 days following surgery (high 74⋅2 per cent, middle 68⋅8 per cent, low 60⋅5 per cent). After adjustment, 30-day mortality remained higher in middle-income (odds ratio (OR) 2⋅78, 95 per cent c.i. 1⋅84 to 4⋅20) and low-income (OR 2⋅97, 1⋅84 to 4⋅81) countries. Surgical safety checklist use was less frequent in low- and middle-income countries, but when used was associated with reduced mortality at 30 days. Conclusion: Mortality is three times higher in low- compared with high-HDI countries even when adjusted for prognostic factors. Patient safety factors may have an important role. Registration number: NCT02179112 (http://www.clinicaltrials.gov)

Seismic Response of Deep Foundations Subjected to Liquefaction-Induced Lateral Spreading: Integrated Research and Practical Implications

Integrated research is described on the response of pile foundations subjected to lateral ground deformations associated with sand liquefaction. Field evidence, experiments and analysis have indicated that pile bending and other response aspects depend on a complex manner on – and are very sensitive to - several parameters including the specific foundation and structural system, the free field ground deformation, and soil aspects such as layering and properties of nonliquefied and liquefied soil strata. The research presented focuses on sites where the liquefiable soil reaches the ground surface (especially important to bridge foundations), showing that in this case the permeability and stiffness of the soil play a major role on pile response to ground deformation. Full-scale and centrifuge experiments with advanced instrumentation as well as Finite Element and Discrete Element numerical simulations, are described and integrated to show the main phenomena affecting soil and pile response. Possible implications are discussed for the analysis, design and retrofitting of deep foundations against lateral spreading