Seismic Response of Columnar Reinforced Ground

Ground improvement using stiff columnar reinforcement, such as stone, jet-grout and soil-mix columns, is commonly used for mitigation of seismic damage in weak ground. Seismic shear stress reduction in the reinforced soil mass is often counted on for reducing liquefaction potential. Current design methods assume composite behavior of the reinforced soil, where the shear stress reduction is based on the ratio of the columnar stiffness relative to the soil as well as the area replacement ratio. This implicitly assumes that the stiff columns will deform in pure shear along with the surrounding soft soil. Three dimensional dynamic finite element analyses were performed to better understand the column deformation and shear stress reduction behavior. The analyses focused on the deformation modes of the stiff column during shaking and the stress transfer mechanisms between the column and the surrounding soft ground. These analyses showed that the seismic behavior of columnar reinforced ground is more complicated than widely thought, and importantly, that current design methods may greatly over-estimate the shear stress reduction the columns provide. The study found that stiff columns do not behave as pure shear beams as implicitly assumed by current methods, but that their behavior is a combination of shear and flexural behavior. Further, the results indicate that the mode of deformation of the columns significantly influences their effectiveness in reducing shear stresses in the reinforced soil. For most common applications, the columns deform in combination of flexure and shear. The net effect is that stiff columns typically achieve only a small percentage of the shear stress reduction implied by area-replacement ratio methods that assume composite behavior for reinforced ground. In summary, columnar reinforcement provides little or no seismic shear stress reduction and current methods may be unconservative. The results of this analytical study are presented in this paper and the implications in terms of the current design practice are discussed

Seismic Performance of Soil-Mix Panel Reinforced Ground

Ground reinforcement methods such as stone columns, jet grouting, and soil mixing are commonly used to improve subsoil conditions for seismic mitigation. In most cases, the purpose of this improvement is for foundation support and/or liquefaction mitigation. Additional benefits of the improvement, such as possible reduction in seismic ground motions, are not explicitly considered in NEHRP/IBC code provisions for establishing site classification and seismic design motions. Such reductions, if present, can have significant payoff. Reduced seismic loads on the super structure result in lower seismic design levels and reduced construction costs. It is conceivable that the cost of ground improvement, typically about 5-15% of total construction costs, may be more than offset by lower overall costs resulting from reduced ground motions used in design. Ongoing research and analytical studies suggests that some soil improvement techniques using stiff reinforcing elements have the potential to reduce the intensity of earthquake shaking beneath structures. Of particular interest, our dynamic finite element modeling suggests that stiff ground reinforcements arranged in latticetype panels (i.e. soil-mix and jet-grout panels) has great potential. Such panels may significantly reduce ground motions and improve NEHRP/IBC site classification. This paper presents and summarizes results from preliminary dynamic three-dimensional (3-D) finite element analyses of soil-mix panel reinforced ground. Results are shown for a series of analyses where typical soil-mix panels are installed at replacement ratios of 24% and 36%. The improvement was found to cause reductions in spectral acceleration of up to 40% in comparison to unimproved ground conditions, especially for structural periods less than 1.0 second. A variety of geometrical configurations such as different replacement ratios, improvement depths as well as panel stiffnesses are currently being studied by the authors to provide further insight into the phenomenon

Numerical Modeling of Columnar Reinforced Ground 1999 Kocaeli Earthquake Case History

The Kocaeli Earthquake (M=7.4) struck Turkey on August 17, 1999 and caused significant damage along Izmit Bay. Following the earthquake, the authors investigated the field performance at improved soil sites. Of particular interest was the Carrefour Shopping Center that was under construction during the earthquake. The reclaimed site is underlain by strata of saturated soft clays, silts, and liquefiable loose sands. Small-diameter jet-grout columns had been installed at close spacings to reduce settlements and prevent liquefaction-related damage beneath footings and mats. Nonlinear dynamic three-dimensional finite element analyses were conducted to model the reinforced ground at Carrefour. The results show that the primary benefit of the columns was different than first suspected. That is, we initially thought the higher composite stiffness of the reinforced ground led to reduced seismic shear stresses and shear strains in the soil mass. However, the numerical results show that the reinforced ground did not behave as a composite mass during shaking due to strain incompatibility between the soil and stiff columns. The results indicate that the columns did not significantly reduce seismic shear stresses and strains (and thus pore pressures) in the soil mass. The effectiveness of the jet-grouting at Carrefour was more related to the vertical support the columns provided that prevented seismically-induced settlements. The implication is that commonly-used design methods and assumptions may lead to overestimates of the effectiveness of ground reinforcement for mitigating seismic damage

Field Evidence and Laboratory Testing of the Cyclic Vulnerability of Fine-Grained Soils During the 1999 Kocaeli Earthquake

Significant earthquake-induced settlements occurred in saturated fine-grained soils at the Carrefour Shopping Center in Turkey during the 1999 Kocaeli Earthquake (M=7.4). Most of the settlement was due to the undrained cyclic failure of silt/clay (ML/CL) and highplasticity clay (CH) strata within the subsoil profile. Each suffered about 1% vertical strain. Extensive laboratory testing on undisturbed samples from these silty and clayey strata has been performed to investigate this behavior. The laboratory testing included monotonic and cyclic simple shear tests, triaxial tests and conventional 1-D consolidation tests. Considerable pore pressure increases have been measured during cyclic simple shear test which was later followed by significant reconsolidation settlement. It was found that significant pore pressures begin developing in these soils at cyclic stresses at about 50% of their monotonic shear strength. This transition in behavior with high pore pressure development and subsequent post-cyclic volume changes corresponds to about 0.5% cyclic shear strains. The study demonstrates the limitations of generalized liquefaction screening methods, and dispels the common misconception that high plasticity soils cannot generate high pore pressures and fail under cyclic loading. Test results indicate that the soils at the site can generate significant pore pressures when shaken at levels expected to have occurred during the Kocaeli Earthquake. The findings from this study are inline with the limited number of studies on this topic. Fine-grained soils, if shaken hard enough, can suffer strength loss and reconsolidation settlements. The challenge remains to better understand such phenomenon and incorporate this into engineering practice. This paper presents the observed ground failure at the site, site characterization studies and following laboratory testing program

Internal Structure and Breakage Behavior of Biogenic Carbonate Sand Grains

This Study Investigates the Mechanical Behavior of Biogenic Carbonate Sands from Puerto Rico at Grain-Scale Level. Micro-Computed Tomography Has Also Been Used to Get Insights on the Internal Structure of These Particles Before and after Loading. the Crushing Strength of These Particles Are Smaller Comparing to the Values Reported for Silica Sands. It Has Also Been Shown that These Particles Have Complex Internal Structure Including a Network of Pores Connected with Channels. This Study Also Demonstrates the Effect of Intragrain Structure of Biogenic Carbonate Sands and Shows How Internal Grain Structure Plays a Role on Particle Fracture

Liquefaction Potential of Railway Embankments

This paper presents an overview of the nature of train-induced vibrations and discusses the liquefaction potential of railway embankments under such low-level vibrations. The paper also presents the results of static and dynamic finite difference numerical analyses performed for a simple railway embankment geometry. The liquefaction potential for the railway embankment foundation was estimated using the results corn FLAC numerical analyses, as well as a cyclic shear stress liquefaction resistance approach using a modified cyclic resistance ratio curve. Liquefaction of railway embankment foundations was found to be possible. However, based on the majority of reported failures the liquefaction potential remains low unless the train-induced vibrations are coupled with factors such as loose foundation, and sudden rise of pore water pressures due to poor drainage, flooding, or heavy rainfall

Amplification of Earthquake Ground Motions in Washington, DC, and Implications for Hazard Assessments in Central and Eastern North America

The extent of damage in Washington, DC, from the 2011 MW 5.8 Mineral, VA, earthquake was surprising for an epicenter 130 km away; U.S. Geological Survey “Did-You-Feel-It” reports suggest that Atlantic Coastal Plain and other unconsolidated sediments amplified ground motions in the city. We measure this amplification relative to bedrock sites using earthquake signals recorded on a temporary seismometer array. The spectral ratios show strong amplification in the 0.7 to 4 Hz frequency range for sites on sediments. This range overlaps with resonant frequencies of buildings in the city as inferred from their heights, suggesting amplification at frequencies to which many buildings are vulnerable to damage. Our results emphasize that local amplification can raise moderate ground motions to damaging levels in stable continental regions, where low attenuation extends shaking levels over wide areas and unconsolidated deposits on crystalline metamorphic or igneous bedrock can result in strong contrasts in near-surface material properties

The Thermal Behaviour of Three Different Auger Pressure Grouted Piles Used as Heat Exchangers

Three auger pressure grouted (APG) test piles were constructed at a site in Richmond, Texas. The piles were each equipped with two U-loops of heat transfer pipes so that they could function as pile heat exchangers. The piles were of two different diameters and used two different grouts, a standard APG grout and a thermally enhanced grout. Thermal response tests, where fluid heated at a constant rate is circulated through the pipe loops, were carried out on the three piles, utilising either single or double loops. The resulting test data can be used to determine the surrounding soil thermal conductivity and the pile thermal resistance, both essential design parameters for ground source heat pump systems using pile heat exchangers. This paper uses parameter estimation techniques to fit empirical temperature response curves to the thermal response test data and compares the results with standard line source interpretation techniques. As expected, the thermal response tests with double loops result in smaller thermal resistances than the same pile when the test was run with a single loop. Back analysis of the pile thermal resistance also allows calculation of the grout thermal properties. The thermally enhanced grout is shown to have inferior thermal properties than the standard APG grout. Together these analyses demonstrate the importance of pile size, grout thermal properties and pipe positions in controlling the thermal behaviour of heat exchanger piles

Analysis of friction induced thermo-mechanical stresses on a heat exchanger pile in isothermal soil

Copyright © 2014 SpringerIn most analytical and numerical models of heat exchanger piles, strain incompatibilities between the soil and the pile are neglected, and axial stresses imposed by temperature changes within the pile are attributed to the thermal elongation and shortening of the pile. These models incorporate thermo-hydro-mechanical couplings in the soil and within the pile foundation, but usually neglect thermo-mechanical couplings between the two media. Previous studies assume that the stress changes imposed by temperature variations in a heat exchanger pile are mainly due to the constrained thermal elongation and shortening of the pile. Also, several recent approaches utilize spring models that focus only on the soil-pile interface in modeling temperature-induced stresses in a heat exchanger pile and implicitly ignore the effect of the full displacement field on soil-pile interaction. By contrast, in this paper, interface elements are introduced in a numerical model of a heat exchanger pile, analyzed in axisymmetric and stationary conditions. The pile is subjected to a uniform temperature increase, with free top and fixed top conditions in elastic and elasto-plastic soil profiles. Simulation results show that the constrained vertical elongation is the most detrimental factor for pile foundation performance. However it is worth noticing that while mechanical constraints (e.g., fixed top and/or fixed bottom) impose maximum stress increases at the ends of the pile , interface effects result in maximum stresses around the mid-length of the pile. This preliminary study indicates that soil-pile friction does not increase pile internal stresses to the point where it would be necessary to over-dimension the foundation pile for heat exchanger use. Furthermore, one cannot expect a significant gain in foundation performance due to the improvement of soil-pile frictional resistance as a result of increased lateral stresses at soil-pile contact. Additional numerical analyses are ongoing, in order to investigate the role of the degree of fixity induced by the building on the heat exchanger pile, and to extend these preliminary analyses to transient operational modes and cyclic thermo-mechanical loading of the heat exchanger pile

Numerical Modeling of Columnar-Reinforced Ground Behavior During Dynamic Centrifuge Testing

Predicting the response of soil profiles during earthquakes is one of the major challenges in geotechnical earthquake engineering. The presence of reinforcing elements such as stiff columns adds further complexity to the problem due to the interaction of these stiff elements with the surrounding ground. This research presents the results of advanced numerical simulations of dynamic centrifuge tests performed on a columnar reinforced model with a loose sandy profile. The model was subjected to earthquake base motions of varying intensities to investigate the reinforcing mechanisms of soil-cement columns. Numerical simulations were performed using the finite element computational platform OpenSees with pressure dependent multi yield (PDMY02) constitutive model. Simulated and measured values were compared for seismic intensity, excess pore water pressure and ground settlement at different locations within soil profile. The calibrated numerical model was able to realistically predict the response of reinforced ground