Browsing by Subject "Liquefaction"
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Item Effect of prefabricated vertical drains on pore water pressure generation and dissipation in liquefiable sand(2010-05) Marinucci, Antonio; Rathje, Ellen M.; Stokoe II, Kenneth H.; Wilson, Clark; Gilbert, Robert; Zornberg, JorgeSoil improvement methods are used to minimize the consequences of liquefaction by changing the characteristics and/or response of a liquefiable soil deposit. When considering sites with previous development, the options for soil improvement are limited. Traditional methods, such as compaction and vibratory techniques, are difficult to employ because of adverse effects on adjacent structures. One potential method for soil improvement against soil liquefaction in developed sites is accelerated drainage through in situ vertical drains. Vertical drains expedite the dissipation of excess pore water pressures by reducing the length of the pore water drainage path. For more than thirty years, vertical gravel drains or stone columns have been employed to ensure the excess pore water pressure ratio remains below a prescribed maximum value. In recent years, the use of prefabricated vertical drains (PVDs) has increased because the drains can be installed with less site disruption than with traditional soil improvement methods. To date, little-to-no field or experimental verification is available regarding the seismic performance of sites treated with PVDs. The effectiveness of PVDs for liquefaction remediation was evaluated via small-scale centrifuge testing and full-scale field testing. A small-scale centrifuge test was performed on an untreated soil deposit and on a soil deposit treated with small-scale vertical drains. Compared to the untreated condition, the presence of the small-scale vertical drains provided numerous benefits including smaller magnitudes of excess pore water pressure generation and buildup, smaller induced cyclic shear strains, reduced times for pore pressure dissipation, and smaller permanent horizontal and vertical displacements. In addition, full-scale in situ field experiments were performed in an untreated soil deposit and in a soil deposit treated with full-scale PVDs using a vibrating mandrel as the dynamic source. In the untreated test area, the maximum induced excess pore pressure ratio reached about 0.95. In the treated test area, the vibratory installation of the first few drains generated significant excess pore pressures; however, significant excess pore pressures were not generated during the vibratory installation of additional drains because of the presence of the adjacent drains. Additionally, the vibratory installation of the drains caused significant settlement and significantly altered the shear wave velocity of the sand. Dynamic shaking after installation of all of the drains induced small accelerations, small cyclic shear strains, and negligible excess pore water pressures in the soil. The results of the field experiment indicate that the prefabricated vertical drains were effective at dissipating excess pore water pressures during shaking and densifying the site.Item Factors influencing the lateral spread displacement from the 2011 Christchurch, New Zealand earthquake(2015-05) Deterling, Olivia Catherine; Rathje, Ellen M.; Cox, Brady RLiquefaction induced lateral spreading during earthquakes poses a significant hazard to infrastructure and has severe consequences. It is critical for geotechnical earthquake engineers to be able to accurately predict lateral spread movements. Empirical and semi-empirical models and various liquefaction index parameters are available to help predict the potential for damage and movements associated with liquefaction and lateral spreading. In this thesis, the available models and liquefaction index parameters were investigated using the observed lateral spread displacements from the Christchurch earthquake in New Zealand. The analysis includes investigation of the Youd et al. (2002), Bardet et al. (2002), Rauch and Martin (2000) and Zhang et al. (2004) displacement models; and the liquefaction potential index (LPI), settlement indicator (Sv1d) and liquefaction severity number (LSN) parameters. The results of this study show that both the Youd et al. (2002) and Bardet et al. (2002) models predict lateral spread displacements much smaller than observed in Christchurch, most likely due to the relatively small magnitude of the Christchurch earthquake (M[subscript w] = 6.2) relative to the earthquake magnitudes included in datasets used to develop the Youd et al. (2002) and Bardet et al. (2002) models (i.e., Mw >= 7.5). The Rauch and Martin (2000) and Zhang et al. (2004) models predicted lateral spread displacements more similar to those observed. The Rauch and Martin (2000) model predicts the average lateral spread displacement over an entire slide area, as opposed to predicting lateral spread displacement at a point, which may have contributed to the more favorable comparison.Item Liquefaction-induced lateral displacements from the Canterbury earthquake sequence in New Zealand measured from remote sensing techniques(2016-05) Secara, Sorin S.; Rathje, Ellen M.; Cox, Brady RLiquefaction is a significant earthquake hazard that can generate large horizontal displacements associated with lateral spreading and these displacements cause considerable damage. To improve our understanding of liquefaction-induced lateral spreading and the models that can be used to predict the associated displacements, the collection of high quality field data on lateral spreading displacements is essential. Remote sensing techniques, in particular optical image correlation using satellite imagery, can be used for this purpose. This thesis investigates optical image correlation of satellite images as a remote sensing technique for this purpose using images from the 2010-2011 Canterbury earthquake sequence in New Zealand. Optical image correlation uses two optical images – one before and one after the investigated event – to measure displacements that have occurred between the time of the two image acquisitions. The correlation analysis calculates the horizontal displacement at a specified spacing, and the displacements are post-processed and filtered to attain the final displacement field. The displacement results from optical image correlation agreed favorably with qualitative field observations of the severity of liquefaction and lateral spreading, as well as the general crack patterns along the Avon River. A more quantitative comparison was performed using field measured displacements along four linear transects that extended perpendicular from the Avon River. The displacements from optical image correlation also agreed favorably with the field measured displacement profiles, although the optical image correlation displacements somewhat larger than the field measurements. This discrepancy occurs because field measurements are based on discrete measurements of crack width, while the optical image correlation are based on average displacements over larger areas and include displacements associated with ductile movements that may not result in cracking. The results from this research show that optical image correlation of satellite imagery pairs can provide accurate and detailed measurements of horizontal displacements due to liquefaction and lateral spreading. This approach can be used to create more complete and detailed databases of liquefaction-induced movements, which can be used to improve current predictive models for lateral spread displacements. Future post-earthquake investigations and research should make use of optical image correlation to document the displacements associated with liquefaction.Item Measuring liquefaction-induced deformation from optical satellite imagery(2014-05) Martin, Jonathan Grant; Rathje, Ellen M.Liquefaction-induced deformations associated with lateral spreading represent a significant hazard that can cause substantial damage during earthquakes. The ability to accurately predict lateral-spreading displacement is hampered by a lack of field data from previous earthquakes. Remote sensing via optical image correlation can fill this gap and provide data regarding liquefaction-induced lateral spreading displacements. In this thesis, deformations from three earthquakes (2010 Darfield, February 2011 Christchurch, and 2011 Tohoku Earthquakes) are measured using optical image correlation applied to 0.5-m resolution satellite imagery. The resulting deformations from optical image correlation are compared to the geologic conditions, as well as field observations and measurements of liquefaction. Measurements from optical image correlation are found to have a precision within 0.40 m in all three cases, and results agree well with field measurements.Item The performance of lateral spread sites treated with prefabricated vertical drains : physical and numerical models(2013-05) Howell, Rachelle Lee; Rathje, Ellen M.Drainage methods for liquefaction remediation have been in use since the 1970's and have traditionally included stone columns, gravel drains, and more recently prefabricated vertical drains. The traditional drainage techniques such as stone columns and gravel drains rely upon a combination of drainage and densification to mitigate liquefaction and thus, the improvement observed as a result of these techniques cannot be ascribed solely to drainage. Therefore, uncertainty exists as to the effectiveness of pure drainage, and there is some hesitancy among engineers to use newer drainage methods such as prefabricated vertical drains, which rely primarily on drainage rather than the combination of drainage and densification. Additionally, the design methods for prefabricated vertical drains are based on the design methods developed for stone columns and gravel drains even though the primary mechanisms for remediation are not the same. The objectives of this research are to use physical and numerical models to assess the effectiveness of drainage as a liquefaction remediation technique and to identify the controlling behavioral mechanisms that most influence the performance of sites treated with prefabricated vertical drains. In the first part of this research, a suite of three large-scale dynamic centrifuge tests of untreated and drain-treated sloping soil profiles was performed. Acceleration, pore pressure, and deformation data was used to evaluate the effectiveness of drainage in reducing liquefaction-induced lateral deformations. The results showed that the drains reduced the generated peak excess pore pressures and expedited the dissipated of pore water pressures both during and after shaking. The influence of the drains on the excess pore pressure response was found to be sensitive to the characteristics of the input motion. The drainage resulted in a 30 to 60% reduction in the horizontal deformations and a 20 to 60% reduction in the vertical settlements. In the second part of this research, the data and insights gained from the centrifuge tests was used to develop numerical models that can be used to investigate the factors that most influence the performance of untreated and drain-treated lateral spread sites. Finite element modeling was performed using the OpenSees platform. Three types of numerical models were developed - 2D infinite slope unit cell models of the area of influence around a single drain, 3D infinite slope unit cell models of the area of influence around a single drain, and a full 2D plane strain model of the centrifuge tests that included both the untreated and drain-treated slopes as well as the centrifuge container. There was a fairly good match between the experimental and simulated excess pore pressures. The unit cell models predicted larger horizontal deformations than were observed in the centrifuge tests because of the infinite slope geometry. Issues were identified with the constitutive model used to represent the liquefiable sand. These issues included a coefficient of volumetric compressibility that was too low and a sensitivity to low level accelerations when the stress path is near the failure surface. In the final part of this research, the simulated and experimental data was used to examine the relationship between the generated excess pore water pressures and the resulting horizontal deformations. It was found that the deformations are directly influenced by both the excess pore pressures and the intensity of shaking. There is an excess pore pressure threshold above which deformations begin to become significant. The horizontal deformations correlate well to the integral of the average excess pore pressure ratio-time history above this threshold. They also correlate well to the Arias intensity and cumulative absolute velocity intensity measures.Item Pore pressure response of liquefiable soil treated with prefabricated vertical drains : experimental observations and numerical predictions(2012-05) Tsiapas, Ioannis, 1986-; Rathje, Ellen M.; El Mohtar, ChadiPrefabricated vertical drains represent a soil improvement technique that achieves liquefaction mitigation by decreasing the drainage path length and hence expediting the dissipation of excess pore pressures. When evaluating the required spacing between vertical drains to achieve the desired reduction in pore pressure response, simplified design charts or more sophisticated finite element analyses are used to predict the pore pressure response. These charts and programs have not been evaluated in terms of their accuracy because there exists little data with which to compare the numerical predictions. More recently, the effectiveness of prefabricated vertical drains for liquefaction mitigation has been evaluated via small – scale centrifuge testing performed on untreated soil deposits and on soil deposits treated with vertical drains. In particular, the performance of the soil deposits subjected to sinusoidal motions and actual earthquake recordings was tested. The main goal of this research is to compare the experimental observations of pore pressure response from the centrifuge experiments with the numerical predictions. The comparison focuses on the average excess pore pressure ratio (r_(u,avg)) that was developed in the location of a vertical pore pressure array in both the untreated and drain – treated sides of the models. In parallel, a parametric study is performed for the numerical predictions in order to study the effect of each input parameter that influences the pore pressure prediction, namely the effect of soil properties, ground motion characteristics and drain parameters. The numerical predictions are found to provide reliable predictions of the pore pressure response despite the simplicity of the constitutive model employed. The numerical predictions of r_(u,avg) time – histories are generally in good agreement with the recorded values in the centrifuge experiments. In most of the cases, the numerical model managed to predict the same maximum average excess pore pressure ratio, which is the parameter that is used in drain design. To incorporate any uncertainty on the soil properties or on the characteristics of shaking, the use of a smaller pore pressure threshold for drain design is recommended.Item Undrained, monotonic shear strength of loose, saturated sand treated with a thixotropic bentonite suspension for soil improvement(2010-08) Rugg, Dennis A.; El Mohtar, Chadi Said; Rathje, Ellen M.Liquefaction is a phenomenon that occurs in loose saturated sand deposits that are subjected to earthquake loading. This phenomenon can cause massive displacements and significant destruction. Many methods for mitigating liquefaction have been proposed and investigated including compaction, drainage, and grouting. One such liquefaction mitigation technique involves the addition of bentonite fines to the pore spaces of a loose, saturated sand via permeation of an engineered clay suspension. This method of soil improvement has provided the basis and motivation for this research. Also, the effect of plastic and non-plastic fines on the static and cyclic response of sands is somewhat contradictory throughout the literature. Thus, the primary objective of this study was to characterize the affect of an engineered bentonite pore fluid on the undrained monotonic response of loose, saturated Ottawa sand in order to determine its feasibility for use as an effective method for liquefaction mitigation. The permeation of engineered bentonite suspensions is proposed as a passive site remediation technique. Thus, the suspensions were delivered to loose Ottawa sand specimens in the laboratory by permeation in a newly designed three-way split mold. This split mold was used to create easily tested specimens that would have an initial soil fabric similar to that expected after permeation in the field. The bentonite suspensions were treated with sodium pyrophosphate to reduce the initial yield stress and viscosity in order to allow for permeation. Three different bentonite suspensions were utilized throughout this study each having different properties and delivering slightly different amounts of bentonite to the loose, saturated sand. The affect of this engineered pore fluid on the undrained shear response of loose, saturated Ottawa sand was compared to the undrained shear response of clean sand and dry-mixed sand and bentonite. The specimen preparation method (dry-mixed or permeated) was shown to have a significant effect on the response of the sand specimens. While the dry-mixed specimens produced larger and more sustained positive pore water pressures than the clean sand (resulting in an increased tendency to flow), the permeated specimens showed a marked decrease in the generation of excess pore water pressures, displayed a more dilative response, and thus resulted in a soil structure that was less likely to flow. Finally, the results of tests on specimens permeated with engineered bentonite suspensions show that there is little to no change in the effective friction angle at critical state. A method for effectively testing permeated soil specimens was developed in this study. This method has laid the framework for further investigations into the use of engineered bentonite suspensions for liquefaction mitigation by permeation grouting.