Conference Paper

Centrifuge Experiments to Evaluate the Seismic Performance of Levees on Peaty Soils in the Sacramento-San Joaquin Delta

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Abstract

Two large scale 9 m radius centrifuge tests were conducted at the NEES@UCDavis experimental facility to understand the interaction between soft peat and stiffer levee fills, to gain insight into the volume change behavior of organic soils under cyclic loading, and to study deformation potential of saturated sandy levees resting on peat due to liquefaction of the levee fill. This study has potential applicability to the Sacramento-San Joaquin Delta where unengineered levee fills rest atop soft compressible peat soils. This paper focuses on the interaction between a nonliquefiable clay levee resting atop soft peat during application of a sine sweep base excitation. Transfer functions derived from the data were compared with results of 1-D ground response analyses. The 1-D analyses capture reasonably well the first mode response of the levee-foundation system, but do not capture a prominent rocking mode. The rocking mode has not been previously observed experimentally for embankment systems and can impose substantial local demands on the levee fills and foundation peat near the edges of the levee.

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... Crack Formation Wall Rotation Wall Failure Centrifuge modeling has also been used to study the seismic performance of levees. A clay levee built on a peat layer of the Sacramento delta was modeled at 57g by Cappa et al. [38]. ...
Thesis
Earthen levees are constructed worldwide as flood control infrastructure and are important components of public safety in the areas they protect. Among the factors which negatively impact the stability and serviceability of earthen levees, internal deteriorations are known to be the most critical type of structural problem. A common type of internal deterioration, animal burrows dug into earthen levee embankments are reported to be the cause of failure in many cases of earthen levees breaches. Globally, the annual cost of damage resulting from failure of earthen structures and associated infrastructure due to invasive wildlife activities is estimated to be many billions of dollars. Understanding the mechanisms of wildlife-caused earthen levee failure is a key component of preventing future breaches. Much of the literature in the area of nuisance wildlife investigates the ecological and environmental impacts of animal activities and habitat, however, studies related to failure mechanisms of earthen structures due to invasive wildlife activities are limited and require a geotechnical engineering perspective. This research aims to identify the mechanics that govern the progress of failure within wildlife-induced levees deteriorations. Investigation of the impact of animal burrows on the hydraulic performance and stability of levee structures is performed using centrifuge modeling. Scaled-down earthen levee models with both landside and waterside burrows as well as a benchmark intact levee model are built and tested at 35g during the centrifuge experiments. The centrifuge experiments are monitored and recorded for deformation, seepage, and pore pressure measurements. Particle Image Velocimetry (PIV) analyses are performed on series of images captured during the centrifuge flights to calculate global deformation of the levee models. Finite element models are developed based on the experiments and used to conduct parametric studies on the impacts of burrow configurations on the stability of the deteriorated levees. The studies investigate key parameters which governs levee safety: burrow length, burrow depth and levee side slope ratio. Details and results of the experimental and numerical work are presented in this thesis along with conclusions and recommendations for future research.
... The rigid container has transparent side walls to enable the acquisition of videos during testing, which was important for this project and outweighed the undesired boundary conditions imposed at the rigid soil/container contact. Studies comparing measured free field responses with results obtained through 1D site response analyses showed good agreement and suggested minimal influence of boundary effects on the overall specimen behavior (Cappa et al., 2014c). Figure 2b shows the placement of the container on the centrifuge arm with its respective global coordinate system. ...
Article
Four large-scale centrifuge tests were performed at the [email protected] equipment site to study the cyclic behavior of levee structures resting atop soft organic peat. The model configurations using a non-liquefiable levee focused on the seismic deformation potential of peat during primary consolidation and secondary compression. The tests performed with a sandy levee studied the liquefaction potential of saturated loose sand fill overlying soft peat as well as the levee-peat-interaction under different loading conditions. The models were subjected to scaled ground motions representative of the Sacramento/ San Joaquin Delta. System instrumentation consisted of linear potentiometers, pore pressure sensors and accelerometers. Slow data recorded at 1 Hz document the settlements during spin up, application of ground motions, and spin down. Fast data sampled at 4,167 Hz measured the dynamic response of the system, the excess pore pressure increase and immediate settlements. The project is archived at the NEES data repository under nees.org/warehouse/project/1161.
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The seismic response of a stiff levee structure placed on soft peat foundation soil is studied using centrifuge testing. The model levee was 5.1 m tall in prototype dimensions and rested atop 6.1 m of peat. The model was shaken with a suite of earthquake ground motions scaled to various intensities. Vertical and horizontal displacement records obtained from accelerometer arrays embedded in the peat were interpreted using bilinear quadrilateral interpolation to obtain in-plane components of the Cauchy strain tensor at each time increment. A direct-simple-shear-equivalent shear strain invariant, γDSS,eq, was computed from the tensors. Values of γDSS,eq as high as 7% were observed for input accelerations of 0.52g. Residual excess pore pressures were mobilized at shear strain amplitudes higher than 1%, reaching maximum residual pore pressure ratios near 0.2. The observed relationship between the residual excess pore pressure ratio and mobilized shear strain agrees reasonably well with results from a laboratory simple shear testing program. Vertical strains in the levee toe region were 2.5 times larger than beneath the crest attributable to levee rocking under seismic loading. These vertical strains contribute to γDSS,eq and therefore constitute an important demand on the underlying peat.
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This paper describes a method to compute the mobilized shear modulus, G, and damping ratio, D, using the nonlinear fit of experimental transfer functions obtained at different depths in centrifuge models, with the analytical expression of the amplification function for a viscoelastic soil layer on a rigid base. The corresponding shear strain,γ, is computed as a function of the particle velocity and shear wave velocity. The sources of potential error in the determination of G, D, and γ embedded in the proposed method are identified and discussed in comparison with two other methods that have been proposed in the literature, based either on the determination of the time lag of accelerations between two accelerometers or on the evaluation of the shear stress-strain cycles from acceleration time histories recorded at different depths in the model. The performance of the threemethods is evaluated using the experimental data obtained from nine centrifuge tests on dry sand. The values of G obtained by the proposed method compare well with the results oflaboratory and literature data; D values are more dispersed and slightly higher than the literature data.
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Seismic site response and site effects models are presented for levees in the Sacramento-San Joaquin Delta where the subsurface soils include thick deposits of highly organic soils. Sources of uncertainty that contribute to the variation of seismic wave amplification are investigated, including variations in the input ground motions, soil profiles, and dynamic soil properties through Monte Carlo simulations of equivalent-linear site response analyses. Regression models for seismic wave amplification for levees in the Delta are presented that range from a function of peak outcrop acceleration alone to a vector of response spectra ordinates and soil profile parameters. The site effects models were incorporated into a probabilistic seismic hazard analysis for a representative location, and the relative impacts of the various models on the computed hazard are evaluated.
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Regression models are presented for the dynamic properties of highly organic soils. The models are based on a database of triaxial and resonant-column/torsional-shear cyclic loading tests on thin walled tube samples mainly retrieved from the Sacramento-San Joaquin Delta. The soils in this database range from highly fibrous peat to amorphous organic clays with organic contents OC ranging from 14–81%, water contents ranging from 88–495%, total densities ranging from 1.056– 1.450 Mg/ m 3 , and effective consolidation stresses vc ranging from 11– 135 kPa. The secant shear modulus G and equivalent damping ratio were modeled as variables dependent on the shear strain amplitude c , consolidation stress vc , and OC. The residuals of the regression models were analyzed against other predictor variables including undisturbed density , loading frequency f, and number of loading cycles N. A regression model for was developed, and conditional probabilities were used to improve the estimation of G and when measurements were available. The database of in situ measurements of shear wave velocity V s was used to adjust the regression model for in situ conditions. Variances and correlations in the regression models are presented.
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A primary source of uncertainty in any evaluation of the seismic stability of the Sacramento-San Joaquin Delta levee system is the site response characteristics of the shallow organic soils that commonly underlay the levees. This paper provides an overview of recent research on the site response characteristics of organic soils using centrifuge and numerical modeling. The centrifuge modeling effort included the development of techniques to measure the shear wave velocity profile for a centrifuge model while in-flight. One- dimensional site response analyses using an equivalent linear procedure were performed with the measured shear wave velocity profiles and the modulus reduction and damping relationships determined from prior laboratory studies. Good agreement was obtained between the numerical simulations and the centrifuge model recordings.
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Full-text available
Regression models are presented for the dynamic properties of highly organic soils. The models are based on a database of triaxial and resonant-column/torsional-shear cyclic loading tests on thin walled tube samples mainly retrieved from the Sacramento-San Joaquin Delta. The soils in this database range from highly fibrous peat to amorphous organic clays with organic contents (OC) ranging from 14-81%, water contents ranging from 88-495%, total densities (rho) ranging from 1.056-1.450 Mg/m(3), and effective consolidation stresses (sigma(')(vc)) ranging from 11-135 kPa. The secant shear modulus (G) and equivalent damping ratio (xi) were modeled as variables dependent on the shear strain amplitude (gamma(c)), consolidation stress (sigma(')(vc)), and OC. The residuals of the regression models were analyzed against other predictor variables including undisturbed density (rho), loading frequency (f), and number of loading cycles (N). A regression model for rho was developed, and conditional probabilities were used to improve the estimation of G and xi when rho measurements were available. The database of in situ measurements of shear wave velocity (V(s)) was used to adjust the regression model for in situ conditions. Variances and correlations in the regression models are presented.
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A highly instrumented centrifuge experiment was conducted at the Univ. of California at Davis, to investigate the seismic response of a saturated dense sand stratum. Nevada sand at about 100% relative density was employed in a laminated (flexible shear beam) container to simulate one-dimensional site response. Among the total of 27 imparted earthquake-like shaking events, peak accelerations near ground surface ranged from 0.03 to 1.7g (in prototype scale), covering linear to highly nonlinear scenarios. This comprehensive set of recorded downhole accelerations is utilized herein to identify variation of shear modulus and damping ratio with shear strain amplitude. The estimated modulus reduction and damping ratio display a confinement dependence. At shear strains below about 0.2%, modulus variation is found in reasonable agreement with the formulae of Hardin-Drnevich and the modulus reduction bounds of Seed-Idriss, while damping is generally higher. At shear strains larger than 0.2%, the shear-induced dilation tendency maintained secant shear modulus at about 20% of its initial value, with a 20% damping ratio approximately. In earlier laboratory experimental studies on loose to medium-dense sands, Vucetic and Matasovic also reported similar trends. Based on the findings, a two-phase (solid and fluid) fully coupled nonlinear finite element program is calibrated and used to conduct numerical simulations of representative weak to strong shaking events. The computational results are in good agreement with the recorded counterparts, and satisfactorily reproduce the important dilation effects.
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Correct evaluation of shear modulus and damping characteristics in soils under dynamic loading is key to both the fundamental understanding of soil behavior and the practical application of soil modeling programs. Dynamic centrifuge tests can contribute significant information about soil behavior, but great care must be taken over the signal processing techniques involved, and the test conditions are different from the laboratory experiments that form the database of existing knowledge. This paper outlines several factors that require careful consideration when deriving stiffness and damping parameters from centrifuge data. Shear modulus and damping degradation curves for a dry sand, saturated sand, soft clay and a model waste are then evaluated to explore some of the factors that are introduced during centrifuge tests. Stiffness is seen to be a more reliable parameter than damping ratio. Damping during centrifuge tests for certain materials appeared to differ from the expected values.
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Significant factors affecting the liquefaction (or cyclic mobility) potential of sands during earthquakes are identified, and a simplified procedure for evaluating liquefaction potential which will take these factors into account is presented. Available field data concerning the liquefaction or nonliquefaction behavior of sands during earthquakes is assembled and compared with evaluations of performance using the simplified procedure. It is suggested that even the limited available field data can provide a useful guide to the probable performance of other sand deposits, that the proposed method of presenting the data provides a useful framework for evaluating past experiences of sand liquefaction during earthquakes and that the simplified evaluation procedure provides a reasonably good means for extending previous field observations to new situations. When greater accuracy is justified, the simplified liquefaction evaluation procedure can readily be supplemented by test data on particular soils or by ground response analyses to provide more definitive evaluations.
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The peak ground acceleration (PGA) of 13.08 m/s(2) recorded at K-NET station in Ojiya city during the 2004 Mid Niigata Prefecture earthquake was 1.5-2 times greater than either of the other nearby stations on a similar soft soil (JMA) and on the outcrop rock (SSI). Those at K-NET during earthquakes with less intensity, however, tended to be smaller than those at JMA. The variation in relative magnitude of PGA with earthquake intensity is probably due to the difference in site conditions between K-NET and JMA. Field investigation including boring, PS logging, and spectral analysis of surface wave methods as well as laboratory tests on undisturbed samples obtained from the sites from this and previous studies reveal near-surface soil profiles and their nonlinear dynamic soil properties at both stations. Equivalent linear dynamic analysis is then performed using the detected soil profiles and properties for the two sites with the motions recorded at SSI as input outcrop motions. It is shown that, despite its very low shear wave velocity of about 50 m/s, the near-surface peat at K-NET exhibits weaker nonlinearity with larger damping at small strains but with smaller damping at large strains than does the near-surface silt at JMA. This might have had strong effects on the extremely large peak acceleration at K-NET during the main shock as well as on the variation in relative magnitude of PGA between K-NET and JMA during earthquakes of different intensity.
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Levees are very challenging engineering structures to study, in part because they are not typically well-engineered structures. Unfortunately, there is little to no guidance as to how to evaluate the seismic vulnerability of levees. This study focuses on systematically studying the dynamic response of levees using a wide range of input ground motions and developing a simplified procedure for the liquefaction triggering evaluation of earthen levees, accounting for the dynamic response of the levee. The study was based on three levee sites representative of three select California Central Valley regions; however, because floodplains tend to generally have similar depositional environments, the study can be extended to other regions as long as some of the principal characteristics are still applicable. A wide range of input ground motions was used to capture and assess the variability in response and performance because of multiple possible earthquake scenarios. Two aspects of the dynamic response and performance of earthen levees are presented in more detail: the site and topographic effects on the peak ground acceleration, and the shear stresses and the cyclic stress ratios for a series of profiles throughout the levee sites. A simplified methodology for assessing the factor of safety against triggering of soil liquefaction is developed, and recommendations for applying the methodology are presented and discussed.
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A model levee resting atop soft compressible peaty organic soil in the Sacramento/San Joaquin Delta was shaken by forced vibration to study the seismic deformation potential of the underlying peat and measure dynamic levee peat interaction. Forced vibration testing occurred over a frequency range of 0 Hz to 5 Hz and produced force amplitudes applied to the embankment crest that induced elastic to nonlinear levee-foundation responses. Available data include acceleration records from sensors mounted on the model levee and on the ground surface near the model levee, and acceleration and pore pressure measurements from sensors embedded in the underlying peat. A remote data acquisition system measured settlements and pore pressures over a span of more than a year, encompassing time before and after the dynamic testing. Small pore pressures were generated in the peat during testing although embankment settlements from cyclic loading were small.
Conference Paper
We investigate through laboratory testing the volume change characteristics of peaty organic soil from Sherman Island, California under static conditions (consolidation, secondary compression) and post-cyclic conditions. Incremental consolidation tests indicate the material to be highly compressible (Cc = 3.9, Cr = 0.4) and prone to substantial ageing from secondary compression (C a/Cc = 0.05 following virgin compression). Strain-controlled cyclic triaxial testing of the peat finds the generation of cyclic pore pressures for cyclic shear strain levels beyond approximately 0.5-1.0%, with the largest residual pore pressure ratios r ur (cyclic residual pore pressure normalized by pre-cyclic consolidation stress) being approximately 0.2-0.4. Post cyclic volume change occurs from pore pressure dissipation and secondary compression. The level of post-cyclic secondary compression increases with rur. Many of these phenomena have not been documented previously and suggest the potential for seismic freeboard loss in levees due to mechanisms other than shear failure.
Final Report for the Washington State Department of Transportation
  • S Kramer
Kramer, S. (1996). "Dynamic Response of Peats." Final Report for the Washington State Department of Transportation, WA-RD 412.1