Search

Many submarine landslides occur through marine sensitive clay layers, and the strain softening behavior plays a key role. The changing soil properties induced by various disturbances will have marked impact on earthquake performance of submarine clay slopes. In the present study, undrained triaxial compression tests on reconsolidated samples of a soft sensitive marine clay were conducted, and a soil disturbance dependent simple constitutive model is proposed for describing post-peak nonlinear strain softening behavior of sensitive clays based on experimental results, and then is implemented in QUIVER code to perform one-dimensional seismic response analysis of mild infinite slopes, to address the relative importance of soil disturbance effect on the site response and sliding displacement of submerged slopes during seismic loading. Detailed laboratory test descriptions and results of a soft sensitive marine clay, model formulations and comprehensive computational efforts are presented. Parametric study of site response analysis focusing on the effects of soil disturbance degree and depth on generic clay grounds with varying sloping angles subjected to harmonic and earthquake loadings are performed. The predicted acceleration amplification, maximum shear strains and permanent displacements provide insight of earthquake performance of submarine sensitive clay slopes. The main findings are drawn based on several key concepts. First, undisturbed sensitive clay is typically stiffer and stronger than disturbed clay consolidated at the same stress level and hence, unless strain softening is triggered, structured sensitive clay will perform better than insensitive clay of the same residual strength. Second, post-peak softening at shallow depth will lead to strain localization on a thin layer, which is the main cause of sudden change of upward wave propagation characteristics and large permanent displacement at seabed surface of mild submarine clay slope. Thirdly, soil disturbance degrees and depths have very marked effects on seismic response and displacement performance of sensitive clay slopes, and such effect will be further enhanced by slope angle or earthquake intensity, which should be cautiously treated in engineering design.

Nonlinear site response of deep offshore clay deposit plays an important role in changing the characteristics of ground motions when subjected to strong shaking, especially when different amplification behaviors of acceleration and displacement are considered. The paper describes a series of centrifuge shaking table tests and numerical simulations to investigate the behavior of deep offshore clay deposits subjected to earthquake loadings. The centrifuge model tests were performed on a slightly overconsolidated clay deposit at UC Davis. A suite of shaking tests were conducted including impulse step wave testing, frequency sweeps, a small “elastic” earthquake event and a “ductility” level large earthquake event. Accelerations, displacements and pore pressures were measured in the soils (free field) throughout the test program. For the elastic level excitations, about a 33% amplification was observed between the input acceleration at the base and the measured accelerations near the top of the clay deposit. However, for the ductility-level earthquake excitation, which had peak input acceleration at the base of 0.46g, acceleration near the top of the deposit was reduced to 0.07g. These observations are correctly simulated by DEEPSOIL with proper soil models and parameters back analyzed from the centrifuge model test. Then systematic 1-D site response analyses are performed on synthetic clay deposit models with 30 to 120 m depths using the validated DEEPSOIL program, and the amplification characteristics of acceleration and deformation induced by base excitation with different intensities and frequencies are analyzed in both time and frequency domains. The results reveal that for deep soft offshore clay deposits subjected to large earthquakes, significant acceleration attenuation may occur near the top of deposit due to soil nonlinearity and even local shear failure; however, significant amplification of displacement at low frequencies is expected regardless of the intensities of base motions, which suggests that for displacement sensitive offshore foundations and structures, such amplified low-frequency displacement response will play an important role in seismic design.

The 2008 Wenchuan earthquake caused widespread liquefaction of gravelly soils (i.e., gravelly sands and sandy gravels), which phenomena show many differences compared to those of typical sands. To study the liquefaction mechanism and triggering evaluation of gravelly soils during this event, comprehensive case studies with measurement of shear wave velocity on both liquefied and non-liquefied sites were carried out according to a suite of recommended procedures for site investigation, field testing and data processing. Three typical cases were provided to illustrate the distinct characteristics of ground motion, liquefaction manifestation, soil profiles and field testing, and explore the possible mechanisms of liquefaction. Eighty-one well identified case histories were obtained, which feature the updated estimation of ground motion parameters from the USGS ShakeMap and the probabilistic treatment of the datasets. The preliminary analyses on liquefaction triggering boundary curves, shear wave velocities of these case histories and the applicability of the Ishihara criterion (1985) indicate that the gravel content plays a key role in the liquefaction mechanism and the consequences of gravelly soil deposits, and the liquefaction evaluation of gravelly sands and sandy gravels should be treated separately.

Strong earthquake events often result in liquefaction-induced ground movements such as settlements and lateral spreads that are a major cause of damage to buildings, bridges, and lifelines. This paper focuses on the subject of liquefaction-induced lateral spreads. Case histories with detailed cone penetration test (CPT) data are derived from the 2010–2011 Canterbury, New Zealand earthquake sequence and used to examine an existing empirical model for liquefaction-induced lateral spreads. The results confirmed the existence of a substantial discrepancy between the predicted and observed lateral spreads, indicating a need for improved models that consider uncertainties in the input parameters. Using a maximum likelihood analysis of the derived case histories, a new empirical probabilistic model was developed for the estimation of lateral spreads in the Christchurch, New Zealand area. Emphasis is placed on assessing the probability of exceedance of limiting displacements for design against the threat of lateral spreads. The new CPT-based probabilistic model with detailed formulations is presented along with an illustrative example. The limitations of the model are discussed.

The development of physical modeling for mega engineering prototypes requires sophisticated use of the capability of current centrifuge facilities. In this study, centrifuge model tests were performed to investigate the dynamic response of high fill slope by using generalized scaling law in an improved way. Firstly the two-stage generalized scaling law was partly re-organized by considering the separate similitude of dynamic strain for the virtual 1 g field, to solve the inherent problem of the scaling of soil strain when the derived prototype strain exceeded some important thresholds. Then a series of centrifuge model tests were conducted under different centrifugal accelerations to model a high fill slope with a prototype height of 100 m by the approach of “modeling of models”. Dynamic responses under sine waves with different amplitudes were monitored and the re-organized generalized scaling law was validated for nonlinear dynamic response with shear strain less than 1%. It was observed that site amplification of high fill slope was affected by both soil nonlinearity and soil strata non-uniformity, where soil nonlinearity led to smaller acceleration amplification under stronger excitation and soil strata non-uniformity contributed to the acceleration de-amplification and larger displacement amplification at the weak sublayer. The large dynamic displacement near the slope surface consequently contributes to the development of lateral cracks after strong shaking. Seismic response under earthquake motions revealed that high fill slope with inclined non-uniform strata was characterized by multiple vibration modes and lower fundamental frequency, which was more vulnerable to earthquake motions with abundant frequency components. This study provides a more physically meaningful use of the similitude of dynamic stress-strain relation in centrifuge modeling for large-scale geotechnical problems under dynamic loadings.