Yannis K. Chaloulos

Insight into the Seismic Liquefaction Performance of Shallow Foundations

The mechanisms that control the seismic liquefaction performance of shallow foundations are identified for the special, still com- mon case of a clay crust separating the foundation from the liquefied ground. For that purpose, foundation response is first analyzed with the nonlinear dynamic finite-difference method and consequently evaluated in connection with published field and experimental evidence. Insight is given into the excess pore-pressure buildup under the foundation, the seismic settlement accumulation, the static-bearing capacity degrada- tion, and the inertia-induced interaction with the superstructure. It is thus shown that a naturally or artificially created nonliquefiable soil crust may effectively mitigate the detrimental effects of liquefaction and allow for a performance-based design of surface foundations, without

Analysis of Liquefaction Effects on Ultimate Pile Reaction to Lateral Spreading

Current design methods for piles in liquefied ground assume that the ultimate soil pressures acting on the pile are drastically reduced relative to the reference values in the absence of liquefaction. However, there is controversy about the adopted design parameters and their effects. Furthermore, it has been experimentally shown that soil pressures are not always reduced, but they may increase well above the recommended design values because of flow-induced dilation of the liquefied soil around the upper part of the pile. In view of this, the pile response is simulated in this paper using a three-dimensional, fully coupled dynamic elastoplastic numerical analysis. The methodology is first verified against results from centrifuge experiments and consequently applied parametrically for various pile, soil, and seismic excitation characteristics. It is thus shown that dilation-induced negative excess pore pressures are indeed possible for common pile and soil conditions at the upper segments of the pile, having an overall detrimental effect on pile response. It is further found that, apart from the commonly considered effect of relative sand density, ultimate soil pressures are affected by a number of other dilation-related parameters, such as the effective confining stress, the permeability of the sand, and the predominant excitation period as well as the pile diameter and bending stiffness. To quantify the relevant effects, new multivariable relationships are established and subsequently evaluated against the empirical methodologies that are currently used in practice.

Kinematic interaction of piles in laterally spreading ground

All current empirical approaches for pile design in liquefied soils agree that the ultimate soil pressure on the pile is drastically reduced relative to the reference ultimate pressures, in the absence of liquefaction. However, there is disagreement with regard to the extent of the aforementioned reduction and also controversy about the pile and soil parameters which control it. For instance, well documented experimental data from centrifuge tests show that significant negative excess pore pressures may develop due to the dilation of the liquefied soil that flows around the upper part of the pile, thus enhancing ultimate soil pressures well above the recommended values. In view of the above objective uncertainties, the problem was analyzed numerically using a 3D dynamic procedure. Namely, FLAC 3D was combined with the NTUA Sand constitutive model, for dynamic loading and liquefaction of cohesionless soils, and was consequently used to perform parametric analyses for various pile, soil and seismic excitation characteristics. To ensure the validity of the predictions, the numerical methodology was first verified against the afore mentioned centrifuge experiments. It is thus concluded that dilation-induced negative excess pore pressures are indeed possible for common pile and soil conditions encountered in practice. As a result, apart from the relative density of the sand, a common parameter in most empirical relations, a number of other dilation related factors influence also the ultimate soil pressure, such as: the effective confining stress, the permeability of the sand and the predominant excitation period, as well as the pile diameter and deflection. Furthermore, it is shown that dilation effects are more pronounced at the upper and middle segments of the pile, having an overall detrimental effect on pile response. Finally, a preliminary evaluation of numerical results shows that the development of a new methodology for the evaluation of p–y response in laterally spreading soils which would incorporate the above effects is feasible.

Pile Design in Laterally Spreading Soil: Feedback from Numerical Predictions and Model Test Results

The main findings are summarized of a systematic research effort regarding the response of pile foundations in laterally spreading soils. The incentive of the research were findings from properly scaled (with regard to the pore fluid) centrifuge experiments suggesting that severe soil dilation may occur at the upper part of the pile, as a result of large soil-pile relative movement, causing soil pressures to significantly increase. In essence, this observation negates current design practice which univocally accepts that soil liquefaction drastically reduces seismic demands. To further explore this new evidence, a 3D nonlinear numerical methodology was developed and tested against the aforementioned experiments. On this basis valuable feedback was first gained with regard to current numerical techniques, with most important the need of a new type of tied-node boundaries for the simulation of submerged infinite slopes. Comparative analyses, with the old and the new boundaries, revealed that the former (which also reflect the kinematic response of the laminar box containers employed in model tests) can significantly underestimate soil pressures imposed to the foundation. In the sequel, the numerical methodology was applied parametrically (for various soil, pile and excitation characteristics) and a new set of multivariable relationships was statistically established for the practical estimation of ultimate soil pressures applied to the pile. Compared to existing empirical relationships, the new ones can capture the aforementioned dilation phenomena, which may develop at the upper part of the pile in the case of relatively low permeability soils (e.g. fine-grained, silty sands) and have an overall detrimental effect on pile response.