STATIC AND DYNAMIC RESPONSE OF SOIL-PILE SYSTEM CONSIDERING SOIL UNCERTAINTIES

Abstract

The growing need for the civil infrastructure and the multi-storeyed structures in the megalopolises require reliable foundation systems ensuring public safety. Pile foundations, categorized under deep foundations is mostly preferred for the high-rise structures. Pile foundations are generally designed and constructed in group, are also considered whenever there is the occurrence of weak soil in the underlying depths of the subsoil. Uncertainties prevail in most of the engineered systems and are often observed in the geological and geotechnical aspects, in turn represent the substantial risk factor allied with the project. The uncertainty associated with the soil properties i.e., the parameter uncertainty is incorporated with the use of lognormal probability distribution pertaining to the parameter’s coefficient of variation. The major aspects of the pile group foundations include the bearing capacity calculation and the corresponding pile behaviour when subjected to the static and dynamic loads. The literature review revealed research gaps, with the lack of studies taking into the account of soil uncertainties in the CPT based bearing capacity calculation, combined axial and laterally loaded response and dynamic studies. The thesis discusses the influence of uncertainty associated with the soil properties on the above-mentioned aspects of the pile group foundations under static and dynamic conditions. The Cone Penetration Test (CPT) is site-specific and it is one of the predominant tests in geotechnical investigations used for varied construction facilities. The evaluation of axial load carrying capacity of pile is one such use. In the present study, the influence of spatial correlation in the CPT data towards the load carrying capacity of pile group in sandy soil is examined. The in-situ test approach involving the spatial correlation of the CPT data in the analytical formulations is established in the study. The inclusion of spatial correlation intervening the spatial averages of CPT data obtained along the soil profile for the pile length, and at its base is studied. The failure probability and its relationship with the factor of safety is illustrated based on the Rackwitz’s first-order reliability method (FORM). The results of the analyses have shown that, the inclusion of spatial correlation imparts a significant effect on failure probability and which when ignored will result in an unsafe design. A test pile group is analyzed to exemplify the reliability assessment of axial pile load carrying capacity by considering the spatial correlation of CPT data. This approach can be utilized for the safer pile design practices within the probabilistic framework.Pile foundations generally experiences various combinations of lateral and vertical load components i.e., axial and lateral loads when static loads are concerned. The present finitedifference study examines the static response of the pile group subjected to the combined axial and lateral loads in the cohesionless soil. A 2×2 pile group embedded in the cohesionless soil is considered in the study. The behaviour of the pile group relates to the properties of soil medium, especially the fiction angle and the elastic modulus. It is well known that the soil, being the natural material, comprises of uncertainties in its parameters. The elastic modulus of the soil is considered for the inclusion of parameter uncertainty in this study using the random variable approach and random field approach. The random variable approach is performed by assigning the random values of the soil property for each Monte Carlo run. The random field approach utilizes spatial variability modelling using Cholesky matrix decomposition technique and thereafter, Monte Carlo simulations are performed for every realization. The thesis investigates the influence of difference approaches on the lateral deflection, bending moment of pile group under the simultaneous action of vertical and lateral loads. And the incurring soil reaction due to the lateral resistance of piles when the lateral load is applied is also determined, envisaging the importance to study the p-y curves at different depths. The thesis addresses the statistical variation in the form of probability density function and cumulative density function of the pile bending moments and lateral deflections. The influence of scale of fluctuation in both the horizontal and vertical directions is also studied. The stochastic responses indicate that the parameter uncertainty of the soil properties and their coefficient of variation plays a significant role in the responses of soil-pile system. This thesis studies the influence of dynamic loads on the 2×2 pile group embedded in two-layered soil using the three-dimensional finite-difference program, FLAC3D, utilizing the high-performance computational facility at the Indian Institute of Technology Roorkee. A three different earthquake motions were selected to perform the seismic analysis and the corresponding pile group deflection, pile bending moments were determined for the various liquefiable soil depths. The influence of uncertainty in the shear modulus of the soil is studied in random variable and random field approaches. Monte Carlo simulations were performed by assigning the statistical generation of random values in the random variable approach. In the random field approach, the Cholesky matrix decomposition technique is utilized in modelling the spatial variability with the prescribed scales of fluctuations in both the horizonal and vertical directions. The stochastic responses of the pile group displacement and the pile bendingmoments were studied for the three different coefficients of variation of shear modulus. The amplification of the accelerations has also been evaluated stochastically. The study encompasses the influence of spatial correlation on the CPT based axial pile group capacity, deterministic and statistical variations of the pile group responses under both the static and dynamic loadings. Traditional pile group design process involves only the deterministic responses, whereas the present study provides meticulous insights on the stochastic analysis and responses of the pile group foundations which might be useful for the geotechnical fraternity towards reliable design practices.