Sumanta Haldar

Reliability measures for pile foundations based on cone penetration test data

Abstract: The in situ behaviour of pile foundations is considerably influenced by variability in soil properties. Cone penetration (CPT) data are often used to determine the pile ultimate capacity. A wider range of values of the ultimate capacity are predicted when different CPT-based methods are used, as compared to using pile load test results. The present study considers inherent soil variability, measurement, and transformation variability. The undrained shear strength obtained from CPT data is considered to be a random variable. An approach to obtain load–settlement curves and the associated statistics from CPT data is suggested. Component reliability indices, based on ultimate limit state (ULS) and serviceability limit state (SLS) criteria, and system reliability indices combining ULS and SLS are evaluated. The variability in the pile–soil interface parameters and pile ultimate capacity is quantified in a Monte Carlo framework using the measured data. The effects of variability, scale of fluctuation, and limiting serviceability settlement on the reliability of pile foundations are also examined. A geotechnical database from the Konaseema site in India is utilized as an example. It is shown that the reliability based design of pile foundations considering spatial variability of soil, along with the variables associated with pile–soil interface properties, enables a rational choice of design loads.

Seismic response of soil-pile raft-structure system

AbstractThis paper presents an initial effort to investigate seismic response of soil-pile raft-structure system considering soil-structure interaction effect. In general, structure and piled raft under seismic load are designed considering fixed base condition. However, soil flexibility may result significant changes in the response of soil-pile raft-structure system. The study considers one storey system consisting of a mass in the form of a rigid floor slab supported by four columns. The piles are modelled by beam-column element supported by laterally distributed springs and dampers. This simple model used in present study is adequately tuned to exhibit reasonably accurate dynamic characteristics while compared to the existing well accepted methodologies. The study shows that soil-structure interaction leads to considerable lengthening of period though the lateral shear in columns are not significantly changed. However, the shear in piles is significantly increased due to SSI effect as inertia of the c...

Response of Vertically Loaded Pile in Clay: A Probabilistic Study

This study presents the response of a vertically loaded pile in undrained clay considering spatially distributed undrained shear strength. The probabilistic study is performed considering undrained shear strength as random variable and the analysis is conducted using random field theory. The inherent soil variability is considered as source of variability and the field is modeled as two dimensional non-Gaussian homogeneous random field. Random field is simulated using Cholesky decomposition technique within the finite difference program and Monte Carlo simulation approach is considered for the probabilistic analysis. The influence of variance and spatial correlation of undrained shear strength on the ultimate capacity as summation of ultimate skin friction and end bearing resistance of pile are examined. It is observed that the coefficient of variation and spatial correlation distance are the most important parameters that affect the pile ultimate capacity.

Buckling and bending response of slender piles in liquefiable soils during earthquakes

Design of pile foundations in seismically liquefiable soils involves identifying the appropriate failure mechanisms. Piles in liquefiable soils are conventionally designed against bending failure due to lateral loads arising from inertia and/or lateral spreading. This is strong evidence that there is another mechanism, which the code does not consider, that may govern the failure of these foundations. In this paper, the response of a single end bearing pile in liquefied soil with and without the effect of axial load has been presented. The effect of liquefaction is incorporated in the pile–soil interaction through nonlinear analysis using the finite difference program Fast Lagrangian Analysis of Continua (FLAC). The method of analysis is carried out using the well documented failure of Showa Bridge piles which failed during the 1964 Niigata earthquake. The response of the pile is also evaluated using dynamic analysis. The need for proper identification of failure mechanisms as well as design guidelines is highlighted.

Influence of dynamic soil-pile raft-structure interaction: an experimental approach

Traditionally seismic design of structures supported on piled raft foundation is performed by considering fixed base conditions, while the pile head is also considered to be fixed for the design of the pile foundation. Major drawback of this assumption is that it cannot capture soil-foundation-structure interaction due to flexibility of soil or the inertial interaction involving heavy foundation masses. Previous studies on this subject addressed mainly the intricacy in modelling of dynamic soil structure interaction (DSSI) but not the implication of such interaction on the distribution of forces at various elements of the pile foundation and supported structure. A recent numerical study by the authors showed significant change in response at different elements of the piled raft supported structure when DSSI effects are considered. The present study is a limited attempt in this direction, and it examines such observations through shake table tests. The effect of DSSI is examined by comparing dynamic responses from fixed base scaled down model structures and the overall systems. This study indicates the possibility of significant underestimation in design forces for both the column and pile if designed under fixed base assumption. Such underestimation in the design forces may have serious implication in the design of a foundation or structural element.

Resistance Factors for Laterally Loaded Piles in Clay

Sustainable engineering requires that engineering products are not only socially acceptable, economically feasible, and environmentally friendly, but also safe against all possible serviceability and ultimate limit states. Laterally loaded piles are mostly designed against the serviceability limit state of excessive lateral deflection. This study takes into account the uncertainties associated with the response of laterally loaded piles and focuses on their load and resistance factor design based on the serviceability limit state of excessive lateral deflection. Resistance factors are obtained for laterally loaded piles embedded in clay deposits in which the soil properties are assumed to be random variables. Pile capacities are determined based on a specified allowable lateral deflection at the pile head. The pile load-displacement curves are generated using the p-y method. Model uncertainties and bias factors are incorporated in the analysis. The uncertainties associated with the lateral capacity, for a specified lateral head deflection, are quantified, and the probability distribution of the lateral capacity is determined using Monte Carlo simulations. The applied dead and live loads are assumed to follow normal and lognormal distributions, respectively. First order reliability analysis is then performed using the distributions of loads and capacity to determine the load and resistance factors.

Beam on Spatially Random Elastic Foundation

Beams on elastic foundations have been mostly analyzed using the Winkler model, in which a series of disjointed elastic springs are used to represent the foundation soil. However the two-parameter Vlasovs foundation model gives better representation of beam-soil interaction since it incorporates normal and shear resistance of soil. Previous studies examined response of beams on elastic foundation model considering model parameters as deterministic quantity. In contrast to previous studies, this paper examines the response of beams resting on elastic soil media with spatial random variations of soil Young’s modulus. The randomness in the soil properties is incorporated in the two-parameter Vlasovs foundation model and, subsequently, the beam responses are obtained. Statistics of beam response are obtained by Monte Carlo simulation approach. It is observed that the soil inherent variability alters the beam response significantly.

Probabilistic Seismic Design of Pile Foundations in Non Liquefiable Soil by Response Spectrum Approach

The behavior of pile foundations in non liquefiable soil under seismic loading is considerably influenced by the variability in the soil and seismic design parameters. Hence, probabilistic models for the assessment of seismic pile design are necessary. Deformation of pile foundation in non liquefiable soil is dominated by inertial force from superstructure. The present study considers a pseudo-static approach based on code specified design response spectra. The response of the pile is determined by equivalent cantilever approach. The soil medium is modeled as a one-dimensional random field along the depth. The variability associated with undrained shear strength, design response spectrum ordinate, and superstructure mass is taken into consideration. Monte Carlo simulation technique is adopted to determine the probability of failure and reliability indices based on pile failure modes, namely exceedance of lateral displacement limit and moment capacity. A reliability-based design approach for the free head pile under seismic force is suggested that enables a rational choice of pile design parameters.

Probabilistic analysis of load-settlement response from pile load tests

The evaluation of variability in ultimate pile capacity from the load-settlement data is useful in the context of code calibration and reliability based design in pile foundations. This paper examines the applicability of two non-linear analytical methods to calculate the load-settlement response of piles using actual test data in terms of percentage deviation from the measured capacity. The degree of agreement associated with each method with respect to field test data is quantified using two different failure criteria (FHWA and Eurocode) for determination of the ultimate load of pile. The analytical methods are used to quantify the variability associated with the soil-pile interface parameters and ultimate capacity using Monte Carlo simulations, which is useful in load-resistance factored/reliability design of pile foundations. Study reveals that variability depends on the method of analysis, percent deviation of prediction from measured values and failure criteria.

Reliability-Based Design of Pile Foundations

Properties of natural soil are inherently variable that influences the design of pile foundation. Apart from inherent variability, uncertainty in pile design arises due to measurement of soil properties in the field or laboratory tests and model error. Furthermore, long-term performance of foundation also depends on fluctuation in loads and resistances both in temporal and spatial scales. Hence, a probabilistic model for the assessment of response characteristics of foundation is necessary. The objective of reliability-based design is to quantify probability of failure or reliability of a foundation system considering variability in the design parameters and associated safety. This article primarily addresses the reliability-based design methodologies of pile foundation under static and dynamic loading. The significance of consideration of variability in soil parameters in the design of pile foundation pertaining to building foundation and offshore structures is highlighted.