Factors Affecting the Reliability of Augered Cast-In-Place Piles in Granular Soil at the Serviceability Limit State-Article

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The study evaluates reliability-based design of augered cast-in-place (ACIP) piles in cohesionless soils at the serviceability limit state (SLS), where settlement exceeds an allowable value. It uses an expanded database of 87 axial load tests and a probabilistic hyperbolic model to represent pile load–displacement behavior.<br /><br />The model’s two curve-fitting parameters, which describe initial stiffness and asymptotic resistance, were found to be strongly correlated with each other and with pile slenderness ratio (D/B). The researchers transformed the parameters to remove their dependence on D/B, then modeled their uncertainty using correlated lognormal distributions. They compared equivalent-normal and non-parametric correlation methods and used the first-order reliability method (FORM) to calculate reliability indices.<br /><br />Reliability estimates became relatively consistent with sample sizes above about 40 tests, although uncertainty in the estimates continued to decline as sample size increased. The choice of correlation coefficient had little effect on reliability estimates for sufficiently large samples.<br /><br />A parametric study examined allowable displacement, uncertainty in predicted pile capacity, and slenderness ratio. Uncertainty in the capacity prediction model generally affected reliability more than uncertainty in allowable displacement, highlighting the value of accurate capacity prediction. Slenderness ratio had the strongest overall influence: reliability decreased substantially for more slender piles, so pile geometry and its relationship to load–displacement behavior must be included in SLS assessments.<br /><br />The findings support using reliability-based methods to account explicitly for uncertainty in pile response, while emphasizing the need for adequate test data, appropriate modeling of parameter correlation, and attention to pile geometry.

Keywords

augered cast-in-place piles

ACIP piles

serviceability limit state

reliability-based design

cohesionless soils

probabilistic hyperbolic model

load-displacement behavior

first-order reliability method

pile slenderness ratio

settlement