Ultimate limit state reliability-based design of augered cast-in-place piles considering lower-bound capacities

Abstract

The use of augered cast-in-place (ACIP) piles for transportation infrastructure requires an appropriate reliability-based design (RBD) procedure. In an effort to improve the accuracy of an existing design model and calibrate appropriate resistance factors, this study presents a significantly revised RBD methodology for estimating the shaft and toe bearing capacity of ACIP piles using a large database consisting of static loading tests in predominately granular soils. The proposed design models are unbiased, as opposed to those currently recommended. Based on the reasonable assumption that a finite lower-bound resistance limit exists, lower-bound design lines are developed for shaft and toe bearing resistance by applying a constant ratio to the proposed design models. Resistance factors are calibrated at the strength or ultimate limit state (ULS) for ACIP piles loaded in compression and tension for two commonly used target probabilities of failure with and without lower-bound limits. For piles loaded in compression, separate resistance factors are calibrated for the proposed shaft and toe bearing resistance models. The inclusion of a lower-bound limit for piles loaded in tension results in a 24%–50% increase in the calibrated resistance factor. For piles loaded in compression, the application of a lower-bound limit results in a 20%–150% increase in the calibrated resistance factor, and represents a significant increase in useable pile capacity. Although the impact of a lower-bound limit on resistance factor calibration is directly dependent on the degree of uncertainty in the distribution of resistance, this effect is outweighed by the type of distribution selected (i.e., normal, lognormal) at more stringent target probabilities of failure due to differences in distribution shape at the location of the lower-bound limit. A companion paper explores the use of the revised ULS model in a reliability-based serviceability limit state design framework.

Serviceability limit state reliability-based design of augered cast-in-place piles in granular soils

“…µ δ a , COV(δ a ), COV(Q' app ), D/B, ψ Q ). The use of β = 2.33 in this example is to aid comparison to the ULS capacities described by Reddy and Stuedlein (2016) in the companion paper; however, reliability indices for SLS should likely be lower owing to the reduced consequences of "failure" (i.e., exceeding the target displacement) for the SLS. For example, Eurocode 7 (e.g., Orr and Breysse, 2008) includes SLS provisions of β = 1.5 (or p f = 6.7%) over a 50-year service life.…”

“…Because very short ACIP piles are rarely constructed, the piles in this database were limited to D/B ≥ 20; the maximum D/B was equal to 68.5. The details of the piles in the load test database are provided in the companion paper (Reddy and Stuedlein 2016).…”

“…One objective of this study is to link RBD of ACIP piles at the ULS with that at the SLS through the ACIP pile-specific design models developed in the companion paper (Reddy and Stuedlein 2016). Past studies on ACIP piles by Phoon et al (2006) and Phoon and Kulhawy (2008) have sought to incorporate the accuracy and uncertainty associated with a ULS capacity prediction model into reliability assessments at the SLS using the Meyerhof method.…”

“…The goal of this study is to use the ACIP pile-specific ULS design models presented in the companion paper (Reddy and Stuedlein 2016) to investigate reliability-based SLS design of ACIP piles installed in predominately granular soils. Those case histories described in the companion paper characterized with high quality load-displacement (Q-δ) curves were used to investigate foundation reliability at the SLS.…”

Statistics of model factors in reliability-based design of axially loaded driven piles in sand

“…Nevertheless, the ULS still received most of the attention and more studies should be performed to develop reliability-based serviceability limit state design, as the SLS is often the governing criterion in foundation design (Becker 1996;Phoon and Kulhawy 2008;Wang and Kulhawy 2008;and Uzielli and Mayne 2011). At the ULS, a consistent load test interpretation criterion is used to produce the measured resistance and then, the resistance factor in LRFD is commonly calibrated from the statistics of the resistance model factor (AbdelSalam et al 2012;AbuFarsakh et al 2009AbuFarsakh et al , 2013Motamed et al 2016;Ng and Fazia 2012;Ng et al 2014;Paikowsky et al 2004;Reddy and Stuedlein 2017a;Stuedlein et al 2012;and Tang and Phoon 2018a, b). It is natural to follow the same approach for the SLS, where the ultimate resistance is replaced by an allowable resistance that depends on the allowable displacement (Zhang et al 2008).…”