Reliability of API, NGI, ICP and Fugro Axial Pile Capacity Calculation Methods

Abstract

The API RP2A (RP2GEO) and ISO 19902 guidelines include four CPT-methods for calculating the axial capacity of piles in sands. The guidelines require that if newer methods are to be implemented in design, the same level of safety shall be documented for these methods as for existing methods. The designer is required to select an appropriate safety factor when using the newer design methods. The challenge lies in deciding which safety factor will ensure a consistent safety level for different soil conditions and pile dimensions. To evaluate the required material factor, the probability of failure was quantified in two case studies for piles designed with the API method and with the newer NGI, ICP and Fugro methods. A calibration of the required material factor for a target probability of failure of 10 -4 /yr was also performed. The results show that the annual reliability index and probability of failure vary with the axial pile capacity calculation method. The study provides a contribution to the discussion on the reliability of the API, the NGI, the ICP and the Fugro methods. The material factor needs to be associated with the characteristic soil parameters selected for design. A large number of case studies should be added to quantify the reliability and the required material factor for each pile capacity method. The findings on margin of safety and the definition of the characteristic shear strength have important implications for the design of piles offshore and can result in significant savings.

Calibration of partial safety factors for offshore foundation design

“…Typically, the annual maximum storm-induced load on the foundation follows a Gumbel (Type I) extreme value distribution (Lacasse et al, 2013a):…”

“…In the analyses described herein, the characteristic foundation capacity was assumed to have a lognormal distribution, based on a study reported in Lacasse et al (2013aLacasse et al ( , 2013bLacasse et al ( , 2013c. A parametric study of the uncertainty in the soil was performed by including values of coefficient of variation, CoV (defined as ratio of standard deviation to mean value), of 15, 20 and 25%.…”

“…The calculations of the deterministic and probabilistic capacity involved six steps (Lacasse et al, 2013a(Lacasse et al, , 2013b(Lacasse et al, , 2013c:…”

“…The analyses in the Lacasse et al (2013c) paper indicate that the uncertainty in the model used to calculate the capacity can have a very significant, sometimes overwhelming, effect on the resulting probabilistic capacity and the ensuing probability of failure. The model uncertainty in a calculation method was also quantified in terms of a mean (or bias), standard deviation and the probability density function that best fitted the data.…”

“…where  represents the modelling and statistical uncertainty in load (effect) calculations and 'scale' is the scaling factor to be used on Q ult to obtain the selected target annual failure probability. On the basis of three case studies of offshore jackets with piled foundations (Lacasse et al, 2013a),  was assumed to have a normal distribution with standard deviation  of 0.1 and mean  of 1.…”