Reliability-Based Approach for Foundation Design without Site-Specific Soil Borings
Authors: Gilbert R.B. , Gambino S.J. , Dupin R.M. 
Published: October 2008

A reliability-based approach is presented to design an offshore pile foundation without a site-specific boring. The essence of this approach is to provide the same level of reliability for the pile foundation, whether or not a site-specific boring is used in its design. Design charts are provided to apply this approach within the frameworks of both the Load and Resistance Factor Design code and the older Working Stress Design code. The design charts relate design factors to the magnitude of uncertainty in pile capacity resulting from the lack of a site-specific boring. This reliability-based approach can be applied both to design a new pile foundation and to requalify an existing pile foundation. It can also be used to make cost-benefit decisions about the need for and optimal amount of additional site information, such as soil borings or geophysics. Examples are provided to demonstrate its use both in the design of piles and the design of site investigation programs.

Introduction

The most recent version of API RP 2A (Ref. 1) for the design of pile foundations incorporates a reliability-based approach. Load and resistance factors are established to achieve a target level of reliability for the foundation. These factors were developed assuming that a soil boring is drilled at the site, that pushed sampling methods are used and unconsolidated-undrained (UU) triaxial tests are performed to determine the undrained shear strength for clays, and that the density and grain size for cohesionless soils is determined. However, it is not always possible both in terms of cost and time to drill a site-specific boring. Examples include platforms that are moved off of the site of a boring before construction, older platforms up for requalification that were designed with either no site-specific boring or a poorer quality boring (e.g., a boring drilled using driven sampling methods), and the potential of characterizing a new field with a representative set of borings and geophysics without trying to place a boring at the precise location of every planned structure.

The objective of this paper is to establish a reliability-based approach to design a pile foundation without a site-specific boring. The essence of this approach is to provide the same level of reliability for a pile foundation, whether or not a sitespecific boring is used in its design. The greater the uncertainty in site-specfic soil properties, the greater the conservatism required in design to achieve a target level of reliability. Application of this approach within the framework of both the newer Load and Resistance Factor Design (LRFD) code and the older Working Stress Design (WSD) code is discussed. Examples are provided to demonstrate its use both in the design of piles and in the design of site investigation programs.

Design Reliability

The design reliability for an axially or laterally loaded pile is defined as the probability that the load on the pile does not exceed the capacity over the design life. For example, consider a case-study offshore field with a normally to slightly overconsolidated clay layer extending more than 300 feet below the mudline.