Design and Construction of Continuous Flight Auger (CFA) Piles

Dan A. Brown, Ph.D., P.E., Steven D. Dapp, Ph.D., P.E.,
W. Robert Thompson, III, P.E., and Carlos A. Lazarte, Ph.D., P.E.
FHWA Geotechnical Engineering Circular 8
April 2007

This manual presents the state-of-the-practice for design and construction of continuous flight auger (CFA) piles, including those piles commonly referred to as augered cast-in-place (ACIP) piles, drilled displacement piles, and screw piles. CFA pile types, materials, and construction equipment and procedures are discussed. A performance-based approach is presented to allow contractors greater freedom to compete in providing the most cost-effective and reliable foundation system, and a rigorous construction monitoring and testing program to verify the performance. Quality control (QC)/quality assurance (QA) procedures are discussed, and general requirements for a performance specification are given.

Methods to estimate the static axial capacity of single piles are recommended based on a thorough evaluation and comparison of various methods used in the United States and Europe. Group effects for axial capacity and settlement, and lateral load capacities for single piles and pile groups are discussed. A generalized step-by-step method for selecting and designing CFA piles is presented, along with example calculations. An Allowable Stress Design (ASD) procedure is used.

Drilled Shafts:
Construction Procedures and Design Methods
(pdf format)

Federal Highway Administration
FHWA IF-99-025
August 1999

This publication, the classic on the subject by Michael W. O'Neill and Lymon Reese, is FHWA's primary reference of recommended construction procedures and design methods for drilled shafts. It was written as a resource for participants in a short course covering the topic of construction and design of drilled shaft foundations for bridges and other structures. The emphasis in this documetn is on providing relatively comprehensive information for engineers who already have some experience with drilled shaft construction and/or design. The initial chapters coer an overview of the characteristics of drilled shafts, site investigations for drilled shafts (to collect information for both constructin and design) an details of drilled shaft construction. These chapters are followed by several chapters on the design of drilled shafts in soil and rock for both axial and lateral loading, with examples. Both ASD and LRFD principles are addressed. Details of design calculation procedures are provided in the appendices. Procedures for performing load tests, an important component of design, are then reviewed, following which model construction specifications and prepared and discussed. The later chapters of the document deal with construction inspection, structural integrity testing, repair of defective drilled shafts and cost estimation. The chapter on inspection includes acceptance criteria and is intended to complement other short courses and documents on drilled shaft construction inspection.

P-y Curves for Laterally Loaded Drilled Shafts Embedded in Weathered Rock

(click on the title above to download)

M.A. Gabr, R.H. Borden, K.H. Cho, S.C. Clark, and J.B. Nixon
North Carolina State University
FHWA/NC/2002-008
December 2002

In areas of weathered and decomposed rock profiles, the definition of soil parameters needed for the analysis and design of laterally loaded drilled shafts poses a great challenge. The lack of an acceptable analysis procedure is compounded by the unavailability of a means for evaluating the weathered profile properties, including the lateral subgrade modulus, which often leads to the conservative design. Results from this research revealed that currently proposed P-y approaches to design drilled shafts embedded in weathered Piedmont profiles do not provide reasonable estimates of load-deflection response. Results in this report are used to develop and validate a procedure for the analysis of laterally loaded drilled shafts embedded in a weathered rock mass. The developed procedure is based on the P-y method of analysis in which the shape and magnitude of the P-y function are defined. The research proceeded along four complementary tracks: i) Finite Element modeling , ii) Laboratory work, iii) Field testing using full scale shafts; field work also included estimation of in situ modulus of subgrade reaction using “rock” dilatometer, and finally iv) Performance predictions. The proposed P-y curves are developed as hyperbolic functions. A method to evaluate in situ stiffness properties of the weathered rock by utilization of the rock dilatometer, as well as by using geologic information of joint conditions, RQD, and the strength properties of cored samples, is proposed. A computational scheme for lateral behavior is advanced by which different lateral subgrade responses are assigned in the model based on the location of the point of rotation. Above the point of rotation, a coefficient of lateral subgrade reaction is assigned on the basis of evaluated modulus as computed from rock dilatometer data or from index geologic properties. A stiffer lateral subgrade reaction is assigned below the point of rotation in order to model the relatively small shear strains in this region. Predictions based on the proposed Py model for weathered rock show good agreement with field test results, which were performed in various rock profiles. The proposed method is also verified by comparisons with published results of an additional field test. Concepts of the proposed weathered rock model have been encoded into the computer program LTBASE.

Static and Dynamic Lateral Loading of Pile Groups
(pdf format)

Transportation Research Board
NCHRP Report 461
2001

This report contains the findings of a study to develop and validate an improved design method for pile groups under static and dynamic lateral loads. The report includes recommendations for estimating the distribution of load to piles in a group and provides guidance on analytical methods for predicting dynamic response. The material in this report will be of immediate interest to bridge engineers and geotechnical engineers involved in designing pile and drilled shaft foundations to resist lateral loads.

The principal force experienced in transportation structures during an earthquake, a hurricane, or a vessel impact is transient horizontal loading. These loads must be transmitted to the structure’s foundation. State-of-the-practice design for lateral loading of pile and drilled shaft foundations uses beam-on-elastic-foundation analysis. In this analysis, load shedding from a pile to the soil is represented by “p-y springs” in which the soil response is modeled as a series of discrete nonlinear springs. The p-y springs currently used in these analyses were developed primarily to determine the load-shedding behavior of single piles subjected to static loads. The use of p-y springs in the analyses of pile groups subjected to static and dynamic lateral loads had not been validated.

The objective of this research was to evaluate and extend current design methods for groups of piles and groups of drilled shafts subjected to lateral loads associated with earthquakes, hurricanes, and vessel impacts. Under NCHRP Project 24-09, Auburn University conducted experimental and analytical studies of pile groups. Through a series of field tests, the researchers determined the distribution of lateral loads to the individual piles in a group and verified that the pile-group response can be predicted analytically using p-y springs. Experimentally determined multipliers are used to adjust the magnitude of load carried by each row of piles in the group. The findings from this research could significantly increase confidence in and reduce the cost of foundations subjected to dynamic loads.