Yang Ke, University of Akron
Drilled shafts socketed into rock are widely used as foundations for bridges and other important structures. Rock-socketed drilled shafts are also used to stabilize a landslide. The main loads applied on the drilled shafts are axial compressive or uplift loads as well as lateral loads with accompanying moments. Although there exist several analysis and design methods especially for rock-socketed drilled shafts under lateral loading, these methods were developed with assumptions without actual validations with field load test results. Some of the methods have been found to provide unsafe designs when compared to recently available field test data. Therefore, there is a need to develop a more rational design approach for laterally loaded drilled shafts socketed in rock.
A hyperbolic non-linear p-y criterion for rock is developed in this study that can be used in conjunction with existing computer programs, such as COM624P, LPILE, and FBPIER, to predict the deflection, moment, and shear responses of a shaft under the applied lateral loads. Considerations for the effects of joints and discontinuities on the rock mass modulus and strength are included in the p-y criterion. Evaluations based on comparisons between the predicted and measured responses of full-scale lateral load tests on fully instrumented drilled shafts have shown the applicability of the proposed p-y criterion and the associated methods for determining the required input of rock parameters.
In addition to the development of a hyperbolic p-y criterion for rock, a method for predicting lateral capacities of drilled shafts in rock and/or soils is developed for assessing the safety margin of the designed shafts against the design loads. A computer program LCPILE is developed using VC++ to facilitate computations. An elastic solution based on a variational approach is also developed for determining drilled shaft elastic deflection due to applied lateral loads in a two-layer soil layer system. The computational algorithm was coded in a Mathematica file for easy application.
Finally, Briaud’s method for deriving p-y curves of rock from pressuremeter or dilatometer test results is evaluated using available field test data. A modification to the Briaud’s method is recommended for applications in rocks.
Ravi P. Aurora and Lymon C. Reese
Texas State Department of Highways and Public
Research Study 3-5-72-176
The behavior of axially loaded drilled shafts which derive most of their resistance to compressive loads from clay-shales is studied. Four instrumented test shafts were loaded to failure using a new type of reaction system in which all the tension steel could be recovered after testing. Three shafts were tested in Montopolis near Austin, and one shaft was tested in Dallas. On the basis of detailed analyses of field data as well as laboratory and field evaluation of the shear strength of soils, the load transferred to the clay-shale has been correlated to the shear strength and in-situ dynamic penetration resistance of clayshales. A design procedure, with indications of its limitations, has been suggested for computing axial capacity with drilled shafts in clay-shales.
Murad Y. Abu-Farsakh, Ph.D., P.E., Qiming Chen, Ph.D., P.E., and Md Nafiul Haque
Louisiana Transportation Research Center
The Load and Resistance Factor Design (LRFD) calibration of deep foundation in Louisiana was first completed for driven piles (LTRC Final Report 449) in May 2009 and then for drilled shafts using 1999 FHWA design method (O’Neill and Reese method) (LTRC Final Report 470) in September 2010. As a continuing effort to implement the LRFD design methodology for deep foundations in Louisiana, this report will present the reliability-based analyses for the calibration of the resistance factor for LRFD design of axially loaded drilled shafts using Brown et al. method (2010 FHWA design method). Twenty-six drilled shaft tests collected from previous research (LTRC Final Report 449) and eight new drilled shaft tests were selected for statistical reliability analysis; the predictions of total, side, and tip resistance versus settlement behavior of drilled shafts were established from soil borings using both 1999 FHWA design method (Brown et al. method) and 2010 FHWA design method (O’Neill and Reese method). The measured drilled shaft axial nominal resistance was determined from either the Osterberg cell (O-cell) test or the conventional top-down static load test. For the 30 drilled shafts that were tested using O-cells, the tip and side resistances were deduced separately from test results. Statistical analyses were performed to compare the predicted total, tip, and side drilled shaft nominal axial resistance with the corresponding measured nominal resistance. Results of this showed that the 2010 FHWA design method overestimates the total drilled shaft resistance by an average of two percent, while the 1999 FHWA design method underestimates the total drilled shaft resistance by an average of 21 percent. The Monte Carlo simulation method was selected to perform the LRFD calibration of resistance factors of drilled shaft under strength I limit state. The total resistance factors obtained at different reliability index were determined and compared with those available in literature. Results of reliability analysis, corresponding to a target reliability index of 3.0, reveals resistance factors for side, tip, and total resistance factor are 0.26, 0.53, and 0.48, respectively for the 2010 FHWA design method and 0.39, 0.52, and 0.60, respectively for the 1999 FHWA design method. The side and total resistance factors calibrated using the 2010 FHWA design method are less than those calibrated using the 1999 FHWA design method.
Murad Y. Abu-Farsakh, Xinbao Yu, Sungmin Yoon, and Ching Tsai
Louisiana Transportation Research Center
The first report on Load and Resistance Factor Design (LRFD) calibration of driven piles in Louisiana (LTRC Final Report 449) was completed in May 2009. As a continuing effort to implement the LRFD design methodology for deep foundations in Louisiana, this report will present the reliability based analyses for the calibration of the resistance factor for LRFD design of axially loaded drilled shafts. A total of 16 cases of drilled shaft load tests were available to authors from Louisiana Department of Transportation and Development (LADOTD) archives. Out of those, only 11 met the Federal Highway Administration (FHWA) “5%B” settlement criterion. Due to the limited number of available drilled shaft cases in Louisiana, additional drilled shaft cases were collected from state of Mississippi that has subsurface soil conditions similar to Louisiana soils. A total of 15 drilled shafts from Mississippi were finally selected from 50 available cases, based on selection criteria of subsurface soil conditions and final settlement. As a result, a database of 26 drilled shaft tests representing the typical design practice in Louisiana was created for statistical reliability analyses. The predictions of total, side, and tip resistance versus settlement behavior of drilled shafts were established from soil borings using the FHWA O’Neill and Reese design method via the SHAFT computer program. The measured drilled shaft axial nominal resistance was determined from either the Osterberg cell (O-cell) test or the conventional top-down static load test. For the 22 drilled shafts that were tested using O-cells, the tip and side resistances were deduced separately from test results. Statistical analyses were performed to compare the predicted total, tip, and side drilled shaft nominal axial resistance with the corresponding measured nominal resistance. Results of this showed that the selected FHWA design method significantly underestimates measured drilled shaft resistance. The Monte Carlo simulation method was selected to perform the LRFD calibration of resistance factors of drilled shaft under strength I limit state. The total resistance factors obtained at different reliability index were determined and compared with those available in literature. Results of reliability analysis, corresponding to a target reliability index of 3.0, reveals resistance factors for side, tip, and total resistance factor are 0.20, 0.75, and 0.5, respectively.
Lymon Reese and Michael O’Neill
Texas Highway Department
Research Project 3-5-65-89
In recent years drilled shafts have come into increasing use as foundation elements due to the economic advantage they afford. Prior to 1965 little information had been acquired concerning the magnitudes of skin friction and end bearing that are developed by drilled shafts. Consequently, design procedures, reflecting the lack of information, have been conservative, often allowing no skin friction at all. Between 1965 and 1971 several instrumented drilled shafts were installed and load tested by the Center for Highway Research at various sites in Texas. The results of the tests were analyzed and, together with a thorough review of the work of other investigators, were used in establishing realistic criteria for design values of side resistance and base capacity. A step-by-step procedure incorporating these criteria was developed for use in designing shafts in predominantly clay soils. This procedure includes the effects of construction technique and shaft geometry and is intended for use in the design office.
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
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.
Nien-Yin Chang, Ph.D., P.E., Principal Investigator (P.I.) Hien Nghiem, Research Assistant (R.A.)
Federal Highway Administration, Central Federal Lands Highway Division
Drilled shafts are increasingly being used in supporting critical structures, mainly because of their high-load supporting capacities, relatively low construction noise, and technological advancement in detecting drilled shaft anomalies created during construction. The critical importance of drilled shafts as foundations makes it mandatory to detect the size and location of anomalies and assess their potential effect on drilled shaft capacity. Numerical analysis was conducted using Pile-Soil Interaction (PSI), a finite element analysis program to assess the effect of different anomalies on the axial load capacities of drilled shafts in soils ranging from soft to extremely stiff clay and loose to very dense sand. The investigation included the affect of anomalies of various sizes and lengths on both structural and geotechnical capacities. The analysis results indicate that the drilled shaft capacity is affected by the size and location of the anomaly and the strength of the surrounding soil. Also, nonconcentric anomalies significantly decrease the structural capacity of a drilled shaft under axial load. The resulting drilled shaft capacity then equals the smaller one of the two capacities: structural or geotechnical.
We also have the previous document, Drilled Shafts:
Construction Procedures and Design Methods (FHWA IF-99-025). It was the last version to be written by Michael W. O’Neill and Lymon Reese, both of whon passed away the following decade.
Federal Highway Administration
Dan A. Brown, Ph.D, P.E., John P. Turner, Ph.D, P.E., and Raymond J. Castelli, P.E.
FHWA Geotechnical Engineering Circular #10
This manual is intended to provide a technical resource for engineers responsible for the selection and design of drilled shaft foundations for transportation structures. It is used as the reference manual for use with the three-day National Highway Institute (NHI) training course No. 132014 on the subject, as well as the 10th in the series of FHWA Geotechnical Engineering Circulars (GEC). This manual also represents a major revision and update of the FHWA publication on drilled shaft foundations co-authored by the late Michael O’Neal and late Lymon C. Reese, published in 1988 and revised in 1999. This manual embraces both construction and design of drilled shafts, and addresses the following topics: applications of drilled shafts for transportation structure foundations; general requirements for subsurface investigations; construction means and methods; LRFD principles and overall design process; geotechnical design of drilled shafts for axial and lateral loading; extreme events including scour and earthquake; LRFD structure design; field loading tests; construction specifications; inspection and records; non-destructive integrity tests; remediation of deficient shafts; and cost estimation. A comprehensive design example (Appendix A) is included to illustrate the step-by-step LRFD design process of drilled shafts as foundations for a highway bridge.
J. Erik Loehr
Center for Transportation Infrastructure and Safety/NUTC program
Missouri University of Science and Technology
NUTC R243-2 / R244-2 / R245-2 / R246-2 / R247-2
These guidelines were developed as part of a comprehensive research program undertaken by the Missouri Department of Transportation (MoDOT) to reduce costs associated with design and construction of bridge foundations while maintaining appropriate levels of safety for the traveling public. The guidelines were established from a combination of existing MoDOT Engineering Policy Guide (EPG) documents, from the 4th Edition of the AASHTO LRFD Bridge Design Specifications with 2009 Interim Revisions, and from results of the research program. Some provisions of the guidelines represent substantial changes to current practice to reflect advancements made possible from results of the research program. Other provisions were left essentially unchanged, or were revised to reflect incremental changes in practice, because research was not performed to address those provisions. Some provisions reflect rational starting points based on judgment and past experience from which further improvements can be based. All of the provisions should be considered as “living documents” subject to further revision and refinement as additional knowledge and experience is gained with the respective provisions. A number of specific opportunities for improvement are provided in the commentary that accompanies the guidelines.
J. Erik Loehr, Ph.D, P.E., Antonio Marinucci, Ph.D, P.E., Peggy Hagerty Duffy, P.E., Jesús Gómez, Ph.D, P.E., Helen Robinson, P.E., Tayler Day, Andrew Boeckmann, Ph.D, P.E., and Allen Cadden, P.E.
This objective of this report is to provide recommendation for the cost-effective design and construction of post-grouted drilled shafts in appropriate transportation applications. The work included review of available technical literature; consultation with those currently involved with post-grouting, collection, and interpretation of results from full-scale load tests on post-grouted and conventional drilled shafts; investigation and evaluation of load transfer and improvement mechanisms for PGDS using numerical models, conduct of field load tests intended to evaluate improvement due to pre-mobilization alone; and development of recommendations for design and construction of PGDS based on the collective work. This report summarizes the current state of practice for post-grouting of drilled shafts in the U.S., documents the analyses and evaluations performed to evaluate alternative improvement mechanisms for PGDS, and describes the findings and recommendations developed based on the work performed. The report concludes with recommendations for future work needed to improve post-grouting practice for transportation infrastructure.
Appendices A through D supplement this report, and are available by request from the FHWA
Improvement of the Geotechnical Axial Design Methodology for Colorado’s Drilled Shafts Socketed in Weak Rocks
Naser Abu-Hejleh, Michael W. O’Neill, Dennis Hanneman and William J. Atwooll
Colorado Department of Transportation – Research
Drilled shaft foundations embedded in weak rock formations (e.g., Denver blue claystone and sandstone) support a significant portion of bridges in Colorado. Since the 1960s, empirical methods and “rules of thumb” have been used to design drilled shafts in Colorado that entirely deviate from the AASHTO design methods. The margin of safety and expected shaft settlement are unknown in these methods, however, both are needed for the implementation of the new and more accurate AASHTO Load and Resistance Factor Design (LRFD) method in CDOT design guidelines. Load tests on drilled shafts provide the most accurate design information and research data for improvement of the design methods for drilled shafts.
As a part of the construction requirements for the T-REX and I-25/Broadway projects along I-25 in Denver, Colorado, four Osterberg (O-Cell) load tests on drilled shafts were performed in 2002. The bedrock at the load test sites represents the range of typical claystone and sandstone (soft to very hard) encountered in the Denver metro area. To maximize the benefits of this work, the O-Cell load test results and information on the construction and materials of the test shafts were documented, and an extensive program of simple geotechnical tests was performed on the weak rock at the load test sites. This includes standard penetration tests, strength tests, and pressuremeter tests. The analysis of the all test data and information and experience gained in this study were employed to provide: 1) best correlation equations between results of various simple geotechnical tests, 2) best-fit design equations to predict the shaft ultimate unit base and side resistance values, and the load-settlement curve as a function of the results of simple geotechnical tests, and 3) assessment of the CDOT and AASHTO design methods.
Chao-Kuang Hsueh, San-Shyan Lin, and Shuh-Gi Chern
Department of Harbor and River Engineering, National Taiwan Ocean University
In addition to nonlinear soil behavior has been assumed in conventional analytical method, performance of the laterally loaded drilled-shaft is also strongly influenced by possible concrete cracking, steel yielding, and shaft/soil separation and interaction. However, these factors are often neglected in the most of available methods and studied results. The main purpose of this paper is to investigate the importance of the above-mentioned effects on lateral performance of drilled reinforced-concrete shaft. The finite element code, ABAQUS, which is available in the three-dimensional analysis is adopted for taking into account the nonlinearity in shaft and soil materials, pilesoil interaction and nonlinear geometric properties of the shaft/soil system in the analysis. Also, infinite elements are used to simulate unbounded boundary condition. One of the lateral pile load tests results of high-speed railway system in Taiwan is used to simulate the real behavior of drilled shaft subjected to lateral load. The numerical results show that the nonlinearity of material and geometry strongly affects the shaft deflection, steel stress distribution, concrete cracking state, soil uplift in front of the shaft, and separation between shaft/soil interfaces that is behind the shaft and along its depth.
Liquefaction-Induced Downdrag on Continuous Flight Auger (CFA) Piles from Full-Scale Tests Using Blast Liquefaction
Deep foundations typically support dead and live loads through a combination of positive skin friction acting along the sides of the pile and end-bearing at the toe of the pile; however, during seismic events, negative skin friction (or downdrag) can potentially develop. Downdrag occurs because of increases in effective stress caused by pore pressure dissipation and settlement by the liquefiable soil layer(s) relative to the pile. This phenomenon creates a dragload that the pile must support in addition to its permanent service pile head load. The depth of the dragload extends to the neutral plane. The neutral plane is the depth where the settlement of the pile equals the liquefaction settlement of the soil and where the axial load in the pile is the greatest. The elevation of the neutral plane is found by trial and error such that the service load plus negative friction should equal the positive friction plus end-bearing resistance.
Fellenius reviewed many case histories involving down- drag on piles and concluded that the dragload does not change the ultimate bearing capacity of a pile foundation, but it should be added to the dead load in design.(1) In addition, the designer must also still determine that both the structural capacity of the pile and the toler- able settlement is not exceeded. For seismic events, it has been found that assuming skin friction equals zero in the layer(s) of surrounding soil in which liquefaction occurs will result in little error. The American Association of State Highway and Transportation Officials recommends that the residual strength of the soil in that zone be assumed when estimating negative skin friction in a liquefied layer(s).
To evaluate the skin friction of CFA piles, a blast-induced liquefaction test program was conducted at a site in Christchurch, New Zealand. During the tests, load versus depth was measured in three instrumented, full-scale, 0.6-m diameter CFA piles of vari- ous lengths. The test program included two sets of blast-induced liquefaction, the first without load on the piles and the second with load on the piles. The blasts liquefied a 10-m layer of sand along the length of the pile. The axial load distribution along the length of the pile because of negative skin friction was measured after liquefaction and reconsolidation. This report focuses on the test results of the loaded piles as com- pared to conventional design practice.
Tom Armour, P.E., Paul Groneck, P.E., James Keeley. P.E. and Sunil Sharma, P.E., PhD.
The use of micropiles has grown significantly since their conception in the 1950s, and in particular since the mid-1980s. Micropiles have been used mainly as elements for foundation support to resist static and seismic loading conditions and less frequently as in-situ reinforcements for slope and excavation stability. Many of these applications are suitable for transportation structures. Implementation of micropile technology on U.S. transportation projects has been hindered by lack of practical design and construction guidelines.
In response to this need, the FHWA sponsored the development of this Micropile Design and Construction Guidelines Implementation Manual. Funding and development of the manual has been a cooperative effort between FHWA, several U.S. micropile specialty contractors, and several State DOT’s This manual is intended as a “practitioner-oriented” document containing sufficient information on micropile design, construction specifications, inspection and testing procedures, cost data, and contracting methods to facilitate and speed the implementation and cost effective use of micropiles on United States transportation projects.
- Chapter 1 provides a general definition and historic framework of micropiles.
- Chapter 2 describes the newly developed classifications of micropile type and application.
- Chapter 3 illustrates the use of micropiles for transportation applications.
- Chapter 4 discusses construction techniques and materials.
- Chapter 5 presents design methodologies for structural foundation support for both Service Load Design (SLD) and Load Factor Design (LFD).
- Chapter 6, which was supposed to present a design methodology for slope stabilization, is not included in this version.
- Chapter 7 describes micropile load testing.
- Chapter 8 reviews construction inspection and quality control procedures.
- Chapter 9 discusses contracting methods for micropile applications.
- Chapter 10 presents feasibility and cost data.
- Appendix A presents sample plans and specifications for Owner Controlled Design with Contractor Design Build of the micropiles, and/or micropiles and footings.
Martin Jones, Groundation
Presented at the Annual Meeting of the Deep Foundations Institute, Hamilton, Ontario, 1987
This paper outlines the development of small diameter grouted piling, to deal with foundation problems in difficult soils conditions and in very restricted access areas. A number of applications are outlined to show the way in which these piles have been used over the last sixteen years in Canada.
(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
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.