Identifying Existing Quality and Complete Databases of Small and Large Diameter Axial Deep Foundation Load Tests
Quality and complete foundation load test databases are needed by researchers in reliability calibration to develop load and resistance factors of geotechnical design methods and by designers to evaluate and improve the geotechnical design for production foundations in their projects. Reliability calibration is the best option to calibrate geotechnical design methods of foundation and thus for implementation of LRFD. These datasets are generated when initial site characterization information is combined with foundation performance testing to provide a comprehensive and complete record of site character, predicted performance, and measured performance of foundation systems.
Reliability calibration allows for improved correlations when variance associated with geology, practice, materials, and other factors can be reduced. Locally calibrated resistance factors usually provide favorable values for geotechnical design and construction methods within the code. Most of the LRFD resistance factors used by States nationwide are those included explicitly in the code and developed from fitting to legacy Allowable Stress Design performance, past experience, observation, and judgement rather than reliability-based design.
Foundation load tests provide the basis for developing improved geotechnical resistance factors for use in design. These tests provide actual performance measurements such that the predictions of performance can be compared for their accuracy, precision, and appropriate levels of risk and reliability.
Complete load testing datasets include three types of information
• Soil/rock information around the load test: This geotechnical site characterization data includes measurements used to determine the foundation design properties. The information also includes metadata such as the means, methods, and test procedures used to obtain measurements (e.g., SPT in-situ tests, sample extraction and lab testing, CPT, DMT, PMT, etc.). Documentation should also include design methods (direct/indirect) including local methods not covered in AASHTO.
• Test Foundation information: location, layout, dimensions, type, and construction and installation information, quality control (acceptance testing information).
• Load testing information: test procedure (including time after installation, duration, and rates of loading), test data and results (load-settlement curves)
Applications of Load Testing Datasets are:
• By Designers: On a specific project, load testing may be used to allow designers to adopt more favourable resistance factors which are currently available (either locally or as provided in the LRFD code). In these cases, usually a minimum number of tests are employed to meet the requirements of the code. Additional tests may be needed if site conditions are variable and a single test is not representative over the whole project. The goal is to provide a more accurate/economical design for the project by increasing the load on the foundations (reducing the amount of conservatism and allowing a given foundation to utilize more of capacity for load rather than in ‘reserve’ to account for uncertainty).
• By Researchers: As part of a local, regional, or national LRFD calibration effort, datasets of load testing information can be used as a basis for statistical reliability-based calibration and validation of resistance factors. Resistance factors can be examined based on types of input data, design methods and proposed equations or relationships and types of construction control. The programmatic work is usually performed as a research project, but the funding sources and organizations can vary (FHWA, TRB, NCHRP, AASHTO, DOTs, Pooled Funds, etc…). Improved practice on a programmatic level, using a statistically significant testing samples requires a significantly larger collections of datasets, as new resistance factors are being developed. Once those new calibrated factors exist, and are validated, then additional projects need only a relatively economical project level of testing to apply the new factors.
Load tests results can also be obtained from program-delivery project testing. Current highway engineering practices tend to value load test results for individual projects, rather than also for future reliability calibration. Lack of appreciation for the programmatic application of load tests can result in lower long-term value. The data may not be archived well or be of sufficient quality (accuracy) for use in the reliability calibration. Variations in test procedures and protocols, type of data collected, and the format of data collection provide challenges in using the data as a resource for future use.
Relatively few reliability-based resistance factors for deep foundations have been developed by AASHTO and state DOTs. This is likely due to the expense associated with a comprehensive load testing program or a lack of data with appropriate similarity to DOT practices which would provide a sufficient sample side for a statistically sound reliability-based derivation. High-quality, accurate, and complete load tests, representative of particular practices and geologies, are needed to improve the resistance factors meaningfully as compared to the existing aggregated national factors, usually based on a wide range of geology and deep foundation geometries, materials and installation practice.
Quality and complete load tests for small and large diameter deep foundations (driven piles and drilled shafts) provide valuable source information to expand aggregated databases used in the statistical analysis needed for new geotechnical resistance factors. The DFLTDv2 provides a resource where additional load testing information can be added. Federal and State Agencies Transportation agencies (e.g., state DOTs) and universities should leverage the value of existing and future load testing by contributing information to the this or other State DOT databases.
Load tests results can also be obtained from transportation projects. The current highway engineering practices emphasize the use of load test results for individual projects, not for future reliability calibration. This could lead to two main problems with the load test data obtained in these projects: a) data is not complete or of good quality (accuracy) for use in the reliability calibration; and b) the information is not reported or complied for future use. There are variations in the type of data collected at load test sites and the procedures followed for obtaining these data by various State DOTs.
In summary, only a few reliability-based resistance factors for deep foundations have been developed by AASHTO and state DOTs because of a lack of quality, accurate, and complete load tests for deep foundations. Quality and complete national and regional load tests for small and large diameter deep foundations (driven piles and drilled shafts) are still needed in the USA. Federal and State Agencies Transportation agencies (e.g., state DOTs) and universities should work together to develop these databases as described next.
Develop quality and complete national and regional axial load test databases in the USA for small and large diameter deep foundations (driven piles and drilled shafts).
This research study will help improve geotechnical design of deep foundations by providing improved, statistically representative, reliability-based designs, rather than basing current designs on legacy practices. More efficient foundation systems with an appropriate reliability are anticipated; locally calibrated factors should reflect decreased variability and increased confidence, resulting in less costly foundations. Reliability calibration requires quality and complete foundation load test datasets. Although databases of load testing data exist, as described in the literature search, there is a need to encourage more load testing and add additional datasets for the continued development of more complete databases which reflect more geographic locations, practices, materials, pile types, site characteristics and performance test measurements. A national protocol to aid in consistence procedures and report data systematically would help states leverage additional programmatic value at project load test sites. Improved standardized processes can aid local calibration efforts. Current resistance factors are based on broad aggregations of data reflecting a broad range of geologies, materials, and practices or past experience and judgement, and not a robust reliability calibration relevant to states’ particular means, methods, and geology. As such, existing factors may contain excessive conservatism, especially for states with particular practices where resistance factors could be optimized.
Federal Highway Administration (FHWA) has led the development of the Deep
Foundation Load Test Database (DLFTD) (Version 1.0 2007) that led to
development of the first generation of AASHTO LRFD factors in the NCHRP Report
507. In early 2017, the FHWA released the updated FHWA Deep Foundation Load Test Database (DFLTD v.2). Instructions
on how to install and use the DFLTD are provided in the associated User Manual
The DFLTD v.2 replaces the previous DFLTD (v.1) and also adds new information
on over 150 load tests on Large Diameter Open-End Piles (LDOEPs). Due to lack of quality
load test data in LDOEPS, no recommendations are developed for dynamic load
test, wave equation, and dynamic formulas and for concrete piles, so this
database need improvements and updates. The DFLTD v.2 can be used by Federal and State
agencies, universities, consultants and contractors, design engineers and
planners, and research and development professionals. The database is
relational where the records can be queried in numerous ways to include
foundation type and size, subsurface soil information, and location. Toward
developing quality foundation load test databases, the FHWA published in a 2015
TRB paper titled _“Development and Use of High-Quality Databases of Deep
Foundation Load Tests” _(Abu-Hejleh et al., 2015).
addition, several State DOTs and researchers have developed and are now
developing their own foundation load test databases (e.g., Florida, Iowa,
Louisiana, California, New Mexico and Illinois, see the above 2015 FHWA TRB
FHWA Publication No. FHWA-HRT-17-034, FHWA Deep Foundation Load Test
Database Version 2.0 User Manual, updates the Deep Foundation Load Test
Database with LDOEP information. (FHWA-HRT-17-034).
FHWA Publication No. FHWA-SA-91-042, Static Testing of Deep Foundations,
provides the following recommendations
on when to perform a pile load test.
FHWA Publication No. FHWA NHI-10-039, Implementation of LRFD Geotechnical
Design for Bridge Foundations, outlines the calibration process within the
example of a state-based research effort to locally develop and calibrate a new
dynamic pile driving formula with an associated resistance factor tuned to
state practice is presented in the publication MN/RC 2014-16, Load and
Resistance Factor Design (LRFD) Pile Driving Project -Phase Two Study
(Paikowsky et al. 2014).
research efforts include (as a representative example):
a Resistance Factor for Mn/DOT’s Pile Driving Formula_, (S. Paikowsky et al 2009)
and Resistance Factors for Driven Piling in Illinois_, (J. Long, A. Anderson, W. Kramer, 2014)
design for driven piles based on a pile load test program in Illinois : phase 2_, (J. Long, A. Anderson, 2014)
OF LRFD GEOTECHNICAL PRINCIPLES FOR PILE SUPPORTED BRIDGES IN OREGON: PHASE 1_, Final Report, OTREC-TT-09-0, (Trevor D. Smith, Peter
Dusicka, Portland State University, 2009)
Florida Department of Transportation's (FDOT) Pile/Shaft Design Procedures
Based on CPT & DTP Data_, (D.
Bloomquist, M. McVay, Zhihong Hu, 2007) Published
Agreement Between Static Method and Dynamic Formula for Driven Cast-In-Place
Piles in Wisconsin_, (J. Long, 2013)
of FHWA Pile Design Method against the FHWA Deep Foundation Load Test Database
Version 2.0_, (Nikolaos Machairas,
Gregory A. Highley, M. Iskander, 2018), Transportation Research Record
This development requires performing the following tasks:
Conduct a survey of State DOTs for load testing information and inquire:
- a. Do you have project-level deep foundation load tests available with information that could be added to a national database but has not been?
- b. Do you have any deep foundation research testing available?
- c. If you were interested in locally calibrating (improving the resistance factor to a more favorable value) a resistance factor, what would be your primary interest 1) type of foundations (piling, shallow foundations, drilled shafts), 2) size of foundations (e.g., large diameter, 4’ diameter pile,…), 3) type of geomaterials (sand, clays, IGM), and 4) type of geotechnical tests (SPT or CPT).
- d. If your state was identified as one with limited load testing, would you be interested in performing additional high-quality load testing to fill in gaps and calibrate your practice? If so, would you consider new, more accurate methods of site characterization (CPT) as part of that process.2. Locate and identify existing deep foundation load testing resources. Start with resources listed in Section 2. Then, survey state DOTs and other agencies to locate and document existing deep foundation load testing information available from US sources. Existing load testing databases, university research test sites and project-based load tests (which may or may not be collected into databases) should be located and the information from these tests compiled.
Develop a methodology to assess existing deep foundation load testing records, archives, or databases for “completeness” and “accuracy” such that the tests can be classified on these quality measures. Develop necessary processes or procedures to review existing data from previously conducted load tests and judge them to be complete and accurate, such that those that are can be used (see next task)
From the deep foundation load testing resources (identified in Task 2), find all the load tests or databases which meet the completeness and accuracy criteria from the previous task. Using the methods developed in this study, evaluate the existing load testing data and classify the information with respect to completeness and accuracy
Assemble a new database which contains the load tests, databases, or information that meets the complete and accurate criteria.
Evaluate summary information from the complete and accurate load tests identified in the previous tasks, including (see the three type of collected at load test sites presented in Section 1)) the location of the tests, test type, type and size of foundation tested, soil stratigraphy, construction and quality control method and other information that may be of interest to search. Identify poorly represented data types where further research may be suggested.
- Based on the outcomes, determine if there are underrepresented data sets, such as particular pile types, installation methods, geographic regions, soil types, or other classifications where additional research opportunities appear in greatest need.
- Identify recommendations for future work based on (a) and state surveys (first task).
Develop procedures, instructions, and guideline specifications (which can be included in State DOT requirement document language) for updating the database of complete and accurate testing, created as part of this work. Provide a mechanism to keep this research work current.
Implementation of the results of this research study can be immediate. State DOTs, consultants, and researchers have access the existing national Deep Foundation Load Test Database (v2). The study recommendations can be implemented through collaboration between national and state transportation agencies (AASHTO, FHWA, State DOTs, ASCE, DFI, ADSC, and PDCA) to aid in the expansion of the database with additional datasets.
Cosponsoring Committees and Endorsements
Committee on Geotechnical Instrumentation and Modeling Committee (AKG60);
Committee on Soil and Rock Properties and Site Characterization Committee (AKG20); Committee on Foundation of Bridges and other Structures (AKG 70);
Ohio DOT; Alabama DOT; New Mexico DOT; Missouri DOT; Louisiana DOTD; Louisiana Transportation Research Center; North Carolina DOT; New Hampshire DOT; Colorado DOT
|Sponsoring Committee:||AKG70, Foundations of Bridges and Other Structures
|Research Period:||24 - 36 months|
|RNS Developer:||Dr. Naser Abu-Hejleh, P.E., FHWA, firstname.lastname@example.org|
|Index Terms:||Databases, Load tests, Pile foundations, Foundation engineering, Geotechnical engineering, |
Bridges and other structures