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API REPORT 80-29 Document Information:
Title
Review and Compilation of File Test Results Axial File Capacity
American Petroleum Institute
Publication Date:
Mar 1, 1981
Scope:
INTRODUCTION
Estimation of the axial load capacity of driven friction piles is a major problem for engineers. Difficulties arise because of a number of factors, e.g., severe changes in the properties of soils caused by pile driving and reconsolidation, drag down of soil from one layer into another, and pile-soil interaction during loading. As a result of such problems there is a potential significant margin of error in the predicted pile capacity. If the engineer adopts a convervative approach, the pile foundation is safe against failure but may become uneconomical. Use of an excessive factor of safety may also lead to severe problems in pile installation. In the terrestrial environment these design problems are often minimized by driving piles to rock, i.e., avoiding friction piles, or by performing load tests. For large diameter offshore piles there have been very few load tests, thus requiring that careful consideration be given to design methods.
Two distinctly different approaches may be used in attempting to reduce the margin of error in the predictions. In what might be termed the "engineering mechanics" approach the designer attempts to develop an accurate model for the behavior of the whole system. Appropriate data are fed into the solution algorithm and pile-soil response is predicted. In the second approach, which might be termed an "empirical" or "engineering" approach, the engineer collects useful and relevant experience of the profession and tries to apply greatly simplified models, coupled with experience or judgment factors, to the prediction of pile behavior.
The engineering mechanics approach needs to be pursued with vigor because it offers a hope for a reasonably accurate solution at some future time. There is a need for improved models for soil behavior, for models to describe the process of pile installation, for economical ways of obtaining numerical output from the model, and for means of measuring the relevant soil properties. There must also be diagnostic tests in the laboratory and field to validate the analytical approach. Although useful results can be obtained in the short term, the goal may be perceived as essentially long term.
The empirical approach must also be pursued vigorously because it deals with the immediate need to find simple methods of analysis suitable for use now. It may also uncover field effects that need to be taken into account in the engineering mechanics approach.
A common need for both approaches to pile design is a readily accessible data bank of pile load test results. Essentially every author who has presented recommendations for a new or revised design method established a data bank and compared predictions made using the new method with measured results. These data banks have been different with the result being that the new methods have not been checked against a common diverse set of data. In addition the time, effort, and cost of repeatedly searching the literature is wasteful and impractical for both the researcher and the practiging engineer designing a pile foundation.
We have made a search of the published literature and collected data for 507 pile load tests, under axial compressive or tensile loads, where the piles were loaded essentially to plunging failure and the strengths of the soils were "measured". The measurement might have involved comparatively sophisticated tests, e.g., field vane or laboratory triaxial tests, or simple more empirical tests, e.g., field standard penetration tests or laboratory pocket penetrometer tests. Photocopies of fifty-nine papers have been collected into a "library" that is available for general use. The reference list is provided in this report.
In some cases the time involved in rereading numerous references is out of proportion to the potential benefits and the engineer may desire to examine only a summary of the relevant data. For each load test we have prepared a summary sheet listing important pile, soil, and load test data from the original paper.
To facilitate selection of load test results from a data bank that is expected to grow to a much larger size, all relevant data were stored on magnetic tape and a computer program was written to select load tests from the data bank in accord with a set of user input criteria in terms of pile material, shape, and size, and in terms of soil type, method of defining soil strength, and range in soil strength. Desired load tests may, therefore, be accessed using the original papers, the hard copy summary sheets, or summary tables printed by the computer. The program will be discussed in Chapter 3 of this report.
Once the data bank was set up, an effort was made to compare the measured pile capacities with capacities predicted using widely used methods of analysis. Four methods were used for piles in clay and five for piles in sand. It was found that the methods have not always been used in engineering practice in the same way they were defined in the original paper. Some methods were intended for only certain pile types but could just as well be applied to other types. In many cases a series of special problems arise when a method is applied, e.g., what is the "end area" of tapered piles, for H piles should the steel/soil contact areas or the area of an enclosing rectangle be used, and how should open ended pipe piles be analyzed? We will assign a name to each method (the name used in the computer program) and define the application of each. Reference to source literature will be provided but in many cases the method we have used will differ slightly from that of the original author. We will note such differences and explain our choice.
Analyses of pile capacities for a large number of load tests is tedious and when hand analyses are performed there is the danger of occasional errors. To minimize these problems the computer program was extended to perform the analyses and then plot measured versus computed capacities and perform simple statistical calculations. No users' manual or program listing is included because the program is under continued development.
The results of numerous analyses are presented and discussed in Chapter 4. In order to estimate the relative accuracy of the various predictive methods, the data were divided into subsets by pile type, soil type, method of measuring soil strength, and, for clays, the range of strength. In addition, tension tests were considered separately from compression tests, and tapered piles from straight piles. Computer generated plots of measured versus computed capacities were prepared and handmade frequency diagrams are also used.
Efforts were made to draw conclusions about the relative merits of the different analytical methods. However, the major effort this year involved collection of published data and development of the program. Interpretation using the existing data bank is made difficult by several problems. First, the number of tests used in any one plot may become too small to be significant when certain combinations of selectors are used. Second, in some cases the data used in estimating the validity of a method are essentially the same as the data used by the original author in formulating the method. Third, if a set of data is divided into several subsets in accord with a certain criterion, the data in the subsets may not have all other criteria the same and thus not be comparable. For example, a set composed of all piles in clay could be subdivided into subsets composed of tapered and untapered piles but the subsets could not be compared directly if the data for tapered piles were mainly for low-capacity timber piles in soft or medium clays whereas the data for untapered piles were mainly for high-capacity steel pipe piles in stiff to hard clays. Inclusion of unpublished load test data will expand the data bank and make comparisons more meaningful.
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