Advances in Designing and Testing Deep Foundations: In Memory of Michael W. O'Neill

Ecclesiastes 3 reminds us, "For everything there is a season, and a time for every matter under heaven: a time to be born and a time to die." Certainly, God knows when that time should be; but it is difficult for us to accept that Michael W. O'Neill's (better known as Mike) time arrived and we will miss him. Mike's service includes committees; mentoring; and continuing education via short courses, seminars, and lectures, from which we, as the geotechnical community, all benefited. We are challenged to continue Mike's legacy by emulating his altruistic spirit in maintaining our geotechnical family.

This paper re-visits the compensated piled raft foundation system, and outlines a simplified approach to the analysis of both conventional piled rafts and compensated piled raft foundations for the support of structures on very soft clays. Two cases are considered: the first where only applied load acts on the foundation, and the second where both applied load and externally imposed ground settlements act. It is demonstrated that the use of compensation, via excavation of the soil and embedment of the raft, can lead to significant reductions in settlement compared to normal (uncompensated) piled rafts. Importantly, when ground settlements occur, the use of compensated piled rafts can lead to significantly reduced differential settlements between the structure and the ground, compared to the case where the structure is supported by end bearing piles. Indeed, the latter may be counter-productive and lead to the not uncommon situation where the building stands well above the surrounding ground. A simplified design approach is proposed, and it is demonstrated that this simplified approach can lead to a computed behavior which is consistent with past experience in Mexico City.

Deflections of laterally loaded pile groups are strongly affected by the degree of pile head fixity. If the cap provides perfect restraint against rotation, deflections are only about 25% as large as the deflections for no rotational restraint at the tops of the piles. The degree of rotational restraint is governed by (a) the rotation of the piles with respect to the cap, and (b) the rotation of the cap itself. This paper reviews available information on pile head fixity and proposes a simple method for considering pile head fixity in analyses of pile groups. The method includes consideration of nonlinear soil behavior, pile-soil-pile interaction, the resistance to lateral loads provided by pile caps, and the degree of pile head fixity. The proposed method can be implemented through computer programs such as LPILE and COM624P, and spreadsheets based on the Characteristic Load Method.

The use of continuous flight auger (CFA) and drilled displacement (DD) piles in the U.S. market has increased significantly in recent years. This paper describes some of the advantages and limitations of these drilling systems compared to other deep foundation systems. Geotechnical and project considerations are described which favor the use of CFA and DD piling. In addition, conditions are discussed which may be problematic for this type of construction or which may limit the practicality and competitive advantages of CFA and DD piles.

There have been several interesting new advances in the design, construction, and testing of auger pressure grouted (APG) piles recently. The new construction techniques involve the addition of displacement piles (no spoils) and partial displacement piles (some spoils) that utilize many of the same installation procedures as traditional APG piles. These new pile types are more applicable where soft to firm clay and loose to medium dense sand soil conditions are present. The new testing and quality assurance procedures available include (1) automated pile installation monitoring equipment, (2) non-destructive integrity testing and (3) grout maturity strength testing procedures. In addition to new design methods, analytical methods are now used that can utilize the installation effort or drilling resistance measured when these piles are installed to provide a much higher level of confidence of the capacity and integrity of these pile types.

This paper examines several issues related to the behavior of augered cast-in-place (ACIP) piles under axial compression loading, including the: (a) load-displacement response, (b) method to interpret the "failure load" from the load-displacement curves, and (c) prediction of capacity using conventional geotechnical parameters. Case histories of field load tests were used herein. First, the generalized load-displacement behavior was examined, and it was found that the basic load-displacement characteristics of ACIP piles are comparable to those of other types of drilled foundations. Then the compression capacity was evaluated in a consistent manner. The results were similar to drilled shafts, but some key differences were noted. These results will be useful in evaluating overall ACIP behavior.

Non-displacement augered cast-in-place (ACIP) piles are being increasingly used in construction because they can be installed relatively rapidly with minimum environmental impact and have the flexibility to quickly adjust pile length for varying soil conditions as compared to the other deep foundation systems. Due to lack of understanding of ACIP pile behavior, in many instances, the design capacity of the ACIP pile must be verified by performing one or more full-scale load test on site before installing the production ACIP piles. Hence there is a need to better quantify the performance of ACIP piles in various soil conditions. In this study the load-displacement behavior of eight full-scale ACIP piles with diameters in the range of 410 to 460 mm was investigated. The length of the piles varied from 9.1 to 21.3 meters and the measured pile capacities varied from 1520 to 2670 kN. A two-parameter non-dimensional hyperbolic relationship to represent the non-linear load-displacement behavior of ACIP piles has been verified with the full-scale test data available from eight tests performed in clay, sand and mixed soil profiles in the Texas Gulf Coast region. The parameters required for the model were related to the in-situ soil properties obtained from the CPT tests and the pile dimensions. The ultimate load capacity predicted by the hyperbolic model was higher than that of the LPC method. The ratio of maximum applied load during the load test to predicted ultimate capacity using the hyperbolic relationship varied from 0.78 to 0.97 with an average of 0.86. The displacement needed to mobilize half of the ultimate pile capacity of the ACIP piles was in the range of 0.35 to 1.5% the diameter of the pile.

In the desert southwest of the United States (Arizona and parts of New Mexico, California, Nevada, Utah and Colorado), the soils are often dense and cemented. The design procedures for drilled shafts described by the American Association of State Highway and Transportation Officials (AASHTO) or the Federal Highway Administration (FHWA) do not incorporate the effect of density or cementation. Use of β-α methods cited in these publications often result in an over-design of the drilled shafts with associated greater costs. The FHWA publication by O'Neill and Reese suggests classifying dense or stiff soils as intermediate geomaterials (IGMs) and proposes alternative design methodology. Based on results of the recent large-scale load tests in Arizona reported in this paper (2.44 m diameter test shaft) and the results of tests in New Mexico described by Chua et al. (1.37 and 1.83 m diameter test shafts), it appears that the actual skin friction in desert southwest soils is much higher than that predicted by the equations for IGMs. This paper presents the results of the test program performed for the Arizona Department of Transportation (ADOT) that included a large scale load test on a 2.44 m diameter, 41.1 m deep shaft to re-design large diameter drilled shaft foundations for the I-10/I-19 Traffic Interchange (TI) in Tucson, Arizona. The methodology implemented on this successful project may be useful for future projects in dense and variably cemented soils.

Underreamed footings, sometimes referred to as belled shafts or belled piers, are frequently used in areas where geological conditions are favorable for their installation. Underreamed footings are sized to provide resistance to compression, uplift and lateral loads. In this study, the load-displacement behavior of two full size underreamed footings load tested to failure in compression and three in uplift were investigated using finite element analysis. The subsoils at the test sites were stiff jointed clay. Due to the relatively high permeability of the jointed clays, four of the load tests were performed under partially drained conditions. Finite element analysis using effective stress parameters allowing for partial dissipation of pore pressures between loading intervals was used to compute the load-displacement behavior of the footings. The at-rest earth pressure coefficient and permeability of the layers were found to be important parameters to model the measured behavior of the underreamed footings.