TRANSPORTATION RESEARCH RECORD 1633 Paper No. 98-1059 117 Using Dynamic Pile Testing To Evaluate Quality and Verify Capacity of Driven Piles PHILLIP A. WALTON AND STEPHEN L. BORG Pile testing using a pile driving analyzer is a nondestructive method of evaluating the support capacity provided by the soil to a pile. The New York State Department of Transportation’s (NYSDOT) approach to incorporating this testing procedure into a project is discussed from design into construction. Specification and contractural responsibilities are addressed, as are typical applications of the test method. By judiciously using dynamic pile testing, NYSDOT is able to cost-effectively link the design process to construction. Informed decisions are possible regarding the field performance of driven piles with respect to pile-soil support capacity, hammer operation, driving stress, damage to piles during driving, and verification of assumptions made for wave equation analysis of piles. The New York State Department of Transportation (NYSDOT) relies on a spectrum of tools to achieve a cost-effective and safe pile foundation design. Dynamic pile testing (DPT), also referred to as the Case Method, is used as a nondestructive quality control on deep foundations, with its primary purpose to determine the support capacity of the pile-soil system. The static pile load test (SPLT) still is perceived by many engineers as the most reliable method for verifying the capacity (quality) of deep foundations. An SPLT performed on a pile loaded to failure provides data about the magnitude and distribution of the ultimate static soil resistance. However, it is a lengthy process to design and construct a reaction frame and reference system with telltales. In contrast, the DPT avoids most of the contractor delays associated with an SPLT and, consequently, much of the cost involved with verifying the quality of the driven pile foundation. Additionally, it gathers dynamic information about the hammer-pile system as the pile is driven. As a result, the pile driving analyzer (PDA) largely has replaced the need for the SPLT on NYSDOT projects. It is used most often for the following reasons: • • • • • Verifying capacity during initial driving and after setup; Measuring pile driving stresses; Detecting pile damage; Monitoring hammer performance; and Verifying wave equation analysis of piles (WEAP) assumptions. The NYSDOT dynamic pile testing program is operated out of the main office in Albany. Two PDAs are used to cover work for the entire state. NYSDOT has used this technology since it was commercially available in 1975 and has owned and operated PDAs since 1978. Testing is conducted exclusively by properly trained engineers under the guidance of a staff who have maintained a continuity of PDA experience for more than 20 years. New York State Department of Transportation, Geotechnical Engineering Bureau, Building 7, State Campus, 1220 Washington Avenue, Albany, NY 12232-0863. DESIGN PROCESS Analysis NYSDOT uses standard methods in the design of the soil support capacity of individual piles. For cohesionless soils, borings are taken with driven samples and an analysis is made by using the sampler blow count, visual inspection of the samples, and all other available information about the site. Methods presented by Nordlund (1) are used to make an estimate of the pile’s static capacity in shaft resistance and end bearing, both during driving and after driving stops. A check is made to ensure the pile foundation settlement criteria are satisfied for the structure. In cohesive soils, the design follows the same general approach with occasional triaxial test information, except the shear strength after remolding and regain in strength must be estimated. A method, usually Tomlinson’s (2), is used to estimate the soil support capacity of the pile at time of driving and after setup. A check is made to ensure that the estimated settlement of the foundation is within tolerable limits for the structure. In both cases, assumptions are made regarding the behavior of the soil in its short-term and long-term support capacity of the chosen pile. With the pile type chosen and its length estimated, a check is made for drivability with the WEAP program using hammers typical in New York. For sensitive soils, the WEAP analyses are run for initial driving conditions and with strength gain of the soil. Should unusual driving criteria be necessary, notes are placed in the plans to assist the contractor in selecting a pile driving system. Specifications The NYSDOT standard specification has an item for dynamic pile testing. The use of this item is decided a job-by-job basis. The test requirement is written into the foundation design report for the project. A note is placed in the contract documents stating the frequency and circumstances for each test, always with the stipulation that additional tests may be ordered by the project engineer. All tests as stated in the contract note are performed by representatives of the state of New York. NYSDOT geotechnical engineering personnel perform the testing and evaluation. This is followed up immediately with a recommendation to the NYSDOT structures division. The standard specification requires the contractor to submit each combination of proposed driving systems for approval. Only the approved specific driving systems may be used, not an equivalent. All piles are driven to the criteria established by NYSDOT, whether derived from WEAP or PDA. The specification allows for rejection of pile driving hammers if they do not meet expected operation standards in the field. 118 Paper No. 98-1059 CHOOSING PROJECTS FOR TESTING Not all pile installations warrant dynamic pile testing. In NYSDOT, approximately 20 percent of pile supported structures have dynamic testing on a representative sample of foundation piles. Most often, testing programs are incorporated into the contract documents. However, on occasion the pile driving does not progress as anticipated and thus tests are added during construction. Planned tests are performed for several reasons, as described in the following sections. Verifying Capacity During Initial Driving and After Setup Some projects are chosen for dynamic testing because the designer has low confidence in the predicted pile-soil interaction. Some common soil conditions and structure designs that necessitate frequent dynamic pile testing fall into the following categories. Questionable Density of Sand and Gravel NYS uses an analysis for friction pile foundations in sand and gravel that relies on an estimate of φ, the angle of internal friction, of the overburden soils. If the soil contains a significant amount of large gravel, the standard penetration test can be unreliable as a tool for estimating φ. Judgment tempering the selection of φ, based on experience and knowledge of the deposition process, may leave great doubt as to what is an appropriate value for this parameter. In the case of H-piles driven in similar deposits, the state of the effective stress at the pile-soil interface could be highly unpredictable. Because of these uncertainties in the design parameters, the engineer may require a dynamic pile test to verify capacity. The strength gain or setup in sand and gravel may not be a significant factor and a retap of the piles most likely would be unnecessary. Cohesive and Silty Soil In cohesive and silty soil, resistance to the pile hammer during driving can be very low, largely because of the effects of remolding of the soil and its loss of strength. The rate of strength gain as pore pressure dissipates is not predicted easily but is estimated on the basis of experience with similar soil deposits. In many cases, lowcapacity piles can be designed by using conservative values for the strength of the soil and the adhesion value of the soil to the pile. These often suffice for an economical design to support the structure if the loads are small. A pile encountering little resistance during driving often does not instill confidence that it has adequate support capacity, although the designer may have confidence that adequate strength gain will occur with time. It is wise to provide field testing to verify the soil behavior and pile capacity. In this situation, a designer likely would specify that the pile be tested during the initial drive and subsequently tested again (the pile is retapped) after a suitable waiting period. In many cases, the setup time does not have to be long; often 24 hours is adequate to demonstrate sufficient strength gain. On occasion, a waiting period more than 24 hours long is needed if a more definitive measurement of capacity is needed. The need for a TRANSPORTATION RESEARCH RECORD 1633 longer waiting period would be specified in the contract documents so the contractor could bid accordingly. Limited Foundation Redundancy with Pile Capacity Uncertainty Some lightly loaded substructures have very limited redundancy in the foundation. A structure with integral abutments (single row of piles) and pier bents falls into this category. If the integral abutment piles are driven through variable soils without a well-defined bearing layer that can be reached at a reasonable depth, some check of the pile capacity is prudent. Measure Pile Driving Stresses Piles Subject to Damage Inspection personnel may be unable to detect pile damage located below ground surface. Precast concrete piles are particularly susceptible to damage from tensile stresses, especially when the pile is penetrating layers of low end bearing resistance. In situations like this, the engineer would specify dynamic testing of the first piles driv-en, after using wave equation analysis to approve the hammer-pile-soil system. The pile driving analyzer would be used during driving of the first piles to verify the WEAP assumptions and to check driv-ing stresses and detect possible damage to the pile during initial driving. Detecting Pile Damage Problems detecting pile damage are uncommon largely because of controls placed on the construction operations specifically to prevent damage. When used to assess possible damage, dynamic pile testing falls into the unplanned category. Typically, precast concrete piles can suffer damage if driving is not controlled. Steel H-piles can break on occasion at the splices because of improper welding procedures. In both cases, the dynamic test equipment may be able to provide indications that damage had occurred. Monitoring Hammer Performance The PDA commonly is used to monitor hammer performance (energy transferred to the pile top). This can be checked against the transferred energy calculated for a given blow count and resistance in WEAP. Preignition in a diesel hammer is when combustion occurs before the ram strikes the anvil. This condition cannot be detected by general observations. Higher-than-normal combustion pressure develops, causing a cushioning between ram and anvil. The ram will rise to the desired stroke without the required energy transfer to the pile. The PDA will detect this condition readily. Verifying WEAP Assumptions The PDA should be used in addition to WEAP when an extended model or complex analyses are available—for example, piles driven with a follower and open-ended pipe piles. Walton and Borg APPLICATIONS On occasion, when a thin-walled, closed-end pipe pile is driven in soft to firm silty clay, the resistance to penetration is observed to be far greater than expected. It has been conjectured that the dynamic effect at the pile toe during driving creates a dilation-and-contraction cycle on the soil pores. This dynamic contribution to penetration resistance will not be present under static loading. The PDA can be a valuable tool for diagnosing this condition. On some projects, NYSDOT will conduct an SPLT. On one project for which prestressed concrete piles were used, the SPLT test was rejected because of poor data. The contractor was given the option to run the static test again or redrive the pile with dynamic monitoring. The dynamic pile test was chosen and it resulted in cost savings and avoided delays. Frequently, pipe piles will be driven in silty material below groundwater. To verify capacity at the estimated length, the pile is restruck the day after it is driven. The silt may be so sensitive to a dynamic impact that a significant amount of resistance is lost in the first hammer blow. Often, this resistance is counted on to achieve the required ultimate pile resistance. In these cases, an observed blow count is insufficient and dynamic pile testing is needed. Measurements taken with a PDA can be used to calculate the resistance contribution from toe bearing versus the pile shaft. For instance, where the pile toe may end up on steeply sloping rock, it cannot always be counted on for resistance under a long-term static loading. The pile could be load rated for just shaft resistance if adequate initial drive and restrike monitoring is done. However, if end bearing was required with the toe of the pile driven to a steeply sloping rock surface, an SPLT would be more appropriate for capacity verification. A common limitation of the PDA is in calculating the ultimate resistance of a pile in a low-toe-displacement situation. In the case of a large-diameter concrete cylinder pile, the impact hammer may have sufficient energy to progress the pile but not enough energy to reach the toe displacement at which the ultimate end bearing is felt. Thus, the ultimate resistance of the pile may be significantly underpredicted and an SPLT test would be required. INTERPRETATION The PDA engineer should be well-trained and have access to engineers who are highly experienced in this discipline. It is important to keep current on the latest trends, technology, and work done in this field. Achieving good data quality in the field is the most important factor in gaining reliable results. It is important that the test results agree with the boring information, the pile static analysis, and the SPLT, if available. Paper No. 98-1059 119 The Case Pile Wave Analyses Program (CAPWAP) is a computer program that refines the PDA data through wave-matching techniques. Certain assumptions are made with the PDA about the dynamic behavior of the soil in calculating the static contribution to driving resistance. The dynamic contribution to driving resistance is accounted for, and calculations for static shaft and toe resistance acting on the pile are made. Most PDA methods for calculating the static pile capacity require input of an assumed soil damping factor. The magnitude of the damping factor may significantly affect the calculation for static soil resistance. Therefore, it is advised in most cases to run CAPWAP analyses of the data. The current state of practice in NYSDOT is to perform CAPWAP analyses for all tests. If the pile is rejected because of inadequate capacity, one of three things generally will occur: 1. Wait longer with additional restrike test (usually applies to piles in cohesive soils). 2. Splice and drive the pile deeper with DPT monitoring. 3. Drive additional piles in the footing. On acceptance of the pile, driving criteria is established for the remainder of the substructure on the basis of the blow count and hammer operation of the test pile. CONCLUSIONS Dynamic pile tests are an effective tool for ensuring quality deep foundations that are installed by using impact driving methods. They are an extension of the design process whereby assumptions made about capacity are verified using measurements taken while driving the pile into the ground. They have the added ability to monitor pile stress and to detect damage, as well as to measure the performance of the driving system. Finally, in those cases in which the WEAP computer program cannot appropriately model the hammer-pile system, or in which soil information is inadequate, dynamic testing in the field serves to validate the expected performance of the deep foundation. REFERENCES 1. Nordlund, R. L. Bearing Capacity of Piles in Cohesionless Soils. Journal of the Soil Mechanics and Foundation Division, ASCE, SM-3, May 1963. 2. Tomlinson, M. F. The Adhesion of Piles Driven in Clay Soils. Proc., 4th International Conference of Soil Mechanics and Foundation Engineering, London, 1957. Publication of this paper sponsored by Committee on Foundations of Bridges and Other Structures.