Design and Load Testing of Large Diameter Open-Ended Driven Piles (2015)

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119 APPENDIX D Interview Notes of Private Practice

120 NCHRP 45-05 Interview Notes – D. Michael Holloway, Ph.D., P.E. - Consultant Page 1 of 2 Agency: Industry Participants: D. Michael Holloway, Ph.D., P.E. – Consultant Interview Date: April 22, 2014 The discussion focused on the key issues that Dr. Holloway believes are important for the industry to consider or solve with respect to LDOEPs. Many of his observations are from his extensive pracƒce in California. General Background Adopƒon of widespread use of LDOEP (pipe pile mostly) in California did not really begin until after the 1989 Loma Prieta earthquake. One design change after the quake was to design pile foundaƒons to not fail – to remain elasƒc. The pile-structure connection could be damaged, but the foundaƒons needed to survive undamaged. When applying the new design methodology, CALTRANS adopted load factors that were overly conservaƒve. Driving to Rock Many sedimentary rocks where hard driving of LDOEPs occurs can result in breakdown of the composition of the rock. We tend to see relaxaƒon of base resistance due to the breakdown of the rock, resulting in less base resistance than esƒmated (substantial toe relaxaƒon). Restrikes frequently show a decrease in base resistance under these circumstances. Piles needed to be driven 1 to 1.5 meters in restrike into the rock to see base resistance increase again in some cases. Careful evaluaƒon of the impact at the rock interface is necessary for these low displacement piles. Driving Behavior Need to recognize that how an open pile drives is dependent on how the stresses are ge–ng to the toe of the pile – the failure mechanism at the toe. With thin wall pile, wave equaƒon analysis will someƒmes indicate that stresses are not very large relaƒve to the yield stress, but piles still have problems with collapsing during driving. This is usually due to poor driving alignment, resulting in ovaling and collapse of the piles due to transverse/eccentric stresses. Stresses at the toe need to be less than half of the yield stress of the steel to accommodate the eccentric forces encountered at the toe. Plug Behavior Plugging or absence of plugging dominates the behavior, so understanding if it is plugging or not is key to understanding how the pile will drive.

121 NCHRP 45-05 Interview Notes – D. Michael Holloway, Ph.D., P.E. - Consultant Page 2 of 2 Adding toe treatments to make driving easier (say an inner ring to cut and loosen soil and reduce plugging) will reduce nominal base resistance of the pile. If the pile is to derive significant base resistance, care must be taken when adding means to reduce the plug to make driving easier. There is not a clear understanding of how the soil inside the pile behaves and how much of the fricon resistance is derived inside the pile and how much outside. An old rule of thumb is to assume 2/3 of the fricon is outside the pile and 1/3 inside the pile, as detected in high strain testing. The “devil is in the details”. Plugging must be addressed wisely. Don’t reduce plugging if it is needed for meaningful toe resistance. Dynamic Testing and Wave Equaon MOdeling Very difficult to accurately assess plugging effects using dynamic testing. Static or pulse loading tests should be considered to help with plug behavior evaluaon. A worthwhile approach would be to either do an uplift test, or drill out the soil in the center of the pile, and then do dynamic testing to quantify fricon distribution along the pile shaft. There are some investigators who assume that larger diameter piles made of higher strength steel will have a greater wave speed than the typical 16,800 ’/ sec wave speed customarily used for steel piles. Improper use of higher wave speed can lead to missing damage near the toe that is actually occurring. To evaluate plugging when using an internal plate to fix the plug in place, add soil mass to the pile in the WEAP model for the portion below where the plug is expected to form. Static Axial Capacity Calculations Dr. Holloway tends to use static analysis methods for a rough esmate of axial resitance/capacity. Experience and dynamic testing/wave equaon analysis interpretaons tend to provide be–er esmates of pile resistance. A major problem in pracce is using different methods for the base and side resistance. For example, using Meyerhoff for side resistance and using Nordlund for base resistance. The approach to the pile-soil behavior for each method is different, so mixing methods can lead to poor predicons. Many designers do not account for residual stress in the pile analysis. This leads to over-estimating side resistance and under-esmating base resistance. In many cases load tests are essentially “proof” tests confirming that the structure load can be supported, neglecng to apply axial loads to failure. However, this does not help provide a clear understanding of the true resistance and how the soil-pile interaction really behaves, which can lead to significant inaccuracy in making adjustments to the pile toe elevations.

122 NCHRP 45-05 Interview Notes –Mike Muchard, P.E. – Applied Foundation Testing, Inc. Page 1 of 1 Agency: Industry Participants: Mike Muchard, P.E. –Applied Foundaon Testing, Inc. Interview Date: April 24, 2014 The discussion focused on Mr. Muchard’s experiences with testing LDOEPs for both offshore and on-shore structures. Soil plug behavior biggest issue for open end piles not driven to rock. Will pile plug or remain unplugged - high pile acceleraon during driving will allow pile to cookie cut and not form a plug.- Uncertainty in predicng design capacity o If unplugged, pile acts as fricon pile o If plugged, pile acts as a displacement pile o Is fricon same on open end piles as other pile types Predicng drivability always a huge question Will you need a driving shoe when bearing on rock Is pile capacity verification with dynamic testing reliable? Dynamic testing may under predict or over predict due to plug behavior. For successful testing of LDOEPs, it is important to understand pile behavior. Things to consider for the testing program include o Durability of pile and testing equipment o Waterproofing of sensors and equipment o Protecon system for sensors Interpretaon issues o In concrete piles, accounting for residual stresses from manufacturing o Spiral Welded Pipe – placement of instrumentation and interpretaon of results o Temperature before and after installaon - thermistors must be incorporated Static or Statnamic tests tend to be more reliable for evaluang pile capacity than dynamic testing methods. Improved instrumentation means we can now obtain reliable strain measurements on pipe piles to help answer some of the questions about soil plugging.

123 NCHRP 45-05 Interview Notes – Steve Saye, P.E. - Kiewit Engineering Co. Page 1 of 2 Agency: Industry Participants: Steven Saye, P.E. – Kiewit Engineering Co. Interview Date: April 21, 2014 The discussion focused on the key issues that Mr. Saye believes are important for the industry to consider or solve with respect to LDOEPs Ability to Calculate Axial and lateral resistance is poor There is much we don’t understand about the behavior of LDOEPs. Analysis methods are thought to significantly underestimate pile resistance of LDOEP’s. Our methods of axial analysis don’t appear to adequately capture the impact of construcˆon pracˆces for these larger piles. For piles above 36 in diameter, there are not very many well documented cases. Assembling good, well documented case histories of these piles should be helpful to DOT’s and Industry. Weld Quality and Inspecˆon Mr. Saye is aware of a case where welding of splices for 42 inch diameter pipe resulted in significant quality issues. Further guidance to DOT’s regarding the inspecˆon and details required for welding of splices for steel LDOEP’s is merited. Pile Resistance in Sand (Cohesionless Soil) Generally observe LDOEP piles not plugging – the piles are advancing as a “cookie cu–er” into the soil. Design methods based on smaller piles don’t adequately predict resistance of LDOEP’s. Vibratory Hammers For very long, spliced pipe piles, a common pracˆce is for a contractor to install the first section with a vibratory hammer to set the pile. Design does not always take this method of installaˆon into account. The effect of vibratory installaˆon on pile resistance is not well understood. Significant differences in opinion occur regarding the damaging impact of vibratory hammer installaˆon of piles in clay, or not. No good comparisons of the actual effect of the vibratory hammer on pile installaˆon in clay are available. The Corps of Engineers Specifications allow vibrating up to 50% of the pile length on some of the hurricane protecˆon projects in New Orleans. Many other standard specifications do not address the use of vibratory hammers.

124 NCHRP 45-05 Interview Notes – Steve Saye, P.E. - Kiewit Engineering Co. Page 2 of 2 Spiral Weld Pipe PDA procedures need special a ention for spiral weld pipe. Experience where PDA tests were run with 2 accelerometers and 4 strain gauges. The static load test and 14 day restrike (aer load test) were significantly different. Another project showed be er comparison of PDA and static load test results when 4 accelerometers and 4 strain gauges were used. Effects of Delays for Splicing in Clay Potential damaging impact on long-term pile resistance from significant delays for splicing. If contractor sets first section of many piles, allowing weeks between the driving of the first section and the re-start of driving of the first piles that were set, does the re-driving have a nega‡ve impact on the pile shaft resistance in that first section? In clay, the remolding of the soils along the pile could result in lower pile resistance than expected or would be available had splicing and re-start of driving occurred within a short ‡me period rather than weeks. Differences between Concrete Cylinder and Steel Pipe Very few cases of comparing the two types of LDOEP’s on the same site to investigate the differences in driving behavior, plugging, and pile resistance in the same condi‡ons. Efforts need to be made to get data made available from the few recent projects, such as the Inner Harbor Closure project in New Orleans. Research Needs An assembly of good quality, instructive case histories or database of LDOEP behavior with cone penetration tests to characterize the soil condi‡ons. Comparisons of dynamic and static load tests to be er correlate dynamic testing and wave equa‡on analyses with static resistance. Assemble current pile inspec‡on and acceptance criteria. Defining the pile movement corresponding with the selected pile load test capacity for the LDOEP static load tests. Evaluate the impact of vibratory pile installa‡on on the capacity of piles, including LDOEP’s. Evaluate the effect of delays in pile installa‡on for splicing on the side resistance capacity of LDOEP’s.

125 NCHRP 45-05 Interview Notes – Bob Stevens, Ph.D., P.E. – Fugro-McClelland Page 1 of 2 Agency: Industry Participants: Bob Stevens, Ph.D., P.E. – Fugro-McClelland Marine Geosciences, Inc. Interview Date: April 24, 2014 The discussion focused on the experiences of Dr. Stevens based on his 36 years in design and construc…on of LDOEPs for both offshore and on-shore structures. One of the key concepts is to determine if the pile will plug or not. Dr. Stevens early work included investigating analyses of plugging behavior by evalua…ng the accelera…on of the soil mass in the pile with respect to the accelera…on of the pile. Iner…al forces need to be evaluated in the wave equa…on analyses. His offshore experience (over 500 sites including 250 platforms) indicates that plugging rarely occurs during driving. o He has seen piles driven 200 to 300 feet in clay with no plugging o The accelera…on of the pile is almost always greater than the soil mass under the large forces from the hammer on the pile. o Very few …mes he has seen the use of plates or other devices to cause the pile to plug at a set depth, he believes they have been successful. o While the pile usually won’t plug during driving, the behavior under static loading will usually be plugged behavior. Proper use of PDA equipment to monitor and test LDOEPs is essential. o Larger diameters require 4 transducers and 4 accelerometers to be—er average the stress-…me history across the pile. o The transducers are very robust and can withstand the long driving …mes and heavy impacts of these piles. Es…mating static resistance with API RP2 method is probably the best approach and entirely applicable to transporta…on structures. Dr. Stevens consulted on bridge projects using the API methodology (Bay Bridge in San Francisco, Trans Tokyo Bay Bridge in Japan). o API regularly working to update the procedures based on data from the field. o Recent updates include modifica…on to the design of piles in sands based on CPT test data. This procedure provides be—er es…mates of pile resistance in very dense sands. o Care does need to be exercised when es…mating the mobilized end bearing for drivability analysis. While static tests are not common for LDOEPs, when they are performed, Dr. Stevens usually sees very good agreement (within 5%) of pile resistance determined from CAPWAP analyses of dynamic test results and static load tests. When driving through soŸ rock (very hard clay, shale, siltstone, gypsum, etc.), the toe stresses and toe displacements must be very carefully monitored. o Stop driving if toe stress reaches 80% of yield stress of pile.

126 NCHRP 45-05 Interview Notes – Bob Stevens, Ph.D., P.E. – Fugro-McClelland Page 2 of 2 o Stop driving if toe displacement turns negave – indicates toe is being damaged/crushed. o Important to check drivability using 90% end bearing, looking at stresses at toe, modeling a fixed-end condion. Current state of pracce appears to not consider the long-term strength gain of piles in clay. o Pore pressures can connue to dissipate over years, slowly increasing pile resistance. o Did restrikes on a 24in diameter pile 30 months after load test and had significant increase in resistance. o Also did restrikes on 96in diameter piles 24 months after driving. o The 14 day restrike or static test of a pile is not the maximum or nominal resistance of the pile. o Current pracce disregards a lot of resistance available for piles in clay.

127 NCHRP 45-05 Interview Notes – Sco Webster, P.E. – GRL Engineers, Inc. Page 1 of 2 Agency: Industry Participants: Sco Webster, P.E. – GRL Engineers, Inc. Interview Date: May 2, 2014 The discussion focused on the experiences of Mr. Webster based on his 33 years in construc­on and testing of LDOEPs for both offshore and on-shore structures. While the majority of Mr. Webster’s experience has been with construc­on and testing, his observations from the numerous offshore projects he has been involved with indicate that design methods are conserva­ve to the actual pile resistance available. In many cases, significantly more resistance is available than considered in design. o Some­mes the willingness of the owner/designer or the ­me needed to demonstrate and use higher resistance is not available. o With offshore construc­on, the ­me window for comple­ng founda­ons is very narrow (sometimes as lile as 2 to 3 days). Thus, the ­me to verify long-term resistance is not always available. o Another consideration of the offshore industry is that the difficulty of delivering materials to a platform loca­on means that the provided pile lengths need to achieve the needed resistance with no margin for splicing, etc. Thus, es­mates of resistance tend to be conserva­ve to avoid problems during construc­on. The increased use of LDOEPs is a result of the significant improvements in pile driving equipment making installa­on of these piles possible. The hydraulic hammer provides the ability to install the larger piles. Some pile sizes may not be possible with diesel hammers. Design loads are increasing, especially for extreme events such as vessel impact, scour, and in the case of offshore structures, cyclic loading and fa­gue. Pile plugging during driving is a behavior we do not fully understand and needs more investigation to fully understand it. o It is often treated as if the choice is one or the other: plugged or unplugged. o The actual behavior of the pile is somewhere in between. Whether a pile will plug during driving or not is extremely difficult to predict. o One debate in the offshore industry is the propor­on of skin fric­on development between the outside and the inside surfaces of the pile. o How plugging affects driving is related to pile diameter, soil type, and the hammer selection, not one single factor. o The blow of a hydraulic hammer results in higher pile accelera­on than that from a diesel or steam hammer. The hydraulic hammer accelera­on is almost similar to that of a vibratory hammer. Plug behavior for long term static capacity also still needs to be looked at to fully understand the behavior.

128 NCHRP 45-05 Interview Notes – Sco Webster, P.E. – GRL Engineers, Inc. Page 2 of 2 Due to the higher loads required, the limit of the ability of some hammers is being pushed to install these piles. With high loads it can be difficult to have a hammer of the appropriate size to verify or test resistance. The lower soil resistance during driving allows for the use of a smaller hammer than may be needed to demonstrate the full available resistance of the pile.

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