involved with the original design and construction, the project team learned that final as-built lengths of the piles were two to three times the design lengths. The engineers developed preliminary foundation designs for the new bridge for the river piers (Piers 4, 5 and 6) and land piers (Piers 1, 2, 3, 7, 8 and 9). The factors influencing foundation type selection were depth to rock, loading, predicted scour, pier/pile cap locations and constructability considerations, among others. At the river piers, bedrock is about 350 ft (107 m) below mudline. Pile caps for the river piers were planned at the water line (about 50 ft [15 m] above mudline), and the design considered predictions of as much as 30 ft (9 m) of scour. Consequently, stiff foundation elements were a requirement at these locations. Initial design alternatives included 6 ft (1.8 m) diameter concrete-filled steel pipe piles driven open-ended 120 ft (37 m) below mudline and 8 ft (2.4 ft) diameter uncased concrete drilled shafts embedded 160 ft (49 m) below mudline. Foundation elements would be embedded in the glaciofluvial and glaciolacustrine deposits to generate resistance through a combination of side friction and end bearing. The design team also considered post-grouting beneath the concrete plug in the steel pipe piles to increase end-bearing resistance. They developed a layout of eight elements per pier (either pipe piles or drilled shafts) and estimated initial loading to be approximately 3,300 kips (1,497 tonnes) per element (all loads shown are strength limit state factored loads). At the land piers, depth to rock ranges between 220 and 280 ft (67 and 85 m) below ground surface. Piers 3 and 7 were at the shoreline, and the bottom of the pile caps were within several feet of the mudline. The team planned permanent sheeting around these pile caps to mitigate scour effects. Initial design included steel HP14x117 friction piles to depths of approximately 110 ft (34 m). The initial number of piles and loading varied from pier to pier, but in general there were approximately 80 piles per pier, and loads were as high as 374 kips (170 tonnes) per pile. Depth to rock at Piers 1 and 8 ranged between 50 and 110 ft (15 to 34 m), so the engineers planned to use end-bearing steel HP14x117 piles for these locations. At Pier 9 (not shown on profile), spread footings on soil were considered adequate. Design Phase Test Programs After consulting with the Rhode Island Department of Transportation (RIDOT) and Federal Highway Administration (FHWA) regarding the difficult nature of the silty soils at the site, Cofferdam for HP pile installation the project team recommended that design phase load test programs be implemented to accomplish the following: • Determine resistances of H-pile, pipe pile and drilled shaft elements by static load testing and dynamic testing. • Compare predicted resistances from static prediction and dynamic methods to the static load test results. • Determine unit skin friction and end-bearing resistances for H- pile, pipe pile and drilled shaft elements. • Examine drivability and constructability characteristics of the test elements. • Use test program data to evaluate foundation types and capacities, and estimate tip elevations for foundations of the proposed piers. Ultimately, the engineers implemented two design-phase load test programs (over $4 million), allowing the project team to test innovative pile designs and make critical design adjustments before construction began. The first test program (TP1) was in 2006. TP1 consisted of installing and statically load testing in axial compression one HP14x117 and one 3.5 ft (1 m) diameter concrete-filled open-ended steel pipe pile (before and after internal pile grouting). A 6 ft (1.8 m) diameter open-ended steel pipe pile was also installed, but a static load test was not performed because the engineers expected the failure load to be too high to achieve economically. The engineers planned to construct a drilled shaft and statically load test it, but the shaft was not successful and was therefore eliminated from the design. Based on the static load test results of TP1, the engineers increased the number of friction H-section piles per pier. In addition, the engineers recommended static load testing of 6 ft (1.8 m) diameter pipe piles, as the dynamic testing indicated lower than expected resistances. Subsequent to TP1, Dr. Samuel Soil profile at Sakonnet River Bridge Paikowsky put forth an innovative design for pipe piles as an attempt to increase the resistance of the steel pipe piles. (At the time, Paikowsky was with GTR and i s cur r ent l y wi th GeoDynamica, Inc.) His suggestion was to install an internal, recessed plate DEEP FOUNDATIONS • JAN/FEB 2015 • 13
