Modified sheet pile abutments for low volume road bridges

Iowa Highway Research Board Project TR-568 was initiated in January 2007 to investigate the use of sheet pile as an alternative foundation component for Low Volume Road (LVR) bridges. A total of 14 different sites were initially investigated in several counties as potential candidates for the construction of demonstration projects utilizing sheet pile abutments. Based on site conditions, three sites were selected for demonstration projects; these are located in Black Hawk, Boone, and Tama Counties. Each of the demonstration projects utilizes a different experimental abutment system.

Sheet pile, typically used for retaining structures in the United States, has been used as bearing piles in Europe for the past 50 years and is a potential alternative for use as the primary component in LVR bridge substructures. To investigate the viability of axially-loaded sheet pile abutments, a demonstration project was constructed in Black Hawk County, Iowa. The project involved construction of a 40 ft, single-span bridge utilizing axially-loaded sheet pile as the primary foundation component. The site chosen for the project consisted of primarily silty clays underlain by shallow bedrock into which the sheet pile was driven. An instrumentation system (consisting of strain gages, deflection transducers, earth pressure cells, and piezometers) was installed on the bridge for obtaining live load test data as well as long term performance data.

Live load testing of the bridge structure was performed on November 3, 2008 by placing two loaded trucks (approximately 24 ton each) at various locations on the bridge and recording data. Maximum axial stresses occurring in the piles were approximately 0.5 ksi and were comparable to estimates made by analysis for a design lane-load distribution width of 10 ft. Flexural stresses, in general, were significantly less than those estimated by analysis and maximum values were approximately 0.2 ksi. Earth pressures recorded during live load testing (with maxima of approximately 100 psf) were also significantly lower than earth pressures estimated by analysis. These results suggest the method of analysis for lateral earth pressures applied to the sheet pile wall was conservative. Long-term monitoring of the bridge from November 2008 through February 2009 was also performed; the datalogging system was damaged by flooding in March 2009 and subsequent long-term monitoring was terminated. Variations in earth pressure over time were observed with the largest variations in earth pressure occurring behind the abutment cap. The earth pressures experienced cycles that varied in magnitude from 50 psf to 1500 psf, suggesting long-term loading due to freeze/thaw cycles of the soil and the thermal deformation of the superstructure elements may be the critical factors in the design of sheet pile abutment and backfill retaining systems rather than vehicular live loads.

The demonstration projects in Boone and Tama Counties were designed using a geosynthetically reinforced soil backfill with a sheet pile backfill retention abutment system. Each of the bridge superstructures is supported by spread footings bearing on the reinforced soil mass abutment systems. The bridge superstructure in Boone County is a 100 ft long, three span J30C-87 continuous concrete slab bridge while the superstructure for Tama County utilizes two 89 ft railroad flatcars bolted together. Structural monitoring systems (including strain gages, earth pressure cells, and piezometers) were developed for load testing and long-term monitoring of these projects as well. Construction of the project in Boone County was completed in fall 2009 and live load testing was subsequently performed on November 13, 2009. Maximum flexural stresses experienced in the sheet pile elements were 0.08 ksi and were significantly lower than estimated by analysis. Vertical and horizontal earth pressures in the backfill (with maxima of 410 psf and 50 psf, respectively) were also lower than expected, suggesting a conservative design approach. Construction of the project in Tama County was completed in August 2010 with subsequent load testing performed in October 2010.

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