United States Department of Transportation
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Field demonstration of new bridge approach slab designs and performance.
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2014-06-01
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Abstract:The Louisiana Department of Transportation and Development (DOTD) has initiated a major effort to minimize the bridge end bump problem associated
with differential settlement. As a result, a new design for the approach slab was proposed, which requires increasing the slab flexural rigidity (EI), and
using reinforced soil foundation (RSF) to support the slab and traffic loads at the roadway pavement/approach slab joint (R/S joint). Reinforcing the soil
underneath the strip footing will result in increasing the soil’s bearing capacity and reducing the embankment settlement by redistributing the loads
imposed by the slab and traffic over a wider area. Bayou Courtableau Bridge was selected as a demonstration project to evaluate, validate, and verify the
new bridge approach slab design method proposed in a previous study. The east and west approach slabs at Bayou Courtableau Bridge are 40 ft. long by 40
ft. wide. The west approach slab was designed using the proposed new method with slab thickness of 16 in., while the east approach slab was designed
using the traditional method with slab thickness of 12 in. The pavement end side of the approach slab was supported by a 4.0-ft. wide strip footing with the
soil underneath it reinforced by six layers of geogrid placed at a vertical spacing of 12 in. The geosynthetic reinforced soil below the strip footing was
designed according to the methodology proposed in previous study. The west approach slab was instrumented with pressure cells to measure the pressure
distribution underneath the footing and contact pressure underneath the slabs. Strain gauges were used to measure strain distribution along geogrid
reinforcement and sister-bar strain gauges to measure strains within the slab. The east approach slab was instrumented with pressure cells to measure the
contact pressure underneath slab. Two static load tests were conducted on both the west and east approach slabs at two different times after construction.
The performance of the approach slabs, including deformation and internal stresses of concrete slab, contact stresses between slab and embankment, stress
distributions within reinforced soil foundation, and strain distributions along the geogrid, was monitored during the tests. The test results indicated that the
west approach slab (with new design) lost most of its supports from the soil; while the east approach slab (with traditional design) kept losing its contacts
from the soil starting from the bridge side towards the pavement side after about a year and half. The roughness profiles show better performance of the
new approach slab system with much lower International Roughness Index (IRI) values. The year and a half monitoring program at Bayou Courtableau
Bridge demonstrated much better performance of the new approach slab design system (west approach slab) compared to the traditional design.
The magnitude and rate of embankment settlement at Bayou Courtableau Bridge site was also monitored (for the two embankment sides) during and after
the construction, and the results were compared with the laboratory calculated settlements and the piezocone penetration test (PCPT)-based settlement
prediction methods. The results showed better predictions using PCPT methods as compared to laboratory results.
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