University of Vermont Transportation Research Center

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The American Association of State Highway and Transportation Officials (AASHTO), along with some other federal and state guidelines, suggest a maximum soil fines (particles finer than 0.075 mm) content in granular structural backfill be used behind bridge abutments and retaining walls. This fines content limit is currently set at 6 percent (by weight) by the Vermont Agency of Transportation (VTrans) and is usually between 5 and 12 percent in most states, according to a canvassing of state Department of Transportation (DOT) practices. The fines content limit is an attempt to assure a free-draining backfill condition so water is not retained behind the structure, thereby eliminating the need to design the abutments and retaining walls for hydrostatic pressures. It appears that this maximum fines content is adopted largely as a rule-of-thumb considering that hydraulic conductivity of a soil is expected to decrease with increasing fines content. In Vermont and many other regions the availability of high-quality structural backfill with naturally low fines content is declining, which warrants an evaluation of whether granular backfill materials with greater than 5% fines content could be successfully used in practice. This research project was set up with two broad over-arching goals. The first goal was to verify that the backfill and drainage details currently used in cast-in-place concrete cantilevered retaining walls and bridge abutments on VTrans projects perform as expected and that the backfill has the engineering properties assumed in the design. The second goal was to find the most cost effective backfill details. To evaluate the above two overarching goals, the specific objectives of this research were to: (1) survey other state Departments of Transportation on their practices for abutment and retaining walls; (2) study the effects of fines on a typical granular structural backfill by performing hydraulic conductivity and shear strength tests at varied non-plastic fines contents; (3) monitor differential water levels between the stream and the backfill at two field sites; (4) analyze the collected data and develop specific recommendations for VTrans; and (5) prepare this final report. To assess if any differential water pressures exist in existing cast-in-place reinforced concrete retaining walls installed by VTrans, a field monitoring program was implemented at two sites in Vermont. The laboratory investigation included flexible wall, hydraulic conductivity tests on a granular structural backfill with 0, 5, 10, 15, 20, and 25% non-plastic fines content at 41, 83, and 124 kPa (6, 12, and 18 psi) confining pressures followed by consolidated drained triaxial compression tests for obtaining associated drained shear strength parameters of these gradations. The 15.2 cm (6 in.) diameter specimens were prepared at optimum moisture content and 95% of maximum standard Proctor density. Some tests were conducted at 90% of maximum standard Proctor density. To enable a comparison with respect to modified Proctor maximum densities, modified Proctor tests were also performed for all base soil-fines content mixtures. The experimental results were compared with relevant studies found in the literature. The results of the field monitoring program were inconclusive. The results of the laboratory investigation indicated that a non-plastic fines content up to 10% may be justified in structural backfill specifications for retaining walls and abutments