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Format:
Print
Author:
Gomez, Brian W.
Dept./Program:
Civil Engineering
Year:
2013
Degree:
NA
Abstract:
The American Association of State Highway and Transportation Officials (AASHTO), along with some other federal and state guidelines, recommend a maximum soil fines (particles finer than 0.075 mm) content in granular structural backfill used behind bridge abutments and retaining walls. This 5% fines content limit is 6 percent (by weight) in Vermont and 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 contents could be successfully used in practice. A related question is how far the structural backfill should extend behind an abutment or a retaining wall. State DOTs have differing specification requirements ranging from a vertical limit to a 1.5H:1V slope from the heel of the wall footing. Clearly, the amount of material needed varies drastically and consequently the cost. The literature does not provide explanations for the fines content limits and extents of the select backfill.
In this research, saturated hydraulic conductivities of a granular structural backfill with 0, 5, 10, 15,20 and 25% non-plastic fines content at 95% relative compaction with respect to Standard Proctor test (AASHTO T-99, ASTM D-698) were determined in the laboratory. These flexible wall permeability tests were conducted on 15.2 cm (6 in.) diameter and about 30.5 cm (12 in.) high triaxial specimens at 41,83, and 124 kPa (6, 12, and 18 psi, respectively) confining pressures employing flow pumps under constant flow conditions followed by consolidation drained tests for-obtaining associated drained shear strength parameters of these backfill gradations. In addition, finite element simulations including systematic parametric studies were also conducted to investigate optimal geometry of the backfill.