United States Department of Transportation
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Nonlinear load-deflection behavior of abutment backwalls with varying height and soil density.
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2011-12-01
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Abstract:We address the scaling of abutment wall lateral response with wall height and compaction condition through testing and analytical work. The
analytical work was undertaken to develop hyperbolic curves representing the load-deflection response of backwalls for two backfill material types
(clay and sand) as a function of wall height. The scaling of backwall resistance with height is expressed by an exponent n applied to a normalized
wall height; we find that the height scaling exponent can be taken as 1.05 and 1.56 for the considered clay and sand backfill materials.
We tested two wall-soil specimens with identical characteristics except for the level of compaction of the sandy backfill. One specimen
(denoted T8.0-1) has as-compacted relative densities ranging from approximately Dr = 0.4-0.6 (which was lower than specified) and the other (T8.0-
2) had a high level of compaction of Dr = 0.9-1.0. Other than the degree of compaction, the two specimens are essentially identical in terms of
dimensions, material gradation, and boundary conditions imposed during testing. The wall height in these tests is 8.0 ft (2.4 m), which represents an
approximate upper-bound backwall height; in previous work we tested a similar specimen with a height of 5.5 ft (1.67 m). The backfill material is a
well graded silty sand known in the construction industry at SE-30. The boundary condition imposed on the test is horizontal displacement towards
the backfill without rotation (torsion or rocking) or uplift.
The modest-Dr specimen (T8.0-1) exhibits nearly elastic-plastic response with negligible strain softening. The peak resistance was
approximately 700 kips (3114 kN), which corresponds to a passive earth pressure coefficient of Kp=10, and occurred with a wall-soil interface
friction that is approximately half of the soil friction angle. The high-Dr specimen (T8.0-2) exhibits a strongly strain softening response with a peak
resistance of approximately 1650 kips (7340 kN) and large-strain (approaching residual) capacity of approximately 1100 kips (4900 kN). These
capacities correspond to Kp values of 24 and 17 for peak and large-strain conditions, respectively.
Using shear strength parameter derived from triaxial drained strength testing, log-spiral hyperbolic (LSH) simulations of the backfill response
are performed that modestly under predict the peak specimen responses. However, the degree of underprediction is modest relative to the substantial
differences in capacity between specimens T8.0-1 and T8.0-2. Those large variations in capacity are well captured by the analysis, suggesting that
the LSH method can account for the effects of compaction condition on the wall capacity. The LSH simulations are also able to capture variations in
specimen response for different wall height.
We recommend that the height scaling effects in future versions of the SDC be modified to more realistically capture the different trends for
cohesive and granular backfills. For granular backfill, stiffness and capacity should scale by an exponent n = 1.5-2.0. We also recommend that
compaction condition be considered in the specification of stiffness and capacity.
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