“Underlying” Causes for Settlement of Bridge Approach Pavement Systems

A comprehensive field study of 74 bridges in Iowa was conducted to characterize problems leading to poor performance of bridge approach pavement systems. Subsurface void development caused by water infiltration through unsealed expansion joints, collapse and erosion of the granular backfill, and poor construction practices were found to be the main contributing factors. To characterize the problem, International Roughness Index and profile measurements from several sites were used to show that approach pavement roughness is several times higher than the average roadway condition and are most severe at the abutment-to-approach pavement intersection and transverse expansion joints due to large $(5–10\mathrm{cm})$ joint widths. Further, a settlement time history was documented at one bridge site by measuring the approach slab pavement elevations periodically after completion of bridge construction, revealing a progressive settlement problem under the approach pavement. To better understand the void development under the approach pavement, laboratory compaction tests were performed on granular backfill materials from various bridge sites to quantify their saturated collapse potential in the postconstruction phase. These tests revealed collapse potential of backfill materials in the range of 5–18% (based on volume) with the high values for poorly graded sandy backfill materials, indicating significant settlement problems. Based on the research findings, some relatively simple design and construction modifications are suggested which could be used to alleviate field problems for similar bridge approach pavement systems.