“Underlying” Causes for Settlement of Bridge Approach Pavement Systems David J. White, A.M.ASCE 1 ; Mohamed M. Mekkawy 2 ; Sri Sritharan, A.M.ASCE 3 ; and Muhannad T. Suleiman, A.M.ASCE 4 Abstract: 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 rough- ness 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 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 settle- ment 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. DOI: 10.1061/ASCE0887-3828200721:4273 CE Database subject headings: Bridge abutments; Drainage; Erosion; Backfills; Pavements. Introduction Problems with bridge approach pavements are widespread and require investigations inclusive of the approach pavement system i.e., pavement layers, joints, backfill, drainage systems, etc.. Better solutions to alleviate this problem are needed to reduce maintenance costs, improve ride quality, and eliminate hazards to drivers. Over the last decade, the “bump at the end of the bridge” problem has gained increased national attention. In 1997 NCHRP Synthesis 234 Briaud et al. 1997 identified many of the major causes of bridge approach settlement in the United States. The study estimated that 25% of the bridges nationwide suffered from bridge approach settlement with an annual maintenance cost of at least $100 million Briaud et al. 1997. A subsequent survey by Hoppe 1999 reported that 44% of the state DOTs consider bridge approach settlement a significant problem. In Iowa the maintenance costs associated with repairing and replacing bridge approach pavements usually exceed the available funds—a sce- nario likely common to many state DOTs. Bridge approach settlement has been investigated previously by a number of researchers Schaefer and Koch 1992; Briaud et al. 1997; Hoppe 1999; Abu-Hejleh et al. 2006, focusing on both superstructure and substructure components. According to these studies, lateral movement of the bridge abutment and settlement of the embankment are considered amongst the primary reasons for the problem. Lateral movement of the abutment is a result of the bridge superstructure expanding and contracting with seasonal temperature fluctuations. This lateral movement affects the inte- gral abutment bridges much more severely than nonintegral abut- ment bridges. In integral bridges, as the temperature increases, the bridge superstructure moves the abutments toward the retained soil causing high lateral stresses, which can reach stress levels as high as the passive pressure limit Schaefer and Koch 1992. As the temperature decreases, the abutments move away from the soil, creating a void between the abutment and backfill material. The presence of the void can intensify soil erosion, increasing the size of the void under the approach slab Schaefer and Koch 1992; Wahls 1990. Greimann et al. 1986 reported that the lat- eral movement of the bridge that occurs in integral abutment bridges also affects the pile stresses and can potentially reduce the vertical load carrying capacity of the piles. According to Ardani 1987, embankment settlement is also a leading contributor to approach settlement problems and is caused 1 Assistant Professor, Dept. of Civil, Construction and Environmental Engineering, Iowa State Univ., 476 Town Engineering Building, Ames, Iowa 50011-3232. E-mail: djwhite@iastate.edu 2 Graduate Research Assistant, Dept. of Civil, Construction and Environmental Engineering, Iowa State Univ., 405 Town Engineering Building, Ames, Iowa 50011-3232. E-mail: meks@iastate.edu 3 Associate Professor, Dept. of Civil, Construction and Environmental Engineering, Iowa State Univ., 406 Town Engineering Building, Ames, Iowa 50011-3232. E-mail: sri@iastate.edu 4 Lecturer, Dept. of Civil, Construction and Environmental Engineering, Iowa State Univ., 490 Town Engineering Building, Ames, Iowa 50011-3232. E-mail: suleiman@iastate.edu Note. Discussion open until January 1, 2008. Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and pos- sible publication on May 8, 2006; approved on September 29, 2006. This paper is part of the Journal of Performance of Constructed Facilities, Vol. 21, No. 4, August 1, 2007. ©ASCE, ISSN 0887-3828/2007/4-273– 282/$25.00. JOURNAL OF PERFORMANCE OF CONSTRUCTED FACILITIES © ASCE / JULY/AUGUST 2007 / 273