ASSESSMENT OF MITIGATING EMBANKMENT SETTLEMENT WITH PILE-SUPPORTED APPROACH SLABS

Problems involving highway bridge approach settlement have been observed at many sites in Louisiana. In southeastern Louisiana, where subsoil settlement potential is the greatest, the bridge structures are usually lengthened in order to reduce the height of the approach embankment. On some major structures, pile supported approach slabs have been used to improve the settlement profile of the approach slab. However, during the years, many pile supported approach slabs have performed well, while others have settled enough to create bumps at the interface with the bridge or roadway. In many other areas of the state, the Louisiana Department of Transportation and Development (DOTD) has implemented accelerated settlement techniques such as preloading in association with wick drains, with some promising results. This research study has identified the factors that contribute to total approach settlement in pile supported approach slabs in southeastern Louisiana. The study involved examination of over 100 pile supported bridge approach slabs. Results of the study indicated that the main parameters that influence the performance of pile supported approach slabs are the height of embankment, subsoil conditions, surcharge height and duration and the length of the piles used for support. The main factor affecting slab settlement is downdrag, or negative skin friction, load imposed on the pile due to the weight of the roadway embankment. Therefore, settlement performance of a pile supported approach slab could be mitigated by selecting the length of piles along the approach slab to yield an ideal settlement profile. The study has developed a database (LAPS) and a spreadsheet program (TU-DRAG) which may be used by DOTD road and bridge engineers for design of approach slabs. In addition, a rating system was developed for assessment of the conditions of the interfaces along the approach slab based on the International Roughness Index (IRI).