Integral-abutment bridges: A complex soil-structure interaction challenge

Integral-abutment bridges (IABs) are a unique and interesting case study in soil-structure interaction. They were developed to solve long-term structural problems that occur with traditional bridge designs but, as an unanticipated result, created new problems of a geotechnical nature that manifest themselves only after an IAB is placed in service. One problem associated with IABs that tends to develop relatively rapidly is ground subsidence adjacent to abutments. This results in the classical 'bump at the end of the bridge' or potential failure of the approach slab if one is used. The other IAB problem develops in the longer term and involves the irreversible buildup of lateral earth pressures on the abutments due to a soil-mechanics phenomenon called ratcheting. These pressures can lead to structural failure of the abutments. With the increased worldwide use of IABs in recent years, there is widespread interest in developing design solutions to the in-service deficiencies exhibited by IABs as they are designed currently. Fundamental to achieving this goal is developing a clear understanding of how and why the geotechnical problems develop. Research indicates that the primary cause of all problems is irreversible displacement of the soil retained by the abutments of IABs. Research also indicates that design solutions can be achieved using a variety of geosynthetics in an innovative, synergistic fashion. First and foremost, a self-stable zone of material must be established behind each abutment. This can be achieved using either a reinforced soil mass or some type of geofoam (EPS blocks or foamed concrete). Secondarily, an EPS-geofoam geocomposite must be placed between this stabilized zone and the back of the abutment to act as a compressible inclusion. This inclusion serves both as an expansion joint to accommodate seasonal displacement of the abutment as well as to provide insulated drainage. An important benefit is that these solutions can be implemented as part of either new construction or the rehabilitation of existing IABs.