Integral bridges, comprising a continuous bridge girder (i.e. deck) integrated to a pair of abutments without using hinged and movable shoes (i.e. bearings), have been constructed to alleviate several inherent drawbacks of conventional bridges. It is shown that this conventional type of integral bridge still has the following problems: (1) large residual settlements in the backfill, developing a bump immediately behind the abutments, and the development of high residual earth pressure on the back of the abutments by seasonal thermal expansion and contraction of the girder, as well as by traffic loads on the backfill; and (2) large detrimental deformation of the backfill by seismic loads. To alleviate these problems, it is proposed to reinforce the backfill with geosynthetic reinforcement that is firmly connected to the full-height rigid facings (i.e. abutments). A newly proposed integral bridge, called the GRS integral bridge, is constructed in stages: first, geosynthetic-reinforced backfill; second, pile foundations (if necessary); third, full-height rigid (FHR) facings (i.e. abutments); and finally a continuous girder integrated to the top of the two abutments, without using shoes. A series of static cyclic loading tests, laterally on the facing and vertically on the crest of the backfill, and shaking-table tests were performed on models of the conventional and new types of integral bridge, as well as two conventional bridge types comprising RC gravity-type abutments and geosynthetic-reinforced soil-retaining walls, both supporting a girder via shoes. The test results showed high static and dynamic performance of the GRS integral bridge, despite its simple structure and construction procedure, and therefore its low construction cost.

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To evaluate the dynamic stability of different bridge types, the results from a series of shaking-table tests on small-scale models of the following bridge types were analysed within the framework of the single-degree-of-freedom theory: (1) a conventional bridge (CB), comprising a girder (i.e. deck) supported via a pair of movable and fixed bearings (i.e. shoes) by gravity-type abutments (without a pile foundation) having unreinforced backfill; (2) a GRS-RW bridge, comprising a girder supported via a pair of movable and fixed bearings by a pair of sill beams placed on the crest of a pair of geosynthetic-reinforced soil-retaining walls (GRS-RWs) having a stage-constructed full-height rigid facing; (3) an integral bridge (IB), comprising a girder integrated to a pair of abutments (without bearings) and unreinforced backfill; (4) a GRS integral bridge, comprising a girder integrated to the abutments (in the same way as the IB bridge) while the backfill is reinforced with geosynthetic layers connected to the facings (in the same way as the GRS-RW bridge); and (5) a GRS integral bridge with a cement-mixed soil zone of rectangular prismatic or trapezoidal shape immediately behind the facing. The following is shown: the stability of the bridge against dynamic excitations increases: (1) with an increase in the initial natural frequency via an increase in the initial stiffness; (2) with a decrease in the decreasing rate of stiffness during cyclic loading (i.e. an increase in the dynamic ductility); (3) with an increase in the damping energy dissipation capacity near and at failure; and (4) with an increase in the dynamic strength. With the GRS integral bridge, the structural integration and geosynthetic-reinforcing of the backfill, as well as cement-mixing of the backfill immediately behind the facings, all contribute to the evolution of these four factors. The natural frequency can then always be kept much higher than the predominant frequency of ordinary design earthquake motion, the response acceleration is kept sufficiently low, and the dynamic stability can be kept very high.

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