The term abutment usually implies not only a support for the bridge, but also what is virtually a retaining wall for the bank behind it. In the case of an arch bridge, the thrust of the arch is invariably so great that there is never any chance that the pressure of the earth behind the abutment will throw the abutment over, and therefore the abutment never needs to be designed as a retaining wall in this case; but when the abutment supports a truss bridge which does not transmit any horizontal thrust through the bridge, the abutment must be designed as a retaining wall. The conditions of stability for such structures have already been discussed. This principle of the retaining wall is especially applicable if the abutment consists of a perfectly straight wall. There are other forms of abutments which tend to prevent failure as a retaining wall, on account of their design.

Fig. 72. Typical Abutment with Flaring Wing Walls.

Fig. 72. Typical Abutment with Flaring Wing Walls.

235. Abutments With Flaring Wing Walls

These are constructed substantially as shown in Fig. 72. The wing walls make an angle of about 30° to 45° with the face of the abutment, and the height decreases at such a rate that it will just catch the embankment formed behind it, the slopes of the embankment probably being at the rate of 1.5:1. If the bonding of the wing walls, and especially the bonding at the junction of the wing walls with the face of the abutment, are properly done, the wing walls will act virtually as counterforts and will materially assist in resisting the overturning tendency of the earth. The assistance given by these wing walls will be much greater as the angle between the wing walls and the face becomes larger.

236. U-Shaped Abutments

These consist of a head wall and two walls which run back perpendicular to the head wall (see Fig. 73). This form of wall is occasionally used, but the occasions are rare when such a shape is necessary or desirable.

237. T-Shaped Abutments

As the name implies, these consist of a head wall which has a core wall extending perpendicularly back from the center. The core wall serves to tie the head wall and prevent its overturning. Of course such an effect can be produced only by the adoption of great care in the construction of the wall, so that the bonding is very perfect and so that the wall has very considerable tensile strength; otherwise the core wall could not resist the overturning tendency of the earth pressure against the rear face of the abutment.

Fig. 73. U Shaped Abutment.

Fig. 73. U-Shaped Abutment.