Masonry arch bridges derive a significant proportion of their strength from the backfill that is used to cover the arch and provide a level surface for traffic. This remarkable gain can be of the order of a factor of 10 when comparing a backfilled arch with a bare arch and derives from two main effects:
(i) Loads applied at the surface of the fill spread through the fill before they act on the arch, thus diminishing their effect. 

(ii) The weight and strength of the fill acts to restrain movement of the arch.

 

Comparatively little is known about this complex soil-structure interaction problem, and it is the aim of this project to improve our understanding of the following: 
(i) Prediction of the gain in ultimate strength due to the presence of backfill of different types e.g gravel /clay / mixed soils.

(ii) The scope to strengthen bridges by providing a near surface strong layer e.g. reinforced concrete or reinforced soil.

(iii) The effect of the backfill on cyclic loading performance of arch bridges. Such loading can lead to progressive deterioration of the arch, however the fill is anticipated to mitigate the effects of this.

(iv) Methods that might be used to characterise the backfill in existing arches to allow improved assessment of the bridge carrying capacity.

 

To achieve these aims, medium and full scale (3m span) test arches will be constructed and backfilled with a range of soil types and then subjected to a range of loading conditions. The tests will be highly instrumented to record deformations, loads and stresses throughout the model. A particular feature of the tests will be that they will be undertaken in low-friction, clear sided test chambers (up to 8m x 2.5m x 1m) that allows viewing of the soil-arch deformations. Digital photographs of the tests as they progress can then be processed to determine exact movement of the soil using state-of-the-art Particle Image Velocimetry (PIV) techniques.

 

The scale of the largest tests is significant requiring construction of a 3m span arch using X bricks and placement of 27 tonnes of backfill. The load required to collapse such an arch is approximately 140 kN equivalent to the weight of approximately 7 cars (assuming a car with a mass of 2000kg).