The effect of bridge approach slabs on pavement deformation

Abstract

Because there is a difference in stiffness between a bridge deck and the materials used in the approach embankment, traffic loading will cause the pavement to deform relative to the bridge deck and form a ‘bump’ at the end of the bridge. This causes drivers’ discomfort and accelerated damage to the pavement. The use of concrete approach slabs constructed so as to slope down from the bridge deck beneath the pavement has been proposed as a means of alleviating this problem.

In this thesis, experimental and theoretical work have been carried out to investigate the behaviour of approach slabs under repeated loading.

The work is broadly divided into six sections :

1. The development of a testing facility for the testing of model approach slabs under controlled conditions.

2. The assessment of the capability and performance of the testing facility.

3. The planning and conduct of the laboratory testing for the investigation of the deformation behaviour of pavements subjected to traffic loading.

4. The formulation of a three-dimensional finite element program using Fourier transforms for the analysis of the problem.

5. The planning and conduct of monotonic and cyclic drained triaxial tests for the investigation of residual strain behaviour.

6. The analysis of the residual strains in the model embankments and the development of a numerical method for the prediction of permanent displacements.

A testing facility has been developed to provide simulated traffic loading on model pavements underlain by approach slabs. Three series of tests have been carried out to investigate the effect of the orientation of the approach slab on the deformation behaviour of the pavements.

It is found that with the use of inclined slabs, the deformation in the pavement surface is more gradual than for the case where a horizontal slab is used. In other words, the bump usually formed at the end of a horizontal slab can be eliminated.

A finite layer method is developed in this thesis, which provides an efficient means for investigating by the moving wheel. the cyclic stresses generated in the road embankment The effect of the approach slab orientation and the pavement stiffness on the soil response can be examined.

Cyclic triaxial tests have been conducted using a fully automated GDS triaxial testing system. The aim of these tests was to investigate the soil residual strain behaviour. The cyclic stress paths have been estimated from the finite layer analyses.

The triaxial test results showed that the residual axial strain after the first loading cycle increased linearly with the logarithm of the number of cycles (for the 50 loading cycles carried out for each test). This relationship was to be dependent on the stress level and the gradient of the stress path.

The residual strains in the model pavements have been backfigured from two of the tests. The residual strains after the first loading cycle were found to vary non—linearly with the logarithm of the number of loading cycles. However, the non—linearity was less pronounced for the initial 50 cycles or so.

A stress path method was developed to utilise the cyclic test data for the prediction of residual axial strains in the pavement layers. Based on this method, the residual strains in the pavement layers after the first loading cycle can be computed. The residual deformations are then obtained by integrating the strains obtained for all the layers. This method was applied to one of the model pavement tests and the results are found to agree reasonably well with the observed values.

There are a number of improvements that can be made to this method of prediction, for instance, the incorporation of a non-linear stress-strain law to predict the first cycle deformation. Also, the measurement of the actual stresses acting in the model pavements would be useful in determining the appropriate stress paths for subsequent cyclic tests.