Numerical Simulation of Pavements Behaviour from Accelerated Tests

Erlingsson (2007) describes two thin pavement structures that were tested in ac­celerated testing by using a Heavy Vehicle Simulator. Both were surface dressed structures, one with 20 cm thick unbound base course layer and the other with the base course divided into a 10 cm bitumen stabilized base over 10 cm unbound base. Both structures were instrumented to estimate deflections, strains and stresses in various locations inside the structure. A numerical analysis was also carried out to simulate the response behaviour of the structure that could be compared with the actual measurements. The simulation was performed using different techniques: 3D and 2D axi-symmetric analyses, finite element and multi layer elastic theory, linear elastic and non-linear elastic base behaviour. The results were further used to model the permanent deformation development in each layer. A cross section of the two structures is shown in Fig. 11.8.

Figure 11.9 shows the induced vertical stress under the centre of a single tyre load for both structures where the axle load is 120 kN, or close to one conventional axle load (11.5 ton in the EU), and the tyre pressures is 800 kPa.

Fig. 11.8 Two test pavement structures: (a) IS02 is an unbound structure and (b) IS03 is a bitumen stabilized structure

Note: The instrumentation used for the response measurements is shown as well.

One can see in Fig. 11.9a the importance of taking into account the non-linear base behaviour for the unbound structure IS02. The linear analyses overestimate the stresses in the upper part of the structure, compared with the two non-linear analyses. In the structure with the bitumen-stabilized base, Fig. 11.9b, this is not as prevalent and both the linear as well as the non-liner analyses capture the overall response of the structure quite reasonably.

Finally Fig. 11.10 shows the results of the predicted as well as the measured accumulated permanent deformation for the base, sub-base and the subgrade layer as a function of load repetition for both pavement structures. The response over the first 300 000 load repetitions are shown.

A simple power law assumption was used in calculating the permanent deforma­tion. This seems to give a satisfactory agreement between the numerical simulations and the measurements for both structures. The largest deviation took place during the early part of the test but, thereafter, the rate of increased permanent deformation was quite similar between the analyses and the actual measurements.

It is also interesting to compare the measured and calculated permanent defor­mation of the two structures. Adding the three curves of Fig. 11.10 together gives the total permanent deformation, i. e. rutting, in the unbound part of the structure.

Number of passes, N Number of passes, N

Fig. 11.10 Prediction versus measurements of permanent deformation development for the three unbound layers as a function of load repetition for both pavement structures IS02 and IS03

The one with the upper part of the base stabilized with bitumen shows a total of about 14 mm of deformation after ca. 300 000 passes, but the other with un­bound base shows almost a 40 mm deformation after the same number of passes. This indicates quite a different “lifetime” of the two structures. This difference in “lifetime” does not prevail in the measurement and calculation of vertical stresses, where stresses at the top of the subgrade are almost the same for both structures (see Fig. 11.9).