THE EFFECT OF THE STEEPNESS OF THE SIDESLOPE ON RUTTING

The objective of HVS-tests was to study the influence of the road cross section and edge effects to the structural strength and permanent deformations of low-volume roads. The purpose of the test results was to verify design methods and calculation models. The second purpose was to verify laboratory tests in a full-scale model test. The tests was financed by Finnish Road Administration. The test consisted of three test sections: one without slope, one with 1:3 slope and one with 1:1.5 slope. The test sections were constructed and instrumented in autumn 2000. The area was insulated during the winter and test was performed one year later. The structures were designed to be equivalent to the structure of a low volume road. All test sections consist of a thin asphalt surfacing of 40 mm, a 400 mm base layer of crushed rock and a 200 mm subbase layer of gravel. The gravel includes fine-grained particles, so the water capillary rise was evident. Instrumentation and measurements were mainly focused on the dynamic and permanent deformations in pavement layers and slope in both vertical and horizontal direction during test loadings. Besides that the water content, pore water and earth pressures were measured. The deformation characteristics were determined with triaxial tests from test samples of each layer. The instrumentation was similar in all sections. Most test instruments seemed to work well during the test and give reliable results. The tested structures were designed to stand up only about 15,000 passes. The structures were constructed carefully according to the quality requirements of Finnra. The quality of the construction was followed with level control, density and bearing capacity measurements. The thicknesses of the layers were even and they fulfilled the quality requirements clearly. The densities of layers did not fulfil totally the quality requirements. The bearing capacities were only about 20 - 35% of the Finnra's requirements. Test parameters and environmental conditions, including the water table regulation, were controlled during the HVS test. All sections were tested identically. At the beginning of test, the water table was 50 mm under the clay surface. At the end of test the water table was elevated to the top of the gravel layer during the test and to the centre of crushed rock. Static and cumulative pore pressures were monitored with transducers. A super single wheel was used as a loading wheel and the load was increased with a step of 10 kN from 30 to 50 kN. The wheel loaded structure in three different positions: 400 mm, 700 mm and 1000 mm from the edge of the slope. One test step consisted of 600 passes in each position. The pavement response to the moving wheel load with several offsets was measured and finally the pavement performance was evaluated with accelerated testing. The permanent and dynamic deformations were followed up with Emu-Coil sensors from the lowest part of crushed rock, gravel and from the two topmost parts of clay (thickness of each 200 mm). The permanent horizontal deformations were also followed from the side slopes. HVS-profilometer measurements were used to monitor the deformations on the asphalt layer. Deformations were measured after each 600 passes. It was assumed that the asphalt layer (50 mm) did not deform during the test and all deformations happened in unbound layers. The distribution of permanent deformations were calculated for each 200 mm layer. From 63 to 80% of the permanent deformations happened in the base layer of crused rock (400 mm) and it was distributed quite evenly between the upper and lowest 200 mm. The gravel layer undeneath deformed about 8 to 13% and the upper part of the clay layer (400 mm) deformed with 4 - 6%. There were significant deformations in sloped structures and it can be assumed that the loading situation in the steepest structure with high water table was quite near to the failure. The damages of the sloped structures were easy to notice and the depth of the ruts were over 40 mm. A significant part of the relative vertical deformations of the structure with 1:1.5 slope concentrated to the upper 400 mm part of the structure. It is obvious, that the failure surface develops to this part of the structure with the highest deformations and continues towards the slope. The measurements of the horizontal displacements from the side slopes confirm this idea. The horizontal displacements of the gentle slope (1:3) concentrated near the surface, while no clear signs of the failure surface was detected.