Raphael HEINIGER

Contributions to the numerical simulation of compaction of non-cohesive soils by means of oscillating rollers

The oscillating roller is a dynamic compacting machine mainly used for nearsurface compaction of asphalt and soil in road construction of earthworks (dams and embankments). The core of an oscillating roller is the drum. An oscillating moment induced by two opposite rotating eccentric masses, whose shafts are mounted eccentrically with respect of the drum axis and applied in the centre of the drum, leads to a cyclic forward and backward motion of the drum. Through friction in the contact surface between the drum and the soil, shear forces are applied to the subsoil, which propagate in the form of waves in the subsoil resulting in soil compaction, i.e. the pore volume is reduced. The tangential application of forces to the soil reduces vibrations in the surrounding area in comparison to vertically vibrating vibratory rollers. In the case of vibratory rollers, roller-integrated Continuous Compaction Control (CCC) firstly proposed by Adam [2], is used as standard to optimize the compaction process. CCC assesses the soil compaction based on the change of motion behavior of the drum. A theoretical validation and possible optimization of the CCC measurement system for oscillating rollers according to Pistrol [24] is still pending and is subject of current research by Paulmichl [23]. Another focus of ongoing research, which is closely linked to the CCC methodology for oscillating rollers, concerns the optimum machine and operating parameters. The compaction effect should be as high as possible and wear of the drum should be minimized. Based on numerical simulations of a roller-soil system, the present work aims at creating a framework to verify and enhance existing criteria for CCC of oscillating rollers and to optimize machine parameters without the need of further expensive field tests.

At the beginning of this thesis a simplified numerical roller-soil model with linear elastic soil behavior is developed in an effort to better understand the basic behavior of the drum and to identify the impact of machine parameters on the drum´s response. It is shown that all considered parameters affect the accelerations at the drum center. Frequency analyses of the accelerations suggest that individual frequencies can each be assigned to at least one parameter.

With another model, which considers an elasto-plastic top soil layer, in a parametric study optimum machine parameters with regard to drum wear are identified. Qualitatively, proper machine parameters can be determined. To reduce wear of the drum, the oscillation amplitude should be as low as possible, the roller should be extremely light or very heavy and the roller speed should be as high as possible. However, without taking the compaction effect into account, these parameters optimized in such manner are of secondary importance. The same model is used to investigate the influence of material damping on drum-soil-interaction. It is shown that material damping has a decisive influence on the compaction depth of the roller.

In the third model, the hypoplastic material law with intergranular strain concept is implemented for the top soil layer that has to be compacted. To avoid numerical instabilities, viscous damper elements are attached to the surface of the soil. Hence the compaction process for a moving oscillating roller can be simulated. It is shown that a non-cohesive soil is loosened up on the surface during one pass, but is compacted underneath till the lower limit of the 0,5 m thick hypoplastic soil layer. Numerically derived accelerations at the center of the drum show a good qualitative match with measurement results from field tests. With this model, the basis for the further numerical investigations of soil compaction with oscillating rollers has been accomplished.

 

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