Ivan PAULMICHL
Numerical modeling approaches to the oscillation roller-subsoil interaction problem
Soil compaction is a fundamental and critical construction phase of a wide variety of engineering structures, since the quality of fills in foundation work of hall and industrial facilities, soil replacements, dam and base layers in road, railway and airport construction depends on the built-in material and in particular on the realization of earthwork. Dynamic roller compaction has become the common method for proper near-surface compaction to prevent future damage of constructions connected to layered earth structures, failure of long-term pavement performances and increasing maintenance costs. While a static roller uses only its weight to compact filled layers, a dynamic roller enhances the efficiency of subsurface compaction through dynamic excitation of the drum.
Depending on the drum excitation, two basic types of dynamic rollers do exist, i.e. vibratory rollers and oscillation rollers. In a vibrating drum a single unbalance mass, which is attached concentrically to the drum axis, generates a rapidly alternating upward-downward motion of the drum. The subgrade is compacted by the dynamic pressure applied by the drum. The drum of an oscillation roller, as considered in the present thesis, is equipped with two offset eccentric masses, which rotate synchronously in the same direction. The resulting alternating high-frequency forward-backward motion of the drum (oscillatory drum motion) is superposed with the translational roller motion (moving drum under the static axle load). Due to the frictional contact between drum and subsoil mainly dynamic shear forces are transmitted to the soil, which in turn increase the subgrade density, also known as shear force compaction.
The lack of real-time compaction information may lead to both under- and overcompaction and, moreover, to an increased wear of the drum of oscillation rollers. Thus, instant compaction control is of particular importance. A high-leveled quality management requires continuous control of the soil compaction in the entire compacted area, which can be achieved only by work-integrated methods. Roller vibration monitoring has been used for over 40 years during soil compaction to provide what is referred to as Continuous Compaction Control (CCC). CCC has become the standard technology for assessing work-integrated and continuously the achieved compaction by vibratory rollers. For oscillation rollers, however, until recently no mature CCC system did exist, although initial approaches to a CCC system were already proposed almost four decades ago. The recently developed CCC technique has neither been verified by analytical nor by numerical studies. The present doctoral thesis therefore aims to fill this gap by pursuing two modelling strategies, lumped parameter modeling and Finite Element modeling.
The proposed lumped parameter model of the interacting oscillation roller-subsoil system facilitates the response simulation of an oscillation drum with the least numerical effort. The compaction process itself is not captured, but different degrees of compaction are considered by varying the soil stiffness. The roller is represented by the oscillation drum and its viscoelastic connection to the roller frame. In the chosen modeling strategy, the curvature of the soil surface below the drum is prescribed. In this way, also the vertical drum acceleration can be computed. The discrete viscoelastic subsoil model consists of a vertical and a horizontal Kelvin-Voigt element. Contact between drum and soil surface is described by means of dry friction according to Coulomb's law. As such, the stick-slip motion of the drum can be simulated. The highly nonlinear equations of motion of this three degrees-of-freedom model are derived separately for the stick and the slip phase of the motion. A detailed response study of one selected roller type shows that this model captures the fundamental response characteristics of the drum-subsoil interaction system observed in the field. The results of a comprehensive parametric study based on four different oscillation rollers essentially confirm the compaction indicator for the considered oscillation rollers in a wide range of soil stiffness. The found application limits of this value are clearly influenced by the device parameters and the operating oscillation frequency.
The presented Finite Element model allows for the first time the numerical prediction of both the dynamic response acceleration and the compaction effect of an oscillation roller during near-surface compaction of non-cohesive soils. In the developed plane-strain model, the intergranular strain enhanced hypoplastic constitutive model captures the nonlinear behavior of the soil below the drum. A “protective foil” is applied to the soil surface to ensure the numerical stability of the model solved with the Finite Element software suite ABAQUS/Standard. The derived stresses, strains, and change of the void ratio in the subsoil representative for the compaction effect as well as the dynamic response of the drum center are analyzed in detail. In addition, computed dynamic stress components in the soil and drum accelerations are compared with data recorded in field tests. It is shown that the developed model qualitatively and partially also quantitatively predicts the fundamental response characteristics of the interacting oscillation-subsoil system observed in field tests. The outcomes of a comprehensive sensitivity study confirm that the quantities derived from the drum response are basically suitable as indicators for CCC with oscillation rollers. Moreover, the results impressively demonstrate that the roller speed has a significant effect on both drum response and achievable soil compaction.