## 1 Introduction  
The rheological behaviour of natural soils mixed with additives (sand, natural aggregates, lime, chemical stabilizers) for road structures, so called improved soils, has to be characterized according to the working technology and the dynamic compaction regime of the vibratory roller. For this, depending on the location of the construction site, in Romania since 1998 and until now, the team of researchers and technicians set up for the compaction process' optimisation has developed specific technologies for 12 sites belonging to the Pan-European N-V and S-E road, between Oradea and Constanta. Thus, tests were performed on the construction sites of the Transylvania, Cluj-Oradea, Lugoj-Timisoara, and Bucharest-Constanta highways, in order to identify the physical-mechanical, rheological and technological characteristics of the improved soil layers put into place. In this context, it is mentioned that the great variety of improved soils showed different rheological behaviour from one location to another, being identified composed of linear viscoelastic models, schematised as follows: Voigt-Kelvin, Maxwell, Hooke-Voigt Kelvin, Newton-Voigt Kelvin and Zener [1; 2; 3; 4; 5; 6]. In addition, there were several situations of weak nonlinearity or hysteretic viscoelastic behaviour identified, with insignificant deviations from the dynamic response in the forced vibration regime of the vibratory roller [7; 8; 9]. Based on the \"in situ\" experimental results and the numerical evaluations with the actual field data and vibratory rollers (Bomag, Dynapac, Amman, Caterpilar, Hamm), it was found that the dynamic response can be evaluated based on linear viscoelastic rheological models. In this case, the vibration compaction process is conditioned by the added elements to the natural soil consisting in the content of clay, sand, mineral aggregates with chemical or ecological stabilizers with dosages specific to the applied technology, that essentially modify the elasticity and viscosity of the road structures to be realised [10; 11; 12; 13; 14; 15]. In vibratory compaction, natural and filling soils can be characterized by Voigt-Kelvin linear rheological models, depending on the dynamic response to compaction. The content of clay, sand, mineral aggregates with chemical or environmentally friendly stabilizers modifies, essentially, both the elasticity and the viscosity of the earth layers brought in on the site. Following the many experiments made on at least 25 categories of land with chemical and/or environmentally friendly stabilizers, the conclusion was that the dynamic response of the vibrating compactor roller matches the Voigt-Kelvin linear viscoelastic model (90% of the cases) [16; 17; 18; 11; 12]. Of all the experiments in the mentioned share, after each passage of the vibrating compactor roll, three cases where the elastic and viscosity coefficients, respective of the stiffness viscous damping \(c\), were identified and they can be classified into layers of soil as follows:  
* soils with discrete variable stiffness after each passage of the vibrating roll and constant damping [16; 17; 12; 11]. * soils with discrete variable stiffness and damping after each passage (soils with more than 80% mineral aggregates with a grain of more than 16 mm) [16; 17; 13; 14; 15; 20]. In situ and laboratory experiments were conducted on natural soils with admixtures of lime, cement, black pigment, natural or chemical liquid stabilizers were performed on two categories of construction sites [14; 15; 20]. This article includes the synthesis of the parameter values of stiffness and damping for the three distinct situations, which offer significant and representative dynamic responses of the vibrating compactor roller.