## 2 Literature Review  
In contrast to rigid pavements, the sequence of thicknesses in flexible pavements aims at supporting and distributing the imposed loads through each sublayer; the purpose of this structural logic is to contribute to the attenuation of the stresses generated by traffic on the underlying soil [18]. Although stresses decrease with depth, the unbound soil thicknesses contribute to the overall structural capacity of flexible pavements by more than 50% [19]. Subgrade properties are of considerable importance for the design and rehabilitation of pavements. The procedure aimed at the determination of the pavement structure translates into one of the most significant implementations of soil mechanics knowledge in the field of road engineering. Several theoretical and empirical methods base the calculation of thickness of pavement layers on the strength of the soil foundation [20]. A direct relationship can be determined between the quality of a soil layer, its intrinsic bearing capacity, and the thickness of the layers; in fact, a higher ability of a layer to distribute the traffic load over a greater area is generally linked to the increased quality rate of that soil layer [21]. A reduction in the required thickness of the overlying pavement layers may be permitted as a consequence of the increased strength of the subgrade. Such improvement positively affects pavement life, too [3]. As a general example, the AASHTO Pavement Design Guide [22], an internationally consolidated design method, among several important items such as pavement performance, traffic, and features of the materials includes also consideration of the subgrade soil in defining the method of flexible pavement design. The pavement system must be designed to adequately protect the subgrade from excessive stress and in every operating condition; characterizing soil is a key factor for design purposes [23; 24]. The design of the pavement structure defined by the AASHTO guidelines [22] takes into account the properties of the subgrade soil by the average value of the resilient modulus (M\({}_{\text{R}}\)). Special attention is required for the assessment of the specific conditions of the roadbed soil: concerning the soil types and conditions, some particular situations are pointed out, such as the presence of excessively expansive soils and frost-susceptible or highly organic soils. The high water content, high compressibility, low strength, high swelling/shrinkage, and poor workability pose some restrictions to the potential of local soils to be used in road construction [25; 26; 27]. [PERSON] and [PERSON] [28] laid emphasis on the problems of expansive clay subgrades; in this type of soil, noticeable changes in volume rely on the variation of the moisture content. [PERSON] et al. [29] focused on soft subgrades with poor properties for highway construction, evaluating different methods as stabilizers, including five types of geosynthetics. [PERSON] and [PERSON] [30] carried out a series of laboratory tests to verify the potential of the stabilized silty soil to meet the construction standards as a highway subgrade. [PERSON] et al. [31] investigated the suitability of a controlled low-strength material (CLSM) as a pavement subgrade of high-traffic volume roads. In their experimental study, they used a local soil classified as high-plasticity clay. At the stage of construction, issues related to underlying soil stability may result in time delays and additional costs [29; 32]. In a recent study, [PERSON] et al. [33] examined the aspects related to locally weak subgrade soils and suggested specific stabilization treatments to support both the construction and pavement loading. Replacement of the existing in-situ soil with higher-quality soil to achieve the required bearing capacity of the subgrade increases the cost and time of the operations [13]. In-situ soil use is generally a viable and effective alternative as it minimizes the environmental impacts and contributes to a substantial decrease in project costs [31; 34].