## 1 Introduction  
Road pavement functionality is strictly related to the properties and conditions of several intrinsic and extrinsic factors. Among all the variables affecting functionality of the pavement structure during its service life, the stability of the subgrade significantly affects its performance in the long term. The subgrade, as well as each layer of the pavement system, must satisfy specified structural quality standards in order to prevent permanent deformations, resist shear and fatigue, and safeguard the integrity of the overlying layers. Locally available materials often do not meet these requirements as they lack performance properties to answer both traffic and environmental demands [1, 2]. Subgrade conditions dramatically impact the pavement life, the initial cost of the pavement, as well as its maintenance costs [3]. Stabilization is an effective solution to upgrade the poor native properties of in-situ soil subgrades. Specific engineering properties such as strength, permeability, volume stability, and freeze-thaw durability can be improved in order to obtain a material of required or desired specifications [4]. Although the main engineering advantage of stabilization is to improve the strength and durability of the soil that is unsuitable for use in the pavement structure, this practice can also be used to reduce dust generation, control volume changes, and improve workability [5]. The most widely applied methods for the stabilization of the road subgrades are typically grouped into three categories: mechanical stabilization, chemical admixture stabilization, and reinforcement with geosynthetics [6; 7]. Over the years, several researchers have obtained noticeable results in the implementation of alternative practices and materials [8]. From the sustainability point of view, the aim was to optimize the use of \"marginal materials\", namely, materials that do not meet the quality requirements established by the standards currently in force [9; 10]. Moreover, the additional purpose was to seek cost-effective alternatives that could contribute to the decrease of energy consumption and greenhouse gas emission [11; 12]. Several suitable techniques for stabilizing the pavement subgrade currently involve the use of waste materials such as agricultural waste, wood waste, and construction waste materials or by-products obtained from industries [13; 14]. In the sphere of road construction, an economically attractive example is the application of lignin-based by-products as stabilizing agents of rejected or marginal locally available soils. Together with cellulose and hemicellulose, lignin is one of the main components of lignocellulosic materials; its basic function is to lend rigidity and strength to the structure of cell walls [15]. Commercially, lignin is a waste product, traditionally obtained from the paper and pulp industry. The global production of lignin is estimated to be about 100 million tonnes/year; almost all of this quantity (about 98%) is destined for low-value uses or energy sources such as the generation of fuels, heat, and electricity [16]. A deep bibliographic analysis of previous studies is carried out in the first section of this paper. The application of lignin as a stabilizing agent of poorly performing subgrade soils is not completely consolidated. Among all additives, lime is probably the most widely used concerning subgrade soil stabilisation [17]. Furthermore, the study reports an investigation on the strength and performance characteristics of a lignin-treated claye soil. In particular, the claye soil (A-6) used in this investigation was obtained as a laboratory mix of two soils with different properties: a fine sand S1--(A-3) and a pure claye soil S2--(A-7-6), respectively. The main aim of the mixing of S1 and S2 was to obtain a resultant mixed soil (S12) which can be used as road subgrade but it has poor performance given the presence of the clay matrix (about 50%); therefore, the use of stabilizing agents is required to improve their performance.