# Improving Very High Plastic Clays with the Combined Effect of Sand, Line, and Polypropylene Fibers
## 1 Introduction  
In recent years, Algeria has started a significant program of repair and construction of pavements, in line with the economic dynamics the country is experiencing. Often, these projects are materialized in areas with problematic soils. One of these soils is the clay of Sidi-Hadjeres in the M'sila region. This clay is characterized mainly by high sensitivity to moisture changes; this is a major problem, since the climate in this specific region of Algeria is arid, with hot and dry summers and cold and rainy winters. When such clay is subjected to wet-dry cycles (due to weather or human activities), structures (such as roads)experience serious damages due to excessive or differential settlements, as well as due to the catastrophic action of heave. Randomly distributed fibers are frequently used in ground improvement in light of their benefits (availability, simplicity of mixing with soils, and their contribution to reducing weak planes) [1; 2; 3; 4]. Although fiber-reinforced clays exhibited higher stiffness and ductility than raw clay due to their increased energy absorption potential [5], polypropylene and basalt fibers when used alone have a limited effect on strength improvement [6], while [PERSON] et al. [7] observed an adverse effect of waste fibers (rubber) on the peak strength of soils; [PERSON] et al. also highlighted that with increasing fiber content, the ductility increases, while the swelling pressure decreases. In practice, the engineering properties of cohesive soils can be improved by incorporating non-expansive soils [8] or using chemicals, such as cement, lime, or other industrial additives, to neutralize the deformable character of the clay. Non-expansive soils contribute to a reduction in the deformability of clay minerals by reducing their plasticity, but their effect on strength is rather limited. For example, in the study of [PERSON] et al. [8], when clayey soil was mixed with dune sand (with 40% replacement), the UCS increased by 30% (from 161 to 209 kPa), while the plasticity index decreased by 18% (from 41% to 23%). On the other hand, the improvement in strength with lime (chemical stabilization) is more remarkable; in this respect, [PERSON] and [PERSON] [9; 10] indicated that the strength of clay tripled with the addition of 5% of lime, while the ductility decreased. However, despite the improvement in strength, lime-stabilized clays were characterized by brittle failure causing cracks in pavement layers [11]. The mechanism of strength development for lime-stabilized clay can be explained by the calcium ions (Ca\({}^{\pm 2}\)) resulting from the lime (quick lime (CaO) or hydrated lime (Ca(OH)\({}_{2}\)) reacting with the silica and alumina of clay to form stronger bonds of calcium aluminate hydrates (CAH) and calcium silicate hydrates (CSH) related to the soil particles. These processes are commonly defined in the literature by cation exchanges, agglomeration, and flocculation phenomena [12]. More recently, there has been an increased interest in using more than one of these additives for soil stabilization simultaneously to reduce the limitation of each one when used alone. In this respect, [PERSON] et al. [13] used dune sand along with lime to stabilize a very expansive clay, reporting that the plasticity of clay decreased significantly when these additives were associated. Further, the same authors reported a significant improvement in strength and indicated that the clay treated with both lime and sand exhibits a brittle failure. Similarly, [PERSON] et al. [6], used 6 mm-long polypropylene (PP) fibers and lime simultaneously to enhance the strength of kaolin (liquid limit \(\omega_{1}=52\%\) and plasticity index PI = 26%). Relatively, when kaolin was mixed with 1% polypropylene fibers (no lime added), the peak UCS increased after 28 days of curing from 97 to 150 kPa, while the UCS reached 400 kPa when 9% of lime was added to the previous kaolin-fiber mixture (again after 28 days of curing).