## 1 Introduction
The transition of the soil condition from saturated to unsaturated induces a decrease in hydraulic conductivity as the suction increases, which heavily affects the soil-water dynamics. This indicates that water flow through a saturated soil column is inherently faster than a desaturated soil column [6]. Hence, infiltration through a saturated soil specimen is much more rapid than evaporation, wherein water is transferred from the soil to the atmosphere through a drying surface zone [6]. Some of the methods to determine the unsaturated hydraulic conductivity include approaches based on stationary flow approximations, transient flow analysis, mini-disk infiltrometer, horizontal infiltration, outflow, evaporation, and instantaneous profile [4; 5]. However, directly measuring the unsaturated hydraulic conductivity is time-consuming and costly [5; 10]. Direct measurement may also pose a challenge, since it changes as the soil suction changes, even if the void ratio remains the same [11]. In addition, the soil suction changes as the degree of saturation changes,which is a result of the flow of water that occurs within the specimen [11]. Hence, empirical, semi-empirical, and analytical mathematical models mainly were used for predicting the mechanical behavior of unsaturated soils [10]. Several studies have also proposed models for estimating the unsaturated hydraulic conductivity by bridging the WRC with the hydraulic conductivity curve. Unsaturated hydraulic conductivity may also be predicted using the WRC together with the constant saturated hydraulic conductivity of the specimen [12]. The filter paper method (ASTM D5298) is a practical and efficient technique of determining the soil matric suction, which can then be used to establish the WRC to indirectly estimate the unsaturated hydraulic conductivity. Compared to other direct measurement methods of determining the unsaturated hydraulic conductivity, it is cost-effective, requires minimal specialized equipment, and provides reliable results for various soil types. The changes in the soil-water interactions under unsaturated conditions due to cyclic wet-dry conditions also make the clay cover susceptible to volume changes and desiccation cracks that would increase the permeability. Previous studies have explored various approaches to improving the performance of clay layers, such as incorporating polymers or natural fibers to reduce cracking due to wet-dry cycles and to lower their permeability. Some studies have explored the addition of straw fiber in soils, which showed that there was a decrease in the amount of cracks as the amount of fiber increased [13; 14; 15]. Polymers are also often used in soil stabilization by reducing the rate of water invasion into the soil mass [16; 17]. According to [PERSON] [18], polymers strengthen structures for particles in proximity, and they can also stabilize particles far from each other through flocculation. Furthermore, polymers are also capable of penetrating pore spaces to achieve adequate stability of the soil mass [18]. Polyurethane is a polymer widely used in construction, particularly in ground improvement of expansive soils like clay. Polyurethane is formed from the chemical reaction between polyol and diisocyanate that produces urethane, a repeating unit in the polymer [19]. [PERSON] et al. [19] stated that polyurethane grout in soil nailing decreases settlement, while polyurethane grout as micro piles improves the response of the ground to dynamic forces. In addition, injecting polyurethane foam into the ground improves its strength, stiffness, and bearing resistance [19]. A study by [PERSON] et al. [20] explored the effects of polyurethane resin injection in a dry and desiccated expansive soil mass. The study concluded that the veins of the injected polyurethane resin in the clay acted as a barrier to moisture better than the intact clay. Furthermore, the injected polyurethane resin into the cracked, expansive soil did not increase its swelling potential. In a study by [PERSON] et al.