## 1 Introduction  
Interfacial adhesion refers to the ability of soil particles to adhere to the interface of other materials in the presence of water [1]. The system of soil adhesion comprises three elements: soil, solid surfaces, and their interface [2]. An example of interfacial adhesion commonly encountered is between clay and metal. It is important to differentiate between soil adhesion and soil cohesion, as shown in Figure 1, where the former involves soil particles adhering to other materials, and the latter refers to soil particles sticking to each other [3]. Figure 1: Illustration of adhesion and cohesion of soil. There are two types of soil adhesion: normal adhesion and shear (tangential) adhesion [2]. The applied load is the factor that determines the adhesion type. The normal adhesion is accommodated with normal tensile load, and the shear adhesion is accommodated with shear load, as shown in Figure 2.  
Adhesion behaviour at interfaces is influenced by various factors, including soil properties, material properties, interface properties and test conditions, as shown in Figure 3[4; 5]. Soil properties affecting interfacial adhesion include soil composition, grain size distribution, porosity, specific surface area (SSA), moisture content, water salinity, matric suction, plasticity, consistency and cohesion [4; 5; 6]. According to [PERSON] and [PERSON] [7], shear adhesion and moisture content exhibit a parabolic relationship, as defined in Equation (1). \[I_{c}=\frac{LL-MC}{PI} \tag{1}\]  
where \(I_{c}\) is the consistency index of the soil, \(LL\) is the liquid limits, \(MC\) is the moisture content, and \(PI\) is the plasticity index. Thus, there is a relationship between the consistency index and shear adhesion, which assists in understanding the behaviour of clay adhesion [8]. Material properties that influence interfacial adhesion include the contact area, applied normal load, and roughness of the surface area [9]. Testing conditions, such as the contact time at the soil-material interface, rate of loading, humidity, and temperature of the interface surface, also play a significant role in affecting the behaviour and value of interfacial soil adhesion [6; 10]. The interfacial soil adhesion derives importance from its applications in geotechnical engineering and agriculture, where it can pose potential challenges. For example, pile foundations, as shown in Figure 4, may encounter stickiness issues during the digging process [11], and the cutter head of a tunnel boring machine (TBM) can experience clogging problems at the interface due to interfacial soil adhesion [12; 13; 14]. Figure 3: Factors affecting interfacial soil adhesion. Figure 2: The adhesive tensile strength (normal pull) and the adhesive shear strength (tangential pull). These practical applications of interfacial soil adhesion can lead to unexpected cost implications. Additionally, soil sticking to agricultural machines during harvesting can result in high energy consumption and reduced work efficiency [6; 8]. Therefore, it is crucial to investigate the behaviour of soil adhesion with other materials. Several studies have employed different methods to measure soil adhesion; however, there is no specific method or device exclusively designed for soil adhesion measurement [3]. Soil adhesion can be measured in the laboratory using pull-out tests [12], piston separation tests [15] and shear plate tests [16]. [PERSON] [1] used a specialised apparatus to measure normal adhesion for loam clay and sandy clay under the influence of water surface tension. Their study revealed that moisture content and material type have an impact on soil adhesion. [PERSON] and [PERSON] [12] utilised pull-out tests to investigate clogging issues in TBMs, where normal soil adhesion occurs. It was found that clay minerals and soil consistency index (\(I_{c}\)) affect normal adhesion. [PERSON] and [PERSON] [17] designed a dedicated instrument to measure normal adhesion between clay and steel.