# Development of a Cone Penetration Testing Apparatus with a Textured Shaft
## 1 Introduction  
Pile foundations are essential geotechnical structures supported by the bearing capacity of the pile tip and the interface friction between the pile shaft and the surrounding soil [1, 2]. Since the bearing capacity varies based on the construction method and soil displacement, it is crucial to select an appropriate foundation construction technique considering the site scale and geological conditions [3]. In such a case, the weathered and soft rock layers, which serve as end bearing layers, are often shallow, and pile foundations are typically constructed as end bearing piles. However, due to the limited availability of construction sites, there has been a recent shift to regions where the bedrock is over 50 m deep and the upper layers consist of sedimentary deposits [4, 5]. In these areas, the application of end bearing piles can encounter technical challenges and a steep rise in construction costs due to the increased pile length, making the use of friction piles more appropriate [6]. To enhance skin friction and facilitate easier pile penetration, various studies have investigated modification of the pile surface. For instance, helical piles, which feature helices attached to steel pipes to increase shaft friction, are installed using rotational devices, unlike traditional driven piles. This method reduces noise and vibration and allows construction with small equipment, making it suitable for various site conditions. The changes in ultimate load according to the rotational speed and the spacing between the helices were studied through full-scale tests [7]. The differences in load behavior basedon the geometric conditions of the helices (e.g., the number and diameter) were analyzed through laboratory model tests and numerical analysis [8; 9]. They confirmed that the helical piles showed relatively higher compressive and tensile capacities compared to traditional piles. Furthermore, a numerical study for micropiles with a modified waveform shape was conducted and revealed that the shaft friction increased while the overall pile length was reduced by approximately 15% [10]. The load-bearing capacities of waveform-shaped micropiles compared through centrifuge model tests identified the most effective pile shape for increasing resistance [11]. The construction feasibility and load-bearing performance of waveform micropiles in deep ground conditions were validated through field tests conducted in strata composed of reclaimed soil, weathered soil and weathered rock [12]. Recently, the evolution of biological systems in nature has been applied to engineering systems for innovative designs. In particular, asymmetrically shaped snake skin produces the movement-dependent response. Previous studies analyzed the frictional anisotropy of snake skin-inspired surfaces for different sands under monotonic loading conditions and in bidirectional shearing directions using a modified direct shear apparatus. The results of the interface friction tests showed that the shorter asperity lengths and greater heights significantly increased shear strength. In addition, higher interface friction angles were produced when the asperity height decreased in the shearing direction compared to the opposite direction when asperity height increased [13; 14; 15; 16; 17; 18]. Particle image velocimetry analysis revealed larger soil deformations and dilation induced within the soil when asperity height decreased. This shearing direction enabled soil to latch on scales and increased the contact area and contact soil behind the scales. Eventually, wedges tended to develop at the leading front of the scales where soil displacement, shear strains and volumetric strains are localized. The soil within the wedges developed during shearing experienced a local increase in the mean effective stresses. However, the caudal shearing direction hindered the latching of the soil on the scales and reduces the contact area [13].