# Development of a Cone Penetration Testing Apparatus with a Textured Shaft
## 3 Experimental Results and Analysis
### Effect of Shaft Asperities and Load Direction on Cone Resistance  
Figure 5 illustrates the change in cone resistance over the entire testing time. Note that the negative convention of the measured force indicates the pull-out test. Regardless of the presence of surface asperities, the penetration load initially shows minimal variation due to the compaction effect on the upper layer of the sample but then begins to increase steadily. After 8 s of penetration, the cone with surface asperities exhibits significantly higher penetration resistance compared to the smooth cone, with the maximum penetration resistance of the cone with surface asperities (=2.39 kN) being higher than that of the smooth cone (=1.75 kN). During pull-out (around 30 s), the load is dramatically decreased, with cones featuring surface asperities initially showing higher pull-out resistance and reaching a maximum pull-out load (=\(-\)0.5 kN) at 33 s and then gradually decreasing. In contrast, the smooth cone shows almost no change in pull-out resistance (=\(-\)0.23 kN).  
Figure 6 shows the evolution of the measured load for different surface asperity shapes and load direction conditions. As shown in Figure 6a, penetration resistance is similar up to a certain depth (100-120 mm) regardless of asperity shape and load direction, but beyond this depth, there are clear differences depending on the asperity shape and load  
Figure 4: Experimental repeatability of cone penetration and pull–out testing. The smooth-surface cone is penetrated and pulled out at four different locations in the same test. Note that the negative convention of the measured force indicates the pull-out test.  
direction. The cone with surface asperity exhibits greater maximum penetration resistance than the smooth cone (=1.75 kN) regardless of load direction. Under the shearing A direction (asperity height increases during penetration), the maximum penetration load (=2.38 kN) is lower compared to that under the shearing B direction (=2.83 kN, where asperity height decreases during penetration). During pull-out, the pull-out resistance under the shearing A direction is rapidly changed with increasing displacement, whereas the pull-out resistance remains relatively constant in the shearing B direction. In addition, Figure 6b shows the change in cone resistance with a constant asperity length of L = 20 mm but a reduced asperity height of H = 0.3 mm. Compared to that in Figure 6a, the maximum penetration resistance is similar regardless of the shearing direction (shearing A = 2.39 kN, shearing B = 2.41 kN). In the case with the same asperity height (H = 0.3 mm) but a shorter asperity length (L = 6 mm), the shearing B condition produces a 1.07 kN higher maximum penetration resistance than the shearing A condition. There is a 0.66 kN difference between the two direction conditions during pull-out. A previous study on plate-sand direct shear tests with surface asperities used particle image velocimetry (PIV) to analyze soil movement near the interface during shearing [13]. The analysis revealed that shear failure at the interface formed wedge-shaped soil deformation patterns or uniform shear bands depending on the surface asperity geometry.  
Figure 5: Time series during penetration and pull–out testing.  
Figure 6: \(Cont.\)