## 1 Introduction  
The CFA (Continuous Flight Auger) pile has been widely used in recent years on ground where soil collapse can occur in the borehole due to the groundwater flow. In the CFA pile construction, the grouting is injected direct after excavation of a borehole in the CFA pile and hence the soil collapse can be minimized by confining of grouting in the borehole. Since the CFA pile is mainly constructed on weak ground conditions, studies have been conducted to estimate the bearing capacity and the behavior of CFA piles in various grounds ([PERSON] and [PERSON], 2008; [PERSON] et al., 2009; [PERSON] et al., 2016, [PERSON] and [PERSON], 2016 [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]). In the CFA pile, the loss of ground can affect the bearing capacity of the pile ([PERSON] et al., 1992 [8]). [PERSON] (2020) [11, 12, 13, 14] conducted a pile test on the CFA pile, and the main purpose of the loading test of the CFA pile was to evaluate the strength of the pile induced by the collapse of the soil. Therefore, this paper determined the behavior of the bottom of the pile affected by the soil collapse in the borehole along with the extension of this study. In particular, the roughness of the failure surface was evaluated with pile samples that simulate mixtures of grouting and soils at the bottom of the CFA pile. Studies have been conducted to measure roughness in geotechnical engineering. The shear strength between soil and structure is also affected by the roughness ([PERSON] et al. (2012), [PERSON] et al. (2014) [15, 16]) Therefore, it was considered in the design of structure and development of geotechnical structure ([PERSON] et al. (2017), [PERSON] et al. (2019) [17, 18]). [PERSON] and [PERSON] (1991) [7] measured the roughness profile of rock joints using a laser displacement meter (LDM) and a linear variable differential transformer (LVDT). However, it was focused on measuring the roughness in one dimension. [PERSON] et al. (2006) [6] proposed a new rock joint surface roughness measurement method by using a camera-type three-dimensional(3D) scanner as an alternative to previous methods. [PERSON] et al. (2017) [PERSON] et al. (2017) [PERSON] et al. (2017); [PERSON] et al. (2017); [PERSON] et al. (2017); [PERSON] et al. (2017) developed a stereo photogrammetry computational program in which three-dimensional (3D) soil surface coordinates were derived from digital stereo pair images of soil samples following erosion tests. [PERSON] et al. (2018) [PERSON] et al. (2018) [PERSON] et al. (2018) [PERSON] et al. (2018) presented a spiral sampling method that addresses the shortcomings of the straight-line method. The roughness parameters derived from the spiral scanlines are closely correlated with the Joint Roughness Coefficient (JRC) values back-calculated from direct shear tests. Research to define the relation between measured roughness and the strength of the ground has been performed ([PERSON] and [PERSON], 2018, [PERSON] et al., 2019 [PERSON] et al., 2019 [PERSON] et al., 2019). Studies on the correlation between roughness and the bearing capacity of foundations such as piles have been also conducted ([PERSON] et al., 2016, [PERSON] et al., 2018, 2018). Recently, SMART monitoring technologies have been applied to the various structures and studies to measure the roughness by using laser scanning have been conducted ([PERSON] et al., 2015 ([PERSON] et al., 2015) [PERSON] et al. (2019) [PERSON] et al. (2019) quantified surface roughness using laser scanning with application to the frictional resistance of sand-timber pile interfaces. [PERSON] et al. (2019) [PERSON] et al. (2019) evaluated the displacement of rough structure during the excavation. [PERSON] et al. (2017) [PERSON] et al. (2017) [PERSON] et al. (2017) applied atomic force microscopy (AFM) to define the features of wings scales of butterflies in microscale. In this paper, the roughness of the failure surface obtained by compression testing mixed samples of grouting and soils simulating the bottom of a CFA pile is measured by terrestrial laser scanning.