# Predicting the Pillar Stability of Underground Mines with Random Trees and C4.5 Decision Trees
## 2 Description of Database  
This study utilized the pillar stability database previously collected by [PERSON] and [PERSON] [26], [PERSON] et al. [27] and [PERSON] et al. [28] to determine and compare the performance of the proposedrandom tree and C4.5 decision tree models. The data recently referenced by [PERSON] et al. [18] comprised of a total of 46 pillar cases from USA underground stone mines and Indian coal mines which are shown in Table 1. The database contains pillar width (\(W\)), pillar height (\(H\)), the pillar \(W\)/\(H\) ratio, pillar stress (\(\sigma_{p}\)), and uniaxial compressive strength of the rock (\(\sigma_{ucs}\)). The range of values associated with each input variable is shown in Table 2. It has been widely accepted, among the researchers such as [PERSON] [29], that the input factors selected by [PERSON] et al. [18] is a complete and suitable set to predict the pillar stability. The same factors have been widely used as significant factors by other researchers (e.g., [PERSON] et al. [30]). Table 2 provides the descriptive statistics of each input parameter. The distribution of pillar case histories database is stable (14 cases) and failed (32 cases) according to the failure process and the instability mechanism of pillars. Figure 1 shows three common failure mechanisms for a naturally fractured pillar [31]. These include: (a) failure with the lateral kinematic release of pre-formed blocks because of rising vertical load and lack of confinement, (b) failure resulting from the buildup of inclined shear fractures that transect the pillar, and is prevailing in pillar with relatively low \(W\)/\(H\) ratio, and (c) failure along with transgressive fractures where the fracture inclination angle with the principal loading axis of the pillar exceeds the angle of friction. In all of these mechanisms, a pillar's mechanical response is directly related to the geological structures of the ground and those effects are most notable for the slender pillars. Although, wider pillars are more likely to collapse by a combination of brittle and shearing processes. Geological discontinuities in rock strata are of fundamental importance for analyzing the strength, permeability, and deformability. Characterization of the discontinuity in geometry (i.e., spatial connectivity, persistence length, and aperture, etc.) is the first step towards better understanding of rock masses overall behavior [32]. It is generally agreed that discontinuities in rock profile significantly affect their strength properties. Due to misjudment of the effect of discontinuities on rock strength, several examples may be quoted from the relevant rock failure literature. [PERSON] and [PERSON] [33] reported that discontinuities have a substantial influence with an increase in pillar inclination even at larger \(W\)/\(H\) ratios. [PERSON] et al. [34] attempted to address the problems of the tensile strength of incipient rock discontinuities and quantified this parameter based on laboratory test results. These indicators are considered the main parameters for quantitatively discovering the activities in the context of the pillar, but data collection is the main challenge faced for their applicability. Hence, the five parameters are considered in the current study.