## 1 Introduction  
With further improvements to the Chinese highway infrastructure, more and more highway engineering construction projects in mountainous areas will be carried out. In order to solve the problem of a shortage of subgrade filling materials in mountainous areas, it is urgent to study the engineering properties and applicability of special subgrade materials. In addition to stress-train nonlinear, elastic-plastic, dilatancy (shrinkage), and other properties, weathered granite, a type of special subgrade material, has its own unique engineering property, e.g., easy weathering and particle breakage. The different of weathering process determines the particle size distribution of the in-situ soils. Furthermore, the particle size distribution affects the particle breakage characteristics of soils. There are not many studies concerning the particle size distribution of natural weathered granite soil, but these types of studies are very important, because the particle size distribution has considerable influences upon engineering properties of weathered granite soil such as compatibilities, permeabilities, and strength-deformation characteristics [1]. The characteristics of particle breakage are obviously affected by the weathering process [1]. The structural failure or damage of soil particles caused by loading is the external cause of soil particle breakage. The internal causes of particle breakage in weathered granite are mainly microcracks (or voids) and fragile mineral components (e.g., biotite, feldspar, etc.) [2]. Particle breakage can cause a change in the distribution of soil particles, and then the physical and mechanical properties of soil will be changed [3,4]. As early as the 20 th century, some scholars discovered the phenomenon of soil particle breakage. However, because the number of construction projects was small and the experimental technology was backward at that time, the phenomenon of particle breakage of soil has not received sufficient attention for quite a long time [5]. It was not until one-dimensional compression tests on sand subjected to high confining pressure (96.5 MPa) were carried out by [PERSON] and [PERSON] [6] and its particle breakage was discovered that particle breakage of geotechnical material was extensively studied in the field of engineering. In the past, many methods and indexes were proposed to evaluate the degree of soil particle breakage [7, 8, 9, 10]. Some measured the increase in percentage of particles passing a given sieve size, some measured the decrease in sieve size corresponding to a given percentage passing [11], and some measured the changes in the overall particles [12, 13, 14, 15, 16]. Among them, four measurement methods, proposed by [PERSON] [10], [PERSON] [11], [PERSON] [13], and [PERSON] [14], are widely used in engineering practice. Among these four methods, because the concept of breakage potential (\(B_{p}\)) proposed by [PERSON] [14] is more scientific and easier to measure, the relative breakage index \(B_{r}\) has been widely used in correlative fields. The particle breakage of soils has been studied in previous research by using the proposed particle breakage indexes from different angles [17], but most of these studies were qualitative, and few quantitative studies were carried out using the indexes, which can truly reflect the overall change in the particle size distribution of soil. The relationships between different factors and quantitative breakage indexes were also less studied. The particle breakage properties of coarse-grained soil at high stress levels have been studied quantitatively [18, 19, 20, 21], but less research has been done on geotechnical material (e.g., weathered granite) that can be crushed at low stress levels [13]. Particle breakage of soil is influenced by several factors, such as initial void ratio, grain shape, grading curves, mineral composition, saturation, grain strength, load size, loading path, etc. [22].