Advances in Civil Engineering

Volume 2018, Article ID 6509728, 9 pages

https://doi.org/10.1155/2018/6509728

## A Systematic Method to Evaluate the Shear Properties of Soil-Rock Mixture considering the Rock Size Effect

^{1}State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining & Technology, Xuzhou 221116, China^{2}School of Mechanics and Civil Engineering, China University of Mining & Technology, Xuzhou 221116, China

Correspondence should be addressed to Guangsi Zhao; moc.621@tmucsgz

Received 29 March 2018; Revised 23 June 2018; Accepted 2 August 2018; Published 5 September 2018

Academic Editor: Dingwen Zhang

Copyright © 2018 Minghui Ren et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The soil-rock mixture (S-RM) is widely applied in the geotechnical engineering due to its better mechanical properties. The shear strength, an essential aspect of S-RM which governs the stability and the deformation, is rather necessary to be revealed properly. The extraordinary issue of S-RM compared to fine-grained soils is the grain size effect on the strength analysis. This paper proposes a systematic method to obtain the realistic shear strength of S-RM by detecting the rock size effect. Firstly, based on fractal theory, the rock size was determined as 5 mm by the multifractal property of granular size distribution. Then, based on 2 selected specimen sizes combining the engineering dimension, shear gaps (*T*) effect and specimen size effect on the shear strength of S-RM have been investigated. It is shown that the gap of the direct shear test decides the physical mechanism of particles forming the shear resistance of S-RM based on the variation of apparent cohesion and mobilized internal friction angle. Specimen size effect is weakened by the gap effect considering the boundary effect. Realistic and stable shear strength parameters of S-RM have been researched by a reasonable gap (0.2–0.4*D*, where *D* is the largest particle size).

#### 1. Introduction

Soil-rock mixture [1] (sand-gravel mixture [2], bimrocks [3, 4], and gravelly soil [5, 6]) is a kind of special geological material widely distributed in slopes and landslides and frequently exists in geotechnical engineering works such as embankment dam, foundations, and tunnel excavation. Due to the limited understanding of this special geomaterial, the prediction of landslides and engineering failure became an intractable issue recently, and branches of studies have been carried out. Different from the fine-grained soils, the oversized particles in the S-RM lead to complicated mechanical properties such as the size effect and the structural effect.

The S-RM is commonly regarded as a two-phase material by distinguishing the “soil” and “rock,” and the rock block content is regarded as the crucial input parameter to evaluate its mechanical properties [7–9]. It was found that the increase of rock block proportion can increase the strength of S-RM [10, 11]. Accordingly, for the geomaterials with wide granular size distributions, the demarcation value between “soil” and “rock” is the essential physical quantity to understand the grading properties. However, the demarcation size of particles in related studies is differently arranged as 2 mm [2, 5], 4.75 mm [6], or undefined standard [12], which makes the related results difficult to be compared. Actually, with the increase of large particle size, the fixed “rock” size is not adaptive to calculate the rock proportion of S-RM. For this question, Medley [3] empirically found that bimrocks such as mélanges are scale independent in terms of engineering dimension—a fractal-like characteristic—and proposed that rock size in bimrocks is , where *A* is the engineering dimension. It is assumed that the strength and deformation parameters determined for S-RM with a certain rock proportion can be applied for preliminary engineering design with similar rock proportion depending on the engineering dimension [13]. A series of empirical approaches of artificial S-RM based on the laboratory test have been conducted [7–9], which consider the relative volumetric proportion determined by the characteristic dimension . The rock size considering the researching dimension is more reasonable to evaluate the effect of rock blocks. Nevertheless, it still possesses the insufficiency which targets at the self-similar regularity of “rock” size, neglecting the description of overall particle size distribution. For instance, the number of blocks in engineering dimension may be different depending on the block size even if volumetric block proportions are the same [9]. In addition, for some coarse-grained soils naturally existed or artificially applied in engineering, the largest particle size is limited with engineering dimension. That is to say, an effective method to describe the granular size distribution is vital for the study of S-RM. Only if these questions have been demonstrated, the related research results can be well referred for engineering design effectively.

The shear strength parameters are valuable for design and failure prediction of engineering, and related laboratory investigations have been carried out to understand the shear behavior of S-RM. Based on the in situ shear test, Zhang et al. [14] found that the existence of rock blocks makes the deformation modulus and the internal friction angle of S-RM greater than that of the soil sample, while decreasing its cohesive force. The fracture plane in S-RM, often rounded rocks and formed in soil, is shown in an irregular shape because of the existence of rock blocks [15]. It was reported that the mechanical behavior of coarse-grained soils is influenced by the cemented properties [16]. The influence of particle size on the shear strength of coarse-grained soils, subjected to different gradation of the specimen, was investigated by numerical and experimental direct shear tests [17]. It is widely believed that the shear resistance of soil is affected by various factors, such as soil type, compactness, and grading properties. Most importantly, no matter what the kind of research purpose of S-RM is, the laboratory test method is definitely vital for acquiring the strength properties [18]. The direct shear test, adopted in this study due to simplicity and convenience, possesses some shortcomings such as the fixed failure plane and the nonuniform stress and deformation in the shear box. The shear resistance of gravelly soils basically originates from sliding of particles and particle rolling, so the formation of the shear band closely depends on specimen size and shear gap dimension which represents the opening between shear box halves. And it is reported that the formation of the shear band is the important cause of the scale effect [19].

However, according to the author’s knowledge, only few articles about shear gap effect on shear strength of coarse-grained materials have been published [20]. Shibuya et al. [21] pointed out that the space between the upper box and lower box should be maintained at a constant value slightly larger than the thickness of a free shear band (approximately 10–20 times D50 for the sands). If the opening between shear box halves is too small, a portion of rock particles within the specified shear band will have crush and fracture failures, which causes the overestimation of actual shear resistance of the coarse-grained soil, while a large opening causes stress reduction and material loss at the specimen edge. To this end, this work attempts to acquire the reasonable shear gap of S-RM in the direct shear test, which is meaningful to obtain the more realistic strength.

#### 2. The Fractal Structure of S-RM Gradation

The first aspect of size effect in S-RM is the particle size distributions (PSDs), which can be applied to predict its mechanical and physical properties. The existence of gravels decides the nonlinear characteristics in PSD of S-RM. Figure 1 presents the original size distribution of S-RM generated in the tunnel excavation in Xuzhou, China. The mixture contains the clayey matrix, gravel, and rock blocks, and the largest particle size of the samples is 60 mm by eliminating the oversize rock block. It can be analyzed that the traditional Talbot grading curve is not effective to depict the size grading properties because of the wide range of size distribution of S-RM.